Truss Metal Plate Failure
Truss Metal Plate Failure
(OP)
This past winter Charlotte had a whopper snow storm with almost 6 whole inches of accumulation. I am now looking at my second roof truss failure as a result. Both situations are identical. Churches with scissor trusses spanning 46 feet, both built in the mid-90s. The truss company who did both is out of business. The failure was at the peak metal plate where the vertical chord gave way from the top chords. Failure mode was teeth pull out. Both failures occurred over the winter starting with one truss quickly followed be several others. Walls kicked out a few inches on each side and trusses dropped at least 6" before shoring was installed.
I modeled the truss on RISA and determined the vertical force under design load is 3500 lbs. and under the actual snow load conditions that caused it to fail about 2000 lbs. I did tons of internet research found the ICC report regarding the truss mending plate allowable design capacity. The allowable teeth stress should be about 192psi in SYP#2. I measured the contact area of about 17.5 sq. in which yields about 3360 lb capacity. I figured close enough since I didn't figure any C factors.
As best as I can tell the truss plate connection met the requirements of both the current ASCI/TPI-2007 and the applicable TPI-85 specifications. Upon further reading some engineers have questioned the TPI testing procedures saying they don't account for localized bending stresses and humidity/ temperature stresses that occur in real world conditions. Yet the entire industry follows them. I'm going back to investigate the first truss that failed a little more closely and see if there was some fabrication error but so far as best as I can tell, the actual capacity of the truss plates did not meet the tested/ design values.
Anyone been through this and can help?
I modeled the truss on RISA and determined the vertical force under design load is 3500 lbs. and under the actual snow load conditions that caused it to fail about 2000 lbs. I did tons of internet research found the ICC report regarding the truss mending plate allowable design capacity. The allowable teeth stress should be about 192psi in SYP#2. I measured the contact area of about 17.5 sq. in which yields about 3360 lb capacity. I figured close enough since I didn't figure any C factors.
As best as I can tell the truss plate connection met the requirements of both the current ASCI/TPI-2007 and the applicable TPI-85 specifications. Upon further reading some engineers have questioned the TPI testing procedures saying they don't account for localized bending stresses and humidity/ temperature stresses that occur in real world conditions. Yet the entire industry follows them. I'm going back to investigate the first truss that failed a little more closely and see if there was some fabrication error but so far as best as I can tell, the actual capacity of the truss plates did not meet the tested/ design values.
Anyone been through this and can help?





RE: Truss Metal Plate Failure
Sometimes you get unbalanced snow on one side of the roof that creates worse conditions than the design snow.
Also - snow loading is based on general estimates of density. Some wet snow can weigh a lot more than anticipated.
You probably don't have any recourse to find data on the actual snowfall density but perhaps some insight as to snow drifts on
windward/leeward sides of the roof might help.
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RE: Truss Metal Plate Failure
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RE: Truss Metal Plate Failure
As far as JAE's comment, the roof couldn't take that live load it was designed for. So pinpointing the actual load can be important. Are there other roofs with the same design you could look at? Maybe they are giving early signs of failure that you could observe.
RE: Truss Metal Plate Failure
www.idecharlotte.com
RE: Truss Metal Plate Failure
In any case, as this appears to be a wood failure I would not look at any problems of the plate values. But rather at possible wood or manufacturing problems.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Truss Metal Plate Failure
RE: Truss Metal Plate Failure
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