MacLeod Consulting, Inc.
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MacLeod Consulting, Inc.
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Church of the Presidents repairs roof
November 20, 2000 This magnificent granite edifice was completed in 1828 and is the fourth meeting house of the Church, replacing the wooden Hancock Meeting House on the same site. Alexander Parris was the architect. Over its 350 years, the congregation of historic First Parish has played a most important part in the building of this city, the state, and indeed the nation. Like their ancestors before them, both Presidents John Adams and John Quincy Adams were life-long members of the Church. Their tombs stand in a crypt within the church beside those of their wives, Abigail and Louisa Catherine. Also, the Patriot John Hancock was baptized in this church by his father, the Rev. John Hancock, who served here as minister from 1726 to 1744. The Quincy's were also active members for many generations. See the Church's web site for more information about their history and activities at www.ufpc.org. The Church hired MacLeod Consulting in 1998 to investigate and design repairs to the roof structure over the sanctuary. Under a separate contract, the Church hired architect, Gary Tondorf-Dick, to design slate and flashing repairs to the roof. A sagging roof led to many cracks in the plaster of the domed ceiling. Eight timber scissor trusses made from Eastern White Pine spanned 72 feet to support the roof and ceiling. In the 1890's, the roof had undergone a major repair to mitigate sagging in the top chords. The low pitch of the roof and the domed ceiling make for a shallow depth in the roof trusses resulting in constant, high stresses in the truss chords. Cross grain drying shrinkage in the timbers resulted in distortion at notched lap joints where members crossed one another. This led to truss members binding and prying against each other at critical joints. The combination of high stresses and prying led one to rupture. A redundant cross member in the truss temporarily checked the complete collapse of this truss. These trusses are exemplary samples of timber truss framing before the development of modern techniques of analysis and design (in the United States, this followed in the latter half of the 1800's). The builders showed a keen, intuitive sense of force distribution in such trusses. They applied good judgment in bolt location and the use of jack braces to minimize cross-grain stresses at joints. The chords were made from single timbers more than 50 feet long notched at cross joints. The successful performance of these trusses over 170 years is attributed to their extraordinary craftsmanship. Analysis, however, revealed the members are stressed much too high to safely remain in service. MacLeod Consulting proposed abandoning the timber trusses in place and transferring the roof load to vaulted steel Fink Trusses. The timber trusses would still carry the ceiling dead load through original wood hangers. The new trusses would carry the roof and snow loading. They also would serve as backup to carry the ceiling with rod hangers. The rod hangers were tensioned to share in carrying the ceiling, but the clevises can develop slack when the trusses deflect under snow. This effectively isolates the plaster ceiling from deflection movement in the steel trusses due to transient loads. MacLeod Consulting prepared a performance based design to shore the ceiling and roof so the Church could continue using the Sanctuary while repairs were made. Brand Scaffold provided an excellent shoring system. The Church contracted separately with several contractors to provide steel trusses, carpentry, and roofing work
The design that MacLeod Consulting prepared called for opening the roof along the eaves to gain access to the attic. The contractor removed shingles and sheathing from the eave up ten feet. From field measurements, the steel contractor established work points for a one-size truss to fit all locations. Steel trusses were fabricated offsite and delivered assembled in half-sections. The steel erector then partly unbolted and folded each together about their heels. Crews lifted the collapsed half-trusses by crane and brought them through openings along the eaves. Carpenters cut out the rafters and reframed them one bay at a time to make way for the ironworkers during hoisting. Ironworkers then unfolded the truss half sections and bolted them together. They finished by plumbing and bracing the trusses. Carpenters reset purlins as the steel trusses were installed. Although the design did not call for releveling the roof up into its original position, the work came close to achieving this result.
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