Progressive Roof Collapse Due to CLR Shifting: Summary

[The following is a summary of a more comprehensive article on the topic available for download under our Technical Resources tab.]

-By David R. Bohnhoff, PhD, PE –

Continuous lateral restraint (CLR) is a continuous line of bracing used to provide lateral support to a series of structural elements. By preventing lateral movement, CLR reduces the effective length of a member it braces, thereby increasing the amount of load it would take to buckle the member.

Within post-frame buildings, CLR is commonly used to laterally brace top and bottom truss chords, compression web members, interior and exterior posts and post-to-truss connections. Use of a single row of CLR to brace a series of compressive web members is shown in Figure 1.

Figure 1. Web member CLR with a diagonal brace for prevention of shifting

Figure 1. Web member CLR with a diagonal brace for prevention of shifting

Continuous lateral restraints are effective only if they do not allow movement of the component they are bracing. As soon as there is CLR movement parallel to the CLR (i.e., CLR shifting), the axial compressive strength of the members braced by the CLR is decreased.

CLR shifting generally occurs because (1) the CLR is not properly anchored, (2) the CLR itself is allowed to buckle and/or (3) a CLR connection fails. Of these three, the most common is failure to properly anchor the CLR.

CLR shifting is well known to be a leading cause of progressive roof collapse. Progressive roof collapse is a roof collapse in which the failure of a single structural component triggers a chain reaction of failures that result in a large portion of a roof collapsing onto the contents below. An example of how this occurs is illustrated in Figure 2, which shows five compression web members braced with two rows of continuous lateral restraints.

Bohnhoff_Figure_2a Bohnhoff_Figure_2b
Bohnhoff_Figure_2c Figure 2. Cut-away view showing (a) shifting of two rows of CLR connecting five compression webs (b) subsequent failure of a truss, (c) CLRs being pulled on by the falling truss and (d) resulting progressive collapse.
Figure 2. Cut-away view showing (a) shifting of two rows of CLR connecting five compression webs (b) subsequent failure of a truss, (c) CLRs being pulled on by the falling truss and (d) resulting progressive collapse.

As the magnitude of CLR shifting increases (Figure 2a), a compression web member or chord in one of the trusses will fail (Figure 2b), which significantly increases the load the two adjacent trusses must support (as indicated by the size of the arrows above the trusses in Figure 2b). As the failed truss is pulled downward by gravitational forces, it pulls on the CLRs. As shown in Figure 2c, this action will help straighten the web members on one side of the failed truss (right side in Figure 2c) and cause further curvature in the web members on the other side of the failed truss (left side in Figure 2c). Because of the combination of this action and the additional load the truss must sustain, the truss just to the left of the failed truss will also fail. This action repeats itself until all similarly compromised trusses to the left of the first collapsed truss have failed (Figure 2d).

Given the collapse scenario diagrammed in Figure 2, the first truss to fail would be the one adjacent to that portion of the roof still standing. In addition, the direction of CLR shifting is always toward that portion of the roof still standing. Figure 3, a photograph of such a collapse, verifies the direction of CLR shifting.

Figure 3. Web member CLR shifting is typically in the opposite direction of a progressive roof collapse. The amount of CLR shifting is fully understood when one notes that the compressive web and the top and bottom chords should lie in the same plane.

Figure 3. Web member CLR shifting is typically in the opposite direction of a progressive roof collapse. The amount of CLR shifting is fully understood when one notes that the compressive web and the top and bottom chords should lie in the same plane.

Proper CLR anchorage is generally accomplished by diagonal bracing of the CLR to a roof, wall or ceiling assembly in such a way as to make use of the in-plane stiffness of the roof, wall or ceiling assembly. In short, use a series of diagonal braces to connect the CLR to a roof, wall or ceiling assembly that is parallel to the CLR.

The most common failure in post-frame buildings during construction is the progressive collapse that occurs when the continuous lateral restraints for truss top chords (often permanently placed rows of purlins) simultaneously shift because of a lack of diagonal bracing (Figure 4). In this situation, the simultaneous shifting is almost always caused by wind forces acting on the trusses. In rare instances, the shifting has been caused by construction equipment accidently bumping up against or otherwise striking the roof framing.

Bohnhoff_Figure_8b Bohnhoff_Figure_8a
Figure 4. Plan view of roof framing illustrating how a lack of diagonal bracings results in simultaneous lateral shifting of CLR and truss top chords

The diagonal braces shown in Figure 4 are generally temporary—their function being replaced by nailed- or screwed-down roof sheathing. Where this is the case, it is important that the diagonal braces not be removed until a portion of the nailed- or screwed-down sheathing has been installed. This is especially true for larger buildings because the probability of failure without truss chord or rafter CLR bracing increases exponentially with an increase in the distance the trusses must clearspan, or span without immediate support (Bohnhoff, 2003). Practitioners who transition from residential to post-frame building construction are often not aware of this and thus ignore the need for diagonal bracing in the plane of the roof during construction. This results in job sites like those shown in Figure 5.

Bohnhoff_Figure_5a Bohnhoff_Figure_5b

 

Figure 5. Progressive roof collapses during construction are due to the lack of temporary diagonal bracing on top chord CLR. The image at right is of a conventional stick-frame building with 2-foot on-center trusses.

For additional information on preventing buckling with CLR, uses of CLR in post-frame buildings, CLR shifting and prevention, reducing progressive roof collapse and detailed references, see the main article at www.nfba.org/resources. In general, the best way to reduce the likelihood of a progressive roof collapse is to have a qualified, professionally registered or licensed engineer both design the building and inspect it during and after construction.

David R. Bohnhoff, PhD, PE, is professor of biological systems engineering at the University of Wisconsin–Madison. He can be reached at bohnhoff@wisc.edu.

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Source: Frame Building News, the official publication of the National Frame Building Association

 

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