deflected slab/beam 1

[bazflexure]

Hi guyz!
Has anybody got idea as to how can deflection be reduce of existing slab/beam.
I have a situation where floor system has enough deflections to disturb the servicibility of building.

 

[Zambo]

http://www.nce.co.uk/carbon-fibre-used-to-reinforce-burj-dubai-floor-slabs/478317.article

 

[rowingengineer]

Bazflexure, We will need more information to be of assistance.
As few things I am sure we would like to know,
What is the existing slab beam system?
How old is the structure?
How does the slab deflect? do you have a survey of the floor slab?
why is the deflection a problem, do we need to reduce existing deflections?
how many stories?
Because if we go general on a situation like this you will get many different answers but not a lot of help.

case in point my suggestion would be to prop the slab with more columns, but can we added extra columns?

Tell us the constraints, and what size the project, and then we can help maybe a little.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[Tomfh]

Anyone know more about the Burj Dubai deflections? I never heard about that issue...

 

[asixth]

Tomfh

There was a write up in "New Civil Engineer International" May 2007 issue. I have it with me so I will scan it in and post it tomorrow (I am at home now and don't have access to a scanner). It was a different world back then, everyone had money and wanted to build tall iconic buildings. What a difference 2yrs can make. Is the Burj nearing topping off? It has seem to have fallen away from media attention.

Bazflexure

More information is required, are the deflections excessive in the slab of a flat-plate floor system?

What is the serviceability failure? My building at work has massive curvatures in the floor slab that are noticeable when you walk over them but everyone has seemed to accept them.

 

[bazflexure]

Hi rowingengineer ,
thanks for the reply.This is some information about the project.

-office building.Rehabilitation required.
-Floor system is reinforced concrete slab-beam.
-Deflections are due to creep/shrinkage means longterm deflections.
-ofcourse there are few cracks in beams.
-I need to reduce the defections and increase its flexural stiffness/strength.

 

[patswfc]

I've been told by another engineer (can't back this up with a reference although it seems plausable), that the floors had been designed as PT slabs, however in a late design change they were changed to normal RC slabs, but the designers forgot to increase the slab depth, thus had deflection problems.

The link above states that the remeidal work was carried out to all floors even though it was noticed during the construction. They most likely couldn't rectify the remaining floors by increasing the floor depth as the columns and founds wouldn't be able to support the increased dead load.

 

[BAretired]

You might be able to add steel stiffening beams at strategic locations, but you would need clear space under the slabs which you probably don't have.

External stressing is another possibility.

BA

 

[bazflexure]

what do you mean by external prestressing

 

[hokie66]

External prestressing means placing tendons outside the existing concrete profile in order to change the existing stress condition of the concrete element and reverse some of the deflection. So tendons under a slab would be anchored at the ends and jacked to put an upward force on the slab. This may or may not have an application in your structure. You might want to consult with a post-tensioning contractor in your area.

 

[rowingengineer]

There are very few option in your project. Turning back time is not easy. I personally would look for points where you can get extra supports, (steel beams and concrete columns). Then use Jacking props (heaps of them possibly, but very $$$$) to lift the slab and install the new supports.



Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[BAretired]

External stressing or pre-stressing is pretty much what hokie said it is. In the old days, when a wood beam showed signs of failing, folks would beef it up with a couple of rods attached to the ends of the beam and angling down to midspan. They would place a spacer between the rods and the beam, then start tightening up the nuts or turnbuckles until the beam was back to a horizontal position. It is a very old method of dealing with the problem, but it requires clearance for the bars which you may not have.

BA

 

[asixth]

burj write up in NCE for those interested.

 

[miecz]

I believe this is an example of external stressing for the purpose of strengthening existing prestressed concrete stringers.

 

[BAretired]

Nice pic, miecz.

BA