Lateral Design of CMU Core - Risa 3D

[MJC6125]

My question here is pretty specific to Risa 3D, but anyone might be able to answer based on what you do in other programs or even with hand calculations.

I'm designing a CMU core in Risa 3D. It's a box with 4 sides/walls to it. Because these walls are connected at their corners, Risa 3D will transfer some of the in plane vertical overturning forces of the walls in one direction into the end walls that run the perpendicular direction, and the whole core acts like a tube. As long as the end walls still have net compressive loads, I think this is fine, and the program will design those walls correctly. If however the overturning forces are large enough to cause net axial tension in those end walls, Risa 3D notes that it does not design these walls for net tension. How do others typically approach this situation when designing a CMU core for lateral loads, do you simply hand check to make sure the end walls have enough reinforcement to resist the axial tension load and call it good (in addition to required bending steel of that load case)? Or do you try to force the program to break those walls at the corners, so you're only relying on each individual CMU wall for lateral design and not transferring loads between the walls. Transferring load between the wall corners sounds more accurate, but I would assume it's conservative if you don't rely on it.

Another related question, are there any other checks that you need to do by hand that Risa 3D wouldn't already be checking if you are going to rely on the connection to the perpendicular walls? I.E. transferring the axial overturning forces to those end walls I think would be like a shear flow check. If Risa has already checked the in plane wall for shear design, and you match that reinforcement in all of your walls, does that satisfy transferring the overturning force to those end walls because it would be the same or less unit shear transferred at the perpendicular wall joint?

 

[JLNJ]

If it's a box with four sides, I'd absolutely design it as a tube where one "flange" is in tension and the other is compression. Got a couple of these being built right now.

You need to be a little careful in that these are often stair or elevator shafts with door openings which may or may not line up floor to floor. And, if it's a stair, the openings won't be in the middle of the long walls where you would like them.

I am a RISA3D user and have been for 20 years; however, I don't use it for masonry. Too much "black box" and not enough load path engineering.

 

[JoshPlumSE]

You can introduce a short "gap" between the walls to avoid the vertical shear transfer between them. That seems the most practical and simple way of addressing the problem.

 

[JLNJ]

If you ignore the "flanges" you are giving up 2/3 or more of the wall's inherent flexural strength. Seems like a lot to give up just to get RISA to calculate some simple rebar quantities for you.

 

[JoshPlumSE]

If you ignore the "flanges" you are giving up 2/3 or more of the wall's inherent flexural strength. Seems like a lot to give up just to get RISA to calculate some simple rebar quantities for you.

I don't know the height of the structure. But, I'd argue that in low rise structures (less than 3 stories), you're not giving up much.

Now, for mid or high rise structures, it would be a big deal. But, aren't those usually cast in place concrete instead of masonry?

 

[MJC6125]

Thank you for the responses.

My project is 3 stories of CMU shearwalls on top of 3 stories of Concrete. I was going to look at the concrete section of the core like a tube, so I figured it would be a good project to try to look at the masonry section like a tube also. But maybe I'll try the short gaps in the masonry walls and see if they all still work reasonably and go with that design.

@JoshPlumSE, if you were to try to design a masonry core like a tube, how would you approach the design in Risa 3D. Do a separate hand calculation for the vertical reinforcement in the "flange" walls that have net tension? Is there anything else you can think of that should be checked? I feel like the shear checks should already be satisfied based on the in-plane shear check in the "web" walls. Because the peak shear stress of the in plane "web" walls will be greater than the shear stress at the joint between the "web" and "flange" walls.

If anyone knows of an example calculation of this type of masonry shearwall, I would be interested in learning from that.

 

[bones206]

One approach you could take is to set the boundary condition of the tension flange wall as "free", then add nodes at your actual wall reinforcement spacing as pin supports. That would give you more realistic tension forces in your end wall bars.

The downside is having to modify boundary conditions for each lateral load direction. Maybe a workaround is to set the BC's for all 4 walls as described above, but include intermediate nodes between the "rebar nodes", set as very stiff compression-only springs. But that would make the analysis nonlinear, which isn't a big deal except if you want to be able to transfer reactions into RISAFoundation.