Engineered vs. Prescriptive Wall Panel Design

Feature

Engineered vs. Prescriptive Wall Panel Design

Explore the two different methods used to calculate
a wall panel’s capacity to resist applied lateral loads.

Commentary

For this article, we thought it would be helpful to provide the following commentary in gray. This extended sidebar will run throughout the article and is intended to put the more technical aspects of the piece in layman’s terms, as well as to provide additional perspective on various issues.

This article explores the two different methods used to calculate a wall panel’s capacity to resist applied lateral loads (think wind and seismic): a code-adopted prescriptive method (IRC), and a science-based engineered approach (IBC). 

When a component manufacturer (CM) endeavors to provide an innovative framing solution to a customer’s lateral resistance problem using a shear wall, they must use engineering principles found in the IBC and/or generally accepted engineering practice to design it. 

This article explores how the prescriptive IRC method essentially stacks the deck against CMs who use these innovative framing solutions by implicitly devaluing engineered design and accepted engineering practice. 

Why it does this is straightforward: the IRC provides a built-in competitive advantage for wood structural panels and, therefore, the wood products industry. This advantage has been strategically codified over time to protect traditional construction methods.

How it does this is a bit more complicated. The code development process is political in nature, and the science and reasoning used to develop the building code is not transparent to the marketplace. 

Understanding this issue in detail can help CMs who want to provide innovative wall framing solutions to their customers. Conversely, not understanding this issue hinders CMs by keeping them bound to products that have a built-in competitive advantage.

Very few elements of a light-frame residence have as much engineering analysis behind their design as metal plate connected wood trusses. Typically, trusses in a residential structure are analyzed by a powerful design software program and reviewed by a structural engineer to insure their design is adequate. An extensive knowledge base developed through decades of testing and structural analysis has been used to develop the design procedures and manufacturing process for metal plate connected wood trusses. Given their engineering advantage, it is unsurprising that truss components provide far more efficient and cost-effective designs than conventional framing.

To better meet the needs of their customers, some CMs have added wall panels to their product lines. One would expect, given the advantages of the design software available to create wall panel configurations, wall panels manufactured by a CM would be more efficient than a stick-framed wall built in accordance with the International Residential Code (IRC). However, if one compares the conventional methods for wall panel bracing with those required for an engineered approach, an entirely different story unfolds. 

The two main approaches for constructing light-frame shear walls are the prescriptive provisions, as found in the IRC, International Building Code (IBC), and Wood Frame Construction Manual (WFCM), and engineered design methods, as found in the American Wood Council’s Special Design Provisions for Wind and Seismic (SDPWS) referenced by the IBC. The prescriptive provisions of the IRC specify the minimum construction requirements (sheathing thickness, fastener type and spacing, anchorage, etc.) and the length of wall bracing that are intended to result in a structure that is able to resist the applicable wind and seismic loads. The amount of bracing required by this approach was originally based on historic practice for light-frame construction. In 2009, the prescriptive provisions for wind loads in the IRC were revised to “have a consistent and logical framework to ensure wall bracing capacity meets wind load demand” (ref 1). However, this revision was still calibrated to “align with past successful wall bracing practice” by using an adjustment factor to increase the design values by 20 percent to account for partial overturning restraint and the contribution of the whole building system (ref 1). The prescriptive design approach has the advantage of being simple to apply, but it can only be used for buildings that meet the height, plan dimensions and loading conditions given in the IRC

Commentary

The IRC establishes a “box” that defines the maximum size and loading conditions of a residential building (see Table 1, which outlines the limits of the box). As long as the building being constructed fits inside that box, the building can be built using the prescriptive method. However, just because a building doesn’t fall down doesn’t mean we have a good understanding of why it doesn’t fall down, or even how well it resists the loads exerted on its individual framing elements. So, for good measure, the building code developers initially established what amounts to a “rule of thumb” to follow based on historical evidence and single-element testing, and then added 20 percent as an educated guess on what the additional strength the entire system added to the resistance capacity of a given wall panel.

On the other hand, engineered design methods are based on the principles of engineering mechanics and/or experimental tests of shear wall assemblies. This approach provides a method for calculating the resistance of a shear wall system, which is then compared to the applied wind and/or seismic loads specified by the building code. The resistance has to be greater than the applied load for the design to be adequate. Most commercial buildings and multi-family residences require an engineered design. Compared to the prescriptive approach, engineering design methods require more complex computations. 

It would be expected that the simplifications necessary to develop a prescriptive approach would result in a more conservative solution than a detailed engineered design, which could take into account the unique features of a structure. However, it can be shown that many prescriptive designs are much less conservative than an engineered solution. This is because the IRC is not based on fundamental engineering, but rather is calibrated to “historical acceptable performance,” which has no engineering definition. Since the IRC does not state the fundamental engineering properties or the factors applied to those engineering properties, the reasoning behind the code provisions can be easily misinterpreted, resulting in engineering-related unintended consequences. Greater transparency with respect to the use of generally accepted engineering mechanics and associated test data calibrations used in the IRC need to be provided, so that designers can make good engineering judgments.

Fortunately, SBCA members have full access to proprietary SBCRI shear wall testing that provides insight into the true behavior of shear walls. To better understand shear wall performance and the competitive advantage of the IRC provisions, this article will examine three different concepts of shear wall design: overturning anchorage, shear wall openings (i.e., perforated shear walls) and the wall aspect ratio.

Overturning Anchorage

Commentary

Overturning anchorage is relatively straightforward. The prescriptive code, which again is based on historical performance, requires traditional anchor bolts set in concrete or affixed to the foundation as the means for a wall segment to resist lateral loads. The development of innovative shear wall hold-down hardware has taken off over the past decade (photo at right is just one example). The good news is a CM can take advantage of the innovative hold-down hardware and, as a result, have a great deal of flexibility to design a value-added wall panel system to meet a customer’s needs. This is an area where a CM’s creativity and skill can produce significant innovation. For instance, if a CM knows the precise loads flowing into the hold-down connection, it’s easy to provide the proper resistance.

Testing shows that there are many times where the uplift load is far less in the building than what the prescriptive or engineered design process suggests is needed. For example, the SDPWS design method predicts a need to resist 4,200 lbs., but the uplift load at a load cell during structural testing indicates the actual load is half of that. It should be possible for a CM to use a less costly anchor, if this is a consistent result and can be quantified and justified.

The prescriptive provisions for wall construction in the IRC do not require hold-downs to prevent braced wall panels from displacing or overturning under shear and uplift loads (ref 2). Instead, braced walls are anchored by bolts spaced 6' o.c. for walls supported by concrete foundations or by three (3) 10d box (3½" x 0.135") nails at 16" o.c. when supported by rim joists, band joists or blocking. For two-story buildings in Seismic Design Categories D0, D1 and D2, and two-story townhouses in Seismic Design Category C, the maximum anchor bolt spacing is reduced to 4' o.c. 

In contrast, engineered shear walls constructed in accordance with SDPWS require the use of hold-down hardware to anchor each end of a shear wall (also known as a segmented shear wall) unless the dead load is sufficient to prevent the wall from overturning (ref 3). 

Currently, there are no codified engineering procedures to design a shear wall that is restrained by anchor bolts only. This is unfortunate given that this assembly has good performance and would provide an economical engineered solution, albeit at lower capacities than a segmented shear wall design using hold-down hardware.

Perforated Shear Walls

Commentary

While it intuitively makes sense to reduce the lateral load resistance capacity of a wall segment because of the presence of window and door openings, the degree of reduction the formulas in SDPWS require appears from testing to be far from precise.

What doesn’t make sense is that wood structural panel resistance performance is overstated in the IRC and SDPWS methods, making the use of engineering not very competitive. Meanwhile, calculating resistance capacity is quite conservative when perforations (window and door openings) are added to the wall configuration.

The SPDWS formulas effectively render any wall segments above or below windows and doors (see areas in red boxes at right), not to mention the doors and windows themselves, as providing little to no resistance. Extensive perforated shear wall testing conducted in SBCRI provides great insight into the error of this assumption.

SBCA’s goal is to use this data to create a wall panel design and QC methodology that will help SBCA members interested in manufacturing walls gain a competitive advantage over stick-built walls.

Being able to take advantage of an increased resistance capacity will make engineered perforated shear walls more competitive against prescriptively built walls. The biggest challenge is developing an approach that is as easy to implement as the prescriptive method in the IRC.

The presence of openings (e.g., windows and doors), called perforations, in a shear wall must be considered in its design. The perforated shear wall (PSW) method contained in SDPWS provides one way to design shear walls with window and door openings. This method only requires a hold-down at each end of the wall instead of placing a hold-down on either end of each wall segment between the openings. 

In the PSW method, an empirical adjustment factor is used to reduce the perforated shear wall capacity to account for the loss of resistance due to the openings and the reduced overturning restraint due to the use of fewer hold-downs. The shear capacity adjustment factor is a function of the area of the openings and the length of the full-height sheathing segments (ref 4). These two variables are combined into a single factor called the sheathing area ratio,  , which can be calculated as shown in the equation below.

where:

Ao = total area of openings in the perforated shear wall

∑Li  = sum of the perforated shear wall segment lengths

h = height of the perforated shear wall

Using the results of experimental shear wall tests, the following regression equation was derived to relate the sheathing area ratio, r, to the shear capacity adjustment factor, C0.

where: 

Ltot= total length of a perforated shear wall including the lengths of perforated shear wall segments and the lengths of segments containing openings

As shown in SDPWS Section 4.3.3.5, the equation for calculating the shear capacity of a perforated shear wall is as shown in the equation below.

where: 

V = shear capacity of the perforated shear wall in lbs.

Co = shear capacity adjustment factor

v = nominal unit shear capacity from SDPWS Table A4.3 

Although the PSW design method was originally calibrated to shear wall test results, recent research has shown the PSW method to predict shear wall capacities significantly below the measured shear wall capacities (ref 5, ref 6). The adjustment factor for the PSW method needs to be calibrated to better fit the test data in order to result in more economical designs.

Aspect Ratio

Commentary

The consideration of what is called “aspect ratio” further highlights the disparity between the prescriptive method and a CM’s ability to design a value-added wall framing solution. The aspect ratio is the relationship between the height of the wall and the width of a wall segment, either between the hold-down and an opening, the corner and an opening, or between two openings. The article points out the IRC limits the wall height to 8', and the allowable wall segment can be at least 24" (2'), hence a ratio of 4 to 1. In practical terms, the prescriptive method allows a window to be placed as close as 24" from the end of a wall or 24" from a door or window without reducing the wall’s resistance capacity.

For a CM using the SDPWS method, the aspect ratio is decreased to 3.5 to 1, meaning the window has to be at least 30" from the end of a wall or 30" from a door or window to avoid reducing its resistance capacity. Further, if anything less than 4' separates an opening from the end of the wall or another opening, the resistance capacity of the wall needs to be reduced further.

Think about all the houses you have contributed product to and try to count on two hands how many of them have their openings more than 4' from the end of a wall or between openings.

So what does all of this mean in practical terms? This article now provides a discrete example of how the prescriptive method arrives at a very different assumed resistance capacity versus the SDPWS engineered method.

The IRC allows wall segments as small as 24" in width to count as part of the braced wall length if they are adjacent to openings less than 64" in height and are part of a continuously sheathed wall. Since the wall height for this provision is limited to 8', a 24" braced wall segment results in a height to width aspect ratio of 4:1. In many cases, the IRC does not require these segments to be restrained by hold-downs. Their only resistance to overturning forces comes from anchor bolts and the surrounding framing members. Unlike the provisions for engineered design, there is no reduction factor applied to these 4:1 aspect ratio segments, regardless of whether they are resisting wind or seismic forces. On the other hand, SDPWS limits the aspect ratio of light-frame shear walls to 3.5:1.

For perforated shear walls, aspect ratios exceeding 2:1 are only allowed if a reduction factor equal to 2bs/h is applied to that segment, where bs and h are the shear wall segment length and height, respectively.

Design Comparison

Commentary

This example makes clear there are serious economic ramifications to the disparity between the two methods used to calculate a perforated shear wall’s resistance capacity.

Using the exact same wall, the IRC states that the wall in the example at right, held down by anchor bolts set 6' apart, has a resistance of 2,242 plf.

In contrast, the SDPWS method would tell a CM that, if they designed this wall using today’s innovative hold-down hardware at the corners, its resistance capacity is reduced to 938 plf, 2.4 times less capacity.

This doesn’t make sense. The IRC significantly reduces the value of the engineered solution. The CM is forced to use more hardware in the hold-downs, but ends up with 2.4 times less capacity.

Based on this example, it becomes clear that, through this disparity, conventional framing methods and products, such as OSB sheathing, have an IRC protected position in the market.

The CM’s ability to provide a cost-effective and innovative solution to their customer is, needless to say, severely restrained.

To illustrate the differences between a prescriptive IRC design and the engineered PSW design method, consider the shear wall shown in the example above. The wall is 11' long, 8' high, and has two 27" by 64" openings. It is sheathed with 3/8" OSB fastened to Spruce-Pine-Fir (SPF) studs spaced 16" o.c. The fasteners are 6d common (2.0" x 0.113") nails spaced 6" o.c. along panel edges and 12" o.c. along intermediate framing members. This shear wall construction is in accordance with the minimum requirements of the IRC. The nominal unit shear capacity for wind loads according to SDPWS is 560 plf x 0.92 = 515 plf, where the reduction factor of 0.92 accounts for the use of SPF framing members.

Following the procedures used to develop the IRC wall bracing requirements, the shear capacity of the example wall can be calculated as the fully restrained shear wall design values from SDPWS (rounded down to 500 plf) times a net adjustment factor of 1.2 to account for the combination of partial restraint and the whole building system contributions to the overall wall bracing performance (ref 1). In addition, a 15 percent increase of this design value can be taken when the walls are continuously sheathed with wood structural panels (CS-WSP) to account for the effect of sheathing above and below window openings and sheathing segments not meeting the minimum length requirements (ref 1). Since the wall height is 8' and the height of the openings adjacent to the segment is equal to or less than 64", the total length of bracing for this wall is 6.5'. Thus, the shear capacity according to the provisions of the IRC is (500 plf x 1.2 x 1.15) x 6.5' = 4,485 lbs. The design capacity is 4,485 divided by 2, or 2,242 lbs. of design lateral load resistance. (See Table 2.)

On the other hand, a shear wall designed using the PSW method in SDPWS requires a reduction in the shear capacity due to the presence of openings, as discussed previously. For the shear wall shown above, the shear strength is reduced by a shear capacity adjustment factor, Co, of 0.81. Since the center pier of the wall has an aspect ratio of 4:1, it is not counted as part of the sum of the perforated shear wall segment lengths. According to the equation for the capacity of a perforated shear wall given above, the shear capacity is 0.81 x 515 plf x 4.5' = 1,877 lbs. The design capacity is 1,877 divided by 2, or 938 lbs. of design lateral load resistance. (See Table 2.)

Shear Capacities & Installation Methods

Commentary

The test standards referenced in the SDPWS method provide an additional illustration of how conventional framing methods are given a competitive advantage in the market. ASTM E72 is a standardized index test for wood structural panels. The ASTM E72 test setup favors wood structural panel performance and its typical failure modes.

In practice, ASTM E72 causes severe stress on the corner connectors of a wall by forcing a rectangle to essentially become a parallelogram through the use of a steel bar that eliminates normal building ductility (you can see the effects of this test in the photo above).

It’s easy to appreciate why the wood structural panel industry worked hard to get the ASTM E72 standard adopted into the IRC. It provides a built-in competitive advantage for wood structural panels and, therefore, the wood products industry.

There are also engineering concerns over the development of the shear capacities that form the basis of the engineering found in SDPWS/IRC. ASTM E72 – Test Methods of Conducting Strength Tests of Panels for Building Construction contains the following notes regarding the evaluation of sheathing materials:

14. Racking Load—Evaluation of Sheathing Materials on a Standard Wood Frame

NOTE 2—These test methods have been used to evaluate design shear resistance of wall assemblies without the involvement of anchorage details. If the test objective is to measure the performance of the complete wall, Practice E564 is recommended.

NOTE 5—Differences in edge distance, angle of fastener, and amount of fastener head penetration into the sheathing may impact the results of the tests and should be consistently installed in accordance with the manufacturer’s installation instructions.

However, ASTM E72 was used to define the nominal unit shear strength of wood structural panels in both the IRC and SDPWS instead of ASTM E564 as clearly recommended above. The SBC Magazine article “Installation & Fastening of Wood Structural Panel Wall Bracing” (March 2014) provides test data that defines the ASTM E564 tested ultimate strengths and goes into greater detail on the fastening-related considerations discussed in Note 5 of ASTM E72.

Conclusions

Commentary

The bottom line is the IRC uses factors to define a shear wall’s capacity to resist lateral loads that are not transparent in the marketplace.

This lack of transparency is a significant problem because engineers are prohibited from understanding the fundamental properties of the design process used by the IRC, making it impossible for engineered solutions to be competitive.

Unfortunately, the wood structural panel industry, led by APA and AWC, and an IRC process dominated by NAHB, have opposed SBCA’s past code change proposals to provide greater clarity on how these adjustment factors were derived and how they should be used. SBCA continues to advocate that providing a transparent understanding of these design properties will lead to better understanding, better design and, ultimately, greater innovation in the market.

When the same wall has a calculated resistance capacity of 2,242 plf using IRC design and framing methods, but only 938 plf using a fully engineered solution, it is clear that innovation in the market is being stifled.

Fortunately, there is hope for CMs. With the extensive shear wall performance data collected through testing at SBCRI, there is empirical evidence on how shear walls actually perform in real-world buildings.

SBCA’s goal is to take that data and work with CMs to develop more accurate engineering design procedures for shear walls. The aim will be to develop design and QC procedures that give CMs a distinct advantage over field framing methods. A transparent, science-based methodology for calculating shear wall resistance capacity will enable more creativity and innovation than the current, non-transparent IRC prescriptive approach.

The Ad Hoc Wall Bracing Committee that developed the prescriptive wall bracing provision in the 2009 IRC spent nearly two years studying the extensive amount of testing on shear walls and whole buildings that has been conducted in previous years. Based on that research, several increases in wall braced resistance capacity have been included in the residential code. Some adjustments, such as the 15 percent increase for continuous sheathing and the use of 4:1 aspect ratio segments, were based on the results of test programs, while other adjustments, like the 1.2 adjustment factor to the design strength, were based on committee member judgments.

One of the driving factors in the 20 percent increase to the design values was a desire to better reconcile the current IRC wall bracing lengths with past wall bracing practices. According to Crandell and Martin, the, “net adjustment factor could be grossly characterized as a ‘calibration factor’ to bring results in line with historic bracing requirements for 1950s or 1960s era 1,500 sq. ft. or less, two story or less, conventionally constructed houses” (ref 1).

SBCRI has conducted extensive research on perforated and segmented shear walls, with both partial and full restraint. Future articles in SBC will discuss the findings from this testing.

References

(1) Crandell J. and Z. Martin. The Story Behind the 2009 IRC Wall Bracing Provisions (Part 2: New Wind Bracing Requirements). Forest Products Society, 19(1), 2009.

(2) International Residential Code (IRC). International Code Council. Falls Church, VA. 2012.

(3) Special Design Provisions for Wind and Seismic (SDPWS). ANSI AF&PA SDPWS - 2008. American Forest & Paper Association. Washington, DC. 2008.

(4) Douglas, B. K. and H. Sugiyama: Perforated Shearwall Design Approach. American Forest and Paper Association, Washington, DC, 1994.

(5)  Dolan, D. J. and A. C. Johnson. Monotonic Tests of Long Shear Walls with Openings. Virginia Polytechnic Institute and State University Timber Engineering Report TE-1996-001. 1996.

(6) The Performance of Perforated Shear Walls with Narrow Wall Segments Reduced Base Restraint, and Alternative Framing Methods. NAHB Research Center, Inc. Upper Marlboro, MD. 1998.