# Similar mode of girder failure may be described the

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Chapter 2 / Exercise 10
Dosage Calculations
Pickar
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similar mode of girder failure may be described. The top flange of a Gerber girder is laterally and torsionally restrained by the connected joist members. The lower flange together with a portion of the web of the Gerber girder is prevented from lateral buckling by the bending stiffness of the web plate and the bending stiffness of the joist chords connected to the girder top flange (Fig. B1). Girder instability through loss of moment resistance by section distortion due to web bending should be design checked. Design procedure proposed: Using Engesser formula (Bleich 31 ), critical end-load for an infinitely long strut [B.1] where, moment of inertia of compression flange about y-y axis spring constant contributed by joist chord and girder web bending and, if joist framing on one side or, if joist framing on two sides taking into account the flexural stiffness of girder web and of the end-panel joist top chords. For symbols, see Fig. B1. Also see Example 1 provided. Since a Gerber girder section is prismatic throughout its entire length, the induced axial flange stress should also vary proportionally with the bending moment diagram along the girder span (i.e. zero stress when moment is zero and maximum when moment is at maximum). A segment of girder bottom flange-web, loaded with zero compression at one end and a maximum compression at the other, may be considered less severely loaded than an end-loaded strut of similar length, cross section and restraint conditions, because an end loaded strut is subjected to uniform compression. To obtain effective design compression to simulate an equivalent end-loaded strut, as used with design expression [B.1], it is proposed that the actual compression, as obtained from the cross-sectional area of the bottom flange including about 15% of the girder web using the maximum support moment, be multiplied by 0.5.
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Chapter 2 / Exercise 10
Dosage Calculations
Pickar
Expert Verified
Fig. B1 Cantilever Girder Stability Check for Slender Girders Note: This design check need not be performed, if a W-shape or a WWF-shape is selected for the girder. 36
REFERENCES 1 Steel Structures for Buildings (Limit States Design), CAN3-S16.1, Canadian Standards Association, 1984 (as revised to 1987) 2 Handbook of Steel Construction, Canadian Institute of Steel Construction, 1987 3 Steel Joist Facts .... Recommended Practice, Second Edition, Canadian Institute of Steel Construction, 1980 4 Cantilever and Suspended Span (CSS), Canadian Institute of Steel Construction, 1987 5 Standard for Steel Roof Deck, Canadian Sheet Steel Building Institute (CSSBI), June 1981 6 Heagler, R. P. and Luttrell, L. D., How to Fasten Steel Deck - Update and Review, Modern Steel Construction, No. 1, 1988 7 Cold Formed Steel Structural Members, CAN3-S136, Canadian Standards Association, 1984 (revised Jan. 88) 8 Commentary on CSA Standard CAN3-S136-M84, Cold Formed Steel Structural Members, Canadian Standards Association, 1986 9 Diaphragm Action of Cellular Steel Floor and Roof Deck Construction, CSSBI, 1972 (currently being revised) 10 Steel Deck Shear Diaphragm Design Manual, Westeel-Rosco Limited (currently Vic West Steel Inc.), 1975 11