Stress Increase for Snow Load

[BadgerPE]

In previous additions of ASD design (specifically AISC and AISI) was it permitted to increase the allowable stress by 33% for stresses caused by snow loading. It has been suggested to me that this is the case, but I was only aware of the allowable stress increase for wind and seismic loading. I am evaluating an existing building (PEMB of course) to see if it will be economical to retrofit for an additional roof load or if an additional support structure is needed. I would like to model the building fully to determine what the original capacity was and therefore can present the owners with a clear view if additional support is needed. Thanks much!

 

[MiketheEngineer]

For wood - yes - Not sure about ASD

 

[JAE]

I've never increased stress levels for snow for steel.

 

[msquared48]

Done that for wind and seismic fr steel, but never snow. Wood - yes, but 1.15.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BadgerPE]

Hmmm....When I reduce the snow load by a 0.75 adjustment factor, the attachment represents the flexural results from my model. It appears that it a large majority of the members are pushed to the limit or even a little overstressed (to be expected when modeling a PEMB) A few comments have said that they personally have never used it, but is anyone aware if the stress increase used to be codified or an industry standard of PEMBs circa 1970s?

 

[structSU10]

As far as I know load combinations have been what they are for a long time. ASCE 7-05 section 2.4.1 gives combinations, but maybe local building code has something different (usually not the case). I think what you are refering to is D + 0.75*(L+S), which shouldnt control for a roof. The controlling combinations for gravity is likely D + S, where S includes any applicable snow drift. Note that for all other combinations with snow, it only needs to be the flat roof or sloped roof snow load.

PEMB's typically dont work to modern codes when checked, so expect to have issues analyzing the existing situation.

 

[msquared48]

In the 1979 Edition of the UBC, section 2303(d), it states that:

"All allowable stresses and soil bearing values specified in this dode for working stress design may be ind=creased one-third when considering wind or eatrthquake forces either when acting alone or when combined with vertical loads. No increase will be allowed for vertical loads acting alone."

This implies no increases for USD concrete design, nor steel design. However a 1.15% increase was still allowed for wood for snow under section 2504(c)4,ii.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Personally, I think an increase in allowable stresses for snow load was a mistake. There are few loads more sustained than snow loads.

BA

 

[BadgerPE]

BA: Are you saying that a 1/3 increase for snow loading was previously permissible by some code?

 

[BAretired]

No, I don't think it was ever as high as 1.33 for snow load. In CSA Standard 086-1970 for timber construction the load duration factor, k_D was 0.95 for continuous, 1.0 for normal, 1.15 for 2 month (which included snow), 1.25 for 7 day and 1.33 for one day duration.

In CSA 086-2001, k_D has only three categories, namely short term (1.15), standard term, which includes snow (1.00) and permanent duration (0.65).

For steel construction, load duration has never been considered.

BA

 

[woodman88]

Personally, I think the snow load 1.15 increase for wood is good for most snow loads in the USA. As it is based on the maximum snow load only occurring once every fifty years plus/minus. There are areas that receive higher snow loads (60 psf or higher) in California/Oregon/Washington that require a 1.00 snow increase. I am sure that there is also many ares in Canada/Alaska that should also use the 1.0 increase for snow.

But, overall IMHO the snow load increase of 1.15 is a good factor for wood.

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.

 

[msquared48]

Just ran across an upcoming projecxt where the snos load is going to be in the order of 800 psf. That will be interesting. Done 400 before, but never 800. No a;;owable snow load stress increses here...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BadgerPE]

"...in the order of 800 psf." Wow! I have done 100 psf before and I thought that was bad enough. Is this in a mountainous region? Who determines that high of snow load?

 

[msquared48]

Local Building Officials and the SEAW (Structural Engineers Association of Washington). Yes, it is literally at the top of a 7000 foot peak, and shedding the snow is not an optioue to the potential of creating an avalanche. Get my drift?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com