Moment Frame on Asphalt / Wheels?

[KootK]

I've got a client who wants a temporary lifting frame that would bear on asphalt at the column bases.

Do I need to consider the column bases as though they are on rollers? Or am I being excessively paranoid? I'm not sure that I have a friction coefficient for this case that I trust. The bases will be sized precisely to preclude their biting into the asphalt excessively. For all I know, the base plates may land on an old oil stain.

If I go ahead with this scheme, is checking the frame for the buckling mode shown below the only thing about the design that would be different from one where the base connections are laterally restrained?

Generally, is this a crazy thing to do? Would you insist on real foundations of some sort?

 

[jhnblgr]

What if you have a track with grooved wheels to provide some lateral resistance

 

[KootK]

To clarify, the present plan is not to use real wheels but, rather, baseplate grillage things that distribute the load. My question was about whether or not I should treat those as wheels analytically. That said, maybe real wheels now... wheel see.

 

[human909]

I designed something similar about 7-8 months ago for temporary lifting/installation works inside an existing structure. I made sure I had a decent frame to ensure stability. I believe the column bases were metal on metal.

Due to the relatively small size I didn't dwell on things too much. I just grabbed my factored loads, mixed in some lateral loads for good measure and threw some 200mm deep beams with decent endplate connections and called it a day. I believe I modelled one side as a roller but I can't say for sure. When site called me up and wanted to increase the safe working load by 33% I could ok it in about 10minutes as I didn't skimp on the members.

Regarding the buckling failure you have drawn I'll say that would be quite unlikely with normal proportions. That is gut feeling only. I suspect you'd reach member capacity before you would buckle like that. My gut feeling comes from the amount of work to do to buckle like that would likely reach member capacity before you reach buckling capacity. So IMO non-linear P-Delta approach would likely be sufficient. **GUT FEELING ONLY**

 

[Greenalleycat]

To clarify, the present plan is not to use real wheels but, rather, baseplate grillage things that distribute the load. My question was about whether or not I should treat those as wheels analytically. That said, maybe real wheels now... wheel see.

I would be treating them as rollers analytically and making sure I was comfortable with that deflection, knowing that the reality would be better for most uses
However, it's just as likely that one side sits on a stone, or a piece of plastic, or some other low friction surface, so it's a semi realistic design case

I think if you have a baseplate (I would gusset it) and a bit of out of plane bracing then you should be sweet as, this doesn't seem like a concerning one as long as the guys doing the lifting don't get stupid with their use of it

 

[Craig_H]

I used to work in the specialized rigging, crane, and heavy haul space. We had hydraulic gantries like this:


Granted, the use case was a bit different: every use had to have an engineered drawing with the specific equipment arrangement, operational limits, etc. Pretty tightly controlled and operated by skilled people.

We considered them fixed at the base, up to a capacity equal to some fraction of the vertical load and the lever arm of doubled skid rail. Exceed that and it's a house of cards. Friction with the ground was accomplished by placing plywood or high friction material between the ground (asphalt, gravel) and the skid track. Skid track had a means for preventing the towers from sliding off the track. Everything had to be dead level, there were limits on wind speed, etc.

The ability to track front to back along the base skid rail, and the option to place skid track on top of the gantry and slide the load sideways, was quite useful in fine tune positioning of equipment and transloading from rail to trailers and vice versa. I expect that your client will want to be able to make similar small movements if they are installing equipment?

I think this arrangement (fixed base, pinned top) produces a less troublesome set of boundary condition forces than fixing the columns to the gantry beams. It looks tippy but the beauty is that with stocky bases, the load magnitude stabilizes everything. The most dangerous loads were lightweight with a big sail area, and the "at capacity" situations with heavy loads were actually very stable.

 

[Lomarandil]

Craig beat me to it. Don't discount the stabilizing effect of a heavy load.

And it's hard to get a static friction coefficient less than 0.1, even in wet conditions.

 

[DBronson]

K:
Have the Owner hire professional machinery riggers - 5 tons is child's play for them. They have fork lifts that can lift way more than that. You can backstop their approach with calc's if necessary.

Regards,
DB

 

[human909]

K:
Have the Owner hire professional machinery riggers - 5 tons is child's play for them. They have fork lifts that can lift way more than that. You can backstop their approach with calc's if necessary.

Regards,
DB

That is assuming you can get fork lifts into the area.

I would presume that if you are designing a lifting frame like this then you are doing it because you can't get normal lifting lifting equipment into place.

That was the case in my previous example and I've designed manually operated lifting frames for 20T loads. (It took muscle and almost half an hour but if it is lifted and placed safely then that is what matters.)

 

[rb1957]

"surely" you're going with the largest McMaster-Carr option (and living with the 10 kip load) ?
Or redesign one, knowing what works for 10kip ?

 

[BridgeSmith]

For all I know, the base plates may land on an old oil stain.

If the pressure is high enough, it may squeeze some oil out of the asphalt, and you'll have an oil slick, anyway, even if there wasn't one before.