Optimum design of steel open web girders

Abstract

Girders are used to support building floors, roofs, and bridges or as members of towers, cranes. When a girder is built with top and bottom members connected with web members it is called an open web girder. (Ref Fig 1) Open web girders can be fabricated from steel, timber, aluminium etc. Among those materials steel is the most common construction material used for girders even though it is a material imported to Sri Lanka. Timber was also used in the past as girders but now it is not commonly used due to non-availability and to protect the environment. Different configurations are used as open web girders (Ref. Fig. 2) and shape varies when used as pitched roof supporting member. (Ref. Fig.3) Most instances, it is useful and preferred to use steel open web girder due to its lightness, factory fabrication facility, simplicity, and time saving than other materials. However there are some disadvantages of using steel girders as well. Use of steel girders for open buildings in aggressive environment such as pavilions in coastal areas increases the maintenance cost. It is always not possible to use steel girders as roof supporting members when effective tying system is required at roof level. In such instances, with the use of steel girder system column sections have to be increased affecting architectural appearance. In designing these girders, the most appropriate structural form have to be selected considering different aspects such as specific functional needs, aesthetics, and economy. Due to high cost involved in construction with steel, it is the duty of the engineers to use optimally designed open web girders without violating other requirements. In Sri Lanka most of the standard steel sections are not freely available and therefore economising is not always practicable. Steel sections available in the market are imported from different countries and hence may not be compatible with commonly used British Standards. This study attempts to analyse most commonly used structural form the open web girder type for pitched roofs.