Hi!
I am new with Direct Analysis Method so I decided to run an example of a frame we saw in class with Staad Pro. The example in class was developed by a linear first order analysis with increased moments factors B1 and B2. Unfortunately, after running the DM in my model I didn’t get the same results. I really don’t know where is my mistake. But I noticed that the statics check for the combination of loads performed with DM does not verify equilibrium between loads and reactions. Can someone help me?
As an alternative to DM, I thought about running a P-Delta (small) analysis with reduced stiffness and notional loads. But how can I modify the elastic modulus E later on so as to check the code with the nominal modulus instead of the reduced? Is there anywhere in the parameters to define a new stiffness?
Thanks in advance. I attach the concerning model.
Rodolfo.
The AISC code suggests the Direct Analysis to be performed by either magnifying the moments reported by the first order analysis (B1 - B2 method ) or by performing the rigorous iterative second order analysis( P-Delta Analysis).
The first approach is an approximate method, whereas the second one is an exact method based on second order iterative process. So, one can expect certain degree of deviation of the results generated by both methods.
Secondly, you have mentioned that you are trying to see the statics check result for the combination case.
But, it is not suggested to use the "Load Combination" for any non-linear analysis, so for the Direct analysis.
Instead of Load combination, try using the "Repeat Load" case.
Regarding the second part of your query, the idea is to manually reduce the the axial and flexural stiffness to account for the material non-linearity as specified by the DAM. So, you can either use the cracked section factor to reduce the A and I value or reduce the E value while performing the P-Delta analysis. But, there is no such way available in Staad to use one value of E for analysts and another E value for design operation.
For more detail on Direct Analysis Method, please refer to the following link
communities.bentley.com/.../10606.direct-analysis-and-its-implentation-in-staad-pro.aspx