Precast concrete is a very popular material for vertical loadbearing elements such as walls and lift-shafts. It allows rapid erection on site whilst retaining the benefits of off-site fabrication.
There are two main types of achieving a connection at horizontal joints, grouted bars, and bolted wall shoes. Of these, wall shoes are by far the most popular as they provide an immediate load carrying connection.
Wall shoes are made by several companies, but the principles are the same for each supplier. Different sizes/capacities are available, and suppliers provide technical data to help select the appropriate size, with tensile capacities up to 900 kN or more.
A wall shoe connection consists of three separate elements.
Wall shoe: There are slight variations between suppliers, but the basic shoe consists of either a plate with a slot in it, or a short length of steel hollow section, both with reinforcement bars welded-on as anchorage. Those with a slotted plate will only take tensile forces. Hollow section is used as it offers considerable tolerance during erection, up to ±40mm. It also allows shear forces to be taken, unlike the slotted plate type. The size and length of the bars varies according to the capacity. Bars should not be cut without reference to the manufacturer.
Foundation anchor: This is simply a threaded socket or a stud, again with anchor bars welded-on. Variations in the number and shape of the bars are available to suit conditions such as a shallow foundation. The socket systems are preferable to studs since they are not susceptible to damage from site vehicles, nor cause trip hazards. As an alternative to a foundation anchor, it is allowable in certain conditions to use two wall shoes as this offers even greater tolerance.
Connecting bolt: This consists of a threaded rod together with a rectangular anchor plate, nut and washer. The material is high strength (grade 8.8) and ‘ordinary’ threaded rod should not be substituted.
The connecting bolt is screwed into the foundation anchor. The upper wall element is then positioned so that the top of the bolt projects through the wall shoe. The anchor plate is placed over the shoe, and after final positioning, the nut is tightened. As this forms a structural connection, it is important to ensure that the plate seats properly ‘steel-to-steel’ onto the hollow section with no gaps that might permit crushing of concrete and subsequent movement/cracking. Full torque should not be applied as it can reduce the capacity of the shoe. For larger sizes it is also difficult to apply in practice. Rather than apply torque, a slugging wrench is often used. This is simply hit with a hammer to ensure tightness. Once the nuts are tightened the assembly is grouted. It is this grout that provides lateral resistance, and for this reason it is important that the hollow section is completely filled with high strength, free flowing grout. Some suppliers recommend particular grouts, and in the event of doubt, advise should be sought. The connection will not take shear loads until the grout has reached adequate strength.
As with many connections systems, if co-existent tensile and shear forces are to be considered, there is an interaction between the two forces. It is not possible to have maximum tensile loads and maximum shear loads at the same time. Manufacturers will advise on this, and may also publish simple interaction diagrams as the example shown.
If shoes are being used to connect shear walls giving stability to a building, then the load on each shoe is calculated using ‘bolt group’ theory. This should be done by a suitably qualified Engineer.
When wall shoes are used in horizontal joints, they are frequently used in conjunction with wire-loop connections to connect the vertical joints. In this way very efficient diaphragm action can be achieved.
Although the name may suggest otherwise, wall shoes can also be used to connect other precast elements as well. A combination of wall shoes and wire loops were used to connect the elements in ‘The Dream’, a 22m high iconic statue.