BRIDGES 1999

Robert L. Schwein / December 28, 2000

Suspension bridges are known for their inherent flexibility and ability to sustain transient overloading. Proper structure design takes into account lateral loading from wind and seismic forces. Following the Tacoma Narrows Bridge collapse when the deck became aerodynamically unstable, dynamic uplift effects from wind began to be considered. Not typically accounted for are lateral loads from hydraulic forces imposed by floodwaters on bridge decks.

Such was the case on a small suspension bridge owned by the U.S. Department of the Interior Bureau of Land Management in eastern California.

The one-lane suspension bridge, built in 1937, spans 157 ft across the Merced River at Briceburg, Calif., and SH 140. It serves local residents and seasonal recreational activities. The U.S. Department of Agriculture Forest Service originally designed and managed construction of the bridge. It is a simple design consisting of two 25-ft-high towers founded on concrete abutments, concrete dead-man cable anchorages at each end of the span and a roadway deck of structural steel and timber. The deck is approximately 25-30 ft above normal river level.

During the flood of Yosemite Valley in a January 1997 storm, the river reportedly rose to a level of approximately 4 ft above the bridge deck and was flowing at an estimated volume of over 90,000 cu ft per second. The deck, guardrails and suspender rods trapped debris and vegetation in the flow and witnesses reported a substantial waterfall formed over the deck. Heavy damage to the timber wheel guards, guard rails suspender rods and the extreme lateral overloading inflicted sway bracing beneath the deck.

Big river

Presently, the deck is deformed downstream approximately 17 in. at a kink in the spandrel girders approximately at the one-third span point. Study of the underside of the 3 x 10-in. deck planks showed chafing marks where the cross members contacted. These showed that approximately one-third of the span deformed in a parallel manner or shearing deformation and two-thirds in a rigid manner or rotational deformation. This evidence also suggests that the deck may have been displaced up to 10 ft downstream during the peak flow.

General damage consisted of deformed stiffening girders, bent suspender rods, fractured and distorted sway bracing, loose and skewed timber attachment clamps and bearing damage. One tower was slightly deformed downstream with attendant buckling of the lacing. The suspension cables and anchorages, saddles and suspender rod brackets showed no damage. Some of the upper suspension rod bracket/cable clamps had slipped towards center-span up to 2 in. on the suspension cables due to the lateral forces on the suspender rods.

After an initial inspection, the guard railings were replaced and the bridge was opened to limited traffic. A joint venture of Wellsona Iron & Engineering of Paso Robles, Calif., Forell/Elsesser Engineers Inc. of San Francisco, Calif., and Schwein/ Christensen Laboratories Inc. of Lafayette, Calif., were contacted to perform a nondestructive structural investigation.

Repairs involve heat straightening of the deck stiffening girders, replacement of several of the bent suspender rods, remounting of the bearings, replacement of the sway bracing and various modifications of other elements.

This bridge took all the punishment nature could dish out and survived. When one imagines the deck deflected downstream upwards of 10 ft with a 4-ft waterfall cascading over it, the dynamic loads are almost beyond comprehension. Judging from the many deformed members, it appears that each was working at capacity, doing its best to hold together in this highly balanced and redundant design.

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