Beam Design in ETABS

[AniMhj]

Dear everyone,
I am modelling a building in ETABS. I have modelled shear walls, columns and beams for lateral load resistance. I am getting huge bending moments in beams with shear walls of length half the length of beam. Some beams are failing in flexure after design and check in ETABS.
By taking the bending moments from the model and designing manually, I get the increased size. By changing the size of those failed beams in ETABS, I get the increased forces (which are very high than previous) in beams and thus fails again.
My question is why the forces have increased significantly as overall load in the building does not increase much?
What are the possible solutions for passing those beams?

 

[BulbTheBuilder]

I am not familiar with ETABS but couple of things. Increasing the sizes of your beam increases the self weight so in-turn increasing your forces. Think of the direct stiffness method. Higher stiffnesses attracts higher forces.

Nonetheless, I’d recommend checking your beam in ETABS under self-weight first. If you see abnormalities in the result then check your boundary conditions or your modelling. Are you designing your structure as sway or unsway frame? I ask because you’ve shearwalls and yet you mentioned you’ve modelled columns and beams for lateral lid resistance

 

[structSU10]

My guess is as you make the beam bigger and stiffer the shear wall and coupling beam assembly attract more force and its a big feedback loop.

 

[milkshakelake]

Agreed with the two above. Increasing the stiffness of that beam will make it attract more forces. Try increasing the stiffness of other vertical lateral elements not part of this load path, like other continuous shear walls. Increase their thickness or concrete strength.

By the way, a situation like what you described is a vertical discontinuity in the lateral system. The beam may be subject to amplified seismic forces, at least in US code. So even if it passes at the end, you still have to use an amplified seismic force on it and it might still fail.

 

[AniMhj]

Thank you everyone for your replies.
As mentioned above, increasing the size of beam has only lead to increase in the stiffness of the beam thus attracting more force.
I will try to increase the stiffness of the whole system.