Pick Up Points

[dik]

I have a uniformly loaded element that has to be supported at 5 points. The element is elastic and uniform in section and is 92' long.

Is there a simple support spacing that will yield the spacing of the 5 pick up points so that each pick up is the same magnitude?

I can try it using a uniformly loaded beam and arbitrarily adjusting the support locations to get reactions, but is there a more simple approach?

Dik

 

[rb1957]

intuitively, equi-spaced, with 1/2 a pitch overhanging ... divide the length into 5 pitches, place supports at the middle of each pitch

 

[dhengr]

Dik:
You say... “I have a uniformly loaded element that has to be supported at 5 points. The element is elastic and uniform in section and is 92' long.”

Do you assume no chance of unbalanced loading? Are you trying to make the bending moments in your element the same so you don’t have to cover plate for the first interior negative moments? Or, are trying to make all five supports identical in reaction value? I’m not even sure that you can meet all of these criteria at once, the continuity prevents this. Or, are you trying to lift this whole system without over-flexing your 92' element? And, if lifting, then any sling slope introduces axial loading components into your 92' element.

I would assume with today’s software you might just model a four span beam and start adjusting the span lengths a bit, and in a few iterations get pretty close to what you want. From any beam tables we can see what 4 spans at 23' each looks like, for a starting point.

If I were trying to lift this 92' long element, I would use 2 - 23' spreader beams (maybe some shorter adjusted length from above) over the two end spans. Then I would want to be able to adjust my pick point on these spreader beams to account for the two different reaction values below, and still balance this spreader beam. Above this would be a second, longer spreader beam, with its pick point at the center, and immediately below this pick point is a cable sling with a turnbuckle to pick up your 92' element at its center reaction. This is supposedly a determinate system and we can fairly easily hone in on the forces.

 

[dik]

Unbalanced loading is a non-issue... It's for designing hoisting to pick up a 92' long heavy plate girder... calculated spans are approx: 8'5" cant, 19'2", 18'-5", ... and symmetrical... used Yakov's program and after a few iterations had it within inches...

thanks, Gentlemen

Dik

 

[rb1957]

1) how different is this optimised result from 9.2' cant, 18.4', 18.4' symm ?

2) why so interested in getting equal reactions ?

 

[PMR06]

You could set up an excel spreadsheet to calculate the reactions (starting with moment distribution maybe?), then use the solver add-in to automatically iterate the spacing between supports until all reactions are equal.

 

[BAretired]

I started to work out a closed form solution, but when dik came up with the answer using an iterative procedure, I abandoned it.

rb1957, your proposal does not result in equal reactions.

BA

 

[msquared48]

First and Pike

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

rb: the equal reactions are to minimise the 5 cranes that have to be purchased for the lift. Five 16 tonners are required as opposed to five 20 tonners...

Dik

 

[dhengr]

Dik:
I sure would try to do that lift with two 40 or 50 ton cranes and two spreader beams; much easier to coordinate during the lift and positioning, than five smaller cranes will be. And, I would think you could buy or rent two heavier cranes and operating crews for an amount similar to the cost of five smaller cranes. Although, I haven’t bought any cranes recently. Could you use the final support structure as a jacking base, get the girder into position on the ground, and pull the girder up into position? Would you please comment on why you must use five small cranes and pick points? Are these boiler support girders?

 

[rb1957]

@dik, i'm surprised that moving the supports <1% of the beam length makes at 25% difference in reaction.

@BA, i agree uniformly spaced supports won't be exactly uniform reactions ('cause of internal continuous beam moments), but i'd've thought they'd've been close enough. but a simple enough doubly redundant beam (taking symmetry into account) to solve exactly (a doubly propped cantilever).

 

[woodman88]

Cranes ouch. Maybe it is not an issue (I have never dealt with this type of problem), but you are aware that the cranes will cause the support locations to vary vertically compared to the others. For a 40' pitch truss with center support having a 1/2” vertical difference will create a large transfer of the reactions to the higher supports. Your load transfer will depend on how stiff the girder is. Hopefully your vertical displacement allowable is large enough not to matter or that you can give the allowable vertical tolerance required for the lift to the crane operators to use.


Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dik]

dh... we have inches to spare... not anywhere enough room for spreaders and we are looking at moving a 92' long, 150K weight plate girder 350' in a mine shaft... using Ingersol-Rand ULM2-160 hoists... Dist from beam to hook is approx 16"... for 16T hoist.

woodsman... neither have I, one of the perks of my job... something new, almost all the time.

Dik

 

[BAretired]

I visualize a problem coordinating five cranes lifting together. I like dhengr's suggestion of getting a couple of large cranes. Two sixty or eighty ton cranes would give you a little excess capacity in case a longer reach is required.

BA

 

[rb1957]

can you link the cranes together ? ie control all 5 with a single controller.

 

[dik]

BA... one of my last problems was moving a large telehandler down a mine shaft... There is little to no room, else a crane or two would be ideal... I'm not kidding when I note we have inches to spare... not feet... It's a contractor problem and he has to snug the hoists up appropriately... not electric or hydraulic... the hoists are manual.

And as they move the hoists down the monorail, there will be changes in the hoist loading due to deflection issues... That's why I have a safety factor (+G+). Just trying to minimise the effect.

Dik

 

[dhengr]

Dik:
Are you moving this girder 350', down the length of this tunnel shaft, lengthwise, standing upright, and essentially in a continuous horiz. plane (elevation) or maybe on some slight inclined plane? Thus, the headroom problem for the hoists, as a function of the tunnel headroom. As opposed dropping it down a vert. shaft. And, given the ULM2-160 literature I’m seeing, you have a trolley rail on the roof of the tunnel? If there is any deflection in the trolley rails, and whatever the take-up and locking length increments in those hoists, I’m not sure how you will control the loads on the individual hoists very well. And, you seem to have no way of measuring these forces/loads. Every hoist, and the trolley rail had better be good for (1.5)(16tons), or some such. I would guess the actual girder is stiff enough so it is not really the issue.

What would happen if you kept the/a trolley and trolley track, but replaced the actual chain hoists with hydraulic pulling jacks. These could be piped so that you could control the hydraulic pressure at each jack from one control panel. This allows for pressure adjustment, and load equalization, while various parts are deflecting.

Is there a mine haul, rail track system, on the floor of the tunnel? Could you put this girder on a couple small 4-wheeled trucks and move it down the tunnel length? Then lift or jack it into position. Heck, 8 or 10 temporary panelized sections of ties and track could be installed on the tunnel floor, and you could roll the girder into the tunnel on trucks or just skid it in on the rails on a couple sleds under the girder.

 

[dik]

appreciated... my original solution was to put the plate girder on wheels... everything is stiff enough that once they have the plate girder loaded, the loading on each hoist will not vary by much... we'll see what the contractor intends to do.

Dik