How to Reduce Precast Double Tee Cambers

[JohnRwals]

Hi!

I need to reduce light weight concrete double tee cambers by 1.5"~2"
in order to pour topping concrete with relatively uniform thickness in the high-rise office building.
12DT20 precast double tees are used with 40ft span.

Can you suggest practical solution to reduce cambers at the production plant or construction site.

Thank you!

JR

 

[r13]

I think you shall talk to the supplier/producer for the required camber, and specify it when ordering.

 

[structSU10]

camber is a result of the eccentricity from the prestress force near the bottom of the double tee. it can be counterbalanced by additional strands up high, but it is not typical.

Also remember that camber isn't a set number for any span, it can and will vary tee to tee. Generally I have always made sure the architect accounted for the camber for any topping that was required, and its generally defined as a minimum thickness towards the center of the span

 

[JohnRwals]

Some of these double tees were shipped to the construction site.
And, we are concerned about excessive cambers.

Thanks!

JR

 

[JLNJ]

Once the precast gets to the site you are in a bad spot. The Precaster will say, “Sure, we’ll make you another one, it will be delivered in three months.” They will point to the PCI manual (paid for by Precasters) which has some very loose tolerances for camber.

If you don’t have control of the beast up front, there are not many good choices. Increase the topping depth and mess up your floor heights. Do some selected grinding (ugh).

Ask your Precaster what he recommends - he has run into this a million times.

 

[r13]

As pointed out by JLNJ, discuss with the precaster is the best way.

Are the panels for roof, or interior floor? Any chance you can swap the already delivered panels for later/other location use? The precaster might be nice enough to give you some credits to haul the panels back for re-sale.

 

[cooperDBM]

Have you accounted for the camber reduction from the weight of the topping itself? If not, it could account for a large chunk of what you are trying to remove.

 

[KootK]

Can you suggest practical solution to reduce cambers at the production plant or construction site.

There isn't one, and I feel that your efforts will be streamlined by knowing the truth of that up front. I design TT's regularly for some precast clients.

1) 40' is a healthy span in a vibration sensitive occupancy. Often times, the bending and stiffness demands will leave little choice but for the precaster to use a significant amount of prestress which usually ends up meaning a significant amount of camber.

2) In a very real way, you want there to be meaningful camber in these things. Without it, you'll end up with topping ponding, visible sag that terrifies the villagers, and any number of other possible issues.

3) You can pull down camber a bit with prestressing up top but the effectiveness of that has it's limits. It's also a more effective technique with products like hollow core slabs. Lastly, if the prestressing in the TT is harped, there's very little chance that will be meaningfully offset by a few straight strands in the flange.

With these types of projects, the answer is to anticipate the camber and deal with it in the planning stages by way of an appropriate, variable thickness topping. As a distant, distant second alternative, you can specify a lightly cambered element at the bid stage. If neither of those things have been done on your project then the likely reality is that you've got a problem on your hands that truly is not of the precaster's making.

 

[JohnRwals]

How about this method using concrete block?

 

[KootK]

I can't tell if you're serious or not. I'll take a chance and say that you are for the sake of disseminating additional info:

1) When the blocks are removed, the camber will come back.

2) If you leave the blocks for a long, long time, you'll induce some creep deflection which, given enough time, may also come back.

3) If you depress the TT's, install and top them, you may crack the bejeezus out of your topping when the camber comes back.

 

[Agent666]

I've seen JohnRwal's solution used, put heavy weight on individual double tees units with excessive camber relative to the units beside them, pour topping around this weight. Remove weight, complete missing bit of topping. This was the precasters standard method of dealing with the issue.

It seemed to work. But I hate to think what stresses you are potentially locking in. Obviously if the units are longer spans, then they don't require a huge weight to deflect them in the untopped state by 10mm or so. But if you are talking about larger differential cambers then you're probably not going to use this method.

You best bet is often to not go with the smallest depth for a given span. It will mean more prestress being required which with the lower stiffness relates back to more camber.

With double tees I also make a point of setting down supporting elements by 25mm and accounting for additional topping thickness/weight on the basis of achieving the minimum topping at midspan with a topping screeded to a level (variable thickness). This is not only easier than screeding to a thickness it offers the benefit of hiding any moderate camber issues.

 

[hokie66]

Don't forget the upwards creep. That can affect rideability on a bridge...a lot.

 

[rapt]

You best bet is often to not go with the smallest depth for a given span. It will mean more prestress being required which with the lower stiffness relates back to more camber.

That was going to be my suggestion. Deeper section will require less prestress so less uplift and a stiffer section so upward deflection is reduced both ways.

 

[Settingsun]

2" in 40' span is L/240. Quite an ask.

Why do you need uniform topping thickness, rather than just level finish?

Can you raise the less cambered beams at their supports?

I would expect that reducing camber will generally cost more than designing in a method of dealing with variation.

 

[JohnRwals]

I wonder how bridge industry has dealt with this problem.
I guess this camber difference or excessive camber could be more critical issue in the bridge industry.

Please share your experience and knowledge.

Thanks!

JR

 

[cooperDBM]

Bridges usually use a variable depth haunch between the girder and the deck. It allows for the camber and the difference between the girder profile and the final deck/roadway profile. The camber and profile difference often counteract each other to some extent.

 

[Settingsun]

When the excess camber has been beyond what thinning the slab could correct, I have seen the grout pad under the bearings made thinner. In one case, the bridge deck level was raised to the level needed for minimum slab depth and the road regraded to suit on the approaches.

 

[hokie66]

They don't always deal with it well. Creates quite an uncomfortable ride over the peaks and valleys.