A 45-story tower to house office space is to be constructed in the business dist

Question

A 45-story tower to house office space is to be constructed in the business district. I he figure below shows the subsurface conditions of the proposed site. The structure will be founded on 12-inch square pre-stressed concrete piles with tips at a depth of 50 ft (in the medium dense sand layer). Assume lambda sat = 115pcf for the soft clay and lambda sat = 125pcf for the medium dense sand. Estimate Qsafe for an individual pile with a FS = 3. If the load per column is 300 tons, estimate the number of piles in a pile group supporting one column assuming the group efficiency to be equal to One Draw a pile group layout with an appropriate center-to-center spacing, Check the group capacity using block analysis for the side resistance in the clay and a group efficiency of one (1.0) for the end resistance in the sand.

Explanation / Answer

PART 1-

surface area of a pile- 12*60*2+12*602+12*12*2=3168

Qsafe = 40NLd/DAp+NbarAs/0.50

where,

N= avrage N value of pile tip.

Lb= length of penitration in strata

D= diametre of pile.

Ap= cross section of pile.

N bar = avrage N along the pile shaft.

As = surface area of pile

40 0.56*60/ 12*144+1.36*3168/0.50

1344/1728+8616.95= 8617.8 psf

PART 2-

for finding the number of pile. we have to

with surface area of three

8617.8/300= 28.65

29/12=2.42

pile group will be of 3 piles together

PART 3-

for finding the capacity of pile

Q= q0B2+4*B*L*F

300*720+4*12*60*3=224640 psf

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