×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concentrated Moment on Concrete slab

Concentrated Moment on Concrete slab

Concentrated Moment on Concrete slab

(OP)
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

 

RE: Concentrated Moment on Concrete slab

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."
 

RE: Concentrated Moment on Concrete slab

ugandabob,

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

BA

RE: Concentrated Moment on Concrete slab

ugandabob,

Is this slab new or existing?

RE: Concentrated Moment on Concrete slab

(OP)
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.

RE: Concentrated Moment on Concrete slab

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

RE: Concentrated Moment on Concrete slab

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

RE: Concentrated Moment on Concrete slab

(OP)
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.

RE: Concentrated Moment on Concrete slab

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources