Evaluation of ultimate strength of low-rise steel building frames and components using wind tunnel data.

摘要

Current wind loading provisions of the ASCE 7 Standard include simplified static descriptions of wind loading originally developed to be simple enough for use with slide rule or pocket calculator. In addition, the ASCE 7 wind loads model spatial variation of actual wind loads in an oversimplified manner determined by the need to cover a wide variety of loading situations by a single table or diagram. For these reasons, the simplified descriptions may result in estimates of wind effects that are crude and inconsistent with respect to risk.; A comparative study is presented of the estimated wind load capacities of structural framing systems of low-rise steel buildings based on loading patterns established from aerodynamic databases on the one hand, and on the patterns specified in the ASCE 7 Standard on the other. The estimated capacities provide a realistic evaluation of the strength of low-rise steel buildings subjected to wind loads, and can be used to judge the adequacy of the current wind loading provisions. The estimates are obtained from inelastic finite element analyses of main and secondary framing systems---main wind load-resisting frames and continuous purlins---with ultimate states associated with local and global instabilities. The study includes the effects of wind direction on the strength of main frames and purlins, and it also evaluates the relative merits of several fabrication details for both framing systems in the buildings with respect to wind resistance. The building models are assumed to be located in two hurricane-prone areas along the Atlantic coast: Miami, Florida and Charleston, South Carolina. The basic dimensions of the buildings are the same except for two different eave heights. Also, two different terrain types were considered.; The results of the study indicate that the wind loads in the ASCE 7-93 Standard considerably overestimate the wind loads for frame design, but do not consider effects of wind direction. The ASCE 7-98 and 7-02 Standards show some improvement for the wind loads over the ASCE 7-93 loads in loading pattern, but they still have not been revised with respect to consistency of the wind loading pattern, especially in terms of the magnitude of the wind load. Based on the results of the study, it is suggested that a directionality reduction factor of about 0.9 would be appropriate in frame design.; On the other hand, the ASCE 7 wind loads for purlin design significantly underestimate the wind loads. To ensure safe purlin design, the current load factor lambda = 1.6 for wind load may be desirable to be used without combining it with the directionality reduction factor of 0.85. In addition, the edge distance "a" specified in ASCE 7 may not lead to conservative secondary member design. Therefore, it is suggested that this edge should be doubled for conservative purlin design.; The study was based on the limited number of the buildings with the low profile of the roof slope, resulting in suction on both windward and leeward roofs for wind pressures. Therefore, its application must be limited to the range of buildings that are rectangular in plan with dimensions 100 ft x 200 ft and have the roof slope less than 10°, which is the maximum roof slope of inducing suction on roof for wind pressure.