Erection Bracing for Steel Beam for Bride

[ingdaniel]

Hi all,

I have a steel bridge with 50 m span composed witn 4 beam (2400 m height) bracing together.

The problem is because the weight of the beam is not posible to lift two beam together (with bracing), so we have to lift and install the beams one by one.

If we lift and instal one beam this will failt for Lateral Torsional Buckling.

So we need some bracing only for the installation.

There is any "typical" bracing for this situation? there any book or reference about this?

Hope you can help

Best regards..

 

[bridgebuster]

You might have to put a strong back on the beam if you can't get a larger crane to lift both together.

 

[BAretired]

2400 m height?

BA

 

[JedClampett]

Bride?

 

[paddingtongreen]

What is the reach of your crane, can it lift at the end of the girder, pixcking up haqlf the weight, and reach across to the embankment?

Can you get a second crane and transfer the one end load halfway across?

Can you add a horizontal truss to the top of the girder to provide the buckling capacity?

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[JStephen]

Can you put a truss-type arrangement using lightweight angles and cable on one or both flanges to increase lateral stability? See attached figure.
If you can support a long slender flexible beam at the top flange (points "A" in the attached figure), it's more stable than if supported on the bottom flange- not sure if any design methods take this into account- but possibly arrange to support it at the top temporarily until otherwise stabilized.
These are ideas, not things I've tried, so you're on your own for details and analysis.

 

[gwynn]

Other options would include:

- King posting the first girder
- Launching the girders in pairs instead of crane erecting them
- Use of a second crane (either to erect in pairs or one as a "holding crane" - never used the second myself but have heard/read of it being done)
- Temporary shoring of the girders between the bearings

Without knowing site constraints it is tough to say which solution may be most practical

 

[gwynn]

Just wanted to add to my post above. Launching is probably only worth looking at if the crane you have has the capacity to make it a crane assisted launch and "catch" the girders before they reach the tipping point. If you need to get into fabricating a launch nose and/or counterweighting the girders, it's probably not the best option.

 

[ingdaniel]

Thanks for the answer...

BAretired

2400 mm

paddingtongreen

What is the reach of your crane, can it lift at the end of the girder, pixcking up haqlf the weight, and reach across to the embankment?
Can you get a second crane and transfer the one end load halfway across?

We are gonna use two cranes at the same time, nothing else is possible because the cranes available.

JStephen

this is exactly what we need to do. Any document for this calculations?

gwynn

we cant do any of this because the cranes available.

 

[racookpe1978]

I am surprised the unloaded beams would not be stable against their own weight during a crane lift. How were they lifted and assembled? How where they transported?

 

[BAretired]

Not a trifling question. There are many interesting articles on the internet regarding this subject. The attached link is the U.S. Department of Transportation Highway Administration Handbook on Bracing Design:

http://www.google.ca/url?sa=t&rct=j...d=12&cad=rja&uact=8&ved=0CF8QFjAL&url=http://

http://www.fhwa.dot.gov/bridge/stee...=dOuce1d0FMsGjdFavORNrQ&bvm=bv.75097201,d.cGE

I would try to get bigger cranes and tie two girders together on the ground before lifting them into place. Bracing a single girder is not an easy prospect.

BA

 

[JStephen]

racookpe1978- the original post doesn't necessarily say they'll fail during the lift- but if "we lift and install". I assume additional pick points along the beam could prevent it failing during the lift, but not when it is set in place.

 

[ingdaniel]

Yes,

When is in place will fail.

I was thinking use a horizontal truss to brace the top flange, but i am not sure about how calculate this truss...

 

[dhengr]

Ingdaniel:
In its crudest form, see if you can find a couple 50' long used steel joists. Lay them horiz. on top of the top flg. of your girder, and weld their web diags. to the top flg. This is all to be scarfed off and cleaned up after you get some bracing to another girder in place. This is much like Jstephen’s sketch. The stiffness of the steel joists (their moment of inertia) is related to their ability to restrain the top flg. buckling or torsional buckling of the girder.

 

[BAretired]

dh,

50' won't quite do it. The span is 50 m.

BA

 

[dhengr]

BA:
I was actually assuming that two or three steel joists would be spliced/cobbled together end to end to approach the length of the girder. I suspected that 164' long bar joists would be kinda tough to find. I also assume that they can do some serious bracing (kickers, etc.) at the two bearing points. With the almost total lack of meaningful info. provided by the OP’ers. on many of these questions there are probably a bunch of possibilities if we only had a better picture of the problem. More and more of the OP’s, these days, have just been a big guessing game for lack of important basic problem delineation info. If they staged the first two girders for quick consecutive picks, maybe a couple lighter cranes at the third points of the first girder could act to hold the first girder during the second lift. Could some of the cross bracing (a few of the braces) be installed on first girder prior to its lift, so they would immediately be in place when the second girder was lifted?

 

[gwynn]

dheng
One possible problem with the truss solution is that most/many jurisdictions will not let you weld to a bridge girder. I hope the OP has looked into this for acceptance before going too far down that path.

I suggested the use of a holding crane above, and was told no.

 

[ingdaniel]

We wont weld, will use bolt.

The problem I have now is how estimate the load for design the truss.

Any idea for this load?

Regards

 

[BAretired]

It is not a simple problem. Wind forces would need to be considered. Wind deflection would be magnified by compression in the top flange due to gravity load. Unless the bottom flange is braced, wind forces will act eccentric to the bracing causing torsional loading and possible aerodynamic instability.

If a horizontal truss is used, the compression chord must be braced periodically to prevent buckling. If chords consist of cable, they can't take compression, so in effect, only the leeward truss would be effective with the girder acting as the compression chord.

BA

 

[JLNJ]

Your best solution might be to get a bigger crane. If you are over by just the weight of the bracing, maybe there is a way to tack the two together and lift them as a pair. Then after the others are placed, cut the first one loose, lift it, and install the bracing.