tailieunhanh - Structural Steel Designers Handbook Part 9

Tham khảo tài liệu 'structural steel designers handbook part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | DESIGN CRITERIA FOR BRIDGES In the ASSHTO LRFD Specifications the pressure P ksf is calculated from rv2 P s 1W where V velocity of water fps for design flood and appropriate limit state and CD is a drag coefficient for semi-circular nosed pier for square ended pier for debris launched against pier and for wedge nosed pier with nose angle 90 or less . For ice and drift loads see AASHTO specifications. Buoyancy should be taken into account in the design of substructures including piling and of superstructures where necessary. LOAD COMBINATIONS AND EFFECTS Overview The following groups represent various combinations of service loads and forces to which a structure may be subjected. Every component of substructure and superstructure should be proportioned to resist all combinations of forces applicable to the type of bridge and its site. For working-stress design allowable unit stresses depend on the loading group as indicated in Table . These stresses however do not govern for members subject to repeated stresses when allowable fatigue stresses are smaller. Note that no increase is permitted in allowable stresses for members carrying only wind loads. When the section required for each loading combination has been determined the largest should be selected for the member being designed. The Standard Specifications for Highway Bridges of the American Association of State Highway and Transportation Officials specifies for LFD factors to be applied to the various types of loads in loading combinations. These load factors are based on statistical analysis of loading histories. In addition in LRFD reduction factors are applied to the nominal resistance of materials in members and to compensate for various uncertainties in behavior. To compare the effects of the design philosophies of ASD LFD and LRFD the group loading requirements of the three methods will be examined. For simplification only D L and I of Group I loading will be .