×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

farm building code for wood trusses

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.

RE: farm building code for wood trusses

I have a copy of the 1990 NRCC Farm Building Code but nothing earlier. I'm not sure how many farm buildings met the code in 1985 as I don't believe it was being strictly enforced at that time.

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

As the Load Duration Factor for wood is cumulative, I personally would require the damaged trusses (as the damaged occurred because of a load condition) to be replaced. I would also reccommend that the "un-damaged" trusses also be replaced. Unless I could show that undamaged trusses would not of received the overloaded snow condition.

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

Quote (charliealphabravo)

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.

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

(OP)
Thanks guys. That's just the kind of input I needed.
My assistant is picking up a copy of the 1977 Canadian Farm Building Code that might be interesting.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources