camber in steel box girder skew bridges 2

[shaurya241100]

Hi Guys,

Hope soem one has tackled this issue recently.

I am just designing a 45 degree three girder system steel box girder deck rail bridge where the adjacent tracks are supported on two decks in between these three box girders which comprise of steel decking stiffened by t section below steel deck.

This is regarding camber to these steel boxes.
The bridge is skew but at every square cross section the soffit of the girders must be at the same level so as the achieve a horizontal deck section at any square xsection along the length of the bridge.

If some one has dealt with this kind of issue can you let me know how camber was applied to the girders?

Cheers

 

[ishvaaag]

So I understand this is a fabrication question, no?

Whilst, your camber values need be derived from analysis, being all-steel helps. Anyway as you know you'll only be able to meet your intent for a target hypothesis (this with behaviour being quite that expected, something not clear one is always to get, I yesterday read of some welding engineer doing practical field work for correction of camber by continuos phone exchanges with a seasoned also welding engineer in his eighties).

So engineering judgement will be needed on what kind of cambering will most favour the general behaviour. You may even do some complementary models to retouch nodes and even bracket some variation of the young modulus. Some setups, particularly rigid links and their connections might turn critical upon movement, yet for some quite typical section nothing that won't have as well typical solutions.

 

[shaurya241100]

Thanks Ishvaag.

 

[Ron]

Since these are box girders, the "boxes" can be fabricated to any geometry you want. Specify hold points for the camber and the fabricator will have to work out the fabrication issues to achieve that.

In short, achieving the proper camber is a fabrication issue, based on the specified geometry of the sections.

 

[shaurya241100]

Ron,

Many thanks for this. This is what I was thinking but the question is do you extrapolate the camber for the individual girders and apply that between the farthest support (hold point) of the bridge so as to spacify camber.
Regards,
Shaurya

 

[miecz]

Seems to me that if you specify the camber to take out the dead load the stringers will be level, or uniformely sloped at erection. If you have some residual camber in each stringer at erection, there will be steps in the cross section.

 

[shaurya241100]

Thans Miecz.

My thinking is if I apply a unifom camber to all girders starting from the farthest points the girder will follow the chord of circle depending upon the skew of the bridge. In this case there will be no step if you take a sqare cross section at any point in the bridge span.

 

[miecz]

Oh, I see. You're going to adjust the bearing elevations from stringer to stringer to account for the camber in the adjacent stringer?

 

[shaurya241100]

I guess so because in skew structures you can't have any other option left.

 

[RHTPE]

shaurya241100 - Please consider some of the points raised in the posted article. It focused on skewed plate girders, but the effects of torsion induced in adjacent girders by deflection needs to be considered.


Ralph
Structures Consulting
Northeast USA

 

[shaurya241100]

Ralph,
Thanks.
The discussion here is about steel box girders which are very efficient in carrying torsion but you are right that there must be some consideration for torsion effects also.
What I am still not very sure is that how can you control that twist because it can never happen that the camber for a number of box girders carryong deck and connected together will be same under serviceablity condition. There is always going to be some diffenece in camber and the cross section at any square section can never be in the same horozontal plane in other words the soffit of the girders will not lie in a plane and which is very difficult to satisfy.
cheers