Knee braced connections between columns and beams have been an important part of many different structures throughout the centuries. Portal-type knee braced structures, in different variations, can be found in old bridges, churches and castles. In old industrial buildings, knee braced connections were widely used in the roof truss to column connections in the lateral plane, between columns and struts in the longitudinal direction, and between columns and floor beams. In all of these cases, especially timber construction, knee braced connections provided a perfect engineering solution that increased stability of the structure, created rigid connections between elements and reduced the bending span for beams. Therefore, it is understood why the designers of early crane runway girders for overhead traveling cranes adopted this approach which had worked well in portal-type structures before.12
Crane girders with knee braces are a typical feature in many mill buildings. The majority have survived in service for many years without any problems. However, some of them have experienced fatigue related damage.
The original intended function of the knee brace in crane runway design was to provide longitudinal stability of the crane runway against crane longitudinal forces and to help the columns deliver a horizontal shear to the foundations (Fig. 1).
The secondary effect of the knee brace, acting as an intermediate spring support for the girder, had not been considered. Girders were analyzed without considering this effect while knee braces were only analyzed for run way horizontal loads. In addition to the spring support function, knee braces provide a partial restraint to the crane girder support rotation resulting in a stress reversal in the girder and its supports. This stress reversal could lead to fatigue failure of the girder and/or girder to column connection details. It has been analytically proved that the magnitude of this stress reversal depends, to a great extent, on the relationship between stiffnesses of the girder, knee brace and column. Occasionally, this stiff ness relationship was established by the design in such a way that it resulted in a minor stress reversal which was successfully resisted by the structure, thereby, causing no fatigue failure. .
A computerized design approach is proposed in this article for the analysis of knee braced crane runway girders as a longitudinal frame with special modeling of the knee brace and crane girder supports. The proposed approach helps the design engineer determine the magnitude of the force reversal in the girder/column/knee brace system, check the fatigue stress range in these members and more accurately account for knee brace effects in crane runway modification projects.