Innovations in Plate Girder Design

Plate girders are often used in structures having spans more than 15-20m. Normal plate girders are provided with intermediate and edge stiffeners, to utilize the post buckling strength of the webs. However, they are often expensive due to additional fabrication and welding. Moreover the detailing has to be proper-otherwise their fatigue strength may be reduced. Recent innovations like hybrid girders and corrugated webs coupled with the developments in automatic welding processes have made these girders economical, aesthetic, and faster to fabricate and erect. They offer as viable alternates to reinforced concrete beams in numerous types of long span structures.

Dr. N. Subramanian Consulting Engineer, Gaithersburg, MD, USA

Introduction

A plate girder is basically an I-beam built up from plates using riveting or welding. It is a deep flexural member that can carry loads which cannot be economically carried by rolled beams. Standard rolled sections may be adequate for many of the usual structures; but in situations where the load is heavier and the span is also large, the designer has the following choices, (Fig.1).

• Use two or more regularly available sections, side-by-side.
• Use a cover-plated beam; i.e. weld a plate of adequate thickness to increase the bending resistance of the flange.
• Use a fabricated plate girder, wherein the designer has the freedom (within limits) to choose the size of web and flanges, or
• Use a steel truss or a steel-concrete-composite truss.

Among the options available, the first is usually uneconomical and does not satisfy deflection limitation. The second option is advantageous where the rolled section is marginally inadequate. Therefore the real choice lies between the plate girder and the truss girder. Plate girders have a moment resisting capacity in between rolled I sections and truss girders. Truss girders involve higher cost of fabrication and erection, problems of vibration and impact, and require higher vertical clearance. For short spans (< 10 m) plate girders are uneconomical due to increase in connection cost and rolled I sections are the preferred choice.