Jib Crane Foundation Cracks

[XPDQ]

I am trying to find out where to look to determine if the grout used at the foundation of a pillar type jib crane is satisfactory or not. The crane rated 2 ton with a 36" x 36" base plate sitting on a 2-1/2" thick, 39" x 39" grout slab. Cracks are visible along the thickness of the grout slab at its outer edges, however the cement below the grout is in very good condition. I don't have much of a background when it comes to foundations.

 

[InDepth]

Do you have a photo? Is the grout tapering at an angle away from the pase plate? Did the cracks form before or after loading? What are the grout specifications (strength, etc...)? Was pea gravel placed in the grout mix? Was the grouth tamped in or flowed into place?

Loadwise: What axial, shear, and moment reaction is at the base?

 

[XPDQ]

Here are a few pictures:

http://files.engineering.com/getfil...a605-42eb-ada7-3fe2afdc34ee&file=P9080001.JPG

http://files.engineering.com/getfil...96d6-4926-99d4-cf112622a77a&file=P9080002.JPG

http://files.engineering.com/getfil...586d-40fb-824c-695a4389ac56&file=P9080004.JPG

http://files.engineering.com/getfil...55e4-48d8-b5db-4577754535c4&file=P9080010.JPG

http://files.engineering.com/getfil...902f-44d5-9982-3ac01757b97e&file=P9080016.JPG

http://files.engineering.com/getfil...ed35-45e0-b43a-65a17ed60c67&file=P9080017.JPG

http://files.engineering.com/getfil...83cd-415c-a6e7-e9ebabe6d2a3&file=P9080020.JPG

http://files.engineering.com/getfil...c850-4347-8754-21315cdb605b&file=P9080023.JPG

It is unknown when or how the cracks formed, the crane has been at the location for a hand full of years. The exact grout specifications are unknown, but grout typically used in this type of application is high strength, non-shrink grout. I don't believe the grout contains pea gravel. The mast was set plumb with leveling nuts and then grout was flowed into place, the base plate is 1-1/2" thick and inset into the 3" thick grout (not 2-1/2" as mentioned before).

The axial load along the pillar is approximately 2830# (crane utilizes a 1 ton hoist). The moment reaction is approximately 628 in-kips about the center of the 36" wide base plate.

 

[InDepth]

XPDQ, I think you have a corrosion problem. It looks like the steel rust is expanding and placing lateral stresses on the grout. Are you in a wet environment that also sees freeze thaw cycles?

You may want to perform a closer look at the condition of the baseplate and verify if the thickness is acceptable and there are no major stress concentrators. Looks like it needs the rust to be sand blasted off and the plate coated. You may want to consider anodoc protection to reduce the rate of corrosion. You make also want to look at the bolts closer. Also look at ways to reduce water intrusion betwqeen the baseplate and grout.

In general it doesn't sound like you are having a servicieability problem other than corrosion, correct?

 

[hokie66]

InDepth is correct. For some reason, the grout was placed to the top of the base plate, thus there is a thin, unreinforced ring of grout around the plate. This has provided a reservoir for moisture, which has corroded the edges of the base plate, and the resulting oxide expansion has pushed the grout away.

I would remove the grout which is above the bottom of the plate, along with any other cracked grout. Then the base plate can be prepared and recoated, and the grout reinstated, but only up to the bottom of the plate.

 

[swearingen]

I second hokie's analysis and solution...


If you "heard" it on the internet, it's guilty until proven innocent. - DCS

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[XPDQ]

hokie66, you are right.

https://www.netweber.co.uk/fileadmi...uction/precision_grouting/Probleme_1_p.90.pdf

 

[InDepth]

Problem solved! Good team work guys!

 

[MiketheEngineer]

Might I suggest you hire someone very familiar with this type of problem for an on-site review..