AMERICAN FOREST & PAPER ASSOCIATION American Wood Council Engineered and Traditional Wood Products

Let’s discuss IRC’s wall requirements and talk at length about bracing. Although the program will talk in some depth about the IRC’s prescriptive requirements, it will also address general conditions which necessitate bracing.

Copyright © 2004 American Forest & Paper Association. All rights reserved.

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This eCourse explains different construction types and the behavior of small structures and structural elements under gravity, seismic and wind forces. Principles and typology of lateral load resistance systems are discussed including prescriptive braced wall lines as addressed by the IBC/IRC. An introduction to engineered shear wall design, location, and inspection points is offered. Throughout, this eCourse demonstrates how AF&PA’s new Wood Frame Construction Manual addresses these topics for one- and two-family dwellings. dwellings

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This eCourse seeks to provide some answers, along with background on the reasons why. Listed here are the topics that we’ll explore in this eCourse, beginning with fundamentals.

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Why does the code require bracing?

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Loads on the framing tend to make the framing move. Even vertical loads such as snow loads can make the framing try to rack.

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Most loads on a building g are applied pp by y vertical p pressure or gravity. Roof framing bears on wall framing which bears on the foundation.

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Dead loads is another term used often in the code and is typically used to indicated the weight of the building materials themselves. However, it can have wider meaning with static loads.

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Live loads – another common term – are loads resulting from the use of the building. This includes people in the building, furniture, and other non-fixed features.

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It snows in various regions of the country, and 30 to 70 psf ground snow load provisions are included in the 2001 WFCM (more about that document later). Snow load span tables automatically reflect the consideration of unbalanced snow loads.

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Lateral loads will also be applied pp to a structure. The most common is wind. Seismic forces will also occur in many parts of the country.

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The Federal Emergence g Management g Association ((FEMA)) developed these charts for wind and seismic zones in the United States (although they’re similar to ASCE 7 maps they aren’t). Notice on this wind chart that while the coastal areas around the Gulf of Mexico and Atlantic Ocean are in a high wind zone, so is most of the central part of the country. This apparently is a reflection of severe thunderstorms in the middle of the country which can generate high straight line winds and tornados.

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Wind loads can be resisted by y design. g Although g most tornados can’t be resisted by an economically feasible design, proper bracing can lessen damage to structures that see a near-miss from a tornado.

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The Fujita j scale is used to rate the intensity y of a tornado by y examining the damage caused by the tornado after it has passed over man-made structures. F-0 has light damage while F-5 has incredible damage. Over 80% of all tornados are classified at F-3 or below. The reason for this slide is to illustrate that different tornados pack different punches. The majority of tornados are not the incredible ones. A word of caution, however, these lower strength tornados can cause horrific damage. We don’t intend to imply otherwise.

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This g graphic p shows the impact p of the tornado. Homes in the direct path were completely destroyed while those a block or two away received a variety of damage. The home about three blocks away received minimal damage. No one will argue that it doesn’t matter how we build a structure that is going to receive a direct hit from an F5 tornado. It’s the b ildi buildings on th the periphery i h off such h an eventt and db buildings ildi involved in less severe storms that will perform differently depending on the construction.

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This g graphic p shows the impact p of the tornado. Homes in the direct path were completely destroyed while those a block or two away received a variety of damage. The home about three blocks away received minimal damage. No one will argue that it doesn’t matter how we build a structure that is going to receive a direct hit from an F5 tornado. It’s the b ildi buildings on th the periphery i h off such h an eventt and db buildings ildi involved in less severe storms that will perform differently depending on the construction.

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One other feature of high wind events is wind-borne debris that driven by the wind can cause moderate to severe impact damage. During the La Plata, MD, Tornado, blankets and sleeping bags were found tossed into trees and power poles. In this example, a 3”x6”x 8’ long lumber section skewered the second story exterior wall of a building, penetrating it by several feet.

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Here is a p photo of the p path of an F5 tornado that went through g Oklahoma in 1999. Many homes close to the tornado path survived this tornado. Was this by chance or luck, or did the method of construction have something to do with it?

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Proper design of wood structures to resist high wind loads requires the correct use of wind load provisions and member design properties. A thorough understanding of the interaction between wind loads and material properties is important in the design process.

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Wind-structure interaction is highly complex. Wind can induce a variety of structural responses as a whole building, and on individual components and assemblies, as seen here. Each of these responses needs to be checked for structural integrity as part of the wind design process.

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When wind is applied pp to one side of a structure,, it wants to p push the end wall and roof in the prevailing direction. In addition, the wind wants to pull the opposite end wall. While this is occurring, the foundation acts to hold back the walls. Hence, the wall is subjected to most of the force.

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Seismic loads arise from ground shaking which can be a result of many causes. Earthquakes by far are the most serious and unpredictable seismic load type. Other more predictable seismic loads arise from human-induced activities.

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The primary feature of seismic motion is that its magnitude varies with time. The time variation can be very short, such as a sharp jolt, or longer, such as a slow rumble. Moreover, the motion direction is typically random and constantly changing. Such a behavior can be described in terms of a wave. Using recorded seismic data, we can describe a seismic load mathematically in terms of a wave function of distance and time.

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Waves have three primary characteristics: •amplitude (the magnitude of the wave), •frequency (the number of complete wave cycles per second) or inversely its period (the number of seconds per complete wave cycle), and •duration (the time lapse of the wave).

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Damping describes the decay rate of the wave amplitude as the wave “dies” out. Friction in the wave generating system is an example that causes waves to damp.

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With respect to dynamic response, buildings offer three primary characteristics: •mass of the building or sub-assemblies, •stiffness of the building structural system, •and damping inherent in the building construction. These can be simply modeled as the “lollypop” shown here. If the stick of the lollypop is sufficiently thin (low stiffness) stiffness), and the mass is pulled back and released, the mass will swing back and forth in free motion. The free sway motion can be described by the mass displacement wave shown here, with measurable frequency. This simple sway mode is known as the natural frequency of vibration. Mathematically, the sway motion equation takes the form of a second order differential equation with respect to time.

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In the motion equation, all the components of the dynamic structural behavior are evident. The equation here is written in terms of linear displacement, x, although angular displacement terms (not shown) and other directional displacements may be present. Solution techniques for this equation exist mathematically.

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Solution of the motion equation can lead to a modal result: a series of frequencies at which the structure will freely vibrate if disturbed. An example of this solution can be heard when a guitarist uses fret harmonics (octave pitches) to tune a guitar. In a building, the sway shape takes different forms that correspond to the modal frequencies in the solution. Thus, a structure can have a number of sway modes with associated frequencies of vibration.

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Let’s put the whole seismic problem together now. Ground shaking occurs with a certain acceleration, a, moving the soil under the building. As the soil moves, the building mass wants to stay put due to its inertia, putting a force on the structure equal to the mass times the exciting acceleration. Eventually the mass moves, lagging the exciting acceleration, causing further inertial forces to develop on the structure. This gets even more problematic when the exciting acceleration changes direction, as the mass wants to keep moving (through inertia) in the the original direction of mass movement. This is sometimes referred to as the whipping force.

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If the exciting wave characteristics match any of the building’s modal wave characteristics, then resonance results when the exciting and response systems vibrate in unison. Resonance is very dangerous since the response system normally self-destructs due to its inability to cope materially with the exciting wave. Hence, it is very desirable from a building design perspective to separate building modal response frequencies from any potential exciting frequencies.

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WFCM Chapter 2 Engineering provisions are based on the ASCE 7-98 Equivalent Lateral Force procedure. Building masses/weights are calculated and collected at floor plane levels. The seismic event base shear is calculated from the seismic loads and distributed on the basis of weights at each story as story forces. Finally, story shears are determined for each floor by adding all the story forces above the floor of interest. The story shears are the forces applied to the top of the lateral force resisting system at each floor level level.

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Buildings move under dynamic conditions. Two principle movements are: racking and twist.

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Structures have horizontal surfaces that can be used to transfer loads applied laterally to the structure. An inertial mass load can originate in the surface and transfer the same way. Resulting shear forces develop across the surface, with maximum values occurring at the supported edges of the surface. These maximum “reaction” forces are the lateral forces that are transferred into the vertical building elements below.

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Building forms impact how lateral forces get transferred into the vertical supporting elements. Here’s an example. A floor has a center of mass located somewhere in it. The structural system below provides a torsional stiffness that can also be centered somewhere within the floor plane. If the stiffness and mass centers coincide, then the building will simply rack in the direction of the applied lateral load. If however, the mass and stiffness centers are displaced, the building frame will twist. The greater the displacement the greater the twist displacement, twist. The diaphragm reactions transferred to the top of the shearwalls can also become very large. Thus, good design for lateral performance would suggest that centers of mass and stiffness be kept in as close proximity to each other as possible. This subject is important for a rigid analysis where the stiffnesses of the system components are known known. The WFCM 2001 assumes a flexible analysis: flexible components that lend to a tributary area approach for the loads.

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There is another way - a technique that dates back to early human inhabitation some 10,000 years ago. Archaeological findings prove the theory of light and strong in known seismically active areas of the earth. The theory holds that humans discovered early that heavy things fall down easily when disturbed with catastrophic results. Light things are not disturbed nearly as easily, and are much easier to support and be made strong. Thus, the simple tent has become a common domestic structure to many peoples of the earth in regions that are seismically active active, even to this day day. Wood frame structures tend to fit this philosophy, mainly because of wood’s very high strength-to-weight ratio.

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Now let’s look at some strategies used in buildings to resist lateral forces, primarily through wall framing methods.

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Here’s a summary of what we’ve discussed so far.

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Without being braced in some fashion, wood frame walls tend to rack in response to loads.

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Bracing – whether it’s to resist the day-to-day loads on the buildings and the typical storms or whether it’s to resist high-wind or high-seismic events – is critical to the performance of the building.

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Here's an example of what can happen when the lower floor of a building doesn't have adequate bracing. This was a building that had 9 condo units, some of them 3-stories tall, which collapsed in 40 - 50 mph wind. A structural collapse while under construction isn't unusual because not all of the required bracing may be in place when a storm strikes. But as you can see in this case, the exterior walls were being bricked and framing was almost complete. If you look at the unit on the left, you will see that the lower floor had mostly doors and windows and very little braced wall area, indicating poor design.

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There are fundamentally three ways to stop a frame from racking. We’ll talk in some detail about the application of these methodologies, but all of the bracing materials and systems involve some version of what you see here.

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Let’s talk in some detail about the methods of providing bracing, starting with the use of triangles. Diagonal tension ties create a triangular geometry within the frame that in itself, is a stiffening element. Compression ties are rarely effective, if at all. Diagonal board sheathing, however, works in this mode.

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This is one of the simplest ways of providing lateral resistance to a wall assembly. However, let-in braces require a perfect and well connected fit in order to work properly, which is often difficult to achieve. And, they cannot provide the same capacity as a properly constructed wood panel shear wall.

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Proper installation and connections are key to making this method work.

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A very efficient way to brace walls, and one that was common years ago, is to sheath the wall in diagonally oriented boards.

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Triangular geometry is used profusely in the truss industry for the trusses themselves, as well as the bracing of them in the context of an entire structural roof or floor system. For peaked roofs, trusses are braced in three planes.

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This is a photo of a demonstration assembly that shows the different kinds of bracing. The ground bracing is in dark blue. Notice how the ground bracing is braced with triangles. Also notice how the lateral bracing on the top chords (in orange) line up with the ground bracing making the truss more stable. Photo showing continuous lateral bracing (CLB) and the diagonal bracing needed to restrain the CLB. Diagonals brace the bracing. It shows rows of lateral bracing being tied with di diagonal l bracing. b i Those Th diagonals di l should h ld not be b more than h 20 to 30 feet f apart.

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Temporary bracing is one of the most critical issues facing the wood truss industry when it comes to construction safety. This Truss Technology in Building shows how critical diagonal bracing is when using short spacer pieces for lateral bracing as is typical on construction sites today. The WTCA Warning Poster completes the educational information that is needed at the jobsite to install and brace trusses safely. For more information on bracing contact WTCA or visit the WTCA website at www.woodtruss.com.

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For long term roof system performance the proper installation of permanent bracing is a necessity. This TTB defines the rule of thumb approach for trusses. Where possible, however, the Building Designer should provide a permanent bracing plan. WTCA also has a “Commentary For Permanent Bracing of Metal Plate Connected Wood Trusses” that goes into more detail. For more information contact WTCA or visit our web site at www.woodtruss.com

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Shearwalls are a vertical building element that can resist lateral forces applied at the top of the wall. In a wood shearwall, the panel perimeter nails provide the bulk of the racking resistance through wood bearing and nail deformation when the lateral external force is applied. Horizontal wall sliding is resisted by nailing or other anchorage installed along the bottom of the shearwall sufficient to resist the external lateral force.

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Secondly, the perimeter-nailed panel resists racking through the resisting action of the perimeter nails to the applied racking moment on the panel. Here the nails do most of the work.

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In order to make this concept work, panels must have a height-to-width aspect ratio of less than 3.5 to 1. This ratio is sufficient to develop “racking action” in the shearwall panel. Aspect ratio’s greater than this produce cantilever beam action - a completely different behavior that is much less effective in resisting lateral forces. The concept of aspect ratios is incorporated into prescriptive bracing requirements, but isn’t specifically addressed. It is, however, that basis for limits to minimum widths of various bracing material that we’ll we ll discuss shortly. shortly

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Although they aren’t part of the wall bracing systems, roofs and floors take the loads from the walls and transfer them to the foundation. Aspect ratios for roof and floors aren’t addressed prescriptively, but in engineered design these elements, called diaphragms, have length-to-width aspect ratio limits as wall as limits on openings.

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In its prescriptive provisions the IRC refers to braced walls as braced wall lines. In engineered design, however, the bracing is provided by shearwalls. Shearwalls feature special nailing and hold-down connections designed to resist applied lateral loads in shear and overturning. Minimum wall aspect ratios apply in order to develop “shearwall action” as opposed to “cantilever beam action” when the wall panel aspect ratios become very slim. Typically, the closer to the minimum aspect ratio for a shearwall, the more dense the nail perimeter nail spacing spacing. In shearwalls shearwalls, it is the perimeter nailing that is the most effective in resolving the transferred applied forces. A more convenient method is the use of shearwall systems: panels, or entire walls. Shearwalls feature special nailing and hold-down connections designed to resist applied lateral loads in shear and overturning. Minimum wall aspect ratios apply in order to develop “shearwall shearwall action” action as opposed to “cantilever beam action” when the wall panel aspect ratios become very slim. Typically, the closer to the minimum aspect ratio for a shearwall, the more dense the nail perimeter nail spacing. In shearwalls, it is the perimeter nailing that is the most effective in resolving the transferred applied forces.

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Diaphragms are usually horizontal surfaces that resist in-plane shear forces. Nailing is more dense where the shears are highest – typically in the panels around the diaphragm perimeter.

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In the second general bracing methodology, the perimeter-nailed panel resists racking through the resisting action of the perimeter nails to the applied racking moment on the panel. Nailed horizontal boards with at least 2 nails on the same stud has the same effect, but to a lesser degree. Here the nails do most of the work.

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A third method absorbs the racking moment directly in the rigid joints in the corners. This is called a moment frame since the rigid corners induce bending moments in all the members near the rigid connections, so indirectly, the frame members also resist the racking forces through flexure.

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Proprietary systems such as APA Sturd-I-Frame try to utilize normal construction t ti – a couple l off anchor h b bolts lt and d iintegrating t ti th the h header d and d sheathing with nails. Notice the attention given to making the corners rigid. Construction of these types of assemblies requires careful attention to details.

Proprietary systems such as APA Sturd-I-Frame try to utilize normal construction – a couple of anchor bolts and integrating the header and sheathing with nails to p the rigid g frame jjoint. develop Note a few things that are important to this detail.. If the overall wall height is more than 8 feet (say you build a cripple wall on top of the header), the panel joint should h ld ffallll within ithi 2 ffeett off th the mid-height id h i ht off th the b braced d portion of the wall. 59

Remember this house? Notice the failure of the garage door header at the corners. In dwelling construction, this type of wall design – an opening with small braced wall sections on either side – is ideal for the application of a moment frame.

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Braced wall lines are employed on the building-level scale to develop lateral resistance in two orthogonal directions for the structure.

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Now let’s look at the prescriptive method in which the IRC addresses wall bracing. The code approaches the subject by specifying where the bracing is to be placed, what materials are acceptable, and what quantities of bracing materials are needed.

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Braced wall lines are walls made up of a series of unbraced sections and sections of walls that are braced with acceptable materials in the required amount. These braced sections are called braced wall panels. The braced wall lines are required to be placed in both directions of the floor plan and are required in each story. Often the exterior walls will provide the required braced wall lines. However, when the distance between exterior walls is too large, an interior wall is required to be a braced wall line. The IBC is specific about the maximum distance between braced wall lines being 35 feet. However, for some reason the 2000 IRC is vague. The only mention on spacing is in Section R602.10.11, which addresses spacing g in high g seismic areas. In the ’03 edition, the IBC’s 35 feet provision has been added. Braced wall lines are permitted to have an out-of-plane offset of no more than 4 feet in one direction, with a provision saying that total offset can be no more than 8 feet. This would allow a wall to have a 4 foot offset in each direction.

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The concept in the IRC is to break the structure of the building into boxes with a limited aspect ratio.

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This is a very rough schematic illustrating the general code provisions for the braced wall li concept. line t All exterior t i walls ll mustt b be b braced d wallll lilines. IInterior t i b braced d wallll lilines are required if the spacing between exterior walls is excessive (greater than 35’ in the 2000 IBC and in the 2003 IRC). Individual portions of wall that contain bracing material are called braced wall panels. An offset in the overall braced wall line of 4’ is p permitted. As illustrated in the lower left portion of the floor plan, the code will allow an offset of up to 4’ in each direction, meaning that the bottom exterior wall could have been offset as shown by the dotted line. Openings are permitted at the corners of the braced wall line as shown on the exterior wall on the left. There is some confusion in the code, however, on this account. Section R 602.10.1 permits the first braced wall panel to be 12-1/2 ft from the corner. Then it says that when the bracing begins more than 12’ from the end of the wall a designed collector is required. i d IIt’s ’ unclear l whether h h this hi was iintended d d to require i a collector ll off some sort ((what’s h ’ termed a drag strut in seismic design) if the braced panel is placed in that permitted 6” beyond 12’-0”, or whether the intent is for a collector to be required in all instances in which the first braced panel isn’t at the end of the wall.

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The code lists 8 general materials that are acceptable as bracing, and we'll talk more about those in a moment. Not all of those materials, however, are acceptable as bracing materials in all instances. The seismic category or wind speed zone, as well as which story is being braced, will determine whether a specific material can be used and how much of the wall must be braced using that material.

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Here are the 8 general bracing materials. The numbers that you see here are the numbers that you'll see on a table that we'll talk about in a moment. We’ll also talk in some detail about the various materials. These are the materials that the code accepts outright. That's not to say that there aren't other materials that will provide adequate wall bracing. However, any other material must be addressed under the alternate methods and materials provisions of the code.

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Add sheathing N ili pictures Nailing i t

Attaching the bracing materials properly is critical; none of the materials will function properly if fasteners are improper (either too few or of the wrong size). It’s important that care be given to compliance with these requirements.

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This is a relatively formal way to say that we’re not sure exactly what resistance to lateral loads are being provided by prescriptive bracing. We know from experience that it works under the limitations of conventional construction. However, since the wall isn’t formally designed and it lacks elements of a shearwall, such as connections to the foundation or floor, it’s hard to quantify the resistance to any exact degree.

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In a formal shearwall design, we can quantify the shear resistance in bracing material; in fact, the code provides those numbers for everything but let-in bracing. But because the overall resistance to racking in conventional construction isn’t completely understood, we don’t know exactly what shear resistance is being provided by the bracing material itself. Here are some estimates of the shear strength of the 8 allowed bracing materials applied according to the IRC. The widely varying numbers explain why different materials must be provided in different amounts amounts.

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Now,, lets talk about the details of the various methods of bracing. g

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One of the bracing methods accepted in the code is 1x4 let-in bracing such as you see here.

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R602.10.3(1) calls for the let-in bracing to be no less than 45 degrees from the horizontal and no more than 60. It also calls for the 1x4 to be let-in to the top and bottom plates as well as the studs.

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But let-in bracing will be effective only if it's installed properly. Here you see an example of poor installation. The notches in the studs are so wide that the 1x4 isn't being held tightly. If the wall tries to move the wide notches are going to permit some racking before the 1x4 comes into contact with the edge of the notches. And it also appears that some of the nails may not be properly driven into the studs. Notice that the nail head in the yellow circle appears to fall right on the outside edge of the stud, meaning that probably only one of the nails is providing any resistance.

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As mentioned before, diagonal boards provide a very efficient bracing material, but that method has fallen out of favor because of the availability of panel products.

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Wood structural panels are very desirable bracing materials, particularly when used in lower floors of multi-story buildings or in buildings subject to high lateral loads. Additionally, the use of these panels to completely sheath a building will offset some other problems that we’ll talk about in a moment.

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Keep in mind that structural fiberboard and particleboard must be manufactured in accordance with standards referenced in the code. While they might look like the materials used in cheap, short lived furniture, compliance with those standards assures a long lasting structural product.

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Gypsum wallboard is acceptable as a bracing material, but because it’s brittle and easily crushed, it’s values are limited and longer lengths of it are required to provide racking resistance.

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Keep in mind that the prescriptively permitted stucco cited in code is traditional portland cement stucco and not EIFS or other stucco-looking material.

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Because it has a rather soft surface, it’s important to prevent overdriving of fasteners. If over driven, two problems are created: 1. If driven far enough, the amount of the nail shaft that bears on the wood fiber to resist shear is less, lowering the shear resistance value. 2. Broken surface permits moisture get into the panel around the fastener, creating decay decay.

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Table 602.10 in the IRC specifies the amount of bracing material that is required and where the bracing is to be applied. Here is the first of the table. Notice that it’s predicated on the SDC and/or wind speed for the location in question and the story to be braced. Then the acceptable types of bracing, identified by the numbers on the previous slide, are listed. The final column talks about the amount of bracing panels required and the location of the panels. The lower seismic zones – A, B & C -- require the least amount of bracing. Zones D1 and D2 require much more because of the intensity of seismic forces. Where wind speeds exceed 110 mph, engineered shear walls are required.

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The site of the building must be reviewed for applicable Seismic Design Category and Design Wind Speed. The lower seismic zones – A, B & C -- require the least amount of bracing. Zones D1 and D2 require much more because of the intensity of seismic forces. Where wind speeds exceed 110 mph, engineered shear walls are required.

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Here we see the acceptable types of bracing which are based on which story is being braced. Notice that all 8 bracing methods are acceptable in 2-story buildings and in the top 2 stories of a 3-story building. But the let-in 1x4 is not acceptable in the bottom story of a 3-story building.

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The table specifies the amount of bracing material that is required and where the bracing is to be applied. Notice that for lower stories the overall amount of bracing varies with the type of bracing material and with loads carried from floors and walls above. The higher floors require less bracing because they carry less load.

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The table contains a provision that is in conflict with Section R602.10. That section permits the first braced panel to start at a point 12 (or 12-1/2) ft from the corner. The table says that the bracing is required at the end of the wall. This appears to be a correlation error. The intent of the IRC drafting committee was to permit the bracing panel to be placed some distance from the end of the wall. This table was, in all likelihood, borrowed from some other source and this “at the corner” statement was erroneously retained.

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And there’s a more minor problem with the text of the table. As you see here, the code seems to require both the use of Method 3 and the other Methods. That’s obviously not the intent, but the use of “and” rather than “or” makes it somewhat confusing for people who tend to read literally.

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As previously stated, alternate bracing methods and materials may be accepted. However, it’s important that their acceptance be based on the knowledge that they will perform adequately when loaded.

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The point to make here is that the provisions of Table R602.10.3 and Section R602.20-4 -- which addresses minimum width of bracing panels -- is going to mean that often narrow wall sections won't be acceptable as braced panels in braced wall lines.

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The code stipulates a minimum length of braced wall panel for most of the materials. This means that the short sections of walls, even if they’re wood structural panels (Method 3), can’t be counted toward the required amount of bracing. This is an example of a built-in aspect ratio limitation similar to the 1:3.5 aspect ratio we talked about for engineered shearwalls.

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However, the IRC permits narrow all sections if the building is sheathed overall with wood structural panels.

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Here’s a summary of that provision. This will permit narrower braced wall panels, but keep in mind that there will still be some limitation on how narrow they can be.

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While we all know intuitively y that a fullyy sheathed house outperforms one with only minimal bracing units, we also know it by experience. The house fully-sheathed with plywood in the foreground performs much differently than this foam sheathed homes in this 2003 high-wind event. This type of performance difference was also confirmed by NAHB testing in 1998.

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Do the braced wall line provisions apply to garage walls? Yes. If the narrow walls on the sides of the garage opening can’t comply with the minimum widths required for the bracing material chosen – even if the building is fully sheathed – there are options permitted by the code. However, they are very restrictive.

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Here’s a summary of the alternate braced wall system provisions of the code that were intended to be used in the narrow sections of wall on either side of a garage opening.

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But it may be that there are proprietary methods of bracing that will work in the narrow walls by garage door openings.

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APA – The Engineered Wood Association recently had a code change proposal approved that will revise the alternate braced panel section of the code to permit wall construction similar to what you see here. This will permit a more user-friendly and less expensive alternative in lieu of what’s currently in the code.

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Although the prescriptive bracing requirements in the IRC address construction in some high-seismic areas, wind is limited to areas with a design wind speed of less than 110 mph. If the building is located in a higher wind area it must be designed according to ASCE 7, must comply with the provisions of SSTD 10, or it must comply with the engineering-base prescriptive provisions of AF&PA’s Wood Frame Construction Manual.

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Diaphragms and shear walls are engineered systems designed to resist lateral loads typically of higher magnitudes. These systems typically require involvement of a design professional. In this segment, we’ll review important features of these.

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Shear walls are engineered wall bracing that are designed to withstand high forces. Often these are used with design professional input.

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Here are other conditions that will trigger the use of an engineered solution such as shear walls.

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In the design of irregular structures, guidance for designing irregular structures as separate structures or inscribed is provided. Typically, the inscribed technique can only be used for wind design. Splitting into separate structures can be used for either wind or seismic design, and often is the preferred method.

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Now we come to Section 2305, Lateral Force Resisting System.

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This section applies to the ASD and the LRFD methodologies.

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Section 2305.1 contains these provisions.

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The use of diaphragms to withstand lateral forces is an integral part of both ASD and LRFD design methodologies. Section 2305.2 contains a deflection formula that can be used for diaphragms.

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Similarly, a deflection formula for shear walls, and other provisions for shear walls, are found in 2305.3.

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This is one of the helpful diagrams in the chapter, which helps to define the height and width of shear walls and shear wall segments in walls with openings. These definitions are used in the determination of the aspect ratios for the shear wall

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A shear wall (wall brace) is essentially a deep, thin cantilevered beam projecting from the foundation that is subjected to one or more lateral forces, such as those due to wind or seismic activity. Listed are the primary components of a complete shear wall design.

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The code provides two alternative methods for the design of shear walls. There is the traditional method, in which force transfer around wall openings is actually designed. The other alternative is the perforated shear wall method, which is the result of recent research about the shear capacity of shear walls where there is no specific provision for transfer of force around the openings

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The idea behind the perforated shear wall method is that even though overturning restraints are not provided at the solid panels between openings, there is still inherent overturning resistance which contributes to the capacity of the shear wall overall. The openings, of course, do cause a reduction in capacity. . . and this table gives the reduction factors based on the amount and size of openings.

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Reassessment of the inherent shear capacity of shear walls under wind loads led to a 40% increase in shear capacity of Wood Structural Panels (WSP) for wind design only.

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The shear capacity of a shearwall segment sheathed on both sides to resist wind loads only is additive.

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In proof, APA tested 3 walls: WSP only, gypsum only, and a combination of the two.

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The results showed the shear capacity of each wall using traditional shear capacities and procedure, compared to adjusted shear capacities and addition of interior sheathing. Walls tested showed 2-3 times greater capacity than the earlier methodologies allow for.

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Perforated shear walls have reduced shear capacity from the traditional segmented wall, but interior holddowns have been eliminated – a usually beneficial feature.

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Three sources were used in development and substantiation of perforated shear walls, including Sugiyama, APA-The Engineered Wood Association, Virginia Tech, and NAHB Research Center tests.

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From these tests, correlation is very good and conservative.

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Shear capacity is reduced using these effective shear capacity factors, adopted into the Standard Building Code. Critical features in this table are percent of full height sheathing, and maximum unrestrained opening height.

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To demonstrate the effects of each of these three design methodologies. First look at an example of the traditional shearwall using the unadjusted shear capacities. Note that a dozen holddowns are required along the length of the first floor.

The WFCM uses 60% of the dead load to resist wind uplift uplift. In most cases cases, the wind uplift is much greater than 60% of the dead load, and will require more than the dead load alone to offset the wind force component. For seismic loads, the holddowns are conservatively sized in the WFCM to meet the shear capacity of the shearwall.

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WFCM 1997 Supplement Table 3B is used to determine shearwall capacity. Assuming SPF framing with G=0.42, 8d nails, 15/32” Structural sheathing, and nailing of 6”/12” and 4”/12” along the panel edges.

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With increased shear capacities (40% increase) and summation of dissimilar materials (100 plf for gypsum) the required sheathing has been reduced, however there are still numerous interior holddowns required.

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By using the perforated shearwall method the amount of sheathing has been increased, however holddowns are only required at each end of the shearwall. The WFCM uses 60% of the dead load to resist wind uplift. In most cases, the wind uplift is much greater than 60% of the dead load, and will require more than the dead load alone to offset the wind force component. For seismic i i lloads, d th the h holddowns ldd are conservatively ti l sized i d iin th the WFCM tto meett the shear capacity of the shearwall.

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Using the perforated shearwall method, shear capacity is reduced using effective shear capacity factors. Assuming an 8’ wall height, window openings are H/2 or 4’ and door openings are 5H/6 or 6’8”. Interpolation is permitted based on percent full-height sheathing in the wall.

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Here are the wall assembly assumptions used for the development of the preceding methodology. A cooler nail is also known as a drywall nail.

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Finally, overturning may be a problem, especially for high aspect ratios. These skinny panels usually develop high overturning forces at the bottom corners of the walls that need to be resisted with the installation of special hold-down hardware.

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Back to the 2003 Missouri storms which fell into the F3 or less category, our damage assessment team, which was made up of members of the American Association of Wind Engineers, estimated that 50% of the failures initiated with poor lateral capacity of walls enclosing two-car garages, such as this one. These are the types of storms where how a home is built can make k a bi big diff difference.

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…so don’t forget the holdowns. You’ll find a handy table in 2001 WFCM Table 3.17F.

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Here’s a handy list for referencing design aids for anchorage against wall sliding and/or uplift. Roof/truss anchorage are also included in the Table 3.x list, among other assembly connections. Many of these tables will give a connector capacity, or a connector spacing as a result.

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Hold-downs are required to prevent a wall panel from overturning. Holddowns may also be used elsewhere to prevent uplift, and to tie the structure load path together to the foundation. Typical calculations are provided for hold-down connections in AF&PA’s LRFD Manual, Example 7.7-1.2.

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The conventional construction bracing g requirements q of the code are required to handle lateral loads as determined by the building’s site location. Handling lateral loads in California is a whole different ballgame, as can be seen from this residential construction detail. We have lateral loads and the code rightly stipulates that our structures need d tto h handle dl th them appropriately. i t l

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Floor diaphragm members must be properly secured at the diaphragm periphery to properly transfer shear forces. These figures show details at the foundation...

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…while these apply to intermediate stories.

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….and when the forces get large, the connecting hardware gets more interesting.

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Here is a glulam beam tension connection good for up to 75 kips.

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When we get to the larger structures or higher loads, naturally engineered construction will be referenced.

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…and here are the referenced standards that will apply.

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The IBC and IRC both reference engineered construction. The IRC references the WFCM, which is engineered construction. The IBC also references engineered construction, but much of the prescriptive provisions of the IBC are still based on conventional construction. Some of the tables in both the IBC and IRC have an engineering base. To clarify the scope of the two documents: IRC = detached and attached one- and two-family; IBC = multifamily, although some designers have used the lateral bracing provisions for business and office occupancies occupancies, as well well. The limiting factor is the 40 psf floor loading.

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To aid designers, the WFCM 2001 Workbook (free from www.awc.org) has been developed. It is a real design example using a real house in a real location and the workbook provides tabulated calculations with complete references to WFCM and blank worksheets for future designs of your own.

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