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Exterior wall considerations speaks to what are exterior wall considerations and types of exterior wall considerations.
This a secondary but equally important secondary set of building design requirements.
Wind Force Resistance
The exterior wall of a building must be adequately strong and stiff to sustain the pressures and suctions that will be placed upon it by the wind. For low buildings, which are exposed to relatively predictable winds, this requirement is fairly easily met.
The upper reaches of taller buildings are affected by much higher winds whose direction and velocities are often determined by the thermodynamic effects from nearby buildings. High suction forces can occur on some portions of the exterior wall, especially near the corners of the building.
Water Vapour Control
The exterior wall of a building must retard the passage of water vapour. In the heat of summer or the cold of winter vapour moving through a wall assembly may condense inside the assembly and cause staining, loss of insulating value, corrosion of metals and decay of wood.
The exterior wall must be constructed to resist the diffusion of water vapour and to restrict the leakage of moisten laden air in order to prevent the transfer of water vapour to parts of the wall where it may condense.
Several different kinds of forces are always at work throughout a building, tugging and pushing both the frame and the exterior wall, thermal expansion and contraction, moisture expansion and contraction and structural deflections. These forces must be anticipated and allowe for in the design of a system building enclosure.
Thermal Expansion and Contraction
The exterior wall of a building has to withstand movements due to changes in temperature at several levels. Indoor/outdoor temperature differences can cause warping of cladding panels due to differential expansion and contraction of the indoor and outdoor faces.
The exterior wall as a whole, exposed to outdoor temperature variations expands and shrinks constantly with respect to the frame of the building which is usually protected by the exterior wall from temperature extremes. The building frame itself expands and contracts to some extent, especially between the time the exterior wall is installed and the time the building is first occupied and its indoor temperature is controlled.
Exterior Wall Considerations
Moisture Expansion and Contraction
Masonry and concrete exterior wall materials must accommodate their own expansion and contraction that is caused by varying moisture content. Bricks and building stone generally expand slightly after they are installed.
Concrete blocks and precast concrete shrink slightly after installation in a building as their curing is complete and excess moisture is given off. These movements are small but can accumulate to significant and potentially troublesome quantities in long or tall panels of masonry or concrete.
In smaller buildings wood cladding components are the types of components most susceptible to moisture movement.
The exterior wall must adjust to structural movements in the frame of the building. Building foundations may settle unevenly, causing distortions of the frame to sag slightly. Wind and earthquakes forces push laterally on building frames and wrack panels attached to the faces. Long term creep causes significant shortening of concrete columns and sagging of concrete beams and slabs during the first year or two of a building’s life.
If building movements due to temperature differences, moisture differences, structural stresses and creep are allowed to be transmitted between the frame and the exterior wall. Wall system components may be subjected to forces for which they were not designed, which can result in broken glass, buckled cladding attachments.
In extreme cases, the building frame may end up supported by the exterior wall, rather than the reverse, or pieces of cladding may fall off the building. A number of provisions for dealing with movement from all these causes are evident in the details of exterior wall systems as presented.
Exterior Wall Considerations
The exterior wall of a building can interact in several ways with building fires. This has resulted in a number of building code provisions relating to the construction of building exterior wall systems, as summarized at the end of this chapter.
To maintain the visual quality of a building, its cladding must weather gracefully. The inevitable dirt and grime should accumulate evenly, without streaking or blotching. Functional provisions must be made for maintenance operations such as glass and sealant replacement and for periodic cleaning, including scaffolding supports and safety equipment attachment points for window washers.
The cladding must resist oxidation, ultraviolet degradation, breakdown of organic materials, corrosion of metal components, chemical attack from air pollutants and freeze thaw damage of stone, brick, concrete, concrete block and tile.
Exterior Wall Installation
The exterior wall system should be easy to install. There should be secure places for the installers to stand, preferably on the floors of the building rather than on scaffolding outside.
There must be built in adjustment mechanisms in all the fastenings of components of the wall system to the frame to allow for the inaccuracies that are normally present in the structural rame of the building and the wall components themselves.
Dimensional clearances must be provided to allow the wall components to be inserted without binding against adjacent components. Most important there must be forgiving features that allow for a lifetime of trouble free enclosure function despite all the lapses in workmanship that inevitably occur. These features such as air barriers and drainage channels to get rid of moisture that has leaked through a faulty sealant joint or generous edge clearances that keep a sheet of glass from contacting the hard material of the frame even if the glass is installed slightly askew.
Exterior Wall Considerations