Camber in Steel beam

[Arun4567777]

Hi!

I recently receievd a design of a beam spanning 30m with intermediate splices every 12 m. They have given camber to a beam for Dead Load. Instead of cold camber technique, they have given slope in members to achieve that camber. Is it ok?

 

[KootK]

So the camber has been created by introducing "kinks" at the splices then?

 

[BridgeSmith]

I guess that could work, but probably considerably more expensive than heat cambering, or fabricating a plate girder. Beam splices, welded or bolted, are expensive. If there are fabricators in your area that can cold-camber the beam, that could be an option, but I suspect it would be too large of a section and too long. For steel bridge girders that length, plate girders would be our go-to option.

 

[Arun4567777]

Thanks for reply.
Yes they have introduced kinks.

 

[Arun4567777]

Their is a limit on transportable length upto 12m. So anyway they have to provide splices.

 

[BridgeSmith]

I guess if the kinks at the splices get it close enough to the final profile, I suppose it would work. I assume the limit on transportable length means a couple of bolted field splices?

 

[Arun4567777]

Yes a couple of bolted field splice.

 

[MotorCity]

Maybe I am not understanding this correctly, but if the "camber" is achieved with (I assume) bolted splices, this does not work.
What you end up with is a series of short beams daisy chained together that will give the appearance of camber, but due only to the beam sagging as a result of "slop" in the bolt holes.
The camber should come out under load, not under the beam sagging on its own due to gravity. If the splices are welded, then it makes sense.
If the splices were made with slip critical bolts, I'd still call this proposal suspect.

 

[Arun4567777]

You are on point. They are making slip critical connections for splice. How do I logically explain to them that this is not the way to do it. Technically if I I need to have a camber of some inches in upward. By providing kink we are achieving the same. How to logically explain it to them.

 

[MotorCity]

You could create two beam models.....one using a continuous beam and one using an internal hinge at each splice location.

You should be able to demonstrate the difference by comparing the moment and deflection diagrams.

 

[BridgeSmith]

Using fully tensioned bolts, that produce a rigid splice, the required camber can be approximated, but it's obviously not going to get you to a smooth curve or straight line. It will deflect as a single beam (as long as the moment doesn't produce forces that exceed the friction capacity). You can superimpose the beam deflection under load onto the assembled shape to see what the final shape should look like. It will have a couple of peaks where the splices are. It's up to you to decide if that's functionally and aesthetically acceptable.

 

[FLCraneBuilder]

We have shipped up to 140' ft long - In the US

 

[EdStainless]

Any country that has wind turbines has a way to ship 115'+.
This approach will result in a lumpy beam when you are done.
You need to figure out if it matters.

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[Retrograde]

Providing camber by introducing kinks at the splices is a method I have seen used many times.

I understand that many fabricators often prefer this method as it is easier to achieve than heat cambering.

Personally I have no issue with it.

 

[Arun4567777]

Thanks all for all the responses.

 

[BridgeSmith]

We've had bridge girders up to 140' fabricated and shipped, but we try to keep the pieces to 125' or less, since there's only one fabricator in our region that can go beyond the 125', and there's alot of federal regulatory hoops to jump through for 'sole source' contracts.

Anyway, even if the length limitation applies, if the camber needs to be more exact than can be achieved with kinking at the splices, then a 3 piece plate girder may be the most economical option. Very precise geometric control of the girder shape is regularly achieved with plate girders.

 

[Tomfh]

We’ve done kinks with bolted slip critical connections a number of times. I don’t have a problem with it.

 

[Agent666]

What you're describing is utilising a "Preset", not "precamber". Simply altering the unloaded geometry to account for structural deflections.

If you take issue with the splices somehow affecting things simply because of the preset, then you presumably should also take issue with a straight beam with splices. Which sounds a bit flawed as that's commonplace right?

Personally I have the same opinion as Retrograde, not an issue if done right.

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