farm building code for wood trusses
farm building code for wood trusses
(OP)
Hi guys,
I am considering whether to repair or replace some trusses over a farm building (shop built in 1985) that were damaged by snow overload in central Alberta Canada. Under current codes the existing trusses are looking to be pretty severely overstressed. If I had to guess, I would say that either the trusses were designed for 2 ft on center but installed at 4 ft on center; or perhaps the snow load was greatly reduced under an old farm building provision such as slippery roof, heated building, low importance factor, blanket farm exemption, etc. I only have the 1995 NRCC Farm Building Code and I am wondering if there is some older code or exemptions that might explain why an otherwise well built and braced roof is so inadequate.
A related consideration is whether or not the trusses are allowed to be repaired back to original condition under some type of grandfathering or whether the loads and provisions of current codes apply. I'm pretty sure I know the answer to that but I would appreciate some feedback.
Thanks in advance.
I am considering whether to repair or replace some trusses over a farm building (shop built in 1985) that were damaged by snow overload in central Alberta Canada. Under current codes the existing trusses are looking to be pretty severely overstressed. If I had to guess, I would say that either the trusses were designed for 2 ft on center but installed at 4 ft on center; or perhaps the snow load was greatly reduced under an old farm building provision such as slippery roof, heated building, low importance factor, blanket farm exemption, etc. I only have the 1995 NRCC Farm Building Code and I am wondering if there is some older code or exemptions that might explain why an otherwise well built and braced roof is so inadequate.
A related consideration is whether or not the trusses are allowed to be repaired back to original condition under some type of grandfathering or whether the loads and provisions of current codes apply. I'm pretty sure I know the answer to that but I would appreciate some feedback.
Thanks in advance.






RE: farm building code for wood trusses
Structural design of low occupancy farm buildings was to be carried out in accordance with the NBC except as follows:
(a) Allowable stresses in working stress design were permitted to be increased by 25%.
(b) The importance factor on the effect of factored loads other than dead loads in limit states design was permitted to be 0.8.
For low occupancy buildings, the load sharing factor was applicable for trusses spaced at 1220mm o.c. This permitted an increase of 10% in allowable stresses.
Reduction in snow load for roofs with slippery cladding were permitted.
Prior to 1985, the ground snow load was lower in some parts of the province than shown in the 1985 Alberta Building Code. In 1971 for example, the ground snow load for Edmonton was only 27 psf as compared with the current 1.7 kPa (35.5 psf).
Taking all of these factors together, it might account for why your trusses fall short of present day code requirements.
BA
RE: farm building code for wood trusses
Also if you can show that the snow load that cause the damage lasted less that one or seven days and would/could not again occur per the current building code. Then an increase in the Load Duration Factor from a snow load (1.15) to a one day (1.33) or seven day (1.25) could be used. But per your post the trusses still would not work using this.
But, I have also worked under engineers who would disagree with this.
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: farm building code for wood trusses
If it is a shop building, I don't think it falls under the farm building code. If it does, the farm building code is administered by Alberta Agriculture and Rural Development and they could be asked for a ruling.
If it falls under the Alberta Building Code, the structure would be regarded as deficient and would need to be brought up to an acceptable standard as determined by the Director of Building Standards.
BA
RE: farm building code for wood trusses
My assistant is picking up a copy of the 1977 Canadian Farm Building Code that might be interesting.