Risa moment frame-Shear

[xkcstructural]

When you perform shear wall design you go from roof down and do tributary area loading at each diaphragm elevation and as you go down you add these up. When you are designing a moment frame in RISA do you still need to add up the shear at each elevation as you go down, or can you just put in the shear that the diaphragm experiences at each elevation and the program knows to sum these? I think its the latter because your reaction at the bottom should sum to be the total which Risa does, if you compound the loads as you go down then the reaction is way bigger, but I am being told that I still need to compound and add them as I go down cause the program can't figure it out.

 

[trevorshiloh]

My understanding is that the program creates transient loads applied to each member based on the diaphragm loading at each level, so you wouldn't compound them as you go down. It should be easy to verify by hand whether the reactions in Risa match what you expect when you add up the diaphragm loads at each level.

By the way, I think if the program required you to compound them at each level, the reaction at the bottom would always be 0.

 

[Celt83]

You also need to include the overturning moment generated by the shear wall above the level you are looking at otherwise the two systems are not statically equivalent.

What is the context in which the person told you this, the statement may be almost correct if you are looking at a piece of the frame in isolation but then you would also need to apply the tension/compression and any moments at the joints from the frame above the isolated location. Need to maintain static equivalency.

If you are modeling the full multi-story frame and you were told the above then that person has no idea how the direct stiffness method functions.

 

[trevorshiloh]

Yeah, that's what I was thinking. If these frames/levels are modeled individually it would make sense to do a 1-for-1 transfer of all the forces from level to level, not just shear. There will be no way the program will be able to "figure it out" between different models. When doing design of individual elements such as shear walls this also applies, as what Celt said above.

 

[driftLimiter]

As with all models, it is prudent to check the summation of forces in each direction at the base level.

A quick verification that the base reactions sum up to the applied load at each level can demonstrate what others are saying here.

These verifications are crucial, especially when using the program to generate the loading.