The next bridge investigated was the Gilbert street bridge, which spans the Vermillion river at Danville, Illinois, just to the southwest of the main business section of the city.
This bridge is about 1,050 feet long and was built by the Chicago Bridge Company in 1893. It is made up of two 275 ft.-0 in. subdivided deck Warren truss spans, two 4-panel deck Pratt truss spans, four 3-panel deck Pratt truss span and four tower pans. The Warren truss spans are supported on stone masonry piers, and the other spans are supported on four towers, one rocker bent and the abutments. The tower posts are set on square stone bases.
The trusses are 20 feet center to center. The roadway is 22 feet wide with a 4 1/2 foot walk on each side.
The floor system is made up of 6x16 in. wood joists spaced 2 feet center to center with a floor of yellow pine plank and creosoted wood block paving. The floor is new, having been laid in the fall of 1910.
Cooper's Highway Bridge Specifications were used in the investigation of this structure and it is rated as a Class A-2 bridge.
The vehicle traffic over this bridge is very much the same as the traffic over the Woolen Mills bridge, consisting principally of the hauling of coal and brick from the mines and brick yards just outside the city. For several years an interurban electric line crossed this bridge, but the tracks have been removed.
It was noted that the efficiencies of practically all the members of the deck Warren trusses are low and that they are comparatively uniform throughout the truss. This would indicate that this bridge was designed for a lighter load than that assumed in the investigation. The laterals and wind bracing are all very efficient.
The efficiencies of the 4-panel deck Pratt truss are about the same as for the Warren trusses, except for the middle section of the top chord, which show an efficiency of only 55%
The 3-panel Pratt trusses have sections practically the same as the 4-panel Pratt trusses ami would, therefore, have proportionately higher efficiencies.
The lowest efficiency in any of the posts of the tower bents was 84% The posts are made up of 4 angles laced, forming a box column.
The .joists have an efficiency of 68%. The floor beams have an effiiency of 55% for uniform and 37% for concentrated loads.
There are expansion rollers under the shore ends of the Warren trusses. These rollers have a calculated efficiency of 96%.
The wood floor system is in good condition, having been renewed in the fall of 1910. The steel work, with some few exceptions which will be discussed, is in very good condition.
The lower struts in the owers are badly corroded and in some cases the lower end of the post is in bad condition. Some of the struts were badly damaged when the new floor was being laid. The old joints were dropped from the floor above and evidently some of them struck the tower struts. Some repair work has been done on the bases of the towers by encasing the strut and the base of the post in concrete. This kind of repair work is of doubtful value, unless the concrete is designed to take the stress formerly taken by the steel, as the steel may continue to corrode inside the concrete until it is entirely gone.
The expansion joint under the right hand end of the second Warren truss was renewed in the fall of 1910. The rollers in this joint are only 8 inches in diameter, which is entirely too small for the length of span. The old roller nest, when removed, was in such condition that it was impossible for the rollers to turn, and evidently they had not turned for a number of years. The new rollers will be in the same condition in a short time if they are not carefully watched and kept clean. The other expansion joint has not been repaired or cleaned and is of little value. The portal struts in the Warren trusses are badly corroded. They are made up of four angles laced, and are placed in such position that one angle forms a trough which holds water, thus causing corrosion to take place rapidly.
The efficiencies of the members in this bridge are low for the loadings assumed. This bridge carries heavy wagon traffic and at one time carried an interurban electric line. The tracks for the interurban line have been removed and there is little likelihood of their ever being replaced. The wagon traffic is not continuous and there is slight chance for the bridge ever to become fully loaded. If the bridge is repaired, the traffic regulated, and the bridge carefully maintained, it should be safe for some time to come.
This structure should be put in safe condition by renewing or reinforcing both the struts and bases of the posts in the towers, and by renewing the damaged struts in the Warren trusses. The entire structure should be thoroughly scraped to remove all old paint and rust, and then given two good coats of paint. Car tracks should never again be permitted on this bridge, and wagon traffic should be regulated whenever there is a tendency toward congestion.