Nonlinear Database-Assisted Design Improves Wind Resistance of Low-Rise Buildings
Dat Duthinh, William P. Fritz
This paper presents an improved method to design buildings to resist high wind. The method consists of using databases of pressures measured in wind tunnel tests and applying these pressures in nonlinear structural analyses, hence the name nonlinear database-assisted design (NLDAD). Available databases of experimental measurements provide structural designers with an envelope of peak loads for all critical sections of a structure, and thus greater confidence in the safety of the design than if a few idealized pressure distributions specified by building codes were used. Unlike earlier approaches, the approach used in this paper targets all potentially critical cross sections of the structure being designed. Precise knowledge of the load cases required enables optimization of the structure, thus leading to more efficient use of materials, which translates into lower cost and less demand on the environment, as well as improvement in safety. NLDAD is not proposed for routine office calculations, but rather for calculations intended to develop standards provisions that reflect more correctly than current methods based on linear response do, the capacity of structures to withstand wind and gravity loads. To illustrate the method, one frame of a steel warehouse originally designed to satisfy ASCE 7-93 wind loads, was investigated. Updated ASCE 7-02 loads were used in this investigation, as well as wind tunnel measurements. The paper describes the selection of load cases from the experimental database using a linear beam model. Next, a much more detailed finite-element model was analyzed under the selected load cases into the large displacement, post-yielding range, with account taken of local plate buckling and initial imperfections. By using NLDAD, improvement in wind resistance of 30 % was achieved, at the cost of 3.6 % increase in weight. Most of the performance improvement was attributable to structural modifications that were partly enabled by nonlinear analysis. Conversely, for the same wind resistance, a decrease in weight of 10 % or so is possible with NLDAD. The governing load case from the wind tunnel pressure database confirms the validity of ASCE 7-02 pressures. When initial deformations are accounted for, the structure still passes ASCE 7-02 Standards only to the extent that wind load factors specified in the Standards may be construed to be associated with ultimate strength rather than allowable stress design. The margin of safety mentioned in ASCE 7-02 Commentary is overly optimistic.