Expert: Arshad Khan - 10/19/2010

Question
QUESTION: dear sir,
as per my previous question dated 15-10-2010 about twin towers on etabs.my movement joint is NOT within basement.the joint is only between the towers above ground floor level.u told me that i should model all system in one model,great, but u told me to make individualy every building alone, i didn't understand!!.
and when i model the whole structure in one model , i get 2 periods (Tx and Ty) coming from the mass participant for the tallest building (on these 2 periods the lower building dos not move) these 2 periods are then used to calculate the static base shear wich is important to calculate the scaling of results right? so if i made 2 individual model why modeling then one model including the whole structure?espacially that i need dynamic analysis for the whole ssystem
again and again sorry and millions of thanks sir

ANSWER: Hi Robert,

I think you need to refer to the local applicable code for this. It is possible that the code stipulates that you use the results from the taller tower and apply that to the whole system, including basements and lower tower.

The alternative would be to generate 2 models, assuming a joint through the basements and then apply the individual base shears. In order to best simulate actual behaviour, it seems logical that you should place the joint closer to the smaller tower so that the mass of the basements attached to each tower is proportionate (same proportion for each) to the mass of the tower- hope you understand.

---------- FOLLOW-UP ----------

QUESTION: dear dir,
i'm using the IBC 2006 code, what do u think according to your great and large experience, is it right to make one model only?

if i have to make 2 model can i make the base shear from ground level without a joint through basement? my basements are rigid enough i think

thanks and respect

Answer
Hi Robert,

I am not too familiar with IBC 2006, but using my engineering judgement, I would say that you should introduce a joint at the location I suggested (i.e. nearer the smaller tower, so both have proportionate masses attached as basements). You should certainly not consider the base of the towers at ground floor level, unless the basements are in bedrock and there is no chance of any lateral movement of the floor slab/diaphragm at ground level.