STEEL REINFORCEMENT FOR RECTANGULAR FOOTING WITH BIAXIAL BENDING

摘要

This paper illustrates the calculation of the amount of reinforcement in a rectangular footing subjected to biaxial bending with a given allowable maximum soil pressure underneath the footing foundation. The procedure first involves the determination of the footing capacity curve for a given soil pressure to resist the external vertical loads applied to the foundation. This capacity curve covers all cases of loading including tension on part of the footing area. The external vertical loads are already determined to be inside the envelope of the footing capacity curve. The methodology is similar to the procedure illustrated in papers by the author and published in ISEC-01, SEMC2001, ISEC-02 and SEMC2004 international conferences for calculating the concrete resisting forces in a rectangular section. For footing foundation the pressure is linear as opposed to the parabolic stress used in those papers. On this curve is plotted the maximum bending moment uplift capacity of the footing which occurs when part of the footing area is in tension. The steel reinforcement to be provided for the rectangular footing is calculated from this maximum moment minus moment caused by any overburden pressure on top of footing. The footing is treated as a reinforced concrete beam. Ultimate strength capacity equations for beams were already published in a book by the author and will be used to determine the amount of reinforcing bars for this footing. The component of the resultant moment uplift is obtained from the capacity curve when the capacity axis is along the diagonal of the rectangular footing. The footing reinforcement is obtained from the net maximum moment uplift. This method will provide an efficient design of a rectangular footing foundation for bridge structures. Microsoft Excel '97 worksheet was used to plot the rectangular footing capacity curve.