New Jib Crane on Existing SOG

[dik]

Does anyone have a good reference to the installation of a new jib crane on an existing slab on grade. Something that allows me to take a look at the axial load of the crane and the base moment and determine the adequacy of a thin (aka maybe 5" or 6" unreinforced concrete slab (in good, uncracked condition). I've had numerous of these lately and have just been looking at bearing, and concrete shear based on a bolt circle. I'll write up an SMath program for the condition and will post it.


Dik

 

[Brad805]

Most of our jib cranes come from Spanco. They have some foundation information.

https://www.spanco.com/products/jib-cranes/

 

[r13]

Most jib crane came with base p1ase plate attached, and having predetermined bolt holes drilled. The tasks left for you are to check the strength of the slab, and determine the length and method of bolt installation. Depending on the lift capacity, you might need to provide pedestal, or concrete pad to increase the bearing area though.

 

[dik]

The problem is that I often get jib cranes with small loads... less than 1T, with a 15' boom, and the client wants to install it on a 5" slab... The bolt anchorage and bearing are normally not an issue. Third project like this in the last two months.


Dik

 

[r13]

Yes, bearing might be fine, but you might have difficulty to develop tensile capacity with post installed epoxy anchor in 5" thick unreinforced slab. Example: Assume lift load 1000#, and 8" lever arm for anchor bolts (2), T_bolt = 1000*15*12/(8*2) = 11250#/bolt. Hope I didn't make mistake.

 

[dhengr]

Dik:
30ft-kips is no small moment, plus some potential impact. And, don’t forget that many slabs are less than full thickness, without considering any cover for the A.B. embedment. I’d opt for saw cutting a square section out of the slab and putting in a flush, and designed for purpose footing.

 

[Brad805]

We always cut out the slab as I never have reliable information for the existing.

 

[ldeem]

I just looked at the Spanco slab requirements and they list (among other items) #4 bars 14" OC or wire mesh per ACI 318. I wonder if they are relying on steel reinforcement for slab bending? If so and given slab reinforcement location is almost always at some random depth this seems like a partially unwise criteria.

Edit - the PDF lists wire mesh as an alternative but the website specifically excludes wire mesh.

 

[dik]

Compared to some of the stuff I see... it's tiny... I'm trying to find a rational method of checking the slab to accommodate it.


I'm trying to avoid that I have no issue with 8" slabs for small loads and moments... but have had several 5" ones and want to find a method to prevent cutting out a chunk of slab... I've been checking the shear on the bolt circle and using about 1/4 of the plain conc shear strength... but, there has to be a better method... yesterday a project came in with 4-1/2" slab... and said it wasn't possible... didn't even allow the Hilti embedment length.

The fastener isn't the issue at all... that's the easy stuff...

Dik

 

[KootK]

TR-34 is what I've been considering the state of the art as of late. I'm not sure that even that tackles a significant point moment though.

I don't love this for thin slabs where you're not able to confidently identify the size, spacing, and vertical disposition of any reinforcing. In the absence of reliable reinforcing, you're essentially left relying on the flexural tension capacity of a shrinkage prone concrete element to stabilize a lifting load against overturning. Even just typing that out makes me feel dirty.

 

[hokie66]

I think there are good reasons why you can't find any analysis tools for that. Why risk an expensive apparatus, and create a potential safety issue, just to save a bit of concrete work?

 

[r13]

I'll add a concrete pad with a few u bars to increase embedment length and pull out strength.

 

[Once20036]

I`ve also seen a couple of these recently and was faced with a similar challenge - similar size, similar loads, similar slab, etc.

I took a "clean slate" approach and designed a spread footing with OTM. It ended up being 4'x4'x24" thick, governed by maintaining positive bearing pressure under 50% of the footing area. This criteria was selected by engineering judgement to minimize rotation at the top/tip of the jib crane.

Then looking at the problem at hand, did I think that my 6" slab could provide similar restraint? Nope...
Would I consider an 8" slab with mesh to provide similar restraint? Nope...
Would I consider an 8" slab with 1 layer of bars? Probably not...

 

[dik]

TR-34?

Was treating it as plain concrete with a SF of about 4 for shear and tension... any reinforcing is just added icing on the cake. I know it's a bit risky but no catastrophic failure. I've seen 4'x4'x4' 'blobs' installed below for light loads... Many jib crane suppliers have that as a typical foundation, with the passive soil pressure helping to resist the overturning.

Maybe it's a bad idea... The e value is about 9'...

Dik

 

[KootK]

 

[Once20036]

I`m a fan of TR-34, I believe it's the British standard.
Its a much more rational, transparent method than a lot of other SOG design tools.
The results (for interior single point loads) are similar to the old US Army methods, but has procedures for things like multiple loads, point loads on edges and corners of slabs.
As KootK indicated above, I don't think there's anything for point moments.

 

[dik]

Thanks...

Dik

 

[r13]

dik,

I would simplify the task to two checks.

1. Bolt embedment length.
2. Direct shear capacity of the existing slab. I would not do the fancy analysis, but simply assume the toe plate alone taking the compressive force, C = M/d, the distance "d" is the lever arm between the bolts in tension and the plate toe, usually it is pre-determined by the jib crane manufacture. The resistance force is F = v_a*t_s*b_eff, in which v_a is the allowable shear stress for plain concrete, t_s is the slab thickness, and b_eff equals b_p+t_s. From the comparison of demand and capacity, you can easily determine the required thickness of the slab, thus decide corrective action.

 

[Once20036]

Retired13 - no consideration for flexure or where that 10ftkip of overturning goes?

For reference, here are two manufacturers that recommend big footings (4'+) for 500# loads on a 15' arm:

https://www.contrx.com/contrx-cranes/products/floor-mount-jib-crane-a-series/

https://www.gorbel.com/docs/default...tenance-manuals/fs-jib.pdf?sfvrsn=fe4b0b19_21

I`m still having a hard time seeing how a thin slab is an adequate substitute.
If it was easy, the manufacturer's would know it and sell it.

 

[dik]

I've used them for 10 and 12" slabs... and checked it out... was just wondering if this could be done with thinner slabs... had a 5" and my initial reaction was 'never'... but wanted some means of qualitatively checking. Still a no go... I've seen many of the Gorbel drawings and they have a 4'x4'x4' blob, or similar... and, yes it their typical detail shows this blob, it could be risky doing otherwise.


I calculated the moment of inertia and section modulus with an annular shape with a thickness of 0.01". Calculated the force and divided it by 1" and the depth of the slab... this gave me the shear stress in psi. I then used a LF of 4.0 and LRFD design to see if the shear was OK... I can adjust the bolt circle, slab thickness, load factor, and concrete strength to see if it works against the applied moment and axial load... seems to work well, but 5" slab is 3x overstressed based on a LF = 4. Really a no-go.


Dik