3 Westgate Street, Southery.

PE38 0PA. United Kingdom.

studio@anthony-johnson-engineering.co.uk:

Telephone

01945 660504

Title

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Historic Conservation & Structural Engineering

Structural design for Historic Buildings can be a tricky beast. Traditional building materials introduce additional factors that need to be taken into account during the design process. Consequently, structural calculations become a “secondary” checking process to other concerns, bourne out of a need to consider the fabric of the building and the intrinsic issues bourne out of how the building interacts with the environment and the long term stabilization of building materials.

Sag of the roof, pushing out on the top of the wall, a tie rod and ceiling binder pulling in the opposite direction and the wall splits about the weakest point- right next to a window opening.

A Large Barn conversion project, currently being undertaken in conjunction with the practice in Suffolk clearly reflects these issues. The barn, formed of timber framing, solid masonry dwarf walls and flint infill panels lends itself to a structure that was not built with the modern Building Regulations in mind. A lack of foundations, replicated through rubble fill and rammed earth, coupled with local drainage ditches has resulted in the rotation of dwarf masonry wall panels, with external flint facing panels introducing weak points through a lack of cross-bonding between adjacent bricks. Shrinkage of the underlying CLAY bearing strata has introduced movement which has been exacerbated by out-of-plane axial loads applied by the timber framing set to sole plates to the head of the dwarf walls. Thrust to the head of full height masonry wall panels by the cut timber roof has lead to parting of masonry panels, with rotation about their base.

Outward rotation of wall panel due to shrinkage of clay ground and out-of plane loads from Supported timber structure.

This in of itself would ordinarily result in a re-build. However, graded structures impart a limitation on the type of works that can be carried out. Direct replacement, like for like is not possible, unless formerly agreed with the Conservation officer and as such, an alternative has to be sought. In the case of the damaged wall panels, underpinning was considered a viable solution, with strips foundations designed, which would be formed from mass filled concrete, poured to excavations formed under the existing walls. Constructed in sections, reinforcement would be added to tie seach section together, ultimately forming a concrete strip under the whole wall panel. Building up from the strip, to the underside of the traditional wall would then be carried out with a Limecrete, thereby forming a barrier between the traditional structure and the modern forundation. It would also allow the foundation to be set at a depth that would permit the formation of a limecrete floor slab internally. Finally, Helifix reinforcing bars would be used to stitch the cracks in the traditional masonry, with lime mortar re-pointing reinstating the traditional finish.

Remedial Works to wall panel, with underpinning calculated with achieve suitable bearing pressure, at a depth where shrinkage of the clay is limited due to a lack of change in pore water pressures.

Environmental damage is significant, with timbers clearly having failed due to beetle infestation and moisture ingress over many years. Roof timbers have snapped in two, resulting in the complete collapse of many sections of the roof, allowing exposure of internal elements, leading to further decay and failure of structural elements.

Converting the structure for residential use will require significant re-building works. However, given that the building is grade II listed, its important to retain as much of the heritage as possible. As a result, design works need to be sympathetic, avoiding the direct replacement of structural elements, except where their function is paramount to the stability of the structure. For example, failed studs should be supplemented with additional supports, rather than directly replacing them, timbers should be spliced wherever necessary.

Failed rafters, rotten studs, wood worm all leading to a loss of structural strength and ultimately structural failure

However, there is the option to make good to repair works that have previously introduced defects into the structure. Prior re-building works with modern portland cement based mortars and modern brick sets have introduced hard surfaces and dis-continuoud joints within the masonry wall panels. Such areas can be broken out and re-built, keying in traditional brick sets with Lime Mortars, ensuring continuity to the wall panel, thus allowing loads to distribute evenly through the structure.

Modern Portland Cement based mortar and No Cross Bonding forming a weak plane in the masonry
richardanthonyj