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1、3.2 Limit State Design,Limit state design requires the structure to satisfy two principal criteria: ultimate limit state承载能力极限状态 serviceability limit 正常使用极限状态,Ultimate Limit State,A structure is deemed to satisfy the ultimate limit state criteria if all design values of load effect combination荷载效应组合
2、 are below the design values of bearing capacity承载能力calculated for the section under consideration.,Ultimate Limit State,structural collapse of all or part of the structure ( very low probability of occurrence) and loss of life can occur. Loss of equilibrium of a part or all of a structure as a rigi
3、d body (tipping 使倾斜, 推倒, 使倾覆 , sliding of structure).,Ultimate Limit States,Rupture of critical components causing partial or complete collapse. (flexural, shear failure).,Ultimate Limit States,Formation of a plastic mechanism - yielding of reinforced forms plastic hinges at enough sections to make
4、structure unstable. Instability cased by deformations of structure causing buckling of members. Fatigue - members can fracture under repeated stress cycles of service loads (may cause collapse).,Serviceability Limit State,A structure is deemed to satisfy the serviceability limit state when the const
5、ituent elements do not deflect by more than certain limits laid down in the building codes, the floors fall within predetermined vibration criteria, and crack widths in concrete are kept below specified dimensions.,Simply supported Beam,Cantilever beam,Serviceability Limit States,Functional use of s
6、tructure is disrupted, but collapse is not expected More often tolerated than an an ultimate limit state since less danger of loss of life. Excessive crack width leakage corrosion of reinforcement gradual deterioration of structure.,Serviceability Limit States,More often tolerated than an an ultimat
7、e limit state since less danger of loss of life. Excessive deflections for normal service caused by possible effects malfunction of machinery visually unacceptable,Serviceability Limit States,More often tolerated than an an ultimate limit state since less danger of loss of life. Excessive deflection
8、s for normal service caused by possible effects damage of nonstructural elements changes in force distributions ponding on roofs collapse of roof,Serviceability Limit States,More often tolerated than an ultimate limit state since less danger of loss of life. Undesirable vibrations vertical floors/ b
9、ridges lateral/torsional tall buildings Change in the loading,Characteristic Loads 荷载标准值,The expected maximum magnitude of load is referred to as the characteristic load. dead load Self weight of the structure and any permanent fittings and finishes; Magnitude can be estimated on the basis of materi
10、al densities and component sizes. Constant magnitude and fixed location,Dead Loads,Examples: Weight of the Structure (Walls, Floors, Roofs, Ceilings, Stairways) Fixed Service Equipment (HVAC, Piping Weights, Cable Tray, Etc.) Can Be Uncertain. pavement thickness earth fill over underground structure
11、,Characteristic Loads 荷载标准值,live load associated with occupation and use of the building The magnitude of live loads cannot be quantified with any precision and it is therefore necessary to make some allowance公差 based upon the type of activity to be performed within the building.,Characteristic Load
12、s 荷载标准值,wind load The magnitude of wind loads is closely related to wind speed design wind speed represents the maximum likely speed at a particular geographical location based upon statistical data. shape and dimensions of the building and its local environment.,Wind Loads,Design wind pressure, p =
13、 qz * G * Cp qz = Wind velocity pressure G = Gust Response Factor风振系数 Cp = External pressure coefficients (accounts for pressure directions on building),earthquakes pressures,Partial Safety Factors荷载分项系数,The characteristic/normal values of loads It is assumed that in ninety-five percent cases the ch
14、aracteristic loads will not be exceeded during the life of the structures,Partial Safety Factors荷载分项系数,Prevent structural failure when structures are also subjected to overloading. depending on the nature of loads or their combinations, and the limit state being considered. Design load = Characteris
15、tic load x Partial safety factor for load,characteristic strength of a material 材料强度标准值,Characteristic strength of a material obtained is the strength of that material below which not more than five percent of the test results are expected to fall,partial safety factor for the material,GB 50010-2002
16、 states that partial safety factor for concrete =1.4 partial safety factor for steel reinforcement = 1.1 when assessing the deflection, the material properties such as compressive strength should be taken as those associated with the characteristic strength of the material. 正常使用极限状态-材料强度标准值,Limit State Design,Ultimate limit states resistances to bending moment, shear force, axial thrust, torsional moment at every section shall not be less than thei
