The great hall roof of the medieval palace at Westminster, which now adjoins the British Houses of Parliament, was erected c. 1395 by Hugh Herland and stands as one of the great achievements of English monumental carpentry. When the roof was built to span the vast un-aisled interior of nearly 68 feet, it was 50 percent wider than any previously known hall in England. Hence, this unique framing of unprecedented scale has long been regarded as a technological tour de force uniting engineering skill and visual elegance. For the last century and a half, Herland's framing, combining a great arch with a "hammer-beam" bracket, has been discussed for its remarkable structure, and a number of ideas have been suggested by English and Continental experts concerning how the great weight of the roof is sustained and which members of the truss are structural as opposed to those which are mainly decorative. Yet, despite recent analyses by prominent engineers, there is no consensus of opinion as to how the frame actually carries its load. From a review of the major literature, which forms the first part of our study, a number of important questions arise concerning the nature of the timber frame, the function of its primary members, and, particularly, the precise structural relationship between the roof and its supporting masonry. In order to address and resolve the major structural and historiographical issues, we have undertaken a new and more complete study of the Westminster roof that combines archaeological and historical research with two types of interactive model analyses. The initial model was carefully constructed of timber at 1:10 scale to conform as closely as possible to a transverse frame of Herland's truss. This model, instrumented with electric-resistance strain gauges, was used to determine the general behavior of the truss under loading and with three kinds of support conditions, corresponding to the conflicting theories examined in the literature. Using the data from the timber-model testing as a guide, we then refined the analysis with a numerical (computer) model. Our results have clearly resolved the major questions raised by previous studies and indicate that the roof derives its primary support from the masonry wall at the level of the heraldic corbels, upon which rest the massive timber wall posts. The load of the roof is carried nearly halfway down the walls by the combined action of the heavy hammer posts and the great arch rib. We also found that the angel hammer beams are in tension and that they function, significantly, to take horizontal forces off the top of the wall. Moreover, these findings are consistent both historically and structurally with other examples of English carpentry that preceded Herland's brilliant design for Westminster Hall.

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