API REPORT 81-40 Chord Stress Effects on the Ultimate Strength of Tubular Joints

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API REPORT 81-40 Document Information:

Title
Chord Stress Effects on the Ultimate Strength of Tubular Joints

American Petroleum Institute

Publication Date:
Dec 1, 1982

Scope:

ABSTRACT

Ten tests were conducted on double-tee tubular joints to study the effect of chord axial and bending stresses on ultimate joint strength. Three separate branch loading conditions were considered: axial load (4 tests), in-plane bending (3 tests), and out-of-plane bending (3 tests).

The current API Specification uses a Qf factor to account for chord stress effects. Comparison of the API formula with the results of the axial branch tests indicated that the Qf factor is unconservative. Examination of the tests upon which the API Qf formula is based indicated a significant variation in thinness ratio, γ, when compared to the tests reported herein, so a new ultimate strength formula is suggested which considers the γ effect

where f is the extreme fiber compressive stress in the chord and Fy is the yield point of the chord material. Qf is taken as 1.0 when f is tensile. When combined axial and bending stresses occur in the chord, it was found that an average Qf, obtained by using the maximum and minimum extreme fiber stress in the formula above, correlated with the test results.

For in-plane bending, the chord stress effects were more significant than those for axial loading. For out-of-plane bending, the chord effect was less significant. The following two formulas were developed from the test data,

The new recommendations gave Qf factors within 5 percent of experimental values.

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