Crane outriggers sat on slab on grade 3

[Sense Check Me]

I need to design a slab-on-grade that can withstand 4 outrigger loads from a 130t mobile crane. I've designed a fair few crane pads on compacted gravel, but never a permanent concrete area.

So far I've got spreader mats under each outriggers to spread the (huge!) point loads. I've checked the rc slab for punching shear and taken load spread at 1:1 down into the underlying sub-grade for bearing capacity checks. For good measure I've also checked spanning for local depressions (future voids) under the slab.

Now, a few thoughts I've got. I conferred with snr colleagues and they reckon the ground is very good (its 1m thick well compacted imported fill) so the ground directly under the outriggers pads is more likely to behave as rigid and the whole area unlikely to act in flexure between the outriggers.

I've been requested to keep the slab 200mm (8") thick. I've added bottom rebar for the punching shear, and top rebar to control shrinkage cracking (but with control joints located between the 4 outriggers).

Does the approach sound sound? Or am I missing fundamentals like spring stiffness and ignoring the slab could acting in bending. Welcome your thoughts.

I can try and upload a sketch if it helps

 

[OldDawgNewTricks]

Yes, I think you are missing fundamentals like spring stiffness (subgrade modulus) and slab flexural stresses. 8" thickness sounds way too small.

 

[dhengr]

Sense Check Me :
Certainly, you do have to consider the soils conditions, and probably want to get a GeoTech guy involved to advise you, not just your “snr colleagues” opinions, to get this ‘damn thing done.’ I think it probably can be done. Whether an 8" reinforced slab is enough, you must determine. Is this a lifting situation that happens often, and where you can exactly dictate the location of the outriggers, literally painting four sq. boxes on the slab for the outrigger locations? Remember the possibility of most of the weight being on only two outriggers (50-50) or maybe even (30-70) at ultimate loading. Get the crane manual to see what the worst outrigger loads are. You already have a steel foot pad which comes with each outrigger, say 4'x4' sq., right? Maybe rotate that sq. foot pad 45̊ to get a longer cantilever span of the steel foot pad. Put that on a short crane mat made up of 6 - oak 8"x8" timbers; and the short mat rests on a perpendicular longer mat made up of 6 or 8 - timbers (number and size to be determined). This way you might be able to reinforce and/or thicken the slab in very specific areas. If the location is random, then you have to look at the whole slab, and stay away from slab edges and joints, but you still have a better/larger defined footprint dictating the slab and rebar. If this is installing a large piece of equip. and then possibly replacing it in 20 years, you might look at a tighter FoS. This is not long duration loading and the primary concern is crane stability, and the loading that causes. You now have several layers of beams on elastic beams, on the conc. slab on an elastic foundation, and a much larger punching shear or bending area to help you out. Each layer’s deflections determines the loading on the next lower layer.

 

[Sense Check Me]

Thanks for the replies.

It's provision for a maintenance lift (probably once every 20 years), very much a temporary case. Yes - I think we could dictate the position of the outriggers.

I've got the Lift Plan which provided the max and min point loads under each outrigger (the max. = 50t approx.). The crane supplier would provide the proprietary mats that travel with the crane. I've been advised to take 1.5m x 1.5m (5' x 5'). A statement to be added on as-built drawings and demarcated (painted) on site, I expect.

If this were a gravel-only area, I'd simply design the pads independently - taking appropriate load spread through spreader mats/grillage and check the mats for bending and the ground for safe bearing capacity.

With this being RC it's making me think harder. I consider the room provided for the slab is quite small - 12m x 12m (40ft x 40ft), the outrigger centres are 7m x 8m (23ft x 26ft) so close proximity of the outriggers to the slabs edge (punching shear of the slab) is governing the reinforcement requirements atm. There's a few issues which mean the slab isn't quite sitting centrally on the slab, due to shallow services under the slab.

 

[Sense Check Me]

Because of the overall size of the slab, I had thought about the requirement for control joints (contraction, saw-cut type) for shrinkage - to prevent undesirable cracking-patterns. But not sure if this is a wise decision now (cutting top reinforcement).

 

[Sense Check Me]

I guess what makes me think there is some mileage in my colleagues approach, is that if this were an existing concrete area (well constructed, but unknown reinforcement quantities), I suspect that the only checks that would be undertaken would be to take a 1:1 load spread through the concrete and check the ground for safe bearing capacity. This possibly alone to ensure the crane stability, not addressing the structural capacity of the concrete (taking it to be non-structural).

 

[HTURKAK]

- If this is new construction, is it possible to install the equipment with the crane supported on compacted gravel then pour the SOG?


In this case, crane operation area is quite small. I would go for a dedicated area with thick 12 in, dictate the outriggers position and provide CJ only..

 

[Sense Check Me]

Good points!

It's new construction - the intention is we would leave site providing an RC slab-on-grade such that a maintenance crane could use it in say 1 year or (possibly even 5-10 years time!). However, I'm now toying with the idea of just leaving a gravelled area, and no RC slab at all, so long as this can be agreed with the client. I think this would simplify matters and eliminate the potential structural behaviour issues/queries discussed above.

Giving it more consideration, it may prove quite challenging to design the slab for bending as I only have max and min loads for each outrigger, so possibly not enough information to analyse how the slab would be behave for a particular loading/slewing case. Possibly I'm wrong there, it could be akin to a tower crane base analysis.

The advice from the site team is that it would be particularly challenging to sit the crane directly onto top of 4 individual RC pads, due to manoeuvrability of the crane - hence the idea to have one big area.

Hmmmm.....interesting one this!

 

[HTURKAK]

Just info.
A few decades ago, i have designed a steel plate storage bldg and left the storage yard compacted gravel. The thick steel plates ( slabs ) stacked on the 200 X 200 timbers . The average loading under the stack was 250 kN /m2..

 

[Sense Check Me]

Unfortunately it appears the client is expecting an RC slab (not a gravelled area) and that is what must be provided. I've got plate bearing tests undertaken on the underlying stoned area and they are showing a well-compacted, firm ground.

Having spoken with a few more colleagues, they akin this problem to how would one approach the outrigger load acting on existing RC road (e.g. where we couldn't control the reinforcement provided), in which case the consensus was to design for:
1. 1:1 load spread through RC slab
2. check safe bearing capacity of subgrade
3. check punching shear

and would not be concerned with the structural flexure of the road / slab between the outriggers.

 

[EdStainless]

We had a similar set up at a plant, but ours was intended to be used every other year.
There were four pads roughly 6'x6' that were rather thick for the crane outriggers.
The entire area was paved because it was a traffic path.
Put the paving was separate from the pads.
Yes, over time it became slightly uneven.
But grinding of raised edges was easier then maintaining a high traffic gravel area.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Sense Check Me]

Thanks everyone.

I've got a meeting with the site team later to discuss. I think the preference is now:
1)have an large RC slab area (as agreed with client)
2)consider 4 individual pads cast incorporated into the slab area (challenging to possibly get the outriggers on centrally due to manoeuvrability of the crane)
3)if option 2 can't be adopted, revert back to a slab cast over full area and consider loads acting locally through slab only.

This discussion has been really insightful, greatly appreciate the replies.

 

[OldDawgNewTricks]

Having spoken with a few more colleagues, they akin this problem to how would one approach the outrigger load acting on existing RC road (e.g. where we couldn't control the reinforcement provided), in which case the consensus was to design for:
1. 1:1 load spread through RC slab
2. check safe bearing capacity of subgrade
3. check punching shear

and would not be concerned with the structural flexure of the road / slab between the outriggers.

Long, long ago when I was an undergrad learning solid mechanics and structural analysis, my professors repeatedly drilled into my head that an analysis should account for all of the following basics:

1. Equilibrium (static or dynamic)
2. Compatibility (of strain/displacement)
3. Constitutive (stress/strain) material laws
4. Boundary Conditions

Your design procedure satisfies Equilibrium but falls short for the other criteria. The Boundary Conditions should account for the loaded area of slab being continuous with the adjacent slab, Compatibility dictates that the subgrade will deflect under load, and the Constitutive properties of the slab will cause flexural stresses in the slab. Therefore, your analysis is a lower bound theorem solution that may not "collapse" but will likely experience significant cracking as the real conditions adjust themselves until they conform to your idealized model.

You might want to read up on Professor Harold Westergaard's original research (1920's - 1940's) for point loading on slabs-on-grade that forms the basis for many modern design methods. Other good (U.S) references are ACI's Design of Slabs on Grade, UFC's Concrete Floor Slabs on Grade Subjected to Heavy Loads, Ringo's Designing Floor Slabs on Grade, PCA and WRI design methods, etc.

 

[Sense Check Me]

Thanks for that OldDawgNewTricks - that does make sense. I'll read the reference material recommended.

Would the logic thus also extend to the situation of a crane set up on an existing RC road? Could the RC road also be expected behave in such manner in which the flexural stresses occurring throughout the slab could cause cracking / damage to a road? It's something that never occurred to me to consider.

Out of curiosity (I'm keen to learn!)....how would one approach the design of such a situation (when little maybe known about the reinforcement quantities and construction)?.

 

[OldDawgNewTricks]

Yes, in general it would apply to a concrete road as well as a site slab-on-grade. There are a few differences for roads (additional base courses used to improve the subgrade modulus, more frequent load cycles accounted for in the design procedure, etc.) but the basic theory is similar.

If truck operators are instructed to operate on concrete slabs, they will often make the property owner sign a waiver stating that the slab is designed for the truck loading and/or the truck operator's company is not liable for any damage that is caused. The owner will not be pleased if a lot of cracking results the first time the slab is loaded.

Here's a reality check (since your username is Sense Check Me): I would expect that your proposed slab thickness is appropriate for typical truck loads (highway truck, concrete mixer truck, fire truck, etc.) to drive across. However, the outrigger loads for a 130-ton mobile crane might be on the order of 4 or 5 times larger than the axle loads for those trucks. Even with outrigger pads/blocking/cribbing to distribute the load over a larger area, I would still expect you to end up with a much thicker slab than you have proposed if the loaded portion of the slab is continuous with the adjacent slab.

 

[Sense Check Me]

Again, thankyou for responding and challenging the logic behind this.

The outrigger pads exert approx. 20 t/m2 onto the rc slab (50t over 1.5m x 1.5m mat area). The sub-base is well prepared (well-graded, well-compacted) and is at least 1m thk of mot type 1 (installed before I was involved in the project) and has plate bearing tests giving deflections of less than 3mm for an applied load of 75 t/m2.

I guess I'm imagining the concrete to behave more like a packing material, rather than a flexural member.

At present, I'd prefer site to cast 4 individual thicker concrete pads, but there's limited space, shallow services and concerns if the crane would be able to centrally load the pads in future.

 

[OldDawgNewTricks]

A couple comments on your numbers:

1. The outrigger load seems quite small for such a large crane. Typically, outrigger design loads will exceed 60% of the rated lift capacity (and sometimes exceed this value by quite a lot). If you are sure that this is the maximum outrigger load that will ever be needed, you probably want to specifically list the limiting outrigger load in your design criteria rather than just listing a 130-ton crane. It might be a good idea to even provide pavement striping to designate the permissible outrigger locations and to mark the allowable outrigger load so there is no confusion later.

2. You should ask the geotechnical engineer if it is OK to use the plate test results directly for design or if any correction factors are needed to determine an allowable subgrade modulus value to use in calculations. The raw numbers are very high.

Even if all of the above numbers are legitimate, I doubt a 200 mm slab pencils out so I would still want to increase the slab thickness and also specify a concrete mix with a high flexural strength. Or provide the separate thicker pads and separate them from the rest of the slab with isolation joints.

 

[Lomarandil]

I agree that it would be good to confirm the applicability of the plate beating test (or whether adjustment factors are intended)

But I disagree about the slab experiencing significant flexural stresses. In a location like this with a seemingly very competent subgrade, and the use of crane pads (which add some beneficial flexibility and redistribution), I see very little load being carried into the slab. Rather, the pad and concrete both serve to deliver load into the subgrade.

The practices of checking punching shear and subgrade adequacy are the common industry standard of practice, and I've successfully applied those methods above sensitive concrete with cranes much larger than 130tn.

----
just call me Lo.

 

[Denial]

If you are looking to get at least a feeling for the likely slab bending moments (positive and negative) in the general vicinity of the applied load, I have developed a pair of spreadsheets that perform rigorous analyses of a slab on an elastic ("Winkler") foundation under the action of one or more localised applied pressure loadings.[ ] One is for the case where the slab and the foundation extend infinitely in both horizontal directions.[ ] The other is for the case where they extend in only one direction, ie the slab has a single infinitely long straight edge and the loadings are applied reasonably close to that edge.[ ] The spreadsheets can be downloaded from my web site at

http://rmniall.com