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# special moment resisting frame and dual sustems

## special moment resisting frame and dual sustems

(OP)
hi all

It will be the first time I use the special moment frame in order to stabilize the building consisting of 2 floors I am working on. I am reading some literature about this topic. One of the questions which comes to my mind:
If there is a possibility to add some walls here and there, will it become a dual system in condition to detail the walls as special structural walls ? If the frame should resist 25% of the lateral force, does the wall have any requirement regarding the percentage of force they must be able to resist ?

Thanks

### RE: special moment resisting frame and dual sustems

#### Quote (structural87 (Structural)(OP))

...... will it become a dual system in condition to detail the walls as special structural walls ? If the frame should resist 25% of the lateral force, does the wall have any requirement regarding the percentage of force they must be able to resist ?

The requirement for dual system , the moment resisting frame shall carry at least 25% of the lateral load. The wall can be OSW or SSW. Pls look the code Table 12.2-1 Design Coefficients and Factors for Seismic Force-Resisting Systems, DUAL sytems, D and E.

### RE: special moment resisting frame and dual sustems

#### Quote (structural87)

If there is a possibility to add some walls here and there, will it become a dual system in condition to detail the walls as special structural walls ?

That's right, although the system is meant to work with the walls as the primary lateral system and the moment frames as a backup system of sorts.

#### Quote (structural87)

If the frame should resist 25% of the lateral force, does the wall have any requirement regarding the percentage of force they must be able to resist ?

There is no requirement for the percentage going to the walls other than it should be in proportion to the stiffness of the walls relative to the stiffness of the other lateral elements employed.

### RE: special moment resisting frame and dual sustems

(OP)
Thank you guys.
In a moment resiting frame, ignoring the slab out of plane stiffness is totally OK right ? So no need to detail the diaphragm slab correct ?

Assuming another scenario, If beams are omitted, is it still OK in assuming the slab as a membrane ( transferring only in plane forces (tension, compression) ?
Thanks for the help

### RE: special moment resisting frame and dual sustems

#### Quote (structural87 (Structural)(OP))

In a moment resiting frame, ignoring the slab out of plane stiffness is totally OK right ? So no need to detail the diaphragm slab correct ?

Yes..the slab could modelled assuming semi-rigid diaphragm and should be modeled using shell elements which are semi-rigid in plane, and the out-of-plane stiffness should be modified to near-zero.

#### Quote (structural87 (Structural)(OP))

Assuming another scenario, If beams are omitted, is it still OK in assuming the slab as a membrane ( transferring only in plane forces (tension, compression) ?
Thanks for the help

If beams are omitted, the remaining system for the seismic loading will be essentially shear walls . Still one cannot assign response modification factor for RC slabs. Check your local code...some codes allow the use of hidden beams which are ductile detailed for flat slab systems. However, in order to satisfy drift requirements, you will need shearr walls which will be primary seismic load resisting system.

### RE: special moment resisting frame and dual sustems

(OP)
So whenever you have flat slab, even with assigning only in plane stiffness, will you still need to detail your slab in terms of chords, collectors and anchorage to the vertical elements?

### RE: special moment resisting frame and dual sustems

#### Quote (structural87)

So whenever you have flat slab, even with assigning only in plane stiffness, will you still need to detail your slab in terms of chords, collectors and anchorage to the vertical elements?

In general, yes. Whether moment frame or shear walls, the diaphragm is performing an important structural function and ought to be designed for the forces induced in it. This is probably even more important than usual in a dual system building because you're passing a lot of forces back and forth between systems via the diaphragm. The one caveat that I'll add to this is for the kind of building where every column in the entire floor plan is part of a moment frame. Diaphragms demands do tend to be quite modest for that kind of buildng.

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