Results of SRSS Manual Combination

[Anonymous]

Good evening,

I noticed strange values in the Reactions Table when I defined Manual Combination contains seismic cases with Quadratic Combination (SRSS) option activated in the Combination Parameters. So I defined manual combination contains only one case a seismic case as I said with quadratic combination (SRSS) option activated. I expected that the results in this combination will be similar to the only case I made the combination of, but it wasn't the same in all fields. These screenshots may illustrate my point more clearly.

Where does the value in Sum of reac. in Case 9 (The Combination) come from? Mustn't it be the same value in Case 5?!!

And thank you in advance.

[Artur.Kosakowski]

Hi @Anonymous

I'm not sure if I understand the objective and the execution of this exercise. Could you attach the rtd file you were working with?

[Anonymous]

Hi @Artur.Kosakowski

Actually my main goal is to apply seismic forces in two directions on a symmetrical building using Response Spectrum Method according  to UBC 97. And I want CQC for Modal Combination and SRSS for Directional Combination.

So I defined two seismic cases in X direction and Y direction, and defined manual combination contains these two cases with the options as in the screenshot.

 

I couldn't explain the value in Sum of reac. for the combination! As I said even when I use one case in this combination I get the same strange value. Nonetheless in Stories table I get different value which is the expected value.

 

I want to know how the Sum of Reac. value is calculated  because what I got from other posts that this is the value considered as the Base Shear ( correct me if I am wrong! ). Also I want to be sure that defining the combination with these options in the screenshot is giving me what I want ( I mean CQC for Modal Combination and SRSS for Directional Combination ). And if there are other tips about this topic I would be so grateful.

Thank you for time.

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

Where does the value in Sum of reac. in Case 9 (The Combination) come from? Mustn't it be the same value in Case 5?!!

For the manual combination as you defined it I think they should be the same indeed. Investigating your model I can see what causes the difference - it is the scaling of the base shear. As far as it can be seen the scaling is done for the manual combination but somewhat differently. What is actually done needs the further investigation by the development team. 

If you need to define manual combination like this you may consider altering the load factor rather than scaling the base shear.

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.

[Anonymous]

Hi @Artur.Kosakowski

It's a shame actually if I can't use this feature I mean Scaling Base Shear in this application, it could save me further manual calculation. Anyway it could be done manually as you said.

Thanks again.