Load distribution

[MacGruber22]

Preface: this is not new construction - it is support for deconstruction of a crane that needs to cross a defunct bridge to be disassembled.

I have a 5'x5' concrete pile cap girder (pier) spanning 23' that is 200% over-stressed, and I am going to place a steel girder on top of it to span in between piles. I have fairly heavy loads (two 500 kip spaced 3-ft from each other (on rail), with the rail near the center of the pier span) I am trying to look at this in two ways...
1. Size the steel beam to carry all of the load to the pile caps, and provide space for the beam to deflect without touching the existing concrete pier.
2. Place the steel beam directly on the concrete pier and distribute the load to the steel beam AND concrete pier girder based on their relative stiffness. The idea would be to generate a steel beam stiffness that removes enough load from the concrete girder so that the girder is not over-stressed. I am limited by a 36" deep steel beam.

Thoughts? Is #2 valid? If so, I am wondering is the steel beam section required (for stiffness) will be more than needed for the strength to carry the two point loads own its own (without the concrete girder supporting any load). I hope I am being clear. Thanks!

 

[jike]

I would recommend that you totally isolate the steel beams from the concrete girders, in other words, let the steel beams carry 100% of the load.

The reason is that the concrete girder most likely has either camber or is deflected......how could you possibly take this into account?

 

[MacGruber22]

I was think about that. I planned on grouting between the steel beam and the girder.

 

[SlideRuleEra]

Concur with Jike. I've have had occasion to field correct efforts by others who tried to share load between two different support systems. Neither system was adequate by itself. That type of shared load is a recipe for TROUBLE.

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[MacGruber22]

Thanks, SlideRule.

 

[KootK]

You could use both sections concurrently. I'd think that even shimming at third points would enforce enough displacement compatibility for your purposes. Maybe directly beneath the loads would be best. Consider:

1) For strength design of the steel, you'll want a lower bound concrete stiffness estimate.
2) For stiffness design of the steel, you'll want an upper bound concrete stiffness estimate.
3) For strength design of the concrete, you'll want an upper bound concrete stiffness estimate.
4) If the concrete can yield in a plastic fashion (under-reinforced), perhaps you can forgo 1-3). You'd get some cracking of course.

While you can do this, it might be more trouble than it's worth.




The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jreit]

When you say the bridge is defunct, what do you mean exactly? The piers are in acceptable condition but only the cap (girder) is not?
If it has not been maintained, it might be difficult to reliable estimate its structural capacity.
I would not want to rely on the concrete girder at all to transfer any load, let alone something of this magnitude.
Also, could catch the attention of local officials who might have issues with it.

 

[MacGruber22]

KootK: that is exactly how I *was* going to do it, but you're right - it is likely more trouble than it is worth.

I mean "defunct", as the bridge will be demolished (or) rebuilt (if a future use can be found) after we are done using it to haul these cranes from the connecting pier to the shore. I think the exact definition of the "bridge" is a high water platform? The property (existing steel mill) is being converted into some other use (yet to be determined) and the current owner (developer) is not too worried about keeping the bridge serviceable in the near future. Their main concern is dragging the cranes off the pier, scraping the steel, and finishing their site demo.

Also: The existing concrete had cathodic protection that was in service till about 2-years ago. We feel better about their condition because of that.

You can see the bridge to the left of the leftmost crane.

 

[KootK]

Oh my... that's pretty cool.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dhengr]

MacGruber22:
Are the cranes still powered? Could you ground a barge, or one end of the barge, at the end of the crane pier and drag the cranes, one at a time onto the barge, and then to shore? I don’t see anything that looks much like a bridge to the left of the cranes, which isn’t a long distance across the water, from the cranes. I do see conveying equip. and associated structures which would seem to prevent moving the cranes to the left. Have these structures been removed? And, are you planning on dragging the cranes into the picture on the structure which carried the conveyors to shore? Can the rail trucks (bogies) make the turn from their pier rails to the rails heading for the shore? How do you know that the piles will carry 500 or 1000kips when the load is right over them? There are three lengths of crane pier multi-girder spans, and if you wanted to get rid of the whole mess at once, you could center a crane over each pier girder span length; then run a barge under the bottom of the pier multi-girder span, lift the barge and haul the whole mess to shore, in three large moves.

 

[MacGruber22]

It *is* very cool.

The cranes are no longer powered - electricity was shut off from the entire property before they started demolition.

They are fighting using barges - that is their impetus for having us analyze that bridge. Look again between the pier and the land - you can see two of the concrete pier griders sticking out. it does look very small from this angle. All of the conveyors and buildings have been removed - it looks like a deserted island at this point. And, yes they plan on dragging the cranes from their current positions onto new rail spliced onto the existing (175# ASCE) that extends onto shore. The picture is quite deceiving, as there is plenty of room for all of this to happen in a straight line.

"How do you know that the piles will carry 500 or 1000kips when the load is right over them? There are three lengths of crane pier multi-girder spans, and if you wanted to get rid of the whole mess at once, you could center a crane over each pier girder span length; then run a barge under the bottom of the pier multi-girder span, lift the barge and haul the whole mess to shore, in three large moves."

How do I know that the piles will carry that? You mean, how do I know they will be able to? I don't know if they will be able to - we are in contact with a geotech who was involved with load testing piles here. I doubt you could squeeze a barge under the bridge, but I am not quite sure I fully understand your procedure.

Attached is my preliminary framing plan - maybe this will help a bit. Again, they are really fighting getting barges involved, for some reason.