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The Track to Span 3: Genesis of the innovative Manhattan West Platform

 

Photos courtesy Entuitive

Photos courtesy Entuitive

by Mike Hillcoat, P.Eng., CCCA

There is a wide hole in the ground at the corner of 9th Avenue and 33rd Street in Manhattan; it has been there for more than a century. More commuters pass through it than any other hole in the ground in North America. For those 430,000 people who cross through it every day, that hole is the brief glimpse of light they get after they have passed through the tunnel under the Hudson River. It is a sign they are about to enter Penn Station; they have made it to New York City.

Despite its prime location, development of this parcel of land has been thwarted over the years precisely because of this hole in the ground. This changed in 2008, when Brookfield Properties watched a steady migration of interest toward Manhattan’s Middle West Side, spurred by interest with the recently opened High Line elevated park and the buzz over the Hudson Yards development.

The developer announced it had a vision for the hole in the ground. Its plan was to cover the tracks and develop the adjacent properties with office towers with a common podium to create a new neighborhood combining commerce, lifestyle, recreation, and transportation where the tracks used to be.

This vision was not easy to realize. When structural engineers first looked at the prospect of constructing a 73-m (240-ft) wide platform over the 15-track-wide rail corridor, they quickly looked to break the span into manageable bays, with intermediate columns down to track level to break the span.

When Brookfield approached the railroads seeking permission to construct dozens of footings sandwiched between the tracks, it swiftly became evident the proposal was impractical. Amtrak and Long Island Railroad’s (LIRR’s) train schedule in and out of Penn Station was not just busy—there were simply not enough hours in the week to get all the trains in and out. Opportunities for closing individual tracks for extended periods to allow footing construction would be extremely limited. The railroads could only offer one three-hour window per week, on Sunday mornings between 2:00 and 5:00 a.m.

It was clear Brookfield’s project team would have to get creative to unlock the site’s value. In 2010, the developer put forth a challenge to a few of its preferred structural engineers used to design its buildings around the world. Since the company operates primarily in the buildings business, the solutions that came back typically involved the one thing that just could not work: columns.

Overlooking the site for the 650,320-m2 (7 million-sf) Manhattan West mixed-use development project located in the Hudson Yards District, the platform takes shape with Spans 1 through 3 already erected, and Span 4 being assembled under the launcher (left). Above, bonded topping slab concrete is being poured over Spans 3 and 4.

Overlooking the site for the 650,320-m2 (7 million-sf) Manhattan West mixed-use development project located in the Hudson Yards District, the platform takes shape with Spans 1 through 3 already erected, and Span 4 being assembled under the launcher (left). Above, bonded topping slab concrete is being poured over Spans 3 and 4.

A view of the launcher from above as it is being erected over the temporary protection platform. Photos © Tom Arban. Photos courtesy Entuitive

A view of the launcher from above as it is being erected over the temporary protection platform. Photos © Tom Arban.
Photos courtesy Entuitive

 

 

 

 

 

 

 

 

 

 

The clear-span solution
There was one proposal that addressed the column issue. Barry Charnish, a Toronto-based engineer who had recently helped Brookfield in engineering a unique climbing scaffolding system to replace marble stone cladding on its 72-storey First Canadian Place tower in Toronto, presented a column-free platform solution.

Charnish—who had just started a new engineering firm, Entuitive—suggested the 73-m (240-ft) wide platform could be built in a single span using deep post-tensioned concrete box girders. The technology was not uncommon in bridge construction; typically bridges two boxes wide, each would carry one direction of traffic. Charnish proposed the platform could be a bridge 16 box girders wide.

Given the idea’s ambition, Charnish had to develop the design to a state where it could be presented to Amtrak and LIRR with confidence. With his newly formed firm, Entuitive, he helped develop the initial design for the post-tensioned precast girders, originally specifying three walled box beams.

The three walled sections were abandoned for two-walled box girders to allow the box’s interior to house ventilation fans, and act as plenums for ventilation of the tracks to be covered below. The depth and layout of the box sections were limited to clear the train electrification cables, and the catenary structures supporting them. These constraints on the box girder section depth meant a concrete strength of 65,500 kPa (9500 psi) for the segments, and that the post-tensioning reinforcement had to be pushed to the limits with 122 kg/m2 (25 lbs/sf)—more than four times what would normally be used in bridges of this span.

The concept was then shared with bridge design and construction firms to solicit feedback. Rizzani de Eccher’s bridge division, and McNary Bergeron & Associates, provided valuable input on constructability, and a collaborative relationship developed to move the design forward.

Commuter train traffic under the completed launcher and temporary protection platform.

Commuter train traffic under the completed launcher and temporary protection platform.

This view of the launcher from the north shows temporary rails for assembly of precast segments.

This view of the launcher from the north shows temporary rails for assembly of precast segments.

 

 

 

 

 

 

 

 

 

 

 

Constraint leads to innovation
Entuitive and McNary Bergeron collaborated to develop a design for the post-tensioned precast concrete platform supported on deep reinforced concrete capping beams, on caissons socketed into the rock below track level. Brookfield presented the column-free scheme to the railroad authorities, and, with the help of an animated video prepared under Entuitive’s direction, it accepted the idea.1 The single three-hour window per week, however, still stood. The project would have to be built during that narrow period without disrupting train traffic.

These constraints drove the design. In addition to the challenges of construction at track level, it is also generally forbidden in the rail industry for construction to occur above active tracks without protection. Entuitive and McNary considered various schemes to allow the bulk of the girders to be fabricated offsite and then ‘launched’ out over the tracks in the three-hour closure window the railroad authorities would allow. The final scheme involved the construction of a Temporary Protection Platform (TPP)—a steel bridge over the tracks on which precast segments would be assembled and post-tensioned.

Another unique innovation on the project involved determining how to launch these spans into their final position from the TPP. Entuitive envisaged a massive rolling gantry, similar to those in container terminals and shipyards, that could lift the assembled spans off the TPP, drive them into place, and lower them onto the capping beam.

Entuitive designed continuous reinforced concrete beams to be built just beyond the north and south sides of the tracks, which would act not only as the permanent support for the platform, but also support a track on which the launching gantry could drive. A specification was developed for the launcher, identifying speeds, lifting capacity, tolerances, and safety factors.

At this stage, Entuitive and McNary had refined the weight of the individual platform girders to approximately 2400 tons—about the weight of 150 public buses. The next step was to find a contractor or equipment manufacturer that could design and build a machine that could span 73 m (240 ft), lift 2400 tons, and drive with that load suspended from it.

Italian company Rizzani de Eccher’s specialized construction equipment manufacturer, DEAL Equipment, designs massive machines purpose-built for the site conditions and constraints of a single project.2 In the case of Manhattan West, the machine it would have to design would only need to be used 16 times.

The project required line drilling rock for capping beam construction adjacent to the active rail lines. Photos © Tom Arban. Photos courtesy Entuitive

The project required line drilling rock for capping beam construction adjacent to the active rail lines. Photos © Tom Arban. Photos courtesy Entuitive

Rail traffi c through the site is constant. This means that every construction operation requires complete, thorough consideration to avoid impact on rail operations.

Rail traffic through the site is constant. This means that every construction
operation requires
complete, thorough
consideration to
avoid impact on rail operations.

 

 

 

 

 

 

 

 

 

 

 

Construction begins
Foundation construction commenced at Manhattan West in August of 2012. The site is fortunate to have super-dense and contiguous Manhattan Schist rock, at a high elevation, upon which to bear the foundations. This means Brookfield’s hole in the ground is prime land to construct super-tall skyscrapers with minimal foundation depth. Manhattan’s skyline mirrors the elevation profile of the Manhattan Schist, with the tallest buildings located downtown and midtown where the rock approaches the surface, but with typically low-rise construction through Greenwich Village and SoHo where the rock dips down considerably. The prospect of building the deep foundations down to the rock required for tall buildings has always been cost-prohibitive.

While the rock was a boon for Brookfield’s long-term plans of two high-rise commercial towers and a high-rise residential building, it presented challenges early on in the construction of the platform. Drilling caissons 10.7 m (35 ft) into the rock on the north and south sides of the site, below the track bed, to support the capping and launcher beams was a somewhat slow process, but access was relatively straightforward.

To construct the TPP, however, two spread footings had to be excavated between active tracks. Since track closures were not an option, it was impossible to use excavation equipment. The ballast had to be excavated by hand, excavation shoring installed tight against the rail ties on each side, and then the arduous process of hand-chipping rock capable of sustaining 40 tons per square foot down 1.8 m (6 ft) in a 1.5-m (5-ft) wide slot between trains began.

Due to the space constraints, only two people could work in the hole at a time. Excavation for these two footings took several months each. It was through this process Brookfield fully realized a platform with dozens of columns could likely never have been built—it was on the right track with the clear-span approach.

Fabricating the launcher
Meanwhile, in Italy, the equipment manufacturer was putting the final touches on its launching gantry. The primary girders were each about 4.6 m (15 ft) deep, and had to be spliced in two in height and into 10.7-m (35-ft) long segments to fit the individual pieces into shipping containers bound for New York by sea.

The hydraulic lifting components, and the wheel groups which would drive the launcher, were complete, but Brookfield was hesitant to allow anything to be shipped without testing to verify the equipment was actually capable of lifting the precast spans. It quickly became apparent, however, a full-scale load test in Italy was impractical. The cost to construct footings that could accommodate the same loads as New York’s Manhattan’s Schist, on the organic soils of the northern Italian province of Udine, and the logistics of creating a test block matching the load of 2400 tons was just not conceivable, even for the engineers who had designed the massive, intricate launcher.

Performance tests proving out the hydraulics and electronics of all the operable parts were conducted at the manufacturer’s shop, but the full-scale load test could not be completed until the launcher was fully assembled in New York City, over the TPP and the busy rail lines.

By the fall of 2013, the cap beams that would support the spans and the launcher were complete, the Temporary Protection Platform was erected, and Jersey Precast, the contractor who had been casting the segments that formed the spans, was ready to ship to the site. The segments were massive; they were 3.5 m (11 ½ ft) tall, wider than 9.1 m (30 ft), and weighed up to 56 tons per segment. The geometry was governed by shipping considerations, including the width and weight restrictions imposed by the bridges leading into Manhattan. However, when the segments arrived at the site, they were dwarfed by the scale of the launcher, which was now being erected.

The manufacturer’s chief engineer, Daniele Tosoratti, was the mastermind behind all aspects of the launcher. He was onsite to oversee the erection at Brookfield’s request. The rationale was since the launcher could not be tested before shipping, it was critical to have the person who knew every aspect of the machine onsite for the erection, commissioning, and in-situ load test.

As it came together, the launcher was turning heads on Manhattan’s West Side. The bright yellow girders towered above the tracks, and with the winch gantry erected atop the girder, the Launcher stood 33 m (108 ft) tall above the capping beam, and a total of about 12 storeys above the tracks below. New Yorkers have seen a lot, but this kind of equipment was unique in an urban construction environment.

Setup and operation of the secondary components such as the segment manipulator, which picked up the individual segments and assembled them on the TPP, went smoothly. The first span that would be assembled, post-tensioned, and launched would be Span 3. Post-tensioning and grouting of the span were also completed with only minor challenges, the most significant being the relentless cold temperatures the winter dealt. Despite these challenges, Rizzani was close to being on schedule, and were narrowing in on a date to launch the first girder.

Preparation and challenges
Brookfield wanted to give the railroad authorities as much advance notice as possible, since the launch meant all the electrified catenary cables that power the trains into Penn had to be shut down. A date was set for early January, but before anything could proceed, that full-scale load test had to be completed.

Tosoratti returned to New York for the final commissioning and preparation for the load test. Every component of the machine had built-in safeguards, shut-offs, safety mechanisms, and redundancies, and Tosoratti had to ensure all were performing perfectly before the 2400-ton span (plus the six additional segments placed on top as an added factor of safety) could be lifted. There was an army of workers on the launcher 24 hours a day during the week leading up to the scheduled launch of Span 3. However, the load test kept getting pushed back. It was of monumental importance to demonstrate confidence and preparedness to the railroad authorities to execute an operation like this, and it was becoming clear the launcher was just not ready.

About 48 hours before the scheduled launch, Rizzani pulled the plug, with Tosoratti making the call. There was an issue with some low-voltage communication wiring that was used to ensure the two ends of the launcher lifted simultaneously and drove in unison. The wiring and connectors were from Italy, and compatible equipment could not be found in New York. It was a safeguard, but nothing could proceed until every piece was in place. Rizzani said it could get what it needed shipped and installed by the end of the weekend. Brookfield gave them a week.

The railroads were accommodating to moving the date. If all parties were in agreement on one thing, it was safety. The launch was rescheduled for the night of January 15, 2014.

The above photo illustrates the installation of shear studs and conduit for the precast platform bonded diaphragm concrete topping at the Manhattan site. Photos courtesy Entuitive

The above photo illustrates the installation of shear
studs and conduit for the precast platform bonded
diaphragm concrete topping at the Manhattan site.
Photos courtesy Entuitive

Placing reinforcement in the north capping beam that will support the platform structure and the launcher rails.

Placing reinforcement in the north capping beam that will support the platform structure and the launcher rails.

 

 

 

 

 

 

 

 

 

 

Ready for launch
On January 14, Rizzani was ready to perform the load test. It could be completed during the day, with the tracks below in operation, because the test would be performed over the Temporary Protection Platform. The hydraulically powered lifting devices picked the precast span off the TPP, and drove it a meter or so to prove the wheels would not seize under the enormous load. Luckily, the test was anti-climactic—there was neither creaking and groaning nor the ‘bang’ structural engineers expect when temporary shoring is removed from a long-span structure and the bolts simultaneously slip into bearing. It just lifted and rolled and proved it was ready to do its job.

At approximately 1:00 a.m. on January 15, Amtrak cut the power to the tracks leading into Penn Station. The launcher was aglow with flood lights, with a faint beeping being the only sound to signal it was ready for operation. Span 3 was lifted off of the TPP, and was hanging from the launcher when Rizzani got the ‘all clear’ to run it out over the tracks. It rolled smoothly and quietly; within a few short minutes, it was dangling above the bearings that would permanently support it on the capping beam. Lowering the span was a slower process, with position and elevation having to be precise to align with the bearings and with the subsequent spans.

By 5:00 a.m., the launch of Span 3 had been completed, and trains started rolling through Brookfield’s hole in the ground, and in to Penn Station. Commuters noticed something different that morning. It might have been that that glimpse of light signaling their arrival in New York got a slight bit shorter, or the bright gray concrete bridge overhead contrasting with the adjacent retaining walls blackened by a century of exhaust. It was clear something new was happening above the hole. For Brookfield, it was just the first monumental step toward developing towers on a site once thought to be unbuildable.

Notes
1 To see the video, visit www.manhattanwestnyc.com/content/innovation/deck_engineering_and_technology-29602.html. (back to top)
2 Rizzani convinced Brookfield it was the right fit for the job when it commented the work involved “only” 2400 tons. (back to top)

Mike Hillcoat, P.Eng., CCCA, has more than 15 years of experience in contract administration and construction, with a strong focus in the transportation, commercial and hospitality sectors. His recent projects include the Manhattan West Platform and two Greater Toronto Area (GTA) projects: VIVA Bus Maintenance Facility and the Union Station Revitalization. Hillcoat is a Professional Engineer in the Province of Ontario, a Certified Construction Contract Administrator, and a member of Construction Specifications Canada (CSC) and the American Institute of Steel Construction (AISC). He can be reached at mike.hillcoat@entuitive.com.

Designing Stone Wool Ceiling Assemblies

All images courtesy Rockfon

All images courtesy Rockfon

by Corey Nevins

Specifiers have an increasing number of choices for commercial ceiling systems. Among the performance considerations for selecting the most appropriate for a particular application are acoustics, fire performance, humidity resistance, hygienic properties, dimensional stability, indoor air quality (IAQ), and light reflection. Added to these are choices pertaining to design aesthetics, ease of installation, maintenance, durability, sustainability, and cost.

Stone wool ceiling panels and metal suspension systems meet these selection criteria for both new construction and renovation projects throughout North America. The material was discovered on the islands of Hawaii, where it occurs naturally as a by-product of volcanic activity. The primary rock involved is basalt, the earth’s most abundant bedrock. The igneous material forms by the rapid cooling of lava from eruptions on the sea floor. Seismic activity, including the earth’s volcanoes, produces 38,000 times more rock material than is used by the world’s largest producer of stone wool.

The typical production process for stone wool begins with the fusion of this volcanic rock at a temperature of 1500 C (2732 F). Emerging from the furnace, the melt runs out of the bottom and onto a spinning machine, where wool is whipped into thin strands, similar to making cotton candy. The strands form ‘wool,’ held together with minor amounts of organic binders.

Stone wool ceilings offer good sound absorption, high light refl ectance, fi re protection, and humidity resistance. These panels are well-suited to create modular ceiling designs, such as long corridors.

Stone wool ceilings offer good sound absorption, high light reflectance, fire protection, and humidity resistance. These panels are well-suited to create modular ceiling designs, such as long corridors.

Now a fleecy web, the material is gathered and formed; the number of layers varies depending on the final product’s desired structure and density. The layered fibers then move to a curing oven. Once cured, the wool emerges with non-directional fibers that contribute to its multiple performance characteristics of the stone wool products. In addition to ceiling panels, stone wool’s unique combination of thermal, fire, and acoustic properties make it suitable for:

  • blown insulation in cavity walls;
  • rolls of loft insulation;
  • pre-formed and faced pipe sections; and
  • wall slabs.

A mineral fleece and water-based paint are layered on top of the stone wool to produce the finished ceiling panels. The stone wool products proceed to cutting saws, finishing and packing equipment, or are led to off-line equipment for special treatment. The majority of the waste created during the production is fully recyclable.

Use of suspended ceilings
Since the 1950s, drop ceilings have been the preferred method for concealing HVAC vents, electrical wires, plumbing pipes, phone cables, and security lines in interior commercial buildings. These suspended, interconnected ceiling systems consist of a metal grid comprising cross-tees and main runners.

The main runners are suspended by hanger wires from the structure above, and wall channels or angles provide a clean look throughout the perimeter. Panels are used to conceal the plenum—hiding the visible structure, suspension system, HVAC, and other equipment, while providing simple access for future maintenance.

The suspension ceiling system is selected for aesthetics, maintenance, and specialized performance such as fire resistance, seismic mitigation, or limited accessibility in security applications. For all ceiling designs, specifiers should check the suspension systems are manufactured to ASTM International standards. On request, suspension manufacturers may provide reports from the International Code Council (ICC) and third-party seismic performance testing and certification reports.

Corrosion resistance is also a priority for metal suspension systems supporting stone wool and other ceiling panels. The industry standard is 23.8-mm (15/16-in.) galvanized steel for suspended metal ceiling grids; most may be specified with a minimum of 25 percent recycled content.

While the ceiling panel’s size, orientation, color, finish, and edge largely determine the overall aesthetic, changing the size of the grid’s face also changes the appearance. For example:

  • a 14.28-mm (9/16-in.) narrow face diminishes the distinction between grid and panel for a more monolithic look;
  • adding a 3.17-mm (1/8-in.) slender, center regress with a ‘bolt-slot’ design accentuates the shadow between panel and grid;
  • mitered intersections provide crisp, continuous lines for a uniform ceiling plane;
  • wide-face 34.92-mm (1 3/8-in.) ceiling suspension offers bolder expression of the ceiling grid modules, especially at high elevations; and
  • in curved drywall applications, radius systems create concave and convex shapes, including barrel-vaulted ceilings.
When a sound wave hits a surface, part of the energy is refl ected, part of it is absorbed by the material, and the rest is transmitted. Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces.

When a sound wave hits a surface, part of the energy is reflected, part of it is absorbed by the material, and the rest is transmitted. Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces.

The noise reduction coeffi cient (NRC) refers to a surface’s ability to reduce noise by absorbing sound. NRC is important in areas where high levels of noise (like a photocopier) are present.

The noise reduction coefficient (NRC) refers to a surface’s ability to reduce noise by absorbing sound. NRC is important in areas where high levels of noise (like a photocopier) are present.

 

 

 

 

 

 

 

 

 

 

 

Specifying acoustic comfort
According to the World Health Organization (WHO):

noise seriously harms human health by causing short- and long-term health problems. Noise interferes with people’s daily activities at school, at work, at home and during leisure time. It can disturb sleep, cause cardiovascular and psychophysiological effects, hinder work and school performance and provoke annoyance responses and changes in social behavior.1

Therefore, it could be argued design professionals have a duty to create acoustic comfort and well-being for the occupants of their buildings. Stone wool can help with two primary components of acoustic comfort: speech intelligibility and noise reduction.

The material’s airflow resistance and density contribute to its high noise absorption properties. The fibers’ size and non-directional orientation lead to stone wool’s inherent sound-absorbing qualities. The measures and concepts discussed in this article provide a foundation for understanding the relationship between stone wool’s characteristics as a material and achieving acoustic comfort.

Speech intelligibility
One important component of acoustic comfort and sustainability, speech intelligibility refers to a listener’s ability to hear and understand a speaker in a room or space. It is measured as a signal-to-noise ratio, expressed in decibels (dB). For this application, the signal typically is speech and the noise usually is everything else in the background.

Reverberation time
An important factor for creating speech intelligibility, it is defined as the time it takes for the sound pressure level to decrease 60 dB below its original level. In most situations (excluding unamplified music performance), a lower reverberation time improves speech intelligibility and acoustic comfort. For most rooms requiring speech intelligibility, mid-frequency reverberation time should be between 0.50 and 1.00 seconds when the room is unoccupied.

Noise reduction coefficient
The noise reduction coefficient (NRC) indicates a surface’s ability to reduce noise by absorbing sound. It is calculated by averaging the absorption coefficients from the 250-Hz, 500-Hz, 1-kHz, and 2-kHz octave bands. It varies between 0.0 (i.e. absorbs very little sound) and 1.0 (i.e. absorbs a lot of sound). NRC is one of two important variables in determining reverberation time (the other being room volume). A higher NRC indicates more noise reduction (or sound absorption) and leads to lower reverberation times and greater speech intelligibility. Stone wool ceiling products typically have an NRC of 0.85 or higher.

Background noise
Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces. It can also include sounds generated by the building’s systems or even those reverberating too long inside the room.

Speech intelligibility
Factors influencing speech intelligibility include:

  • speech signal’s strength and clarity;
  • sound source’s direction;
  • level of background noise;
  • room’s reverberation time and shape; and
  • listeners’ hearing acuity and attention span.

Reverberation time depends on two main variables: the volume of the room and the amount of sound-absorbing materials. As volume decreases or as the amount of sound-absorbing materials increases, reverberation time decreases and speech intelligibility generally increases. Since the volume of the room often depends on functional and aesthetic criteria, reverberation time is often solely dependent on the amount and efficacy of sound-absorbing materials.

In many cases, placing sound-absorbing materials on the walls is not desirable due to its tendency to get damaged, dirty, or worn because of occupant contact. As a result, whether speech intelligibility is poor, fair, or good can highly depend on the ceiling specified. This is why acoustic standards and guidelines for schools, hospitals, offices, and other types of facilities have minimum NRCs of 0.70 and up to 0.90. Stone wool ceiling panels, more than other panels made of less-absorbing materials, help ensure projects comply with acoustic performance requirements in these standards and guidelines.

Even if reverberation time is appropriate, speech intelligibility can be low if the background noise in the room is too loud. Speech intelligibility equates to a high signal to noise ratio. Consequently, it is also important to ensure noise from the exterior, other interior spaces, and from the building’s systems is controlled.

Noise reduction
In other rooms or spaces like open offices, cafeterias, libraries, and gymnasia, speech intelligibility is not the primary acoustic goal; rather, the push is for overall noise reduction for stress relief and concentration. Noise reduction equates to an overall decrease in sound pressure level from loud continuous noise (e.g. traffic noise transmitting into the building), as well as event-specific noise (e.g. a crying baby). The sound pressure level in a room depends on the strength of the sound source, the room’s size, and the quantity and quality of sound-absorbing surfaces.

Just 30 decibels of periodic noise can be disturbing to sleep or concentration. Conversational speech is generally between 50 to 70 dB. Noise with sound levels of 35 decibels or more can interfere with speech intelligibility in smaller rooms. This is demonstrated by a phenomena known as the ‘cocktail party effect,’ whereby as noise levels get louder and louder, people try to talk louder and louder to be understood. Despite their efforts, speech intelligibility decreases and acoustic stress increases. It is not until someone leaves the ‘party’ that they realize just how agitated they were as their muscles begin to relax, heart rate slows, and respiration deepens. Stone wool, because of its high noise-absorbing characteristics, also helps achieve the overall noise reduction goals.

Whether sound reduction is needed for speech intelligibility or overall acoustic comfort, blocking noise that could be in the plenum above the ceiling can also be important in some instances. As more acoustics standards and guidelines place minimum noise control criteria on wall constructions (i.e. sound transmission class [STC]), the need for ceilings to block noise from adjacent spaces traveling via the overhead plenum is becoming less frequent. This is because achieving the minimum STC wall requirements necessitates the walls be extended up to, and sealed against, the underside of the deck above them. However, in the cases where the walls do not extend full height, or where there may be noisy mechanical equipment in the plenum, the ceiling also may need to block noise from transmitting into the space below them.

Ceiling attenuation class (CAC) indicates the ceiling’s ability to prevent airborne sound from traveling between adjacent rooms when the demising walls do not intersect with the structural deck above. CAC is also a good measure to judge how much protection is offered against noisy mechanical equipment in the plenum. The higher the CAC value, the greater the ceiling’s blocking capacity. A CAC value of 35 dB is considered to be moderately high and may be specified for stone wool ceiling panels. When even higher sound-blocking capacity is required, stone wool ceiling panels can be specified with a CAC value up to 43 dB in combination with a high NRC of 0.85.

Insulation infl uences the sound level in the receiving space, helping provide more privacy between rooms and better concentration in the adjacent room.

Insulation influences the sound level in the receiving space, helping provide more privacy between rooms and better  concentration in the adjacent room.

In practice, there is a strong link between sound absorption and room-to-room sound insulation. This link may not be accurately refl ected in laboratory testing. In practice, two ceilings with the same ceiling attenuation class (CAC), but different NRCs, produce different levels of perceived sound insulation. The ceiling with the highest NRC will do a better job of lowering the sound pressure in both the sending and the receiving room.

In practice, there is a strong link between sound absorption and
room-to-room sound insulation. This link may not be accurately
refl ected in laboratory testing. In practice, two ceilings with the same ceiling attenuation class (CAC), but different NRCs, produce different levels of perceived sound insulation. The ceiling with the highest NRC will do a better job of lowering the sound pressure in both the sending and the receiving room.

Total sound insulation is the ability of a total construction (e.g. partitions, ceiling, fl oor and all connections) to prevent sound from traveling through the ceiling void and through building elements. Sound insulation of ceilings is measured using CAC, while walls are measured using the sound transmission class value (STC).

Total sound insulation is the ability of a total construction (e.g. partitions, ceiling, floor and all connections) to prevent sound from traveling through the ceiling void and through building elements. Sound insulation of ceilings is measured using CAC, while walls are measured using the sound transmission class  value (STC).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fire performance
Every second counts once a fire has started. Specifiers know choosing the right building materials can delay the spread of fire and provide the vital extra minutes needed to save the occupants and limit the damage.

Given its volcanic origins, stone wool can withstand temperatures up to 1177 C (2150 F). It is non-combustible, will not develop toxic smoke, and does not contribute to the development and spread of fire even when directly exposed to fire.

Ceiling panel products are required to be tested for surface burning characteristics to Underwriters Laboratories (UL) 723/ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials. Testing requires 7.31 m (24 ft) of material to be exposed to a flame ignition source in a Steiner Tunnel Test to determine how far the fire will spread during 10 minutes, and how much smoke is developed during this period.

The test was developed by Al Steiner of UL and has been incorporated as a reference into North American standards for materials testing. The progress of the flame front across the test material is measured by visual observation, while the smoke emitted from the end of the test assembly is measured as a factor of optical density. A Flame Spread Index and a Smoke Developed Index are calculated from these results. Both indices use an arbitrary scale in which asbestos-cement board has a value of 0, and red oak wood has a value of 100.

Many commercial applications require a Flame Spread Index of 25 or less and a Smoke Developed Index of 50 or less. Products labeled “FHC 25/50” (Fire Hazard Classification 25/50) or “Class A” (ASTM E1264, Standard Classification for Acoustical Ceiling Products) fulfill these requirements. Stone wool ceiling panels may be specified to meet the most stringent requirements with a maximum Flame Spread Index of 0 and a maximum Smoke Developed Index of 5.

Humidity and hygienic attributes
Humidity can weaken the structure of certain ceiling materials, causing them to sag and, in extreme cases, even fall out of the suspension system. This often happens in buildings under construction where the building is not yet temperature- and humidity-controlled, or materials have not yet dried. Additionally, humidity levels are naturally high in wet rooms like kitchens and sanitary areas, and moisture problems may occur.

The stone-wool core in acoustic ceiling panels can be specified as hydrophobic, which means it neither absorbs water nor holds moisture. This makes the ceiling panels ‘sag-resistant,’ even up to 100 percent relative humidity (RH) and in temperatures ranging from 0 to 40 C (32 to 104 F). The material is dimensionally stable and does not warp, curl, or cup. It also neither rots nor corrodes. Further, its characteristics remain unaltered over time, maintaining its dimensions and physical characteristics throughout a building’s lifecycle.

Since stone wool is inorganic, it also does not promote the growth of mold or bacteria. North American studies show a relationship between mold and damp conditions, and an increase in allergic reactions, along with eye, nose, and throat irritation.2 They have also been associated with litigious concerns that some commercial building owners have termed ‘sick building syndrome.’

Twenty-three percent of office workers experience frequent symptoms of respiratory ailments, allergies, and asthma. The impact has been an increased number of sick days, lower productivity, and increased medical costs. The economic impact is enormous, with an estimated decrease in productivity around two percent nationwide, at a cost of $60 billion annually.3

Helping maintain cleanliness, stone wool ceiling panels may be specified with a smooth, non-textured finish that can be vacuumed with a soft brush attachment. Specially treated hygienic and medical surface finishes allow cleaning with water and some diluted disinfectants, such as chlorine, ammonia, and quaternary ammonium. In some cases, specially treated surface finishes on stone wool ceiling panels allow for more intensive cleaning, such as steam cleaning twice a year following a defined protocol.

Sustainability
In addition to being composed from the earth’s most abundant bedrock, stone wool ceiling panels can contain up to 42 percent recycled content. When removed, undamaged stone wool products may be reused or recycled for other projects. However, if recycling, one should be observant of recycling plant locations.

Stone wool is an excellent thermal insulator and contributes to energy-efficient buildings. Stone wool ceilings’ reflective, smooth surface also can play a significant role in enhancing energy efficiency through better light distribution. The health benefits of natural light include a more positive mood, improved productivity, and lower absenteeism.4 Maximizing use of natural daylight may allow a reduction in the number of lighting fixtures. The subsequent lowering of electric loads may reduce cooling costs.

Further contributing to sustainable goals, stone wool ceiling panels may be specified with UL Environment’s Greenguard Gold Certification for low-emitting products. Certification is only given to products compliant with the associated requirements, which among others include stringent limits on emissions of more than 360 volatile organic compounds (VOCs).

UL Environment states indoor air can be two to five times more polluted than outdoor air. Greenguard Gold criteria incorporate health-based emissions requirements as denoted by the U.S. Environmental Protection Agency (EPA), the State of California Department of Public Health’s Section 01350, and others.

More than 400 green building codes, standards, guidelines, procurements policies, and rating systems give credit for Greenguard products. Certification also fulfills the low emission requirements of the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) v4 program, and the Collaborative for High Performance Schools’ Criteria (CHPS) for low-emitting materials.

Stone wool, the core material of stone wool ceiling products, can withstand temperatures up to 1177 C (2150 F). It is made from basalt rock and is non-combustible; it will not contribute to the development and spread of fi re.

Stone wool, the core material of stone wool ceiling products, can withstand temperatures up to 1177 C (2150 F). It is made from basalt rock and is non-combustible; it will not contribute to the development and spread of fire.

Stone wool acoustic ceiling products that have been certifi ed to GreenGuard Gold certifi cation standards for low chemical emissions into indoor air during product usage are suitable for environments such as schools and healthcare facilities.

Stone wool acoustic ceiling products that have been certified to GreenGuard Gold certification standards for low chemical emissions into indoor air during product usage are suitable for environments such as schools and healthcare facilities.

Aesthetic design
Beyond sustainability and performance, there are numerous aesthetic considerations in selecting the best stone wool ceiling panels to achieve the desired architectural expression.

Shape
The shape of a stone wool panel’s edge significantly contributes to the ceiling’s overall appearance. Demountable options include:

  • square lay-in—cost-effective, provides easy access to the plenum, and mounts in standard suspension systems;
  • tegular—square or angled, hangs on a visible and recessed suspension system that creates a shadow between the tiles, and mounts in standard suspension systems;
  • semi-concealed—appears to float under the suspension system, the profiled edge and deeply recessed grid profiles presents an elegant shadow (an effect emphasized by specifying the suspension system in black); and
  • concealed—hides the suspension system to create a monolithic appearance, but only some concealed panels are demountable.

Another option is the direct-mount assembly, where ceiling panels are directly bonded to the structural soffit or an existing ceiling surface. These systems are for areas where ceiling heights do not permit the use of the suspension setup.

Panels are not limited to two dimensions of squares and rectangles; they may be formed into three-dimensional cubes. Baffles and clouds provide an alternative solution for rooms where contiguous ceilings are unsuitable. They are suited to thermal mass applications where the soffit needs to be left exposed. They can be used as part of a retrofit or to create a design feature.

A wide range of sizes contributes to the design freedom and flexibility offered with stone wool ceiling panels. By combining different module sizes, even small rooms may seem larger and long corridors can seem less distant. The line of a ceiling impacts the perception of a space and creates focal points that may show direction, outline an object, or divide a large space into more comfortable zones.

Horizontal lines convey stability, grounding, and direction. Vertical lines, on the other hand, also communicate stability, as well as pillar-like attributes of strength and balance. Diagonal lines are perceived as dynamic and transformational with overtones of freedom, while curves are considered playful, organic, and soothing.

Texture and color
Based on today’s design styles, stone wool ceiling panels are preferred in smooth and lightly textured surface finishes. This gives the impression the ceiling is lighter in texture, weight, and color. White and neutral tones are the most popular color choices for interior ceilings. For more vibrant spaces, stone wool ceiling panels can be specified in a breadth of other hues.

A viewer’s perception and relation to a color changes depending on whether it stands alone, is dominating a space, or if it is in play with other colors. It also is influenced by the quality and quantity of light hitting it.

Colors evoke physical and psychological reactions, and the brightness or color temperature creates different moods and ambiance: Warm colors—such as red, orange, and yellow—are considered stimulating. Cool colors—like blue, purple, and light green—generally have a calming effect.

Spatial perception is also affected by color. Lighter hues tend to make spaces seem bigger, while darker ones can make spaces feel more intimate. A dark ceiling will seem lower than it really is, or—when installed high enough above—simply disappears.

Color schemes also can indicate the purpose and usage of a space with boundaries and transitions. Consideration should be given to how the visual stimulation in a space will be perceived by the brain to evoke a desired response. This is of utmost importance in environments where varied spaces have different tasks and functions, to avoid any confusion that can cause stress in the occupants.

Segment-specific demands
Color certainly has a place in educational settings, but aesthetics may need to be secondary to performance requirements. Fire performance and indoor air quality are top-of-mind, and acoustics also need to be of primary importance. Classrooms in the U.S. typically have speech intelligibility ratings of 75 percent or less, meaning every fourth spoken word is not understood.5 Loud or reverberant classrooms may cause teachers to raise their voices, leading to increased teacher stress and fatigue.6

In school activity areas, stone wool ceiling panels may be specified with both a high acoustic performance and impact-resistance. The panel’s reinforced surface withstands tougher-than-average wear and tear, as well as frequent mounting and demounting.

Along with durability and flexibility for future redesign, health care facilities seek products with easy-to-clean surfaces to support infection control. Most Methicillin-resistant Staphylococcus Aureus (MRSA) infections occur in people who have been in hospitals or other health care settings and are resistant to the antibiotics commonly used to treat ordinary staph infections.7

Stone wool ceiling panels designed for medical use have been classified Class 5, or better, in accordance with International Organization for Standardization (ISO) 14644-1, Cleanrooms and Associated Controlled Environments−Part 1: Classification of Air Cleanliness. Those that have specially treated medical and hygienic surface finishes also help mitigate:

  • MRSA bacteria resistant to antibiotics and responsible for post-surgery infections and septicaemias;
  • Candida Albicans, which is yeast responsible for skin infections and pneumonias; and
  • Aspergillus Niger, which is mold responsible for pneumonias.

Noise also contributes to patients’ slower recovery times. Studies show high levels of sound have negative physical and psychological effects on patients by disrupting sleep and increasing stress.8

With respect to auditory privacy, acoustic performance not only is relevant to patient decency and respect, but also to the protection of corporate intellectual property, and to increased concentration levels in working environments. After surveying 65,000 people over the past decade in North America, Europe, Africa, and Australia, researchers at the University of California-Berkeley reported more than half of office workers are dissatisfied with the level of speech privacy, making it the leading complaint in offices everywhere.9

Conclusion
From acoustics to fire performance and aesthetics to sustainability, stone wool ceiling systems provide the versatility and attributes to meet the varied requirements of commercial and institutional buildings’ new construction and renovation projects.

Notes
1 Visit www.euro.who.int/en/health-topics/environment-and-health/noise. (back to top)
2 Visit www.hc-sc.gc.ca/ewh-semt/air/in/poll/mould-moisissure/effects-effets-eng.php(back to top)
3 See William J. Fisk’s “Health and Productivity Gains from Better Indoor Environments,” from the 2000 edition of Annual Review of Energy and the Environment. Visit www2.bren.ucsb.edu/~modular/private/Articles/Fisk%20HealthandProductivity%202000.pdf(back to top)
4 For more, see Vanessa Loder’s article, “Maybe Money Really Does Grow on Trees,” in the May 4, 2014 edition of Forbes. Visit www.forbes.com/sites/vanessaloder/2014/05/04/maybe-money-really-does-grow-on-trees/2(back to top)
5 See Classroom Acoustics, by Seep et al, published in 2000 by the Acoustical Society of America (ASA).(back to top
6 See Tiesler & Oberdörster’s 2008 article, “Noise: A Stressor? Acoustic Ergonomics of Schools,” in Building Acoustics (15 [3]). (back to top)
7 Visit www.mayoclinic.org/mrsa(back to top)
8 See “Sound Practices: Noise Control in the Healthcare Environment?” published by HermanMiller Healthcare in 2009, and “Sound Control for Improved Outcomes in Healthcare Settings,” by Joseph Ulrich, published in 2004 by the Center for Health Design. (back to top)
9 See John Tierney’s article, “From Cubicles, Cry for Quiet Pierces Office Buzz,” in the May 19, 2012 edition of the New York Times. Visit www.nytimes.com/2012/05/20/science/when-buzz-at-your-cubicle-is-too-loud-for-work.html. Also, visit www.cbe.berkeley.edu/research/index.htm(back to top)

Cory Nevins is Rockfon’s director of marketing, leading the company’s continuing education and training programs to keep commercial building team members updated on acoustic stone wool ceiling panels, specialty metal ceiling panels, and ceiling suspension systems. He has nearly 20 years of experience in the building products industry, the majority of which has focused on ceiling systems, and a bachelor’s degree in marketing from Miami University in Oxford, Ohio. Nevins can be contacted at cory.nevins@rockfon.com.

The Benefits of BIM for Interior Steel Framing

Image © BigStockPhoto/AndreasG

Image © BigStockPhoto/AndreasG

by Mike Murzyn 

Building information modeling (BIM) is quickly becoming a formal procedure for modern steel construction. From software that optimizes the building envelope with information on dead load and structural load inputs for wind, seismic, and other requirements, to programs enabling sustainable design by addressing energy efficiency and green product specification, BIM processes are being adopted across the country.

Recent research conducted by McGraw-Hill Construction shows BIM adoption in the construction industry expanded to more than 70 percent of architects and engineers in 2012, compared to 17 percent in 2007 and 49 percent in 2009.1 However, while use is increasing, the concept of component-specific BIM for applications such as wall, floor, and ceiling systems is only beginning to gain traction among design and building professionals. Sometimes referred to as ‘add-ins,’ these tools allow specific products and systems to be directly imported into a larger BIM design and shared with the entire construction team, enhancing project coordination and collaboration.

Component-specific BIM for wall design
Accounting for nearly 60 percent of all metal studs in the United States, interior, non-structural wall partitions are one of the most common applications for steel framing—specifically light-gauge studs.2

In this example, the conduit and wall framing have been planned to work together. The bank of electrical outlets was known in the design phase. As a result, each trade was able to understand how the elements should fit in the field. Not only does this look clean, but the coordinated effort also allowed it to pass inspection and prevent delays in the field. Images courtesy ClarkDietrich Engineering Services LLC

In this example, the conduit and wall framing have been planned to work together. The bank of electrical outlets was known in the design phase. As a result, each trade was able to understand how the elements should fit in the field. Not only does this look clean, but the coordinated effort also allowed it to pass inspection and prevent delays in the field. Images courtesy ClarkDietrich Engineering Services LLC

Steel framing used for interior wall partitions comes in various shapes, thicknesses, and sizes. Each of these components has a specific function in the wall assembly. Selecting the correct size and thickness depends primarily on spacing of framing members and the wall’s height. Other considerations throughout the selection process include the application of the wall finishes, whether they will be applied to one or both sides, full height for composite design, and any applicable impact resistance requirements.

Through use of building information models, each element of a wall assembly can be created as an ‘intelligent’ object containing a broad array of product information in addition to its physical dimensions. Every element in the BIM project knows how it relates to other objects and to the overall design. This helps manage complex plans from multiple trades, as well as identify and avoid potential clashes or inconsistencies before reaching the jobsite. This is of particular importance as the wall framing phase of a project can significantly impact several other trades.

Additionally, the level of detail available through BIM add-ins is of value to architects and engineers looking to develop data-rich 3D images of interior spaces or component load-bearing wall framing. It helps in the creation of infinite views, perspectives, schedule data, and facility and operational management for the life of the building. In addition to detail on code requirements and strength, a BIM project that has taken wall types to the next step by using framing add-in software can identify the location of wall penetrations, as well as provide accurate wall shapes and opening dimensions on individual panel drawings.

Wall design and construction
There are hundreds of different wall types being used in steel framing, however, there has been little information about incorporating interior framing into BIM projects until recently.

Building information modeling (BIM) is helping streamline the cold-formed steel (CFS) framing process by providing detailed 3D models to track the framing through the construction process and allowing them to frame around other trades without the mechanical systems being in place.

Building information modeling (BIM) is helping streamline the cold-formed steel (CFS) framing process by providing detailed 3D models to track the framing through the construction process and allowing them to frame around other trades without the mechanical systems being in place.

In response, some cold-formed steel (CFS) framing manufacturers offer BIM add-ins allowing users to seamlessly integrate a significant amount of wall data into new or existing models, improving comprehensiveness. The programs can also eliminate the need for temporary wall libraries requiring users to change each individual wall element to accommodate updates.

When considering different wall framing BIM add-ins, it is important to ensure the program includes all the information the design team needs, such as:

  • detailed wall assembly data with product information;
  • type and number of wall sheathing layers;
  • overall wall width with the ability to add resilient channel and/or wall insulation;
  • recycled content details for projects pursuing points under the Leadership in Energy and Environmental Design (LEED) program;
  • product submittal sheet links;
  • fire test data, including Underwriters Laboratories (UL) test number;
  • sound test data (e.g. sound transmission class [STC] performance rating); and
  • limiting height tables based on stud spacing, deflection, and interior lateral load.

In many cases, when a wall type is created, it displays the actual materials and assembly needed for correct installation. This amount of programming and detail allows the design team to see exactly how the wall needs to be constructed, what the limitations may be, and how it fits into the overall building’s construction. However, it is important to remember not all manufacturers’ BIM software add-ons are the same, and users need to be aware some systems may be more robust than others.

During the construction process, a common change order is for the removal or relocation of partition wall framings. Partition framing removal and relocation occurs due to design changes by the owner or architect, or to accommodate unanticipated intrusions, such as mechanical, electrical, or plumbing penetrations. Unfortunately, every time these changes take place, it increases the cost and time spent on the project.

However, through BIM and component-specific add-ins, building professionals can virtually identify these clashes and design the necessary changes prior to the contractor putting labor on the job. Add-ins now offer tools to simultaneously detect multiple clashes with mechanical/electrical/plumbing (MEP) and structural members into wall types, then automatically create openings in the wall around these clashes. This extra level of detail opens the lines of communication between members of the design team and increases the likelihood of open, productive conversations where changes can take place in real time. Wall framing BIM add-ins can include a wide range of information relevant to installation, code requirements, LEED guidelines, and future maintenance.

Large BIM projects typically have onsite work stations allowing workers of all trades view 3D models to identify and avoid potential clashes or inconsistencies.

Large BIM projects typically have onsite work stations allowing workers of all trades view 3D models to identify and avoid potential clashes or inconsistencies.

Accurate estimates and best practices
The integration of a BIM wall framing add-ins greatly benefits structural engineers and architects looking to increase their participation with the design team and produce interior drawings more efficiently and accurately. The ability to look at a BIM object or wall type and quickly understand a wall’s construction, fire and sound requirements, and limiting heights and design helps the engineering team more effectively answer questions about performance characteristics and structural integrity as well as other associated building elements.

Bid accuracy is essential. By accurately building interior walls using BIM, the framing contractor can confidently describe construction costs and immediately anticipate areas that may require additional work to prevent clashes. This information helps the contractor stay in line with the project’s budget, and better understand the time and materials needed for a particular phase. Possessing this level of information while bidding on a project can help other stakeholders easily process the data and showcase how the interior walls integrate with other plumbing, electrical, and HVAC components.

BIM also has benefits after the structure is complete. Providing as much detail as possible to facility management and building owners can help reduce costs for the lifetime of the building and make maintenance and updates more cost effective over time. A small example would be the ability for the facility management to be able to see wall framing backing locations for cabinetry or hand rails. What makes BIM unique is its ability to integrate information from various stages of the building lifecycle and easily communicate this data to the appropriate members of the construction team.

To increase efficiencies and reduce confusion, it is important BIM models and drawings provided to the construction team are as complete and detailed as possible, which encourages active and collaborative coordination between all involved parties.

As BIM continues to gain traction for projects of all sizes, there are three best practices for any individual or project team to consider:

  • discuss how to organize the BIM model;
  • decide on the level of detail to include; and
  • share the design intent with other professionals who play a role in construction and may interact with the model.

By incorporating a wall framing add-in into the larger BIM projects, design teams are ensuring there is an elevated level of detail and awareness regarding the installation of the interiors, resulting in fewer changes and less confusion.

This proprietary add-in incorporates and fi lters detailed information on wall elements and design properties, such as UL assemblies based on fi re-rating requirements, sound transmission class (STC) ratings, and limiting height design.

This proprietary add-in incorporates and fi lters detailed information on wall elements and design properties, such as UL assemblies based on fi re-rating requirements, sound transmission class (STC) ratings, and limiting height design.

This proprietary BIM add-in incorporates detailed information on wall elements and design properties, such as Underwriters Laboratories (UL) assemblies based on fi rerating requirements, sound transmission class (STC) ratings, and limiting height design.

This proprietary BIM add-in incorporates detailed
information on wall elements and design properties, such as Underwriters Laboratories (UL) assemblies based on fi rerating requirements, sound transmission class (STC) ratings, and limiting height design.

Conclusion
The sustainable qualities of cold-formed steel framing make it a natural fit for high-performance buildings. With various environmental and economic benefits, incorporating steel framing into a project can result in labor and cost savings for the construction team. Additionally, when using BIM add-ins, such as those for designing wall assemblies, specific system components can be seamlessly integrated into an easy-to-share information-rich model.

BIM opens the door for architects to pass models with various wall elements and design properties onto contractors with the assurance all materials will work together within the overall building design. This type of interactive platform, in which details have been linked together, gives architects and contractors a truly collaborative framework to successfully design even the most challenging wall assemblies.

 Notes
1 For more, see McGraw-Hill Construction’s The Business Value of BIM in North America SmartMarket Report. Visit at www.construction.com/about-us/press/bim-adoption-expands-from-17-percent-in-2007-to-over-70-percent-in-2012.asp. (back to top)
2 See M.Reisdorf’s “Light Gauge Metal Stud Framing” at buildipedia.com/aec-pros/construction-materials-and-methods/light-gauge-metal-stud-framing-planning-and-practices(back to top)

Mike Murzyn is a technical product and marketing manager for cold-formed metal framing manufacturer, ClarkDietrich Building Systems. He was a key developer in the company’s building information modeling (BIM) add-in wall-type creator. Murzyn has more than 15 years of experience with design, engineering, product, and software development. He can be contacted via e-mail at mike.murzyn@clarkdietrich.com.

Mix Design Fundamentals: Considerations for concrete for slabs-on-ground

Photo © BigStockPhoto/Theerapol Pongkangsananan

Photo © BigStockPhoto/Theerapol Pongkangsananan

by Paul Potts

“Concrete cracks and nothing can be done about it” is a common refrain when the material cracks unexpectedly. However, it is too often an excuse when poor design or improper placement has resulted in excessive random cracking. The real problem is too much mix design water, a lack of welded wire reinforcement, insufficient aggregate, or inadequate curing methods.

Grade-level concrete for hospitals, schools, and commercial buildings is something that can be walked on, supports vehicle traffic and other moderate loads, and provides a hard surface for floorcoverings. Interior concrete should not randomly crack or curl excessively to the point grinding is required before the floor finish can be installed. Similarly, exterior concrete should not randomly crack or deteriorate prematurely from freeze-thaw cycles. Using less water and more aggregate, making the right choices about reinforcement, and ensuring proper contraction jointing will improve the outcome.

This polished concrete floor was designed with synthetic fiber reinforcement but without welded wire reinforcement (WWR). Photos courtesy Paul Pott

This polished concrete floor was designed with synthetic fiber reinforcement but without welded wire reinforcement (WWR). Photos courtesy Paul Pott

It is a well-established belief among some architects, structural engineers, and contractors that strength is the defining characteristic predicting the quality of concrete. Another common erroneous assumption is synthetic fiber reinforcement can replace welded wire reinforcement (WWR) to make concrete less disposed to long-term shrink cracking. Others believe more, not less, cement is better, or the main impact of adding more aggregate to the mix involves making it more expensive. None of this is true.

Clearing incorrect info
It is not uncommon to base the specification for concrete on strength and slump. Unfortunately, strength does not get tested until several days after hardening; and slump, which has a strong relationship to placement, is only loosely related to the most desirable qualities of hardened concrete.

In this author’s first years as construction administrator, he was taught slump was an indication of water content, but there is not direct relationship between the two. As the Portland Cement Association’s (PCA’s) Design and Control of Concrete Mixtures points out, slump tests are simply measures of consistency—in other words, the ability of fresh concrete to flow.

The desirable qualities of slab-on-grade concrete are resistance to curling and shrinkage cracking, along with finishability, flatness, strength, and durability. In northern latitudes, ‘durability’ particularly means resistance to freeze-thaw cycles. To better ensure these qualities, the designer must consider total water content, size and quantity of well-graded aggregate, type of reinforcement, and timeliness of applying curing and cutting construction joints as controlling factors. The factors in designing concrete with these qualities are:

  • low water-to-cement (w/c) ratio;
  • minimum total cement content needed (with the understanding more cement requires more water);
  • size and weight of aggregate in a yard of concrete;
  • entrained air in exterior concrete; and
  • presence of WWR.

According to the PCA handbook, almost every quality of concrete will be improved by reducing the total water content in the batch. The w/c ratio is the weight of water divided by the weight of cement. To minimize total water content, designers must start by specifying a low water-to-cement ratio (e.g. 0.45 w/c ratios for interior concrete and either a 0.45 or 0.40 for exterior concrete). To further minimize total water, they should opt for the lowest practicable cement content (e.g. 5-1/2 sack). At the same w/c ratio, the less cement in a batch of concrete, the less total water needed.

According to Chapter 9 (“Designing and Proportioning Normal Concrete Mixture”) of PCA’s Design and Control of Concrete Mixtures:

For any particular set of materials and conditions of curing, the quality of hardened concrete is determined by the amount of water used in relation to the amount of cement. The less water used, the better the quality of the concrete—provided it can be consolidated properly. 

Concrete with a water to cement (w/c) ratio of 0.45 and below is stiff when it comes off the truck—otherwise, it may have been tampered with.

Concrete with a water to cement (w/c) ratio of 0.45 and below is stiff when it comes off the truck—otherwise, it may have been tampered with.

 

Using more—rather than less—large aggregate (e.g. 900 to 1000 kg [2000 to 2200 lb]) reduces the total cement and water needed, improves resistance to cracking and curling, and increases the strength at the same time. This is also economical, as aggregate is cheaper than cement.

It is important to keep in mind there is a limit to how low one can go with cement and water because there must be enough water (or substitute) to allow concrete to flow into place. Further, there must be enough cement to promote the finisher’s work. The effect low water has on finishing work can be counteracted with water-reducing admixtures (WRAs).

Water content, wire reinforcement, and cracking
Excess water (i.e. more than required for hydration) improves concrete flow, making the material more economical to place. However, water not hydrated by the cement eventually becomes ‘bleed water’ and evaporates. The evaporation reduces the total volume of the slab, causing the slab to shrink in overall dimensions. More excess water available for evaporation means more shrinkage and more potential for long-term shrink cracking.

When the concrete’s shrinking volume is restrained by a pipe, masonry corner, steel column, or the drag of an uneven subgrade, there may be stresses beyond the material’s tensile strength to resist—as a result, shrink cracks develop. (Tensile strength is only 10 per cent of compressive strength.)

Welded wire reinforcement buttresses the tensile strength of concrete by distributing the stresses over a wider area where it can be better controlled. Some architects and engineers believe synthetic fiber mesh can be substituted for welded wire in concrete for the same crack-resisting properties. However, synthetic fiber mesh is only effective at preventing plastic shrink cracking—hairline cracks that develop before final finishing.

To clear up confusion about the purpose of synthetic fibers, PCA published the following statement on its website:

Plastic fibers should not be expected to replace wire mesh in a slab-on-ground. However, although not affecting joint spacing, plastic fibers are used to reduce plastic shrinkage cracking.1

While finishing is not any more difficult with low w/c concrete, placement, and screeding require an ambitious crew.

While finishing is not any more difficult with low w/c concrete, placement, and screeding require an ambitious crew.

Mix design
‘Strength’ is the reciprocal of the w/c ratio—the lower the ratio, the higher the concrete strength. While it may be prudent for legal reasons to specify a minimum strength, it is just as well as to develop the mix design for slab-on-ground applications by specifying the maximum sack content, water-cement ratio, and aggregate quantity/size. Strength follows the water-to-cement ratio as surely as day follows night.

A w/c ratio of 0.045 and 5-1/2 sack content with 900-kg (2000-lb) of large aggregate with welded wire reinforcement is a good standby mix design for an interior slab-on-grade. The strength will fall around 30 N/mm2 (4500 psi). A w/c ratio of 0.040 and 5-1/2 sack specification with 900 to 1000 kg (2000 to 2200 lb) of aggregate and six per cent air with welded wire reinforcement is a durable exterior mix design, but this concrete is stiff and may need a water-reducer to aid placement. The strength will be around 35 N/mm2 (5000 psi).

Pop-outs in exterior concrete are the result of waterlogged soft stone or chert in concrete mixtures that, owing to their lighter density and porousness, absorb water and float up near the surface during finishing operations, then explode during the first freeze cycle. Most concrete mix designs limit soft stone and chert to less than one percent (if permitting it at all). However, even this one percent can leave an awful-looking mess on the surface of a new driveway or sidewalk. To avoid pop-outs in exterior concrete, one should use 21AA crushed limestone aggregate where it is available. When limestone is unavailable, the soft stone and chert in natural stone aggregate should be limited to less than one percent.

Contraction joints versus random cracking
Contraction joints in slab-on-ground designs are for aesthetic purposes—without contraction joints, cracks would occur randomly. Contraction jointing limits the cracking to where it is least objectionable and, in the case of thin-set terrazzo, epoxy, and urethane floorcoverings, at locations that can be coordinated with joints in the surface materials. Still, random cracking can be preferable in some scenarios.

For example, the appearance of carpet, vinyl, rubber, and linoleum can be improved by allowing the concrete to randomly crack instead of adding contraction joints with their inherent curling problems. To control the width of random cracking, one should start with a low water-to-cement ratio mix and require WWR. Random cracks in concrete are free of curling, and a low w/c ratio combined with welded reinforcement minimizes the width of cracks.

In a random crack system, attention must be paid to corridor intersections that may need contraction joints to prevent excessively wide cracks from developing where the long corridor runs and restraints at corners can produce quite large random cracks, regardless of the mix design and WWR. Concrete slabs for wood floors on sleepers or rubber cushions can be designed the same way.

It is not an option to allow random cracking under hard tile, epoxy floorcoverings, and thin-set terrazzo bonded to the concrete, because any cracks in the substrate will telegraph through to the surface of the bonded floorcovering (Figure 1). Concrete under bonded floorcoverings must be reinforced with WWR or a mat of steel reinforcement to reduce shrinkage cracking. Such components lower the risk of shrinkage cracking by restraining contraction during set time and reducing the width of cracks once the concrete sets. While some structural engineers may not see the need for WWR in non-load-bearing slabs-on-grade, its importance for reducing curling and cracking must not be overlooked.

telegraphed crack

This shrink crack telegraphed through the thin-set terrazzo and became the subject of litigation and costly court-ordered repairs. (A core sample can be seen at right/below.)

This shrink crack telegraphed through the thin-set terrazzo and became the subject of litigation and costly court-ordered repairs. (A core sample can be seen above.)

Vapor barrier and curing considerations
A vapor barrier should be placed under all slabs-on-ground that will receive floorcovering. Properly consolidated concrete is waterproof, but not vapor-proof. If a vapor barrier is not used, any moisture under the slab migrates upward by capillary action through the concrete to eventually degrade the floorcovering adhesive. Most floorcovering manufacturers consider the lack of vapor barrier a defective concrete installation—many void their warranties on that basis.

While some specifiers recommend a blotter layer of granular material between the vapor barrier and the slab, in practice this has met a lot of resistance, principally because of the difficulty of keeping the blotter layer intact while completing the mechanical and electrical underground and preparing the concrete pour.

According to Chapter 9 in PCA’s Floors on Ground handbook:

Other specifiers believe that no blotter layer is needed and that concrete should be placed directly on the vapor retarder. The idea is that concrete slabs should be cured from both the top and the bottom. A granular layer between the vapor retarder and the concrete creates a potential water reservoir that could cause moisture problems at a later date. 

A properly installed vapor barrier directly under the slab has other benefits beyond reducing the transmission of moisture vapor. First, the polyethylene film acts as a slip-sheet between the underside of the concrete and the sub-base; this reduces drag on the slab and decreases shrinkage cracking. Further, when combined with timely surface curing, a polyethylene vapor barrier under the slab is an asset, retaining moisture in the concrete to improve hydration during setting. Whenever a vapor barrier under the slab is used, the water-to-cement ratio should be 0.45 or less.

It is especially important to start curing as soon as practical after finishing operations are complete. Where concrete is placed directly on a vapor barrier, a double application of cure-only compound should be applied at right angles to each other. In cases where the floorcovering requires an adhesive, cure-only compounds should be used rather than cure-and-seal products—sealing the concrete interferes with the bond between the concrete and the floorcovering or topping.

Low water-to-cement concrete is harder to move around and requires considerably more handwork to get it in place. Placement and spreading of low water-to-cement ratio concrete can be improved by adding water-reducing admixtures (WRAs)—however, excessive use can cause more drying shrinkage. These are rarely required by the specification, but are included in the specification as an alternate product that may be used.

Low water-to-cement concrete is harder to move around and requires considerably more handwork to get it in place. Placement and spreading of low water-to-cement ratio concrete can be improved by adding water-reducing admixtures (WRAs)—however, excessive use can cause more drying shrinkage. These are rarely required by the specification, but are included in the specification as an alternate product that may be used.

Aggregate
Larger-size and greater quantities of coarse aggregates work in several ways to reduce cracking and curling, and improve the economy of concrete. Coarse aggregates, less than 50 mm (2 in.) in size, cost about half as much as the cement in concrete. The larger aggregates leave less room that must be filled with cement paste so they reduce the total water required in the mix. To avoid pop-outs in exterior concrete, the designer should specify crushed limestone aggregate where it is available.

More and larger aggregates reduce shrinkage cracking, and reduce curling. The weight and dimension of larger aggregates put a drag on the movement of the materials within concrete during the shrinkage stage, and reduce overall contraction thereby reducing shrinkage cracking. Greater quantities of coarse aggregate magnify these benefits—900 kg (1984 lb) of 32 mm (1 ¼ in.) coarse aggregate is a good starting point for specifying the aggregate in slab-on-ground concrete.

Conclusion
Concrete designed with low water-to-cement ratios and the maximum size and content of large aggregate, reinforced against shrinkage cracking with properly spaced contraction joints will have fewer random cracks and less curling. To that end, the purpose of this article is two-fold:

  • encourage the concrete mix designer to think in terms of the low w/c ratios and aggregate content instead of strength and slump as the starting point for the concrete mix design; and
  • encourage the use of WWR where crack mitigation is important, cautioning against misapplication of synthetic fiber as countermeasure with regard to long-term shrink cracking.

Notes
1 Visit www.cement.org/tech/faq_fibers.asp. (back to top)

Paul Potts has worked as construction administrator for Kingscott Associates, a school design/construction firm. A licensed MasterSpec writer and a consultant to the construction industry in Michigan, he is currently working on the reorganization of the Michigan State University Cabinet Shop and finishing construction administration work on the New Community Building and Pavilion project for the City of Potterville. Potts can be reached at paulpotts1@comcast.net.

Enhancing Energy Performance with Balcony Thermal Breaks: On the Right Track with Chelsea Green

Thermal break connections are being used in the balconies on the 11th through the 14th floors to further enhance the building’s energy performance.

Thermal break connections are being used in the balconies on the 11th through the 14th floors to further enhance the building’s energy performance.

A birds-eye view of concrete pour after balcony thermal break connections are installed at Chelsea Green.

A birds-eye view of concrete pour after balcony thermal break connections are installed at Chelsea Green.

 

 

 

 

 

 

 

 

 

A few blocks from the High Line, the restored elevated railway bed that now sports pedestrian walkways amid a landscape of greenery, New York City’s Chelsea Green is a 14-story luxury condominium from Alfa Development.

Designed by Stephen B. Jacobs Group (with DJM Construction and structural engineers WSP Cantor), every aspect of the 6875-m2 (74,000-sf) concrete structure is intended to consider its impact on the environment. Green attributes are found everywhere from the cabinets and HVAC to the rainwater irrigation system and vegetated roof. To ensure ultimate efficiency, light-emitting diodes (LEDs) have been mounted throughout the building and solar shades installed above the windows.

Additionally, thermal break connections are being used in the balconies on the 11th through 14th floors to further enhance the building’s energy performance. Recently approved by the NYC Department of Buildings, these 52 concrete-to-concrete thermal breakmodules are being installed on 10 of the 2.3 x 4.9-m (7 ½ x 16-ft) balconies at Chelsea Green to deal with thermal bridging otherwise occurring where the envelope is penetrated.

Each balcony at Chelsea Green is cantilevered out 2.3 m on 203-mm (8-in.) thick tapered concrete slabs.

“Traditional balcony attachments deal primarily with only the structural cantilever and, as a result, transmit exterior temperatures to the interior floor slabs, adding to the energy use of the unit,” said Alfa’s senior project manager, Frank Mattiello. “This thermal bridge effect can be felt when walking barefoot in one’s apartment—even when the heating or cooling systems are in operation.”

The structural thermal breaks provide load-bearing thermal insulation for these slabs and transfer bending moment stress and shear forces. Their integrated hanging and tensile reinforcement mitigates use of other costly elements like stirrups or hooped mat.

“This is a major breakthrough for combating thermal bridging in New York City residential buildings,” said Stephen B. Jacobs Group’s Omalawa Abdullah-Musa. “The process for getting this product incorporated into the project was challenging, since it was relatively unknown to most structural engineers here. Chelsea Green has set the tone for future projects, and we are looking forward to spreading the word about this innovative technology.”

To read the full article, click here.