Explanation: A plate girder is deep flexural member used to carry loads that cannot be economically carried by rolled beams. When load is heavier and span is also large, there are three options : i) two or more I0sections, connected appropriately (ii) plate girder (iii)truss girder. Of the above alternatives, first is uneconomical. So, plate girder or truss girder can be used. Plate girder is used since cost of fabrication of truss girder is high.

Explanation: When load is heavier and span is also large, either plate girder or truss girder can be used. But, plate girder is preferred because of the disadvantages of truss girder. The disadvantages of truss girder are higher cost of fabrication and erection, problem of vibration and impact, requirements of higher vertical clearance and costly maintenance.

Explanation: Plate girders are built-up flexural members. Their bending resistance can be increased by increasing the distance between flanges. This also increases the shear resistance as web area increases.

Explanation: When the loads and span are large, plate girder sections either with riveted/bolted connections or welded connections may be provided. The number of flange plates can be increased depending upon the moment to be resisted. If depth is limited and loads are too large, welded box plate girder is provided. A box girder with riveted/bolted connections can b e provided but it is too costly as compared to welded one. Box girders have great resistance to lateral buckling.

Explanation: An ideal bolted plate girder section consists of flange angles and cover plates for compression flange and only flange angle for tension flange. The various elements of riveted/bolted/welded plate girder are : web plate, flange angles with or without cover plates for riveted/bolted plate girder and only flange angles for welded plate girder, stiffeners – bearing, transverse and longitudinal, splices for web and flange.

Explanation: The limit states considered for plate girder are yielding of tension flange and buckling of compression flange. The compression flange buckling can take place in various ways, such as vertical buckling into the web or flange local buckling. Flange buckling can also be caused due to lateral-torsional buckling.

Explanation: At high shear locations in the girder web, usually near supports and neutral axis, the principal planes will be inclined to longitudinal axis of the member. The principal stresses will be diagonal tension and diagonal compression along the principal planes.

Explanation: The principal planes will be inclined to longitudinal axis of the member at high shear locations in the girder web. Along the principal planes, the principal stresses will be diagonal tension and diagonal compression. Diagonal tension does not cause any problem but diagonal compression causes the web to buckle in the direction perpendicular to its action.

Explanation: Diagonal compression causes web to buckle in the direction perpendicular to its action. This problem can be solved by any of the following ways : (i) reduce depth-to-thickness ratio of web such that problem is eliminated, (ii) provide web stiffeners to form panels that would enhance shear strength of web, (iii) provide web stiffeners to form panels that would develop tension field action to resist diagonal compression.

Explanation: As web begins to buckle , the web loses its ability to resist diagonal compression. The diagonal compression is transferred to transverse stiffeners and flanges. The vertical component of diagonal compression is supported by stiffeners and horizontal component is resisted by flanges. The web resists only diagonal tension and this behaviour of web is called tension field action.

Explanation: Intermediate stiffeners can be used to develop tension field action and improve shear capacity of web. The main purpose of these stiffeners is to provide stiffness to the web rather than to resist the applied loads. Additional stiffeners called bearing stiffeners are provided at points of concentrated loads to protect the web from the direct compressive loads.