Research On Wind Load Characteristics And Optimal Design Of L-shaped Tall Buildings

Along with the development of science and technology, lack of lands for urbandevelopment gradually increases. Tall buildings are becoming higher, lighter andmore flexible and their shapes tend to be more complex. As a key factor of structureand glass curtain design, wind loads and their optimization during the structuraldesign of tall buildings are not neglectable. However, researches on wind loads havebeen so far focused on square or rectangular tall buildings, the research on wind loadsof irregular plane buildings has rarely been reported. Based on the wind tunnel test ofan L-shaped tall building for its wind-resistant design, wind pressure distributions andwind load characteristics were firstly analyzed in detail. In order to obtain moregeneral results, a series of typical L-shaped tall building rigid models were adopted inwind tunnel tests for simultaneous pressure measurements. According to the testresults, an in-depth study on wind load characteristics of L-shaped tall buildings wascarried out. Finally, optimal design of wind loads on L-shaped tall buildings waspresented by the optimum criteria method. The research content in this thesis can besummarized as follows:Both synchronous multi-pressure sensing system (SMPSS) technique and highfrequency force balance (HFFB) method were used for CAARC standard tall buildingmodels. Improved method for frequency response function of tube system andmodification of data smoothing process were presented. Test results were comparedwith those from different wind tunnel all labs over the world, it was verified that thetest equipment, flow field simulation, data acquisition as well as data processing inthe architectural and environment wind tunnel laboratory of Hunan University arereliable. Moreover, based on the test results, empirical formulas for the powerspectrum density (PSD) of wind loads including along-wind, across-wind andtorsional directions were proposed, which could be applied to the CAARC standardtall building model.Wind pressure distributions and wind load characteristics acting on an L-shapedtall building in practice were investigated. By use of the integral of wind pressures onthe model surfaces, the global shape factors which are useful for wind-resistant designof structures were derived. The probability distribution characteristics of thefluctuating wind pressures in negative pressure zone were analyzed. The results showed it is not reasonable to estimate the extreme wind pressures by Gaussiandistribution. Wind induced responses and equivalent static wind loads (ESWLs) werediscussed and the results showed that the first-order mode is enough to provideaccurate displacement response, but at least the first three modes are needed to keepthe accuracy of the acceleration response. ESWLs at along-wind direction are mainlymade up of mean wind loads while ESWLs at across-wind direction are controlled byinertial wind load. Wind-induced dynamic factors computed from the wind tunneltests were compared with those calculated from the new and old edition load codes inChina, it has been found that wind-induced dynamic factors in the new edition loadcode are larger than those in the old edition load code. The test results are smallerthan those in the new edition load code at the bottom of the building. As the heightincreases, the test results increase and even exceed those in the new edition load codeat the middle-upper part of the building.Eight L-shaped tall building rigid models with different geometric dimensionswere tested at four typical terrains by SMPSS technique. Based on the test results,characteristics of wind pressure distributions on the surfaces of the L-shaped tallbuildings were studied. The effects of wind direction, turbulence intensity, aspectration, slenderness ratio and the height of measure point were discussed. Shapefactors of wind load for every elevation at typical wind directions were summarized.The power spectra and coherence functions of L-shaped tall buildings wereinvestigated in frequency domain. In terms of the skewness and kurtosis test method,a classification criterion of the non-Gaussian district on the surface of L-shaped tallbuildings was proposed. Reasonable estimates were made for the peak factors by useof the generalized extreme value (GEV) theory.The coefficients of base bending moment and three-component forces acting onthe L-shaped tall buildings were studied, and the worst wind directions wereidentified. PSDs of base bending moments and three-component forces under theworst wind direction were discussed. Taking the aspect ratio and terrain type as thebasic variables, mathematic models of wind loads at along-wind, across-wind andtorsional directions which are useful for L-shaped tall buildings were proposed by useof the nonlinear least-square method. Meanwhile, mathematic models of thecoherence function of along-wind to torsional and across-wind to torsional were alsopresented. Numerical examples were adopted to verify the reliability and accuracy ofthe mathematic models.Based on the principle of virtual work and Rayleigh quotient, mathematic models for wind-resistant optimal design of L-shaped tall buildings were presented. Themodels took the sectional dimensions of structural elements as design variables, thetotal weight as the objective functions and wind-induced response as constraintconditions. The optimal criterion (OC) method applied to the optimal progress wasadopted with using the Kuhn-Tucker condition. The effectiveness and economics ofthe optimization proposed in this study were testified by an L-shaped tall building inpractice. The results showed that the stiffness of the building improves significantlyafter the optimization, the total weight decreased about18.1%and the EWSLs atacross-wind direction decreased almost9.03%.In this study, the combination of wind tunnel tests and theoretical analysis wereadopted to investigate the wind effects and the optimal design of L-shaped tallbuildings. The outputs of this study are expected to provide valuable information andreference for the wind-resistant design of L-shaped tall buildings and the revision ofwind load codes in the future.