Portal Frames in Etabs

[odysseusky]

If someone can shed some light on this, I would really appreciate it.


I have a structure where 4 levels of concrete slabs are supported by several identical portal frames (also concrete), and each floor is identical, so why do the bending moments of the beams get bigger as the levels go up? Shouldn't the bending moments of beams on each floor be the same? because apparently the loadings on the them are the same for each floor.

here's a couple screen shots to illustrate the problem the

http://imgur.com/a/IXIpz

ive also attached the etabs model, if anyone wants to see whether there's any error in the model that's causing this.

thanks a lot in advance

 

[prasadrmb]

As I think this could be due to the elastic shortening of columns. Axial load of columns increases with towards bottom of the structure. When axial load is high, there may be higher elastic shortening. Then support moment will reduce. When we go up, axial force decreases and shortening reduce. Then moment will increase

In addition, at upper floor, columns are not continuing. Therefore, beam/column moments will increase.



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