Serviceability ASCE7-10 appendix c

[braves25]

Hello all,

I am needing to check drift for my structure and am looking at appendix C to use a "reduced" 10 year wind load to calculate drift. My normal wind speed is 115 MPH per figure 26.5-1A. I am using ASD load combinations (2.4.1). From figure CC-1 for 10 year wind, the wind speed is 76 MPH. When I go to run my numbers for drift using the 76 MPH wind speed, (also looking at equation CC-3), will the ASD load combinations give faulty results? In those equations there is an 0.6 factor before all wind loads. How does this work with equation CC-3 and the 76 MPH provided in the figure for 10 year wind? I do not want to be overly conservative on designing my moment frames and use the 115 MPH wind speed if drift controls the design. I'm using RAM to model my structure and have added load cases/combinations for serviceability using the 76 MPH wind speed. Thanks!

 

[JAE]

The 0.6 factor on wind (W) is for the "ultimate" wind loads from the main body of the standard (i.e. 115 mph).

The commentary for Appendix C - section CC1.2 provides equation CC-3 which is described as a load combination that [blue]"can be used to check short term effects"[/blue] and in which the wind load is [blue]"based on serviceability wind speeds in figures CC-1 through CC-4."[/blue]



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[RWW0002]

The 0.6 is to convert 50 yr ultimate wind pressures to 50 yr. service wind pressure. If you want to check drift against 10 yr service-level wind you can use the 10 yr wind maps and still apply the load cases with the 0.6 factor.

This is similar to using the 0.7 factor on top of ASCE7-05 (service level) wind loads to adjust to 10 yr wind. If you want a sanity check run a check with ASCE7-05 wind speeds (90mph) and multiply the resulting pressures by 0.7. You should arrive at a similar answer.

 

[RWW0002]

JAE is correct. The 76 mph wind speed appears to be service-level wind, not ultimate. The 0.6 factor should not be applied. I should have checked my math before posting. Sorry about that.

 

[braves25]

That was the direction I was leaning....would I also take off the 0.6 on the dead load in the ASD equations? Or just simply use the ASD combos from the ASCE 7-05 code?

 

[RWW0002]

Queue rant about load and importance factors being included in windspeed maps.. It only leads to confusion about what is actually going on "behind the scenes". That being said, I should have checked myself before posting.

Personally, I still apply a 0.7 factor to my service-level wind loads from design for deflection and drift checks to arrive at 10 year wind pressure. (This adjusts for 10 yr wind but leaves the importance factor in the wind pressures - generally conservative but not overly so).

 

[JAE]

For the overturning/uplift check you'd still want to keep the 0.6D in the load combination (i.e. 0.6D + 1.0W)
That provides an inherent safety factor of about 1.5 on overturning.

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