Interpretation of Beam Vibration Amplitude Measured with Strain Gages

The ratio of the flexural strain to deflection for a simple vibrating beam is different from the ratio of the midspan flexural strain to the midspan deflection for a simple beam with a static load applied at the center. Therefore if a strain gage, bonded to the extreme fiber of a beam, is calibrated in terms of deflection by applying a static load, the strain gage will not indicate the true deflection while the beam is freely vibrating. It is calculated that the strain in a freely vibrating beam per unit deflection is 82.3 percent of the static strain per unit deflection at midspan for a simply supported beam. Similar calculations for a beam fixed at both ends give a value of 71.4 percent. Vibrational tests were conducted on three joists of Douglas-fir (Pseudotsuga menziesii (Mirb ) Franco) and one steel beam with ends fixed. The ratios obtained varied between 68.7 and 71.9 percent in comparison with the theoretical value of 71.4 percent, thus confirming the theoretical calculations.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member