This bridge is built in a style known as a Concrete Channel Beam. A channel beam is an upside down U-shaped beam made out of concrete. They are precast in a factory, then transported to the site to be installed on top of the bridge piers and abutments. The bridge deck can be built direct on top of the flat surface of the channel beams. This was a quick and economical way to build a bridge that was commonly used in the 1950s and 1960s. Many such bridges have serious deterioration some 40 and 50 years after they were built. Engineers have been searching for an economical way to repair these structures without having to replace entire bridges. This bridge, however, appears to be in very good shape and has not yet suffered that type of deterioration.
The photo above is looking west down the length of the bridge deck of the West 36th Street Bridge. Knollwood Mall would be behind the photographer.