Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.119
TABLE 9.23 Allowable Service (Unfactored) Loads on Drilled Piers, kips* (Continued)
Safe allowable service-load bearing pressure on soil, psf
Bell dia, ft Bell area, ft 2 10,000 12,000 15,000 20,000 25,000 30,000
12.5 122.72 1227 1473 1841 2454 3068 3682
13.0 132.73 1327 1593 1991 2655 3318 3982
13.5 143.14 1431 1718 2147 2863 3578 4294
14.0 153.94 1539 1847 2309 3079 3848 4618
14.5 165.13 1651 1982 2477 3303 4128 4954
15.0 176.15 1767 2121 2651 3534 4418 5301
15.5 188.69 1887 2264 2830 3774 4717 5661
16.0 201.06 2011 2413 3016 4021 5027 6032
16.5 213.82 2138 2566 3207 4276 5344 6415
17.0 226.98 2270 2724 3405 4540 5675 6809
17.5 240.53 2405 2886 3608 4811 6013 7216
18.0 254.47 2545 3054 3817 5089 6362 7634
*ƒc1 � 0.33ƒ�c .
NOTE: Bell diameter preferably not to exceed 3 times the shaft diameter. Check shear stress if bell
slope is less than 2:1. (Courtesy Concrete Reinforcing Steel Institute.)
pression times 85% of the compressive strength of the standard test cylinder.
ƒ� c
The 15% reduction from full strength accounts, in part, for the difference in size
and, in part, for the time effect in loading of the column. Capacity of a concentrically
loaded column then is the sum of the loads on the concrete and the steel.
The ACI 318 Building Code applies a strength-reduction factor � � 0.75 for
members with spiral reinforcement and � � 0.70 for other members. For small
axial loads (Pu � 0.10ƒ�A c g,
where Ag � gross area of column), � may be increased
proportionately to as high as 0.90. Capacity of columns with eccentric load or
moment may be similarly determined, but with modifications. These modifications
introduce the assumptions made for strength design for flexure and axial loads.
The basic assumptions for strength design of columns can be summarized as
follows.
1. Strain of steel and concrete is proportional to distance from neutral axis (Fig.
9.50c).
2. Maximum usable compression strain of concrete is 0.003 in/in (Fig. 9.50c).
3. Stress, psi, in longitudinal reinforcing bars equals steel strain �s times 29,000,000
for strains below yielding, and equals the steel yield strength ƒy, tension or
compression, for larger strains (Fig. 9.50ƒ).
4. Tensile strength of concrete is negligible.
5. Capacity of the concrete in compression, which is assumed at a maximum stress
of 0.85ƒ�c , must be consistent with test results. A rectangular stress distribution
(Fig. 9.50d) may be used. Depth of the rectangle may be taken as a � �1c, where c is the distance from the neutral axis to the extreme compression surface
ƒ� �4000 ƒ�
and �1 � 0.85 for c psi and 0.05 less for each 1000 psi that c exceeds
4000 psi, but �1 should not be taken less than 0.65.
In addition to these general assumptions, design must be based on equilibrium
and strain compatibility conditions. No essential difference develops in maximum
capacity between tied and spiral columns, but spiral-reinforced columns show far