LRFD VERSUS ASD SEISMIC ???

[rnorth]

given in contract docs....
unfactored ASD axial load of 10 kips

to design the connection with LRFD
we take factored axial load = 1.0 x E
where E = 10 kips

or .....
factored axial load = 1.0 * (E/0.7)

0.7 being the ASD load factor
If a document (dwg or spec) mentions unfactored ASD load, in this case seismic, this is taken as E, correct?
thanks in advance for any opinions....

 

[JAE]

I think you might expand on the information you are providing so we can help better.

Is this a column, part of a building, bridge, beam?

Seismic loads taken from many building codes (I'll use the IBC as that is what I'm familiar with ) provide seismic design methods that can provide seismic load demands on buildings and parts of buildings.

The value calculated with the IBC is E, which is the load effect considered as an ultimate load...thus the load combinations for LRFD use 1.0 x E, since E is already an ultimate load effect.

If you wish to use ASD, then you would take you (ultimate) E and multiply by 0.7 to get an equivalent "unfactored" seismic demand load.

Whether the contract documents you refer to already used the 0.7 factor should be verified. If they refer to it as an ASD axial load then they probably have already done this: 0.7 x E. However, I'd verify to be sure.

Also - you refer to connection design - some connections require the overstrength factor applied.

 

[weab]

If it says just the word "unfactored", I'd believe that the 10 kips is "E", especially if you are using UBC or IBC.

If it says the words "unfactored ASD", then I would interpret that as ASD. You must multiply by 1.4 to design by LRFD. I would say that ASD is clear. I would think that the wording was chosen by one of us older guys who often equate "unfactored loads" to mean service loads.

In my humble opinion, UBC/IBC did a disservice to engineers when it began calculation all loads via service loads except for seismic- stupid! If they had not done that starting with UBC97, you wouldn't have had this question.

 

[JAE]

Well, a seismic event, in and of itself, is an ultimate event to begin with (unlike all the other types of loads).

You don't add a safety factor to a load demand that is already beyond what the structure can take without damage. That would be stupid.

You instead design ductility into the structural components such that the building bends like crazy but doesn't fall down. Adding a SF to E would make no sense as the building is designed to yield at perhaps 0.333E or less.

 

[weab]

I understand that seismic is an ultimate load, and yet you can still factor it down to some service level for design purposes using ASD. I think it is counter intuitive to include 1.0 factors for anything but service level loading. To me, this implies that there is no safety factor of the load using LRFD.

Ultimately, this is no big deal, really. But this issue alone is the root cause of rnorth's question.

 

[JAE]

Once you read and understand the philosophy behind the derivation of seismic demand values, intuition ceases to be an issue.

But your point is respectfully taken.

 

[steellion]

It is ambiguous enough that clarification should be provided in the construction documents clearly stating what the design forces are, especially if designed using ASD if seismic loads control. If I were on the fabrication side of things, I would assume that "unfactored" meant 0.7E and not reduce it further.

 

[rnorth]

thanks for the input. helpful to hear opinion of others.