Kentucky Transportation Center personnel conducted measurements of live stresses on structural members of the I 65 (John F. Kennedy Memorial) Bridge over the Ohio River. That work was performed in conjunction with an extensive flaw evaluation of the bridge by Hazelet & Erdal Consulting Engineers.

The measurements were performed by placing strain gages on the upper chord (an H-beam) and a vertical post and transverse strut that were framed into the upper chord at PP63 on the West truss. Measurements were performed using battery-powered data logging instruments that were capable of unattended strain measurement. The units were used to monitor live stresses induced by routine traffic. Specific tests included short duration time-history measurements on the three structural members, a short duration strain gage rosette measurement on the upper chord and a day-long stress histogram measurement also on the upper chord.

The time-history measurements were intended to measure the magnitude of live stresses in the beams. Multiple gages were installed on opposite faces of the structural members for those tests to gain insight into the nature of forces acting on them. The rosette test was performed to measure the principle stresses acting on the upper chord and, thereby, ascertain the nature of forces acting on the upper chord. The stress histogram measurements were conducted to determine the magnitudes of live stresses and number of stress cycles over an extended period.

The time-history test data revealed low magnitude live stresses at all the test locations. The maximum tensile live stresses measured were: 1) 1,147 psi for the upper chord, 2) 2,230 psi for the vertical post and 3) 580 psi for the transverse strut. The rosette test yielded a maximum principle stress of 1,120 psi. The stress histogram data indicated that the variable-amplitude live stresses acting on the bridge were comparable to a constant-amplitude live stress of 1,660 psi at a rate of 773,864 cycles per year.

Comparisons of stress data taken on the upper chord indicated that it was subject to some non-axial forces. Similar measurements taken on the vertical post indicate that it was subject to transverse forces possibly induced by the strut.

Based upon the low stress magnitudes measured during the tests, it appears that live stresses generated by traffic do not have a significant impact on the structural integrity of the bridge.

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