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PDCA received this question through the Ask PDCA function on Piledrivers.org.

A two-story building is used for this example. A quick approximate analysis may be obtained as follows: 
Column Load Rundown, using an "Interior" column will typically yield the largest load for a building
Assume column grid and also pile grid spacing to be 20 feet on center (a good range for all building framing systems is 20 feet to about 24 feet square for column layouts)
DLroof = 25psf; DLfloors = 35psf; DLstr.slab(first floor) = 75psf (implies an 8-inch thick concrete structural slab, not a slab on grade); LLroof = 50psf, LL2nd = 50psf; LL1st = 125psf 
Column Load Rundown using the information on the line above is as follows:
i. Tributary Area of an Interior Column = (20')*(20') = 400-sf
ii. DLtotal = [(400sf)*(25psf+35psf+75psf)] = 54,000lbs = 54kips
iii. LLtotal = [(400sf)*(50psf+50psf+125psf)] = 90,000lbs = 90kips
iv. TLtotal = 54kips + 90 kips = 144kips
v. As you can see, these loads have been developed as allowable loads because they have not been factored up per LRFD. Keep them in ASD format to match the soil using ASD.
Unfactored Column Load Rundown for an Interior Column = 144 kips
Using a Factor of Safety = 2.5 (judgement required here of an experienced engineer) yields an Ultimate Load Requirement of (2.5)*(144kips) = 360 kips = 180 tons. This is the ultimate capacity to seek in the field driving piles. 
At this point, information about the soil strate underlying thebuilding project is required; for this example, none has been provided.
Knowing from experience, this is a relatively lightly loaded pile and with good soil (1) 12.75-inch o.d. x 0.375-inch thick wall pipe pile could possibly carry this load. However, you must ask the question, "Is it a short or slender pile, including seismic effect for liquefaction?"
For short columns/piles, the Axial Capacity is (Fy)*(Ag) = ???kips and buckling is not considered
For slender columns/piles, the Axial Capacity shall be determined by the unbraced length and the provisions of AISC are often used and suitable. For liquefiable soils, coordinate the unbraced length with a geotechnical engineer or use your own judgement.
If the building column to be supported is part of the Superstructure Lateral Force Resisting System (LFRS), you may want to add extra piles purely for redundancy because you must avoid collapse. When doing this, reduce the depth of pile penetration because each pile takes a portion of the 360kips (the example load used here).
The controlling load combination used to evaluate stability under seismic load is often used as DL + EQ, requiring a factor of safety of 1.25 to be achieved from the soil before instability of the structure is a concern.
To determine the cost, you need to have something tangible to price such as the type and number of piles, dimensions of grade beams, floor slab dimensions and reinforcing, etc. t

Posted in PileDriver Magazine. Tagged as Issue 3, 2018.

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