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

 

[r13]

Once,

Flexural behavior of SOG is everybody's guess. Locally, the concentrate load effect is basically taking care by tension (on bolt), and compressive (shear on concrete) as bearing usually is not a concern for this type setup. +/-F = M/d, isn't moment in the picture?

dik,

You won't have any problem to set the load on slab signed off by me, and it's part of the reason I've never specified SOG less than 8" , and usually doubly reinforced. The tendency of future installation always in my mind when dealing with industrial facilities. The type of business/production usually provide good clue on the type of equipment on latter expansions.

 

[dik]

retired13... this is an existing building that's been re-purposed... and, I like 8" slabs, too, but have done 5" ones too for light loading conditions.


Dik

 

[KootK]

If you wind up doing this with a 5" slab, I'd recommend load testing it to 150% of the rated capacity before putting it into service. There will be human bodies on the line with this and, clearly, nobody knows of a robust checking methodology for this situation.

 

[hokie66]

This should not be encouraged. Think of inexperienced engineers who might read this and say, "if these old guys are prepared to do it, why should we not?"

 

[r13]

dik,

I understand your situation, so the recommendation to check the strength of the existing concrete, and method to strengthen it as the situation dictates. I still prefer to provide thickened pad rather than cut out the slab and build independent footing, which is more suitable for heavier equipment, not for 1Ton jib crane. Though the 15' arm is somewhat out of the norm, maybe limit it to 10'-12' is more realistic.

 

[dik]

I just wanted to see if there was a methodology... I won't use a 5" slab...

Dik

 

[haynewp]

I would use a fine mesh FE with uplift point loads for the anchors and a uniform bearing for the opposite of the base plate if I wanted to look at the stresses. Model springs for the subgrade modulus below and check if the slab moment is more than the tension modulus of rupture with a safety factor. In general, I would never have thought to support a jib crane from a 5" slab.

 

[dik]

haynewp:

I wouldn't normally... but, was asked if it could be done and my initial response was no... I was wondering if there was a methodology of determining the slab thickness required for anchorage. I've anchored small jib cranes to 10 and 12" slabs, and even 8" slabs for very small loads... with a little care and lots of 'wiggle' conditions.


Dik

 

[r13]

dik,

Since the installation is on an existing concrete floor. The first instinct is to install epoxy anchors. As stated previously, you can get the approximate tension rather fast, from that, you can get required embedment length from the manufacturer's catalog, say Hilti.
But, you will find out that the demand of slab thickness could be controlled by the effect of compression, as epoxy anchor can develop very high strength in very short length.

 

[haynewp]

The anchorage is one part but then the effects on the surrounding slab is another. I would try what I suggested to show the surrounding slab can't handle it.

 

[Settingsun]

TR34 assumes reinforcement in some equations according to the answer I received elsewhere earlier this week.

The slab bending moment will be between half and one times the crane base moment. How wide a strip is needed to resist that in comparison to the anchor layout?

 

[dik]

haynewp:
Anchorage is the least of my concerns... looking at shear issues based on the bolt circle, plain concrete shear and flexure and anchorage to adjacent slab, if I have to cut a footing... didn't realise it could be so interesting... right now what I have done is made a thickened part of slab, looked at the plain concrete shear on the bolt circle using the thicker concrete as well as plain concrete flexure on the width of the thicker slab and a means of dowelling the thicker portion to the thin slab using dowels as tension items and not shear items... will continue through the weekend... might be able to go to a 4'x4' thickened part that my only be a foot or so thick instead of 4'... This also gives me a means of checking a thicker slab to see if it works OK. By using variables with SMath I can change various parameters to see how things change.

steveh49:
Picked up a copy of TR34 a couple of days back... will look at it this weekend... at first glance, it appears to be a good reference.


Dik

 

[Once20036]

Once,
Flexural behavior of SOG is everybody's guess. Locally, the concentrate load effect is basically taking care by tension (on bolt), and compressive (shear on concrete) as bearing usually is not a concern for this type setup. +/-F = M/d, isn't moment in the picture?

I`ve spent a lot of time wondering about that myself. Ultimately, there's a distance between the tension bolts and the compression bolts, and that's the moment that I`m talking about being transferred to the slab on grade.

If you assume a 1000# lift at 15', that's 15ftkip. Assuming a base plate with 4 bolts at 2' centers, you have 7.5k of tension, or 3.75k per bolt.
Taking the sum of moments about your compression bolts, you have 7.5k tension * 2ft = 15ftkip of force to be resisted by the slab.
Where does this 15k of tension go? It would need a 14' x 14' x 6" chunk of slab that needs to mobilize. If the slab cracks before then it doesn't matter how strong your bolts are.

Alternatively, ignore the tension argument - if the slab was good for 15ftkip I`d be OK with it. The last slab I checked was good for ~0.5ftkip with typical light crack control WWF.

 

[dik]

I'm looking at cutting out a square section of slab and making it thick enough that the shear on the concrete 'bolt circle' (not a rectangular bolt pattern) is OK. Looking at the width of the square section in plain conc flexure and looking at the shear at the edge of the rectangular section for the thinner slab and using shear friction to connect the thin slab to the thick slab... Then look to see if the actual anchorage is OK... the anchorage is small and the least of my concerns.

Looking at developing a methodology that I can use for other slabs and see if I can write up a simple SMath program to accommodate it.


Dik

 

[r13]

I still think build a raised concrete pad is simpler than cutting concrete. But, well, you make the call. By the way, unless dowel is provided, I wonder where is the shear friction is coming from, not by pouring concrete against each other though.

 

[dik]

I was planning to use dowels... as tension elements not as shear elements. To raise the slab, it's a matter of bonding the topping to it, I would suggest. It's still a work in progress...

Dik

 

[r13]

Dik,

I guess the left is you have proposed, that I suggest dowel is required to hold the new concrete in place and resist the applied load. My proposal is on the right. Correct me if I am wrong.

 

[dik]

Pretty much that... and minimal tripping and housekeeping issues... the approach on the right may be less costly.


Dik

 

[dhengr]

The raised concrete pad makes 20-30 sq.ft. of important/usable floor space immediately around the jib crane unusable, but they still pay rent on the space. The crane is servicing equipment or process line which likely needs/uses that space to function well. That’s pretty false economy. The dowels allow you to bring some of the existing slab into the moment/stability calcs.

 

[Once20036]

Both of those options are pretty reasonable to me.
The third that we've done in the past is a flagpole type foundation. Core a 24" hole through the slab, dig, and pour a new reinforced pier. It can minimize impact to the floor space compared to the square options I`ve looked at.