EFFECT OF REALISTIC, SOMEWHAT RANDOM MASS AND STIFFNESS ON TALL BUILDING RESPONSE TO WINDS AND EARTHQUAKES

摘要

Breaking the (artificial) symmetry caused by exactly repeated floor masses and stiffnesses in tall buildings, and repeated truss structures (bridges, space vehicles), can cause dramatic changes in the response to external forces. The fact that some buildings survive earthquakes and other very similar ones, do not, may be in part due to these effects. Building response for various dampings, various randomizing of parameters, various heights and excitations are given. Variation of +/- 20% in parameters can cause displacements (RMS or maximum) of up to two to three times those found from fully symmetrical structures (i.e., with no variation in parameters). Buildings of 3 to 50 floors are considered; the greatest effects are seen for low frequencies near resonances for all structures. Largest effects are found for the tallest buildings which can experience amplifications of a factor of four (over motions for fully symmetrical structures). We consider the parameter groupings which produce the largest effects, hoping thus to offer guidance for avoidance of catastrophes. The largest excursions appear for the low modes, where by chance extra mass has appeared at some antinodes (and similarly for stiffness).