Concentrated Moment on Concrete slab

[ugandabob]

Hi,

I'm checking a 10" slab for the installation of jib cranes. The jib cranes will apply a concentrated moment on the slab, near mid-span. The jib cranes will be located in several places; sometimes in column strip, others in the middle strip.

How much of the slab contributes to the resistance of this concentrated moment? The base plate is about 24"x24". The slab span is abour 24ft in each direction.

The Canadian code doesn't offer anything, and textbooks seem to ignore concentrated moments/loads on slabs.

Thanks

 

[rowingengineer]

I would take the same width as a point load on a slab. see thread507-223271


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[rowingengineer]

I would then design this as a beam of this width spanning along the strips. detailing would need to ensure the loads are transfered about the attachment point.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[BAretired]

ugandabob,

Are you attaching the jib crane to the slab with through bolts?

BA

 

[csd72]

ugandabob,

Is this slab new or existing?

 

[ugandabob]

It is an existing slab. I don't think the anchorage is up to us; i think the jib crane provider designs the connection. We just need to check the slab.

If not, we would likely go with a bunch of long adhesive anchors. We don't have access to the slab underside; there is a large water tank directly below that covers the entire floor area.

 

[BAretired]

I don't think tributary width is a valid concept in this instance. A concentrated moment will produce tension in the top on one side and tension on the bottom on the other.

If the jib is located in the middle of a middle strip, there is probably no top steel, so you have to ensure that the combined stress is not tensile in the immediate vicinity of the jib base.

You also have to ensure that the combined tensile stress on the bottom does not exceed the capacity of the bottom steel.

BA

 

[csd72]

There is also a potential punching shear issue that needs to be checked.

 

[ugandabob]

BA: The drawings specify top steel (minimum) at the intersection of middle strips, so I think we'll be alright. From what I can tell, the top steel is there for deflection only; I don't see any way the slab would bend upwards at these locations.

I ignored distributing the moment. I checked the 610mm wide slab strip directly below the base plate only. The top bars in that width are sufficient.

The moment is not very large. The factored moment that the jib crane put into the slab is about 11.3 kN.m. When I checked service load stresses, the net tensile stress (jib crane tension minus dead load compression) was less than the cracking stress of the concrete, but I don't want to rely on that. Without the top steel present, I would have tried to have the jib cranes moved (they are not installed yet).

csd72: I checked punching and we are OK

The jib cranes are not very large; they will not carry more than 1000 lbs.

Thanks for the tips everyone.

 

[csd72]

ugandabob,

I was thinking more of the moment component of the punching shear, i.e you have an increased punching shear on the shear boundary where the plate is bearing on the slab.