John Speller's Web Pages Marent Gulch Trestle (NPR)

John Speller's Web Pages - US Railroads
Marent Gulch Trestle (NPR)
The Marent Gulch trestle stands four miles west of Missoula on the Northern Pacific Railway. The original timber trestle was built in 1882 and was in use at the opening of the railroad. "It was built in alternate spans of 20 feet and 50 feet, two bents being braced together under the short spans, so as to form timber piers. The 50-foot spans were Howe trusses with wooden angle-blocks, which deflected so much that it was found necessary to reinforce them before the completion of the iron viaduct. It was only to be expected that such a structure would almost certainly be destroyed by fire, and to preempt this an iron replacement viaduct was commenced only two years after the original had been completed." This was designed by George S. Morison (1842-1903), one of the outstanding bridge engineers of the period. Alfred Noble (1844-1914) was Resident Engineer and supervised the works during construction. "The dimensions of the new viaduct were fixed in part by those of the old viaduct. The distance from center to center of piers in the new viaduct was made 140 feet, which brought the piers under the centers of the alternate long spans of the old wooden viaduct. The distance between the centers of the trusses, or the width of the piers at the top, was made 20 feet, this being the same as at Portage, but the object of adopting this width was to raise the new iron trusses without disturbing the old wooden ones. The new bridge consists of five spans, each 116 feet 8 inches long; of four towers, each measuring 23 feet 4 inches on top, and four girder spans of 30 feet each, making the total length of the entire structure from outside to outside of iron work 796 feet 8 inches. The total height from top of masonry to top of stringer is 201 feet 9 inches, this being at the high side of the two central towers. The viaduct is straight, but is built on a grade of 1.962 per cent." [See Transactions of the American Society of Civil Engineers, Vol. 25 (September 1891), p. 500.]
The original temporary timber trestle at Marent Gulch, constructed in 1882
George S. Morison (1842-1903), engineer of the second, iron Marent Gulch bridge, constructed in 1884
Morison's permanent iron trestle, 1884
Another view of the three central spans of the original timber trestle shortly before replacement in 1884
Site Contents Untitled