Author Archives: CS Editor

What the 2015 International Building Code Means for Wood Construction: Part I

by Buddy Showalter, PE

Photo © iStock. Photo courtesy AWC

Photo © iStock. Photo courtesy AWC

The recent approval of the 2015 International Building Code (IBC) is of interest to design/construction professionals as it often means expanded options for structural applications. However, understanding the latest changes and allowances in various jurisdictions can also be a daunting prospect.

This article is the first part of a series that will discuss four new code-referenced standards, explaining what the latest building code updates mean for the specification of traditional and engineered wood product applications in buildings. These products are a selection being opted for more often by developers and building designers for sustainable and cost-effective projects. Each publication has been approved by the American National Standards Institute (ANSI) as American National Standards and each is referenced for wood design in the new IBC.

In this installment, the 2015 National Design Specification (NDS) for Wood Construction and the dual format allowable stress design (ASD) and load and resistance factor design (LRFD) standard used to design wood structures worldwide will be explored.

At a high level, the 2015 NDS has been updated to reflect the following:
● incorporation of cross-laminated timber (CLT) in several chapters, including a new product chapter specific to CLT;
● new terminology for laminated strand lumber (LSL) and oriented strand lumber (OSL);
● clarification that withdrawal design values for lag screws exclude the length of the tapered tip;
● inclusion of char rates for CLT and structural composite lumber; and
● relocation of reference to Special Design Provisions for Wind and Seismic (SDPWS).

The primary update to the 2015 NDS is a new product-design chapter for CLT—an engineered wood building product designed to complement light- and heavy-timber framing options. Due to its high strength and dimensional stability, CLT can be used as an alternative to concrete, masonry, and steel in many building types.

Having gained popularity in Europe over the past 20 years, CLT is now available to North American building designers. It offers the structural simplicity needed for cost-effective buildings, as well as benefits such as fast installation, reduced waste, improved thermal performance, and design versatility. It can be used in a wide range of applications, including mid-rise, urban infill, industrial, educational, and civic structures.

The new CLT chapter is consistent with other product chapters originally included in previous versions of the NDS, but most closely modeled after Chapter 9 for wood structural panels. The applicable product standard for CLT is ANSI/APA PRG 320, Standard for Performance-rated Cross-laminated Timber, and applicable design values are to be obtained from manufacturer’s literature or a code evaluation report.

Other changes reflected in the 2015 NDS specific to CLT include:
● general connection provisions revised to accommodate CLT in Chapter 12 (“Dowel-type Fasteners”);
● new sections applicable for wood screw and nail withdrawal from end grain of CLT;
● additional sections to address determination of dowel bearing strengths for fasteners installed in CLT; and
● new placement provisions for fasteners and lag screws (see image below).

This image demonstrates end distance, edge distance, and spacing requirements for fasteners in the narrow edge of cross-laminated timber (CLT). Fastener placement provisions are based on CLT cross-section dimensions, as opposed to individual laminations within the CLT. End distance, edge distance, and spacing requirements for fasteners in the panel face of CLT should be designed in accordance with existing National Design Specification (NDS) for Wood Construction requirements for these fasteners in other wood products. Data from 2015 National Design Specification (NDS) for Wood Construction. Images courtesy AWC

This image demonstrates end distance, edge distance, and spacing requirements for fasteners in the narrow edge of cross-laminated timber (CLT). Fastener placement provisions are based on CLT cross-section dimensions, as opposed to individual laminations within the CLT. End distance, edge distance, and spacing requirements for fasteners in the panel face of CLT should be designed in accordance with existing National Design Specification (NDS) for Wood Construction requirements for these fasteners in other wood products. Data from 2015 NDS for Wood Construction. Images courtesy AWC

Chapter 13 of NDS (“Split Ring and Shear Plate Connectors”) clarifies provisions for the design of these types of connections is not directly applicable to CLT. Possible considerations for their use, as part of an engineered design, would need to include requirements for end and edge distance, spacing, and effects of perpendicular crossing laminations.

Chapter 14 (“Timber Rivets”) consistent with design of shear plate and split ring connectors in CLT, clarifies provisions for design of timber rivet connections are not directly applicable to CLT.

Table_16_2_1_B

A char rate model for CLT based on observations from testing was incorporated in the 2015 NDS fire design chapter. Accordingly, a new effective char depth equation and table for CLT were added. Section properties can be calculated using standard equations for area, section modulus, and moment of inertia using reduced cross-sectional dimensions. The dimensions are reduced by the effective char depth, achar, for each surface exposed to fire.

Chapter 16 (“Fire Design of Wood Members”), includes a char rate model for CLT based on observations from testing. Accordingly, a new effective char depth equation and table for CLT were added (see table at right).

Terminology for engineered wood products LSL and OSL are also added to Chapter 8 (“Structural Composite Lumber”). Structural composite lumber products LSL and OSL are addressed in ASTM D5456, Standard Specification for Evaluation of Structural Composite Lumber Products, but have not previously been defined within NDS. Added product definitions for LSL and OSL are consistent with those in ASTM D5456.

Chapter 16 was also revised to address structural composite lumber products such as parallel strand lumber (PSL), laminated veneer lumber (LVL), and LSL.

Finally, along with the updated NDS, the NDS Supplement: Design Values for Wood Construction new design values for southern pine lumber are incorporated. The American Lumber Standard Committee Board of Review approved changes to design values for all grades and sizes of visually graded southern pine and mixed southern pine lumber with a recommended effective date of June 1, 2013. Additionally, new and revised grades of machine stress-rated lumber and machine-evaluated lumber are also included in the 2015 NDS Supplement. Both the 2015 NDS and its supplement are available for download on AWC’s website.

Next month, an update on 2015 Special Design Provisions for Wind and Seismic (SDPWS), outlining new considerations and criteria for proportioning, designing, and detailing engineered wood systems, members, and connections in lateral force resisting systems will be available.

Buddy Showalter HeadshotBuddy Showalter joined the American Wood Council (AWC) in 1992, and currently serves as vice president of technology transfer. His responsibilities at AWC include oversight of publications, website, helpdesk, education, and other technical media. Showalter is also a member of the editorial boards for Wood Design Focus, published by the Forest Products Society, and STRUCTURE magazine, published jointly by National Council of Structural Engineers Associations (NCSEA), American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI), and Council of American Structural Engineers (CASE). Before joining AWC, Showalter was technical director of the Truss Plate Institute. He can be reached at bshowalter@awc.org.

Choosing a special inspector

SPECIFICATIONS
Michael A. Matthews, PE

In a previous article, this author provided a look at the concept of special inspections. In advising the owner in the selection of a special inspector for a project, it is important the registered design professional consider several factors. It is the position of structural engineers—as articulated by the Council of American Structural Engineers (CASE) Guide to Special Inspections and Quality Assurance, and as per the recommendation of the Investigations and Oversight Committee on Science and Technology—the onsite presence of the structural engineer of record (SER) is mandatory. Therefore, having the SER serve as the special inspector wherever possible makes sense.

It should be noted when the SER or the designated agent is the special inspector, the concern identified by congress to bring the SER onsite during construction is addressed. In cases where the SER is not local to the project, this is rarely economically practical. Therefore, it would be more economical to use a local firm as the special inspector, if designated by the SER, which can access the project site more efficiently.

Regardless, the special inspector should have experience in performing the types of special inspections required for the project and a good track record of scheduling and completing special inspections in a way that does not interfere with the general contractor’s schedule—construction delays can be costly to the owner. Further, as the emphasis of the special inspections program is to bring structural engineering knowledge and expertise to the jobsite, the special inspector’s experience should be scrutinized by the local building officials to ensure knowledgeable inspectors will be onsite.

Installed “in accordance with plans and specifications”
All too frequently, an underqualified special inspector who is uninformed of the building code requirements is engaged by an unaware owner. Further, such inspectors do not meet the minimum requirements of the ASTM E329, Standard Specifications for Agents Engaged in the Testing and/or Inspection of Materials used in Construction. These underqualified special inspectors frequently lack the structural engineering expertise necessary to certify critical structural components. Further, they frequently elicit inspection contracts from owners based on lump sum contracts, or with promises of a limited number of site visits. Logically, it would be impossible to know the number of inspections required to inspect a building before construction commences, as it depends on the contractor’s means and construction methods, as well as adherence to the schedule.

Additionally, these underqualified special inspectors often poorly inspect required items, or altogether overlook required items in inspections as they are too focused on just completing the task. These underqualified special inspectors typically document inspections with broad statements, which generally indicate a location (e.g. north wall), type of inspection (i.e. wall reinforcing inspection), and a generic statement (e.g. installed in accordance with plans and specifications).

These brief reports provide minimal useful information and can potentially provide a false sense of security that what was constructed was truly in accordance with the plans and specifications. Further, these broad statements are rarely qualified with reference to specific items that were inspected, such as reinforcement size and specific location or the installation of any special items, such as embeds that might have been required in reinforced concrete.

Additionally, it should be noted field modifications are made to every project. Whether because of the use of an alternate product or because of an oversight by the contractor, as-built construction is never perfectly identical to the original design. However, these underqualified special inspectors rarely alert the SER to field changes that may or may not be acceptable under the original design intent.

Unfortunately for the SER, these underqualified special inspectors are frequently used, thereby diluting the original reason for such special inspections. From the SER’s perspective, inspection reports providing generic statements such as “Installed in accordance with Plans and Specifications” are of no use. Further, in the event of a failure, the SER is placed in the undesirable position of having to defend their design when these inspections may have likely approved faulty construction. This has historically led to long and costly failure investigations, as well as the ensuing litigation costs.

Adding value to a project
Aside from the obvious improvement in public safety, special inspections can provide additional value to both the registered design professional and owner. This value can be envisioned as a quality assurance (QA) plan that checks the construction of the critical structural and life-safety components against the construction documents and shop drawings. A poorly executed special inspections program enlisting unqualified and inexpensive inspectors can be worse than no special inspections at all, because it leaves the owner, design team, building official, and public with a false sense of security that what was constructed met the intent of the approved construction documents.

Registered design professionals who advise owners to choose a skilled special inspection service benefit by reducing the likelihood of future insurance claims. Construction deficiencies can be identified in the field during proper special inspections and often corrected before they become costly, time consuming, or dangerous when the SER or his designated agent serves as the special inspector. Generally, timely identification of construction deficiencies and quick development of solutions can lead to quick repair of deficient conditions at minimal expense.

Quality special inspections provide added assurance to the owner and the design team the structure is constructed as it was designed and with the materials specified in the drawings. Building owners can further benefit by having a special inspection record details the construction of critical building design elements. Often, this record will include photographic documentation and true as-built conditions. The as-built conditions denoted in the special inspections field reports become more valuable when future additions or changes in use of the structure are considered.

Michael A. Matthews, PE, is president/CEO of the Structures Group (TSG), a consulting engineering firm specializing in a diverse range of skills focused in the areas of structural engineering, forensic analysis, special inspections, risk analysis, and independent plan reviews. He has been a licensed engineer for nearly 30 years, and is licensed in 20 states, as well as the District of Columbia. A sought-after speaker on topics related to structural engineering and forensic analysis, Matthews has presented seminars at Kansas State University, American Institute of Architects’ (AIA’s) ArchEX conference, Virginia Building and Code Officials Association (VBCOA) Annual Conference, and the annual Virginia Engineers Conference. He can be contacted by e-mail at mmatthews@thestructuresgroup.com.

AISC design guide announced

American Institute of Steel Construction (AISC) has released Design Guide 29, Vertical Bracing Connections–Analysis and Design, as a resource for various design professionals. The document provides a comprehensive approach to common bracing system design based on structural principles. Topics covered include brace-to-gusset connections, orthogonal and non-orthogonal connections, eccentric braces, connections at column base plates, and gusset plate stability. More than a dozen complete design examples are also included. The guide is available for download at www.aisc.org/dg.

MPI guide now available

The Master Painters Institute (MPI) has released a resource for selecting system options on both interior and exterior surfaces. Architectural Painting Specification Manual includes descriptions for nearly 50 interior and exterior surfaces of different qualities, as well as information on appropriate surface preparation for various applications. Dewpoints, wood types, paint-spread rates, and concrete masonry unit (CMU) standards are also discussed. An MPI Approved Product List outlines how products match up to criteria with full descriptions. Prepared by industry experts, and overseen by a North American advisory board, the manual is intended for architects, engineers, designers, and specification writers. To read the document, visit www.paintinfo.com/mpi/store/archman.shtml.

Aluminum design guide published

The Aluminum Association has released the 10th edition of the Aluminum Design Manual—a reference for design/construction professionals working with the material in structural applications. The guide is referenced in the 2015 International Building Code (IBC), and it allows engineers to create safe and innovative structures using modern, lightweight, corrosion-resistant aluminum alloys. Chapters of the guide include “Specification for Aluminum Structures,” “Commentary to the Specification for Aluminum Structures,” “Material Properties,” “Section Properties,” and “Design Aids.” Illustrated design examples of more than 30 structural design calculations are also included. To purchase the guide, visit www.aluminum.org.