The responses flooded in after our July story about rare photos from the construction of the Ann Richards Congress Avenue Bridge, which was built in 1910, expanded and updated in 1980, and still proudly spans Lady Bird Lake.

One particular message stopped us in our tracks.

“Here’s a nugget for you,” reader William Hughes writes. “The wooden forms used to cast the concrete arches in the Congress Avenue Bridge still survive. They were recycled as roof trusses in the building now occupied by another firm, Pfluger Architects, down on Riverside Drive, east of Congress Avenue.”

Could this be true? Those wooden trusses, which looked so striking, almost prehistoric, in the rediscovered photographs have been hiding in plain sight a short stroll from the American-Statesman newsroom?

Of course we knew the handsome former warehouse at 209 E. Riverside Drive. Shaped somewhat like a wartime Quonset hut, the structure had been the home of Rockford Business Interiors, founded in 1952 and now known as McCoy-Rockford Commercial Interiors. The project that transformed the building from a warehouse into a showroom was honored in 2000 by Preservation Austin for its adaptive-reuse design by TeamHaas Architects, which did something similar when it rehabbed the old 1959 Palmer Auditorium into the Long Center for the Performing Arts.

These days, Rockford is next door at 211 E. Riverside Drive, while Pfluger Architects, which specializes in designing schools, colleges, universities and specialty facilities, enjoys the light-filled building shaped like a half-barrel partially held up by hinged trusses.

So we contacted Brad Pfluger, president of the firm, who told us by phone that his trusses were actually metal, and they had been recycled, not from the Congress Avenue Bridge but from the 1942 Lamar Boulevard Bridge.

This rang a bell.

The Lamar Boulevard Bridge, started in 1941, is an open-spandrel deck arch bridge constructed of poured-in-place reinforced concrete. It was added to the National Register of Historic Places in 1994. Instead of wood, the trusses used for the concrete forms were made of steel and, in fact, were designed to be reused.

To check them out, we tromped down to the lobby of the building, where an incredibly helpful receptionist, Donya Fagan, not only made us feel right at home but also tried to dig up more information about the building’s history.

A little display in the lobby told more about the building’s signature trusses.

“The unusual shape of the center of each arch is actually a giant hinge,” the display text reads. “The arches were designed as the formwork of the Lamar street bridge and were hinged to fold for removal after the concrete arches of the bridge were cast. The seats, or bases of the arches — which are called bow trusses — are solidly bolted atop stout concrete piers. The lateral walls of the original warehouse were non-structural ‘infill’ of concrete block and steel sash windows set between the piers.”

Clearly these are not the trusses used to form the concrete arches on the older Ann Richards Bridge.

“There's no one old enough to remember, of course,” says Michelle Dippel, south and central office leader and vice president for HNTB, the design and construction company that built the 1910 Congress Avenue Bridge and also provided those fabulous photographs of the bridge under construction, “and no records one way or the other.”

Thank goodness HNTB saved the photos.

While at the Pfluger offices, staring up at the Pfluger trusses, I came to understand something about the even earlier 1876 Congress Avenue Bridge, the one that immediately preceded the 1910 span designed by HNTB. That bridge — seen in pictures on its original stone pedestals and, in other shots, on temporary wooden piers as it made way for construction — is very much visible in some of the photos made during the construction process for the newer bridge.

It turns out that the 1876 bridge is not a trestle structure, as I had written; it, too, is a truss structure. A reader had alerted me to the difference while I was on vacation in July and the subject slipped from my mind.

I say “is” because the 1876 old bridge, as reported, still stands in Richard Moya Park south of the airport and spans the floodplains of Onion Creek. It was moved there in the early 20th century.

It’s worth a visit. As are the Riverside Drive offices of Pfluger Architects.

That way you can compare surviving trusses used for the 1876 Congress Avenue Bridge (now at Richard Moya Park) and the 1942 Lamar Boulevard Bridge.

Hughes, who sent us on this productive search in the first place, appreciated the update and added with a twist on the old Brooks Air-Cushion Appliance advertising: “In the words of that classic ad, don't ‘throw away that truss!’"

MORE ON THE CONGRESS AVENUE BRIDGE

See the view from the Congress Avenue Bridge in the 1800s vs. now

Austin’s first long bridge used ancient pontoon technology

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