What Can Happen to Unbraced Wood Truss Web Members? Part II

Posted by in Case Studies, Wood | January 27, 2020

So, what happens when the web members of a wood truss are not braced laterally as specified by the truss manufacturer?

When a wood truss is designed and constructed by a truss manufacturer, the manufacturer commonly provides drawings that show which truss members require lateral bracing to prevent sideways buckling of the relatively flexible truss members.  This type of buckling is similar to the buckling of a yardstick being pressed down against the floor.

Lateral bracing is especially required on relatively long compression web (vertical and diagonal) members.  Bracing is usually provided by a line of 2×4’s threaded through the truss and nailed to one of the narrow sides of the web member.  The lateral bracing is usually set near the mid-length of the web member.  The addition of the lateral bracing connects all the same web members together so that if one buckles sideways they must all buckle sideways.  See Figure 1 for the eventual result.

Figure 1 Truss collapse. Note relatively clear view
through trusses due to the lack of lateral bracing.
Figure 2 Truss collapse. Top chord buckled from lack of adequate lateral bracing. It is easy to see through trusses
because of the lack of that “dreaded bracing.”

Notice that looking up or through the trusses the view is unobstructed by that “dreaded lateral bracing.”  The lack of lateral bracing is usually pretty obvious and comprehended within minutes of entering the collapsed building.  However,…..

The issue is, “What caused the trusses to collapse or fall?”  It may be the lack of lateral bracing or it could be another cause.  That question often takes a longer to answer as it usually requires that the origin of the failure be identified so that the cause can be determined. 

Another issue in truss collapse is, “Has there been any history for the collapse or falling?”  Further examination of the trusses may identify previous attempts to fix a damaged member or address an on-going issue.  

In one case, we were able to determine that the trusses had started to buckle during construction, were re-plumbed one at a time, and then the inadequate bracing was re-positioned to await the eventual collapse.  Thus, the inadequate repair during the construction phase did not properly address the initial buckling , but merely delayed the eventual collapse.   

If you are involved in a wood truss collapse and want to know what happened, why did it happen, when did it happen, how did it happen, please contact us.  We can help answer those questions.

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