Dynamic Stiffness of Concrete Beams


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Title: Dynamic Stiffness of Concrete Beams

Author(s): Kenneth C. Johns and Michel D. Belanger

Publication: Journal Proceedings

Volume: 78

Issue: 3

Appears on pages(s): 201-205

Keywords: beams (supports); deflection; dynamic structural analysis; moments of inertia; reinforced concrete; serviceability; static loads; stiffness; structural design; vibration.

Date: 5/1/1981

The stiffness and effective moment of inertia of reinforced concrete beams used in dynamic calculations is investigated experimentally by a study of natural frequency of vibration. Beams are statically loaded to various levels into the cracked regime, then caused to vibrate freely with small amplitudes. Natural frequency is observed and the effective dynamic stiffness is calculated. The evolution of this dynamic stiffness with increased static load or bending moment is studied. Neither gross, cracked, nor effective moments of inertia appear adequate to predict the behavior observed. From the limited number of observations available, a formula is tentatively proposed for a dynamic stiffness.