Wind Load Conversion Factor???

[msquared48]

I am dealing with a much higher wind loading than normal for one project and want to make sure I get things right here...

I am used to dealing with ASD loads for wind under ASCE7-05 and IBC 2006. In looking at IBC 2012 and ASCE 7-10, the loads are much higher. I know this has been addressed before in the forums, but the search function is not working for me.

My questions are:

1. Are the 7-10 compared to the 7-05 loads merely about 1.4 times greater, intended for direct application to LRFD equations?

2. Can I merely divide these 7-10 values by 1.4 to get the ASD loads, using the appropriate load equations and material tables?



Mike McCann, PE, SE (WA)

 

[Lomarandil]

I'm no expert here, but my understanding is that the 7-10 loads are at a strength (LRFD) level. There are some other reliability tweaks embedded in the new windspeed maps, something for hurricane regions, and some things as they've refined the concept of building importance.

But at the end of the day, my understanding is that a 7-10 wind load with a 0.6 factor for ASD load combinations should produce about the same result as the old 7-05 wind loads for ASD. (Or a difference of 1.6~=1/0.6)

 

[SteelPE]

As Lomaradil states, the wind speeds have been adjusted to strength level (I guess they had seismic envy). When you are done running through the load combinations with the 0.6 reductions you should end up at about the same loads you had before (you can kind of see this in 1604.3 note f which used to read 0.7 but not reads 0.42) Why they decided to take something that was in one chapter and spread it out over 6 is beyond me.

 

[RPMG]

Lomarandil is correct. Since there was so much reorganization in the update to 7-10, they did not change the content. Demand / Capacity in ASCE 7-05 = ASCE 7-10. They did add a simplified all heights method. This change was made to push LRFD and make wind more comparable to seismic.

1. 7-05: 90 mph wind yields 1.0 Wind (ASD) is equal to 7-10: 115 mph wind yields 0.6 Wind (ASD)

2. Multiply 7-10 values by 0.6 or whatever your ASD combination requires.

 

[wannabeSE]

IBC equation 16-33 converts Vasd to Vult. Do a little algebra (V²)and you end up with 0.6 conversion. Don't forget to consider the Importance factor if it isn't 1 in ASCE 7-05.

However, I think the wind speed maps may have changed in a (very) few areas. So you may want to compare the 2 codes if it is pertinent to your project.

 

[ajh1]

The importance factor is not effectively covered by using a wind speed off of a different map. There are maps for Category 1, Category 2, and Category 3/4. Select the wind speed from the proper map and you've covered the importance factor. There is no actual importance factor in the wind pressure equation now.

 

[JStephen]

I was just checking one site near the Gulf Coast and it actually drops the wind loading quite a bit using the newer standard.

 

[dcarr82775]

1. roughly 1.6 greater, not 1.4
2. mult by 0.6 not 0.7

The numbers for most conditions as essentially the same. The small differences are due to rounding the contours to multiples of 5mph

 

[EngineeringEric]

Very Useful website for wind speeds:

http://windspeed.atcouncil.org/

 

[Triangled]

RTUs, in addition to the above, have a significant new (2010) uplift factor, if I recall correctly after a couple of beers and away from the office...