Washington DC area has a rich stone masonry tradition going back to the founding of our nation and beyond. We have a wealth of images in our library that can inspire new works or simply inform us about the past.
Boulder Bridge: A Happy Accident
This wonderful structure is a one lane bridge on Beach Drive just north of Blagden Road in the heart of Rock Creek Park. It was constructed in 1902, and, although the road surface is supported by a formed and poured concrete arch, the monstrous boulder veneer is constructed to support its own weight, and, had the stone been run the full width of the bridge, no concrete would have been needed at all; although the rise would have likely been more pronounced.
The concrete structure was able to be such a shallow rise by the use of the Melan system. Josef Melan was an Austrian born engineer of prominence at the time Boulder Bridge was being constructed. He was a pioneer in reinforced concrete work, and invented a system of arched steel truss-work that enabled wide spans with low rises to be stable modern structures.
The stones in the 80 foot wide arches measure close to 2 feet thick and as much as 6 feet long. Legend has it that when the engineer specified “man-sized” stone for the project, he anticipated the contractor purchasing large stone that a man could easily pick up and lay. Instead, the contractor used stone the size of a man, and when the architect came to inspect the project, he was pleasantly surprised at the misinterpretation.
There is no mystery in how stone this size are laid. A very beefy wood structure is built beneath the future stone arch, and the stone are set on a bed of sand above the wood form with mortar between the stone. Stone of this size must have been lifted with steel straps and most likely slipped into place from the boom of the delivery truck atop the already complete concrete deck. When the entire arch is laid and the mortar cured, the wood is removed, and the bridge is complete.
My guess would be that the stonework was completed in July or August when the creek would be at a trickle flow. A flood big enough to wreck the wood form (or “falsework”) would destroy all stonework on the bridge if even just one of the arch stones had been left unlaid. The arch had absolutely no strength until every stone in the span was set.
When I first moved to DC from the fast paced Brooklyn, NY, the very thought of a one lane bridge would set me off in derisive laughter. It would have seemed to have just been an act of pokey, poor planning. Does it mean that I’m old if I kinda enjoy waiting for oncoming cars to pass while taking in the view of this lovely design?
References:
January 2010 by John DeFerrari, http://www.streetsofwashington.com/
David P. Billington (1985). The Tower and the Bridge: The New Art of Structural Engineering. Princeton University Press. pp. p136
