Multi-tower building with common podium in Etabs

[Wajahat Latif]

I have a question regarding modeling a multi-tower building with a common podium. The building is 12 stories high and the podium is present in the first 3 stories only. Both towers have the same height. When modeling this in Etabs, should I consider the podium, tower 1 and tower 2 to be three different "towers"? That is, tower 1 (i.e. podium) would be modeled till the 3rd floor and then T2 and T3 would be defined from the 4th floor to top. 

 

I would then find the base shears of these three towers separately and then perform a 100% match with the response spectrum base shears for each of the three "towers" separately.

 

Do you think this is the correct approach? 

 

Also, do you think it's reasonable to connect the two towers with a single rigid podium? Alternately, we can provide an expansion joint in the podium instead and model the two towers separately.

[Wajahat Latif]

Here is a response that I received from Engr. Aung, Director AIT Solutions. Sharing for everyone's knowledge.

"You can consider the podium, Tower 1 and Tower 2 to be three different "towers", using multiple towers option. T3 is podium (Foundation to 3rd floor), T1 and T2 are Tower 1 and 2, starting from 4th floor to roof.

 

However, it is suggested NOT to use ELF base shear from analysis if you have multiple towers in the model.

 

Normally, ELF base shear for each tower above the podium is calculated manually for scaling purpose only. To determine the base shear for scaling of two towers resting on a common podium, it is suggested to compute the design base shear of each tower above the podium (using weight of tower above podium) based on code-specified equations manually.

Then, scale the base shear of each tower above the podium in a 2-tower combined model in different response spectrum cases.

You will have two sets of response spectrum cases: RSX-T1 and RSY-T1; RSX-T2 and RSY-T2.

Use 2-tower combined model to design each tower with corresponding response spectrum cases.

For podium design, you may use the envelope of those cases.

 

For application of wind load in multiple towers, it is suggested to turn on "Allow multiple towers" in the Option menu and assign the diaphragm names separately for each tower.

If the story height of each story is different between two towers, it is suggested to crosscheck the wind base shear of each tower above the podium with manual calculated results.

 

It is okay to connect two towers on a common podium.

It is suggested to check the in-plane forces in podium diaphragm due to movement of towers under lateral loads.

 

If the seismic gap is allowed at the amenity podium floors and no constructability and maintenance issues, you can also provide the seismic gap and analyze the towers separately."

[Badar (BAZ)]

That is a well put response. On which forum did you get this response from?

[Wajahat Latif]

I emailed him. I'm yet to implement this since the building I'm working on no longer shares a common podium now; an expansion joint has been provided. 

However, in reference to making two sets of response spectrum cases (for both towers), I didn't find any option in Etabs where we could explicitly do that. I suppose this only means that after we create 4 regular RS cases (RSX-T1, RSY-T1 and RSX-T2, RSY-T2), we scale RSX-T1, RSY-T1 to match the manually calculated base shear for tower 1 and design this tower against these load cases. Similarly, we do the same for tower 2 by scaling (against tower 2 base shear) and designing against RSX-T2 and RSY-T2. What do you think?

[Badar (BAZ)]

Do you study in AIT? How do you know him?

9 minutes ago, Wajahat Latif said:

I suppose this only means that after we create 4 regular RS cases (RSX-T1, RSY-T1 and RSX-T2, RSY-T2), we scale RSX-T1, RSY-T1 to match the manually calculated base shear for tower 1 and design this tower against these load cases. Similarly, we do the same for tower 2 by scaling (against tower 2 base shear) and designing against RSX-T2 and RSY-T2. What do you think?

That is right. Not only that, a separate set of load combos are required as well for each tower. But, since you have provided a seismic gap, this does not matter now.

[Wajahat Latif]

Thanks.

No I haven't studied at AIT. I attended his online lecture on 8th April that was advertised on AIT Solution's facebook page. Attaching the poster here. 

[Badar (BAZ)]

ok.