ASCE7-05 Vs. ASCE7-10 Wind Loads

[KeySol]

I have been doing some comparisons on the wind loading as set forth in the ASCE7-05 vs the -10 versions. Specifically the C&C but i think this will be the same for MWFRS loading as well.

My concern is this:

I feel like I am missing something. The equation for velocity pressure qz has changed but only by removing the importance factor. so this equation is exactly the same except the design wind speed. This has gone up, i believe it takes into account the building type but not sure what else.

So between the two codes the qz for the -10 is larger than the -05 version. I would be ok with this except the fact that the rest of the coeeficients and variables and equations are exactly the same.

Kh,Kz is the same
Kzt is the same
Kd the same
and oh yeah the design wind pressure equation, and the GCp (again im looking at C&C) and Cpi are exactly the same.

SO at the end of it all i now have pressures that larger using the -10 code. Actaully they much greater because V is squared.

Am i missing something here? Any comments or thoughts on this are much appreciated.

 

[JAE]

They took out the importance factor and used different wind speeds to account for the building category instead.

 

[WillisV]

you also only have to use a load factor of 1.0 instead of 1.6 on W

 

[frv]

Willis V is right.

There was a webinar not too long ago regarding this and in theory, loads should be pretty similar between the two versions (which begs the question, why mess with it in the first place?).

 

[KeySol]

WHat i found was that the loading is now "ultimate" loads. so since i am working on ASD design i would multiply by .6 to get them to the ASD level. When i do this i get almolst the exact same answer as what we did in the -05 code.

 

[WillisV]

That is what I told you in my previous post. Congratulations on finding it.

 

[KeySol]

WillisV

I guess i didnt follow your response. Is there anyway you could elaborate on what your saying?

 

[WillisV]

Umm...pretty easy to follow...but for the sake of completeness:

Under ASCE 7-05 the standard wind speed was 90mph. In the final condition this value is squared in the force equation so you get 8,100. In strength load combinations the load factor on wind was 1.6 (for instance 1.2D+0.5L+1.6W). So 1.6x8,100 = 12,960.

Under ASCE 7-10 the standard wind speed is 114mph. 114mph^2 = 12,996. In strength load combinations for ASCE 7-10 you no longer use 1.6 for wind, you use 1.0 (for instance 1.2D+0.5L+1.0W). So 1.0x12,996 = 12,996.

Comparing these two values show they are almost identical. You will see some differences along the coast etc. but the majority of the country the wind load stays the exact same, it is just presented in an ultimate manner to be multiplied by 1.0 just like seismic loads.

 

[swearingen]

WillisV,

The 7-10 standard speed is actually 115, but it still gives pressures that are very close for the basic wind speed covering 90% of the country. For the hurricane zones, things get out of hand quickly. For an extreme case, let's take Guam:

For 7-05, the speed is 170. Squared, with the 1.6, this comes to 46240.

For 7-10, the speed is 195. Squared, with the 1.0, this comes to 38025,

for a reduction of over 21%!

I'm just curious if they really want us to use 4/5 of the wind loading we've been using in the hurricane zones...




-5^2 = -25 ;-)

http://www.eng-tips.com/supportus.cfm

 

[WillisV]

swearingen,

In short, Yes, they really do!.

There were two main changes made in the wind code for ASCE 7-10, one of semantics, and then a more substantive change:

The semantics change that has been primarily discussed in this thread is to go to 1.0 load factors with higher ultimate wind speeds which results in the same wind load effect (114mph was the draft basis, looks like they made it nice and round for the final ASCE 7-10).

The substantive change as I insinuated in my previous post is a change in actual loading in hurricane zones. A great deal more hurricane data was used in the model for ASCE 7-10 (think of all the hurricane's we have had since ASCE 7-98 which is when the last model was developed) with a more sophisticated attenuation algorithm (how much the wind speed slows down from the eye outwards). Thousands of simulations were run to develop the updated loadings. This is one of those rare cases where the changes are actually due solely to increased knowledge and based squarely on scientific study and not change for changes sake.

Most of this is based on Cook and Vickery's work - for an additional brief overview see:

http://www.coastalcontractor.net/article/238.html

 

[chichuck]

All,

Please see the attached xlsx file.

I've calculated the wind loads for a site in midwestern USA, for all four Occupancy Categories, and compared final pressures in psf for both ASD and LRFD.

http://files.engineering.com/getfil...d1d328b05dcd&file=asce_7_wind_comparisons.xls

Regards,

chichuck