Revolutionary hybrid design goes the distance in Tunable Bridge

November 16th, 2009

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When an architect on a long-span pedestrian bridge project insisted that an arch bridge design be glazed all the way up the arch structure, it got Joe Meheen thinking outside of the box again. Meheen — veteran Denver engineer, multiple patent holder and lifetime friend of the bridge industry — knew the glazing cost, alone, would be formidable. The result of his brainstorming let him keep the desired aesthetics of the arch system but reduce the enclosed portion to the lower truss area only, in a revolutionary, structural hybrid design called The Tunable Bridge from Big R Bridge.

Lightweight, low-cost, easy-to-install, and very attractive, The Tunable Bridge is short on costs, long on possibilities, and, with clear spans of up to 400 feet, is expected to take modular steel bridges to a whole new dimension – especially in light rail and transit applications over major transportation corridors. Its unique design combines the inherent benefits of a hinged tied arch system with those of a standard steel truss system. The structures’ natural frequencies can then be can balanced or “tuned” like a finely-tuned instrument. Here’s how it works.

The steel truss system is designed to support all of the structure’s dead load, plus a small uniform construction load. Longitudinal, tightening rods in the trusses’ lower chords pre-stress the truss modules for dead load before installation. The steel trusses are installed just like standard modular bridges, spanning from abutment-to-abutment or pier-to-pier.

Once all of the dead loads are applied to the steel truss system, the hinged tied-arch modules are installed and attached to the steel truss system. This attachment is via a system of adjustable tension hanger rods between the arches and the steel trusses, along with longitudinal equalizing tension members across the bottom chords of the steel trusses. Adjustments to the hangers and the longitudinal members, allow for “tuning” the structure such that the live and dead loads are balanced between the arch and the steel truss systems. Tightening nuts that are attached to the rods make the adjustment.

With no critical members, this redundant stress sharing increases safety and allows for lighter-scale and lighter-weight structures using less steel. This opens the door for longer, more graceful modular bridges that cost less to manufacture and are easier to install. The modular components of Big R’s Tunable Bridge install quickly — a key consideration for applications over high-traffic thoroughfares or difficult terrain, where long spans and quick installations are required.

The Tunable Bridge design also accommodates the added weight of covered or enclosed areas, reducing winter maintenance costs for their owners and increasing comfort and convenience for their users. Typical of Meheen, are a number of added features. The top and bottom flanges of the truss members are u-shaped to act as rain gutters, so additional gutters are not needed — further reducing weight and cost. Plus, the design has all pins and connectors visible for easy inspections.