Weak Storey 1

[GalileoG]

I am working on a 6 storey reinforced concrete building where the first floor height will be almost three times that of the other floors.

I am worried about having a weak storey especially given the fact that a lot of my bottom storey walls will have major openings (excluding the core shear walls at the elevator.)

What is the best way to compensate for this loss of stiffness? Would flanging my elevator core walls really give me that much of a boost in stiffness? Increasing column sizes? I am trying to come up with a reasonable solution for the architect.

I guess the best way is for me to run a couple of trial runs on my analysis software and compare lateral deflections. Before I do that, I am curious as to how you folks usually compensate for stiffness when you come across something like this.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[kslee1000]

I think utilizing as many walls, in a ruly manner, as you can, is your best shot. Columns are important, but have limited contribution. Try a few runs with multiple of unit "I's", you will grab the idea very quick and clear.

 

[civilperson]

Add right angle walls to major axis shear walls for additional stiffness.

 

[GalileoG]

Civilperson, you mean to flange my walls?

I agree. I think this would really help. I won't be able to run my trial runs until Monday though.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

Three times the height of the other floors? Wow, that is some hefty storey! You need some serious stiffness in that first floor, Clansman. It is difficult to suggest anything specific when you provide no specifics. How about a plan of the main and typical floor?

BA

 

[hokie66]

When you have a building like that, you really have to look at it as a 5 storey building atop a podium structure. The two are totally different, and deserve separate consideration. Take the loads, both vertical and horizontal, from the 5 storey building and apply them to the podium structure.

 

[oxbridge]

The concrete floor stiffness (around the core) is considerably less than the stiffness of the lift core (as a whole), and therefore I've always been told to ignore any contribution it makes to that lateral stability (makes the calculations easier too). For me the floor is only relvant to the location of where the lateral load is applied. Obviously for buildings where the foors are equally spread vertically, the lateral load is normally applied as a UDL. So the only difference i see with your situation is that you'll apply more lateral load at 1st floor level. (Probably won't make much difference to the re-bar required)

Also, remember to increase the buckling lengths of your columns between ground and 1st-can significantly increase the column size.

 

[msquared48]

"Add right angle walls to major axis shear walls for additional stiffness."

The Architect will love this... DO IT!






Mike McCann
MMC Engineering

 

[slickdeals]

Given the height of the first story, you should be able to get a few hefty beams to act as a moment frame. I am assuming that this is a flat-plate structure with concrete shear walls for lateral support. You may need to use a dual system (shear walls + moment frames) on your first floor to get to the target stiffness you are seeking.

Like BA says, You will need some hefty columns to be able to participate in the lateral stiffness. Otherwise, small columns will only end up "going for the ride" with the core walls.

If you are in a seismic zone, you may incur more of a penalty due to the soft story effect.

 

[GalileoG]

Thanks all.

After unending negotiations with the architect, we settled with using a few deep beams and flanging some of my walls to achieve my target stiffness.

Thanks again for your help folks.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[KootK]

Because of shear lag, you would have to continue the flanged walls into the second story in order to count their contribution in the second story wouldn't you??