The Building Conservation Directory 2021

54 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 1 C AT H E D R A L C O MM U N I C AT I O N S USING DRILLED ANCHOR TIES In some instances the need to positively anchor scaffolding to historic fabric will be impossible to avoid. The appropriate system will depend on a number of factors and on a protected building or monument the proposal should be agreed in advance with the statutory authorities (see page 25). Guidance about drilled-in anchor fixings, their suitability for different substrates, installation and testing are provided in TG4:17 Anchorage Systems for Scaffolding . If scaffolding is likely to remain in situ for an extended period, ties should be avoided. Where they are unavoidable consideration should be given to upgrading fixings to stainless steel and ensuring that appropriate separation barriers are in place to avoid a reaction between the stainless steel fixings and the scaffold (bimetallic corrosion). Mild fixings are not recommended as corrosion may cause staining on the building fabric and damage associated with expansive ‘rust-jacking’. This may also reduce the capacity of the fixing. Temporary lateral compression loads, which should be detailed in the scaffold construction information, are either applied via ties or by the butting ends of structural transoms. The contractor will need to install plastic caps as a minimum. The load may be further spread by timber pads and additional protection provided by carpet tiles. COMMON TYPES OF ANCHOR TIE ‘Drop-in’ expanding socket anchors are only suitable for structural concrete and hard natural stone away from free edges due to the expansion forces exerted on the fabric. They are unsuitable for most other walling materials including brickwork due to the risk of cracking the structural elements. Self-tapping screws are suitable for concrete, steel, hard masonry and (subject to manufacturer’s recommendations) timber. When used externally, they are only suitable for durations up to a few years, dependent on exposure conditions. On completion of the project, the screw can be removed. Consideration should be given as to how the hole is made good on completion. Nylon plug anchors with screw-in eyes are suitable for concrete, masonry and (subject to manufacturer’s recommendations) timber to carry a tensile load. They only have the capacity for light duty ties, so more fixings may be required. They are only suited for durations up to a few years. Resin fixed socket anchors are suitable for concrete and masonry and have the benefit of not exerting expansion forces on the building fabric. These types of anchors are only able to carry a tensile load. Resin socket anchors cannot easily be removed. Through bolting with spreader plates may be the only way to achieve an adequate pull- out force on poor quality masonry or timber structures. It works by spreading load across a greater area of the fabric. It would however be better to avoid loading historic fabric in such poor condition. A modular access tower being constructed at Cromford Mill, Matlock: in this case the height to width ratio meant that fixings into the masonry were unavoidable.