The Technical File | Groundforce Groundforce Shorco | Technical File Issue 3.0 Rolling Strut Systems - Basis of Design Rolling strut (RS) linear support systems are available in either slide rail or trench box formats and offer a number of advantages over their traditional fixed strut counterparts in terms of strength and functionality. The system employs a single (rolling) strut assembly at the front and rear of the plates which as the name suggests, is able to slide or roll up down vertically. Much greater versatility can thus be achieved enabling larger and longer pipes to be installed with rolling struts compared to standard trench boxes. The multi position rolling strut concept does however, add a level of complexity to the determination of structural capacity (or overall resistance to lateral earth pressures) of the unit. One can appreciate that the overall box capacity will be dependant on the position of the strut within the post. It is therefore “safer” to specify its capacity when the strut is positioned in such a way that it induces the most onerous forces in the supporting components. This is generally with the strut at its uppermost position. As with traditional trench boxes, capacity or resistance is usually quoted in terms of allowable uniform panel pressure. Minimum values for RS system resistance are quoted in sections 10.2 and 10.3. Should the designer wish to carry out an analysis with the strut positioned lower down in the posts to give higher system capacity, it is necessary to carry our a more rigorous structural analysis as described in the following sections. Failure mechanisms Both RS slide rail and boxes incorporate three basic structural elements; posts, panels and struts. In the case of boxes the posts are integral within the panels themselves. There are three main potential failure mechanisms in a simple box assembly namely: Inward bowing of the box panel between the struts - Bowing moment (denoted M1). Bending of the post under the strut - kicking moment (denoted M3). Failure of the strut; see page 10.1.2. Design considerations for panels The overall structural capacity of the box or slide rail is the least value to induce failure due to the following: Bowing of the panels (M1). This is determined by the bending strength of the panel in the plane of bending. Panels are typically of cellular construction and it the strength is proportional to the section modulus of the component parts in the plane of bending (or bowing). Section through a typical box panel indicating kicking moment (M3) Plan view of box panel indicating bowing moment (M1) Rolling strut slide rail system (single bay only is illustrated) Rolling Strut Box Unit 10.1.1