Wind loads, return periods and averaging time, Conversions? 3

[IJR]

Pals

Can anyone help me determine a 50year wind from a 10year wind?

Does averaging time used in determining wind velocities matter much? I mean while ASCE uses 3 second, will that be VERY different if it were 10minutes?

Respects
IJR

 

[UcfSE]

Yes, it's different.

ASCE 7 provides tables for converting 50-year winds given in the document to other return periods. I assume that can work backwards.

Curious, why would you want to do this? Did someone give you 10-year winds and ask you to design for 50-year based on that information?

 

[IJR]

Exactly UcfSE

And another example. In Russia they use 5-year winds, and if you have to use any US code, you have to go back and get a 50year equivalent.

Respects
IJR

 

[Lion06]

AISC also publishes the conversion from 50 year wind to 10 year wind.

 

[IJR]

StructuralEIT and UcfSE

AISC? let me have a look at that. What AISC? The steel code?

ASCE says you multiply 50yr wind speed by a factor for a duration Tr. This is in table C6-7. This table makes sense because factor for Tr=50 is 1.00.

In that table factor for Tr=10years is 0.84.

I take that to mean I can factor down the 50yr wind speed to 0.8V where V is the 50year wind speed as given in that code

Am I right here?

respects

IJR

 

[UcfSE]

Yes. V₁₀=0.84*V₅₀. If you have V₅₀>100 the factor is a little different, as you can see.

 

[IJR]

Thanks UcfSE

you friends are always great.

 

[Lion06]

If you have the 50 year wind load already calc'd, just multiply that by 0.84^2 since the velocity term is suared in the wind load calc. The 0.84 factor is for velocity not force. This will give you a wind force conversion factor of 0.71.

 

[IJR]

Thanks StructuralEIT

I have got that. I once blindly factored wind pressures by 0.7 while calculating wind induced deflections which I was told should be based on 10yr not 50yr wind.

Now I fully know why

My next concern would be when use of lower return periods are warranted.

respects

IJR

 

[Lion06]

DesignGuide 3 (AISC) has a good deal of serviceability considerations and the applicable loads. Generally speaking, you only use a 10-year wind for serviceability issues (such as drift) and the 50-year wind for strength issues.

 

[IJR]

Additional thanks StructuralEIT