Tower Crane Foundation Design- Accounting for Torsion

[IAstateengineer]

I am working on a design for a spread footing foundation system for a Liebherr 630 EC-H 20 Litronic tower crane. I am wondering how to design for the torsion that would be expected from this type of crane. I have the value for the torsion created by the crane when in operation. Any tips or references to get me started would be great.

 

[JedClampett]

I'm not sure exactly what torsion you're talking about, but if it's twist around the base, couldn't you just calculate the resistance due to friction? The torsion might seem like a big number, but the crane and foundation are going to weigh a whole lot.
If you're worried about attachments, that's a different calculation.

 

[Mccoy]

IAstateengineer, if by torsion you mean the moment around the vertical axis of the foundation, and the analysis by torsional impedance, one reference I know is Gazetas 1991 in Foundation Engineering Handbook edited by Fang. Torsion should be the Mz moment in the attached drawing. Pls note the author(s) treat the case of static and dynamic conditions.

Also pls see

http://ssi.civil.ntua.gr/downloads/...ANCES OF SURFACE AND EMBEDDED FOUNDATIONS.pdf

http://www.etcg.upc.edu/docencia/po...983SDEE_FoundationVibrationsSTATEoftheART.pdf

 

[hokie66]

I'm like Jed. Once you design the footing for overturning, I think it will always be big enough to resist the torsion due to movement of the crane. I've done a lot of them, and never worried about torsion. Maybe I have been wrong.

 

[IAstateengineer]

The torsion I am talking about is the slewing torque moment about the centerline of the tower.

 

[Ron]

Check the rotational friction at the bottom of the footing and the resisting soil pressure from the depth of the footing.....I think you'll see it isn't an issue. Much of the torsion from the swing of the crane is attenuated as it moves down the structure. It works as a rotational spring, not a rigid structure.

 

[chicopee]

I don't deal with tower cranes, however, scanning "Cranes and Derricks" by Shapiro, 2nd edition, the section Moments and Horizontal Loads starting on pg.327 details wind loads that can create torsion and reveal some simple mathematics. In that same book, you may find other section that could deal with the same subject. The same auther may have updated editions of this book.

 

[chicopee]

One more thing, crane manufacturers do impose maximum wind conditions during lifts and prohibit sideway loading of cranes.

 

[Mccoy]

I agree that by considering together base friction and side passive resistance of foundation (if it isn't of a circular section) a thick enough footing or mat should yield a high shear resistance.

An altrnative would be to use the Gazetas impedance method, since the twisting moment which means IAstatengineer is the same torsional impedance in Gazetas article.

I would model an elastoplastic spring, that is, until a certain rotational displacement is reached we are in the serviceability field, then we reach the ultimate state of plasticity hence soil collapse.
Dynamic torsional stiffnes for a square footing would be Kt= 8.3*GB³ [kN*m],where G is operational shear modulus and B side of square footing. This is a lower bound solution since no embedment is considered.
I find this interesting because I've never dealt with torsional moments. The engineer should choose a critical acceptable rotational displacement (in radians) beyond which we have plasticity or unacceptable strain. Theta=Mz/Kt.
According to you guys, which would be a critical treshold for rotational displacement theta in our crane example?

 

[WinelandV]

The first time I designed a tower crane foundation (tcf), my boss had me look at this for my own benefit. For an embedded spread footing, you get so much mass that between friction on the base and passive resistance on the sides(due to the base section anchors, most of these foundations are at least 4' deep, with 6' being normal), it's really not a controlling limit state for the design. Granted, all the tcf's I designed had the sides backfilled and compacted with some good granular soil. If it's a spread footing sitting on the surface, then you should definitely pay special heed to the friction req'd and spec a material underneath that can develop the friction factor you need. By far, for a spread tcf, the overturning moment about the diagonal controlled the length and width required. The "cranes and derricks" book that Chicopee referenced covers the analysis of that condition.

Of course, if your tcf is supported on piles, you can pass the torsion directly to the piles as shear.

 

[IAstateengineer]

Thanks for the responses guys. It makes sense that this wouldn't be a controlling limit state for the design assuming the foundation was embedded sufficiently. This being said, I still need to look at it. I assume the appropriate way to look at this would be to turn the torsional moment into a series of forces at each leg by de-coupling the torsional moment about the tower crane base. Using the dimensions of the four legs I should be able to de-couple and determine the force components at each leg. Assuming these forces are less than the horizontal force on the tower (which I have given to me from the manufacturer of the crane) I could consider them insignificant to the design.

 

[Mccoy]

IAstateengineer, just out of technical curiousity, which is exactly the design torsional moment you have to deal with?

 

[IAstateengineer]

The slewing torque moment about the center of the tower is 477,940 ft-lbs.

 

[Mccoy]

That value in metrics should be 648 kN m if my conversion is not off the beam.

With a secant G of 30000 kPa (4351 psi), which is not a rigid soil and a 3 m (9 feet) square section footing, I get a 1*10^-4 radians of rotational deformation, which definitively doesn't sound critical, and this without an embedment.

Unless I'm seriously wrong, this may be why the torsional moment does not govern design unless it's extremely large.
I'm kind of nonplussed because I was expecting a larger deformation...