Design Values Matter: Make Sure You Fully Understand Why
Design Values Matter: Make Sure You Fully Understand Why
It really isn’t overly dramatic to say that the structural building components industry lives and dies by accurate raw material design values. However, for many years, this industry has more or less taken design values for granted because it was assumed that the wood products and steel industries stood behind the design values ascribed to their products without exception. Also, it had been so long since lumber design values had been revised dramatically, upwards or downwards, that the 2012-2013 Southern Pine (SP) design value devaluation caught virtually everyone in the industry off guard.
This high-profile action, combined with data obtained through recent full-scale testing conducted at the SBC Research Institute (SBCRI) on braced wall panels, brought into sharp focus for the SBCA Executive Committee and SBCA Board of Directors the need for an industry policy articulating exactly why raw material design values are so important to the products this industry manufactures. To gain an understanding of why this policy (sbcmag.info/dvp) is needed, what it communicates to the marketplace, and, ultimately, what it promises about the future, let’s break the policy down into its basic elements.
Design Values = Load Resistance
No matter the species, component manufacturers (CMs) purchase and rely on the accuracy and reliability of many different lumber design properties, including: bending (Fb); shear parallel to grain (Fv), compression perpendicular to grain (Fc^), compression parallel to grain (Fc), tension parallel to grain (Ft), and modulus of elasticity (E and Emin). Depending on the type of structural load resisting component (roof, floor or roof truss) and a lumber member’s location in that component (top chord, bottom chord, or web), one or more of these design values are critical to the structural performance of that component and will control the ability of the lumber member to resist anticipated loads.
As the policy articulates in its initial paragraphs, CMs don’t simply purchase lumber; they actually purchase the design values attributed to that lumber. Those design values, in the case of SP, are determined by the Southern Pine Inspection Bureau (SPIB) and then approved by the American Lumber Standards Committee (ALSC).
During the component design process, technicians working for CMs utilize component design software that assumes reliability of lumber design values, which have been input into the American Wood Council National Design Standard’s (NDS) engineering mechanics-based resistance equations.
NDS standardized engineering equations assume that the resistance to applied loads is equal to or greater than the applied load, along with a typical factor of safety, such as two. The concept that the load divided by the lumber’s load resisting properties is less than or equal to 1.0 implies that the engineering properties for lumber are accurate and reliable. In other words, it is difficult, within the mathematics of engineering, to have 1.0 really be equal to 1.0 plus or minus 30 percent. If design values are low by 30 percent for any reason, the expected factor of safety is also not going to meet end-use expectations. Based on this, the factor of safety could now be 1.5 versus 2.0.
These same engineering mathematics fundamentals apply to component design software, however, in a more complicated manner because the math used in design
software is based on a more sophisticated matrix method or finite element engineering analysis. These equations also assume that lumber properties accurately represent resistance. The real difference is that there is less chance for human error when software-based engineering math is used, as opposed to the “hand calc” engineering math used through the NDS.
Naturally, the engineering math used to determine accurate raw material properties extends to all building materials used to provide structural resistance, whether they are connector-plates, wood structural panels (e.g. OSB, plywood, etc.), LVL, I-joists, hangers or fasteners.
For CMs to effectively meet customer needs and build market share, innovation and optimization of engineering resistance must have suppliers who stand behind their raw material design values as accurate and reliable. Whether it is visually-graded lumber, truss plate steel, or a machinery safety bar, it is necessary to put in place the quality control systems needed to assure this reliability.
Regular Testing of Raw Materials
Since accurate and reliable raw material design values are so critical to the components manufacturing industry, it is vital that raw material producers conduct regular testing of their resource to verify those values. An example of this need for regular lumber testing is found in Appendix A of Supplement 13 to the SPIB Grading Rules, 2002 Edition, approved by ALSC in January, 2013:
Wood is a natural product subject to variations in geography, climate, specific site characteristics, silvacultural practices, and harvesting decisions. Its strength properties are not only anisotropic (vary by principal axis) but also can vary with proximity to the center of the tree. These characteristics complicate the assignment of individual pieces into design value groups based on the visual appearance.
Wood, being organic, is subject to variability. Fortunately, this isn’t news to anyone. Lumber testing procedures for all North American species were recently reviewed by ALSC to ensure that testing programs are regularly conducted in a manner that ensures the design values ascribed to visually-graded lumber is a representation of the entire current population of trees harvested. As SPIB also states in Appendix A, “there is great variability present in design values assigned through visual grading means.”
Alternatively, machine graded lumber (MSR/MEL) is tested and graded by a machine, which assesses the actual strength properties during the production process of each piece of lumber. Whether it’s by directly measuring the stiffness of the lumber or its density, MSR/MEL has the advantage of providing more accurate and reliable lumber design values. Further, the quality assurance process for MSR/MEL lumber ensures that every single piece of MSR/MEL lumber has been assessed and can be used in engineering equations with assurance that these equations will produce reliable resistance. That assurance allows CMs the flexibility to be more confident in the creative and innovative techniques that their truss designers use to meet customer needs.
Regular Testing of the Building Code
Raw material design values are not the only thing that needs to be tested and verified on a regular basis. As the SBCA design value policy points out, there are building-code adopted wood product prescriptive applications that should be tested and verified as well.
It is not disputed that the model building code is a set of “ICC-consensus-based” provisions. (ICC is not an ANSI or ASTM consensus standards development process.) However, when these requirements provide prescriptive design guidelines and values based on outdated test data, problems arise. Historical test data were based on test assembly boundary conditions that do not reflect real building performance.
For example, historical test data uses a steel beam to apply lateral load across the top of a wall assembly. This steel beam is supposed to represent floor or roof framing. This steel beam boundary condition test result is used to establish code-based resistance to lateral load design values. By definition, these design values will be much different than what a real structure experiences.
When this testing and design value development information is not transparently understood, it is difficult, if not impossible, to make engineering judgments with respect to actual resistance. In addition, only a handful of engineers in the U.S. really know the substance behind the judgments made in the development of the building code language, which everyone otherwise has assumed is correct.
Engineering should be straightforward and easy to understand: apply a load and provide enough resistance to safely transfer that load to the ground. Recent empirical test results
show that code-adopted prescriptive design values are not accurate. This information should be easy to understand and well known so that engineering load resistance judgments can be made based on real resistance and all the factors that may have been applied to that resistance to make the code based tables work.
From 2009 through 2012, Qualtim, Inc. conducted (with more testing ongoing) 470+ ASTM E564/ASTM E2126 full-building tests, associated data analysis and finite element modeling to help understand true building braced wall panel performance. The results conclusively demonstrate that the IRC’s prescriptive 3/8", 7/16" and 15/32" wood structural panel design values are overstated by a factor of 1.8 for both walls without interior gypsum wallboard applied and those with interior wallboard.
This is a significant inaccuracy because fundamental shear wall design values, as defined in the building code, are overstated within the building code. The building code then becomes law, as municipalities adopt the code, and the 1.8 code implied design value factor is not explicitly defined. In the case of braced wall panels, this places the engineered designs that CMs undertake at a distinct competitive disadvantage where products other than OSB and plywood are used because no one knows that a code-implied design value factor of 1.8 has been applied to the code-based structural resistance provided.
When these code-adopted factors are not transparent or well understood by the engineering or building code community, engineered designs become devalued in the marketplace because they are viewed to be more expensive. Therefore, specifiers and construction product purchasers default instead to the prescriptive, building code approved alternative.
SBCA’s design value policy argues that greater transparency is necessary to provide a fair playing field for making good performance-based engineering decisions and accurate building code approval judgments. As the policy states, “it is in in the best interests of both the construction industry at large, as well as the truss and component manufacturing industry in particular, that engineering, and thus construction, be entirely based on tested and accurate raw material load resistance data.” This approach will not only ensure reliable building performance and more accurate factors of safety, it will also allow for greater innovation through the use of accurate and empirical-data formulated judgments.
Comply with State-of-the-Art
One of the most difficult aspects of raw material design value changes is the impact it has on prescriptively engineered conventional light frame construction, or stick-framing, when compared to the engineering that goes into truss design. For instance, when the new SP design value changes go into effect June 1, 2013, all engineered truss designs that are sealed by a professional engineer have to use the new SP design values, unless the Building Designer authorizes that a specific set of lumber design values can be used for the specific project. From a reliability, knowledge and liability perspective, CMs have no choice but to use the new design values to calculate everything from maximum spans to panel lengths to joint locations and plate sizes. Doing otherwise would introduce arbitrary engineering and construction defect liability exposure.
Likewise, building owners and contractors who use prescriptively engineered conventional framing are required to use the new design values by the SP effective date assigned by ALSC. Unfortunately, by either ignorance, oversight or neglect, they may not choose to use the effective SP design values. Many local framers may simply continue to build as they have done in the past, despite the fact that the SP span tables, adopted into the building code, have dramatically changed on the SPIB effective date. In some instances the local building inspector may not know of the changes either, and the unsuspecting local framer is effectively allowed to not follow the SP effective date state of the art. This is yet another example of not knowing, oversight or neglect.
Older span tables using outdated design values are still readily available and will be cause for confusion. If a local framer unknowingly picks up an old floor joist span table, and compares this span table to a corresponding span table for floor trusses (which use SP effective date design values), joists appear to be both stronger and able to span further than trusses and this may result in joists being a less expensive floor system. This apples-to-oranges comparison is possible again because of the SP effective date oversight process.
For either a local framer or building inspector to know of the design value changes and the absolute need to comply with the SP effective date state of the art, yet ignore the change and construct using old design values is an example of neglect, perhaps gross neglect. It is further neglect for either an industry resource or trade association to advise the marketplace that the use of old design values is appropriate as long as a building inspector chooses not to enforce the change. This “only change if you get caught” mindset is dangerous and constitutes a blatant disregard of the testing behind the lumber design value changes, engineering principles, the SP effective date “state-of-the-art” standard, and good old-fashioned common sense.
To ensure a more level playing field for the components industry, SBCA’s policy advocates that all manufacturers, sellers, specifiers, purchasers and users should be held to the same SP effective date “state-of-the-art” standard, regardless of the actual enforcement mechanism.
Striving for a Reliable Field
This new SBCA policy draws into sharp focus the many challenges facing the structural building component industry with regard to raw materials and other construction products. There are several facets of this issue that have the consequence of reducing the value of engineering. This clearly creates less incentive for CMs to innovate based on using their engineering skills to the full extent possible. This also allows competitive advantages to be inequitably built into the marketplace by codifying them into the law of the land (building codes).
Striving to understand and appreciate these impacts, and then actively participating in SBCA’s efforts to address them in the marketplace, may be one of the most important activities you can do as a CM. Deeper knowledge will help you craft business strategies that will place your business in a better position to compete with prescriptively engineered conventional construction (stick framing).
Given that our industry is in the component design and engineering business, which is closely related to the building design business, accurate and reliable engineering is central to every CM’s future success. A key SBCA mission is for engineering services to have ever-increasing value to the markets we serve.