Effect of Chord Cm Length on Strength of T Joints
Authors: Madros M.S.Z. , Zettlemoyer N. , Healy B.E. 
Published: October 2008

A study has been conducted to examine the influence of can length on the static strength of compression loaded T-joints. Finite Element models of the T-joints are developed and analyzed the SESAM/ Pretube, PATRAN(r)), and ABAQUS? computer programs. The ultimate loads from the analyses are compared with recommended values in API (1993) and other codes. In general, it appears that the API recommended ultimate strength values for T joints with low B should be modified. Also, the API derating equation for X joints with short cans is conservative for T joints. However, a derating equation is required for T joints.

INTRODUCTION

Circular tubular joints involving a chord and at least one brace are sometimes strengthened by providing a thicker chord section or can right at the intersection. Early versions of API did not provide any means to calculate the effect of various can lengths on strength. It was assumed that the standard practice of extending the thickened section one-quarter of the chord diameter (but at least 12 inches, see Fig. 1) beyond the brace was sufficient to develop the full strength estimated by the given capacity equations. However, a study by Vegte et al 1showed that this approach could be un conservative for X joints.

API RP2A2, 20th Ed. 1993, adopted the derating equation that already existed in AWS3 to calculate the effect of short can length on the ultimate strength of an X joint, Section 4.3.4 gives the capacity for a joint with chord member outer diameter, D, and can length, L,, as :

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in which P(1) is the nominal strength obtained using the chord thickness away from the joint, and P(2) is obtained using the thickness at the joint.

In this paper, the effect of the length of thickened joint cans on the capacity of tubular T joints is investigated. Finite element techniques and standard computer programs are used to model and analyze the joints. First, pilot studies are conducted to verify the accuracy of the analyses. X joints with configuration equal to those used in the Vegte1 study are analyzed and compared with his results. Then, a T joint model is analyzed and the results compared to an experimental data4. Finally, to assess the above form of derating equation for T joints, a series of T joint models are developed and analyzed. The numerically determined ultimate loads at various can lengths are compared to values recommended by API and other codes of practice.

The study of can length effect on joint strength naturally leads to the question of the limiting length beyond which fiuther increase will not result in further increase in joint strength. The results obtained from the numerical studies are compared to some suggested effective length expressions.

FINITE ELEMENT ANALYSES

The results herein are based purely on numerical modeling using standard fifth element computer programs. However, the predicted behavior is first verified against experimental and other published numerical results.

Methodology

The methodology used in modeling the joints maybe summarized as follows.