A non-loadbearing wall used as a protective screen over the structural frame of a building. The materials used are varied: they include aluminium, steel, and especially glass. In mediaeval military architecture, the term referred to the defensive outer wall of a castle.
Curtain wall is a term used to describe a building façade which does not carry any dead load from the building other than its own dead load. These loads are transferred to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, wind forces acting on the building, seismic forces, and its own dead load forces.
Curtain walls are typically designed with extruded aluminum members, although the first curtain walls were made of steel.
Curtain walls differ from storefront systems in that they are designed to span multiple floors, and take into consideration design requirements such as: thermal expansion and contraction;
History
Medieval curtain wall
Curtain wall is used to describe the set of walls that surround and protect the interior (bailey) of a medieval castle. These walls are often connected by a series of towers or mural towers to add strength and provide for better defense of the ground outside the castle, and are connected like a curtain draped between these posts. Examples of curtain walls as part of castles are at Muchalls Castle, Scotland and Dunstanburgh Castle, England, the latter of which is in a ruined state.
Modern curtain wall
Prior to the mid-20th Century, buildings were constructed with the exterior walls of the building supporting the load of the entire structure. With the advent of the structural concept of shear walls and building cores, the exterior walls of buildings no longer had to support high dead loads and could be designed as much lighter and more open than the brick and steel facades of the past. This gave way to increased use of glass as an exterior façade, and the modern day curtain wall was born.
The first curtain walls were made with steel mullions, and the glass was attached to the mullions with tape and urethane sealant. The first curtain wall installed in New York City was this type of construction (see Lever House).
Similarly, sealing methods and types have evolved over the years, and as a result, today’s curtain walls are high performance systems which require little maintenance.
Stick systems
The vast majority of curtain walls are installed long pieces (referred to as sticks) between floors vertically and between vertical members horizontally.
Unitized systems
Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. Unitized curtain wall has the advantages of: speed;
Rainscreen principle
A common feature in curtain wall technology, the rainscreen principle theorizes that equilibrium of air pressure between the outside and inside of the "rainscreen" prevents water penetration into the building itself.
Design
Curtain wall systems must be designed to handle all loads imposed on it as well as keep air and water from penetrating the building envelope.
Loads
The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building. In the case of curtain walls, this load is made up of the weight of the mullions, anchors, and other structural components of the curtain wall, as well as the weight of the infill material. Additional dead loads imposed on the curtain wall, such as sunshades, must be accounted for in the design of the curtain wall components and anchors.
Wind load
Wind load acting on the building is the result of wind blowing on the building. This wind pressure must be resisted by the curtain wall system since it envelopes and protects the building. A scale model of the building and the surrounding vicinity is built and placed in a wind tunnel to determine the wind pressures acting on the structure in question.
Seismic load
Seismic loads need to be addressed in the design of curtain wall components and anchors. In most situations, the curtain wall is able to naturally withstand seismic and wind induced building sway because of the space provided between the glazing infill and the mullion. In tests, standard curtain wall systems are able to withstand three inches (75 mm) of relative floor movement without glass breakage or water leakage.
Snow load
Snow loads and live loads are not typically an issue in curtain walls, since curtain walls are designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so, these loads may need to be considered.
Thermal load
Thermal loads are induced in a curtain wall system because aluminum has a relatively high coefficient of thermal expansion. This means that over the span of a couple of floors, the curtain wall will expand and contract some distance, relative to its length and the temperature differential. In unitized curtain wall, a gap is left between units, which is sealed from air and water penetration by wiper gaskets.
Blast load
Accidental explosions and terrorist threats have brought on increased concern for the fragility of a curtain wall system in relation to blast loads. Murrah Federal Building in Oklahoma City, Oklahoma, has spawned much of the current research and mandates in regards to building response to blast loads.
Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a bomb attack. As such, blast resistant curtain walls must be designed to withstand such forces without compromising the interior of the building to protect its occupants. Since blast loads are very high loads with short durations, the curtain wall response should be analyzed in a dynamic load analysis, with full-scale mock-up testing performed prior to design completion and installation.
Blast resistant glazing consists of laminated glass, which is meant to break but not separate from the mullions.
Infiltration
Air infiltration is the air which passes through the curtain wall from the exterior to the interior of the building. The American Architectural Manufacturers Association (AAMA) is the governing body in the U.S. which sets the acceptable levels of air infiltration through a curtain wall. This limit is expressed (in America) in cubic feet per minute per square foot of wall area at a given test pressure.(Currently, most standards cite less than 0.6 CFM/sq ft as acceptable).
Water penetration is defined as any water passing from the exterior of the building through to the interior of the curtain wall system. To test the ability of a curtain wall to withstand water penetration, a water rack is placed in front a mock-up of the wall with a positive air pressure applied to the wall. This represents a wind driven heavy rain on the wall. Field tests are also performed on installed curtain walls, in which a water hose is sprayed on the wall for a specified time.
Deflection
One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. Deflection limits are also necessary to control movement at the interior of the curtain wall. Building construction may be such that there is a wall located near the mullion, and excessive deflection can cause the mullion to contact the wall and cause damage. Also, if deflection of a wall is quite noticeable, public perception may raise undue concern that the wall is not strong enough. A deflection limit of L/175 is common in curtain wall specifications. Say a given curtain wall is anchored at 12 foot (144 in) floor heights. The allowable deflection would then be 144/175 = 0.823 inches, which means the wall is allowed to deflect inward or outward a maximum of 0.823 inches at the maximum wind pressure.
Deflection in mullions is controlled by different shapes and depths of curtain wall members. The depth of a given curtain wall system is usually controlled by the area moment of inertia required to keep deflection limits under the specification. it is a separate criterion in curtain wall design and analysis. For example, the advantage of some curtain wall designs is the ability to span more than one floor (commonly known as twin-span or multi-span, as opposed to single or simple span).
As mentioned above, the deflection of aluminum is three times greater than an equivalent steel shape under the same load.
Thermal criteria
Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good conductor of heat. This translates into high heat loss through aluminum curtain wall mullions. These breaks provide a significant decrease in the thermal conductivity of the curtain wall.
Thermal conductivity of the curtain wall system is important because of heat loss through the wall, which affects the heating and cooling costs of the building. On a poorly performing curtain wall, condensation may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls.
Infills
Infill refers to the large panels that are inserted into the curtain wall between mullions.
Glass
By far the most common glazing type, glass can be of an almost infinite combination of color, thickness, and opacity. Presently, 1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). Transparent glass usually refers to vision glass in a curtain wall. Opaque glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall.
Stone veneer
Thin blocks (3 to 4 inches (75-100 mm)) of stone can be inset within a curtain wall system to provide architectural flavor.
Panels
Metal panels can take various forms including aluminum plate;
Louvers
A louver is provided in an area where mechanical equipment located inside the building requires ventilation or fresh air to operate. Curtain wall systems can be adapted to accept most types of louver systems to maintain the same architectural site lines and style while providing the necessary functionality.
Windows and vents
Most curtain wall glazing is fixed, meaning there is no access to the exterior of the building except through doors. However, windows or vents can be glazed into the curtain wall system as well, to provide required ventilation or operable windows. Nearly any window type can be made to fit into a curtain wall system.
Fire safety
Fire safing and smoke seal at gaps between the floors and the back of the curtain wall are essential to slow the passage of fire and combustion gases between floors. Spandrel areas must have non-combustible insulation at the interior face of the curtain wall.
Maintenance and repair
Curtain walls and perimeter sealants require maintenance to maximize service life.
Exposed glazing seals and gaskets require inspection and maintenance to minimize water penetration, and to limit exposure of frame seals and insulating glass seals to wetting.
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