New API Equation for Grouted Pile-to-Structure Connections

ABSTRACT A new equation for the design of axial load transfer in grouted pile-to-structure connections, with or without shear keys, has been written into the latest edition of API RP 2A. The general form of the equation was obtained from an ultimate strength formulation for a predicted failure mode, then safety factors were applied. The reliability of the equation was studied by comparison with the results of 147 tests, and results are presented. The paper al sodiscusses 1 imitations on the equation, compares it to the equation in the U. K. Department ofEnergy Code and presents some numerical examples. ORIGIN OF NEW EQUATION In 1982 the American Petroleum Institute (API) Task Group on Fixed Platform Criteria asked member Demir 1. Karsan to form a mini committee on grouted pile-to-structure connections in order to review the current state-of-the-art and identify possible changes in recommended design practice. The first meetings of this new committee generated considerable discussion of grouted pile-to-structure connections, both with and without shear keys, and of tests which had been performed on this type of connection. Committee Chairman Karsan and member Nat W. Krahl reviewed the available test data and, in late 1982, presented to the committee two equations, one for estimating the ultimate strength of such a connection, and the second a modified form which could be used for design. The ultimate strength equation was based upon test observations that two separate mechanisms occur during the failure of a grouted tubular connection containing shear keys: first, a slippage between the steel and grout, and second, a crushing of the grout against the shearkeys (see Fig. 1). Such a connection normally fail s in a ductile manner, so that the ultimate strength of the connection is the sum of the two separate sources of strength. Before final failure occurs, diagonal cracks tend to open across the grout, frequently between diagonally opposite shear keys, or from one shear key to the opposite pipe. The ultimate strength equation presented to the committee stated that;(Mathematical equation available in fullpaper) EVALUATION OF NEW EQUATION BY API COMMITTEE Eqs. 1 and 2 were of sufficient interest that committee members issued a call for all available test data of grouted pile-to-structure connections so that a comprehensive reliability analysis could be performed on the equations. Additional data were indeed made available to the committee, so that data from a total of 201 separate tests were accumulated. The committee was of the opinion that a minimum unconfined grout strength should be specified for actual field connections, and a minimum unconfined strength of 2,500 psi (17.25 MPa) on 28-day old specimens was chosen. Hence, in comparing Eq. 2 to the test data, all tests were excluded in which the measured grout strength was less than 2,500 psi (17.25 MPa). 147 tests remained, including 62 tests of plain pipe (without shear keys), and 85 tests of pipe with shear keys.