Tag Archives: wood

Wood design nominees sought


Saavedra Gehlhausen Architects’ design of the Anderson Japanese Gardens Visitors’ Center (with engineering by T.E. Meyer & Associates) includes a low sloping roof with long overhangs and copper roofing. The post-and-beam timber structure, common to Japanese architecture, is expressed on both the exterior and throughout the interior. Douglas-fir was used for all of the wood components, including columns and beams, glued-laminated timber (glulam) structural deck, trim, and Shoji panels. The Rockford, Illinois, project was awarded by WoodWorks last year. Photo © Daniel G. Saavedra

WoodWorks, a division of the Wood Products Council, is now accepting nominations for its awards program celebrating the design, engineering, and construction of non-residential and multi-family buildings.

For the awards program, which has no nomination fees, special consideration is given to recently completed buildings, projects employing wood as a structural element, and designs exemplifying new opportunities for wood construction.

“The Wood Design Awards provide an opportunity for architects and engineers to showcase cutting edge projects that leverage wood’s beauty and versatility,” said WoodWorks executive director Jennifer Cover, PE. “What sets a project apart is its ability to demonstrate ingenuity and the unique attributes of using wood in design, such as cost savings, sustainability and structural performance.”

Specific categories range from education buildings and commercial design to government projects and innovative engineering. Entrants across these categories will also be selected to receive regional excellence awards. The deadline for nominations is September 30. For more information, visit www.woodworks.org.

Design of Fire-resistive Exposed Wood Members

Photo courtesy Structurlam

Photo courtesy Structurlam

by Bradford Douglas, PE, and Jason Smart, PE

Designing for life safety is complex and multifaceted, and fire-related issues comprise a large portion of model codes. For nearly 15 years, a mechanics-based design method has been used in the United States to estimate the capacity of exposed wood members using basic wood engineering mechanics.

This mechanics-based design method first appeared in the 1999 version of Technical Report (TR) 10, Calculating the Fire Resistance of Exposed Wood Members (TR 10). This method was then incorporated into new design procedures of the 2001 National Design Specification (NDS) for Wood Construction. These procedures were later adopted into the model building codes through reference to the NDS for calculating fire resistance of wood members. The procedures have been used extensively for design of large, exposed wood members, but are now also beginning to be employed for estimating structural fire resistance of smaller exposed wood members.

Excerpt from Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members, Table 1 (one-hr). This shows tabulated one-hour design load ratios, Rs, for flexural members exposed on three sides. [CREDIT] Data courtesy American Wood Council

Excerpt from Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members, Table 1 (one-hr). This shows tabulated one-hour design load ratios, Rs, for flexural members exposed on three sides. Data courtesy American Wood Council

Wood buildings can be designed to meet rigorous standards for performance, which is why the International Building Code (IBC) allows the material’s use in a wide range of building types—including structures that are taller and have more area than some designers realize. Table 601 of the IBC shows the required fire resistance of building elements (i.e. structural frame, walls, floors, and roofs) for each construction type. Ratings are given in hours. The exception is Type IV, where the wood structural elements are assumed to have inherent fire resistance due to their required minimum dimensions—in other words, no fire resistance rating is necessary except for exterior walls.

Fire resistance is a measure of the length of time an assembly or structural element can sustain a given load when subjected to a standardized fire exposure condition. Fire resistance of wood members and assemblies may be established by any one of five means listed in IBC Section 703.3, “Alternative methods for determining fire resistance.” The most common methods are tested assemblies and calculated fire resistance.

Excerpt from DCA 2, Table 2 (two-hr), showing tabulated two-hour design load ratios, Rs, for compression members exposed on four sides.

Excerpt from DCA 2, Table 2 (two-hr), showing tabulated two-hour design load ratios, Rs, for compression members exposed on four sides.

Fire design of exposed wood members
The fire resistance of exposed wood members, including lumber, glued-laminated (glulam) timber, and structural composite lumber (SCL), may be calculated using provisions of NDS. This allowable stress design approach is referenced in IBC Section 722, “Calculated Fire Resistance.” The design procedure allows calculation of the capacity of exposed wood members using basic engineering mechanics.

Actual mechanical and physical properties of the wood are used, with member capacity directly calculated for a given period—up to two hours. Section properties are computed assuming an effective char rate (i.e. βeff) at a given time (i.e. t). Reductions of strength and stiffness of wood directly adjacent to the char layer are addressed by accelerating the char rate by 20 percent. Average member strength properties are approximated from existing accepted procedures used to calculate design properties. Finally, wood members are designed using accepted engineering procedures found in NDS for allowable stress design. (The design procedures presented in NDS Chapter 16 are not intended to be used for the design and retrofit of structures after a fire.)

For sawn lumber, glulam, and SCL, the nominal char rate (i.e. βn) is typically assumed to be 38 mm/hour (1.5 in./hour). For a given time in hours, the effective char rate is then:

β_(eff=) 1.8/t^0.187 in./hour

To calculate section properties of wood members, the effective char layer thickness (i.e. aeff), for structural calculations is computed as:

a_(eff=) 1.8t^0.813 in.

This photo showcases glued-laminated lumber (glulam). Photos courtesy Structurlam

This photo showcases glued-laminated lumber (glulam). Photos courtesy Structurlam

The 2012 and earlier versions of IBC have also contained an empirical calculation method for estimating the structural fire resistance of wood beams and columns exposed to a standard fire exposure for up to one hour. However, this empirical method has been removed from the 2015 IBC in favor of provisions contained within NDS Chapter 16 that are much broader in application and leave less room for design error.

Basis for NDS chapter 16 approach
AWC’s TR 10 contains full details of the NDS method as well as design examples.1 TR 10 was recently revised to incorporate a new section that supports the use of the design method with smaller dimension sizes associated with lumber joist floor assemblies. It also has revised design examples to match the 2012 NDS.

The revised design tables in Appendix A allow more accurate calculation of fire resistance of columns with any slenderness ratio. They also eliminate tabulation of special cases that can be misapplied, such as:

  • deleted beams exposed on four sides that are assumed to be fully braced throughout the fire rating;
  • columns only exposed on three sides, but assumed to be unbraced; and
  • tension members that do not resist flexure due to member dead load.

Finally, a new Appendix B that calculates the fire resistance of single-span lumber joists for any design stress ratio when joists are exposed on three sides and braced on the top edge was incorporated.

Simplified approach
AWC’s Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members has been revised to replace the empirical design equations currently in the 2012 IBC with simplified design information developed in accordance with the code-approved NDS fire design procedure for exposed wood members. The tables and examples have been rewritten for consistency with the approach outlined in the 2012 NDS and TR 10.

Supporting arches were used at the Boy Scouts of America Camp Fife project.

Supporting arches were used at the Boy Scouts of America Camp Fife project.

For beams and columns stressed in one principal direction, simplifications can be made to allow creation of load ratio tables. These tables can then be used to determine the structural design load ratio at which the member has sufficient capacity for a given fire resistance time. Tables in DCA 2 give load ratios corresponding to one-hour, 1.5-hour, and two-hour fire resistance ratings for specified member dimensions.

All tabulated load ratios apply to standard reference conditions where the load duration factor, wet service factor, and temperature factor equal 1.0 (CD=1.0; CM=1.0; Ct=1.0). For more complex calculations where stress interactions must be considered, or where standard reference conditions do not apply, designers should use the provisions outlined in TR 10, along with the appropriate NDS provisions.

Flexural members
Design load ratios (i.e. Rs) for fire design of flexural members for various beam sizes are tabulated in DCA 2 tables for one-hour, 1.5-hour, and two-hour fire resistance ratings. The Rs values given in these tables apply to three-sided exposure in which the beam’s top edge is protected from fire exposure (e.g. protected by the underside of a floor or roof). These tabulated values apply to flexural members loaded in bending about one axis only, and are continuously laterally supported along the compression edge. The dimension ‘d’ is the actual cross-sectional dimension measured in the direction normal to the axis about which bending occurs, and is not necessarily greater than ‘b’ (Figure 1).

To use the DCA 2 tables, the designer only needs to know the required fire resistance rating (FRR), the structural load ratio (Rs), and the beam dimensions. For example, when a beam is exposed on three sides and is protected by a floor deck bracing the compression edge, it is required to have a one-hour FRR. When the design professional wants to use a 152.4 x 330.2-mm (6 ¾ x 13 ½-in.) beam being loaded in bending about the strong axis, DCA 2 Table 1 (one-hour) shows a 152.4 x 330.2-mm beam would calculate to have a one-hour fire resistance rating if loaded to its full design load (Rs=1).

The entrance of the Rocky Mountain Elk Foundation building. [CREDIT] Photo courtesy Structurlam/OZ Architects

The entrance of the Rocky Mountain Elk Foundation building. Photos courtesy Structurlam/OZ Architects

OZ Architects used structural composite lumber (SCL) for the Rocky Mountain Elk Foundation building in Missoula, Montana.

OZ Architects used structural composite lumber (SCL) for the Rocky Mountain Elk Foundation building in Missoula, Montana.

Compression members
Design load ratios, Rs, for fire design of columns are calculated as the product of two ratios, Rs1 and Rs2. Values of Rs1 and Rs2 for various column sizes are tabulated in DCA 2 for one-hour, 1.5-hour, and two-hour fire resistance ratings. For cases in which the product of Rs1 and Rs2 is greater than 1.0, Rs should be taken as 1.0. These tables apply to cases in which all four sides are exposed to the fire; however, values calculated using these design load ratios may be conservatively applied when one or more sides of the column are protected. Both‘d’ and ‘b’ represent the dry dressed cross-sectional column dimensions. The dimension ‘d’ is the actual dimension measured in the direction perpendicular to the axis about which buckling is being considered, and is not necessarily greater than ‘b’ (Figure 2). The designer should consider buckling about both axes and use the lesser design value.

The tabulated Rs1 values are calculated based on a square column cross-section having dimensions ‘d’ by ‘d,’ and therefore must be multiplied by Rs2 for any column that is not square. Where ‘b’ is less than ‘d,’ Rs2 will be less than 1.0; and where ‘b’ is greater than ‘d,’ Rs2 will be greater than 1.0. The Rs1 values are derived using the more conservative value of the parameter ‘c’ from equation 3.7-1 of the NDS (for long columns, c=0.9 results in lower Rs1 values; for short columns, c=0.8 results in lower Rs1 values). This allows the design load ratios to be used with sawn lumber, structural glulam, or SCL. The Rs1 values are also based on the assumption that Emin’/Fc*= 350. As a result, the design load ratios (Rs) may conservatively be used for all species and grades where the ratio of Emin’ to Fc* is greater than or equal to 350.

CS_July_2014.inddTo use the tables, the designer only needs to know:

  • required fire resistance rating;
  • structural load ratio;
  • column dimensions;
  • unbraced length of the column; and
  • end support conditions.

For example, in a case where an exposed column is required to have a two-hour FRR, the designer may want to use a nominal 14×16 column with actual dry dimensions of 330 x 381 mm (13 ¼ x 15 in.). The column will have an unbraced length of 3.35 m (11 ft) in both directions, and can conservatively be assumed to have pinned end conditions on each end of the column (Ke=1.0).

Since the unbraced length is the same in both directions, buckling would tend to be about the 13¼-in. dimension. The effective length (i.e. Le) would be 3352 mm (132 in.) and the Le/d ratio would be 10. From DCA 2 Table 2 (two-hour), a square column that is 13¼ x 13¼ in. with Le/d=10 would have an Rs1=0.39. A 13¼ x 15-in. column buckling about the 13¼ in. dimension would have an Rs2=1.11. Therefore, the 13¼ x 15-in. column would calculate to have a two-hour fire resistance rating when loaded to 43 percent or less of its full design load (Rs = [Rs1][Rs2] = [0.39][1.11] = 0.43).

Timber decking
DCA 2 also provides tabulated design load ratios (Rs) for various decking types and thicknesses, corresponding to one-hour, 1.5-hour, and two-hour fire resistance ratings. The dimensions ‘b’ and ‘d’ given in these tables are the actual dry dressed dimensions of each individual member. The Rs values given in DCA 2 Table 3.1 are applicable to butt-joint timber decking fully exposed on one face and partially exposed on the sides, in accordance with NDS Section 16.2.5.

The Rsvalues given in DCA 2 Table 3.2 are applicable to double and single tongue-and-groove decking exposed only on one face. These calculation procedures do not address thermal separation.

Constr-Specfr-Douglas-Smart-Fire-140506_Page_3To use the tables, the designer only needs to know the required fire-resistance rating, the structural load ratio (Rs) and the decking type and dimensions. For example, a timber deck is required to have a 1.5-hour FRR, and the designer wants to use 4x tongue-and-groove decking. From DCA 2 Table 3.2, the nominal 4x tongue and groove decking (76.2 mm [3½ in.] actual thickness) would calculate to have a 1.5-hour fire resistance rating if loaded to 23 percent or less of its full design load (Rs=0.23).

Where a specified fire resistance rating is required, Section 16.3 of the NDS requires connectors and fasteners be protected from fire exposure. This protection can be achieved by any of the following:

  • wood members having dimensions sufficient to prevent the char front from reaching the connectors and fasteners for the duration of the required fire-resistance rating time—the char front can be calculated as: a=1.5t^0.813 in.;
  • fire-rated gypsum board having a finish rating greater than or equal to the required fire-resistance rating; and
  • any approved coating having a fire rating greater than or equal to the required fire resistance rating time.

Structural fire design provisions have been incorporated in Chapter 16 of NDS, which is referenced in Section 722.1 of the 2012 IBC as a method of calculating fire resistance of exposed wood members. A comprehensive discussion of this mechanics-based design procedure can be found in Technical Report No. 10, while DCA 2 provides a simplified design aid of this code-approved fire design procedure for several typical applications with exposed wood members. 2

1 For more information, TR 10, Calculating the Fire Resistance of Exposed Wood Members, is available at www.awc.org/pdf/TR10.pdf.
2 Additional information on building code requirements for wood can be found in the American Wood Council’s Code Conforming Wood Design documents, available at www.awc.org/codes/ccwdindex.php.

Brad Douglas, PE, joined the American Wood Council (AWC) in 1986, and serves as its vice president of engineering. Douglas directs a program aimed at developing state-of-the-art engineering data, technology, and standards on structural wood products, systems, and assemblies for use by design professionals and building officials to assure safe and efficient design and use of wood. He is a graduate of Virginia Tech. Douglas can be contacted at bdouglas@awc.org.

Jason Smart, PE, joined AWC in 2013 and is manager of engineering technology. Smart focuses on development and support for new and emerging technologies and related changes to design and model building code standards. He is a graduate of Virginia Tech with degrees in civil engineering, wood science and forest products, and timber engineering. He can be reached at jsmart@awc.org.

Overlooked Considerations for Windows and Curtain Walls

Photo © BigStockPhoto/Zhiwei Zhang

Photo © BigStockPhoto/Zhiwei Zhang

by Derek B. McCowan, PE, and Douglas R. Pac, EIT

The primary factors most designers consider when selecting window and curtain wall assemblies for their projects are well-understood: cost, aesthetics, and thermal performance, to name a few.

There are some other important, though often overlooked, considerations that can be important to weatherproofing, performance, and durability. This article will discuss some of these lesser-known factors and some of the related issues and challenges.

Material selection for windows
Since nearly all curtain wall and storefront systems exposed to weather are fabricated from aluminum, this article’s section focuses on window materials. The common material types selected for commercial windows include vinyl, aluminum, wood, aluminum-clad and vinyl-clad wood, fiberglass, and steel. Each has several advantages and disadvantages, along with considerations that affect performance and durability.

Vinyl windows are typically the most inexpensive of the aforementioned material options; they are often chosen when appearance is less of a concern. Aside from their look, vinyl windows have proven to be an economical and rot-resistant option for residential construction applications where small punched openings are required.

Nevertheless, the authors recommend specifying commercial-grade windows over residential ones, even on wood-framed residential projects. Vinyl is not a high-strength material and therefore typically cannot accommodate large openings where high wind loads are present. (Vinyl is often reinforced with aluminum for improving strength.)

Welded vinyl window frame corner and nailing fl ange. Images courtesy Simpson Gumpertz & Heger

Welded vinyl window frame corner and nailing flange. Images courtesy Simpson Gumpertz
& Heger

Key benefits of vinyl windows often include:

  • heat-welded frame corners that provide good resistance to water penetration (Figure 1);
  • wept glazing pockets that allow water bypassing glazing seals, as well as condensation, to drain to the exterior; and
  • continuous nailing flanges at the perimeter of the windows for easy installation and reliable connections of surrounding weather barriers to the window frame.

Another attractive feature available with some systems is the use of a fully welded ‘master frame’ around the perimeter of multi-unit assemblies, such as two or three windows ganged together. The continuous, welded perimeter frame can help eliminate the sensitivity of gang mullion end conditions that are often problematic with respect to leakage.

Like vinyl windows, aluminum windows are attractive to building owners because they are highly resistant to corrosion, and are also strong and durable. With this increase in strength and durability generally comes added cost.

Although there are exceptions, aluminum windows rarely come equipped with wept glazing pockets; unlike many of their vinyl counterparts, they rely on sealant at mitered or coped frame corners (Figure 2), making a sill pan flashing an essential feature of a window system design—even shop-applied sealant is sensitive to workmanship (i.e. workers in a factory) and sealant can degrade and lose adhesion over time.

The added strength of aluminum, combined with the various options that are often available (e.g. receptor framing, reinforcing mullions, and gang mullions) generally make aluminum windows a good option where large multi-unit and ribbon assemblies are needed.

Wood windows are often used in residential and historic applications, but are susceptible to rot, especially when less durable species like pine are used without being maintained. Durable wood products like mahogany are a great option, but may be cost-prohibitive. Aluminum-clad or vinyl-clad wood windows are often specified when the owner and/or designer desires the appearance of wood on the interior while obtaining the durability/rot-resistance of an aluminum or vinyl window on the exterior. The concept is appealing, but there are weaknesses of some clad wood windows that often go unrecognized.

Sealed aluminum window frame corner.

Sealed aluminum window frame corner.

Joints in the cladding are either treated with sealant or left unsealed (Figure 3). If seals at the cladding joints and/or glazing seals are poorly fabricated, not well-maintained, or not installed, water can bypass the cladding and reach the wood core. Once water reaches the wood core, it can become trapped due to a lack of weep provisions within the cladding itself (with many systems, no weep paths are provided). This can lead to rotting and leakage to the interior.

Many manufacturers treat the wood with a preservative that can help extend the material’s life. However, the preservative does not solve the underlying problem. Other products like aluminum may carry similar risk of leakage due to failure of glazing seals, but they do not pose the same risk of frame deterioration.

Perimeter detailing
Well-considered fenestration perimeter details are critical for the window, curtain wall, and storefront system’s air and water penetration resistance. Industry professionals who conduct leakage investigations on problem buildings know the perimeters of glazed systems are often the source of problems. Flashing and perimeter sealant detail options can vary depending on the type of fenestration chosen and, in particular, the shape of the frame members at the system perimeter. If the perimeter detailing is not coordinated with the intended fenestration type, issues can arise. The failure of coordination may arise as a result of inconsistency between the drawings and specifications, or due to a substitution.

For instance, the authors have reviewed designs where the drawings showed pressure-glazed curtain wall mullions and related flashing details, but the specification called for an interior-glazed storefront system. Storefront systems have different mullion geometries and work differently from curtain walls—therefore, flashing options are quite different.

Situations like this can go unnoticed until it is time to fabricate and install the storefront system, at which time the installer may realize it is not possible to install the perimeter details shown in the drawings. Similar situations arise when fenestration systems are changed after the design is complete (i.e. value engineering or substitution requests), or when an acceptable ‘or equal’ system is selected by a subcontractor rather than the ‘basis of design’ (BOD) one.]

Unsealed joint in aluminum cladding.

Unsealed joint in aluminum cladding.

Continuing with the curtain wall versus storefront example, a common and reliable perimeter flashing condition for curtain walls consists of a flexible sheet membrane, such as silicone sheet or ethylene propylene diene monomer (EPDM) membrane, extending from the surrounding weather barrier into the glazing pocket of the curtain wall (Figure 4). The perimeter mullions of storefront systems generally do not have an open glazing pocket to facilitate direct access for tying in the perimeter flashing, so the glazing pocket flashing detail is not feasible. If the project team does not realize the discrepancy until the glass-and-metal systems are already fabricated and installation is imminent, issues can result, and the opportunity to achieve a reliable perimeter flashing can be lost.

Similarly, if a stick-built, pressure-glazed curtain wall system and associated glazing pocket flashing concept is specified, but a pre-glazed unitized curtain wall system is provided, the flashing design will no longer work (even with an excellent high-performance unitized system). When units are pre-glazed, the glazing pockets will not be accessible, and a continuous starter sill will be present along the sill condition, necessitating a re-design of the flashing system.

This topic is not only applicable to storefront and curtain wall systems, but it also holds true for the different window types discussed in this article. For another example, if flanged windows are envisioned by the design team though block-frame windows are purchased for the project, the original detailing, sequence of installation, and even the intended weatherproofing materials may no longer work.

Performance data
Designers often rely on manufacturer-published performance data when selecting fenestration systems. Some of the more common test data in which specifiers are interested include thermal performance, air and water penetration resistance, and structural performance. The data is useful for relative comparisons between similar base systems, but does not ensure the exact glazing assembly specified for a project will have the same performance level.

For instance, data is often published for a particular window model of a particular size with a standard configuration (i.e. a single window unit of a standard size is tested, not various sizes or any multi-unit assemblies). In most cases, this exact window size will not be installed on any given project. The data may not be applicable for:

  • larger windows (often, bigger windows have more trouble meeting performance requirements);
  • windows of a different configuration (e.g. multi-unit assemblies ganged together, with or without a receptor frame);
  • windows with additional features such as muntins; and
  • windows with different glass types/thicknesses.
Flexible membrane fl ashing at perimeter of curtain wall is shown in the upper photo. Flexible membrane fl ashing extending from weather barrier into curtain wall glazing pocket is depicted in the lower photo.

Flexible membrane fl ashing at perimeter of curtain wall is shown in the upper photo. Flexible membrane fl ashing extending from weather barrier into curtain wall glazing pocket is depicted in the lower photo.

Often, untested, unrated components—such as gang mullions, sill receptors, and muntins—are where problems arise during field testing and in service.

For example, on a recent project, a window advertised as an “80-psf window” (i.e. 3830 Pa) was barely able to meet a 40-psf (i.e. 1915-Pa) design wind load. This was mainly due to window sizes being larger than the assembly tested by the manufacturer, along with the inclusion of components like muntins that were advertised as standard, but were not included in the tested assembly. The authors have also observed frequent water test failures at ganged mullions that were advertised as standard components, but also not included in the tested assembly.

Special care is needed when specifying and detailing a fenestration system—be it a window, storefront, or curtain wall. One should not only consider appearance, economics, and thermal performance, but also give consideration to other factors with special focus on project-specific detail considerations.

Are large punched openings or ribbon openings desired? How important is rot-resistance when compared to appearance? How proactive will building owners/managers be with respect to maintenance? What is the likelihood a different glazed system will be proposed after the design work is complete, and how will this process be managed? These are just a few questions to consider on future projects.

Derek B. McCowan, PE, is a licensed professional engineer in Massachusetts with national engineering firm Simpson Gumpertz & Heger. He has 12 years of experience investigating building envelope failures, consulting on new design projects, and providing construction administration and performance testing services. He has a graduate degree in civil engineering and is member of the American Society of Civil Engineers (ASCE) and of ASTM International Committee C24 Building Seals and Sealants. McCowan writes and presents frequently on various building enclosure topics and has experience as a guest lecturer at various local universities. He can be reached at dbmccowan@sgh.com.

Douglas R. Pac, EIT, works for Simpson Gumpertz & Heger. He has six years of experience investigating envelope failures, designing envelope repairs for restoration/renovation projects, providing construction administration services, and consulting on new design projects. Pac has a master’s degree in civil engineering from the University of New Hampshire. He can be reached at drpac@sgh.com.

LCT ONE: A Case Study of an Eight-story Wood Office Building

All images courtesy Cree GmbH

All images courtesy Cree GmbH

by Nabih Tahan, AIA

For buildings to perform better, the construction industry must change the way it designs and builds. Lifecycle Tower (LCT) ONE, an eight-story wood office building in Austria, is both a prototype and proof of a concept that demonstrates an innovative process and building. Through its use of wood, LCT ONE focuses on reducing the negative impact of buildings on the environment, while improving comfort and indoor air quality (IAQ) for occupants.

LCT ONE began as a research and development project based on the lifecycle assessment (LCA) of buildings. The motivation was to find a substitute for traditional construction practices that can have negative impacts on local and wider environments. The population is growing and the trends are shifting. People are moving from rural to urban areas. Cities must find new ways to grow around transit systems with sustainable developments that do not deplete resources and harm the environment. Therefore, cities must readapt to the current changes.

The sustainable wood and concrete hybrid building is now occupied and showcases a way of achieving mid-rise and tall buildings that can go up to 100 m (328 ft) and 30 stories.(Figure 1).

Motivation for research project
A sustainable strategy must consider the entire lifecycle of a building and its materials. This includes resource extraction, material production, construction, operation, demolition, and recycling. Since the Industrial Revolution, progress in developing cities translated into more concrete and steel produced with oil and coal, to build tall and mid-rise buildings. LCT ONE was developed to introduce alternatives by substituting renewable resources for fossil fuels, along with systems and processes that can yield better building performance.

Exterior and interior photos of LCT ONE.

Exterior and interior photos of LCT ONE.

Forestry carbon cycle
Wood is a renewable resource that grows from the sun—it is the ultimate solar product. Trees absorb carbon while providing oxygen. Modern timber-based products, such as engineered lumber, are available worldwide. Heavy glued-laminated timber (glulam) members are stable, will not shrink and twist, and can be pre-cut and prefabricated to exact tolerances, which are airtight, resulting in saved energy. At the end of a building’s life, the wood can be reused for other purposes and later turned into fuel and energy.

Further, use of wood is carbon-neutral. In other words, growing forests absorb carbon from the atmosphere and wood products store carbon. It remains stored when wood products are recycled into other products. At the end of their life, bioenergy is produced from these products, as well as from mill and forest residues and reforestation ensures the carbon cycle continues.

Ecological backpack
Selection of building materials should be linked to the use of natural resources, including raw materials (renewable and non-renewable), energy, water, and land. To specify products for the LCT system, data was collected and calculations performed to measure the total amount of natural resources required to produce a certain product or building. Since trees grow above the ground, it is resource- and energy-efficient to extract and produce wood as a building product. Wood has a much lower ecological ‘backpack’ than traditional materials, such as concrete and steel.1

Prefabricated construction process
Austria has a long history of prefabricating high-performance building components out of wood. The process begins with computer-aided design (CAD) software used to cut lumber using computer-numerical-controlled (CNC) machinery. The members are assembled in a carpentry shop, under a controlled environment where windows, insulation, sheathing, vapour retarders, and finishes were installed. The components are made to tight tolerances, can be quickly assembled onsite, and meet the most stringent blower door test requirements. Modern timber technology is available and can deliver high-performance buildings using renewable resources (Figure 2).

Prefabricated timber wall and floor panels for LCT ONE.

Prefabricated timber wall and floor panels for LCT ONE.

Operation and maintenance
Existing buildings consume a lot of energy during their operational life. To reduce consumption, the LCT system was developed according to the stringent Passive House standard introduced in Germany. The strategy is to drastically reduce consumption before relying on renewables. This is the surest path to reach zero net energy. Highly efficient solar and mechanical equipment have an ecological backpack, therefore ‘less is more.’ No need to heat and cool is more sustainable than heating and cooling with renewables or high-efficiency equipment.

The energy modeling software program Passive House Planning Package (PHPP), is an accurate tool for predicting heating demand and peak heating load in low-load buildings. The PH standard is based on energy performance (kWH/m2/year [kWH/sf/year]); in Europe, the predicted energy consumption during the design phase has proven to be accurate when compared to actual consumption during operation.2 These metrics and strategies are valid across different climate regions of the world.

To guarantee maintenance and durability, a building science consultant is part of the integrated design team and advises on the permeability and diffusion of the entire building enclosure. The most important aspect is airtightness, which prevents air and moisture from entering the building enclosure. Additionally, the building’s exterior finish material is always installed on a rainscreen, creating a ventilation layer behind it and allowing any water penetration to drain before it reaches the building enclosure.

Urban mining
All building products originated from mining the earth. At the end of their ‘lives,’ buildings are typically added to landfills and new materials are mined. The LCT research project strived to develop a solution where reusable materials are saved from landfills. Urban mining conserves our natural resources, eliminates potential energy costs, and greenhouse gas (GHG) emissions. When a building has reached its full useful life, urban mining of the LCT system can be activated, extracting materials to reuse, recycle, and convert into bioenergy, thus protecting landfills from unnecessary waste.

System and product development
As a result of the research phase, LCT ONE was designed and built as a proof of concept. The goal was to develop a system and products that can be used on any urban infill projects, but with wood replacing concrete and steel, where possible. The goal was to introduce an industrial process for buildings—similar to the process used by car and computer companies to design and build their products. Instead of miles per gallons, building performance can be guaranteed in kWh/sf/year.

To reach this goal for tall, large-volume wood buildings in urban settings, the LCT system integrates:

  • planning;
  • offsite production and onsite assembly;
  • use and future conversion;
  • dismantling; and
  • recycling of buildings.

Structural system
The LCT system was developed as a core and shell that acts as the structural system and enclosure of a tall, large-volume wood building. This system is analogous to the ‘Intel Inside’ of a computer. It is the hidden operating system on which each manufacturer relies, but each computer looks and performs according to the manufacturer’s design and specifications. Similarly, the core and shell of the LCT system can be looked at as the ‘LCT Inside’—each architectural and engineering team can design the building according to its own aesthetics, integrating the site and client’s program requirements into the design.

Posts and hybrid slabs (left) and LCT ONE interior during construction (right).

LCT ONE interior during construction (left) and posts and hybrid slabs (right).

The core is where the elevators, stairs, wet rooms, and shafts are located. It serves as the building’s stiffening element. While wood is the optimal choice as a material for the core, concrete and steel can also be used until codes enable creation of taller wood buildings.

The gravity loads are carried by a series of heavy-timber glulam posts on the exterior of the building spaced approximately 3 m (10 ft) apart. These posts are exposed on the building’s interior, adding to the warm aesthetic of the space. For fire protection, the size of the post is increased beyond the structural requirements. Approximately 38 mm (1 1/2 in.) of wood for one-hour fire protection is added to each exposed surface of the posts, which creates a charring layer in case of a fire. Wood burns ‘safely,’ because based on fire tests, calculations, and simulation, predictions can be made on how long wood will withstand the flames—therefore, the building codes allow this additional thickness in recognition of the superior fire endurance demonstrated by large wood beams and columns in fires. (Figure 3).

Hybrid wood/concrete floor slabs span about 9.1 m (30 ft) between the exterior heavy timber posts and the core, transferring all lateral forces from the former to the latter. The benefit of a hybrid system is it takes advantage of the properties of each material to meet all the structural, fire, acoustic, and thermal requirements using the least amount of resources and energy. The hybrid slabs were tested in a full-size fire chamber and passed a two-hour fire test.

International Building Code (IBC) requires ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials. However, in this case, the tests were performed in Europe according to International Organization for Standardization (ISO) 834, Fire Resistance Tests—Elements of building construction, which is the governing full-scale furnace test.

The design of the slabs provides a built-in fire separation between each floor because there is no wood-to-wood contact between floors.

Integrated building enclosure
The building enclosure is then integrated with the core and shell to give the building its aesthetic appearance. The façade is a curtain wall that withstands wind loads, but not gravity forces. The seismic forces are transferred from the glulam posts to the hybrid slabs through a hinged connection.

Building enclosure and integrated mechanical, electrical, and plumbing (MEP) systems.

Building enclosure and integrated mechanical, electrical, and plumbing (MEP) systems.

The enclosure can be made of any material, but many prefer wood, as it is a renewable resource. The windows, insulation, water, air barriers, vapor retarders, and exterior finishes are designed with the collaboration of a building scientist, mechanical engineer, and exterior wall manufacturer. These sections take into consideration the orientation of the building, as well as the energy performance and standard demanded by the client.

Integrated MEP Systems
Similarly, the mechanical, electrical, plumbing (MEP), and fire protection systems can be integrated within the core and shell and optimized according to the building orientation and enclosure. The systems can be prefabricated and are easily accessible between the structural members. (Figure 4).

Planning process
As a prototype, LCT ONE is proof of the Lifecycle Tower concept. Its foundation and core were built onsite with traditional reinforced concrete construction. The building enclosure was timber frame and the floor elements were made out of the wood/concrete hybrid slabs.

To guarantee performance, the LCT concept is designed to automatically meet the requirements of certification programs. However, as a prototype, it was necessary to compare the LCT system to mainstream certification programs. As mentioned, LCT ONE was designed to meet Passive House and received official certification from the Passive House Institute in Germany. It also applied and received Deutsche Gesellschaft für Nachhaltiges Bauen (DNGB) Gold certification from the German Sustainable Building Council.

Energy standards and certifications
The goal was to meet the energy requirements of Passive House because it has proven to be accurate in predicting actual energy consumption. In collaboration between the engineers and building enclosure manufacturer, the building envelope—as well as the mechanical, electrical, and ventilation systems—were designed to optimize the building performance according to the given location and orientation. By using the PHPP software, multiple reiteration were attempted to optimize the balance between the orientation, building enclosure, mechanical, electrical, and renewable energy systems.

Part of the production process of hybrid slabs and wood in metal frames is shown here.

Part of the production process of hybrid slabs and wood in metal frames is shown here.

The building orientation was governed by the property’s existing location. The orientation created a negative effect on the energy balance. The building would have performed better if it could have been rotated by 90 degrees. To mitigate the effect of the orientation, the thermal performance of the building envelope was improved by increasing the thickness of the wall and insulation and specifying higher-performing triple-glazed windows.

Building enclosure
The building enclosure consisted of prefabricated timber frame walls, where the insulation, windows, and sheathing were installed offsite. The connection and intersection between all wall, floor, and roof elements were designed to minimize thermal bridging. Insulation was applied on the exterior of the window frames to decrease heat losses through thermal bridging. All joints were taped to be airtight in order to meet the Passive House blower door test requirements. This test is one of the three requirements needed to meet Passive House certification. It measures infiltration air flow at a pressure difference of 50 Pa. The requirements stipulate it cannot exceed 0.6 air changes per hour (ach) at 50 Pa.

The shape, size, and number of windows were optimized for low heat loss in the winter and low heat gain in the summer, as well as to reduce demand for artificial lighting throughout the year. Tilt-and-turn operable windows were specified to allow for natural ventilation—as they open to the interior, they allowed exterior shading devices to be installed for reducing heat gain in summertime.

Passive heating and cooling
Passive heating is achieved by large windows in the staircase (eastern orientation). The morning sun heats up the concrete wall and heat is stored in the wall’s thermal mass. Passive cooling is achieved by operating the chiller machine in a free-cooling-mode during most of the year. Further, the optimized ratio between transparent windows and opaque walls, as well as deep window reveals, prevents the office building from overheating in the summer.

The production of timber frame walls is shown here.

The production of timber frame walls.

Building system
The following building systems were used on LCT ONE:

  • heating system: district heating system—renewable-fueled combined heat and power;
  • cooling system: conventional chiller machine with enhanced free-cooling-option;
  • hot-water system: highly efficient, decentralized water boiler on each floor;
  • heat recovery ventilation: central system installed in basement with carbon dioxide (CO2) sensors on every floor that control the amount of air introduced;
  • lighting: fully automated and daylight-dependent lighting system (including motion detection), automated dimming and zoning, and daylight-dependent shading operation and positioning;
  • services: fully automated building services system;
  • controls: motion and window detector controlled heating and cooling, as well as CO2 sensors;
  • waterless urinals; and
  • photovoltaics (PV): 10-kW (peak) rooftop system—prepared for future installation of a 10-kW PV on the southern façade, which will be required to become a zero net energy building.

Blower door tests were conducted in two stages. The first was a random test at two floors performed after the installation was completed. (The core was excluded.) The result was 0.35 ach at 50 Pa. Before commissioning, a blower door test was performed on the entire building, including the core. The results were 0.55 ach at 50 Pa, meeting the Passive House standard.

Construction process
While foundations and concrete core where being built onsite, the wall elements and the hybrid wood/concrete floor slabs were produced offsite. The assembly of the wall and hybrid floor elements took eight days onsite, one floor per day.

Producing slabs
The industrial production of the hybrid wood/concrete slabs took place in a precast concrete shop. The heavy timber glulam beams were supplied by a lumber manufacturer who cut them accurately with CNC machinery and attached the required metal fasteners and connectors. The beams were delivered to a precast concrete manufacturer, who placed them in metal forms, added metal reinforcement and poured the concrete. This process was repeated daily for each form. The advantage of this system is it allows the concrete to cure offsite and prevents additional moisture into the building. Additionally, prefabricated slabs are assembled quickly onsite (i.e. eight minutes per slab) to tight tolerances (Figure 5).

LCT ONE on day two, four, and eight of the installation process.

LCT ONE on day two, four, and eight of the installation process.

Producing walls
The walls were produced in a local carpentry shop. Engineered lumber was used for all wood members, including studs. The panels were produced on tables, horizontally, where the timber frame, sheathing, and insulation were installed. After standing up the walls, the windows were installed. All joints, including around the windows, were sealed airtightly with high-performance tapes (Figure 6).

After production, all the slab and wall elements were shipped to the site and assembly began. Five skilled carpenters were able to assemble all the components, water- and airtight in eight days, for all eight stories (Figure 7).

To verify the energy standard will be met, blower door tests were performed twice as mentioned—once when the installation was done and again before commissioning.

LCT ONE begins with the premise the building industry does not automatically have to rely on concrete and steel for all urban buildings; it demonstrates there is the opportunity to substitute timber for many applications.

Wood is a renewable resource. Essentially, while one building is in construction, the sun is producing the timber for the next building. Timber technology has advanced, where modern industrial machinery and processes make it possible to erect timber buildings quickly, economically, and according to all building regulations and high-performance standards.

LCT ONE is pioneering a new way of building, based on guaranteeing performance. It was developed according to a system that can be the shell and core, while still offering flexible design solutions and architectural and aesthetic possibilities to make each building unique. By following this ‘system approach,’ the performance of buildings can be guaranteed, similar to the performance of cars, computers, and other products manufactured through industrial processes.

The LCT system can be applied as a worldwide solution. With its introduction in Europe and now beginning in North America, it serves as an inspiration to wean the traditional building industry away from only fossil-fuel-intensive products and systems. Less-developed countries, especially those with forests, can adapt the LCT system to modernize their building industry. They can introduce new sustainable forestry management policies and begin manufacturing modern, engineered lumber products to build high-performance timber buildings. A new process of education to create new green jobs and affordable housing solutions could be an alternative to attempting to develop building solutions that rely on fossil-fuel-based resources that less-developed countries do not have and cannot afford.

1 For more on the ‘ecological backpack’ concept, see M. Ritthoff et al’s Calculating MIPS: Resource Productivity of Products and Services, at epub.wupperinst.org/frontdoor/index/index/docId/1577. See also K.-H Robèrt et al’s “Strategic sustainable development–selection, design, and synergies of applied tools” in the June 2002 issue of Journal of Cleaner Production. (back to top)
2 For more information visit, www.cepheus.de. (back to top)

Nabih Tahan, AIA, is an international architect, Passive House consultant, and vice president of business development for Cree Buildings. For more than 30 years he has honed his knowledge in architecture, energy efficiency, and sustainable timber-based construction methods through work in Austria, Ireland, and the United States. In 2005, his own retrofitted home became the first Passive House home in California. Tahan also acts as the North American ambassador for Cree Buildings, and educates architects, developers, building engineers, and municipalities on the potential of tall wood buildings. He can be contacted at nabih.tahan@creebuildings.com.

A Sound Decision

Wood brings acoustic value to structures
by Michael Heeney

In the sea of concrete and granite that people have come to expect from buildings in Washington, D.C., one structure showcasing wood stands out from the crowd. When Arena Stage at the Mead Center for American Theater reopened in 2010, it was the capital’s first modern structure of its size to use heavy timber components. It was also the country’s first project to use a hybrid wood and glass enclosure to envelop two existing structures. Designed by Bing Thom Architects (with Fast+Epp Structural Engineers, Clark Construction, and StructureCraft Builders Inc.), the structure has a lobby large enough to hold up to 1400 patrons from all three theaters out at the same time. To warm that huge space and absorb sound, the design team again used stained poplar for the wood soffit on the lobby ceiling.
Photos © Nic Lehoux. Photos courtesy Bing Thom Architects

When designing a commercial structure, it is important to consider the situational aspects and parameters before selecting the most appropriate building products. While limitations such as budget and availability often sit at the forefront of these decisions, factors like aesthetic details and desired outcomes must be taken into account. One of the chief considerations for many projects should be the acoustics, encompassing everything from sound transmission to absorption and reverberation. Continue reading