What Can Happen to Unbraced Wood Truss Web Members?

Posted by in Construction & Shortcuts, Structural terms, Wood | September 21, 2015

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 BWT-1 for the two lateral braces provided at the bottom of the trusses we discuss today.

Figure BWT-1 Two lateral braces on bottom chord of trusses.

Figure BWT-1 Two lateral braces on bottom chord of trusses.

Obviously, this is not the complete solution.  The lateral bracing also requires a few diagonal members that prevent all the web members from buckling sideways together.

In the trusses in Figure BWT-2 and 3 there is no lateral bracing for the relatively long diagonal web members that are in compression – and so, they buckled.

Figure BWT-2 Diagonal web members buckled.

Figure BWT-2 Diagonal web members buckled.

Figure BWT-3 Diagonal web members buckled.

Figure BWT-3 Diagonal web members buckled.

As the old maxim says, “For the want of lateral bracing the roof framing was lost,” or something like that.

Considering the length of the unbraced web members, it is amazing that the service life of the trusses was not cut short by buckling and fracturing under the previous weights of ice and snow that were probably greater than the one they buckled under.  However, I suspect that these relatively long and flexible web members exhibited buckling under past snow loads and the repeated buckling loosened the metal end plate connections, effectively reducing the stiffness of the web members, and thus, reducing their capacity to resist buckling.  Eventually, the buckling resistance of the weakened web members became less than that required for the weight of ice and snow on the day of the failure.

“And now you know the rest of the story.”

Disclaimer