Confusion from the ASCE 7-10 and CBC 2019 Wind Code

[chorner26]

OK so I have always multiplied out the wind loads from ultimate and multiply by CBC (equation 16-33 on page 23 of the CBC. ie if the wind pressure is 25 psf ult *Squareroot(0.6) = 0.77*25psf =19.25 psf.
But the confusion comes when applying ASCE 7-10 load combination 6 ____ D+ 0.75L+0.75(0.6W)+0.75(Lr or S or R1. The question is do I multiply by 0.6W or by 0.77W ID it is ultimate 25psf*0.6= 15 psf which is very different than 19.25 psf?

 

[phamENG]

For ASCE 7 and the IBC, (√0.6)V[sub]ult[/sub] converts the ultimate wind speed to the "V[sub]asd[/sub]" wind speed. It doesn't say it specifically, but if I remember correctly this conversion is meant to approximate the fastest mile wind speed. This measure of wind speed was used in the US prior to ASCE 7-98, and several prescriptive codes still use it (flag poles, radio antennae, certain wood framing guides, etc.).

I have no idea why you would multiply the calculated wind load by that, but I'm also not familiar with the CBC.

If you're designing something that requires you to use the IBC and/or ASCE 7-10, then you calculate W using the 3-second gust V[sub]ult[/sub] for the appropriate risk category and then multiply it by the stated factor. Don't introduce anything extra. This goes for ALL load combinations including wind - not just #6.

 

[Harbringer]

If you're designing something that requires you to use the IBC and/or ASCE 7-10, then you calculate W using the 3-second gust Vult for the appropriate risk category and then multiply it by the stated factor. Don't introduce anything extra. This goes for ALL load combinations including wind - not just #6.

Agreed

Also 2019 CBC uses ASCE 7-16. Certain wind provisions have changed from 7-10.

 

[Aesur]

I agree with phamENG, the specified equation in the CBC is Vasd = V*sqrt(0.6), this is a conversion from ultimate wind speed to ASD level wind speed, ie mph, not psf. The reason this is squareroot is because your pressure is 0.00256*(Factors)*V^2, and your load combinations from LRFD to ASD is 0.6W for ASD, therefore this is a way to approximate the ASD level wind speed in mph. When using the combinations you use the "Ultimate" level which is the V calculated with your local wind speed. The Vasd is not really used much from a current design perspective and is more of a conversion back to older codes or other codes that still use the older format.