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CONCRETE BEAM CAMBER 3
- Thread starter NKK
- Start date
I would think that most college text books would discuss camber. Also, common practice is to camber beams where span/depth ratios exceed those indicated in Chapter 9 of ACI 318. Camber can "remove" initial dead load deflections, long term creep deflections, and even some live load deflections if desired. It is usually up to the specifying engineer to determine when and how much camber is required for various beams or slabs.
NKK,
Camber does not reduce the total deflection, only the "seen" deflection.
Remember that anything attached to the member being cambered will not benifit from the camber. The total deflection from the poured position will still be the same as for a non-camber member. The only difference is the amount of deflection above or below the horizontal.
If curtain walls are being attached or brittle partitions such as brick walls or brittle finishes placed on the member, camber will not help at all in reducing the deflection that the attached materials experience.
Camber is only useful for members such as parking garage beams where it may be desirable to reduce the amount of deflection that can be "seen" i.e. the deflection from the horizontal. By cambering by half of the total expected deflection then the "seen" deflection will only be half of the deflection, but the total deflection is still the same, just measured from a different starting level.
Camber does not reduce the total deflection, only the "seen" deflection.
Remember that anything attached to the member being cambered will not benifit from the camber. The total deflection from the poured position will still be the same as for a non-camber member. The only difference is the amount of deflection above or below the horizontal.
If curtain walls are being attached or brittle partitions such as brick walls or brittle finishes placed on the member, camber will not help at all in reducing the deflection that the attached materials experience.
Camber is only useful for members such as parking garage beams where it may be desirable to reduce the amount of deflection that can be "seen" i.e. the deflection from the horizontal. By cambering by half of the total expected deflection then the "seen" deflection will only be half of the deflection, but the total deflection is still the same, just measured from a different starting level.
Well, I agree with you, rapt, that camber doesn't reduce or eliminate the total deflection but simply changes the starting point of the beam....but that is the whole point of cambering. If you have a very long span, you don't want your starting point to be a sag in the beam and thus the floor. Cambering does apply to more than just parking decks and it is not just for visual improvement of your structure.
Floors in occupied buildings should be fairly flat under self-weight so that cabinets, walls, etc. will be able to sit square. A flat roof should be cambered where there is concern that long-term deflections (creep in concrete for instance) can expose your structure to ponding. Cambering beams that receive a subsequent concrete deck helps avoid extra thick slabs at midspan due to finishing the concrete flat...thus avoiding excessive dead loads and loss of safety factor.
Cambering isn't something that you do every day, but it isn't just for looks.
Floors in occupied buildings should be fairly flat under self-weight so that cabinets, walls, etc. will be able to sit square. A flat roof should be cambered where there is concern that long-term deflections (creep in concrete for instance) can expose your structure to ponding. Cambering beams that receive a subsequent concrete deck helps avoid extra thick slabs at midspan due to finishing the concrete flat...thus avoiding excessive dead loads and loss of safety factor.
Cambering isn't something that you do every day, but it isn't just for looks.
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- #7
JAE
I am simply stating what many forget about cambering. Explaining the limitations. You did not mention it in your response. The person you were answering might not have realised it and could get into trouble because of it.
I have seen examples where unthinking designers have not realised this. Someone who designs a 15m span slab edge which will deflect over 110mm so they specify a precamber of 90mm and think they have designed a nice relatively flat slab with only 20mm deflection over 15000mm. Then they connect a glass curtain wall to it. OOPS.
Same thing with your office space. Deflections after camber look ok, still hogging upwards under initial self weight. Until they erect lots of stiff (but not brittle) partitions and, as the slab deflects due to cracking, shrinkage, creep etc, we have nice gaps at the bottom or the top of the partitions , depending on whether or not the partition man bolted them to the floor, and the doors do not close properly. All because the designer thought he was only going to get 10mm or 20mm deflection and ended up with 120mm.
Yes, camber has it's uses but we must understand its limitations. Properly designed non-cambered PT members perform much better than cambered RC members in terms of deflection control.
I am simply stating what many forget about cambering. Explaining the limitations. You did not mention it in your response. The person you were answering might not have realised it and could get into trouble because of it.
I have seen examples where unthinking designers have not realised this. Someone who designs a 15m span slab edge which will deflect over 110mm so they specify a precamber of 90mm and think they have designed a nice relatively flat slab with only 20mm deflection over 15000mm. Then they connect a glass curtain wall to it. OOPS.
Same thing with your office space. Deflections after camber look ok, still hogging upwards under initial self weight. Until they erect lots of stiff (but not brittle) partitions and, as the slab deflects due to cracking, shrinkage, creep etc, we have nice gaps at the bottom or the top of the partitions , depending on whether or not the partition man bolted them to the floor, and the doors do not close properly. All because the designer thought he was only going to get 10mm or 20mm deflection and ended up with 120mm.
Yes, camber has it's uses but we must understand its limitations. Properly designed non-cambered PT members perform much better than cambered RC members in terms of deflection control.
- Thread starter
- #8
Thank you for all of your comments. Here is my challenge: the floor constructed as a RC waffle slab (150 psf) has to be leveled at the end of construction so the mobile shelves rails can be installed and have to stay leveled when all the live load of 300 psf is applied so the shelves can be operated properly. I guess it's matter of finding the balance like with everything else in life.
NKK,
Depending on your definition of "levelled", this is not possible for a suspended slab.
You need to get the Mobile Shelf supplier to give you more realistic limitations, or tell him it is not possible. Precamber cannot help you and neither can prestress. You cannot avoid live load, shrinkage, creep and temperature deflection, after the slab is poured and stripped, no matter what suspended structural system you use.
You only solution is a slab on ground and you will have to support it on rock, not soil of any sort. Then you might have a chance of complying.
The concretors cannot even finish the slab surface to comply with this, forgetting about deflections.
Depending on your definition of "levelled", this is not possible for a suspended slab.
You need to get the Mobile Shelf supplier to give you more realistic limitations, or tell him it is not possible. Precamber cannot help you and neither can prestress. You cannot avoid live load, shrinkage, creep and temperature deflection, after the slab is poured and stripped, no matter what suspended structural system you use.
You only solution is a slab on ground and you will have to support it on rock, not soil of any sort. Then you might have a chance of complying.
The concretors cannot even finish the slab surface to comply with this, forgetting about deflections.
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- #10
NKK, I think the rail suppliers are using layman's language when they say level rather than insisting on 0mm or even 0.00mm sag. What you will need as the designer is to get their acceptable tolerances. If these are not realistic then you will have to go back to them. I had the same problem with rails on a factory floor. The machine supplier said they had to be dead level. We then agreed a tolerance but it wasn't realistic for the concrete guys so the rails were put in afterwards on a levelling grout bed to within the 5mm requirement. However you have a bigger problem because you have a suspended slab. Perphaps you could get rid of the dead load deflections first and then put the rails in on a levelling screed. That way you only have to deal with the imposed loads and creep. The longer they can wait to put in rails the more creep you can get rid of. Carl Bauer
3111davi
Structural
- Jul 23, 2002
- 2
Has anyone ever experienced problems with the finishers screeding off the built in camber in the formwork because they were using a lazer level screed to finish the concrete slab. Thus reducing the specified clear cover for the reinforcing steel. How is the typical concrete beam with camber finished?
jheidt2543
Civil/Environmental
The slab should be finished to a constant thickness, not a level plane. If this is explained to the finishers, they will have no trouble in screeding the slab to the correct elevations. Basically, it is the reverse of screeding in a slope to the slab.
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