Post Elastic Behaviour and Moment Redistribution in a Double Span LTP200 Steel Trapezoidal Sheet

Abstract

Cold Formed Steel Trapezoidal sheets are very popular in Sweden as a roofing system for stadiums and arenas. To effectively utilize the span and field moment capacities, Lindab International uses Gerber system of joints for one of their products LTP200 Trapezoidal Profile. In this joint, a hinge is placed in span at a predefined location near the internal support where the bending moment is theoretically zero. Within the working limit states, this joint works very well as it is not subjected to any moments. But as the section over the mid-support fails, the stiffness of this section decreases dramatically and so does its moment capacity, which is normally the case with cold formed section. As the moment capacity of this section drops, there is a redistribution of bending moments in the whole system and the joint will be subjected to moments in this post elastic scenario. Previously, the joint was not able to sustain these bending moments. Now, Lindab is modifying the design of this joint by using a combination of screws and sheet overlap. In this design, the joint will work as a hinge in working limit states, but as the section over midsupport fails, contribution of overlap is attained and the joint will now work as continuous sheet. The amount of moments this joint is subjected to; depends on the variation of stiffness of the section over mid-support and reserve capacity of this section. The aim of this thesis is to study the behaviour of the section over mid-support after its elastic limit and make conclusions about inelastic reserve capacity and stiffness variation. In the end, a moment redistribution diagram will be made for a double span system at ultimate state which will enable the design of joint against the varying internal forces.

Cold Formed Steel Trapezoidal sheets are very popular in Sweden as a roofing system for stadiums and arenas. To effectively utilize the span and field moment capacities, Lindab International uses Gerber system of joints for one of their products LTP200 Trapezoidal Profile. In this joint, a hinge is placed in span at a predefined location near the internal support where the bending moment is theoretically zero. Within the working limit states, this joint works very well as it is not subjected to any decreases dramatically and so does its moment capacity, which is normally the case with cold formed section. As the moment capacity of this section drops, there is a redistribution of bending moments in the whole system and the joint will be subjected to moments in this post elastic scenario. Previously, the joint was not able to sustain these bending moments. Now, Lindab is modifying the design of this joint by using a combination of screws and sheet overlap. In this design, the joint will work as a hinge in working limit states, but as the section over midsupport fails, contribution of overlap is attained and the joint will now work as continuous sheet. The amount of moments this joint is subjected to; depends on the variation of stiffness of the section over mid-support and reserve capacity of this section. The aim of this thesis is to study the behaviour of the section over mid-support after its elastic limit and make conclusions about inelastic reserve capacity and stiffness variation. In the end, a moment redistribution diagram will be made for a double span system at ultimate state which will enable the design of joint against the varying internal forces.