How to determine casted slab can take the vehicle load? 1

[neaveyoung]

Recently we are planning to park a mobile crane on the casted rc slab. But how we can check / determine if the rc slab can taking this load? I'm trying to check the deflection and punching shear on rc slab. Is these checking enought to determine the slab can taking the load? Thanks for advice.

 

[Qshake]

Is the slab recently placed? If so, you'll need information on the concrete compressive stress from cylinders or cubes before you'll have anything to check.

If the slab has been there awhile but unintended for crane loading, then you'll need to verify the compressive strength of the concrete and reinforcing. If it is a slab on grade you'll need the subgrade modulus too.

Then you'll need to verify the loading. Will the crane be supported by outriggers entirely or share loading with rubber tires. Or is the crane a track type?

Depending on your familiarity with various software you can develop a model of this or use theory of plates to solve. There are a lot of stress tables/charts for point loaded slabs (if you have outriggers) but the uniform line load may be a tad more difficult. I would think it's in Roarks Book of Stress/Strain.

If there are outriggers then chances are you will have quite a bit of dunnage under each rigger to distribute the load out over a larger area of concrete. This will help prevent punching shear. Deflection will be dependent on the subgrade, loading and flexural rigidity of the plate/slab.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BAretired]

Check moment capacity, shear, punching shear, deflection and vibration. Be very careful to consider the loads very carefully including impact if the crane is operating atop the slab. Consult OSHA regulations pertaining to Mobile Cranes.

If you are in doubt as to how to proceed, get an experienced engineer to check it for you. Note that there have been many failures of concrete slabs supporting mobile cranes.

BA

 

[BAretired]

Here is what can happen if you're not careful.

BA

 

[Qshake]

BAretired - I take it that is an elevated slab?

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[msquared48]

Not now!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Qshake]

Msquared48 - I agree - more like an elevator shaft with no elevator and one big useless peice of junk baring the shaft!

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BAretired]

Here are a few more pics of the same accident and also some discussion.

http://www.heavyequipmentforums.com/showthread.php?t=8316

BA

 

[neaveyoung]

Thanks all.
The suspended slab was casted more then 3 months ago and had achieved the required concrete compressive strength.The crane have outriggers. But base on the crane loading i have check and assume the outriggers will park on the middle on span, then it must required at least 1700mm2 main reinforcement. So could i say that the slab have 2 layers main reinforcement T13-100 and the rebar provide is 1330mm2 x 2 = 2660 mm2? Enclosed the slab details.
Thanks.

 

[hokie66]

No, you can't assume how much reinforcement the slab has. The best source for this information is the drawings by which the slab was built 3 months ago.

 

[cap4000]

Its a 12 inch slab with #4@6 inches on center with a 15 foot span. I suggest you use Finite Element program like RAM or Staad. What are your outrigger loads??

 

[dik]

As noted, you need to determine the loads and the slab capacity. If necessary, you have to add timber/shoring posts beneath to transfer the load to something that can accommodate them.

I come from an era where FEM was not the panacea and the design can be checked using by relatively simple analysis unless the column spacing is pretty wierd...

I didn't look at the info provided, but a 12" slab spanning 15', if there's any continuity, is pretty robust; you still have to 'do the sums...'

Dik

 

[neaveyoung]

hi hokkie, the rebars is base on the construction drawing.
The outrigger load is about 100KN each. I don't have any FE software to analysed.

 

[cap4000]

100Kn = 22.5 Kips. What is the outrigger spacing??

 

[neaveyoung]

Should be about 3000mm.

 

[Qshake]

neaveyoung - All have provided good advice. If you don't have access to the conventional tools a structural engineer would use to solve this problem then perhaps it's best to seek out other professional advice or peer review.

As noted previously you should not be assuming anything.

Any method, Roark's or FEA or even peer review should be used in tandem to provide a check on each method.

The outriggers typically are shored up using mats of 4x4 or 6x6 timbers specifically for spreading load out and avoiding punching shear failure.

Also contractors like to use larger safety factors to ensure they are not going to have a failure such as posted by BAretired. Short of that failure contractor's don't want to be liable for cracks in slabs either. So be conservative.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[cap4000]

What is your bending moment?? Westergaard has a formula for 4 points loads on a slab.

 

[BAretired]

neaveyoung,

Wind forces can alter the crane reactions substantially. They can act in any direction and cannot be ignored. Here are a few links to the subject of wind on mobile cranes.

http://www.google.co.uk/search?hl=&...ff&rlz=1B3GGGL_enCA259CA260&ie=UTF-8&aq=2&oq=

If the slab thickness is 250 mm as shown on your sketch, it is 10", not 12" as earlier stated. I agree with hokie and Qshake that you cannot assume anything. This is not a job for amateurs, but if you decide to tackle it yourself, make sure that your professional liability is paid up.

BA

 

[Qshake]

There is another reference "Cranes and Derricks" by Shapiro and Shapiro that is now in it's 3rd Edition. I have an earlier edition and find it to be a valuable desk reference.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[miecz]

The forces to the slab will depend on the crane used. I think you need to find out which crane will be used for the job. Then go to the crane manufacturer to get the crane reactions for the lifted load and the operating geometry, including the outrigger width and the arm swings.