The Story Behind IRC Wall Bracing Provisions
The Story Behind IRC Wall Bracing ProvisionsJay H. Crandell, P.E.IntroductionThe conventional woodframe construction provisions ofthe 2006 International Residential Code (IRC) (ICC 2006a)may be viewed as a “melting pot” of engineering scienceand building art. A previous article addressed many challenges in reconciling conventional construction practiceswith modern methods of engineering practice (Crandelland Kochkin 2003).In this article, the derivation of the IRC wall bracing provisions is documented and discussed in view of the technical research, engineering analyses, and judgments they embody. The art and science behind accurately understandingconventional wall bracing is still considered to be in its infancy and subject to disparate interpretations, even thoughit has been studied at various times since the early 1900sand especially in recent years (HUD 2000, HUD 2001a, Niand Karacabeyli 2002, Paevere 2002, Crandell and Kochkin2003, Cobeen et al. 2004, Kasal et al. 2004, Crandell 2006,APA 2007, Simpson 2007).BackgroundAny complex set of interrelated engineering design rules(including material strength properties, safety factors, resistance factors, load factors, mapped hazards or designloads, empirical equations, and equations based on principles of mechanics, etc.) is “tuned” to produce a design solution that is considered to represent or effectively calibrate tosuccessful design or construction practice. Engineering isfundamentally an empirical science and, as such, the resultof applying any engineering theory must produce answersthat agree with experience. The process of calibrating or“tuning” design theory to agree with successful experience,hypothetically applied to conventional wall bracing, is illustrated in Figure 1. This tuning process may account forflaws or biases in codified theory for determining loadsand/or resistance for a specific application, such as lightframe house construction. Because the ultimate goal is toachieve “acceptable” performance in a manner that is riskconsistent across all hazard conditions and applications,this process inevitably involves judgment, science, and politics. This observation is especially true in the developmentof the IRC wall bracing provisions, particularly the derivation of wall bracing amounts in Table R602.10.1 of the IRC.Summer 2007Figure 1.—Illustration of calibration of engineering theory tosuccessful conventional construction practice.Unfortunately, it should be recognized that currentbuilding codes and engineering design standards do notprovide analytical procedures suitable to evaluation of conventional bracing methods and building systems. For example, there are no codified design values or equations to predict the structural resistance of conventional (or “partiallyrestrained”) wall bracing systems such as wood let-inbraces, wood structural panels, Portland cement stucco,and other bracing methods recognized in the IRC. Codifiedengineering analysis conventions and data that are available only apply to detailing conditions, such as the use ofhold-down anchors, that are not representative of conventional construction and the complex load-paths involved(Crandell and Kochkin 2003). The degree of error or conservative bias in attempting to apply accepted engineeringconventions to the analysis of typical light-frame buildingsor homes can be substantial (refer to Addendum A to this article which compares three different attempts to reconcilecodified engineering analyses to results from an actualwhole building test).Bracing amounts in IRC Table R602.10.1 for various conventional bracing methods represent one means of addressing the concerns above in a rational manner using judgment3
and the best available technical information during thedrafting phase of the IRC in the late 1990s.1Setting the Stage for the IRCLeading up to the drafting of the IRC (2000 edition), various bracing provisions for conventional construction werein use (CABO 1995, ICBO 1997). With increased concernfor performance of conventional wall bracing in high hazard regions of the country and in the context of modern construction trends (e.g., larger buildings, more open interiorspaces, more windows, and increasing use of narrowerbracing panels not complying with the traditional 4-ft.braced panel width, etc.), these provisions were coming under increasing scrutiny and pressure to conform with engineering practice. Methods for engineering design, however,did not agree with a general perception of successful use ofconventional construction practices, particularly in lowhazard regions. A solution was needed to reconcile thesecompeting truisms.Fortunately, the American Forest & Paper Association/American Wood Council (AF&PA/AWC) staff had discovered ashear wall analysis method developed in Japan some 20 yearsprior (Line and Douglas 1996, Line 2002). The method isknown as the perforated shear wall (PSW) design method.While still requiring a “non-conventional” load path (i.e., ahold-down bracket) at the end of a PSW, the method provideda means of evaluating an otherwise conventionally framedbraced wall system with varying amounts and sizes of wallopenings (including full-height openings representing spacesbetween intermittent conventional brace panels). Thismethod is currently recognized with a number of seeminglyconservative constraints (relative to tested or actual performance) in Special Design Provisions for Wind and Seismic(AF&PA 2005). It is featured in the design examples andwhole building test comparison in Addendum A to this article.Shortly after the PSW design method was discovered, thehomebuilding industry, along with the U.S. Department ofHousing and Urban Development (HUD), became interested in its application as part of a multi-year research program at the National Association of Home Builders (NAHB)Research Center, Inc. (www.nahbrc.org). The research initiative was aimed at developing engineering methods tomore efficiently design conventional woodframe homes.One of the many objectives of this research program was to14In recent years, several advancements have continued to occur inunderstanding the performance of conventionally braced woodframe buildings. While these are not addressed in this article, it isworth mentioning a generalized mechanics-based model under development by AF&PA’s American Wood Council (AWC) staff thatprovides a means to design wall bracing using detailing and loadpaths inherent to conventional construction as well as engineeredconstruction. Ultimately, such a tool may allow conventional construction to be considered as an engineered system on par with perforated shear walls and segmented shear walls, each with their respective trade-offs in performance vs. detailing and constructionefficiency.develop a lateral design method for wood frame homes thatcould be used to answer a growing call to justify and improve conventional wall bracing provisions in the modelbuilding codes. This research initiative occurred at a timeleading up to unification of the three major national modelbuilding code organizations as the International CodeCouncil (ICC) and the drafting of the IRC (2000 edition).Thus, work began immediately to extend the PSW designmethod to applications without requiring hold-down anchors at the ends of braced wall lines. The goal of this effortwas to morph the design method into one which could beused to design the lateral resistance of a truly conventionalwoodframe building. As a result, testing of three-dimensional wall assemblies with corners restraining the ends ofPSW-braced wall lines was conducted. Additionally, testswere run comparing the difference in performance of PSWbraced wall lines with: variations of base connections (e.g., nails vs. bolts), various sizes of openings and widths of bracing segments (e.g., 4:1 segments), and incremental enhancements to framing (e.g., truss plateor strap reinforcements at “weak links”).In addition, a whole building test was conducted(Paevere 2002) to verify performance and investigate accurate means of distributing horizontal forces to conventionalbraced wall lines through a conventional (unblocked) diaphragm without a clearly defined chord member along theeaves (e.g., no fascia board or perimeter nailing of the roofdiaphragm). Most of these studies were reported in an earlierarticle (Crandell and Kochkin 2003). Many of these studiesoccurred after derivation of the IRC bracing amounts in TableR602.10.1, but in general they have served to confirm whatwas done rather than identify glaring deficiencies (although this should not be taken to mean there are no deficiencies or opportunities for improvement).Drafting of IRC Wall Bracing ProvisionsIn the late 1990s, a drafting committee was assembled byICC to prepare a draft of the IRC. This committee includedmembers of the building community, code official community, and general interests. Early in this process, NAHBsponsored development of a revised set of bracing provisions based on some of the earliest results of the researchmentioned previously.At one point, a set of provisions had been developed andwere tentatively approved by the committee. These provisions included seismic bracing amounts (similar to currentTable R602.10.1 in the IRC) plus a separate table addressingwind bracing.In addition, these provisions introduced the concept ofcontinuous structural sheathing (current section R602.10.5of the IRC) and a number of other coordinated features toenable efficient application of the provisions to conditionsthat were considered problematic in modern conventionalconstruction. Furthermore, this early draft increased bracWOOD DESIGN FOCUS
ing amounts in high seismic areas relative to past practice(also allowing only fully sheathed or continuous-sheathedwall systems in high seismic regions) while maintainingprevious bracing methods and amounts used for manyyears in lower seismic hazard regions.The wind bracing requirements introduced significantimprovements, addressing perhaps the major deficiency inpast conventional bracing practices relative to their application to modern homes. These changes required significantconcessions by interests represented on the IRC draftingcommittee. While imperfect, they represented a notable improvement to wall bracing requirements for conventionalconstruction as a whole.Later in the IRC drafting process and as the process continued to attract broader public input, concerns were effectively voiced that the draft bracing provisions did notstrictly conform to recommended seismic provisions of theNational Earthquake Hazard Reduction Program (NEHRP).In the end, the conventional bracing provisions for the IRCwere redrafted to conform more closely to the NEHRP provisions and only a few features from earlier draft bracingprovisions were retained. Features retained from the earlierdraft included seismic bracing amounts (now TableR602.10.1) and the continuous structural sheathing approach (now Section R602.10.5).Unfortunately, the wind bracing table in the earlier draftof IRC bracing provisions was removed from considerationand wind speed limits were more or less arbitrarily added tothe seismic bracing table (Table R602.10.1). In addition, allconventional bracing methods (except Method 1 let-inbracing) were recognized as in the past for use in high seismic regions. While these IRC drafting decisions may havebeen considered advantageous for seismic performance insome regards and questionable in others (Crandell andKochkin 2003), they clearly resulted in wind bracing requirements in the IRC that are dubious and potentially unsafe, especially for large multi-story homes.2Derivation of theContinuous Structural Sheathing MethodThe continuous structural sheathing bracing method (or“R602.10.5 Method”) was conceived and developed, not toincrease the strength of homes, but to provide equivalentbracing performance with less bracing. Thus, it was intended to address a problem in modern housing construction where increased use of windows and doors of largersizes created difficulties in providing adequate space for2Work is currently underway within the ICC Ad Hoc Committee onWall Bracing to address the concern with wind bracing amounts inthe IRC (www.iccsafe.org/cs/cc/ahc-wb/index.html).3 This original intention has been restored and clarified in the recentICC code development cycle for the 2007 IRC Supplement and thefuture IRC 2009 edition. Similar action has already been taken in anumber of states by appropriately amending Section R602.10.5 ofthe 2003 or 2006 IRC.Summer 2007traditional bracing methods (e.g., let-in braces or 4-ft. bracepanels). It was also intended to be used with other codecompliant bracing methods on other braced wall lines provided that:1. any wall line using the R602.10.5 method had a minimum 2-ft. sheathed corner return at each end per IRCFigure R602.10.5 and2. other braced wall lines in the same building werecompliant with provisions applicable to the bracingmethod used.3The continuous sheathed bracing method in the IRC isbased on the PSW design method with some notable differences relative to its current codified form. These are: PSW Shear Reduction Factor (F) – This factor is the foundation of the PSW design method (in its empirical form),and it accounts for the reduction in shear capacity of aperforated shear wall relative to the same wall withoutthe presence of openings (solidly sheathed throughout).The empirical form of the shear reduction factor used todevelop Co factors in the Wind and Seismic standard is F r/(3 – 2r) where “r” is a parameter based on geometricproperties of a wall line that relate to bracing strengthand stiffness (e.g., r 1 represents a shear wall withoutperforations whereas r 0.2 represents a shear wallwith a substantial number of openings and little structurally sheathed surface area).However, various tests of perforated shear walls reported in Crandell and Kochkin (2003) demonstratethat a more accurate (less conservative) prediction isachieved on average by the form F r/(2 – r). This formwas used in developing the IRC bracing amounts for intermittently spaced wood structural panels in accordance with Method 3 (wood structural panel sheathing,per IRC Section R602.10.3) and continuous wood structural panel sheathing (IRC Section R602.10.5).The only analytical difference between these two bracing methods is in the treatment of opening sizes. ForMethod 3, wall areas between intermittent brace panelsare assumed to be openings that occupy 100 percent ofthe wall height. These wall portions between braceswere conservatively assumed to provide no shear resistance (i.e., act like a bare frame without gypsum finishesor other components that are known to contribute to thelateral resistance of light-frame construction).For continuous structural sheathing, the size of openingsmay vary from typical window clear opening height(e.g., 67% of the wall height) to that for a door (e.g.,85% of the wall height) and sheathing is applied continuously between and around wall openings. These assumptions permitted the use of the PSW design methodto back-calculate a length of bracing required for wallswith maximum opening heights corresponding to theseconditions. Thus, the bracing adjustment factors of 0.9and 0.8 in Section R602.10.5 reflect the effect of limited5
opening heights on the performance of a perforatedshear wall relative to a perforated shear wall with opening heights of 100% (i.e., bracing Method 3 in the IRC). Overturning Restraint at the Ends of a PSW – In the Windand Seismic requirements for PSWs, a hold-downbracket is required for full restraint at each end of thewall line. However, testing (Crandell and Kochkin 2003)clearly demonstrated that substantial capacity of a perforated shear wall could be maintained by use of cornersto “partially restrain” the ends of the wall line. To account for the effect of using corners as overturning restraints, the unit shear value for Method 3 andR602.10.5 bracing in the IRC was based on tests of a 12ft.-long wall with a minimum 2-ft. corner return (Dolanand Heine 1997). This resulted in a unit shear capacityof 634 plf being used as the basis for evaluating Method3 and R602.10.5 bracing in the IRC with the PSW designmethod.4 In more recent studies, better methods of analyzing the effect of “partial” end restraint on the shear capacity of perforated and segmented shear walls havebeen developed and are summarized elsewhere (HUD2001b). Uplift Restraint along the Bottom Plate of a PSW – In theWind and Seismic version of the PSW design method, thebottom plate of the wall must be anchored against a uniform vertical uplift force equivalent to the maximumhorizontal unit shear force in the wall line. But, the previously mentioned testing included conventional bottom plate connections (e.g., two 16d pneumatic nails at24 in. oc and anchor bolts at 6 in. oc). These connectionsare not consistent with the Wind and Seismic uplift anchorage requirement, yet the tested PSWs still performed as predicted with uplift restraint equivalent toroughly 50 percent of the unit shear strength of the wallsystem. Because the Wind and Seismic provisions areconservative and came after the IRC bracing provisions,the IRC relies on the uplift restraint provided by conventional bottom plate connections. Aspect Ratio of PSW Segments – In the Wind and Seismicversion of the PSW design method, the aspect ratio ofPSW segments is limited to 2:1 (height:length). Agreater aspect ratio is permitted (up to 3-1/2:1) with areduction in the nominal unit shear capacity used to determine PSW capacity. This reduction applies even ifonly one segment has an aspect ratio greater than 2:1when it is included in the determination of total length46These tests included 1/2-in. gypsum wall board on the interior fastened at 7 in./10 in. oc (edge/field) using 5d cooler nails and7/16-in.-thick oriented strandboard on the exterior fastened with8d common nails at 6 in./12 in. oc with Spruce-Pine-Fir framing at16 in. oc. The tests also used the sequential phased displacement cyclic test protocol which resulted in a limited contribution from gypsum panels at peak load. Response of the corner-restrained wallswas noted as being very ductile.of shear wall segments in a PSW. In the IRC, PSW segments are permitted to be as narrow as 4:1 (with limitson the height of openings adjacent to the PSW segment)with no penalty. This difference is based on testing(Crandell and Kochkin 2003).Derivation of the Minimum Bracing Amounts(IRC Table R602.10.1)The following summarizes key features of the analysisused to derive the seismic bracing amounts in IRC TableR602.10.1. These analysis features are employed in example calculations in the next section. As mentioned previously, the IRC bracing amounts in Table R602.10.1 were notoriginally intended to address bracing amounts for lateralwind forces (a separate wind bracing table was originallyproposed). Allowable Stress Design (ASD) Safety Factor 2.0Note: This safety factor is taken to be representative of traditional design practice and is applied to the average ultimate (nominal) tested shear wall strength. Deflection Limit unspecified (see note below)Note: Deflection limits were not specifically evaluated. Appropriate deflection criteria, particularly when applied tosmall deflections, are very sensitive to variations in boundary conditions represented in any particular index test aswell as wall segment aspect ratio and actual end-useboundary conditions imposed by a real building system.However, in comparison to test data serving as the basis forthe continuous sheathed method, an approximate 0.5 to0.7 percent drift limit at ASD load (50% of ultimate load)may be considered as representative. This range is reasonably consistent with historic deflection criteria for lightframe wall bracing which permitted up to 0.65 percentdrift at 50 percent of ultimate capacity (FHA 1949) or asmuch as 1 perce
The Story Behind IRC Wall Bracing Provisions Jay H. Crandell, P.E. Introduction The conventional woodframe construction provisions of the 2006 International Residential Code (IRC) (ICC 2006a) may be viewed as a “melting pot” of engineering science
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