Wind load on tower cause uplift, down thrust and horizontal shear on base of tower.some legs experience uplift with horizontal shear(one way or both way) and some large down thrust along with horizontal shears at base.While designing foundation, if isolated footing is not adequate and, therefore, mat/ raft is to be considered, how to treat the problem since there be large eccentricity due to overturning action? can any one suggest me the appropriate method of raft/mat designing in this case?
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raft/mat for telecom tower 1
- Thread starter AdhTP
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Have you considered three caissons? How about driven piling in three locations.
For your mat alternative I suspect that bearing capacity at the toe is not most critical, but that with a mat you might need great footing thickness for uplift resistance. Would you then rotate about the toe? I think not. I'd not consider a pressure distribution with any zero presume areas.
For triangular pressure distribution, a common assumption is to use the recommended "bearing capacity" as that toe pressure to minimize toe settlement. Other discussors might not agree.
For your mat alternative I suspect that bearing capacity at the toe is not most critical, but that with a mat you might need great footing thickness for uplift resistance. Would you then rotate about the toe? I think not. I'd not consider a pressure distribution with any zero presume areas.
For triangular pressure distribution, a common assumption is to use the recommended "bearing capacity" as that toe pressure to minimize toe settlement. Other discussors might not agree.
Pile/cassion foundation should be considered as suggested above.
But, as a practice, I will start to estimate the size of the mat foundation:
1. Obtain max e caused by loads on towewr.
2. estimate depth using punching shear relationship (assume bo=4d, h=d+4" or 6").
3. estimate min width by solving (for square footing)
(P/B^2)*(1+6e/B)+unit pressure of concrete <= fa, and
(P/B^2)*(1-6e/B)+unit pressure of concrete >=0.
Choose whichever is larger.
Then check overturning and sliding stability, increase thickness as necessary.
But, as a practice, I will start to estimate the size of the mat foundation:
1. Obtain max e caused by loads on towewr.
2. estimate depth using punching shear relationship (assume bo=4d, h=d+4" or 6").
3. estimate min width by solving (for square footing)
(P/B^2)*(1+6e/B)+unit pressure of concrete <= fa, and
(P/B^2)*(1-6e/B)+unit pressure of concrete >=0.
Choose whichever is larger.
Then check overturning and sliding stability, increase thickness as necessary.
Agree with above, but I would start with checking for overturning and then up the size as required to satisfy bearing pressure limitations.
slab effectively acts as an upside down cantilever under the action of the bearing pressure (dont forget to factor it up).
Also Helical piles are good for these things as their uplift is basically the same as their downward capacity.
slab effectively acts as an upside down cantilever under the action of the bearing pressure (dont forget to factor it up).
Also Helical piles are good for these things as their uplift is basically the same as their downward capacity.
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