Pick Up Points
Pick Up Points
(OP)
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
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






RE: Pick Up Points
RE: Pick Up Points
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.
RE: Pick Up Points
thanks, Gentlemen
Dik
RE: Pick Up Points
2) why so interested in getting equal reactions ?
RE: Pick Up Points
RE: Pick Up Points
rb1957, your proposal does not result in equal reactions.
BA
RE: Pick Up Points
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Pick Up Points
Dik
RE: Pick Up Points
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?
RE: Pick Up Points
@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).
RE: Pick Up Points
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.
RE: Pick Up Points
woodsman... neither have I, one of the perks of my job... something new, almost all the time.
Dik
RE: Pick Up Points
BA
RE: Pick Up Points
RE: Pick Up Points
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
RE: Pick Up Points
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.
RE: Pick Up Points
Dik