Diaphragm Action

[KenStructural]

Hi Guys,

May I have your thoughts in my etabs model particularly the diaphragm action of floor plate (rigid diaphragm), assuming the lateral resisting system are the ones along the perimeter of the building and the rest of the frames are for gravity (internal frames). As shown on the attached sketch (Figure 1) is the axial response of the beams opposing the seismic loads whereas Figure 2 is the modified model having internal beams as gravity beams and to idealize the diaphragm action to transmit the seismic/in plane forces to resisting element (collector/drag-strut) and eventually to lateral resisting system what I did is to release the axial resistance (axial stiffness modifier = 0.01) of the internal beam thereby the seismic load is transferred to the outer frame which is the lateral resisting frame. Is figure 2 model is correct or acceptable.

 

[structSU10]

As long as the diaphragm is designed for the extra work it is doing, that approach makes sense to me.

 

[dcarr82775]

Are the internal frames really frames, or simple spans beams/girders? Seems to me if you actually have moment frames, those frames will take lateral loads.

 

[MrHershey]

Assuming these are actual frames with no moments released, I would tend to design for the moment they'll actually see. You can pick the external frames to be your lateral system if you want and design them for higher forces, but the structure won't really care what you want when it actually sees an earthquake. It'll react however it reacts, so you'll need to make sure you've provided the capacity for it to do so.

 

[KenStructural]

Hi Guys,

Thanks for the input I appreciate it,
@dcarr82775, the internal frames are frame (physical), my intension is that the lateral frames are the ones along perimeter and the internal frames are just gravity frames (gravity beams and columns), please note that the stiffness modification factor for lateral frames are as follows 0.7Ig for columns, 0.35Ig for beams and 0.25Ig for slabs, and the stiffness modification factor for gravity frames (internal frames 0.001Ig for columns and 0.35Ig for beams and also the axial stiffness modifier for gravity beams is 0.01 this way the external frames soak up all in plane forces coming from the diaphragm as shown attached.