Slab-on-grade capacity and outrigger dunnage thickness

[todh]

I had a contractor call me today asking for a rule-of-thumb on determining the dunnage footprint size and thickness for crane outriggers supported by a slab-on-grade. I didn't have one to provide him, but to say that I could do a FEM analysis to figure it out for him.

This contractor has had issues with cracking the slab-on-grade while erecting concrete tilt-up wall panels and he wanted some assistance.

Can you use the "Designing Floor Slabs On Grade" book (by Ringo and Anderson) "Section 5.4 - Using equations to design for column loads" as a starting point to determine the area of a "base plate" required for a given load? Then how would you proceed with determining the dunnage thickness to ensure you distribute the load down to the slab to that full "base plate" footprint?

Any suggestions?

 

[rscassar]

http://wiki.answers.com/Q/What_is_the_OSHA_standard_on_dunnage_under_crane_outriggers

I would think that the outrigger of the crane itself would be the responsibility of the crane manufacturer and designer.

As to whether the slab-on-grade (which is your responsibility) has the capacity to transfer the load to the sub-grade. I would treat the point load the same way that any other point load is considered for a slab-on-grade.

 

[chicopee]

I worked for over 10 years as an engineer for a bridge contractor in Ct. Our lattice boom cranes were used primarily to erect steel girder on many construction sites.

To ensure that the cranes would remain level, we used different methods of cribbage. Some of these methods are outlines in Shapiro's Cranes and Derricks

My JPEG attachment is information that I developed from
that reference; the cribbage is supported by the bare unfrozen ground in that drawing which is different from the concrete slab detailed in your OP. This type of cribbing system was used for relatively light loads withour hydraulic cranes(rough terrain, all terrain etc...)

For outrigger support on concrete slabs, asphalt pavement underwhich sewere lines etc.. would identified, soggy ground and for heavy lifts such as tub girders, our lattice boom and hydraulic crane outriggers would be resting on 1/2" steel plates about 3X the size of the floats; under the steel plates we would have pantoons (12"x12"x10' and some longer which were used on barge steel decks ) bolted together.

Our cribbing systems never failed once. For excellent info. on cribbages get a copy of the above reference.

 

[chicopee]

Check out this newly released thread no. 274-288092 in the Geotechnical engineering forum. Good cribbage under the float but too close to the enbankment or may be the outrigger beam was not fully extended which would be a violation the load chart for"On outriggers". It is hard to tell why soil bearing was overwhelmed.