Design Guide 7 Seismic Load Combination 1

[Zman42]

I am designing a free standing crane in a high seismic area. AISC Design Guide 7 has a Case 3 load combination that includes earthquake loads, where the load is at strength level (E). Can someone explain to me why it wouldn't be at allowable stress level (.7E)?

 

[JAE]

It looks like the whole combination (in Case 3 where the E term shows up) is multiplied by 0.75.

 

[Zman42]

Thanks for responding JAE. I did see that reduction you mentioned. So taking that into consideration you almost have ASCE 7-10 section 2.4.1 basic load combination 6b: D+.75L+.75(.7E)+.75S, except that for some reason you get to reduce your dead load by 25% and the seismic is included at strength level. It doesn't make any sense to me...

 

[CELinOttawa]

It is all about the statictical chances... What are the chances of having BOTH a design level earthquake and design level snow at the same time? Answer: Almost zero, so your code says worst case consideration is 75% of that case.

 

[CELinOttawa]

Oops; Should have been design level LIVE, EQ & SNOW, with best estimate of anticipated dead load... Really the chances are nealy nothing.

 

[wannabeSE]

The 0.75 load reduction looks a lot like a 1/3 increase in allowable stress for load combinations with wind or seismic. This makes me wonder if the earthquake load is a pre 97(?) UBC service level load. It may be worth contacting AISC and asking for clarification and recommendations.

 

[jdgengineer]

I don't have the design guide in front of me, but it may be worth checking when it was published? EQ loading used to be allowable and its possible the combinations in the design guide are outdated. Personally, I wouldn't use the combinations in the design guide as they are not legally adopted. I would use the IBC load combos (or equivalent in your area) and use the design guide as inspiration as to how to apply them with respect to cranes. (E.g. not including lifted load in seismic mass, etc). If the IBC combos produce lower stress, I'd be tempted to also check the design guide combos for sanity.

 

[CELinOttawa]

This is a prime example of why it is just plain stupid to have so many codes... It is bad enough that Canada has a National MODEL Code, and then each province MODIFIES and adopts locally as they see fit. At least it is ONE code methodology that gets adopted; I can't believe that there are so many variations on the number and type of methodologies all adopted disperately throughtout the US.

Weird.

 

[JAE]

Well, in recent years the US has gravitated into the IBC code (replacing the UBC, BOCA and Southern Standard).
That is a singular code but there is still the history back in time of multiple code types and editions.

I would think even in Canada there was the transformation of EQ loads going from service level to ultimate level - but I don't know that to be a fact.