The introduction of a cylindrical branch into a cylindrical pressure vessel (forming a branch-pipe connection, or tee) may reduce the capacity of the vessel to withstand internal pressure. Some form of reinforcement of the connection may be required if the components of the vessel are to regain their former pressure-carrying capacity. Various proposals have been made for the reinforcement of pipe junctions, but it is not known how effective or how necessary those reinforcements are. During the past 20 years increasing efforts in research have been made towards a solution to this problem through elastic analysis of stresses by experiment and theory. However, there is a wide spectrum of service conditions under which the elastic stress field cannot be adequately incorporated in an analysis to predict the strength of the branch pipe connection. These include conditions of static internal pressure or external loadings at ambient temperature, with normally ductile materials, under circumstances where un-serviceability is considered to occur when the permanent deformations are greater than the elastic deformations produced in the vessel. Under such conditions, the limit pressure of the theory of rigid-plastic materials may be a relevant measure of the strength of the connections. During the past decade there has been much progress in the plastic analysis of shell structures, until quite recently confined to axisymmetric problems. As yet, experimental evidence that the theory is applicable is very scarce and relates only to axially symmetric configurations. The present investigation was undertaken to study the actual behavior of branch-pipe connections in the inelastic range with a view to the ultimate development of a limit analysis for these configurations. The first configuration, designated as Model UW-3, was a standard reducing tee. The second configuration was a standard full branch tee designated as UW-4. Both vessels were Schedule 40, ASA B16.9, donated by Taylor Forge & Pipe Works.