Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Ceiling deflection damage caused by excessive snow - who is responsible

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Daryl Richardson wrote:
Dennis,
 
        Your assessment that there was more snow than the code allows for seems reasonable and may well be correct.  There are, however, other possible reasons for having excessive snow loads than your suggestion that the snowfall this year in much more than the statistically expected design snow load.
 
        Some possibilities include the following.
 
1.)    Influence of the mountains.  Snowfall in mountainous regions is difficult to predict.  It's possible that buildings on the specific site should be designed for higher loads than those specified in the code.  This may mean either that the code loads need to be increased or that an experienced engineer should have recognized this and allowed for more snow load than the minimum specified by code.  There were postings on the list (in which I was a participant) dealing with this subject a couple of years ago some of which I saved on my old computer.  I found one private correspondence and forwarded it to the list; you should be able to find more in the archives.
 
2.)    You didn't mention anything about large nearby buildings (with higher roofs), signboards, parapets, penthouses, or other structures that could influence snow drift patterns,  It is not uncommon for these obstacles to double (or more) the snow load.  These effects usually apply to small localized portions of the roof, not to the whole roof (you didn't say how extensively the observed overload effects were distributed).  One possibility: a higher building could have been built after the subject building and it should become the second building owner's responsibility to reinforce the roof of the older building.
 
        I hope this is of some help.
 
Regards,
 
H. Daryl Richardson
Thanks for the reply,
I won't know much more until next week, however, I did find out that the area is only 2300 feet above sea level - certainly at the lower end of the snow region. The roof slope was only 1/4" per foot, so it would seem reasonable that this area had no real snow load. The designer checked with Riverside county for the applicable snow load, but these are usually a guess. I would have contacted the local building official in the region.

Trus-Joist is adamant that they designed to the appropriate loads (live and dead) as presented by the engineer. I have a similar problem with a low slope roof on my own home where the trusses in the garage are spanning 25-feet and there is a noticeable crack in the joint of the Gypsum. I have the same problem on the walls which have plywood below the gypsum. In my case, the walls and ceiling are subject to dramatic temperature ranges since the winter lows can be in the upper 30's at the worst and the summer high can be well over 130-degrees. The location of the crack is at mid span and this is consistent, but he width of the crack is a hairline.
I also know that the company that installed the Gypsum did a very poor job as I had some cracking inside where a truss was flush to a beam. Temperature changes caused the trusses to expand and contract and this was sufficient to crack the paper tape (if they taped the joint at all), but some 8 years ago I repaired the crack with a mesh tape and have not had the problem return since.

What I am saying from this example, is that a 25-foot span on a joist (even a Trus-Joist) at the roof level generally provides a very flexible roof system. The code allowable deflection is around 1.25-inches and this is sufficient to place stress on a paper tape joint. I would be confident that even if this is within code, the chances are that any additional unexpected loading to the roof would be sufficient to crack the tape.

My thoughts are that before his client gets bent out of shape, my friend offer to repair the cracks and do so using a nylon mesh tape - and then offer to evaluate the performance at the end of the summer to see if the crack returns. Trus-Joist should make a report and also follow through as the client is now accusing the contractor (my friend) of substituting a lesser performing TJI product to save money. We all know that the process - even if we run the joists using TJ-Beam - is to have Trus-Joist rerun the system to verify the calculations before they supply the product to the site. The client is angry at this point because he does not feel that claiming the calculations were within allowable bending limits is sufficient - it does not solve the problem.

Doing a crack repair may cost a bit of money, but it will be a lot cheaper than litigating.

Dennis

--

Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net


760.564.0884 (office - fax)

This e-mail is intended to be delivered only to the named addressee(s) and may contain information that is confidential and proprietary. If this information is received by anyone other than the named addressee(s), the recipient(s) should immediately notify the sender by e-mail and promptly delete the transmitted material from your computer and server. In no event shall this material be read, used, stored, or retained by anyone other than the named addressee(s) without the express written consent of the sender or the named addressee(s).