A
curtain wall is a building
façadeA facade or façade is generally one side of the exterior of a building, especially the front, but also sometimes the sides and rear. The word comes from the French language, literally meaning "frontage" or "face"....
that does not carry any
dead loadIn mechanical and structural engineering, live loads and dead loads are two kinds of forces exerted on an object. The concepts are used especially where analysis of real-world objects is required...
from the building other than its own dead load, and one that transfers the horizontal loads (wind loads) that are incident upon it. These loads are transferred to the main building structure through connections at floors or columns of the building. A curtain
wallA wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...
is designed to resist air and water infiltration, wind forces acting on the building, seismic forces (usually only those imposed by the inertia of the curtain wall), and its own dead load forces.
Curtain walls are typically designed with extruded aluminium members, although the first curtain walls were made of steel. The aluminium frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as
daylightingDaylighting is the practice of placing windows or other openings and reflective surfaces so that during the day natural light provides effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy...
. However, parameters related to solar gain control such as
thermal comfortHuman thermal comfort is defined by ASHRAE as the state of mind that expresses satisfaction with the surrounding environment . Maintaining thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.Thermal comfort is affected by heat...
and visual comfort are more difficult to control when using highly-glazed curtain walls. Other common
infillInfill in its broadest meaning is material that fills in an otherwise unoccupied space. The term is commonly used in association with construction techniques such as wattle and daub, and civil engineering activities such as land reclamation.-Urban infill:...
s include: stone veneer, metal panels,
louversA louver or louvre , from the French l'ouvert; "the open one") is a window, blind or shutter with horizontal or, less often, vertical slats, that are angled to admit light and air, but to keep out rain, direct sunshine, and noise...
, and operable windows or vents.
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;
building swayA tuned mass damper, also known as an active mass damper or harmonic absorber, is a device mounted in structures to prevent discomfort, damage, or outright structural failure caused by vibration...
and movement; water diversion; and thermal efficiency for cost-effective heating, cooling, and lighting in the building.
History
Prior to the middle of the nineteenth century, buildings were constructed with the exterior walls of the building (bearing walls, typically masonry) supporting the load of the entire structure. The development and widespread use of structural steel and later reinforced concrete allowed relatively small columns to support large loads and the exterior walls of buildings were no longer required for structural support. The exterior walls could be non-bearing, and thus much lighter and more open than the masonry bearing walls 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 plate glass was attached to the mullions with asbestos or fiberglass modified glazing compound. Eventually
siliconeSilicones are largely inert, man-made compounds with a wide variety of forms and uses. Typically heat-resistant, nonstick, and rubber-like, they are commonly used in cookware, medical applications, sealants, adhesives, lubricants, insulation, and breast implants.Silicones are polymers that include...
sealantA sealant is a viscous material that changes state to become solid, once applied, and is used to prevent the penetration of air, gas, noise, dust, fire, smoke or liquid from one location through a barrier into another. Typically, sealants are used to close small openings that are difficult to shut...
s or glazing tape were substituted. Some designs included an outer cap to hold the glass in place and to protect the integrity of the seals. The
first curtain wallLever House, designed by Gordon Bunshaft of Skidmore, Owings and Merrill and located at 390 Park Avenue in New York City, is the quintessential and seminal glass box International Style skyscraper. It is the pioneer curtain wall skyscraper in New York City. The 92 meters tall building features an...
installed in
New York CityNew York is the most populous city in the United States, and the center of the New York metropolitan area, which is among the most populous urban areas in the world. A leading global city, New York exerts a powerful influence over worldwide commerce, finance, culture, fashion and entertainment...
was this type of construction. Earlier modernist examples are the
Bauhaus' is the common term for the ', a school in Germany that combined crafts and the fine arts, and was famous for the approach to design that it publicized and taught. It operated from 1919 to 1933....
in
DessauDessau is a town in Germany on the junction of the rivers Mulde and Elbe, in the Bundesland of Saxony-Anhalt. Since 1 July 2007, it is part of the merged town Dessau-Roßlau. Population of Dessau proper: 77,973 .-Geography:...
and the
Hallidie BuildingThe Hallidie Building, is an office building in the Financial District of San Francisco, California, at 130 Sutter Street, between Montgomery Street and Kearny Street. It was built around 1917-1918 and is credited as the first building to feature glass curtain walls. It was designed by architect...
in San Francisco.
The 1970’s began the widespread use of aluminum
extrusionExtrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
s for mullions. Aluminum offers the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease.
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. Framing members may be fabricated in a shop environment, but all installation and glazing is typically performed at the jobsite.
Unitized systems
Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. These completed units are hung on the building structure to form the building enclosure. Unitized curtain wall has the advantages of: speed; lower field installation costs; and quality control within an interior climate controlled environment. The economic benefits are typically realized on large projects or in areas of high field labor rates.
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. For example the glass is captured between an inner and an outer gasket in a space called the glazing rebate. The glazing rebate is ventilated to the exterior so that the pressure on the inner and outer sides of the exterior gasket is the same. When the pressure is equal across this gasket water cannot be drawn through joints or defects in the gasket.
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. The building structure design must account for these loads.
Dead load
Dead load is defined as the weight of structural elements and the permanent features on the structure. 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
windWind is the flow of air or other gases that compose an atmosphere . On Earth, wind consists of the bulk movement of air...
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
must be resisted by the curtain wall system since it envelops and protects the building. Wind loads vary greatly throughout the world, with the largest wind loads being near the coast in hurricane-prone regions. For each project location,
building codeA building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of the building codes is to protect public health, safety and general welfare as they relate to the...
s specify the required design wind loads. Often, a
wind tunnelA wind tunnel is a research tool used in aerodynamic research. It is used to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...
study is performed on large or unusually shaped buildings. 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. These studies take into account
vortex sheddingVortex shedding is an unsteady flow that takes place in special flow velocities . In this flow, vortices are created at the back of the body and detach periodically from either side of the body. See Von Kármán vortex street.Vortex shedding is caused when a fluid flows past a blunt object...
around corners and the effects of surrounding buildings.
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
seismicSeismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth...
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. Anchor design needs to be reviewed, however, since a large floor-to-floor displacement can place high forces on anchors. (Additional structure must be provided within the primary structure of the building to resist seismic forces from the building itself.)
Snow load
Snow loads and
live loadsIn mechanical and structural engineering, live loads and dead loads are two kinds of forces exerted on an object. The concepts are used especially where analysis of real-world objects is required...
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 expansionWhen the temperature of a substance changes, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bonds...
. 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. This expansion and contraction is accounted for by cutting horizontal mullions slightly short and allowing a space between the horizontal and vertical mullions. In unitized curtain wall, a gap is left between units, which is sealed from air and water penetration by wiper gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to account for movement. Incidentally, this slot also accounts for live load deflection and
creepCreep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material....
in the floor slabs of the building structure.
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. The bombing of the
Alfred P. Murrah Federal BuildingThe Alfred P. Murrah Federal Building was a United States Federal Government complex located at 200 N.W. 5th Street in downtown Oklahoma City, Oklahoma, United States. The building was the target of the Oklahoma City bombing on April 19, 1995, which killed 168 people...
in
Oklahoma CityOklahoma City is the capital and largest city of the U.S. state of Oklahoma.Oklahoma City may also refer to:*Oklahoma City metropolitan area*Downtown Oklahoma City*Uptown Oklahoma City*Oklahoma City bombing*Oklahoma City National Memorial...
,
OklahomaOklahoma is a state located in the South Central region of the United States of America. With an estimated 3,617,316 residents in 2007 and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, has spawned much of the current research and mandates in regards to building response to blast loads. Currently, all new federal buildings in the U.S., and all U.S. embassies built on foreign soil, must have some provision for resistance to bomb blasts.
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
dynamicIn physics the term dynamics customarily refers to the time evolution of physical processes. These processes may be microscopic as in particle physics, kinetic theory, and chemical reactions, or macroscopic as in the predictions of statistical mechanics and nonequilibrium thermodynamics....
load analysis, with full-scale mock-up testing performed prior to design completion and installation.
Blast resistant glazing consists of
laminated glassLaminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral , between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high...
, which is meant to break but not separate from the mullions. Similar technology is used in hurricane-prone areas for the protection from wind-borne debris.
Infiltration
Air infiltration is the air which passes through the curtain wall from the exterior to the interior of the building. The air is infiltrated through the gaskets, through imperfect joinery between the horizontal and vertical mullions, through
weep holeWeep holes or "weeper holes" are small openings left in the outer wall of masonry construction as an outlet for water inside a building to move outside the wall and evaporate. The term was coined by archaeologist C. Leonard Woolley after finding evidence of weep holes in ziggurats...
s, and through imperfect sealing. The American Architectural Manufacturers Association (AAMA) is an industry trade group in the U.S. that has developed voluntary specifications regarding 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). Testing is typically conducted by an independent third party agency using the ASTM E-783 standard.
Water penetration is defined as any water passing from the exterior of the building through to the interior of the curtain wall system. Sometimes, depending on the building specifications, a small amount of controlled water on the interior is deemed acceptable. AAMA Voluntary Specifications allow for water on the interior, while the underlying ASTM E - 1105 test standard would disqualify a test subject if any water is seen inside. 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 quality control checks are also performed on installed curtain walls, in which a calibrated spray nozzle is used to spray water on the curtain wall for a specified time in order to investigate known leaks or leading up to a validation test like the ASTM E-1105.
Deflection
One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. This translates to three times more
deflectionDeflection or deflexion may refer to:*Deflection *Deflection *Deflection *Electrostatic deflection*Deflection...
in an aluminum mullion compared to the same steel section under a given a load. Building specifications set deflection limits for perpendicular (wind-induced) and in-plane (dead load-induced) deflections. It is important to note that these deflection limits are not imposed due to strength capacities of the mullions. Rather, they are designed to limit deflection of the glass (which may break under excessive deflection), and to ensure that the glass does not come out of its pocket in the mullion. 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.
Deflection limits are typically expressed as the distance between anchor points divided by a constant number. A deflection limit of L/175 is common in curtain wall specifications, based on experience with deflection limits that are unlikely to cause damage to the glass held by the mullion. 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 inertiaThe second moment of area, also known as the area moment of inertia or second moment of inertia is a property of a shape that can be used to predict the resistance of beams to bending and deflection. The deflection of a beam under load depends not only on the load, but also on the geometry of the...
required to keep deflection limits under the specification. Another way to limit deflections in a given section is to add steel reinforcement to the inside tube of the mullion. Since steel deflects at 1/3 the rate of aluminum, the steel will resist much of the load at a lower cost or smaller depth.
Strength
Strength (or maximum usable
stressIn continuum mechanics, the concept of stress, introduced by Cauchy around 1822, is a measure of the average amount of force exerted per unit area of a surface within a deformable body on which internal forces act...
) available to a particular material is not related to its material stiffness (the material property governing
deflectionDeflection or deflexion may refer to:*Deflection *Deflection *Deflection *Electrostatic deflection*Deflection...
); it is a separate criterion in curtain wall design and
analysisStructural analysis comprises the set of physical laws and mathematics required to study and predict the behavior of structures. The subjects of structural analysis are engineering artifacts whose integrity is judged largely based upon their ability to withstand loads; they commonly include...
. This often affects the selection of materials and sizes for design of the system. For instance, a particular shape in aluminum will deflect almost three times as much as the same steel shape for an equivalent load (see above), though its strength (i.e. the maximum load it can sustain) may be equivalent or even slightly higher, depending on the grade of aluminum. Because aluminum is often the material of choice, given its lower unit weight and better
weatheringWeathering is the decomposition of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice,...
capability as compared with steel, deflection is usually the governing criteria in curtain wall design.
Thermal criteria
Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good
conductorIn heat transfer, conduction is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. Conduction takes...
of heat. This translates into high heat loss through aluminum curtain wall mullions.
There are several ways to compensate for this heat loss, the most common way being the addition of thermal breaks.
Thermal breakA thermal break is an element of low thermal conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials.In architecture some examples include the following:...
s are barriers between exterior metal and interior metal, usually made of
polyvinyl chloridePolyvinyl chloride, commonly abbreviated PVC, is the third most widely used thermoplastic polymer after polyethylene and polypropylene. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in...
(PVC). These breaks provide a significant decrease in the thermal conductivity of the curtain wall. However, since the thermal break interrupts the aluminum mullion, the overall moment of inertia of the mullion is reduced and must be accounted for in the structural analysis of the system.
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,
condensationCondensation is the change of the physical state of aggregation of matter from gaseous phase into liquid phase and the reverse of evaporation. When the transition happens from the gaseous phase into the solid phase directly, bypassing the liquid phase, the change is called deposition...
may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls.
Rigid
insulationThe term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer....
is provided in
spandrelA spandrel is the space between two arches or between an arch and a rectangular enclosure.There are four or five accepted and cognate meanings of spandrel in architectural and art history, mostly relating to the space between a curved figure and a rectangular boundary - such as the space between...
areas to provide a higher
R-valueThe R value or R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it or . The bigger the number, the better the building insulation's...
at these locations.
Infills
Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element.
Regardless of the material, infills are typically referred to as
glazing, and the installer of the infill is referred to as a
glazierA Glazier is a construction professional who selects, cuts, installs, replaces, and removes residential, commercial, and artistic glass. Glaziers also install aluminum storefront frames and entrances, glass handrails and balustrades, shower enclosures, curtain wall framing and glass and mirror...
.
Glass
By far the most common glazing type,
glassFloat glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made from float glass...
can be of an almost infinite combination of color, thickness, and
opacityOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
.
For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm)
insulating glassWhen multiple glass panes or "lites" are assembled into units, they are commonly referred to as insulated glass, double glazing, Double Glazed Units or Insulating Glass Units ....
. Presently, 1/4 inch glass is typically used only in
spandrelA spandrel is the space between two arches or between an arch and a rectangular enclosure.There are four or five accepted and cognate meanings of spandrel in architectural and art history, mostly relating to the space between a curved figure and a rectangular boundary - such as the space between...
areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). The 1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch (12 mm) airspace. The air inside is usually atmospheric air, but some
inert gasAn inert gas is any gas that is not reactive with elements.Unlike noble gases, inert gas is not necessarily elemental and is often a compound gas. Like the noble gases the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases...
es, such as argon, may be used to offer better
thermal transmittanceThermal transmittance, also known as U-value, is the rate of transfer of heat through one square metre of a structure divided by the difference in temperature across the structure. It is expressed in watts per square metre per kelvin, or W/m²K...
values. In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Larger thicknesses are typically employed for buildings or areas with higher thermal,
relative humidityRelative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.- Definition :...
, or
sound transmissionAcoustic transmission in building design refers to a number of processes by which sound can be transferred from one part of a building to another...
requirements, such as laboratory areas or
recording studioA recording studio is a facility for sound recording. Ideally, the space is specially designed by an acoustician to achieve the desired acoustic properties...
s.
Glass may be used which is transparent, translucent, or opaque, or in varying degrees thereof.
Transparent glass usually refers to
vision glass in a curtain wall. Spandrel or vision glass may also contain translucent glass, which could be for security or aesthetic purposes.
Opaque glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall. Another method of hiding spandrel areas is through
shadow box construction (providing a dark enclosed space behind the transparent or translucent glass). Shadow box construction creates a perception of depth behind the glass that is sometimes desired.
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. The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size. Common stone types used are:
Arriscraft(calcium silicate);
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
;
marbleMarble is a non foliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite . It is extensively used for sculpture, as a building material, and in many other applications...
;
travertineTravertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from geothermally heated hot-springs. Similar deposits formed from ambient temperature water are known as tufa.-Features:...
; and
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
. The stone may come in several different finishes, which adds many more options for architects and building owners.
Panels
Metal panels can take various forms including aluminum plate; thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; and panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel. Other opaque panel materials include fiber-reinforced plastic (FRP), stainless steel, and terracotta. Terracotta curtain wall panels were first used in Europe, but only a few manufacturers produce high quality modern terracotta curtain wall panels.
Louvers
A
louverA louver or louvre , from the French l'ouvert; "the open one") is a window, blind or shutter with horizontal or, less often, vertical slats, that are angled to admit light and air, but to keep out rain, direct sunshine, and noise...
is provided in an area where mechanical equipment located inside the building requires ventilation or fresh air to operate. They can also serve as a means of allowing outside air to filter into the building to take advantage of favorable climatic conditions and minimize the usage of energy-consuming
HVACHVAC is an acronym that stands for the closely related functions of "Heating, Ventilating, and Air Conditioning"-the technology of indoor environmental comfort...
systems. Curtain wall systems can be adapted to accept most types of louver systems to maintain the same architectural sightlines 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
FirestopA firestop is a passive fire protection system of various components used to seal openings and joints in fire-resistance rated wall and/or floor assemblies, based on fire testing and certification listings....
ping at the
"perimeter slab edge"A building joint is a junction where building elements meet without applying a static load from one element to another. When one or more of these vertical or horizontal elements that meet are required by the local building code to have a fire-resistance rating, the resulting opening that makes up...
, which is a gap between the floor and the backpan of the curtain wall is 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. Some building codes require the mullion to be wrapped in heat-retarding insulation near the ceiling to prevent the mullions from melting and spreading the fire to the floor above. It is important to note that the firestop at the perimeter slab edge is considered a continuation of the
fire-resistance ratingA fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail a host of other criteria, involving other evidence of functionality or fitness for...
of the floor slab. The curtain wall itself, however, is not ordinarily required to have a rating. This causes a quandary as
Compartmentalization (fire protection)Compartmentalization in structures, such as land-based buildings, traffic tunnels, ships, aerospace vehicles, or submarines, is the fundamental basis and aim of passive fire protection....
is typically based upon
closed compartments to avoid fire and smoke migrations beyond each engaged compartment. A curtain wall by its very nature prevents the completion of the compartment (or envelope). The use of fire sprinklers has been shown to mitigate this matter. As such, unless the building is sprinklered, fire may still travel up the curtain wall, if the glass on the exposed floor is shattered due to fire influence, causing flames to lick up the outside of the building. Falling glass can endanger pedestrians, firefighters and firehoses below. An example of this is the
First Interstate Bank Fire in
Los AngelesLos Ángeles is the capital of the province of Biobío, in the municipality of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123.445 inhabitants...
,
CaliforniaCalifornia is the most populous state in the United States, and the third largest by area. California is the second most populous sub-national entity in the Americas, behind only São Paulo, Brazil...
. The fire here leapfrogged up the tower by shattering the glass and then consuming the
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
skeleton holding the glass. Aluminium's melting temperature is 660°C, whereas building fires can reach 1,100°C. The melting point of aluminium is typically reached within minutes of the start of a
fireFire is the rapid oxidation of a combustible material releasing heat, light, and various reaction products such as carbon dioxide and water. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the...
. Firestops for such building joints can be qualified to
UL 2079 -- Tests for Fire Resistance of Building Joint Systems. Sprinklering of each floor has a profoundly positive effect on the fire safety of buildings with curtain walls. In the case of the aforementioned fire, it was specifically the activation of the newly installed sprinkler system, which halted the advance of the fire and allowed effective suppression. Had this not occurred, the tower would have collapsed onto fire crews and into an adjacent building, while on fire. Exceptionally sound cementitious spray
fireproofingFireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...
also helped to delay and ultimately to avoid the possible collapse of the building, due to having the
structural steelStructural steel is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and strength...
skeleton of the building reach the critical temperature, as the post-mortem fire investigation report indicated. This fire proved the positive collective effect of both
active fire protectionActive fire protection is an integral part of fire protection. AFP is characterised by items and/or systems, which require a certain amount of motion and response in order to work, contrary to passive fire protection.-Fire suppression:...
(sprinklers) and
passive fire protectionPassive fire protection is an integral component of the three components of structural fire protection and fire safety in a building. PFP attempts to contain fires or slow the spread, through use of fire resistant walls, floors, and doors...
(fireproofing).
Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully
temperedArchitectural glass is glass that is used as a building material. It is most typically used as transparent glazing material in the building envelope, including windows in the external walls. Glass is also used for internal partitions and as an architectural feature...
glass to allow full fracturing of the panel into small pieces and relatively safe removal from the opening.
Maintenance and repair
Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years. Removal and replacement of perimeter sealants require meticulous surface preparation and proper detailing.
Aluminum frames are generally painted or
anodizedAnodizing, or anodising in British English, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit...
. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating.
Anodized aluminum frames cannot be "re-anodized" in place, but can be cleaned and protected by proprietary clear coatings to improve appearance and durability.
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.
External links