Direct Analysis Method for Response Spectrum Load Case

[if.franco1139]

Good evening. I'm currently trying to implement AISC's direct analysis methodology in Robot.

The requirements for this method as stated on the AISC 360-10 document are:

• Calculation of axial, flexure and shear deformations
• Modelling of geometric imperfections
• Reduction of stiffness due to residual stresses
• K=1
• Second order analysis (P-Delta)

I have found that Robot is completely capable of fulfilling the requirements for both static and dynamic analysys, but it is not clear to me how to implement the requirements for a response spectrum load case and seismic combinations.

Now it is important to recall that response spectrum analysis is classified as linear dynamic analysis, and it uses the results provided after a modal analysis for which the algorithm the program uses, I asume, is of linear nature.

In Sap2000 you can create a nonlinear analysis case and use the resulting stiffness matrix for any linear analysis case, including a modal case and a response spectrum case. Is there any way to do this in Robot?

[Artur.Kosakowski · Accepted solution]

In Sap2000 you can create a nonlinear analysis case and use the resulting stiffness matrix for any linear analysis case, including a modal case and a response spectrum case. Is there any way to do this in Robot?

In Robot the modal analysis uses the stiffness matrix from the case (nonlinear as well) that is defined directly prior to it.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[rigid_joint]

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@Artur.Kosakowski wrote:

In Sap2000 you can create a nonlinear analysis case and use the resulting stiffness matrix for any linear analysis case, including a modal case and a response spectrum case. Is there any way to do this in Robot?

In Robot the modal analysis uses the stiffness matrix from the case (nonlinear as well) that is defined directly prior to it.

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But, how does this software attribute really work?

I have set up an example with a Linear and another one with a Non-Linear case.

On top node, I have placed a vertical Fz=-500kN and a lateral Fx=50kN load.

The top node displacememt results for both cases are right.

Linear Top Displacement = 4.608cm

Non-Linear Displacement = 5.440cm

So, how can I import the modified matrix that it is derived by the Non-Linear Case and run a modal analysis with this modified matrix?

I have already read the post#2 here http://forums.autodesk.com/t5/Robot-Structural-Analysis/P-delta-analysis/m-p/3753044/highlight/true#...

but I cann't make it happen so far. All the times Period stays the same

Here are some screenshots of my models:

Linear

Non-Linear

[Artur.Kosakowski]

Non-Linear

Having case 1 selected press the Change analysis type and set Modal taking into account static forces.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[fareed12]

Is possible to calculate Torsional Amplification factor (Ax ) by dynamic forces from (Response Spectrum Analysis) or need to make procedures to do it? I mean can I get max displacement and avg displacement directly from result 

Dynamic force Response spectrum analysis and used to get Ax=[Umax/1.2Uvag]^2

[fareed12]

Good day I know Robot do not calculate (Ax)direct I mean I can get result direct from Response spectrum (Uavg &Umax)
Because when we Design use dynamic analysis we can not use (Uavg&Umax) directly but need make procedures Excuse see this link(https://wiki.csiamerica.com/pages/viewpage.action?pageId=4554971)