ETABS - Seismic analysis of load-bearing masonry walls

[FVandal]

Hi,

I’m working on the seismic analysis of a load-bearing masonry wall building with ETABS. How can I keep the masonry walls that provide the vertical support of the beams as vertical shell elements, while still do a proper analysis ? If I don’t use any trick the first vibration mode will be excessively low due to the stiffnesss of the walls. However, in real life they will be cracked and will provide no stiffness. It should be noted that up to now I’ve done a Linear Response Spectrum analysis.

I thought about removing the walls and creating ‘’falses’’ columns while adding line loads to consider the mass and it will be my last option. However, I would like to keep the wall shell elements.

Thanks !

 

[mike20793]

There will still be some stiffness associated with the walls even if they are cracked. Under gravity loading, they are likely to not crack, unless because of shrinkage. Even so, TMS 602-08 (ACI 530.1-08) Section 1.9.2 allows for stiffness to be based on uncracked properties. I don't think your client is worried about vibration when there is a lateral event and that's when the masonry would actually crack.

 

[mike20793]

Sorry, for some reason I thought of vertical vibration when you mentioned it, rather than the lateral mode shapes. My statement about using uncracked section properties still remains valid.

 

[wannabeSE]

Have you adjusted the stiffness properties?. The following link is for concrete, but the same concepts should apply to masonry:

https://wiki.csiamerica.com/display...ed+section+properties+for+shear+wall+and+slab

 

[FVandal]

Thank you for your answers,
I solved the problem by adding line loads at the perimeter of the model to take into account the mass of the masonry walls. Then, I applied the following properties modificators :

Membrane f11 Direction = 1
Membrane f22 Direction = 1
Membrane f12 Direction = 0.00001
Bending m11 Direction = 0.00001
Bending m22 Direction = 0.00001
Bending m12 Direction = 0.00001
Shear v13 Direction = 0.00001
Shear v23 Direction = 0.00001
Mass = 0
Weight = 0

With this the walls stiffnesses doesn’t influence the modal analysis results at all.