New concrete bridge tests at FAST

Railway Track and Structures, July, 2004 by Duane Otter

New concrete bridges at TTCI's FAST facility are providing researchers with information that can be applied toward stress reduction.

Test trains are rolling over two newly-constructed concrete test bridges at the Facility for Accelerated Service Testing, Pueblo, Colo. The bridges were constructed to conduct a variety of tests under the heavy-axle-load train in order to reduce the stress state on concrete railroad bridges.

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While the bridges have only been in service for a short time, test results are coming in and preliminary results by Transportation Technology Center, Inc., indicate several opportunities for stress reduction.

Bolted Rail Joint Concrete Ties Concrete Bridge HAL Traffic = High Maintenance

A bolted rail joint was intentionally installed on one of the spans to accelerate the degradation of the concrete ties, granite ballast and concrete span. After 10 mgt of HAL train traffic, the track in the area of the joint required tamping on a daily basis (approximately one mgt). Rate of superelevation loss and surface loss was about 3/4 inch per mgt. The joint was maintained in track until 19 mgt to complete a preliminary round of measurements. The degradation was primarily in the form of ballast breakdown. One cracked concrete tie was noted near the joint, but no damage to the span was noted.

The rail was cut to make the joint. It was then bolted together with conventional joint bars. There was no railhead mismatch, as is often found when a plug rail is bolted into track. In the case of a railhead mismatch, the maintenance requirements and dynamic impact forces (see below) are likely to be even greater.

Bolted joints are bad for bridges, too!

Impact strains higher than 50 percent were measured on the 15-foot span with the bolted joint, as Figure 1 shows. The impacts shown were measured under train speeds varying between 20 mph and 40 mph. It is likely that the amount of impact forces would be even greater over a larger range of train speeds.

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On spans without rail joints, the impacts over the same speed range were significantly lower (Figure 1). Removing unnecessary bolted rail joints from bridges should be one of the first priorities for reducing the stress-state on railroad bridges, whether the bridge has a ballasted deck or an open deck. (RT & S, September 2003, p. 21.) Union Pacific is pursuing efforts to reduce the number of rail joints on its bridges. On ballast deck bridges, removing a joint is typically not much more difficult than removing a joint in open track. On open deck bridges, extra procedures may be required due to tie spacing and rail anchoring concerns. Extra precautions are required also to prevent fire on bridge timbers during welding operations.

Impact reduction = stress-state reduction

Different types of track tie configurations will be installed on one of the bridges in an effort to reduce the amount of impact force transmitted to the concrete spans. After completing a series of measurements with the standard concrete ties, a set of concrete ties with rubber pads was installed on one of the bridges. The pads are designed to reduce the track modulus to a value comparable to that on the approach track. Numerous strain gauges on the spans and foundations are being used to evaluate and compare the impact forces with each type of tie. Plans for future testing include similar evaluations using timber and plastic ties, as well as the possibility of a ballast mat on the bridge deck.

In theory, some of these track construction materials should benefit the bridge in two ways:

(1) Reduce the amount of impact force transmitted from rail to bridge span, and,

(2) Reduce vehicle dynamics due to changes in track stiffness between bridge and approaches.

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Testing at FAST intends to develop, test and quantify these reductions in stress-state.

Better load distribution = stress-state reduction

Figure 2 shows a typical cross section of a FAST concrete bridge span, including locations of strain gauges. Figure 3 shows a typical strain distribution across the bottom of the span. Note that the greatest strain is about 35 percent higher than the lowest strain. Variations in the strain distribution across the width of the span can be caused by several conditions, such as placement of track on the span, fit between spans and caps, bearing properties and cross-section design.

Future research plans call for investigating techniques to make the load distribution more uniform. By doing so, the peak stresses in the span are reduced, hopefully prolonging the useful service life of the span.

Bridges designed with a future in mind

A team of bridge experts in the railroad industry designed the layout for the new concrete bridges at FAST. The team included bridge engineers from each major railroad, plus representatives from AREMA structures committees. Two separate bridges were constructed to double the number of end spans. Experience suggests that end spans encounter the highest impact loads due to the "bump at the end of the bridge" effect.