1.0 INTRODUCTION
An external envelope is constructed for enclosure of a building to protect the occupants, storage items & the interior of the building from outside intrusion (for security), external weather elements (wind, snow, rain etc), whilst producing a comfort factor relevant for the end user/s by ensuring the correct materials & methods are used whilst cohering with the relevant Building Regulations and British Design Standards.
2.0 FADE DESIGN
The fa�ade of a building needs to be designed at the very early stages of the construction process. This ensures that a detailed analysis of the energy input/output, and it’s structural integrity and durability is concluded. If this procedure is not adhered to, complications may occur in the later stages of the construction process due to restrictions on planning applications and the overall design of the building.
‘The functional requirements of a fa�ade are:
- Strength & Stability
- Resistance to weather
- Durability and freedom from maintenance
- Fire safety
- Resistance to the passage of heat
- Resistance to the passage of sound’
[Barry 4, The Construction of Buildings, Fifth edition, Oxford 2001]
3.0 HEAT TRANSFER
The transmission of heat design needs to adhere to Building Regulation Approved Document Part L – Conservation of fuel and power. This regulation is applied to decrease and limit the loss of heat through the external envelope of a building other than dwellings by establishing maximum U Value’s for the overall transmission of heat.
The interior of the building needs to be kept at a comfortable temperature which, needs to be above that of the outside air. If this is the case, the transfer of heat sources to outside air will continue. This happens by the following methods:
3.1 Conduction
The speed or rate of the transfer of the molecule to molecule kinetic energy in the material.
3.2 Convection
The transfer of heat by physically moving the molecules from one place to another.
3.3 Radiation
The transfer of heat through space via electromagnetic waves (radiant energy)
The most effective material of the resistance to the passage of sound is a solid wall. The more dense and thick the material, the more effective the barrier to the passage of sound.
4.0 BUILDING REGULATIONS
All buildings need to conform to the following building regulations:
- LOADING – For the loading, Ground Movement & Disproportionate collapse of building as per Approved Document A – Structure
- FIRE SAFETY – For the fire safety, safe escape and components of the materials used as per Approved Document B – Fire Safety.
- WATER/MOISTURE/INSECT RESISTANT – For the water tightness and prevention of insect infestation as per Approved Document C – Site preparation resistance to contaminants and moisture
- ACOUSTICS – The fa�ade materials will need to be acoustically sound as per Approved Document E – Resistance to the Passage of Sound
- VENTILATION – For the required air changes and adequate ventilation of a building as per Approved Document F – Ventilation
- IMPACT – For the testing of the impact that a fa�ade can take as per Approved Document K – Protection from falling collision and impact
- AIR PERMEABILITY – For the conservation of air and heat transfer as per Approved Document L2A – Conservation of fuel and power
- ACCESS – For the access via doors, windows and maintenance needs of a building as per Approved Document M – Access and to and use of buildings
5.0 STRUCTURES
The load of a building needs to conform to British Standards Design Regulation BS 3699 – Part II
5.1 Steel
Steel is the frame of a building. It suffers from elastic strain when under load and will expand. This needs to be taking into consideration when determining the load of the building, including the fa�ade. To reduce the load of the fa�ade on the structural frame, joints between the fa�ade, interior and structural frame will prevent any damage occurring when there is any movement. The extra lengthening of columns will allow for the elastic movements in the beams and columns.
5.2 Concrete
Concrete is a load bearing material when made in-situ. If made offsite into panels, it would make it a non-load bearing material. For in-situ, you will need to:
- Form a metal framework (re-inforcement) around the starter bars of the foundation
- Install the timber/metal framework shutters both sides of the reinforcement allowing for specification cover
- Install support bard throught formwork
- Bolt strongbacks to formwork and ground for extra support
- Pour concrete in formwork and keep going until desired height is reached
This forms the frame as well as the fa�ade.
Concrete suffers from shrinkage as well as elastic strain. It is very brittle and just breaks under extreme load and cannot take bending.
It is a porous material which allows water to seep in which will rust the steel if re-inforced. It will then expand and push off the concrete which creates spalling (pocket cancer). The steel needs to be galvanised with zinc. This doesn’t make steel absolutely immune to rust. Zinc is a grey in colour and is a sacrificial layer which lasts for approximately 30-40 years. Compression joints and the allowance of the shortening of the designed need to be considered in the early design stages.
Concrete alone is a poor insulator. To increase the u-value, insulation will be required i.e. Insulated back plaster board &/or a Sto-render system. Very little maintenance is required for concrete as it just hardens over the years. For access, a cherry picker, scaffolding or a cradle will be required.
5.3 Glass
Glass needs to conform to Building Regulation Approved Document N – Glazing – Safety in relation to impact, opening and cleaning.
Glass can be used as a load bearing material as well as a fa�ade. Panels of glass are made in factories by casting. It is then brought to site and connected together in-situ to the steel frame by bolting it together. Nowadays, there are many different types of glass from ordinary annealed glass right through solar control glass and even self-cleaning glass. The 3 main one’s being Georgian Wired glass for fire safety, toughened glass for extra strength, tempered glass and toughened glass. Glass retains a lot of heat giving it a high u-value. So the thicker it is, the more heat it retains and expands so enough expansion room is needed.
If thick, glass retains a lot of heat which makes it expand, it will need enough expansion room.
6.0 FADE FORMS, MATERIALS & CONSTRUCTION
When designing the external envelope, post installation movements need to be taken into consideration. This can be caused by thermal movement, settlement of foundations, elastic shortening of columns, edgebeam and sway deflections
6.1 Heavyweight
6.1.1 MASONRY – Stone
Natural Stone Cladding
Stone facades can be made in-situ or in factories and made into pre-cast panels. Stone blocks are either ashlar or rubble masonry. Both can be laid in either coursed or uncoursed height. To hold it in place, a lime mixture, cement or mortar can be used. Veneered stone is glued against concrete blocks to form stone cladding. Slipform is short forms (around 2 feet tall) are placed either side of the wall to form a guide for the wall, stones are placed inside of the forms and concrete poured to hold them together. Mechanical fastening against the steel frame holds the stonework in place.
Design and installation is covered under BS 8298. The thickness is typically 75mm with new standards bringing it down to about 40-50mm thick. This is dependant on the type of stone, structural calculations and performance testing.
The specific environment determines the type of fixings used which should be a graded non-corrosive material such as austenitic stainless steel. The sealant needs to be primed to prevent seeping into the porous material. The life expectancy of the sealant is 15-20 years so will need replacing throughout the building life span. Stone is of high durability so can take all weathering elements but the pollution in the atmosphere and rainwater can cause staining so cleaning is necessary under BS 8221-1. Stone work is relatively low maintenance as the material just hardens over the years so has a long life span. Stonework alone has a poor u-value, therefore, extra insulation will be required.
The photo shown is of 20 Gracechurch Street, City of London. This building shows a stonework fa�ade.
6.1.2 MASONRY – Brick
Cavity Brick Wall
A cavity brick wall is a load bearing fa�ade and consists of two brick wythes separated by a cavity (space) and are bonded together by masonry clip ties. The wall may include insulation from 2″ – 4 1/2″ width, dependant on the specification.
Moisture will penetrate through the outer brick wall so to prevent any damage, flashings are installed in the cavity, through weep holes of the outer brick wall. This will divert the moisture/rain back to the outer environment on cavity trays.
A brick wall alone is a poor insulator. With the double-skin of brick and possible extra insulation material, the cavity brick wall is an excellent thermal insulator. The cavity walls also have excellent fire resistance with all walls having 4hrs or more.
Its load bearing capacity is good but the size of the building will alter this.
6.2 Lightweight
6.2.1 Metal Panels
Metal panels are a non-structural element and can be made of aluminium, steel, stainless steel or copper. The panels are made off-site in factories and either pre-cut to specification or cut in-situ. They are screwed or bolted into a structural framework system i.e. metal studs/lightweight steel frame.
Metal expands a lot so expansion joints need to be installed.
Due to metal panels having a low u-value, they are insulated by specification. The panels can be made up with insulation being sandwiched inbetween two sheets of metal panels. Alternatively, batt insulation can be installed in-situ inbetween the stud wall and metal panels.
To protect the building from air and moisture either a barrier system, rainscreen or drainage system will need to be installed, the latter being the preferred method. This due to high performance and is more aesthetically pleasing. The system is installed inbetween the panel joints.
The panels are not fire rated and are not flammable (dependant on the insulation characteristics).
On it’s own, the panels are not sound insulated. Although, with the thermal insulation being present, this may provide acoustic insulation.
The panels do not have a high impact resistance and are easily damaged. They have high corrosion resistance therefore, requires little maintenance over it’s long life period and 7-20 year intervals of sealant replacement and cleaning will be needed to prevent pitting.
6.2.2 Plastics
UPVC (Unplasticised Polyvinyl Chloride) panels are non-load bearing and made-off-site in factories, taken to site and installed in-situ by bolting it to the structural framework system leaving enough room for expansion joints.
UPVC has very similar properties to aluminium in that is has excellent properties and is very durable.
The panels are tough with a high fire rating, environmentally friendly and maintains all external elements.
Although the panels do not rot, they discolour over time so will need mild detergent cleaning as regular intervals. The panels are generally used for residential/industrial building.
6.2.3 Timber
Timber is an aesthetically pleasing, non-load bearing fa�ade and can be made in-situ or off-site depending on the size of the building. Pre-cast panels are brought to site and bolted to the structural frame system which can be fixed to a steel/concrete frame or brick wall with allowance for expansion joints.
Timber has poor durability as it is a natural product which is open to elements and constant maintenance is needed with a liquid treatment system. It expands in the winter and contracts in the summer causing it to dry out, crack and split.
On it’s own, timber is a poor insulator so needs a separate insulation system. To prevent rain/moisture from penetrating the timber and the infestation of insects, a rain screen will need to be installed between the panel and the timber with the insect screen being installed at the end of the panels.
6.2.4 Fabric
GRP (Glassfibre reinforced Plastic) / Resin Panels
These panels are a non-load bearing fa�ade and are made off-site in factories, transported to site and installed in-situ. They are bolted to a structural frame system allowing for expansion joints; foam compression for GRP and rubber compression joints for Resin. These form a weather seal.
The panels alone had poor u-values so insulation is either installed into the panels or built into the framework.
They are very durable, self-coloured and specially coated so they can withstand all elements.
To maintain the panels, they will need to be cleaned via cherry picker or cradle on a regular 2-3 year basis. The joints are non-perishable but regular checks will ensure the joints are in working order.
6.2.5 Curtain Wall
A curtain wall system is a non-bearing wall of glass, metal or masonry attached to a buildings exterior structural frame.
There are various different systems that are used:
Stick system
Most commonly used. Is a fully framed member and whole framing is predominately constructed in-situ. The frame supported to main slab with the expansion joints in mullions. Stick systems are usually found constructed for shopping malls.
Unitised
Very expensive system. It is of floor to ceiling height with sealant, is very good quality, very durable and an has excellent performance.
If used, this system is very rapid so saves a lot of time but is extremely heavy.
Panellised
Like unitised, is about 6-8m wide. Panels are very expensive and large so need fewer. This system can cover two floors as a time and is very heavy so a good crane needs to be used in line with the correct method statement. This system is generally not used.
Spandrel Ribbon
Spandrel is the opaque part of the fa�ade and is made of ribbon panels with a silicone sealant to allow for expansion. This system is not as common now as it was in the 1980’s.
Structural Bolted Glass
The glass is either bolted to a supporting frame or a number of pieces of glass are bolted together to form a structural glass assembly.
7.0 CONCLUSION
Designing and installing a fa�ade isn’t just a simple task. It is the skin of the building that protects the end user from the outside elements. There are many, many various systems and materials that can be used and is dependant on the location of the building and the atmospheric conditions to how the material will deteriorate. You need to take into account how the size and load of the building and exactly what building is used for.
