TECHNOLOGY :Channel Bridges

Channel bridges are a fascinating solution to a low bridge problem. By raising the girder function of overbridges up into a chunky parapet on the edge the normal longitudinal girders beneath the deck can be eliminated completely and some 90cm (3’ ) or so extra clearance given to the roadway below.

A common old 4.27m (14’) bridge that gets regularly bashed by the top of high truck loads is quite likely to be built on 90cm (3’) steel I-beam girders here in the eastern US. By replacing these girders and the deck with channel bridge sections the highway operator gets 5.15m (17’), all without having to mess with raising the abutments or approach roadway grades.

Channel bridges are a spinoff of French ingenuity in segmental concrete design, the patents being held by Jean Muller Internat, with Bridgetek of Saratoga Springs NY the US licensee. The New York State Thruway was to be the first US customer for channel bridges but it couldn’t get competitive quotes. NY State DOT is now building three channel bridges, after the project got formwork and other development $s from FHWA. Eleven have been built in France.

Transverse sections (parapet to parapet) are precast in a factory. They typically consist of a complete cross section segment with chunky parapet girders each side about 12.2m (40’) long, allowing some 9.75m (32’) of deck providing for two lanes and 1.2m (4’) of shoulder each side. The segments are normally about 2.5m (8’+) wide with a deck thickness of about a foot (30cm). Segments will fit on a regular tractor trailer. No overwide load fuss.

Steel erection frame

Before erection, large longitudinal steel beams are temporarily run along either side of the new bridge to allow all the segments to be swung into place by crane and their position adjusted by nuts on threaded bar from temporary cantilervered hangers. The segments are made structural after post-tensioning cables are strung through ducts in the parapet edge beams and the integral deck section, and stretched. Then the erection steelwork is removed.

It is claimed the system makes for quite fast construction. Cost for now is high with the system new and formwork having to be custom built, but could drop a lot once the pioneering is done. To our eye channel bridges are very handsome, with a strong clean line to them. The maintenance will be mainly a matter of keeping those internal tensioning cables in good shape. The tensioning which keeps the units in compression is also liable to reduce formation of small cracks and spalling. No steel to paint and repaint, or I-beam ledges on which pigeons can sit and shat those banks of corrosive white gunk. (Contact Michael Zicko Bridgetek 518 580 1374 mzicko@aol.com)

Manic Metrics at FHWA

We got onto all this reading the Federal Highway Admin’s excellent publication PUBLIC ROADS, which also deserves credit for its bold efforts to go metric — sort of. The Sept-Oct issue in its article on channel bridges had a rich mix of measures. Lots of ‘m’ for meters and ‘mm’s, then drawings in feet and inches, a table with square feet, but really wild was weight in kilonewtons (kN). As in: “The concrete mix included 3.0 kilonewtons (kN) of concrete, 0.5kN of fly ash, 12.5kN of aggregate...” (p44)

Kilonewtons? It made us think of an old journo pal Maxwell Newton (financial editor of the NEW YORK POST, now deceased, Max not the POST) and we wondered if we’d missed the gram being renamed in the late-Max’s honor. He was a very big guy, not that weight had been his problem, the booze’n wimmin‘n taxes had. Most of us write quietly in private but Max exulted in having a bunch of people around him, and sometimes a telephone call as well, all the while carrying on wild conversations with much irreverant merriment as he banged away at the keyboard. Max would have loved the tag ‘Killer-Newton’ and I can just imagine him exuberantly signing his columns kN.

Noone much so far has honored the amazing Maxwell N, so we emailed FHWA in hope that he’d finally gotten recognition from Uncle Sam.

But the editor of PUBLIC ROADS told us sadly that this wasn’t so, and that the l’il gram is still around as the metric base measure of weight. The authors of his article “Channel bridges” had got a bit bamboozled by metrics, he said, and had just screwed up in specifying those newtons in place of grams.

But who are we to poke fun at people having problems with metric conversion? TRnl has its mixups of measures too, so please focus in on those killer channel bridges. (Bob Bryant PUBLIC ROADS mag 703 285 2443)