huangyhg

unreinforced concrete caissons??
we have a bldg that is 30 stories high on concrete caissons as per the geotech report. the design engineer shows no reinforcing in the caissons that are about 30 ft below grade.......is this common practice?
thanks,
i always have reinforced my drilled piers (caissons) but i have to admit that most were in expansive clay regions where pier uplift forces were possible.
the key might be if there is overturning resistance that the piers are participating in. also lateral shear forces on the piers.
there are aci provisions for unreinforced concrete in 318 but 318 doesn't cover drilled piers.
here are aci's documents on piers that you can purchase:
336.1-01: specification for the construction of drilled piers

336.1-98/336.1r-98: reference specifications for the construction of drilled piers and commentary
336.2r-88: suggested analysis and design procedures for combined footings and mats (reapproved 2002)
336.3r-93: design and construction of drilled piers (reapproved 1998)
any column uplift would obviously dictate reinforcing extending all the way to the base. where lateral shear is on a caisson, reinforcing is usually extended down in the range of 10 to 15 feet until the moment dies out due to lateral soil pressure.
i would think for a 30 story building, you would want reinforcing in each caisson at least in the top.
typically use 6-15m for caissons and 10m@36 ties... right up to 48" diameter unless loading indicates otherwise... bars are for tension... also use 19'-8" long bars... rebar length in these areas is 18m and can be cut into 1/3's...
dik
i have design a number of buildings in australia on piers (caissons) and without reinforcement but there are a number of things that should be checked:
1. check the codes to see what is the allowable tolerance for the placing of the piers and the placing of the superstructure over. if both of these maximums were to occur at once, then would a plain concrete pier be able to take the eccentricity in bending?
2. what is the slenderness of the pier? if it is less than 3' wide then you may contemplate putting in some steel to allow for slenderness effects. depending on the type of soil, it may or may not provide restraint against buckling.
3. is there any later load on the pier? once again will the plain concrete take it in bending?
3. as noted above, what is the soil type, is it expansive,are there any uplift forces on the pier? e.t.c.
if you decide to reinforce it you may not need to reinforce it all the way down - be practical.
thank you to all for your replies....
no uplift but there are lateral wind loads to be transfered.
appreciate your input.
cva1993

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