Design prestressed bridge beam for long term downward camber

[MIKE_311]

The bridge owner doesn't want any downward camber in the long term (i.e. no tension in the PS beam bottom flange over time).

PCI has a procedure for estimating long term camber from creep and shrinkage using multipliers, but states that these multipliers are not applicable to bridges with composite cast in place decks, as it serves to limits the effect of creep/shrinkage the beam, nor should the designer include additional camber for long term effects when the multipliers are used.

My question is, what is the typical practice for considering long term camber in PS bridge beam with composite decks? if it is considered, what is the typical procedure used to calculate/estimate it?

 

[skewl]

To me, camber simply means the deflection of a girder and it is not totally related to the stress in the girder. You can cast a girder with a very large positive camber, put on some load which may lead to positive stress (tension) in the bottom fibre, but still be in a positive camber. Is the "no negative camber" requirement a matter of geometric or for durability (to avoid cracking at the bottom)?

Can you tell me which PCI manual and chapter you are referring to?

 

[bugbus]

There are hand calculations that can be done from various books ("Time-dependent behaviour of concrete structures" by R. I. Gilbert is a good one), but I would normally run this through software like Midas or Autodesk Structural Bridge Design which will work out your long-term deflection in a few seconds (and I believe are based on the same/similar methods anyway). I have found that the software, if used correctly, will give a good prediction of the hog/deflection measured on site.

I agree with kewli that 'no downward deflection' =/= 'no tension in bottom flange'. To me these are different criteria.

Not sure about MIKE_311, but in my local code, the no negative camber / downward deflection rule seems to be mainly for visual/aesthetic reasons. There might also be clearance issues if there is a large deflection over time. But I think practically, if the girders are designed to be fully prestressed (uncracked) under service loads, the long-term deflection will usually be upward anyway, simply due to the amount of prestressing required.

 

[MIKE_311]

Thanks, let me restate, the owner says no downward deflection to ensure there will be no tension in the bottom of the beam (for cracking).

I agree, downward camber does not equal tension. But simply designing for service loads I don't think is enough as the issue is that long term effects (creep/shrinkage) may reduce prestressing forces and thus lessen the camber.

The beams are designed using Leap Bridge Concrete (Conspan), which will estimate the long term camber (using PCI multipliers) but that's overly conservative. So I'm looking to see if there is another method that is preferred or standard practice. I see UDOT discusses this in their manual but it looks like just PCI multipliers in a different table.

To add, the section of PCI Bridge Manual that discusses this is 8.7.1.

 

[3DDave]

That's an arch that requires thrust resistance at both ends to provide only compression loads in the structure.

 

[Settingsun]

The thread title says long term camber but the comments about cracking suggest combined with service load. Are you sure they know what they want and sure you also know? Reeks of them coming back and telling you what you've done isn't what they meant.

Putting aside the composite slab for a moment to simplify the picture, pretensioned bridge girders hog upwards as Gusmurr said. They hog up more over time because the creep effectively reduces the stiffness and tends to dominate over shrinkage warping. This despite the loss of prestress.

 

[Guest090822]

Mike 311 - we used to set the PCI growth multipliers to 1.0 in Conspan. There isn't much guidance on the subject and you are correct, the PCI methods are for beams without a composite deck and aren't applicable to your situation. I've also designed to the requirement of having a minimum camber and it is mainly for aesthetic reasons and the owners (DOTs) insisted. In most cases we had to add a few more strands and/or reconfigure just to meet the minimum camber requirements.

 

[MIKE_311]

. Thanks. Im not sure the owner knows either which is what is leading to our confusion on the subject, and admittedly I am not a PS beam expert. I can design one easy enough but I don't have experience with long-term performance.

Their design manual, used to say, "negative final camber of L/2000". We asked for clarification of "when" is final camber. They came back and said that section of the manual is being revised and would like us to never have any downward camber. Never is long time.

If PS beams tend to end up with more camber over time, then it makes sense to design to no negative at erection, and not introduce more. Is there some research or documentation I can refer to defend that claim?

 

[Settingsun]

I can't help with no-camber design. Australian beams are routinely designed with upward camber to avoid downward camber under sustained loads. The camber is not entirely predictable or uniform even comparing notionally identical girders. We have to specify how the contractor adjusts for the measured variation from the design predictions.

 

[Settingsun]

Here's what you're up against:

Edit: link to document didn't work. I'll upload it.

 

[Settingsun]

https://files.engineering.com/getfi...5ad-23f9174a0f9b&file=Whiting_Bridge_2000.pdf

 

[IDS]

Thanks for the link Steve.

I have done a fair bit of work in this area, and reached similar conclusions; see "Practical methods for the analysis of differential strain effects" attached.

The "Super-T" bridge girders used in Australia are particularly susceptible to differential strain effects, because they have wide thin top flanges and compact bottom flanges, nonetheless these effects can cause significant variation in behaviour in any pretensioned girder, and need to be considered if there is a strict contract requirement for no downward camber.





Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

I'm still not clear on what the criteria is supposed to be. There's the 'no downward deflection from the time of installation' criteria, there's the 'no sag in the final condition' criteria, there's 'no net tension in the bottom' criteria, and then there's the 'no cracking of the concrete at the bottom'. All have different limiting stresses, and there are a few different approaches to accomplish that.

We typically worry more about upward camber causing issues with maintaining the vertical profile, and cracking in the top. Maintaining enough precompression in the bottom of the section usually isn't a problem; it's usually having too much, leading to excessive creep, resulting in excessive upward camber, that we fight with.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[IDS]

Rod - certainly the actual criterion needs to be defined, but I think the important point is that there is no simple method that will provide a reliable estimate of actual deflections.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

Rod - certainly the actual criterion needs to be defined, but I think the important point is that there is no simple method that will provide a reliable estimate of actual deflections.

Agreed. There don't seem to be any simple ways to get good deflection estimates, although from what I understand the complex methods aren't much better. As far as I have understood, the only thing that results in any decent consistency, is specifying a long time to release of the strands/transfer to the concrete, but prestressers really don't like doing that.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MotorCity]

The terms "camber" and "deflection" are being used interchangeably. They are not the same thing. Deflection is simply displacement due to actual load on the structure. Camber is the process of inducing curvature into a member during fabrication such that under the application of actual in service loads, the member will deflect so as to appear flat or straight (i.e. no visible sag).

 

[Settingsun]

IDS, I forgot to mention that I'd seen your article before, back when writing a spreadsheet for age-adjusted modulus calculations. Super-tees are tricky. I once had a contractor call and ask if they had to throw one away because it was sagging under own weight.

MotorCity, what you described is often called precamber (shape under zero load) to distinguish from the shape in the finished structure which is often called camber IME (= precamber + deflection).

 

[BridgeSmith]

I think Motorcity's terminology is good, as far as it goes. However, I would extend the definition of camber to include long term camber increases due to concrete creep under the sustained stress of the prestressing force.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bugbus]

I personally would not consider long-term upward deflection to be 'camber', but it is all just semantics at this point.

In my mind, 'camber' is whatever shape is built into the concrete before any load or prestress is applied to it.

 

[BridgeSmith]

I personally would not consider long-term upward deflection to be 'camber', but it is all just semantics at this point.

It's more than semantics, because the terms are used in the design specifications, so the design engineer needs to know and use the correct terminology for the design, in order to design it correctly and for others to be able to review their work.

In my mind, 'camber' is whatever shape is built into the concrete before any load or prestress is applied to it.

If that was the case for a prestressed girder, there would be no camber, since the casting beds are flat. The beam/girder doesn't camber until the prestress is transferred to the concrete.

Camber is the result of internally applied forces (prestressing) and deflection is the result of externally applied forces (including the selfweight of the beam, if I remember correctly).

Camber is nearly always positive in typical designs, since the strands are near the bottom to resist the applied loads. Deflection is nearly always negative, due to the downward direction of the applied loads. The interplay between the applied loads and the internal stresses can result in an initial shape that is bowed up or sagging down, and unless the internal and external moments are perfectly balanced, the shape will continue to change throughout the life of the beam.

Rod Smith, P.E., The artist formerly known as HotRod10