Ultimate Strength of Tubular Joints: Chord Stress Effects
Authors: Boone T.J. , Yura J.A. , Hoadley P.W. 
Published: October 2008

The results of ten tests on large scale circular tubular joints designed to investigate chord stress effects are presented and discussed. Axial, in-plane bending and out-of-plane bending loads were applied to the branch members of the double-tee specimens, in combination with compressive axial chord loads and chord moments. The results are compared with the previous test work that has been used to develop offshore specifications. It is found that current design equations do not adequately predict chord stress effects on joint strength. New design equations are recommended that better reflect the effect of chord stress on ultimate tubular joint strength.

INTRODUCTION

The current American Petroleum Institute(API) recommendations [1] for tubular joint design are based principally on extensive tests of simply loaded joints; that is, joints where the branch members, which are welded to the continuous chord members, have only axial loads, in-plane moments, or out-of-plane moments. In actual design situations, joints are rarely simply loaded. The chord may also have axial and bending stresses. Japanese researchers have investigated the effect of axial chord stress on branch axial load strength in a limited number of relatively small scale specimens [2]. Their work indicated that compressive chord stresses had a detrimental effect on the branch axial load strength

The API recommendations for punching strength at a tubular joint use a reduction factor, Qf to account for the presence of chord stresses. Qf is defined as follows:(Mathematical equation available in full paper) where is the sum of the applied axial stresses and bending stresses in the chord divided by the allowable chord stress. This factor is derived from one of the Japanese test series in which double-tee specimens were used, and various levels of uniform axial stress were applied to the chord. There have been no experiments in which moment has been applied to the chord. When chord moment is present, the API recommends use of a Qf factor that is determined from the maximum moment stress as if it were applied uniformly across the section. This would appear to be a very conservative assumption, warranting investigation. But more importantly, the Qf factor itself is based on the assumption that the chord stress reduction effect, as determined from a DT specimen with a 4 in. (102mm) chord diameter under one type of branch load, is applicable to the full range of tubular joints used in practice.

This paper presents the results of ten tests on identical large-scale double-tee specimens that were used to investigate the effect of uniform axial and bending chord stresses on the strength of tubular joints subjected to three different types of branch loads: axial compression, in plane bending, and out-of-plane bending. The specimen which is shown in Fig. 1 was selected mainly for ease in applying the variety of loading conditions desired. Also, the double-tee type joint has a simple geometry that shows small scatter in experimental results for axial loading, thus avoiding the need for replicate tests.