ASCE 17-96 para. 4.2.5 Load Combinations & SEI/ASCE 7-02 combinations

[saksgppl]

ASCE 17-96 para. 4.2.5 Load Combinations and recent SEI/ASCE 7-02 combinations -

In reviewing the ASCE 17-96 (Air Supported Structures) guideline, paragraph 4.2.5.1 states that "Load combinations shall be determined in accordance with ASCE 7 as modified in Section 4.2.5.3 to account for unique aspects of internal pressure as both load and resistance"

ASCE 17-96 Para. 4.2.5.3 lists out the load combinations, factors, and internal pressure loading and internal pressure load factors.

For example: 1.4D-1.0Po

0.9D-1.6Po

1.2D +1.3W+(....) -1.0Po

etc...

Note the 1.3 factor on wind load.

Looking at SEI/ASCE 7-02, the load combination factors are different. for example, the factor for wind is 1.6W not 1.3W as in ASCE 17-96.

ASCE 17-96 was released in 1996 so perhaps this is the reason the factors are different from what ASCE 7 calls out now.

The question is: In using the ASCE 17-96 (Air Supported Structures) guide, are the factors listed in load combinations to be replaced with the most recent guidlines issued by ASCE ?

I would tend to think - YES, since the ASCE 17-96 states that ""Load combinations shall be determined in accordance with ASCE 7 as modified in Section 4.2.5.3 to account for unique aspects of internal pressure as both load and resistance".

This tells me that the load combinations given in ASCE 7 should be used, but the Po (internal pressure) term must be used to account for the air structure interal pressure.

Do you agree or should the load combination equations found in ASCE 17-96 be used as stated even if the factors are different from the ones suggested by ASCE 7 ?

Thanks for any thoughts.

 

[JAE]

The previous ASCE 7 wind load factor was 1.3. This was because a wind directionality factor of 0.85 was already in the source wind speeds and wind load W.

With more recent versions of ASCE 7, the 0.85 factor was removed from the W, and placed in the load factor.

So the new load factor changed from 1.3 to:

1.3 x (1/0.85) = 1.529 .... i.e 1.6

So in using any load combination, you should understand where the W is sourced from and use the appropriate load factor.