C O N T E X T 1 3 4 : M A Y 2 0 1 4 21 analysis software was used to assess the adequacy of bracing and strutting in the frame, all braces that would be placed in tension under the application of load had to be isolated from the model so that the loads would be carried solely by compression braces. In modelling Harmondsworth Barn, wind loads were applied perpendicular to the long and short elevations to determine the minimum number of braces in each direction that would be needed to maintain the stability of the structure. Longitudinally, the building presents a relatively small area to the wind (although drag occurs across the top of the roof), and it has a large number of braces. Conversely, the sides of the building are subject to greater wind loads, and the cross frames have fewer braces and struts. The cross frames were modelled with and without the temporary strutting inserted by McCurdy and Company (even though this was not intended to stiffen the frame) to prove that it was contributing little to the stability of the trusses and could eventually be removed. The analysis demonstrated the increasing importance of the buttressing effect of the aisle frames in maintaining the stability of the cross frames towards the central bays of the building, away from the restraint provided by the gable ends. It was determined that the loads could be carried by only two of the aisle braces acting in compression at any one time.Therefore, ensuring that these components could function as originally intended became a repair priority.The analysis indicated that the main action of the nave braces was to prop the tie beam. Structural repairs have been designed and specified to address the defects outlined above. In addition, the temporary strutting in the nave trusses will be removed to enhance the significance of the original timber frame.To do this it will be necessary to strengthen and support the ends of the purlins in a visually unobtrusive way. Replacement of the roof coverings provides the opportunity to do this. In general, the approach to structural repairs has been to use techniques that can be carried out in situ with minimal disturbance to the frame; that involve the least loss of significant fabric; and which can be dismantled should this ever become necessary in the future. In general, the simple and pragmatic repair tradition, already well established at Harmondsworth Barn (and in many other timberframed buildings), of using metal fastenings, straps and brackets to strengthen or restrain components or joints will be followed. However, where suitable, advantage will be taken of modern materials, such as stainless steel and structural resins. For example, some of the dislocated aisle braces are so slender in section that there is insufficient material to rebuild joints, using traditional carpentry repair techniques, which will be sufficiently resistant to buckling. However, an in situ repair, using modern fasteners and epoxy resin adhesives, allows the existing brace to be preserved in its entirety, and its functionality restored. On the other hand, timber-to-timber repairs have been specified in situations where they are the most appropriate and effective solution for a particular defect. In addition to structural repairs, the current contract includes re-roofing with peg tiles and riven oak battens, repairs to the weatherboarding and doors, and maintenance of the plinth.The works will be complete in December 2014, allowing the building to be taken off the buildings at risk register and opened to visitors. The building continues to feature strongly in the life of the Harmondsworth community and is now under the management of the Friends of the Great Barn at Harmondsworth. Visitors to the barn will continue to wonder at its awe-inspiring scale and enjoy this outstanding example of medieval timber architecture. Key Red = Compression Blue = Tension Thickness of hatch indicates magnitude of forces. Figure shows axial forces under lateral wind load only. principal rafter nave purlin stainless steel hanger screwed to purlin Hardwood packing to regain compression bearing Long fully thread screws Toby Murphy is a senior engineer and Iain McCaig a senior architectural conservator at English Heritage. Graphical output produced by stick-frame structural analysis software A detail of repairs to support ends of nave purlins A detail of repairs to aisle braces
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