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Case Study: Arco Building, Keble College, Oxford

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Technical

Architect - Rick Mather

Ref. Brick Bulletin (Summer 1996) and riba Journal (September 1995)

The brickwork is the outer leaf of a cavity wall with a wide cavity containing partial fill insulation boards. The bricks used were 248 x 102 x 41mm which were specially made for the job. Some walling is in stretcher bond and some in soldier courses stack bonded.

The brickwork did not course with the standard, metric-sized, concrete blocks used for the inner leaf and so tie provision had to allow for the bed joints at different levels between inner and outer leaves.

Wall ties are normally built into the bed joints of brickwork and blockwork, not only because it is convenient to do so, but also because the inevitable compressive force that naturally occurs in these joints, due to dead weight of the material and any imposed structural loads, improves resistance to the tie being pulled out. Generally it is not considered good practice to build cavity walling with ties embedded in the vertical cross joints between the masonry units of either leaf of the wall, because these joints are not subjected to loading.

Wall ties can be embedded in vertical cross joints when such location is dictated by specific brickwork detail, but this requires care to ensure effective fixing and that the ties do not slope back toward the inner leaf. The placing of ties in this manner is generally restricted to relatively small brickwork features.

Two-part ties with a sliding anchor feature are available from specialist manufacturers and would have been appropriate, but at Keble an alternative solution was adopted.

Vertical stainless steel dove-tail channels at 600mm centres were bolted to the blockwork inner leaf to locate s/s wall ties with dove-tail ends. For areas of stack bonded brickwork the ties were built in at every horizontal joint (ie 258 mm vertical centres). In general this tie frequency was considered sufficient to give stability to the outer leaf without the addition of bed-joint reinforcement. However, to avoid the interruption of the curved section of the brickwork of the south elevation by vertical movement-control joints, stainless steel ladder pattern bed-joint reinforcement was placed in alternate courses, ie 516 mm vertical spacing.

In the areas of stretcher bonded brickwork, fixing channels were also installed at 600mm vertical centres, but ties were placed at 516mm vertical centres (10 courses). This provided ties at greater density than the conventional 900 x 450 mm spacing pattern.

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