×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Weak Storey

Weak Storey

Weak Storey

(OP)
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.

RE: Weak Storey

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.  

RE: Weak Storey

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

RE: Weak Storey

(OP)
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.

RE: Weak Storey

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

RE: Weak Storey

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.   

RE: Weak Storey

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.  

RE: Weak Storey

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

The Architect will love this...  DO IT!

 
 
 
 

Mike McCann
MMC Engineering

RE: Weak Storey

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.

RE: Weak Storey

(OP)
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.

RE: Weak Storey

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??
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources