The I-235 Freeway through Des Moines, Iowa is roughly 14 miles long and was reconfigured and repaved over six years, beginning in 2002. Three basket-handle arch pedestrian bridges were added over the freeway during this reconstruction. Concurrent with the freeway reconstruction, a four-lane arterial road was also built on the city’s south side with one basket-handle arch traffic bridge as part of that project.
The lighting for all four bridges consisted of metal halide floodlighting, accenting the bridge structures and providing walkway lighting on the pedestrian bridges. The total connected lighting load for all four bridges was 27,339 watts.
The existing lighting required ongoing maintenance and the close placement locations of the accent lighting to the bridge yielded a non-uniform illuminance. Routine maintenance of the pedestrian bridge’s lighting over an active freeway presented additional hazards and challenges.
The four bridges' relighting consisted of an LED area type luminaire with high angle lateral distribution and is rated for upward aiming in a wet location. A hinged stainless steel mounting stanchion was custom designed to mount at the existing locations with the new LED luminaires affixed to each one. Supplemental safety cables attach the luminaire and mounting to the bridge structure in the event of a catastrophic failure. The assemblies' hinging allows the fixtures to be raised to the bridge deck for servicing.
The total new connected lighting load for all four bridges is 6,554 watts, representing a 76% energy reduction with a much greater uniformity and nearly doubled illuminance values.
An image of a basket-handle arch bridge is a component of The City of Des Moines' logo and the new bridge lighting serves to reinforce that impression during hours of darkness. The bridges are highly visible to visitors and residents alike and are a source of great community pride.
KCL Engineering specified CREE OSQ Flood luminaires with WSN "Wide Sign" optics to light the structures. The WSN Optic is ideal for this application. Light is emitted very wide side-to-side, which provides a much more uniform appearance over the original metal halide installation. The distribution is also very narrow front-to-back, so lighting energy is optimized, lighting the structure and minimizing wasted light spilling into the sky.
While the appearance of the installation is a great transformation for these iconic structures, the most compelling story may be energy and maintenance savings. As noted in the Designer’s narrative, the installation consumes 76% less electricity than the incumbent technology. Even with this tremendous savings, structures still appear to be illuminated to a higher level and the color rendering is vastly better. Along with energy savings, the City of Des Moines will see a tremendous maintenance and labor savings. Lamps in the existing installation were replaced every 2-3 years and doing so meant traffic lanes needed to be closed. Work was performed at night, so contractor rates were higher. The new CREE OSQ luminaires should not have to be touched for many years to come. The CREE OSQ fixture carries a 10 year warranty that gave KCL Engineering and the City of Des Moines an even greater level of confidence for this very important piece of the Des Moines metro.