The Sykesville Bypass Bridge is one of only six aluminum girder bridges in the United States and the only one in Maryland, with worldwide significance despite its ordinary appearance. Opened to traffic on 17 October 1963, it remains as originally built, two lanes wide and 294 feet long, with three girder spans of triangular aluminum box beams supported by two reinforced-concrete piers and two abutments. It is the longest aluminum triangular box beam girder bridge in the world. Its construction reflected a period when the United States was expanding its road network and steel had become increasingly scarce, prompting interest in aluminum as an alternative bridge material. Use of aluminum in bridges traced back to 1933, when the deck of Pittsburgh's 1882 Smithfield Steel Bridge was replaced with a lightweight aluminum deck, encouraging engineers to explore aluminum as a primary structural material. From the 1940s to the 1960s, several all-aluminum bridges were built in North America and Europe because aluminum offered lighter weight than mild structural steel, comparable strength, corrosion resistance, toughness in frigid temperatures, and ease of fabrication, though its higher cost limited wider use. Major advances came in the early 1960s with the Fairchild or Unistress Design, conceived by the Fairchild Engine and Airplane Company in nearby Hagerstown, Maryland, which adapted aircraft fabrication principles to bridge construction by using hollow triangular box-beam girders joined into a single lightweight, strong, easily manufactured structural unit; only three such bridges were erected because of aluminum's cost. Based on that design, the Sykesville Bypass Bridge used a 35-foot-wide aluminum girder system made of nine alternately inverted hollow triangular box beams, prefabricated and lifted into place, each 67 inches deep and formed from 0.091-inch-thick rolled aluminum sheets and extrusions. Connected side by side, these beams created a large composite trapezoidal beam supporting the concrete slab deck, which was poured atop a corrugated aluminum sheet running longitudinally over the box beams; once hardened, the concrete deck and aluminum beams acted together as a single composite structural unit. The aluminum alloy used throughout was 6061-T6, a corrosion-resistant alloy still in use today. After four decades of service, the bridge was closed to vehicular traffic in 2004 because leaking roadway joints prematurely deteriorated the concrete piers and galvanic action between the aluminum structure and improperly contacting steel bearing pads damaged aluminum components, problems tied to the bridge's steel and concrete elements rather than to the aluminum structure itself.