Study On Wind-Gravity Coupling Effect Of Extra-high Buildings And Equivalent Static Wind Load

Structures of tall-buildings are becoming light and slender with urbanization developing rapidly recent year. Wind-gravity coupling effect (WGCE) is one of problems which should be to solve in structural designing. WGCE is a phenomenon that horizontal displacement of the high flexible structure induced by wind load is enlarged by gravity. Performance of WGCE in statics is larger displacement. That in dynamics is natural frequencies reduced and response of structure changed. It was reported in engineering that WGCE of extra-tall buildings is notable, so studying on WGCE is necessary.The ambition of thesis is to determine influential factor of WGCE, to find different dynamic characteristics under the influence of gravity and to present a method to calculate WGCE of general extra-tall buildings. In general, works listed below had been finished.The dynamic equation including large deformation and WGCE is concluded using cantilever model. Response of structures can be calculated at any point of time by differential equations. Schedule calculation about the tall-building acted by wind shows amplitude produced by pulse wind is changed. The result including WGCE is larger than traditional result.The along-wind dynamics equation can be written as two equations by average wind load and pulse wind load. Average wind load equation is just as nonlinear static equation which is easy to solve. Pulse wind load equation is nonlinear dynamic equation which can be solve through mode superposition and equivalent linearization. Calculated result indicates gravity-rigidity ratio is an important parameter for WGCE. Natural frequency of structure decreases and response of structure increases with gravity-rigidity ratio of structure. Ground roughness, natural damping and average wind speed little impact on WGCE as value of gravity-rigidity ratio is small. While value of gravity-rigidity ratio is large, WGCE decreases with natural damping and average wind speed. Equivalent static wind load (ESWL) is a general method in structural design. Mean square deviation of restoring force is taken as ESWL by equivalent principle and distributed ESWL along height direction can also be concluded. It is shown that ESWL expression including WGCE contain gravity equivalent wind load that does not exist in common ESWL expression. The designing ESWL can be obtained after crest factors are added to every item of ESWL expression. Wind load factor are different between WGCE's method and traditional method. WGCE makes distribution of wind load factor reduction in middle and lower part of structures and increasing at top part of structures. Gravity-rigidity ratio is an important factor for WGCE expressed in ESWL and other factors are not evident.Across-wind WGCE is similar to along-wind one. Gravity-rigidity ratio is also a decisive factor for across-wind WGCE. Because mechanism of across-wind is different from that of along-wind, across-wind WGCE have some differences to along-wind WGCE. Response of structures increases with Gravity-rigidity ratio as lower average wind load. While average wind load become strong, responses of structures increases with gravity-rigidity ratio at first, then decreasing after reaching peak point. WGCE makes curve of responses to gravity-rigidity ratio turn to left. WGCE of across-wind associates to section aspect ratio for tall buildings with rectangular cross-section. Influence of WGCE first decreases then increases as aspect ratio is less than2and it is no law as aspect ratio is more than2. ESWL of across-wind is similar to that of along-wind.There is torsional wind load besides along-wind load and across-wind load in calculation of general extra-high building including WGCE. In order to obtain general analysis method in frequency domain, FEM equations with WGCE are concluded from3degrees of freedom in every storey. It is shown that natural frequency decreases with gravity-rigidity ratio increasing as mass eccentricity is small. The opposite is the case as mass eccentricity is large. The same conclusion is for rigidity eccentricity. WGCE makes response of eccentric structure increasing in along-wind direction and across-wind direction, but decreasing in torsional direction. Influence of WGCE takes place periodic changes at different angles. As eccentricity is small, eccentric angle have little impact on WGCE. While eccentricity is large, eccentric angle is an important factor to WGCE.Liquid column vibration absorber(LCVA) is used as an example to explain what change of structure with vibration absorber including WGCE. The result is shown that mass ratio is an important parameter that can reduce vibration effectively when it increases. In fact, vibration is decreased very quickly as mass of vibration absorber up to1%?2%of main structure. Section ratio, length ratio and head loss coefficient are also key parameters relative to damping rate. If we want to reach optimal damping rate, natural frequency of LCVA should equal to or near to that of main structure. There is some different about calculating result between WGCE method and tradition method. The general law is that value of WGCE about damping rate shows large as gravity-rigidity ratio is small while it is small as gravity-rigidity ratio increases. In order to get correct result, it should be taken into account WGCE for tall slender structure damping design.It is proposed to use restrict stiffness instead of the code from results of along-wind and across-wind in the paper. That provides experience for engineering design.WGCE is an new research area in wind engineering. Some parts of work are done in the paper. It is suggested to go a step to new research direction about WGCE.