Monolithic Concrete Slabs - posible alternative 2

[WhiteLotus]

I have a design for a monolithic slab that will be 8'x18'x4' that is designed to support a 90 ton piece of equipment. This particular design requires a great deal of field prep and alot of effort in terms of coordination, materials and manpower. This problem is intensified by the fact that this particular design has to be installed 10 to 15 times per year, every year for the next 10 years.

One possible alternative is to use precast foundations (2'x8'x2') stacked in an array (think of the game JENGA!). We currently use these blocks as temporary supports in a storage yard prior to installation.

Since this alternative isn't a monolithic pour, my concern is that I will have to use alternative design criteria with which I'm not familiar.

If anyone has any recommendations for using precast foundations in the manner I've described above or can point to sources that can assist me, it would be greatly appreciated.

The equipment has an expected life span in excess of 50 years and can be considered entirely a dead load. It will not be subject to any horizontal load conditions.

Thank you
R

 

[SlideRuleEra]

What you are describing sounds quite doable. Ninety tons on a foundation with those dimensions should be reasonable. The best analogy for your proposal seems to be a grillage foundation. These were used in the early 20th century. These were typically constructed from steel beams or wooden timbers, often encased in concrete (however the concrete was for protection, not strength). Your precast concrete members, if properly reinforced, could be considered as the grillage. Try to find old engineering reference manuals (1940's or earlier) for information on how to design a grillage foundation. One source might be "Foundations, Abutments & Footing" by Hool & Kinne.

 

[Focht3]

A dead-load-only bearing pressure of 1,250 psf (~60 kPa)? Assembling an array of precast elements sounds reasonable; but what will they support? (I'm a bit concerned about dynamic forces.) And could differential movements cause future problems? (This might be the real 'killer' for your approach.)

While the array elements will weigh about 5 kips (5,280 lbf ~23.5 kN), they will be 'small' in comparison to the footprint of the total foundation. If the larger footing were 16x8x4, you could alternate the orientation of the blocks within the foundation thickness to improve the overall stiffness. Otherwise, the foundation's lengthwise stiffness in both shear and flexure will be much lower than a monolithic pour would be.

A better approach might be to construct an 18x9x4 footing with 9x2x2 elements, and construct some pieces that are 3x2x2 and 6x2x2. This would allow you to stagger the ends of the array elements and improve the composite footing stiffness.

That is, if this remains a viable foundation system -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[WhiteLotus]

The 90ton piece of equipment is a step up transformer for a utility. Under normal operating conditions, it will not be subject to any lateral loads and will be constructed in an enclosure so wind loads will not be of any concern. Thank you Focht3!! I'll try using precast blocks of different sizes in my analysis.

 

[Focht3]

Hmmm,

Transformers do induce vibrations into the foundation - at sub-harmonic to harmonic frequencies of the systems' AC frequency (60 Hz in the U.S.). You will need to discuss this with both the transformer's manufacturer and your geotechnical consultant.

Are the vibrations significant? That's too complicated to answer in this thread. As a friend of mine often says, "Maybe yes, maybe no." By using a "Tinker Toy" assemblage of concrete pieces to construct a foundation, the footing is no longer monolithic. All of the textbook examples assume the shallow footing is an ideally rigid, monolithic element. Your idea violates those assumptions.

And I can see lots of O&M issues with this, particularly over a 50 year service life.

Let us know what you find out - an interesting problem.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[GeoPaveTraffic]

You could install some post-tensioning cables vertically through the individual pieces to increase their ability to work as a unit.