Shear capacity of U-shaped shear walls

[Enhineyero]

Hi all, regarding shear capacity of a shear wall. Usually we assign as one pier (in ETABS) the RC elevator shafts of buildings giving it a U-shaped geometry. It is very advantageous in design of its vertical bars, since combining the walls will give you a much larger inertia, compared to designing the walls individually. however, how does one check the shear capacity of the section?

by theory, doing the same approach and designing it using its "over-all" geometric section should be correct. Doing so will yield a high shear capacity due to a large effective depth. I dont know if its the right approach, because somehow it does not feel right. may be because its giving a large shear capacity. Also how would one take care of the torsion as produce on the pier forces from ETABS.

I tried to find literature on how to do design of U-shaped shear walls, but cant seem to find one, all i can find is rectangular shear walls. can anyone suggest any reference or share their knowledge on this subject.

Any input is greatly appreciated

 

[ishvaaag]

The panel elements should have the proper shear stresses extant ... subdivide hence the faces for local precision of the shear stresses in zones of the U shaped shearwalls. This is more clear in FEM packages where to deal with this matter you simply use (and get results for) plates. It is certain that the reinforcement of plates having generic solicitations is a bit less than popularized but there is some literature about that. The fact that when plate elements and with some programs and not very careful ways of proceeding one could be forfeiting some relevant structural effects (second order deformations, potential buckling of any kind at some place in the cores) and lack of code guide on the matter doesn't help to include in the structural design programs a tool for reinforcement of plate elements including all elements of solicitation.

 

[slickdeals]

In ETABS, if you define the pier as a single pier, it reports the value of the controlling leg. In one direction, you have two legs and in the other direction you have one leg. Assuming you have the same shear in both directions, your controlling element will be the single wall pier and ETABS will report the same reinforcing for all piers.

Torsion in a core will produce shears in the wall, either in addition to or counteracting the wind/seismic shear.

 

[asixth]

The design of the U-shaped shear core will be the same as any other channel section. The flanges with take the overturning and the web will take the shear. The design of the web for shear will follow the code provisions for wall design.

The torsion which is created by the eccentric location of the wall or actions stemming from the torsional mode will need to be rectified by the walls and columns. If there is scope to close the core with a coupling beam will help more so because a closed-section is created (with openings).

 

[Enhineyero]

Thank you all for your replies,

Slickdeals - I assigned the whole U-section as one pier. what do you mean by the value of the controlling leg for the shear. For example the left leg has a value of 2kN, and the right leg 5 kN, ETABS will report in the pier forces only the force of 5 kN for the pier?!? I do not want ETABS to design for me. I prefer to do some manual calculations first.

asixth - how do i go about checking the shear capacity of the wall? using code formula what effective depth would be used?

does anyone have a sample calculation for a U-shaped core wall? It would be very helpful. thanks again

 

[slickdeals]

This is from ETABS's manual. Look in the documentation.

Pier/Spandrel Shear Reinforcing
Displaying the shear reinforcing data provides the maximum required area of
shear reinforcing for both piers and spandrels. For piers, shear reinforcing
areas are displayed for both the top and the bottom of the piers.
Important Note: For piers with multiple legs, the shear reinforcing is
reported for the worst-case leg.
For spandrel shear reinforcing, two types of shear reinforcing are displayed:

 

[asixth]

kN or MN. Those shear forces don't seem correct, 2kN and 5kN shear force would be work with crack control reo. From memory the shear calcs are based on 0.8*Lw where Lw is the length of the wall. I haven't got a code but I think the equations are 0.17*sqrt(fc)+ρ*As*fsy (in metric).

 

[ishvaaag]

Good, slickdeals, I am not practicing by ETABS and so didn't remember such specifics of when I touched that program.

Not just a shear issue, I still would be wary of reinforcements provided by programs for plate elements seemingly critical and with all components of solicitations (say, relatively thin shearwalls in proportionally tall structures and with high solicitations).

 

[Enhineyero]

Thanks for all the replies.

Slickdeals - thanks for posting that, i will try to experiment on the results of etabs using some practice models.

asixth - the forces i gave is just example forces, for the sake of discussion, they are not from analysis.

ishvaaag - Normally i use computer programs for structural analysis. the design i do by manual calculations unless i am confident that the design output of the programs are accurate and correct. That is why i would normally do a hand calculation first.