Design Interaction Curves for Tubular Cantilevers

ABSTRACT To provide a means of developing maximum strength and design interaction curves for tubular cantilevers, a general purpose beam-column computer program is modified and used to first determine ultimate load capacities. A feature is added to the well known pile analysis computer program (BMCOL-76) to account for material nonlinearity and is presented in the paper. Results from this program are used to compare the proposed to existing design checking equations. Since the AISC allowable compressive stress term (Fa) remains in the design checking equation proposed, the AISC effective length factor (k) for typical pile and conductor "add-ons" is valuated. This is done by using the modified BMCOL-76 program to determine the critical buckling load for two typical examples. A procedure for designing pile and conductor "add-ons" is presented with an improved design checking equation and method for determining effective length factors. INTRODUCTION The subject of sizing piles and conductors for hammer placement was prompted some time back when a change in a recommended practice (1) was questioned. The change stated that a lateral load of 10% (reduced later to 2% in supplement 1 to the 13th edition, March 1983) of the hammer weight must be considered in addition to the actual weight of the hammer for conductors driven in a vertical or nearly vertical position. The validity of the 10% lateral load requirement was questioned when a case history review of ten installed vertical conductors showed that not one of the installed conductors would meet such a requirement. Furthermore, the largest lateral load anyone case could withstand, according to the appropriate AISC allowable or combined stress condition, was 5.8% of the hammer weight. In addition, only half of the ten installed conductor cases reviewed could withstand more than a 2% lateral load requirement. The following is a discussion of the investigation conducted on the beam-column design problem of an inclined tubular cantilever subjected to a tip load. INVESTIGATION Although the design procedure question that arose pertained to sizing conductors, it was felt that any possible remedy to be considered should also be compared to the design procedure for pile "add-ons" as well since both are merely examples of structural cantilevers. The support conditions of the conductor and pile examples considered (i.e., continuous beam-columns with one end span a cantilever) are detailed within the structural models shown in figure 5. It will be shown that the design procedure for sizing conductors or nearly vertical members should be similar to that for piles or inclined members. What is proposed is that conductors be regarded as inclined to a realistic out-of-plumb angle instead of subjected to some lateral load. Since the present design procedure for sizing conductors and piles depends on the use of existing AISC (9) interaction equations (1.6–1a, 1.6–1b and 1.6–2) which are for all columns in general, it was thought that a more appropriate interaction relationship for the column cantilever could probably be developed. The conclusion was the realization that the essential AISC checking equation (1.6–1a) has an inappropriate moment modifier for the design of "add-ons" which is adequate for sizing inclined members but not for nearly vertical members. The procedure of selecting an appropriate interaction equation for various boundary conditions and cross sectional shapes is well documented and the approach presented elsewhere (5, 11) is followed to some extent.