Elevator Cores, Coupling Beams and Outbard Shear Walls 1

[mxpengr]

We have many projects where we have 2-3 PT floors of subteranean parking with 1-2 floors of PT above grade and then 5 floors of wood.

Most of these building have more than adequate shear provided in the perimeter basement walls and other exterior concrete walls.

The buildings also happen to have the usual concrete eleveator and stairwell cores.

When analyzing these structures they work without the core walls or with maybe the 3 sides that do not have openings in them.

The city reviewers require us to provide 100% shear walls with coupling beams around these cores (like a typical high rise construction project) even though they are not really needed for lateral resistance. We have done some with 3 shear walls and with lateral releases at the wall with the openings and they seem to be willing to accept that.

I am a little perplexed by this requirement and do not understand why we cant just do a normal wall at the core openings (even without releases)? I realize that these walls will not work laterally and may crack in a seismic event if the deflection is too great, but they have not been considered as part of the load path in the analysis and I would expect that we could make that decision without violating any code issues.

The reviewer hasn't been able to provide an explanation other than - thats just the way it is - and I am curios if anyone here could provide some insight.

Thanks!

Mark

 

[mxpengr]

Ooops! that shoud read "outboard" not "outbard"

 

[msquared48]

Hi Mark... So that's your handle...

Mike McCann
MMC Engineering

 

[csd72]

Maybe they are concerned that the core, being the stiffer element, will actually take all the load. They probably think that a dual system would be safer.

You may want to try and justify your decisions based on specific clauses of the building code, and submit this as an appeal against the decision.

 

[RareBugTX]

If that's what they want, give it to them. The nice thing about structural engineering is that there are so many options and approaches to one problem.That promotes creativity. Once I had a review with a Building Official. The project was a tiny 2 story residential building with new brick veneer facade. Wind controls here and I proved him that unreinforced CMU was enough for my shear and out of plane forces. He didn't buy it "You have to have some reinforced masonry the full height of the facade, He stated". Without looking into the details of your design, I see his point, I think is reasonable for him to have some shear walls at the core, but that's my take on it. Work with them, be nice , and come up to a solution that both satisfies you and them, remember, they are holding the pan by the handle.

Good Luck!

 

[mxpengr]

The problem with "working with them" is that is is expensive and often makes the walls thicker (12" +) which gave the architect fits (so what else is new).

I discussed this with Bijan Aalami of ADAPT (they design big towers all over the world) and he felt like it was completely unnecessary.

I was always taught that the design engineer selects a load path and so long as that is designed properly all the other components were simply redundant.

Relative stiffness is a good point and an exception to my statement above, but we have over 300 LF of shearwall (most of the surrounding basement walls) around the perimeter of the slab. I don't see any way the core would be stiffer unless you specifically design it to be stiffer.

Thanks for the feedback!

 

[mxpengr]

Hi Mike!

 

[msquared48]

And designing it to be stiffer will suck even more load to the small, perforated core, necessating a trial and error analysis.

It is much simpler to disconnect the core from the diaphragm and design the walls for their own lateral loads due to their own weights. This should be the structural engineer's design call as long as it meets code, not the building official. I think that there is a whitepaper on a similar issue from SEAW and I will post it when I find it.

Mike McCann
MMC Engineering

 

[msquared48]

Here is a link to that white paper - a non-binding agreement between SEAW and WABO...

Mike McCann
MMC Engineering