Large uniform crane load over shallow electrical vault 2

[terramatters]

I have a situation where there is a 45-ft-dia. Manitowoc ringer crane being constructed on steel and timber matting over an approx 2-ft-thick soil fill pad over existing on-site roadways designed for H-20 loading at an industrial site. Crane load will be uniform ground pressure of 4,000 psf. An existing 12 ft by 13 ft of unknown height electrical vault underlies the road surface by 2 or 3 feet. No info available on top slab, base and wall thicknesses and reinforcing. Crane will be in place 2 to 3 months.
Any thoughts on likelihood of damage to vault - low, moderate, high? One way to look at the 4ksf load is as 35 ft of fill at 120 pcf; not exactly the same but similar, like a vault buried 35 feet deep. Anybody come across an example of this or observed what actually happended?

 

[JedClampett]

I'm not an oddsmaker, I'm an engineer.
How much repair and inconvenience will result if the vault collapses? Does it service a hospital, a water plant, an operating factory that will have to be shut down? How many refrigerators will shut down?
My opinion is unless you own that vault and will suffer all the consequences if it fails, you should build a slab over it supported below the vault and remove the risk, no matter what the chance of failure.
What I've seen of precast vault design is while they're redundant, they're not robust. A lot of 5 inch or 9 inch thick thick walls and slabs.

 

[dhengr]

That’s potentially a nasty problem. You better learn a whole not more about the make-up of that vault, reinforcing, thicknesses, floor slab/foundation, and the roadway slabs, etc. You should have plans of the vault related to the ringer beam and the tracks on the crane. You should also have some cross sections through all of this, and some knowledge of the soil conditions around the vault.

Your suggestion that a 35' overburden might be similar may be relevant in some ways but way off base in others. This certainly would exert a worst case lateral loading on the vault walls as supported retaining walls; or a max. uniform load on the vault roof slab; and a fairly uniform found. loading. However, the 4k/sf is a nice average load (simplification) the crane guys use for estimating their mat design and soil loading, probably with a FoS of 1.025. But, what happens with the ringer beam spanning diagonally over the vault, no load on the boom, but full counter weight load on the ringer beam right over the vault; or the opposite, full load on the boom out over the vault? That’s quite a concentrated load on two short lengths of vault wall and the footing under these wall portions. What about a track or the ringer beam fairly near the vault walls and its action as a surcharge loading on the supported retaining walls of the vault.

I think you really have to look at and study all these kinds of conditions and loadings to optimize the positioning of the crane and ringer beam. Or else design some foundation improvements to keep these concentrated loads off of and away from the vault.

 

[terramatters]

Thanks very much for your quick and helpful response Jed and dhengr, they confirmed what I was thinking.

 

[msquared48]

I would really engage the expertise of a local structural engineer too here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[chicopee]

Is the ring right over the vault? If it is I would select another site or build a platform spanning the vault for the ring.

 

[terramatters]

Thank you MMC and chicopee. The ring (via its 34 equally spaced steel support feet/pads) sits on a layer of steel matting (8'x16'x11" thick), which overlies a layer of oak matting (4'x20'x12" thick) oriented 90 degrees to the steel matting; hence load will be reasonable distributed once it reaches the vault. Overall the matting measures about 60'x60' in plan. The soil fill foundation pad below the structural matting is about 1 foot thick along one edge, and about 2 feet along the opposite edge, due to the sloping roadway below. The structural engineer for the crane pad has max 3,800 psf from the crane and timber/steel matting applied onto the top of the soil foundation pad during lift of the 110T vessel, and 3,200 psf for other times when the counterweight is the controlling load. [client has been advised several times to get proper evaluation for the vaults and utilities under the crane but oddly has not followed advice; we've given only rough preliminary guidance given limited info on the existing structure conditions]

 

[msquared48]

Don't approve anything here without the client engaging a structural engineer. No reason to be the sacrificial lamb to line his pockets.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

Is this one of those canes that has outrigger legs cantilevering out from under the ring beam or a ton of hydraulic jacks under the ring?

I can't imagine it is cheap to get one of those things in place and erected. That being the case, you might as well hire a SE to design a proper foundation.
As expensive as the crane and engineer might be, cleaning up and paying for a toppled crane has got to be a lot more....not to mention the risk to all those involved.

I was on site for a demolition job once upon a time and some abandoned under ground plant utilities went unaccounted for. The outrigger pad for a 100 Ton rough terrain Grove Crane sunk (12" or so) during operation because of some voids below. No one was hurt, but the ensuing fight between the operators and the engineers in charge of the lift plan was lengthy and cost to the job schedule were large.

 

[racookpe1978]

"Averaged" loads through the pad to "generic dirt" are correct when planning - but only when there is "average dirt" under the pad over the whole extent of the pad.

Here? You've got a bad combination of two problems.

For the actual localized, point loads under the pad as the crane and its rolling counterweight are turned (or the boom and load are lifted while the rig is stationary), the vault and its roof have to act like a bridge. A very low-strength bridge of almost unknown capacity. Failure could be by sagging in the middle of the span, by punching through, or by collapse of the whole roof.

But, then as the rig is further rotated, the transient load won't be over the middle of the vault, but over the ground (fill) around the vault. So that fill needs to resist the new and ,moving compression load - which means the vault wall (and floor) needs to resist the load without further compression. Failure would be collapse of the wall of the vault, or local squishing of the fill itself. In either event, the pad fails (yields) and the crane become unstable.

You can't use "average" reactions to the transient point crane loads under the pad unless the pad itself is so rigid that it will actually create "average" loads under the pad.

 

[ztengguy]

The crane guys dont mind setting up over this, or are they asking for the evaluation?

 

[dhengr]

Terramatters:
I think you got the drift of JedC and my original posts, but to put it another way.... going back to your OP; you have a 12' x 13' void (vault), unknown height, unknown roof or floor slabs, or found. conditions other than the floor slab; only a few feet under a crane mat, and about 4k/sf from the crane on that mat. A very quick calc. (12' x 13') = 156sf x 4k/sf = 624k, on the vault walls and the vault found. somehow, assuming the mat and the ringer beam and its support pedestals can span the 12 or 13' vault, not a certainty. It wouldn’t be unreasonable to add a couple ft. immediately around the vault as adding to its wall loads, thus (16' x 17') = 272sf x 4k = 1088k, on the vault walls. And, this says nothing of the increased lateral soil pressures on the vault walls. And, there will be cable tunnels or conduits of some sort entering that vault, only a few feet under ground.

 

[racookpe1978]

Hmmmn.

I think I see what you mean with that (even rough) calc.

If the span (the vault roof) could hold the force, and even if the vault walls could withstand the side loading from the soil around the vault that I was thinking of, you're predicting the walls themselves (or the walls and their immediate foundation) would need to resist that much force forcing them vertically into the soil. So another failure may be driving the vault walls straight down - assuming they don't blow out sideways first.

 

[terramatters]

To all contributors: thank you all for your input. Client and owner were finally convinced there iss a potential/very likely issue, and they provided access (w/confined space entry permit) for an actual look at this electrical vault; a 13.8 kV one by the way. The site is at a refinery, which I visited last
Thursday (2 hrs away). I entered the vault with the electrical contractor and confirmed all the sizes and dimensions and equipment (9.5' square outside, 8 ft square inside, 6.6' tall inside. 4 conc duct banks entering the vault with close to 50 conduits carrying wires/cables of varying thicknesses. I worked with our in-house structural engineers and we developed a scheme of vertical and horizontal (both directions) jacks of about 40 to 85 kips. After vertical jacks are installed vault will be partially filled with sand to first horizontal jack level (sand used as support due to danger of dropping 12-inch-square by 1-inch thick plates at ends of jacks onto cables and causing short-circuits and possible fatalities and total shutdown of refinery). Sand/jacks will then be added to top of vault. After crane lift is done sand will be vacuumed out. This all assuming scheme is accepted for risk by fire/safety team (have not heard results of meeting on that). There is a sump with pump in bottom of vault, electricians said even if it filled up with water the electrical will not be an issue-all waterproof apparently. Will be interesting to hear how it all works out. We had also recommended ongoing settlement monitoring during test lift and actual lift.

 

[rollerich]

1. Do not put any load on the vault. Locate the vault. The vault is designed probably for 18K axial load which is 9 kips on 2 tires down 3' which is less than 1000 psf.
The crane will break the vault.
Mike Ollerich PE 40 years and I work on a lot of law suites you will fix every thing and pay for down time.

mollerich@ats-sd.com

 

[bridgebuster]

I agree with rollerich. IN NYC street vaults are designed for 600 psf.