ACCIDENTAL TORSION IN BUILDINGS DUE TO STIFFNESS UNCERTAINTY

Discrepancies between the computed and actual values of the structural element stiffness imply that a building with nominally symmetric plan is actually asymmetric to some unknown degree and will undergo torsio nal vibration when subjected to purely translational ground motion. Su ch accidental torsion leads to increase in structural element deformat ions which is shown to be essentially insensitive to the uncoupled lat eral vibration period of the system but is affected strongly by the ra tio of uncoupled lateral and torsional vibration periods. The structur al deformations increase, in the mean, by at most 10 and 5 per cent fo r R/C and steel buildings, respectively, and by much smaller amounts f or a wide range of system parameters. The increase in structural defor mations due to stiffness uncertainty is shown to be much smaller than; implied by the accidental torsional provisions in the Uniform Building Code and most other building codes.