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All concrete in caissons shall have a minimum ultimate compressive strength, fc of 3,000 psi as determined by 28-day cylinder tests unless other periods of time are specified in the contract documents.
(a) The maximum allowable axial stress in the concrete fc shall not exceed 0.25 fc for caissons without permanent shells.
(b) When permanent full length noncorrugated steel casing is used, the designer may consider an increase in allowable concrete stress in accordance with the following empirical formula: The maximum allowable axial stress in the concrete, fc shall not exceed 0.30 fc + 1.5 (tfy/D) for caissons with permanent shells, but not more than 0.40 fc where:
t = shell thickness in inches after deductions for corrosion
D = inside shell diameter in inches
fy = yield strength of the shell steel
(c) When reinforcing steel or structural steel are used to increase its load capacity, the caisson shall be designed according to the code provisions of Chapter 13-148 of this Code “Steel and Metal Construction”, except that strength reductions for length of member need not apply when the caisson has adequate lateral support provided by soil or other positive means.
(d) When the caisson extends through a layer of unstable soil, the maximum design shall be computed as a column with an unsupported length equal to the depth of the unstable layer of soil plus four times the diameter of the caisson.
(Prior code § 70-12.2)