VARIABLE MODE SHAPE HIGH-FREQUENCY FORCE-BALANCE

THE SERVICEABILITY LIMIT STATE ASSOCIATED WITH OCCUPANT COMFORT UNDER WIND INDUCED LATERAL AND TORSIONAL MOTION IS BECOMING AN INCREASINGLY IMPORTANT CONSIDERATION IN THE DESIGN OF TALL BUILDINGS. IN SOME INSTANCES, STRUCTURAL MODIFICATIONS REQUIRED TO PRODUCE ACCEPTABLE PREDICTED RESPONSE CHARACTERISTICS CAN COST HUNDREDS OF THOUSANDS OF DOLLARS OR EVEN MILLIONS OF DOLLARS. WITH THESE KINDS OF COST AT STAKE IT IS IMPERATIVE THAT PREDICTED RESPONSE LEVELS BE AS ACCURATE AND RELIABLE AS POSSIBLE. AEROELASTIC MODEL TESTS AND HIGH-FREQUENCY FORCE-BALANCE TESTS PROVIDE TWO MEANS OF ESTIMATING BUILDING RESPONSE TO WINDS. IN MOST INSTANCES, AEROELASTIC MODEL TESTS ARE IMPRACTICAL BECAUSE OF THE TIME REQUIRED TO DESIGN AND CONSTRUCT THE MODELS. RESULTS OBTAINED AFTER COMPLETION OF THE NORMAL DESIGN PROCESS CAN PRODUCE COSTLY MODIFICATIONS OR LIVELY BUILDINGS WHERE OCCUPANTS MUST ENDURE NUMEROUS INSTANCES OF DISTURBING MOTION. CURRENT HIGH-FREQUENCY FORCE-BALANCE TESTS CAN PROVIDE RESULTS IN A FRACTION OF THE TIME REQUIRED FOR AEROELASTIC MODEL TESTS BUT INCORPORATE LINEAR MODE SHAPES FOR SWAY AND A UNIFORM MODE SHAPE FOR TORSION. THUS, PREDICTED RESPONSE LEVELS MAY CONTAIN SIGNIFICANT ERRORS. FORCE-BALANCE TESTS ALSO DO NOTPROVIDE INFORMATION ON POSSIBLE AEROELASTIC EFFECTS WHICH MAY INCREASE OR DECREASE LOADING AND RESPONSE. RESEARCH IS PROPOSED ON SECOND GENERATION HIGH-FREQUENCY FORCE-BALANCES AND ASSOCIATED INSTRUMENTATION WHICH WOULD OVERCOME THE MODAL SHAPE INACCURACIES BY ALLOWING MODAL FORCES TO BE DETERMINED FOR VARIOUS MODE SHAPES. BASIC RESEARCH IS ALSO PROPOSED ON THE INFLUENCE OF BUILDING MOTION ON MODAL FORCES.