CE248_LN_Loading_and_Gravity_loads - Lecture notes for CE...

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Lecture notes for CE 248 Behavior of plastic design of steel structures Topic: Loading Prepared by: T.Y. Yang Date: August 31 st 2006 Text: ASCE 7 – 02 (or 05): Minimum Design Loads for Building and Other Structures. Scope: “This standard provides minimum load requirements for the design of buildings and other structures that are subject to building code requirements. Loads and appropriate load combinations, which have been developed to be used together, are set forth for strength design and allowable stress design. For design strengths and allowable stress limits, design specifications for conventional structural materials used in buildings and modifications contained in this standard shall be followed.” Provide the minimum load requirements. Used for LRFD or ASD. Allowable Stress Design (ASD): Used by engineers for more than 150 years. Best estimate of the load are applied to a linearly equivalent model. Stress of the model is calculated and checked against “Allowable” stress (usually portion of the yield stress). For example: o Allowable tensile stress for gross steel cross section = 0.6 Fy. This is equivalent of providing a safety factor of 1.67 (Fy/0.6Fy = 1.67). o Different material (steel, concrete) and action (tension, shear, bending, torsion) will have different safety factor. Load and Resistant Factor Design (LRFD): Routinely used by engineers in the design office. Loads are factored to calculate the ultimate load, where the load factors are based on statistical interpolation of measured condition. o For example the load factor for the dead load is greater than the live load. The applied load is then applied to a linearly elastic model. (Courtesy of Prof. Moehle)
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The component capacities are calculated assuming some inelastic behavior of the cross section. Different strength reduction factor φ are assigned for different failure mode. For example: = 0.9 (for bending), = 0.85 (for shear). This is because we want to be more conservative in shear failure than flexure failure.
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CE248_LN_Loading_and_Gravity_loads - Lecture notes for CE...

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