×
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

two way concrete slab middle strip

two way concrete slab middle strip

two way concrete slab middle strip

(OP)
I have an existing two way concrete flat slab built in 1950s. Owner wants to bring the structure to current code for new occupancy. Stress levels work for flexure (shear is a different story), however the bottom rebar splice requirements have changed. I understand the column strip bottom steel must be continuous and I plan to add FRP or something to make that happen. Do I need to also make the middle strip bottom steel continuous since the ACI requires a 6" lap (effectively doing nothing). The old code used in original design has, consequently, a 6" gap in those bottom bars.

RE: two way concrete slab middle strip

In my opinion, it would be excessively onerous to add, and possibly fireproof, FRP just to satisfy the detailing requirements of the modern integrity steel provisions. I'd make a play for a waiver of those requirements, either from the client or the AHJ, whomever is mandading the upgrade.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: two way concrete slab middle strip

Mck26,

Are you sure about the capacity in the 2 directions. There was a long period in the USA where they did not design for full load carried in both directions for slat slabs. I am not sure when the practice changed but I thought it was around the 50's.

Though I agree that it is nice for middle strip bottom steel to be continuous as there is not much negative moment there and it could easily go into tension, I doubt that it would be required for Integrity requirements. I would think that only column strip reinforcement especially in the shear head zone would need to be continuous for that.

RE: two way concrete slab middle strip

Quote (rapt)

There was a long period in the USA where they did not design for full load carried in both directions for slat slabs. I am not sure when the practice changed but I thought it was around the 50's.

Assuming we are both talking about the same thing, whereby the total static moment for a flat slab/plate panel was less than WL/8, then it was later than the 1950's.

Here is an extract from ACI-318 in 1963:



I think this was permissible until the 1971 edition.

RE: two way concrete slab middle strip

Ingenuity,

Yes, that is what I meant, but I thought they joined the real world and recognised the theory of statics a little earlier than that.

There were lots of older flat slabs in the USA that were very under-designed, and I doubt that the steel was elastic enough in those days to allow membrane action to save them!

Looking at the allowed L/D ratios, they must have had deflection problems as well!

RE: two way concrete slab middle strip

Some of the very early (like 1905) flat slab systems (C.A.P. Turner, for example), before these slabs were codified, had significant issues:



With resulting deflections:

RE: two way concrete slab middle strip

(OP)
It was originally designed using the working stress method with an fs = 20 ksi. The steel used however has an fy = 40 ksi. Using strength design method in current ACI with Fy = 40, it works for flexure however punching shear fails. Maybe we will waiver to get exemption from certain requirements, but it is a strict client/AHJ.

RE: two way concrete slab middle strip

mck26,

Since your flat slab was constructed after the 1950's it is probably okay, but worth checking bond and anchorage, especially if justifying the as-built capacity based upon USD, for an original design by WSD.

From CRSI Engineering Data Report #48 'Evaluation of Reinforcing Bars in Old Reinforced concrete Structures'
:

RE: two way concrete slab middle strip

By way of follow up with regards to my post above re ACI-318 1971 and the total static moment. I checked one of my old references, and the 1963 code was the last issue where less than 100% of statics was code permissible.

From: THE DESIGN OF FLAT SLAB STRUCUTRES - AN HISTORICAL SURVEY by K.A. Faulkes:


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