One bridge(twin girder), 2 cranes. What design hoist load? 5

[IJR]

Pals

I have the standard twin-girder bridge that will have two cranes(ie two cabs) running on it, one lifting for the half span, the other for the other half span.

Any literature or specs for what design load the bridge girder be designed for?

respects
ijr

 

[msquared48]

It would be very helpful to know what you are lifting...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[IJR]

Each crane will hoist 44 kips(20 metric tons)

respects
ijr

 

[hokie66]

I think you would have to assume both hoists are loaded at the same time. Otherwise, why have two hoists?

 

[IJR]

hokie66

OK, but then they can not both be found at midspan given such a heavy load.

My assumption is they would be say at a third points.

respects
ijr

 

[hokie66]

I think you would have to find the maximum bending moment just as you would for any set of moving loads. It would be under one of the inside wheels. You can control this somewhat by designing the hoist bumper system to keep the hoists separated.

 

[csd72]

If these are post tensioned girders then you have to be really careful on how you apply the load in order to ensure they are not overstressed.

Remember to check buckling.

 

[DaveAtkins]

Are you designing the girder that the bridge crane travels along (not the two girders that form the bridge crane, but the girder line which spans column to column)?

If so, I think you must assume the hoisted loads are as close to one end of the bridge crane span as possible, not at the third points. The manufacturer should be giving you the maximum wheel load.

DaveAtkins

 

[IJR]

Dave Atkins

I am designing the crane bridge(the two parallel running girders that carry the crane), not the runway girders(the ones over which the bridge moves )

the case is special: two cranes on the same bridge

respects
ijr

 

[paddingtongreen]

You must design for the worst possible load positions. I would be tempted to cover a 60/40 division of the span to allow for future modifications of usage.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[csd72]

Sorry,

Please ignore my above comment as it is based on a misunderstanding.

As paddingtongreen has stated, the design should be based on the worst load positions for shear, bending, deflection and lateral loading. You need to find out what the minimum distance is between the two cranes as well as the closest possible distance to the ends and base it on that.

Shear would obviously be both cranes as close as posssible to one end, e.t.c.

Look up the SCI guide on cranes.

 

[MikeHalloran]

The hoist manufacturer provides drawings that show the distance from the hook to the extremities and faying surfaces of the hoist frame, which in turn determines how close the hook can get to the runway or in this case to the adjacent hook. The "crabs" that carry the hoists on the crane's bridge girders are normally just a little bigger than the hoists.

If you don't now know the hoist geometry and the crab dimensions, it would be conservative to assume that both hooks can and will be at the span center for bending purposes.



Mike Halloran
Pembroke Pines, FL, USA

 

[msquared48]

OK.

As previously stated, the max shear for either of the two girders would be for both 20 Kip loads:

1. as close as possible to one end of the girder, and

2. each hoist as close as possible to one girder.

The maximum moment would be for both 20 Kip loads:

1 with each hoist as close as possible to the center of the girder span, equally spaced from the center line, and

2. with each hoist as close as possible to one girder.

Remember that the hoists carriages travel along the length of the girders, but the hoists themse3lves, depending on the model, can travel transverse to the girders too for better piock and placement capability.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

One thought, can both crabs operate at the same time? if so, that would double the normal braking load (lateral load on the girder).

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

You could use a bumper at midspan to keep the hoists apart and on their own sides. But then, you wouldn't be able to lift in that area.

 

[IJR]

This one is inside a warehouse. Sure no crane will ever lift the 44kip(20metric ton) load, since if ever there is such a load, then sure there is a huge volume that takes up some space.

hokie66, the owner will not want the bumper, and he is the one who specifies.

So I will go ahead and consider both cranes working. It will be a very conservative design.

respects
ijr

 

[hokie66]

Bridge crane designs should be conservative. After all, it's not the steel that costs all the money, it's the mechanical and electrical stuff.