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【正文】 )(b dynamic loading Inertia forces which resist accelerations of the structure in this way are the most important disti nguishing characteristic of a structuraldynamics problem. In general, if the inertia forces represent a signifieant portion of the total load equilibrated by the internal elastic forces of the structure, then the dynamic character of the problem must be accounted for in its solution. On the other hand,if the motions are so slow that the inertia force39。 a nondeterministic analysis Correspondingly is the analysis of response to a random dyna mic loading. In general, the structural response to any dynamic loading is expressed basically in terms of the displacements of the structure. Thus a deterministic analysis leads to a displacementtime history corresponding to the prescribed loading history 。 and b) to prevent collapse before a specified overload has been placed on the str ucture (safety or ultimate condition). Since normally elastic conditions will prevail under working loads, a structural theory based on the assumptions of elastic behavior is appropriate for determining serviceability conditions. Collap se of a structure will usually occur only long after the elastic range of the materials has been excee ded at critical points, so that an ultimate strength theory based on the inelastic behavior of the mate rials is necessary for a rational determination of the safety of a structure against collapse. Neverthel ess, an elastic theory can be used to determine a safe approximation to the strength of ductile struct ures(the lower bound approach of plasticity),and this approach is customarily followed in reinforced concrete practice. For this reason only the elastic theory of structures is pursued in this chapter. Looked at critically,all structures are assemblies of threedimensio nal elements, the exact ana lysis of which is a forbidding task even under ideal conditions and impossible to contemplate under conditions of professional practice. For this reason, an important part of the analyst39。 5 UNIT SIX Text Structural Analysis 一． Structure of Buildings Considering only the engineering essentials, the structure of a building can be defined as the assemblage of those parts which exist for the purpose of maintaining shape and stability. Its prim ary purpose is to resist any loads applied to the building and to transmit those to the ground. In terms of architecture, the structure of a building is and does much more than that. It is an inseparable part of the building form and to varying degrees is a generator of that form. Used skillf ully, the building structure can establish or reinforce orders and rhythms among the architectural v olumes and planes. It can be visually dominant or recessive. It can develop harmonies or conflicts. It can be both confining and emancipating. And, unfortunately in some cases, it cannot be ignored. It is physical. The structure must also be engineered to maintain the architectural form. The principles and tools of physics and mathematics provide the basis for differentiating between rational and irration al forms in terms of construction. Artists can sometimes generate shapes that obviate any considera tion of science, but architects cannot. There are at least three items that must be present in the structure of a building: stability strength and stiffness economy Taking the first of the three requirements, it is obvious that stability is needed to maintain sh ape. An unstable building structure implies unbalanced forces or a lack of equilibrium and a conesq uent acceleration of the structure or its pieces. The requirement of strength means that the materials selected to resist the stresses generated by the l
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