CPD: Design and specification of curtain walling
This CPD covers curtain walling systems, including the types available, their assembly, installation and testing
This CPD is sponsored by Comar Architectural Aluminium Systems. Comar is part of The Parkside Group, comprising four brands - Comar, Alu-Timber, Duco, and Axim - they design, manufacture and deliver
facades. Visit www.comar-alu.co.uk
Part 1: Curtain walling types
There are four main types of curtain walling system: ladder, stick, unitised and structural glazing.
In ladder systems the frames are made up as individual ladders in the factory and fixed together and glazed on site. Height limitations make this system best suited for low rise.
Stick systems are the most popular and cost-effective. Individual mullion and transom pieces are assembled and glazed in situ.
The structural glazing system is similar to the stick system.
Unitised systems are completely assembled in the factory, including glazing or infill panels, before the curtain walling is brought to site. This allows the building to be made weathertight quickly.
Part 2: Assembly
The process of installing stick system curtain walling is begun by constructing the vertical mullions.
The horizontal transoms are then secured to this structure, and these are followed by internal gaskets, which hold the infill panels.
Correct installation of the gaskets is important,because they act as the barrier which keeps out both wind and rain.
Having installed the gaskets, the thermal barrier is added, and following that, the glazing or infill panels, which are secured by pressure plates.
The curtain walling assembly is then finished off with a capping to achieve the desired aesthetic.
Part 3: Design calculations
Calculations in accordance with BS EN 13830 have to be carried out to ensure that the curtain walling will be strong enough to accommodate the wind loadings to which it will be subjected.
The vertical loads imposed by the weight of the system will also have to be calculated. This includes the glass, infill panels, mullions and transoms. It is within this calculation that the dead load for any solar shading should be taken into account.
Deflection in three directions will also need to be considered. It is important to choose mullions and transoms which are able to deflect under the load of forces acting upon the system and are strong enough to sustain this.
Part 4: Drainage
There are a number of ways by which rainwater can penetrate the fabric of a building. Kinetic energy can drive rainwater through the sealed joints of the system and surface tension can cause water to adhere to, and move across surfaces. Water ingress can be prevented through correct drainage and detailing.
Wind pressure can force water into the structure. Pressure equalisation can be used to combat this. The cavity between the outer face of the curtain walling and the inner seals of the glazing is ventilated naturally to prevent a build-up of water. Small amounts of water are allowed to percolate into this area and will run out again in a controlled way through slots designed for this purpose.
Part 5: Movement
The method of controlling structural movement within the curtain walling system needs to be considered. Too great a movement within the structure can result in the breakdown of seals, leading to the curtain walling system failing.
Movement can occur through lateral shifts within the building, through thermal expansion or shrinkage, and through heat build-up within the curtain walling.
This movement is accommodated by the use of slotted holes within the brackets that fix the curtain wall to the structure, and through expansion joints at each mullion. The tolerance for temperature change and all structural movement needs to be allowed for within the construction movement joint.
Part 6: Procurement
The procurement of curtain walling should always involve suppliers from an early stage. They will need to ensure that what has been designed is practical and feasible.
Non-standard components will lead to longer lead-in times.
In order to accommodate heat gain and light control requirements, the glass supplier will also need to be consulted at an early stage.
There is often a discrepancy between the building and the drawings at the construction stage and dimensions should be checked before installation of the curtain walling proceeds. BS5606:1990 accepts that in any production process dimensional deviations are inevitable.
Part 7: Installation and safety
Initially the subcontractors will install the brackets to hold the curtain walling to the structure. Once these brackets are in place, the mullions can be fixed, along with prepositioned brackets to accept the transoms. After the mullions and transoms are in place the glass can be installed.
The contractor is required to comply with the Health and Safety at Work Act 1974. A safe method of erecting the system must be provided.
Glass can be the most vulnerable and dangerous construction material on site, and special provisions for handling it should be made. The glazing should be stored in racks in such a way as to allow water to drain from its surface. Prior to installation, seals should not be subjected to direct sunlight, which can cause degradation and failure.
Part 8: Testing and standards
There are various British and European Standards that curtain walling is tested and constructed to.
The Centre for Window and Cladding Technology’s Standard and Guide to Good Practice for Curtain Walling has become the handbook for UK curtain walling practice.
There are major testing sites for curtain walling in the UK, where systems are tested for air permeability, water tightness and penetration, wind resistance and thermal cycling.
Taywood Engineering is one of the approved bodies for full-scale testing of proposed cladding systems.
Each testing rig consists of a 10m-high open-fronted box, with steel beams to represent floorplates.
The curtain wall is fixed to this structure and sealed. Water is then sprayed on to simulate rainfall, and a specially constructed fan directs blasts of air onto the system, simulating wind speeds of up to 370km/hr.