In our delayed return to things Trussed Arch, we must first acknowledge two Bridgewrights we have already touched on here on the weblog. Both built in this form and tradition, both were contemporaries of the subject of this entry – Timothy Palmer of Great Arch & Permanant Bridge fame, and Theodore Burr, his name forever appended to Arch type bridges, though paradoxically not for his ahead of its time Delaware River masterwork.
Both were Master Builders, an archetype then long recognized – Those not just who had reached the complete command in a “physical” ability in their chosen craft at a skill level beyond their brethren, and also that uncommon ability, to see every layer of intricacy necessary to the planning and management of any project – And somehow also, an innate ability to visualize, design, and then draw, anything, (no matter how complicated) they were planning to build.
We cannot fully explore this Truss Type without discussing Lewis Wernwag, the Bridgewright whose works suggest he was the master of this typology. His prolific life’s work is most often exemplified in descriptions of his 340′ clearspan Schuylkill River masterwork known to history as The Colossus. This best illustrated in Lee H. Nelson’s highly recommendable book devoted to this single bridge – The Colossus of 1812: An American Engineering Superlative.
So much in fact has been written about the man and his works that we’ll not directly discuss his life in any bio type format, nor will we have a full hard look at The Colossus.
“A View of Fairmount and the Waterworks” by John Rubens Smith painted just three years prior to its loss to fire – Image courtesy of The Library of Congress
Our exploration will center around aspects of his design that suggest his understanding of his use of materials, choices which exemplify his complete understanding of his primary chosen material. And then discuss how his work moved beyond the Arch type bridges so common to this early period of bridge innovation
My intrigue with his chosen detailing goes to two things which deeply illustrate his understanding of wood as a building material, details that are today too often overlooked. These being that unwritten bridgewrighting rule of all timber needing be with every opportunity, spaced apart. In the case of The Colossus, in part by wrought iron “Links” These links, a clamp of sorts to serve as part of the Chord assembly, they also secondarily, though no less importantly served as spacers to provide airflow, and prevent large areas of wood to wood contact. ( The Colossus made great use of Iron – 40 Tons – Wernwag purchased an area nailworks & foundry in anticipation of the project ) And though it went unsaid this was obviously also to provide and maximize the ever- important airflow that affects the quick drying of any unintended wetting. The other detail I find interesting is one often cited in descriptions of this bridge, this of all its timber being “Slit through the heart” Always with this citation too much is seemingly made of his reference that this was done to “Show any defect” – Yes this was a secondary benefit of milling smaller baulks from large diameter sawlogs, (as was the norm to the period) this was without doubt also about other benefits wrought from such milling. His chosen six inch breadth is not just about staggering splices, it is also about rapid seasoning and thereby reducing the dead load of the bridge superstructure as quickly as was possible. It was also probable that in the doing at least twice the amount of curved Chord leaves were realized in the milling of his candidate sawlogs. (in all probability naturally curved – We know Palmer chose such for his Chord Lams) And the benefits of such milling do not end there. I would contend “Slit through the heart” is period jargon for what has come to be known as “Free of heart center” One of the betterment’s gained in FOHC milling is a reduced tendency in season checking, greatly reduced both in size and number. Lewis’ long experience in millwrighting would have him understanding what encouraged rot in wet environment constructions. Large leak catching open checks caked full of moisture holding dirt are one of those avoidable encouragements. And the aspect of FOHC that most overlook is the term Center, meaning heartwood, and the complete absence of sapwood. Sapwood is highly susceptible to the fungi that cause Brown Rot, and in my opinion its use should be avoided in the framing of bridges. With longevity and rot susceptibility clearly being high in his mind as he designed The Fairmount, I think it not a stretch to suggest this is an opinion he and I share. We are also of the same mind that Eastern White Pine for reasons which still hold true, is the go to choice in species for bridge framing.
Somehow today those who specify material for wooden bridges and the rehabilitation of historic spans, rarely spec dense ring count or FOHC even though both are readily available in their go to species of Douglas Fir. A species I do not see as across the board appropriate for all truss types, or to certain timber joints. This though is perhaps a discussion for another time.
Some might see some of what I suggest as just so much supposition. (and I am among those who revile revisionism based on groundless conjecture ) However I am in part reacting to such seen in earlier descriptions of Wernwag’s Collossus, and my conjecture is grounded in an intimate knowledge of these constructions, their methods, and their materials.
It is interesting that unlike his early Arch Bridge contemporaries, Wernwag transcended the Arch and also built in what some call true trusses. This was perhaps driven by an interest in keeping costs down by eliminating the needed mass and added complexities in abutments designed to carry Arches. This interest was perhaps driven by movement requiring repair, which developed in his abutment there on the western bank of the Schuylkill.
A detail from Lewis’ Neshinemy Bridge included because it is an example from one of his “Truss” bridges. That, and I simply love Strong-Arcs, (aka – Trussed Beams) and I see them as under appreciated. This, and with that the source material seen here, dates from the mans very lifetime.
As dusk descended on Wernwag’s life and time, a new age was slowly dawning. The role then long existing in mankind (from the dawning of man’s great works, from standing stones to our ancient temples and pyramids, to builders of tunnels & aqueducts to those who designed and oversaw the building of the great Cathedrals) which he had filled so well, that of a highly technical designer who was also a highly skilled rigger and builder, was then stratifying into specialties seen to no longer overlap, and to be seen by some to have very different places on the societal and hierarchical ladder. In truth, we are perhaps little the better for it. It is only with the possession of both skill sets, and both sets of understanding, each equally and finely honed, that anyone can truly be, a Master Builder.