L20_design of beams-fall 10

# L20_design of beams-fall 10 - EAS 209-Fall 2010...

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EAS 209-Fall 2010 Instructors: Christine Human 10/14/2010 1 Lecture 20- Design of Beams for Bending Lecture 20 Design of Beams for Bending For a beam in bending, the largest normal stress occurs at the surface of the beam at the location where the bending moment is a maximum. Today’s Objectives: Design beams for bending Draw more shear force and bending moment diagrams Learn about Load and Resistance Factor Design (LRFD) Today’s Homework: S M I c M m max max = = σ EAS 209-Fall 2010 Instructors: Christine Human 10/14/2010 2 Lecture 20- Design of Beams for Bending A safe design requires that the maximum normal stress be less than the allowable stress for the material used. This criterion leads to the determination of the minimum acceptable section modulus. Many beam sections will satisfy the required section modulus. The beam with the smallest weight per unit length or cross sectional area will be the least expensive and therefore the best choice, provided there are no other constraints such as depth of beam, beam deflection etc. all all m M S max min =

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EAS 209-Fall 2010 Instructors: Christine Human 10/14/2010 3 Lecture 20- Design of Beams for Bending Design Procedure 1. Select material and determine σ all from table of material properties. Alternatively, use . . S F ult all σ = Unless otherwise indicated assume allowable stress is the same in tension and compression 2. Draw shear and moment diagrams to determine |M| max 3. Determine S min from all M S max min = 4. Pick section 5. If beam is not symmetrical and σ all (tension) σ all (compression) you will need to check both M max and M min (see example p7) Rectangular Timber Beam 6 2 bh c I S = = If b or h or h/b is
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L20_design of beams-fall 10 - EAS 209-Fall 2010...

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