Jib crane base mount 1

[Samgg]

Hi everyone,

I need to mount a jib crane on a concrete floor. I have a base plate of 24"x24"x1". The concrete slab is 12" thick and it is armed with steel rods. The torque improve by the jib crane is 160 000 lbf*po. My question is : How can I know if the concrete will withstand the stress?

thank you!

 

[SperlingPE]

What is the spacing between the bolts in your base plate?
What size and spacing of reinforcing in slab?
Where is the reinforcing located (top, middle, bottom) in your slab?
Will you be embedding the bolts or will they be post installed anchor of some sort?

 

[Lutfi]

The jib crane base will see the following loads:

1. Axial load (vertical shear on foundation)
2. Bending moment that will vary due to the amount of horizontal rotation that the job will do.
3. Torsional load depending on the horizontal rotation of the jib and the angle of the wire rope with the vertical.
4. You may even have lateral shears at the column base.

I do not recommend placing the jib on a slab that has the following unknown (at least you did not indicate them):

1. Slab thickness
2. Compressive strength
3. The area of reinforcing steel in the slab
4. The location of the steel within the slab
5. The allowable soil bearing pressure
6. Is the jib going to be fixed at the base or pinned and it the top of the jib column going to be restrained/pinned?

As you see, you may need to do more investigation and have a qualified engineer make the design and recommendations for you. Giving us the bending moment of 160 Kip-Foot is not enough.

Regards,




Lutfi

http://www.cdeco.com

 

[jike]

You need to analyze your slab for current loads (such as DL + LL + whatever additional loads that the governing building code requires) plus the external moment applied by the jib base plus the vertical load of the jib. The internal moment produced by the external moment varies depending where it is applied along the length of the span. For a simple one way slab span, you should be able to calculate the internal moment, by hand, from statics. For a continuous one way slab span, you may have to do it by a computer analysis, since it is a statically interdeterminate problem. The combined moment then needs to be converted to rebar and concrete stresses and compared against allowable strengths or capacities, depending on which code that you are using. Make sure that you also check the shear stresses produced by the up and down moment couple.

For a two way slab, the problem becomes much more complicated and will require a much more sophisticated analysis.

I assume that this is a structural slab and not a slab on grade.

If you do not feel confident in analyzing this by yourself, then I recommend hiring a licensed structural engineer.

I hope this quick outline has helped. Best of luck!

 

[Samgg]

___
o | 3" between the end and the bolt
|
| 9" between the two bolts
o |
|
|
_o_|

For a total 24"x24" by 1" thick

I use Hilti HVA chemical anchor of 3/4" by 6 5/8" long (post installed)

The slab is reinforced by rod #6 spaced bye 12" at the top and the bottom of the slab.

The vertical load is 555lbs and the moment 160 100 lbs*po.

The concrete used is 3000psi.

With this, I'm searching for a simple and general method to calculate how much torsion a concrete slab can hold. I don't need something too accurate, I just looking for an approximative value.

Thank you for your help

 

[steve1]

For a slab on grade situation, assuming a reasonable size slab to prevent gross overturning of the slab, I would think that the failure mode would be punching shear through the concrete. This can be determined using the vertical load and overturning moment from the crane, the base plate size, the thickness of the floor, and the compressive strength of the concrete.

 

[JAE]

Samgg - there isn't a "simple and generic way" to calculate this. This is a complex problem - one that might require FEA or perhaps some reasoned judgements per steve1 above. The punching shear effect could also be considered after determining a proper critical line around the plate (bo in the ACI shear equations).

Be careful here, if you aren't sure what you are doing - get some help.

 

[theonlynamenottaken]

I design alot of davit installations into existing concrete roof slabs (a davit is basically a static lifting point, or a non-motorized crane). Steve1 is correct that the governing failure is normally punching shear. I usually use a slab-column moment transfer calculation for this application (ACI 318-02 11.12.6). Granted my baseplates usually bolt-thru and sandwich the slab so that calculation is a little more applicable. As far as the structural analysis for the bending in the slab this is a very simple FEA problem using plate elements with a couple of beam elements thrown in to apply the moment.

What is the "po" in lbs*po?

 

[Samgg]

The "po" is "in" (inch). Excuse me, my fist language is french so I make some mistake.

Your help is really appreciated. I think I will give my problem to a structural enginneer to be sure of what it is done.
thank you all