20 C O N T E X T 1 3 4 : M A Y 2 0 1 4 However, a number of systematic structural defects were evident: 1. The jowls of nearly all arcade posts had split, some widely. This is a defect which is commonly seen in traditional tie-beam lap dovetail assemblies, and occurs as a result of the transverse and radial shrinkage of green oak timbers as they dry. What appears to have happened in this case is that shrinkage of the arcade plate led to the load from the tie beam (the entire weight of the nave roof) being carried increasingly by the jowl. The resulting eccentric loading of the post created a moment of force which the lap joint was unable to resist, as the dovetail at the end of the tie beam had also shrunk, allowing it to withdraw from its housing in the arcade plate.The jowled posts split as the resultant forces acted on natural planes of weakness caused by medullary rays in the timber. 2. The nave purlins are tenoned into the sides of the principal rafters, not ‘clasped’ between the collar and the principal rafter as was usually the case in Middlesex when the barn was built.This joint employs a diminished haunch tenon used upside down (the earliest examples of conventional diminished haunch tenons are not found until a few years later, in the 1430s, in Oxford and south Oxfordshire). This novel arrangement has not worked well, as the tenons tended to withdraw from the mortices as the purlins deflected under the load of the roof. As the joint is upside down, the tenons are weak in shear and many have broken. Recognition of this defect led McCurdy and Company in the 1990s to insert temporary softwood angle struts and bolsters to prop the purlin ends. 3. On the east side of the barn, settlement and rotation of the central part of the plinth wall on its shallow foundation led to the lateral displacement of the sill beam and wall frame. The distortion of the aisle framing dislocated many of the paired braces from the aisle tie beams. The consequential reduction in buttressing provided by the aisle frame, combined with the problems with the lap dovetail assembly described above, led to the east and west arcade plates being pulled apart by more than 100 millimetres in places, dislocating the joints with nave tie beams and many scarf joints in the arcade plates. However, underpinning of the unstable parts of the plinth wall was carried out in the 1990s and no subsequent movement has taken place. That many of these problems occurred relatively early in the life of the building is evidenced by the antiquity of the repairs carried out to remedy them. Other more localised and recent defects included decay cavities, mainly in the ends of nave tie beams and sill beams, many of which had been enlarged by rats and squirrels. In general, framed structures can be simpler to analyse than masonry ones because load paths are clearer, making it easier to understand what each component is doing. European framing traditions rely on braces and struts acting as compression members to provide stability, because pegged mortice-and-tenon joints can carry only relatively small tensile loads limited by the shear strength of the pegs and tenons. In Harmondsworth Barn many joints of this type failed because they were subjected to tensile loads as a result of movement and distortion of the frame. Therefore, when two- and three-dimensional stick-frame structural Top right: Deflection of the nave purlins has opened the inverted diminished haunch tenon joints which connects them with the trusses.The consequential overloading of the tenon has caused many to fail. Bottom right: Settlement and rotational movement of the east plinth wall has dislocated the sill beam from the aisle sill. Iron straps were provided to restrain further movement. A typical split in the jowl of an arcade post. Note the lateral movement in the lap dovetail joint connecting the tie beam and the arcade plate. Iron bolts have been inserted to restrain the split in the jowl, and a plate and tie rod provided to resist further lateral movement in the lap dovetail assembly. Further repairs of this type, using hand-forged plates, straps and brackets, are to be carried out in the current repair programme.
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