Drag Members

[rn14]

I noticed in the webinar videos the beams between lateral frames are assigned to "gravity". Am I wrong thinking these beams should be "lateral" since they will act as a drag member to get load from the diaphragm to to lateral frames?

 

[Shotzie]

My guess is that the load will still get into the frames regardless of whether the drag members are there or not, and the drag members are excluded from the lateral system in the model to prevent the drag members from affecting the stiffness of the lateral system. I believe sometimes drag members need to be designated as lateral for the sake of getting the diaphragm to correctly load different shear lines. For example, if you know one shear line should see more load than it is in the model then adding a lateral drag strut may allow you to get more load into that shear line.

This is somewhat speculative, so hopefully some members more familiar with the software chime in.

 

[JoshPlumSE]

It depends on what the webinar is trying to emphasize. It may be focused more on the gravity side than the lateral side, so it isn't focused on that sort of thing.

If the diaphragm is to be a "flexible" diaphragm then it is VERY important to make those drag / collector members LATERAL so that the lateral force distribution will be done properly in the RISA-3D model.

Similarly if the diaphragm is "semi-rigid" then it is a good idea to have the drag / collector members in the lateral model. Though it's not a catastrophic problem if they are not in there like it would be for the perfectly flexible diaphragms.

If the diaphragm is "rigid" then it doesn't matter. The diaphragm is going to take the load to the walls or frames regardless of whether those drag/collector members are in the lateral model or not.