How to understand 0.78 or 0.6 in wind speed determination as per API650-2016?

[tmgczb]

In API 650-2016, wind speed is by default determined as per ASCE 7-05.
If ASCE 7-10 is used, wind speed shall be multiplied by 0.8, or wind pressure shall be multiplied by 0.6.

Why?
If ASCE 7-10 is used, is Importance Factor is required or not?
In ASCE 7-10, importance factor is 1, use importance factor from ASCE 7-05?

 

[JStephen]

ASCE 7-05 was based on wind speeds calibrated for allowable stress design, and corresponding wind loads would be increased for LRFD, etc.
ASCE 7-10 (and later versions) was based on wind speeds calibrated for LRFD design, and corresponding loads are reduced for allowable stress design.
Under ASCE 7-10 and later versions, the importance factor has been eliminated by means of using different wind speeds for different Risk Categories.

 

[Snickster]

0.6 is called the shape factor. When air flows over a cylindrical surface the velocity pressure is converted to static pressure but since the surface is curved it is not same as as if it were a flat surface. The actual static pressure felt by the surface is 0.6 times less for a curved surface than a flat surface.

This is explained in all piping stress analysis references as 0.6 factor is used for piping wind loading also.

 

[JStephen]

The 0.6 is not the shape factor. It is ASCE 7-10 load combination factor for Allowable Stress Design, and 0.78 is just the square root of that factor, if applied to wind speed rather than presure.

 

[Snickster]

It is the shape factor. This is why 0.78[sup]2[/sup] = 0.6

 

[JStephen]

See the note at the end of Section 5.2.1(k) of API-650 which explains the difference.

 

[tmgczb]

ASCE 7-05 was based on wind speeds calibrated for allowable stress design, and corresponding wind loads would be increased for LRFD, etc.
ASCE 7-10 (and later versions) was based on wind speeds calibrated for LRFD design, and corresponding loads are reduced for allowable stress design.
Under ASCE 7-10 and later versions, the importance factor has been eliminated by means of using different wind speeds for different Risk Categories.

How should we understand wind speed is calibrated based on a certain design method?
If 0.78 or 0.6 is applied to wind speed calibrated based on ASCE 7-10, does it mean it has been converted to wind speed calibrated based on ASCE 7-05?

 

[JStephen]

Yes, multiplying ASCE 7-10 wind speed by 0.78, or pressure by 0.6, will give corresponding ASCE 7-05 wind speed or pressure.
Note that it is not exact due to the importance factor being included/ excluded and also due to general updating of wind speed maps.

 

[Snickster]

The ASCE Code has evolved over the years and so have the mechanical codes that reference that code for wind and earthquake loads.

Back when I first started design engineering 1984 the code for wind load was ASA A58.1 then became ASCE-7. In the early ASCE codes the wind load was just the theoretical wind pressure = V[sup]2[/sup]/2g multiplied by other factors. The LFRD method I don't think even existed. The V[sup]2[/sup]/2g pressure was multiplied by the shape factor including other factors to get the actual design load on a vessel or pipe. Shape factor for pressure vessels was 0.8 to account for such things as stairways and ladders etc., and for pure cylinders such as pipe was 0.6.

I remember now that somewhere along the way ASCE started including the shape factor in their basic wind loads when the LFRD method was introduced. Mechanical standards adjusted for this. I don't remember the specifics but somehow the different mechanical standards accounted for this in different ways. It was very confusing really to deal with. Programs such as Caesar still used the old method of having the shape factor multiplied by the theoretical velocity pressure.

Here is a pressure vessel calculation using shape factor around 1995 using 0.8 as shape factor:

 

[tmgczb]

Yes, multiplying ASCE 7-10 wind speed by 0.78, or pressure by 0.6, will give corresponding ASCE 7-05 wind speed or pressure.
Note that it is not exact due to the importance factor being included/ excluded and also due to general updating of wind speed maps.

ASCE 7-10 considers different MRI(Mean Recurrence Interval), 300/700/1700 years, for structures of different risk categories.
For different MRI, is the same factor 0.78/0.6 is used?

 

[tmgczb]

Yes, multiplying ASCE 7-10 wind speed by 0.78, or pressure by 0.6, will give corresponding ASCE 7-05 wind speed or pressure.
Note that it is not exact due to the importance factor being included/ excluded and also due to general updating of wind speed maps.

ASCE 7-10 considers different MRI(Mean Recurrence Interval), 300/700/1700 years, for structures of different risk categories.
For different MRI, is the same factor 0.78/0.6 is used?