STUDIES IN GABLE FRAMES FOR INDUSTRIAL BUILDINGS

Abstract

Large span (30-60m) Industrial Roofing systems have traditionally been done through 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. Coupled with this is the height of the columns which further results in increased lateral loads due to wind. Use of closed form tubular sections has helped in meeting some requirements upto some span. In the recent time, however, Gable frames made in high strength steel (300-35OMPa as against 250 for hot rolled section) has become quite popular. The rise of the Gable is generally in the range of 5°-7° as against 10°-15° for trusses. The Cable comprises elements such as Columns, Rafter, Purlins and roof sheeting. All these elements transfer the imposed loads through flexural actions for which I-section is considered quite suitable. To deal with large moment on account of very large spans, the flanges need to separated by a large spacing (700-1300mm). Also, the moment varies in a parabolic profile with respect to span and hence a constant I-section (prismatic section) becomes inefficient in comparison with trapezoidal longitudinal section (non-prismatic section). 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, Gable Frames have been studied for their performance in the span range of 40-60m. 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 flange thickness-to-width, web depth-to-thickness with and without vertical stiffeners have been varied in the suitable range to study their impactli-,_ifluence on thedesign moment resisting capacity, control of deflection and finally the percent capacity utilization at various locations. The results have been presented in a lucid manner in a combination of graphical and tabular form.