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3. Experimental Measurements on the St. Marys Bridge
The field work to make experimental measurements of the vibration characteristics and check certain secondary effects was completed in May of 1968 by the Structures and Applied Mechanics Division of the Office of Research, Federal Highway Administration. Reference 16 titled "The Dynamic Response of the Eyebal Chain Suspension Bridge Over the Ohio River at St. Marys, West Virginia," documents this activity. There are five results of particular interest in the discussions and conclusions of this report:
a. The dynamic behavior of the structure with respect to natural frequencies and mode shapes is in good agreement with the computations performed by Mr. George Vincent, strengthening the conclusion that the aeroelastic excitation of this structure by wind to significant amplitudes was improbable. The pin connections at numerous points in this structure provided a high level of energy absorption, producing a logarithmic decrement of 0.10 for damping in the fundamental vertical mode.
b. The response of the structure to a moving, heavy vehicle showed that there was lateral as well as vertical oscillations generated in the deck structure and that relative lateral motions of the deck system with respect to the eyebal chain were produced. These motions induced bending stresses in the hangers which were superimposed on the tension stresses which these members were designed to carry. The range of frequencies of motion induced by the moving vehicle was from approximately three to seven cycles per second, which is within the range of from 2 to 15 cycles per second to which a seated human is most sensitive. Dynamic increase of strain produced by these oscillations as compared to the static strain produced at crawl speed varied from as low as 20 percent of the static strain in the chain bent post to as much as 250 percent of the static strain in the hangers. It is important to note, however, that even the dynamic strains involved were very small. These live loading effects due to a single vehicle constitute only a small fraction of the total live load stress possible from traffic loading over the full length of the structure, and live load stress in the members is but a small portion of the total stress in the member. For example, the end span hanger at panel point 11 showed a static unit strain of 12 microinches/inch with the load in the middle of the center span, six microinches/inch when the load was in the middle of the side span (both of these strains being the average of the two faces of the hanger), and a dynamic unit strain of 16 microinches/inch with the load passing through the side span. This dynamic increment represents only about 480 psi. of bending stress.
c. Secondary effects in the northwest chain bent post produced live load tension on one corner of this post under certain positions of load, whereas certain other positions of the load produced compression at the same point. The gage at the southwest corner of the chain bent