Snow Load Increase 1

[NewbieStruct]

We do a very few buildings in Arizona. We had done a building in 2013 and we had contacted the city and confirmed the ground snow load to be 40 psf. We have the pdf design criteria from the city that shows the ground snow to be 40 psf. We are doing a new job in the city and we downloaded the same pdf and now it shows the roof snow to be 40 psf. Should we be concerned for the building done in 2013? The city is Williams.

 

[msquared48]

No. But save all your correspondence and records here. Things like this happen all the time as codes change. You are OK as long as you used the loads required at the time.

Mike McCann, PE, SE (WA)

 

[Jerehmy]

it changed from 40psf to 40psf?

 

[NewbieStruct]

Ground psf was 40 originally (we used roof snow of 32 psf using code equations) and now roof snow psf is 40. It is approximately an 8 psf increase.

 

[Splitrings]

It must be a misprint. It would have to be specified as a GSL. The roof load is dependent on the characteristics of the structure.

 

[msquared48]

Not necessarily splitrings. But it would be good to check with the jurisdiction


Mike McCann, PE, SE (WA)

 

[Splitrings]

I suppose the AHJ could step outside the standard codes, but I can't imagine they would want to. If the GSL is 40 psf, the roof load for a cold, flat non slippery surface with an importance factor greater than 1 would be much greater than 40 psf. Maybe their intent was to specify a minimum design roof load?

 

[manstrom]

Sometimes code officials don't like to see the ground snow load reduced per the code.

Sometimes code officials don't the difference between ground snow load and roof snow load.

A call to the jurisdiction is in order. Of course, you will probably get a receptionist who falls into the latter category. Try to email the building official and get it in writing.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[dcarr82775]

Most jurisdictions in my area specify both a ground snow and minimum roof snow, which are both the same. Tradition and historical data is the reason for the minimum roof snow. In reality the ground snow should probably just be increased.

I wouldn't be concerned about the 2013 building.

 

[JAE]

Sometimes code officials don't the difference between ground snow load and roof snow load.

That's been my experience - some AHJ's don't understand the difference.

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[Triangled]

I have a job a few miles away in Flagstaff Arizona where snow load is definitely specified for the roof as 40 psf.

 

[jike]

I agree with Manstrom and JAE. AHJ don't always know the difference between ground snow load and roof snow load and so to one inquiring engineer they will say one thing and the next one something different.

If you are able to get an answer, GET IT IN WRITING.

 

[ajh1]

Having requirements for both a ground snow load and a minimum roof snow load appear to be becoming more common. Typically, the ground snow load is then used only for the purpose of determining items like snow drifts while the overall roof load is controlled by the minimum roof snow load. Both the Connecticut state code and Rhode Island state code have had a requirement of this nature for many years, and yes, in many cases the numerical requirement for both conditions is the same.

 

[Triangled]

Williams is about 35 miles from Flagstaff which changed their roof requirement as a result of a snow event around 2010. I don't know if this applies to Williams too. It's not impossible that the requirement changed from 40 psf ground in 2013 to the current 40 psf roof. See attached report.

 

[Triangled]

2nd try....
Williams is about 35 miles from Flagstaff which changed their roof requirement as a result of a snow event around 2010. I don't know if this applies to Williams too. It's not impossible that the requirement changed from 40 psf ground in 2013 to the current 40 psf roof. See attached report.

 

[bigmig]

I would research the historical snow fall data and try to self-determine the historical load and then compare it to the under conservative 40 psf ground. There are many agencies that track this information from a meteorological standpoint.

In my state, we have over 100 years of data in some sites. Some building departments use values that are way over conservative. Others, are right on and match the data.

 

[NewbieStruct]

It seems the change is very recent. We have the old document from the city that shows the 40 psf ground. We will design the new building for 40 psf roof. We will keep the old design document from the city for our records.