Do you apply Kd to W when dealing with uplift effect?

[Doublest]

ASCE7-05 Table 6-4 says: Directionality Factor Kd shall only be applied when used in conjunction with load combinations specified in 2.3 and 2.4.

This Directionality Factor Kd basiclly reduces the wind load value. I can see its reasonablity by using reduced "W" when calculating combined downward load effect.

However, when calculating upward load effect in 2.3, equation 6: 0.9D+1.6W+1.6H, if still apply this Kd factor to "W", it will make the uplift value smaller and possibily make it underestimated.

Do you guys use Kd factor in uplift load combination calculation like in equation 6?

Thanks.

 

[KBVT]

Yes, the Kd factor should be used in all calculations utilizing the load combinations in Section 2.3 and 2.4 of ASCE 7-05. This came about when they revised the load combinations (LRFD) to have a 1.6 multiplier in lieu of the 1.3 which was present back in ASCE 7-95. Reference section C6.5.4.4 of the commentary.

 

[slickdeals]

The wind directionality factor has a history behind it. The old codes used to have a 1.3 factor for wind load. The current codes have a 0.85 factor for use with the 1.6 factor.

IMHO, you can use the Kd factor along with Equation 6.

See Miami-Dade county's position statement

http://www.miamidade.gov/buildingco...0512 - Wind Load Directionality Factor Kd.pdf

 

[strucguy]

I certainly would...because the code refers to all the combinations given in Section 2.

 

[Doublest]

Thanks guys for your answers.

I raise this question because I have seen some structural engineering design reports in which when checking the anchor bolt pull out (or concrete breakout) strength, they neglect the structure dead load (either because it is unknown or it is minimal), and only wind uplift alone is considered, therefore it is no longer a load combination, so Kd is not applied.

I find the logic here is a little bit funny, with the dead load to help the anchorage, the wind uplift load is reduced by Kd, that makes resultant uplift double lower. while without considering the anchorage help from dead load, the wind uplift load stay high without reduction.

That's why I raise this question here.

 

[ToadJones]

I guess, for me, when weird things like this come up, I check the worst case and go from there....

 

[PMR06]

I don't understand their logic. One could argue they are still using a load combination, they have just assigned D=0.

Either way, their "design" seems very conservative. Now maybe that conservatism is costing money??