NTSB Extended Animation - The Fern Hollow Bridge Accident

This animation with video excerpts describes the circumstances surrounding the
collapse of the Fern Hollow Bridge, which occurred about 6:37 a.m. eastern standard
time on January 28, 2022, in Pittsburgh, Allegheny County, Pennsylvania.
The animation begins with a photograph of the collapsed bridge (viewed looking
west from the east abutment) including four passenger vehicles and a port authority
articulated bus, followed by a satellite image showing the location of the bridge
about five miles east of the city center. A higher magnification satellite image shows
the vicinity of the bridge, which ran east and west and carried Forbes Avenue over
the north of the Frick Park. A picture of the bridge (looking east) shows the two lanes
on the bridge in each direction, with text indicating the bridge opened in 1973.
A series of graphics are used to depict the structure of the bridge. A three-
dimensional model of the bridge is shown, and text annotations indicate the main
girders running east and west, the four legs which are connected by cross-bracing,
and the east and west abutments. The design of the bridge called a K frame is
introduced, and the shape of the bridge being like a letter K tipped on its side is
indicated with a graphic.

Another series of graphics illustrate the construction of the legs, with the components
identified with arrows and text labels. Each leg has an I-shaped cross section, with the
main load-carrying outer flanges connected by a central web plate. Each leg also
includes longitudinal and transverse stiffener plates welded to the flanges and the
web plate. The steel legs narrowed from top to bottom, and then tapered to a shoe
that rested on a reinforced concrete thrust block. The change in taper at the top of
the shoe caused the compression forces carried in the flanges to push outward in that
area. The outward push is illustrated by arrows, with narration that the outward push
was resisted by tension in the web and a transverse tie plate at the top of the shoe.
The tie plate was thicker than the other transverse stiffeners in each leg because of its
critical location at the top of the shoe.

The nature of the weathering steel used in the construction is indicated with text
surrounding the image of the three-dimensional bridge model. The text notes that
weathering steel forms a protective surface coating called a patina, and its use
eliminated the need for painting or coating.