Influence of Creep on Torsional Restraint Provided to Cold-Formed Z-shaped Purlins by Sandwich Panels

Abstract

Sandwich panels supported by purlins are widely used as the main load bearing part of roof and wall cladding systems of steel buildings. Beside of their main function as building envelope, sandwich panels provide lateral and torsional restraint to purlins which positively influences the member resistance. Lateral restraint provided by sandwich panels is equal for both gravity and uplift loads applied on the panel’s surface, however, the torsional restraint may have a different behaviour depending on the direction of loading. While gravity loads increase the contact pressure between the panels and purlins so that the torsional restraint is ensured up to a relatively large rotation of the purlin, uplift loads reduce the contact area which can result in smaller torsional restraint. The paper discusses and compares the different behaviour and addresses the resulting requirements for the experimental setups. Another aspect that is noticeable in torsional restraint, but not in lateral restraint, is the influence of the materials of the insulation core (usually PUR-, PIR- or EPS-foam or mineral wool) of the sandwich panel. These materials significantly differ in their properties from the materials of faces as they exhibit complex behaviour with time-dependent response. Deformations from compressive stress increase over time which is referred as creep. This behaviour is evident both under gravity loads and under uplift loads, which reduces torsional restraint. Creep of the core can also result in the reduction of the preload force in screws, as the core is under compression after tightening. The paper focuses on the influence of creep on torsional restraint provided to cold-formed Z-shaped purlins by sandwich panels. Results of experimental tests are presented, and the influence of creep is evaluated for various types of the core materials of the sandwich panelsSandwich panels supported by purlins are widely used as the main load bearing part of roof and wall cladding systems of steel buildings. Beside of their main function as building envelope, sandwich panels provide lateral and torsional restraint to purlins which positively influences the member resistance. Lateral restraint provided by sandwich panels is equal for both gravity and uplift loads applied on the panel’s surface, however, the torsional restraint may have a different behaviour depending on the direction of loading. While gravity loads increase the contact pressure between the panels and purlins so that the torsional restraint is ensured up to a relatively large rotation of the purlin, uplift loads reduce the contact area which can result in smaller torsional restraint. The paper discusses and compares the different behaviour and addresses the resulting requirements for the experimental setups. Another aspect that is noticeable in torsional restraint, but not in lateral restraint, is the influence of the materials of the insulation core (usually PUR-, PIR- or EPS-foam or mineral wool) of the sandwich panel. These materials significantly differ in their properties from the materials of faces as they exhibit complex behaviour with time-dependent response. Deformations from compressive stress increase over time which is referred as creep. This behaviour is evident both under gravity loads and under uplift loads, which reduces torsional restraint. Creep of the core can also result in the reduction of the preload force in screws, as the core is under compression after tightening. The paper focuses on the influence of creep on torsional restraint provided to cold-formed Z-shaped purlins by sandwich panels. Results of experimental tests are presented, and the influence of creep is evaluated for various types of the core materials of the sandwich panels