Coupled shear wall with pt flat plate design

[JONNH]

I am wondering what others thoughts are on 30+ story coupled shear wall building systems and their design. Typically, we design these in ETABS using shell elements for the flat plates and shear walls. We have found that due to the stiffness of the floor plates, they can greatly impact the building dynamic properties and force distribtuions. Typically, I have designed these types of buildings with two models, one for collecting lateral loads on the shear walls and one for gravity loads, and then post-processing this data for the shear wall design.
Typically, I find one of the most highly stressed elements are the coupling beams. As such, they are always one of the first items I look at, to make sure they are sufficiently sized for shear and moment, before the architect sticks us with an undersized link.
I have been reviewing a job that falls into this category, but there are no coupling beams. When you run the ETABS anaysis with the 8" slabs moedeled as shell elements building drifts and all seem to be fine, and the wall designs will be fine as well. But when I run the system ignoring the slab stiffness, the drifts shoot up. If I look at the stresses in the slab, they are well beyond allowable code limits.
How do others handle this in ETABS? Personally, for a building of this size, I am inclined to design the shear walls and coupling beams for the full lateral demand, and ignore whatever contribution the slab may have. Then once the lateral system is sized, design the for plate for gravity loads, and whatever lateral forces it may see due to compatability. Any thoughts?

 

[hokie66]

I agree with your approach. In my experience, you can't make a normal sized core work for a 30 storey building without coupling beams across the elevator lobbies. The C sections can work in the direction parallel to the elevator doors, but in the other direction, you have to have the coupling beams to utilize the T-C couple of the two sides.

 

[BAretired]

Have you tried reducing the height of the shear walls? In a tall building, the frames in the upper floors are forced to sway excessively by the large rotations in the shear walls acting as vertical cantilevers from the base.

Perhaps if you reduced the height of the shear walls, say by six or eight stories and allow the lateral forces in that region to be carried by frame action, the stresses in the slab would be more acceptable.

Another possibility is to tie the shear walls to remote columns at roof level using very deep beams. This has the effect of reducing the rotation of the shear wall at the roof. The problem is, the architect may not like it.

 

[ishvaaag]

I wouldn't have any problem in using the rigidity of the present flat plate stiffness to meet any requirement, as long they can be properly designed. This is mainly because where I practice (Spain) monolithic structures are the general norm, and most design the building structures for all the solicitations, vertical and horizontal in one model (or only with variations to accommodate allowed economies in say, columns).

However it seems to me difficult with such number of stories plates only 8 inches thick may meet their own demand in collaboration with to the lateral stiffness; hence some proper solution must be found, of which some have been named above.

 

[haynewp]

Is this for seismic or wind?

 

[sfat]

i did a similar building analysis using ETAbs.its a 26 storied building RCC structure.dual system is used.core coupling beams sizes were found critical at the lower levels.i have done the analysis using shell behaviour of 9" flat plate and checked drifts for this case.some people take membrane behaviour in which displacements are higher than what are in shell behaviour.but i think that in reality,building behaves with slabs' shell behaviour ( in RCC cast in place structures)so i have designed cores and columns according to that.and checked displacement values according to this also.
i have then designed slabs by SAFE for gravity loading by exporting it from ETABS model.

 

[slickdeals]

I was at a recent NCSEA winter institute and found two things very interesting in a presentation by Tracy Kijewka-Correa from Notre Dame who is collecting a lot of data from instrumented tall buildings.

It concludes that concrete shear wall/frame buildings are usually a lot stiffer than analysis predicts. Her research states that anywhere between 25-50% of slab is effective in trasmitting lateral forces.