Response spectrum Base reaction vs Story Shear 1

[s3ndy]

Hi everybody,

I created a ETABS model of 40-story building with 8-story podium, analyzed with response spectrum case and tried to find out the response spectrum base shear.
Should I use the value given in the 'building modal info > RS base reaction' or 'display > story shears (at base)' because both are totally different? Which one is correct?(I summed up all the point support reaction but it wasnt the same either).Is it because I modelled the podium too?. I noticed that the value of story shear decrease significantly at the 8th sty (top of podium). Why is that so? (I think the story shear should increase from the top story as it goes down to the base.. correct me if I'm wrong..)

I'm looking forward to your reply, guys.. thanks a lot..

 

[zeemas]

For Understanding scale factor between RS and Static, please check the old threads.

Regarding shear reduction at the 8th story is because of you modeling, you must be modeling walls at all around perimeter up till 8 floor.
The shape and magnitude of the shear and moment diagrams below LEV 8 are a function of the relative stiffness of the LEV 8 diaphragm, basement walls, VLLR system and the support conditions. In order to obtain a realistic lateral load distribution below LEV 8 the diaphragm flexibility must be considered in the analysis. Do not assume that the diaphragms at and below LEV 8 are rigid.

I hope it will help you if this is the case, otherwise let me know.

Zeeshan

 

[slickdeals]

@Zeeshan,
Which country do you practice engineering in? Email me at deviationz (at) gmail (dot) com

 

[s3ndy]

Hi zeemas,

thanks for the reply. Now I am still rerunning my ETABS model using the flexible(semi rigid) diaphragm at the podium story.

How to obtain the response spectrum base shear in ETABS? Should I use the value given in the 'building modal info > RS base reaction' or 'display > story shears (at base)' because both are totally different?

 

[s3ndy]

Hi zeemas,

I have rerun my ETABS model and it gives me reliable base shear result. Thanks for your helpful advice.

The story mass at 8-story podium) becomes much smaller that that in the previous model (when rigid diaphragm was still applied)...What is the reason?