As the title of this guide suggests, it addresses how to fit remedial wall ties into an existing wall. This article focuses on retrofit helical wall ties; explaining what they are, the benefits of selecting them for brick-tie replacement and the technical advantages that the Twistfix CD Tie offers over comparable products.
Helical wall ties are helix-shaped rods having multiple longitudinal threads and a pointed tip. The stainless-steel ties anchor the brick façade of a building to the main walls of the structure. Each tie fixes into a tiny pilot hole, which minimises disturbance to the masonry. The hardness and angle of the winding ridges enable them to cut spiral grooves into a wide variety of masonry components as the tie is driven, by a series of hammer impacts, into brick, block, stone or concrete. Installation is remarkably quick and simple.
Independent test programs, and more than 35 years of use, prove the lasting performance and reliability of helical wall ties. BRE Digest 329 (Table 5) shows them as being suitable for use in all remedial situations, irrespective of the substrate of the building or its fire performance criteria.
The mechanical interlock anchorage of a helical wall tie resists tension and compression loads; it exerts no expansive pressure on the masonry, does not rely on chemical adhesives and is unaffected by temperature extremes or fire.
The cross-sectional profile of the spiral wall tie includes high-profile threads, which maximise grip in the masonry, and deep helical troughs which allow it to accommodate differential movements between inner and outer walls. The twisted helix has multiple natural drip-points which prevent the transmission of water across the cavity.
When under load, the threads interact with the host building material to impart an accumulative cone of stress at a tangent to the helix. The helix distributes loads evenly along the full penetrative length of the tie to ensure the reliability of its connection in a wide variety of construction materials.
One-piece stainless-steel helical wall ties are:
The criteria for establishing which are the best helical wall ties to use for retrofit applications include considering which provides the most reliable performance over a sustained period and which offers the most cost-effective installation in terms of the combined expenditure on products, tooling and labour.
The manufacture of helical wall ties involves the twisting of pre-shaped and work-hardened stainless-steel wire. The twisting of traditional helical wall ties involves clamping one end of the profiled wire and spinning the other (much like twisting a rubber band on a toy aeroplane by spinning its propellor). The method develops a helix, though the pitch helix varies along its length.
In use, the leading end of a ‘spin-twisted’ tie cuts winding channels or internal threads into the masonry. As installation proceeds and the fastener drives further into the wall, its imprecisely twisted ridges tend to widen the grooves. This partial stripping of the internal thread impedes the reliability of the grip that the tie has in the wall.
In contrast, Thor Helical, who manufacture the CD wall ties for Twistfix, use a highly-engineering twisting die technology to control the twist of the helix precisely. The refined, exact twist of the CD Tie (Patent EP1307303) forms tightly mating threads within masonry to ensure a superior interlock along the full length of the tie.
Independent tests for CE Mark compliance show the CD Tie is exceptionally versatile across a wide range of masonry types. Further testing, by Oxford Brooks University, confirms no adverse effect on the performance of the CD Ties, after subjecting them to 100-year service cycles for movement caused by thermal and moisture variations.
Competing helical wall tie systems require multi-part installation adaptors to fit the ties into existing cavity walls. Such lengthy adaptors work with a telescopic action and can be expensive, unreliable, and cumbersome to use.
Conversely, the CD Tie has a unique driving end configuration (Patent GB2439633) which allows safe and controlled installation with low-cost, easy to use tooling. The short one-piece setting tool eases the installation process and ends the need for unwieldy and expensive telescopic setting tools.
Twistfix CD Ties offer:
Product: Thor Helical 9mm diameter CD Tie
Lengths (mm): 180, 205, 230, 255, 280, 305 & 330
Material: Austenitic Stainless steel - (304 & 316)
Ultimate Tensile Strength: => 15kN
|DECLARATION OF PERFORMANCE
(CONFORMS WITH BS EN 845-1:2013)
|Mean Load Capacity
|Dense Aggregate Concrete||7||6||60||150||2870||2700||75|
|Structural Concrete C30||30||7||40||150||2370||2690||50|