Rigidity of pile mat

[allimuthug]

Hi i have a pile mat supported by six piles below the mat.
The loads on the pile mat are almost negligible, however i want to design the reinforcement on the pile mat due to self weight and overburden soil.
Considering a 1 meter width strip spanning between the piles, (self weight of pile mat and overburden soil on the strip)

If the slab is not rigid different procedure has to be adopted to calculate the moment on pile cap.
Kindly correct me if i am wrong also let me know how to find the rigidity of the pile mat. How to check one way shear in the pile mat?

[link

http://files.engineering.com/getfil...fad5-41e8-89c9-4e23515d7ed5&file=Pile_mat.pdf

]Link[/url]

 

[allimuthug]

sorry guys i posted in hurry hence a lot of grammatical and spelling mistakes are there

 

[JoshPlumSE]

It's usually the relative rigidity that you're interested in. How does the relative stiffness of the mat in flexure compare with the stiffness of the piles themselves. You'll have to get the pile stiffness from the geotech.

For the mat, you could make a simple assumption fixed - fixed beam with an end displacement of 1 unit. Moment = 6*EI/L^2, Shear = 12*I/L^3/ That will at least get you in the ball park for your comparison to the pile stiffness.

 

[KootK]

Based on the information that you've provided, I'm not sure that I'd bother worrying about the rigidity. I'd treat it as a one way slab spanning N-S between the pile rows and then design an E-W in slab girder running over top of the pile rows. It'll be fast to design and result in a rational reinforcement pattern.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

Typically, pile mat rigidity is important to ensure that a (more or less) point load on the mat is distributed relatively uniformly to all piling - whether they are near the point load or some distance from it. This means that minimal deflection under load is the a better reference than either moment or shear capacity. As JoshPlum stated, rigidity is a relative value - no definite acceptance criteria.

Comparing predicted pile settlement under load with estimated mat deflection at that pile location will give an idea if rigidity is reasonable. The goal is to have:
Mat Deflection at a Specific Pile Location << Than Specific Pile Settlement at Design Load

The most direct way to minimize deflection is to make the mat "thick" (increase the moment of inertia) compared to its horizontal dimensions. Consider it analogous to the Span / Depth radio of a structural beam.

From your sketch:
Maximum mat horizontal dimension = 5.5 meters
Mat thickness (ignoring the 0.075 meters that the piles are embedded in the mat) = 0.65 meters

"Span" / Depth Ratio = 5.5 m / 0.65 m = 8.5

Based on my experience, and for reasonable loads, a ratio of 10, or less, will be quite rigid. A ratio over, say 12, becomes suspect.

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[Binupattazhi]

Approximately 10 or less as the span to depth ratio can be considered as OK to ensure rigidity of pile cap. However there are no reference to substantiate this