Sliding Failure for a Skid on Ground 3

[Wintio 1]

Hi,

I've had a read through some similar topics and it looks like a few people have issues with sliding failure. The formulas seem straight-forward, I'm just not sure the results I'm getting are realistic. Was hoping someone here might be able to do a bit of a sanity check.

I've got an equipment skid being placed directly on the ground - 2.3m high, 3m wide - with a design wind speed of 43m/s. Even without applying a shape factor that gives me a wind load of 7.5kN.

The equipment weighs 2.5T (24.5kN vertical).

I'm assuming a 0.4 coefficient of friction between steel and ground. My design load case is 0.9DL + Wu.

This gives me a resistance of 0.9 * 0.4 * 24.5 = 8.82kN against a lateral load of 7.5kN. FS = 1.18 - way lower than the suggested 1.5.

There's a fair bit of skepticism around the office about a 2.5T skid sliding away in the wind (and to be fair, the FS is >1, so it shouldn't fail), but does anyone see anything I'm missing?

I've proposed a concrete slab underneath to increase the weight, but we'd like to avoid it if possible. Any ideas?

 

[IFRs]

Is the wind force constant for the entire height all the way to ground?
What would the shape factor be?
Can you increase the friction or add anchor bolts?

 

[MotorCity]

1.18 is lower than the desired 1.5, but I'd let it slide (pun intended)

 

[briancpotter]

I'd probably check sliding failure using 0.6DL + Ws (which should give roughly similar results to 09.DL + Wu), and I'd consider that to have the 1.5 FS built into it (1/0.6 = 1.667). On the other hand, 0.4 isn't a particularly conservative coefficient of friction. Your values for loads look reasonable to me though.

 

[JStephen]

One thing to do is decide if you need to prevent a sliding failure. For a permanent structure, you assume that's required. For something like a barrel or shipping container or vehicle, you don't necessarily assume that's required. Skid-mounted equipment could go either way, I would think.
Informative video:

https://www.youtube.com/watch?v=gx7s3Hn9oVw

 

[dhengr]

Wintio 1:
Note that the 1.5 FoS and the .6DL apply primarily to buildings and structures overturning or sliding, where people life/safety is involved. The .6DL entered the equation along with LRFD and the later IBC’s and later ASCE-7's editions. Before that, it was 1.5 FoS vs. DL and WL at their respective C.G’s, and this is still probably pretty reasonable for field equip. You certainly should know the light weight of your equipped skid, you’ve been lifting it during building and at the work sites. What are the application C.G’s of the two loads? What are the consequences and reasonable limits of any skid movement, either lateral or vertical from settlement? Does this hurt piping and mechanical connections, and can you (must you) design connections to tolerate some of this movement? Does this movement likely relate to people safety? For short term installations, I would want to eliminate the need for a conc. pad if I could, by proper site preparation and skid base design. You could design the primary skid beams for bolting (or bolting and grouting) to a conc. pad; and then also detail for adding 2, 4 or 6 vert. feet (ground penetrating) to add to the lateral sliding stability. These might be 1.5 - 2' long, 8 or 12" channels bolted to the main skid beams when needed, during the process of lifting the skid off the trailer for setting.

 

[TLHS]

First off, look at adjusting your importance factor. That might clear up your problem right there. The consequence of failure is likely low, so maybe drop that down.

Second, is this actually a 'building' that has to follow the building code as written or is there latitude? If it isn't, you can start looking at safety risks and cost/benefit. Maybe you're comfortable reducing your return period because of operational expectations or because it's temporary? Maybe you can look at the 30 second design gust and estimate how far that would actually move the structure and realize that it's fine. Maybe you decide it's an extreme design case and count in resistance from dragging attached equipment with it.

This isn't a very heavy equipment skid for the presented wind area, so maybe you decide that it makes sense to embed something into the ground.

What happens if you backfill to around top of skid? I normally don't count passive pressure in the top layers, but for this kind of thing I'd likely be willing to take some contribution.

 

[TLHS]

Or there's the good old fashioned oil field solution of bolting some 2.5ftx2.5ftx5ft concrete blocks to the thing for weight and calling it a day.

 

[Shu Jiang PEng SE]

Per ASCE7 for low-rise building (<60 feet), the maximum horizontal wind pressure on the wall of flat roof building is 19.2psf corresponding to 110mph wind. The wind pressure on the skid is 19.2x96.188^2(mph)/110^2(mph)=14.68psf=0.70kpa. The force is 0.7x2.3x3=4.83kN.

 

[racookpe1978]

But, if wind sliding is even a remote possibility, is not seismic movement and operation vibration bumping, impact, water hammer more likely?

Even a pump (almost no wind load at all) is bolted down to its foundation to prevent movement from vibration, heat, pipe forces, electric disconnection/shock if the power cable is twisted ...

 

[TLHS]

Seismic movement isn't necessarily an issue in a lot of areas. Plus, if it's in a place where some additional sliding won't hurt anyone, it isn't necessarily an issue. You can likely treat it the same way you'd treat a self anchored tank. The friction weight of the equipment is sufficient and they're not top heavy enough to overturn.

Additionally, in some cases, sliding in an earthquake just may not be a concern. I don't care if an unmanned reclaim water pump in a mine pit somewhere slides around in an earthquake.

Vibration is a serviceability concern. I've seen equipment that runs with reasonably low levels of vibration while being completely unanchored. There's a risk and you could affect machine life and system operation, but there are applications where that's reasonable. For process skids, where the it's not just a big piece of rotating equipment on a skid, you can often analytically prove that vibration is okay in a more conventional way.

Heat and pipe expansion can be an issue, but you can deal with it on the piping detailing side, or by looking at it from a deflection standpoint, by letting the equipment slide around a bit.

You definitely have to make sure a skid with rotating equipment won't flip over on an emergency shutdown or in the startup case. You can probably make an argument that the pipe helps you, there, if you really need to, but I would avoid going down that road.

With transient fluid forces, you need to have a good chat with your piping guys to decide what happens and where the forces are going. This can certainly be a concern.

Basically, your design philosophy moves from the pump being a solidly anchored point, to being something that moves (with resistance) along with the piping system. It changes the way you design the structure, the piping and the pump.

It's not a conventional way of doing things, and I've never seen it done for high criticality stuff, but it's pretty common in mines and other places where you might have large scale piping systems that move around.

Process skids where you have a few smaller pieces of equipment and various other equipment on a skid are more straightforward and can be approached in pretty conventional ways.