Pre-cambering of concrete beam 1

[Murali27]

Dear All

I have a ribbed slab spanning 13m with a 850mm rib depth (to be designed as a simply supported). The long term deflection is around 50mm due to self weight of the slab alone. Can we have a camber 50mm @ center of the span on account of selfweight alone? Is there any limit on pre-cambering of beams?

Section capacity is ok but not the deflection.

Thanks
Murali G

 

[JAE]

Yes, you can camber (and should camber) concrete beams to minimize the effects of dead and long term dead load deflections. Not sure that there is any limit but normally, you would look at the instantaneous dead load deflection and the long term deflection and see what sort of upward "hump" could be tolerated until the long term deflections actually happen.

Normally, I would camber only for the immediate DL deflection and then check to see if the long term deflection is tolerable without cambering.

 

[SlideRuleEra]

There is quite a bit of information on the web about cambering of prestressed members, mostly about bridges (there are limits to what is acceptable)
Here is one document, "Field Verification of Camber Estimates for Prestressed Concrete Bridge Girders"

http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_STR/FDOT_BD545_07_rpt.pdf

http://www.SlideRuleEra.net

 

[Murali27]

Thanks JAE

Short term deflection (Only Dead) = 11 mm
Long term deflection (Only Dead) = 68 mm
This is a conventional cast-in situ beam. This ribbed slab is supporting a landscape area. Even 40 mm would not be an issue (0.2mm crack width will be checked).


Thanks SlideRuleEra for your reference. I will go through.

Thanks,
Murali

 

[rapt]

Murali27

Those numbers do not sound consistent between short and long term. Are these self weight only or including the landscaping as a dead load also. You earlier said 50mm self weight long term.

Based on thiose figures you have a dead load deflection of L/191 which is very high.

Give me the section shape, concrete, design code and loads and I will do a quick run to get deflection estimates.

I agree with JAE that I would not pre-camber more than the initial self weight deflection.

 

[Murali27]

Thanks rapt

The values are for self weight only, but the long term deflection estimated for required steel 2T32

Span 12.5m
Section - T shape

Flange width = 1150mm
Flange thick = 200mm
Web width = 350mm
Web depth = 550 (Excluding flange thick)

Superimposed Dead Load = 10 kN/sq.m
Live Load = 5 kN/sq.m

Code used = BS 8110

Thanks
Murali

 

[SlideRuleEra]

Murali - When calculating the camber be sure to consider that the temporary falsework supporting the forms may deflect during the concrete placement. This would be an allowance in addition to the camber compensating for the self weight of the concrete.

http://www.SlideRuleEra.net

 

[dik]

If used for landscaping, it's not visible and you may want to camber to provide maximum drainage... so water is not ponded...

Dik

 

[dawn836]

Murali27

What is the yield strenght of steel(Fy) and the concrete cubic strenght (Fcu)?

 

[Murali27]

Hi dawn836

Fy = 460MPa
fcu = 45MPa

Thanks, Murali

 

[rapt]

Murali27

2 T32 is not enough for a start. 3.3T32 are required. So I have done the calcs with 4T32.

Also, the rib depth you have defined here is 750 total compared to your first posting which said 850mm. I have done the calcs on 750mm.

I agree with your initial dead load deflection of 11mm but the long term deflection you are quoting is the total long term allowing for self weight, sumperimposed dead load and permanent and short term live load.

Long term self weight only deflection would be about 34mm assuming no higher loading was ever applied.

You will never get these ribs to conform the the BS8110 20mm maximum rule but it is stupid anyway.

Increasing the bottom reinforcement to 5T32 would reduce the deflection to about 57mm and a 20mm precamber would make these ribs work ok.

Or add some bonded prestress would be even more efficient.

 

[Murali27]

Thanks rapt.

I agree with you. Since the landscape will be there on the top of slab, 20mm wouldn't be an issue.

Do you have any spreadsheet for this to calculate the longterm, I used Prokon design module, but it calculates the longterm deflection for the required reinforcement.

Thanks, Murali

 

[rapt]

Murali27

I used a program we develop and market called, funnily enough, RAPT,

http://www.raptsoftware.com.

I still think the best solution would be bonded PT for this. Doing it with RC only is wasting reinforcement to try to make deflections work.