Opt_FEM - STRUCTURAL DESIGN USING FINITE ELEMENTS Nam-Ho...

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1 STRUCTURAL DESIGN USING FINITE ELEMENTS Nam-Ho Kim
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2 INTRODUCTION FEA: determining the response of a given structure for a given set of loads and boundary conditions Geometry, material properties, BCs and loads are well defined Engineering design: a process of synthesis in which parts are put together to build a structure that will perform a given set of functions satisfactorily Analysis is very systematic and can be taught easily; design is an iterative process Creative design: creating a new structure or machine that does not exist Adaptive design : modifying an existing design (evolutionary process)
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3 INTRODUCTION STRUCTURAL DESIGN Structural design: a procedure to improve or enhance the performance of a structure by changing its parameters Performance: a measurable quantity (constraint and goal) the weight, stiffness or compliance; the fatigue life; noise and vibration levels; safety Constraint: As long as the performance satisfies the criterion, its level is not important Ex: the maximum stress should be less than the allowable stress Goal: the performance that the engineer wants to improve as much as possible Design variables: system parameters that can be changed during the design process Plate thickness, cross-sectional area, shape, etc
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4 EXAMPLE Design the height h of cantilevered beam with S F = 1.5 E= 2.9 10 4 ksi, w = 2.25 in. 1) Allowable tip deflection D allowable = 2.5 in. (No need S ) FE equation after applying BCs FE solution F = 2,000 lb L = 100 in h w       2 2 3 2 12 6 0 64 L vF EI LL  32 22 33 46 , FL v Ewh 3 2 allowable 3 3 3 allowable 4 4 3.66 in vD h EwD
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5 EXAMPLE cont. 2) Failure strength = 40 ksi (Need S F ) Supporting moment at the wall Maximum stress at the wall Height calculation with the factor of safety      22 1 1 1 2 2 3 6 4 6 2 EI C Lv L FL  2 max 2 6 h M I wh 2 6 6 4.47 in FF FLS Sw
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6 FSD EXAMPLE CANTILEVERED BEAM w = 2.25, h = 3.5 in. Determine new height using FSD Section modulus and max. stress at the initial design New section modulus using stress ratio resizing 22 3 old 2 2.25 3.5 4.594 in 66 I wh S h  max old 43.537 ksi M max 3 new old allowable 43.537 4.594
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Opt_FEM - STRUCTURAL DESIGN USING FINITE ELEMENTS Nam-Ho...

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