Existing PEMB and New Snow Drift - Code Requirement

[waytsh]

I would appreciate all your opinions on this as a sanity check. I have been asked to look at an existing pre-engineered metal building for snow drift from a new higher length extension. There is no question that I will need to review the purlins and eave struts to the current code for this new snow drift. The frames are a different story and I will explain.

The only two frames affected by the length addition snow drift are the endwall frame and the next frame in. The same frame was used in both locations and the original reactions indicate that both frames were designed for a 25' trib and 30 psf roof snow load. The new length extension is self supporting so the endwall frame only has a trib of 13.125'. The current code only requires a balanced roof snow load of 20 psf. When all is said and done both frames are carrying less total snow load when I use the minimum 20 psf snow load in combination with the snow drift than the original 30 psf.

Can I make the argument that since the new code required snow loading does not exceed the original designed snow loading that no review of the frames is required and they do not need to be brought up to the current code

Thanks!

 

[jayrod12]

Well if the loads used for the original design are higher than the new loads, it meets the current code. There's no question of being brought up to current code as it already meets it.

Why would it have been designed for 30 psf snow if the code level is 20psf? That's my only question.

 

[waytsh]

Often times with pre-engineered metal buildings contractors will order a higher snow load because they are not comfortable with how tight they are designed. I don't know if that was the case here but it is not unusual to see it.

 

[jayrod12]

Fair enough. It's unusual around here. But then again our code minimum snow locally is 36 PSF.

 

[DaveAtkins]

If the new required snow load is less than what the building was designed for, then you are OK. I know some states previously had a conservative snow load for the entire state, and ASCE 7 gives you lower snow loads--perhaps that is the situation you are describing. But I suggest checking one more time to make sure you are calculating required snow load correctly (remember exposure factor, thermal factor, etc.).

DaveAtkins