Monthly Archives: August 2012

Adulation for Bruno

In learning we had this past Fourth of July somehow missed acknowledging his 200th, and in looking for more information on the the good Mr. Pratt, we find adulation for Bruno…

I have several times here linked to full text clips of period bio-obits for historical members of the wooden bridge community. I quite like these as primary source materials, clearly they are written by close friends or associates, within weeks or months of someones passing. As such there are often found interesting nuggets of information, sometimes recorded nowhere else.

Here we learn Thomas Willis Pratt shared more than much in common, in a parallel lives sort of way, this far beyond bridges and bridge design, with others we have discussed here on these pages. Involvement in Railroading, and being often described as and thought of as an engineer, despite a lack of any degree.

We also learn he was involved in the building of Railroads and railroad bridges here in NH, and like his storied wooden bridge predecessor Timothy Palmer he also spanned the Merrimack in Newburyport. The thought that homage to his father Caleb was part of appending his name to the ‘ 44 patent was here affirmed. And somehow, we come to know he was a pamphleteer of sorts, often writing Boston papers using the “Nom de Plume” of Bruno.

Surprisingly, despite the success, even in his own lifetime of the “Pratt Truss” built then, and for decades on into the future, in a variety of materials and in many variations – The Bio suggests “ Mr. Pratt derived little or no pecuniary profit from the invention.” This leaving us to wonder why.

The author displays multiple biases, of the newer is necessarily better variety – Besmirching the name of the good Col. Long, based primarily on some assumption of an adherence of all wooden parts in an all wooden bridge, and seemingly an assumed and accepted superiority of iron over wood as a building material. He then carries on about “The advance of knowledge taught us to modify those notions of the powers of the camber, and of the need of Counters except for short distances each side of the centre of the bridge” – This sadly is to my mind, a first person contemporaneous suggestion, that then as now, design engineers were failing to seek any input from the very people they would conspire with to build their designs – Though calcs and models do suggest that but for those near mid-span, there is no need for Counters to convey loads from Panel to Panel – However, in the process of construction of wooden bridge trusses, with a number of truss types, they are useful in every panel in both the fully controlled development of, and in-service maintenance of camber.

He also (The author) then betrays his own suggested wood is inferior biases, in his description of Pratt’s April ’73 patent No. 137,482 – Though the bio, like the passing of Mr Pratt only follows the patent date by two years, we learn this all wooden truss had already been built in numbers. While most wooden trusses can be produced far far quicker than most people would today expect, I see that part of his description “Ordinarily it could be laid together and prepared for Tree-Nailing in an hour” as a bit of an exaggeration.

This Bio was written almost on the Eve of the Ashtabula Bridge Disaster, which would send shock-waves of change through the Engineering and Bridge Building communities, and I can’t help but wonder if the author perhaps tempered some of his thoughts in response.

– The following is from the Proceedings of the American Society of Civil Engineers Vol. 1 November 1873 to December 1875 –

Less than Hardwired for Hardware

If I were to define what it is I do, the description I see as mayhaps that one most accurately describing it, with both eloquence and in as few words as is possible, is simply to suggest that “I join timber”

My work is to take one of our planets greatest blessings, Trees. And to take their only modestly converted stems, (With variation, but in essence with only four sapwood slabs removed) and then join this blessing into useful configurations. Be it, houses, barns, or bridges.

In most such efforts joining timber, I find myself working to wholly avoid the use of large metal fasteners – This is almost one of the sub-definitions of what it is to be a Timberframer. Wood to wood is what we do.

Bridgewrighting, as allied a trade as it is, is a bit different. Even those Truss types without iron in them – Longs, Paddlefords, Burrs and Towns, still are often peppered with a smattering of bolts…

Many Truss types, share iron as almost an equal partner, equal in effort if not proportion of material used. A Symbiont of sorts, necessary to allow a largely wooden truss, to do what is asked of it.

Pratt’s are one of these “types” – So, echoed in the lyrics of one of my favored songs, with The First Day in August, last years local Timber Framers Guild project – The Wason Pond Bridge marked the passing of its first year, (see July / August ’11 archives) and I recently found myself willingly engaged in a once common bridgewrighting chore – A first of several, wrench in hand scheduled visits, to tighten Truss Rods and assorted Bolts to compensate for expected and predictable shrinkage as the Timber in the Through Truss seasons.

Joints again fully seated, camber re-tuned. All went as hoped for and expected, with but one small exception. The washers on some of the smaller Bolts sunk into the now dry White Pine wood grain as an attempt was made to re-tighten. I chose to replace these. My first thought was to go to the cast Ogee’s found so commonly in this application. Their cost and limited availability saw me turn to the second most common washer type found on Wooden Bridge Bolts – Large Square Flats.

As such explorations often do, I went looking for contemporaneous Rules of Thumb in what was seen as a norm for such hardware. In AJ DuBois’ – The Strains in Framed Structures – We find these not only crunched number, but period proven suggestions in his list of specifications.

As always, I like to point out how White Pine was and is, the favored Species for the framing of Wooden Bridges – Also from the DuBois List – Section VIII

In Jacoby’s – Structural Details or Elements of Design in Timber Framing – A wealth of information is found on the seemingly mundane subject of washers, as they are related to Timber Work.

The restoration of Maryland’s Gilpins Falls, is the only time we’ve worked with “Special Countersunk Washers” and their funky headed friction dependent Bolts. This bolt & cast washer type was also that chosen and used by Nichols Powers in The Blenheim.

The Wason Pond Bridge is fitted with a number of “Malleable Iron Washers” on both the smaller section Truss Rods at Mid-Span, and at the Tie Beam Bolts.

And like many other wooden spans, it is now home to a large handful of Squares.