Shear Wall & Moment Frame Combined Lateral System

[mjs611]

I have searched through the threads and have not found anything that fits my situation. I have a building in which one side has 2 shear walls while the other end has a moment frame. They are connected by a rigid diaphragm meaning I have to find each lateral resisting systems relative rigidity to proportion my lateral loads. I know how to find the rigidity of the shear walls using a simple formula and a moment frame by applying a unit load. My question is how do these relate to each other. Being as they are both different materials.

If anyone knows how to relate the rigidity of a moment frame to a shear wall I would appreciate it. Thanks for your help!

 

[JAE]

Once you find the relative stiffnesses of braces or shearwalls, the structural elastic behavior (and load sharing/distribution) does not depend on the material. Stiffness is just stiffness.

If the brace/wall goes inelastic, then there may be differences in how the load is "soaked up" after the brace/wall cracks or deforms.

 

[csd72]

If you only have 2 shear walls/frames then the stiffness doesnt matter, the load will distribute same as a simply supported beam.

If you have 3 or more walls/frames then the relative rigidity does matter.

 

[mjs611]

JAE:

That does make sense and I do understand the difference now. The issue was that I was using formulas that did not base the rigidity on a unit load rather on h/d ratio and material properties. I found it much easier and more accurate to model each frame/shearwall individually in Risa with a unit load, then take the inverse of the deflection. Thanks for your help.

 

[whyun]

csd72,

When you only have shear lines at each end and none in between, rigid diaphragm will induce torsional moment that will distribute based on the moment arm between the center of mass and center of rigidity (plus accidental torsion due to eccentricity).

One way to see this is in lieu of having moment frames at one end (low stiffness) and shear wall on the other (high stiffness), think of shear wall on one end and essentially no lateral system on the other.

For the original post, question about how the force is distributed is valid, but what is important is when mixing different type of systems in the same direction, the seismic coefficient determined using the system with lower ductility shall be used throughout.

If the lateral force is wind, I agree with csd72.

 

[csd72]

Good point whyun,

What you say is 100% correct if you have 2 perpendicular walls that can provide twisting resistance, otherwise your forces will be distributed proportionately and only your deflections/drift will be different.

 

[whyun]

Even with the absence of perpendicular walls/frames, the lateral elements parallel to the seismic direction will resist torsion. Without doing any math, I predict that the frames will take larger torsional shear compared to the walls, however, I can't predict how considerable it will be relative to the direct shear.

As long as the distribution is done properly and the forces at the frames penalized/amplified, the design should be permissible provided that the structure meets the maximum bound for a torsionally irregular structure.

 

[d2783]

Hi guys,

I have a unique situation.
I have a five storey building with basement perimeter shear walls & moment frames for the rest of the structure.
I am modelling it in ETABS.
Does anybody know how do I perform the analysis with different Response modification factors (R) in one model.
Does anybody have ever expirienced this issue.

Thank you.

 

[csd72]

d2783,

It is bad karma to jump onto someone elses thread, you should start another.

csd