ASD Load Combos vs. Service Load Combos

[steel_possum]

Can anyone explain the difference between ASD load combinations and Service load combinations? I am using ASCE-7-16 LRFD strength load combinations for my design, and trying to set up service load combos as well. I have always been under the impression that ASD is not the same as service, but apparently there is some debate about that. My current way of doing service combos is to take the LRFD strength combos, set all the coefficients = 1.0, and leave out the extraordinary load combos (such as those that include seismic).

 

[driftLimiter]

ASCE 7-16 Section 1.2 defines service loads. And by that definition I would agree with your comment that the LC factors are 1.0 for Dead, Live, Environmental (snow, soil or water), and Self Straining Effects.

**Edit I'd add to that that different combinations can be used when checking deflections per IBC

 

[phamENG]

Right...ASD and service are not the same thing. For instance, when designing with steel you're using Allowable Strength Design. Strength (whether for LRFD or ASD) for wind load is checked at a 700 year MRI for RC2 structures while service load is generally taken to be a 10 year MRI. That can be about 58% less load than the strength check.

Service loads are somewhat discretionary...IBC has basic deflection calculation requirements, but they are not the end all - be all of serviceability checks and lots of problems can develop even when they are followed. So serviceability (and the service level loads used to check it) is a bit subjective.

 

[BulbTheBuilder]

ASD is "Allowable strength design". ASD load combos are strength design load combos. They are usually referenced loads at their nominal or serviceability limit. Long time ago, according to history, these load combos were used to determine load effect under elastic stresses and those stresses were compared to the allowable stress capacity of members at a slightly level below failure either by yielding (I mentioned elastic stresses previously) or ultimate stress limit.

Example. How strong is this chair? Let's load this chair with a man weighing 200lb. If the chair fails/breaks at 200lb we know the strength of that chair. To be safe, we will call out a lower failure and say, this chair can carry a 119lb (using a factor of safety of 1.67) and that's the ASD approach.

Service load combination is used to check the serviceability limit state of the structure and not an ultimate limit. Your structure can fail serviceability prior to the ultimate limit.

D+L are both serviceability and ASD. Something like wind the load combo becomes D+Wa (wind service load eg. 50MRI wind) and not D+0.6W. I'd refer you to CC3 of ASCE 7-16 (I don't have the exact reference off-head so you might want to double check)

 

[WesternJeb]

That is a very, very interesting topic that I haven't considered.

When doing overturning calcs for seismic, I (and others in my firm) have always used .7E + .6*D... Since overturning is a service level check, am I justified in using .7E + 1.0D?

 

[driftLimiter]

Overturning for seismic isn't a serviceability issue its a strength issue. 0.7E+0.6D all the way.

 

[WesternJeb]

Hm.. I agree it isn't a serviceability issue, but it is a service level issue.

 

[phamENG]

If you're using an ASD design philosophy, sure. If I'm using LRFD, though, I'd use the LRFD load combination of 0.9D+1.0E. Unless there's a particular code reference to use the ASD combination? I'm not aware of one, but I never really have to worry about seismic overturning (wind is nearly always an order of magnitude greater for the buildings I design).