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Types of Fixings of Precast Cladding:
Load bearing fixings are designed to transfer the weight of the cladding units together with any services or secondary fixings, such as doors or windows that may be present, to the building structure and are usually designed as pin-jointed supports. These fixings generally take the form of concrete nibs or metal (usually stainless steel) angles.
Restraint fixings are designed to hold panels back and transfer all horizontal forces (such as wind pressure or suction) to the structure. They need to be fully adjustable to facilitate the leveling and alignment of the panel.
These fixings must also be able to accommodate any differential movement between structure and cladding.
Design of fixings in precast cladding:
The basic requirement in any cladding design is that support and restraint to the main structure to be provided. It is important to realize that with any cladding system, once in position and the building complete, access to the fixings is not readily possible and therefore complete integrity of design and material workmanship has to be provided.
The majority of support fixings are formed from bent angel sections. Angels as single elements are basically simple to design. The outstanding leg is designed as a cantilever section.
The design of bolts is one of the most important aspects of fixing design. In load bearing situations a bolt has a number of forces to resist. The downward load produces a shear force, the bending moment on a bracket produces a couple which imposes a tensile pull on the bolt in some circumstances. If there is a large amount of shimming, a moment stress in the bolt can also be induced.
Fixing selection for precast cladding:
The initial selection of fixings is important and several factors should be considered.
- The fixing material must be corrosion resistant.
- The metal should be non-staining when used in direct contact with the cladding material.
- The metal should have sufficient strength to resist the applied loads.
- The metal should normally be specified from standard widths and thickness. Non standard materials are difficult to purchase and also very expensive.
- The metal should have good workability but also keep its strength during fabrication.
Fixing positioning for precast cladding:
Below is a layout of support and restraint fixings on a bottom supported cladding panel.
Green part in the drawing represent restraint fixing and blue part as load bearing fixing.
Restraint fixing: Each panel should have four positioned near the corners, tying the panel back to the structure and transferring any wind forces.
Load bearing fixings: Each panel should have two support positions through which its weight is transferred to the structure.