Curtain wall (architecture)
A curtain wall is an exterior covering of a building in which the outer walls are non-structural, instead serving to protect the interior of the building from the elements. Because the curtain wall
Curtain walls may be designed as "systems" integrating frame, wall panel, and weatherproofing materials. Steel frames have largely given way to aluminum extrusions. Glass is typically used for infill because it can reduce construction costs, provide an architecturally pleasing look, and allow natural light to penetrate deeper within the building. However, glass also makes the effects of light on visual comfort and solar heat gain in a building more difficult to control. Other common infills include stone veneer, metal panels, louvres, and operable windows or vents.
Unlike storefront systems, curtain wall systems are designed to span multiple floors, taking into consideration building sway and movement and design requirements such as thermal expansion and contraction; seismic requirements; water diversion; and thermal efficiency for cost-effective heating, cooling, and interior lighting.
History
Historically, buildings were constructed of timber, masonry, or a combination of both. Their exterior walls were load-bearing, supporting much or all of the load of the entire structure. The nature of the materials resulted in inherent limits to a building's height and the maximum size of window openings.
The development and widespread use of
An early example of an all-steel curtain wall used in the classical style is the Kaufhaus Tietz department store on Leipziger Straße, Berlin, built in 1901 (since demolished).[5]
Some of the first curtain walls were made with steel
The widespread use of aluminium
Systems and principles
Stick systems
The vast majority of ground-floor curtain walls are installed as long pieces (referred to as sticks) between floors vertically and between vertical members horizontally. Framing members may be fabricated in a shop, but installation and
Ladder systems
Very similar to a stick system, a ladder system has mullions which can be split and then either snapped or screwed together consisting of a half box and plate. This allows sections of curtain wall to be fabricated in a shop, effectively reducing the time spent installing the system onsite. The drawbacks of using such a system is reduced structural performance and visible joint lines down the length of each mullion.
Unitized systems
Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. These completed units are installed 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. 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 outer gasket is the same. When the pressure is equal across this gasket, water cannot be drawn through joints or defects in the gasket.
Design concerns
A curtain wall system 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.
- Dead load
Dead load is defined as the weight of structural elements and the permanent features on the structure.[7] 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 may include sunshades or signage attached to the curtain wall.
- Wind load
Wind load is a normal force acting on the building as the result of
- Seismic load
Seismic loads in a curtain wall system are limited to the interstory drift induced on the building during an earthquake. 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 typically able to withstand up to three inches (76 mm) of relative floor movement without glass breakage or water leakage.
- Snow load
Snow loads and
- Thermal load
Thermal loads are induced in a curtain wall system because aluminum has a relatively high
- 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 Building in Oklahoma City, Oklahoma, 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.[10]
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 are 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
Blast resistant glazing consists of laminated glass, which is meant to break but not separate from the mullions. Similar technology is used in hurricane-prone areas for impact protection from wind-borne debris.
Air 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
Water penetration
Water penetration is defined as water passing from the exterior of the building to the interior of the curtain wall system. Sometimes, depending on the building
Deflection
One of the disadvantages of using aluminum for mullions is that its
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 that 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. However, some panels require stricter movement restrictions, or certainly those that prohibit a torque-like motion.
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. 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 one-third the rate of aluminum, the steel will resist much of the load at a lower cost or smaller depth.
Deflection in curtain wall mullions also differs from deflection of the building structure, whether concrete, steel, or timber. Curtain wall anchors must be designed to allow differential movement between the building structure and the curtain wall.
Strength
Strength (or maximum usable
Thermal criteria
Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good
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.
Rigid insulation is provided in spandrel areas to provide a higher R-value at these locations.
Thermally-broken mullions with double- or triple-glazed IGUs are often referred to as "high-performance" curtain walls.[12] While these curtain wall systems are more energy-efficient than older, single-glazed versions, they are still significantly less efficient than opaque (solid) wall construction.[13] For example, nearly all curtain wall systems, thermally-broken or otherwise, have a U-value of 0.2 or higher, which is equivalent to an R-value of 5 or lower.[14]
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. Some common infills include metal panels, louvers, and
Glass
Larger thicknesses are typically employed for buildings or areas with higher thermal,
Glass may be used which is
Stone veneer
Thin blocks (3 to 4 inches (76 to 102 millimetres)) of stone can be inset within a curtain wall system. 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: calcium silicate, granite, marble, travertine, limestone, and engineered stone. To reduce weight and improve strength, the natural stone may be attached to an aluminum honeycomb backing.
Panels
Metal panels can take various forms including stainless steel, aluminum plate;
Louvers
A
Windows and vents
Most curtain wall glazing is fixed, meaning that 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
Firestopping at the perimeter slab edge, which is a gap between the floor and 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. The firestop at the perimeter slab edge is considered a continuation of the fire-resistance rating 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) 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 from heat, 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 1988
Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully
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
Stainless steel curtain walls require no coatings, and embossed, as opposed to abrasively finished, surfaces maintain their original appearance indefinitely without cleaning or other maintenance. Some specially textured matte stainless steel surface finishes are hydrophobic and resist airborne and rain-borne pollutants.[16] This has been valuable in the American Southwest and in the Mideast for avoiding dust, as well as avoiding soot and smoke staining in polluted urban areas.
See also
References
- ^ Cuss, Helena (1 April 2016). "Britain's top 10 maverick buildings". Royal Academy of Arts. Retrieved 8 July 2022.
- ^ "Earliest curtain wall office building". Guinness Book of Records. Retrieved 21 April 2024.
- ^ "Oriel Chambers History". Oriel Chambers. Archived from the original on 23 November 2021. Retrieved 11 December 2023.
- ^ JSTOR 41601861.
- ^ "History". janwillemsen. August 2013. Retrieved 15 March 2014.
- ^ "Omni San Diego Hotel offers breathtaking Bay views through Wausau's curtainwall" (PDF) (Press release). Wausau, Wisconsin: Wausau Window and Wall Systems. n.d. Archived from the original (PDF) on 4 March 2016. Retrieved 11 December 2023.
- ^ 2006 International Building Code, Section 1602.1
- ^ "Minimum Design Loads for Buildings and Other Structures", American Society of Civil Engineers, 2005; Chapter 6
- ^ "Minimum Design Loads for Buildings and Other Structures," American Society of Civil Engineers, 2005; Chapter 7
- ^ "Design of Buildings to Resist Progressive Collapse," UFC 4-023-03, U.S. Department of Defense, 2009
- ^ Testing is typically conducted by an independent third party agency using the ASTM E-783 standard.
- OCLC 52540181.
- ^ Lee, Ivan Yun Tong (29 September 2010). High Performance Window Systems and their Effect on Perimeter Space Commercial Building Energy Performance (MASc thesis). University of Waterloo.
- ^ "BSD-006: Can Highly Glazed Building Façades Be Green?". Building Science Corporation. 11 September 2008. Retrieved 9 November 2022.
- ^ "Technical Report, Interstate Bank Building Fire". United States Fire Administration. Archived from the original on 4 October 2010. Retrieved 21 November 2009.
- ^ McGuire, Michael F., "Stainless Steel for Design Engineers", ASM International, 2008.
External links
- European Commission's portal for efficient Curtain Walling Archived 23 September 2015 at the Wayback Machine
- EN 13830: Curtain Walling - Product Standard
- EN 13119: Curtain Walling - Terminology
- Understanding Curtain Wall & Window Wall differences