Ultimate Strength of Tubular Joints Subjected to Combined Loads

ABSTRACT Nine tests were conducted on double-tee tubular joints subjected to various combinations of axial load, in-plane bending and out-of-plane bending in the branch. These tests along with three reference tests (axial load alone, in-plane bending alone, and out-of-plane bending) were used to study interaction equations for joint design. The static ultimate loads are compared with the API RP-2A interaction equation. It was found that the API method gives conservation results if predicted strengths are used for the reference tests. If experimental strength or accurate theoretical prediction are used, the API interaction equations are slightly un conservative. A new equation is recommended in which the interaction between axial load and out-of-plane bending is almost linear. INTRODUCTION Equations describing the ultimate strength of tubular joints are based on tests of simply loaded joints where branch members, which are welded to a continuous chord member, are subjected to only loads, in-plane bending (IPB) or out-of-plane bending (OPB). A recent study used data from 747 ultimate axial load tests in order to develop axial strength equations [5]. Fewer experimental results are available on tubular connections subjected to bending. Year et a1. includes 16 IPB tests and 17 OPB tests as the data base for ultimate bending strength equations [10]. Tubular joint design has progressed to a point where the available empirical equations yield reasonably accurate estimates of ultimate load for the three basic loading cases. These three cases, however, rarely occur alone because the welded construction used in tubular trusses leads to moment-resisting connections. Therefore, the interaction between the three basic loads needs to be addressed. Design methods, such as the American Petroleum Institute (API) recommendations for tubular joint design [1], suggest an interaction equation for joints based on experience with combined loading on tubular members. For example, in-plane bending and out-of-plane bending moments can be combined vector ally for circular members which have the same bending strength in all directions. However, in joints the bending capacity and stiffness are different in the two bending directions, so moments are nondimensionalized in the API recommendations by the bending strength, Nu " in the respective directions, as shown below:(Mathematical equation available in full paper) A recent experimental study by Stamenkovic [7] on combined axial load and in-plane bending showed a linear interaction relationship, but the test setup itself and the definition of failure used by the researchers give a first yield load, not ultimate strength, as discussed in detail elsewhere [4,6]. Tests combining axial load, in-plane bending, and out-of-plane bending are difficult to perform because the large displacements cause secondary moments which must be properly monitored. In addition, for combined in-plane and out-of-plane bending, the branch does not deflect in the direction of the resultant load because of different joint stiffness in the in-plane and out-of-plane directions. Presently, there are no reliable data which describe the interaction of loads on ultimate joint strength.