Base shear from ETABS 2

[Vonx]

Hi all, I've got a question about the base shear. After you do the response spectrum analysis in Etabs, how do you find the total base shear of the structure(multi-storey building to be more specific)? This is to work out the scaling factor. I was told that you can look at the "Story force" section of the 1st storey of the structure in the output report. Is that correct?

 

[stressed]

No, that's not correct. Under Display menu>show tables, checkbox reactions/support reaction. There is a "summation" base shear reported along with individual restraint reactions. You can control which cases are to be reported using the options on the right side of the show tables.

 

[drile007]

You can get Story shears:
1) Display/ Show Story Response Plots.../ Story Shears
2) Display/ Show Tables.../ Building Output/ Story Shears

Drile007

 

[Vonx]

Thanks guys, that helped.

 

[Vonx]

Hi stressed:
I was able to do the show table thing, thanks for that. However, I could not find the "summation" function mentioned in your last post. I had to manually add all the reaction forces together under same direction to find out the base shear. Would you please give some instructions on this one?

 

[stressed]

Vonx, apologies, I was mistaken in my earlier reply regarding what the "summation" option offers you. I have been working more with SAP lately and I gave you some misleading advice. Here is what you need to do to obtain the summed base shear that you want - First, select all joint restraints at the base of your model, as well as the first floor columns and walls which are connected to those restraints, and assign them all to a group. Next, use Define menu>Section cuts and define a section cut based on the group you just created. After you've run your analysis, using Display menu>Show tables>Section cut forces, you should have the summed base shears. Let me know if that technique doesn't do what you're looking for.

 

[Vonx]

Hi to stressed/drile007:

Thanks for the replies. I have done a comparison using both the story shear method and the section cut forces summation method to calculate the base shear. In my case, the values are very close:

Vx (story force) : Vx (section cut) = 1 : 1.03
Vy (story force): Vy (section cut) = 1: 0.98

As you can see, the differences are within 5%. I tend to believe both methods are valid in terms of calculating the base shear using Etabs. Please give your comments.

 

[GarthW]

Vonx

You should have a look at Display -> Show Tables -> Modal Information -> Table: Response Spectrum Base Reactions. They have had to introduce a special routine to determine base reactions since all results from Response Analysis are positive. Summing base reactions or column shears would be wrong.

GarthW