PPI Forums

For roof panels or skylights, I usually calculate a load combination of uplift wind load minum dead load. But some PEs calculate the combination using uplift wind load plus dead load. I can't verify which one is correct from ASCE 7-02 or 05. Anyone has any suggestion on this? Thanks.

I calculate my net wind uplift with wind LL minus min. roof dead load. There are some fairly well written wind calc's Excel programs you can download for free at buildabout.com.

PEGUO wrote:

For roof panels or skylights, I usually calculate a load combination of uplift wind load minum dead load. But some PEs calculate the combination using uplift wind load plus dead load. I can't verify which one is correct from ASCE 7-02 or 05. Anyone has any suggestion on this? Thanks.

I would use DL only for self-weight. For collateral load (i.e. sprinklers, mechanical, etc.) do not include the weight for uplift.

Always use the total of DL + COLL + Wind for downward forces. This will surprise you how often this controls the design... especially for c&c for IBC 2003.

Structural engineer magazine (gostructural.com) has an article by SK Ghosh that brushes against this question.

It is the engineer's responsibility to determine which load cases govern. IBC 2000/2003/2006 also has a lot of "hidden" text about what constitutes a dead load (i.e. 2/3 of dead load counts) if you look at chapter 16 long enough. There are also requirements for using W wind load determined from IBC, if you use the alternate load cases, meaning, W isn't W as listed in IBC, you have to apply 1.3xW.

Be careful and read the whole relevant portion of the code.

ASCE 7-05 Chapter 2 section 2.4.1 Basic nominal loads using allowable stress design.

0.6DL + W

Wind and DL are in opposte directions. (you did say it was for a skylight on a roof. The wind pressures are almost always up on a roof) If it were on a wall, then use the same equation twice and change the sign of wind beacuse the wind can come from any dorection so the wall could be a leeward or windward wall).

For a skylight use components and cladding pressures.

when we take the uplift into consideration,we often use 0.6W + dead load. due to uplift, we decrease the dead load which may be conservative, it made sense.
just do it, maybe sometimes you do not know which is right,just input several load combinations to test which will control the disign. keep learning from you guys