Truss Analogy for Steel Moment Connections

Unprecedented widespread failure of welded moment connections in steel frames caused by the 1994 Northridge and the 1995 Kobe earthquakes have alarmed the engineering communities throughout the world. Beam-to-column connections in steel frames have been traditionally designed by using classical Euler-Bernoulli beam theory which leads to assumptions that the flanges transfer moment while the web connection primarily resists the shear force. The results of a recent finite element study at The University of Michigan have shown that stress distribution in the vicinity of moment connections drastically differs from the pattern assumed following the classical beam theory. This is in agreement with the boundary effect postulate expressed by the famous Saint Venants Principle. The finite element study has shown that the magnitude and direction of the principal stresses in the connection region are better approximated by using truss analogy rather than the classical beam theory. Accordingly, both the bending moment and the shear force are transferred across the connection near the beam flanges through diagonal strut action. Thus, the beam flange region of the traditionally designed connection is overloaded. This conclusion explains, to a large extent, the recently observed steel moment connection failures. This study uses the finite element analysis results and the truss analogy in order to establish a more realistic load path in the welded steel moment connections, a practical and more rational analysis and design procedure has been developed. The new connection design uses a combination of flange cover plates and vertical rib plates to carry the normal and shear stresses created by the truss action. The design procedure was successfully validated through cyclic load testing of a nearly full size specimen.