FRAMED- TRUSS SYSTEMS FOR LARGE SPAN INDUSTRIAL STRUCTURES

Abstract

Large span (30-60m) Industrial Roofing systems have traditionally been done through conventional simply supported Truss systems using open sections such as Angle, Channel and I-section. This needs a large height for the truss system so as to create high moment resisting capacity by members which are capable of carrying axial forces only. The large space between the bottom chord and the ridge remains unutilized (5-7m height) and provides a very large projected area for the wind pressure to act. As a consequence to this, the truss spacing is reduced so as to deal with comparatively low values of load. Use of closed form tubular sections has helped in meeting some requirements up to some span. In the recent time, however, framed truss made in high strength steel (300-35OMPa as against 250 for hot rolled section) hollow sections has become quite popular. The roof slop of the framed truss is generally in the range of 5°-7° as against l00-150 for conventional trusses. The roofing system of an industrial building comprises elements such as Purlins and roof sheeting. All these elements transfer the imposed loads through flexural actions to rafter of framed truss. To deal with large moment on account of very large spans with force couple action large liver arm (truss depth) is required. Also, the moment in the rafter of the frame varies in a parabolic profile with respect to span and hence a constant truss depth becomes inefficient in comparison with trapezoidal shaped framed truss. This type of manufacturing has now become feasible and cost-effective. This category of work in steel is referred to as Pre-Engineered Building (PEB). In the present thesis work, framed truss have been studied for their performance for the span of 30 and 40m. The rafter inclination has been kept in the low range of 5°-7° with a column height of 6m and frame spacing about 5-6m. The elements of rafter such as Eva depth, ridge depth, and number of panels for various web patterns have been varied in the suitable range to study their impact/influence on the ridge deflection and finally the percent capacity utilization of various members. The results have been presented in a lucid manner in a combination of graphical and tabular