Raft

[Mkhidri]

There is about 8 silos spaced out equally over a rectangle shaft of 10 x 20 meter. I want to calculate ULS and SLS for worst case scenerio, which would include loading half of silos in one side of the raft. So, I want to consider to case: flexible and rigid raft. I know u can treat the raft as a beam in case of flexible raft. Can any body shed some light on this??? I have looked into Elastic solution by davis and poulos, but not sure how to deal with this.!!!!

 

[FixedEarth]

We need more info like:

1) soil/rock description & strength
2) silo height
3) Mat foundation embedment & thickness and
4) Seismic region concerns.

 

[Mkhidri]

Well, what I need is a solution in principle. There is no Seismic concern. mat thickness is 0.45 m. Silos are about 5 to 10 meter high filled with sand. it is on gravel fill and clay ontop of bedrock.

 

[FixedEarth]

Those short heights and no seismic concern, eliminates the need for piled rafts, so now you can concentrate on raft supported by soils.

Allowable bearing capacity would not be critical due to the large mat area. So your problem now becomes limiting total settlement, checking for the large stress bulb at the mat perimeter, and checking for differential settlement between edge and center of the tank.

You can use BOEF equations in Roark's formulas for stress and strain book. Given the 0.45 m mat thickness, you are likely to have a flexible beam but you may get semi rigid beam if mat thickness is closer to 0.6 m.

 

[Mkhidri]

Fixed Earth: Yes, the raft is supported by soil. Can u relate me to any text or publication which i can further educate my self.

 

[FixedEarth]

The best I found is a paper by Horvilleur & Patel, titled "Mat Foundation Design - A Soil Structure Interaction Problem". It is part of great collection of papers in "Design and Performance of Mat Foundations" State of the Art Review, ACI SP-152, 267pp, 1995, by the American Concrete Institute. It has everything you need about Mat Foundations. Maybe others can mention other publications.

 

[Mkhidri]

thanks the holder of FixedEarth account lol

 

[mar2805]

Is this article available for online purchase?

 

[mar2805]

@Mkhidri
Please note that you didnt create worst load scenario.
Only first or only last horiziontal row would create worst scenario since they would create moment around weaker axis (smaller H^3)

Guys one question, I know its a long shot but calculating raft fondation on ground, can it be done "by hand" and not using computer analysis?

 

[AnaAnt]

Hi to all.
Im looking for some tips on beam-raft design process.
Basicly I have few question and would love to get some assistence from more experienced people of this forum.
1) Should downstead beams and slab be poured at once, or is it ok to first cast beams, detail reinforcement overlaping from beam to slab, and then cast a slab.

2.)When defining a perimetar ground beam on top of wich is a bearing wall. Should the width of the beam be determined as it was a strip footing so that bearing capacity under the perimetar beam is always less then actual ground capacity. I dont know if this is maybe conservative because we are delaing with a raft foundation and not with a strip footing.

I hope you guys can spread some light. I thank you very much.
Ana

 

[BigH]

@AnaAnt - start a new post; don't hijack an existing one.

 

[AnaAnt]

Sorry guys :-(
Will do.