This set of Tricky Design of Steel Structures Questions and Answers focuses on “Plate Girders & General Considerations”.
1. A plate girder is used when
a) span is large and loads are heavy
b) span is small and loads are heavy
c) span is small and loads are light
d) span is large and loads are light
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.
2. Why plate girder is preferred over truss girder?
a) plate girder requires costly maintenance
b) higher vertical clearance required for plate girder than truss girder
c) cost of fabrication of plate girder is high
d) 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.
3. Bending resistance of plate girders can be increased by
a) decreasing distance between flanges
b) increasing distance between flanges
c) reducing distance between flanges to half
d) bending resistance cannot be increased
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.
4. Which of the following is economical if depth is limited and loads are too large?
a) rolled section beam
b) truss girder
c) welded box plate girder
d) bolted box plate girder
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.
5. An ideal bolted plate girder section consists of
a) flange angles and cover plates for both compression flange and tension flange
b) flange angles and cover plates for compression flange and only flange angle for tension flange
c) only flange angle for compression flange and flange angles and cover plates for tension flange
d) flange angles for both compression flange and tension flange
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.
6. The modes of failure of plate girder are
a) by yielding of compression flange only
b) by buckling of tension flange only
c) by yielding of tension flange and buckling of compression flange
d) by yielding of compression flange and buckling of tension 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.
7. At high shear locations in the girder web, principal plane will be ______ to longitudinal axis of member
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.
8. Which of the following causes web buckling in plate girder?
a) diagonal tension
b) diagonal compression
c) diagonal tension and diagonal compression
d) neither diagonal tension nor diagonal compression
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.
9. Which of the following statement is correct for reducing web buckling due to diagonal compression?
a) not providing web stiffeners to increase shear strength
b) providing web stiffeners to reduce shear strength
c) increasing depth-to-thickness ratio
d) reducing depth-to-thickness ratio
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.
10. Which of the following is correct during tension field action?
a) web can resist diagonal compression
b) horizontal component of diagonal compression is supported by flanges
c) vertical component of diagonal compression is supported by flanges
d) vertical component of diagonal compression is supported by stiffeners
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.
11. Which of the statement is not true about intermediate stiffeners?
a) they reduce shear capacity of web
b) they improve shear capacity of web
c) they can be used to develop tension field action
d) their main purpose is to provide stiffness to the web
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.
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