first mode period

[tohamy]

hello

I has been created a 2D concrete frame model consist of 9 storey and analyzed it using Eigenvalue analysis to get the period of the first mode shape but i think that the resultant period is very short = 0.292 sec, also i created a similar model using CSI sap program and the period was 1.02 sec !
is it logic ?? or what could be the problem??

[z.gronti]

Dear tohamy,

Please check for the correct modeling of the structure's mass and the settings within the Gravity & Mass tab of the Project Settings module.

[tohamy]

I assigned a concentrated mass at each joint for 2D frame and made the specific weight of materials equal zero to take the mass value from lumped mass only, also i has been choose from the option define mass from frame element and mass elements, is this wright or not?
can i send the model to any one to review it please??

[tohamy]

i want to note also that i need the first mode shape for the 2D frame to be in-plan (x-z) not out of plan

[huffte]

You can send the model to me at tim.huff@tn.gov if you like tohamy. I'll look at it as I find time.

[tohamy]

thank you very much huffte i will send it to you, put i have last question if i want do model the pounding between two adjacent buildings, should i assign the mass as lumped mass at each frame joins?? or it is not important??

[huffte]

Well, your question is a bit unclear, but I will attempt an answer, tohamy. Each frame joint should have the appropriate mass assigned and a lumped mass model would typically suffice in a framed structure with a large portion of the mass concentrated at the floors.. The two buildings would be modeled completely unconnected with the exception of gap links at the floor levels, for example.

[tohamy]

thank you but let me ask the question another way if i want to get the pounding force between two frames, should i represent the mass for frames as lumped mass to get true results or can i make the program calculate the mass from members specific weights and applied loads.
thank you again

[huffte]

I believe that Zoi was correct when she mentioned the mass in the model. Make certain that:

(1) you have the un-modeled mass of the floor slabs, etc. specified in your lumped masses; my suspicion is that this is the problem, but I could be wrong

(2) you can either include self weight of modeled beams and columns in your lumped mass calculation or by specifying the material densities and letting SeismoStruct do the work; either way should give reasonable and similar results

(3) ensure that the Young's modulus values in the two models match

(4) you can select only the x, z, and ry check-boxes in the global mass direction tab of the settings to ensure you only get the modes you want

Best of luck, tohamy.