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STAAD Auto load combination ASCE Load factors for Wind Load

 
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MANU H V
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PostPosted: Mon Jan 21, 2019 10:39 am    Post subject: STAAD Auto load combination ASCE Load factors for Wind Load Reply with quote

Dear All,

As per the code ASCE 7, the load combination for LRFD based design includes a load factor of 1.0 for wind load. But in the staad when generating the auto load combination as per AISC code, in Public category the factor for wind load is taken as 0.8 and 1.6 but all the other load factors are as per the code. Is there any reason for this or can we edit that load factor to 1.0 ?
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Dipyaman_Ghosh
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PostPosted: Tue Jan 22, 2019 4:57 am    Post subject: Reply with quote

Yes, you are right on the observation of ASCE7 load combination factor. However, unless you have mentioned, my observation states that while you are assessing ASCE7 2010 load combinations, STAAD, however is pulling out the load combinations for ASCE7 2005. The load factor for 1.0 for Wind load is mentioned in ASCE7 2010. The reason for that is that the wind speed / basic velocity pressure is designated to be 26~28% more than what you normally achieve as per calculations from ASCE7 2005. You will notice that there is even a factor of 0.5 used for load factor for wind load in ASCE7 2010.

Review some literature on this velocity pressure and I am sure you would get the justification of using 1.0 as a load factor for Wind load. Correct me, if I am being wrong though.

You mentioned a solution and it would work - manually editing the load factors as per your requirements, till an automated solution is received. This should work if in case you are pulling out ASCE7 2010 load combination factors and still getting load combinations from ASCE7 2005. Report it and side by side continue working on with your edited load combination factors.
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MANU H V
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PostPosted: Tue Jan 22, 2019 1:17 pm    Post subject: Reply with quote

Thanks for the reply. I got cleared that the STAAD picked the value of 1.6 for WL from ASCE 7-5.

The thing I didn't get is when we use the code for a site location not included in the ASCE Basic wind speed map, but we have the basic wind velocity for 3 sec averaging time measured at a height of 10 m for a particular site. Is that correct to calculate the velocity pressure with the existing wind velocity without the importance factor as per ASCE 7-10 and then going with load factor of 0.5 and 1.0? or Is there any specified procedure for sites located out of the map ?

Other thing is as I observed the only difference between ASCE 7-5 and 7-10 was the importance factor, whose values are to the max 1.15. But I didn't understand why load factors are reduced from 1.6 to 1 when I value is varied as 1.15 to 1.( Even the square value for I, like to the velocity in the equation qz is 1.32).
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Dipyaman_Ghosh
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PostPosted: Tue Jan 22, 2019 2:28 pm    Post subject: Reply with quote

To your question for importance factor I, I am not sure but I was reviewing somewhere that they are eventually going to take the concept of I and resort to make the load combination compact and precise. This concept though I am not sure and very well can be a rumour. They are going to be done with the importance factor I in wind section completely, or something like that.

For the reduction in factors in WL in 2010 from 2005, the reason being that they are increasing the wind velocity pressure a considerable percentage from 2005 calculation. Even the hurricanes now come into a lower category in ASCE7 2010 compared to what it was compared in 2005. Hence the reduction in the wind load factors as per the behemoths sitting at the ASCE7 facility Very Happy with their innumerable study on wind loadings which is getting balanced out with higher wind velocity pressure.
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MANU H V
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PostPosted: Tue Jan 22, 2019 2:42 pm    Post subject: Reply with quote

Its fine we stick to specifications for load factors value, which they have published after many research work.

My main concern is about the following question:

"The thing I didn't get is when we use the code for a site location not included in the ASCE Basic wind speed map, but we have the basic wind velocity for 3 sec averaging time measured at a height of 10 m for a particular site. Is that correct to calculate the velocity pressure with the existing wind velocity without the importance factor as per ASCE 7-10 and then going with load factor of 0.5 and 1.0? or Is there any specified procedure for sites located out of the map ?"
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Dipyaman_Ghosh
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PostPosted: Tue Jan 22, 2019 3:20 pm    Post subject: Reply with quote

You're absolutely right. ASCE7 was primarily arranged for USA and nearby areas for wind velocity contours. However, if we go through the definition of different kinds of physical parameters important for wind load analysis, we can create / make up crude values of wind velocities of 3 second gust  based on the analogy of the weather conditions of places outside the states.
Secondly, defining authorities like municipalities can issue direct information for the 3-second gust wind velocities which sometimes is the only outside information you need. Rest I can recommend you to make an educated guess.
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