Return to index: [Subject] [Thread] [Date] [Author]

RE: notional load method of second-order analysis

[Subject Prev][Subject Next][Thread Prev][Thread Next]
>in order to take column buckling lengths equal to unity
>and use AISC-ASD design procedures, can we use
>pdelta analysis taking notional forces into account
>arising from imperfections.
 
That depends. The answer to your question is fairly complicated and depends upon just how extensive your second-order analysis is. An excellent reference that will give you guidance on this is ASCE's Effective length and Notional Load Approaches for Assessing Frame Stability: Implications for American Steel Design. See more about it here:
 
 
The AISC Specification requires that second-order effects be considered and provides some guidance as to how to do it. One method is the B1 B2 method. That is good for understanding it, but somewhat more suited to hand-calculations.
 
Some software programs "do" second-order analysis. Some may even do it correctly. Second-order analysis involves both the P-big Delta effects (frame deformations) and P-little delta effects (member curvatures). Since computer programs only track the nodes, the P-little delta effects may be lost or neglected. I've "tricked" software into considering P-little delta by placing an extra node at the mid-height of the columns in the model.
 
Other approaches, such as the notional load approach can be used and are covered in great detail in the reference given above.
 
>shall notional forces be included in p-delta
>combination in conjunction with dead and
>live loads
 
Simplifying things far too greatly, the notional load method relies upon the use of notional or fictitious lateral forces on the frame in addition to the other loads present. Since you are trying to account for the increased column moments due to displacement of the frame and the vertical loads acting through those displacements, I believe the answer to your question is yes.
 
>if buckling lengths are taken as unity with
>an appropriate analysis, AISC requires max
>KL/r as 200 for columns, how can this be
>checked as taking k=1,or is this requirement
>no longer valid.
 
The recommended Kl/r limitation is still applicable. >From AISC LRFD Specification Commentary Section B7, this recommended limit is “based on professional judgment and practical considerations of economics, ease of handling, and care required to minimize inadvertent damage during fabrication, transport and erection.” It is further indicated that this requirement is not strength related.
 
Charlie