WAYS OF REDUCING COST IN CASE OF MINERATE FOUNDATION DESIGN 3

[nedians]

I have to design foundation for a minerate of a mosque having a height of 80ft., width 7ft. Slab are 4inches thick ( as given by architect). Bearing Capacity of soil is 1.5TSF @ 5ft to 8ft. Wind is the major concern and it is taken as 23psf lateral force on the minerate( Speed of wind is our city is taken as 100Mph).
I have very little idea how to perform this design as i haven't found any book on this topic. can any one please guide me some steps and guide ways to have a economical minerate foundation design. i want to know different ways which can help me in getting a economical design.

 

[phuduhudu]

I had a similar question on a tower base. See the soil mechanics discussion area. Hopefully this link will take you to the thread. I had some useful replies.

thread486-6421 a tower beware of using ordinary safe bearing pressures because these often allow for 25mm settlement. Now if you got 25mm settlement on one side of a 2.5m base that makes a rotation of 1 in 100 so for a 25m high tower that multiplies to a deflection at the top of 250mm which may be unacceptable. Then there is the problem of increasing the base moment when the centre of gravity shifts (look at Pisa).

My plan for economy of foundation was to get as much base as far out as possible so I used a cruciform base with fairly heavy reinforcement. I haven't got to detailed design yet so I haven't worked right through it. Anyone elses ideas will be welcome Carl Bauer

 

[IJR]

Without further info I would agree with carlbauer.

You got to widen your bearing area, and depending on your soil data it may mean a wide area at some good depth.

When you say economical, here is what you get: The minerate is going to swing in the air and has to be held in place. In foundation work economy usually means reduced excavation and in my area it also means no piling but in Russia it means go on piling, cause down their everyone sells prefabricated footings or piles and they have to because most soils are wet.

Regards

IJR

 

[Hariharan]

If you are providing an isolated spread footing as it appears,
you have to consider the following:

1. Stability Against Overturning:
Factor of Safety Against Overturning = (Resisting Moment)
------------------------
(overturning moment)
should be greater than 1.5. For square or rectangular footings
you should consider the edge of the footing as the axis about
which moments are computed. For circular footings, it is a bit
dicey, and you may consider an axis about 0.7 R from the
centre. (My judgement, no backup!)

2. Factor of Safety against sliding > 1.5
(friction factor) * Weight
F.O.S = ------------------------------
Horizontal force

3. No tension in Foundation (Soil stress)

This is implicit in (1) above for square or rectangular footings.
Not so evident in other shapes (circular, octagonal, hexagonal).
Not mandatory, but good engineering practice, specifically
for foundations governed by wind loads

4.Maximum Soil Stress < Safe bearing capacity * Stress increase
Factor

Stress increase factor for rare event would be given in the
governing code for your project.

Make sure you calculate the weights and wind load properly.
For OTM calculations, weights should be minimum weights
and for stress calculations, include live load. Select appropriate
shape coefficient for wind load computation. The wind
speed/pressure selected should not be below local statutory
requirements.

If the wind in your location is not directional, i.e., it may act
from any direction, an octagonal founation is my preferred
choice. Square footings will have high stresses in diagonal
wind cases, stability against overturning could be difficult to
establish with circular footings.

Hope this helps. These basics will be available in any book
on Foundation design.

M. Hariharan

 

[oldrunner]

1. How much does the Minaret weigh?
2. In determining your wind load, I would increase the load by some sort of factor to take into account the roughness of the surface of the structures; that is, small openings and protrusions. Don't forget, the 23 psf is probably the wind stagnation pressure and you have to increase the wind load for height, exposure and a gust factor coefficient at the varying heights up to 80 feet. Futher you would also futher increase the pressure for the kind of structure it is; either round or a flat face.
3. Once you determine the forces from the wind, then estimate a footing size, starting with say a depth of five feet with the 1.5 tons (3000 psf) allowable soil bearing value. Your soil consultant should inform you whether this value can be increased for wind loads.
4. The footing can either be square or hexangonal. I like square footings because they are easy to dig and the reinforcing is regular. Check the stablily across the narrow dimension and the soil bearing pressure across the diagonal.
5. Determine the overturning moments about the bottom of the footing and calculate the soil bearing pressure and the factor of safety. Adjust the size of the footing until your limits are satisfied.
6. I don't know how you may be attaching the structure to the foundation, so you would have to interface with the structural designer for the Minaret for this.
7. Is your project in an earthquake zone?
8. Others suggest that you be conservative on your foundation design to minimize settlement and possible tilting. These are very good suggestions. I once had to fix a large concrete structure in Pakistan that started to sink and tilt because the original designer had used 3000 psf for his bearing pressures. Unfortunately, it was determined that there was 95 feet of soft material below the structure as the location had once been part of the Jhelum River.