Grout Failure Under Base Plates

[cranepete]

I have designed a tower crane foundation with dunnage beams on bearing plates locked down with anchors. The bearing plates were installed on approximately 3" of grout. The grout is failing under all four bearing plates. The bearing plates are 26"x24"x1 1/4" and the grout pads are approximately 30" square. I would appreciate any input to this situation. Thank you.

 

[Lutfi]

I have several questions:

1.What strength grout did you specify?
2.What was strength was provided?
3.What is the mode of failure?
4.Does the base plate carry uniform pressure or is there bending moment?


Regards,
Lutfi

 

[cranepete]

Thank you for responding;

1. I did not specify a grout strength
2. I was told that the grout was 10,000 psi.
3. The corner under the side with the max reach has failed on the outboard side of the beam toward the center of the crane, the other corners have cracks near the corners of the grout pad and cracks by the anchors and along the centerline of the pad.
4. The dunnage beams are W14x233 with 15 7/8 flange width, bending in the plate should be minimal. The maximum projection of the plate past the edge of the beam is 5 1/16".

The beams are anchored to the foundation with two (2) 1 3/4" dia. ASTM A772 bars per beam end. The anchors are locked off at 200 kip each for a total of 400 kip at the end of the beam. The max reaction on the base plate is 540 kip. The crane has been up since mid November and the cracking in the grout was noticed last week, approximately 2 months after installation.

 

[SlideRuleEra]

I've had to deal with a lot of grout failure under the base plates of heavily loaded industrial equipment.
1.25" sounds like it is too thin for 26" x 24" base plates in that type service. Often the best thing to do is to replace the baseplates with thicker material. Put them in final position and pressure grout under them to ensure maximum baseplate/grout contact. Rather drastic, but I found out the hard way that fixes and half-way repairs are almost always a waste of time and money.

Best Wishes

http://www.SlideRuleEra.net

 

[VTPE]

Typically you have to add pea gravel to frout when it reaches a depth of 3" or greater. This provides additional bearing strength and less brittle grout. In addition, was the grout placed on a cold day. A foundaiton can be a big heat sink, especially if it is big. The heat will literllay be sucked from the grout into the foundation causing the grout to not set up correctly.