Converting Wind to ASCE 7-05 from Ultimate 1

[WS6 Jim]

ASCE 7-10 and the 2012 IBC use ultimate wind speeds
In ASCE 7-10 "Table C26.5-6" shows that to convert 7-10 speeds to 7-05 speeds, you divide the 7-10(ultimate speed) by sq.rt. 1.6
In IBC 2012 the equation provided to convert to Vasd, is to multiply the 7-10(ultimate speed) by sq.rt. 0.6

Am I correct in thinking Vasd is the same as ASCE 7-05 design wind speed?
If so, it looks like ASCE 7-10 ans IBC 2012 back calculate wind speeds differently. Does anyone know why?
Which is the proper way to back calculate ultimate to nominal wind speeds?

Any help is greatly appreciated!

 

[Once20036]

If you divide by the square root of 1.6, you get 79.1% of the original value. If you divide by sqrt (0.6), you get 77.5%.
Are you concerned by the 2% difference?

If you need the ASCE 7-05 wind load, why not use ASCE 7-05?

The wind force is related to velocity squared, and it's much more straight forward to convert wind force to service levels (for deflection calculations). Would this work for your situation?

 

[WS6 Jim]

The design software I use requires ASCE 7-05 wind speed as an input.
2% will make a difference due to a cost competitive field.
If I use ASCE 7-05 wind speed from ATCouncil I do not get the added benefit of the new contour wind lines.
That is why I need to convert the ultimate speed back to 7-05.
Does anyone know why the methodologies are different?
What is the correct way to convert based on my situation?

 

[mike20793]

The limit state was changed to be more consistent with the ultimate limit state that the seismic forces are based on. To convert to nominal wind speed from the ultimate wind speed, you multiply by the square root of 0.6. This equation is found in the IBC Section 1609.3.1 (Equation 16-33).

 

[Hokie93]

Strictly speaking, you should use the conversion equation in IBC, assuming your state/jurisdiction has adopted IBC as the model building code and has not amended Equation (16-33) in some manner. The Commentary to ASCE 7-10 is not part of the standard. Having said all that, I think dividing by the square root of 1.6 is technically the more rational approach but the 2% difference is minimal. The 0.6 factor comes from the coefficient used in ASD load combinations in IBC and ASCE 7 to convert ultimate wind pressures to service-level wind pressures.