Free Standing Jib Crane bolted to floor

[jike]

I am reviewing a jib crane right now where the manufacturer is claiming that it can be just bolted to the floor (slab on grade) as long as the slab is of a certain thickness and a certain width with no cracks or joints within that area. The problem I see is that they are using a factor of safety of overturning of about 1.2 versus 2.0. They seem to be out there telling clients that no foundation is required.

Thoughts?

 

[Gumpmaster]

I've just bolted a jib crane to the floor before, but I specifically designed the floor to take the additional loads. You don't necessarily need a sepparate foundation, but the floor needs to be checked to make sure it can take the loads and resist the overturning.

 

[jike]

If everything checks out then fine but I believe here they are trying to push their product on the basis of "we don't need a footing".

 

[hokie66]

I would be concerned with any jib crane manufacturer who tells its customers what type of footing it needs. They should just specify the design loads and leave the footing design to the client's engineer. If they are pushing that hard, makes you wonder about the quality of the product they are selling.

 

[msquared48]

I thought that a FS of 1.5 was the minimum by code? Have things changed? To me, 1.2 is not acceptable. I would not accept the liability. The idiot factor, by experience, is at least 1.5.

What is the crane capacity?

Is this a single leg crane or multiple leg?

How thick is the slab?



Mike McCann
McCann Engineering

 

[gwynn]

What country is the manufacturer in?

Are they basing the FS on already highly factored loads in an LSD or similar scenario?

In some cases I've seen low factors of safety on overturning are acceptable, but the loads have already been factored well above anything reasonably expected.

 

[JedClampett]

I can see problems with not only the slab, but with the anchorage to the slab. The post installed anchor manufacturer has minimum depths of concrete to develop the anchor capacity. If it's a 4 inch or 6 inch slab, it's probably not enough.
It's very cheap to saw cut out the slab on grade, dig down and install a proper size and depth cast in place concrete footing with anchor bolts. For the amount of money it would cost, especially compared to the jib crane, I'd strongly recommend it.

 

[jike]

The 1.5 FS for overturning used to be for foundations with wind. The 2.0 FS for overturning used to be for foundations with gravity.

Now that those F.S. have been removed from the codes and replaced with load combinations, there is not a load combination for uplift or overturning caused by gravity loads (unless you want to use 0.6 + W), so I just revert back to what we have used for many years, FS = 2.0.

 

[Bagman2524]

McMaster Carr has a footing design (depth, width & anchor bolt size) for their jib cranes.

 

[gwynn]

Now that those F.S. have been removed from the codes and replaced with load combinations, there is not a load combination for uplift or overturning caused by gravity loads (unless you want to use 0.6 + W), so I just revert back to what we have used for many years, FS = 2.0.

I see nothing wrong with applying the old 2.0 safety factor for overturning, based on specified loads where dead loads govern. However, in a crane design the main overturning loads are live loads, so it is possible a FS of 1.2 is based on 0.8 or 0.85 time the dead loads resisting overturning and 1.5 or 1.7 times the live loads causing overturning. 1.2*1.5/0.85 = 2.1, so if based on factored loads the 1.2 may well be acceptable.