Curtain Wall Engineering CE426: Final Report Jack Greenberg May 6, 2011
Jack Greenberg
5/6/2011
Curtain Wall Engineering The curtain wall is an important development of the 20th century that allowed for taller, economical and efficient construction of buildings. Concisely, a curtain wall is an exterior non‐ load‐bearing wall supported by the structural formwork of a building. 1 A curtain wall, which connects to the structure of the building, is constructed merely to support its own dead weight, transferring vertical gravity and horizontal wind loads to the building structure. Curtain walls have become the norm for building erection, finding application in most buildings that are constructed today. However, curtain wall construction is not actually a new invention; rather, it had existed unnamed for centuries. The Greeks and Romans had developed a rudimentary version of modern curtain walls using nonbearing fills such as brick, stone or concrete inside post‐and‐lintel constructed frames. Likewise, medieval cathedrals employed similar construction, with columns supporting arches that contained curtain walls of stained glass. 2 On a widespread level, curtain wall systems replaced the conventional building technology of bearing walls. As opposed to curtain walls, bearing walls are structural elements of buildings, supporting the actual weight of the building. The invention of steel and concrete framing allowed for the development of curtain wall technology because building materials became strong enough to carry the entire load of the building, no longer needing exterior bearing walls for support. The old masonry bearing walls used to construct older buildings 1 2
Donaldson, Barry. Exterior Wall Systems: Glass and Concrete Technology, Design, and Construction. Hunt, William Dudley. The Contemporary Curtain Wall.
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needed to be enormous to support the loads of the floors above, especially for tall buildings where some walls could be as wide as six feet or more. Therefore, curtain walls have provided an important alternative to bearing wall construction because it enabled the reduction of the size of the exterior wall to a mere three inches, saving time, space and money for developers and allowing the construction of high‐rise buildings. This relatively new wall system lends itself very well to modern practices of prefabrication and mass production allowing for economy and flexibility in the design. The main drawback of constructing a curtain wall is the difficulty in connecting the wall to the structural elements of the building which is easily achieved in bearing wall design since the wall is actually part of the structure. Nevertheless, the curtain wall’s advantages also include a reduced weight, customizable appearance, and faster, more efficient erection making it the most viable option for buildings. The curtain wall functions as a filter for the elements, fire, people, animals, sounds odors, and anything else that might pass into or out of a building. The difficulty in designing a curtain wall is integrating the sometimes conflicting individual functions the wall must provide for the building. For example, controlling the condensation that appears upon the glazing usually complicates the protection against precipitation. Therefore, throughout the entire process of design, careful thought and consideration goes into the formation of each element in the curtain wall. Curtain walls are generally composed of several basic elements, not limited to but including the fill or panel, joints, vapor barrier, backing and insulation. However, some curtain 3
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walls don’t contain all of these elements such as aluminum framed curtain walls that don’t contain a concrete masonry unit backing. Additionally, curtain wall elements are composed of different materials, varying in shape and size. Probably the most difficult elements of the curtain wall to design are the joints that connect the curtain wall. One of the things that a curtain wall is most susceptible to is water leakage. Porous materials such as stone and masonry, allow water to be siphoned through their surfaces to the inside. 3 Nevertheless, the critical location of water entry though curtain walls is at the joints of the façade. The design of the joint is complicated for curtain walls since the joint invariably becomes the flexible link in the wall. It must be weather‐tight, yet free to vary its dimensions with changes in temperature. It must protect against rain while allowing condensation to bleed off. There are vast numbers of joints in the façade making it an even greater design challenge for engineers. Besides for precipitation protection, control against condensation is important for maintaining clear windows and for maintaining the integrity of the insulation of the wall. A vapor barrier is constructed to allow the little water that inevitably enters the building to return the atmosphere. In many curtain wall designs, this provision consists of opening or weeps holes in the base of the unit. Weep holes are difficult to design because they can actually have negative effects on the building if it is not designed properly. If the weep holes are not large enough they can become ineffective since grime and dirt could potentially clog them. Additionally, if the weep holes are too large or are placed to frequently throughout the façade, water can enter the building more easily, defeating the purpose of the holes. Similarly, the 3
Hunt, William Dudley.
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vapor barrier needs to be wide enough to allow room for the deflection of the panels spanning across the wall without commandeering too much of the space of the building. 4 The panels and glazing that span across the building are elements that cover a majority of the building envelope. The critical design challenge in these elements is the large surface area they possess. Probably most importantly, the material chosen for the panel and glazing greatly influences the thermal properties of the curtain wall. Among the biggest concerns in curtain wall design is adequate temperature control. The various types of panels and glazing can have large implications in the money spent on cooling and heating in the building. Additionally, fire protection is controlled by the effectiveness of the panels and glazing. The wall must not allow fires to spread from one building to another or allow fires to spread to different floors of the building. Masonry materials slow down the rate of heat transfer by using a large amount of heat to evaporate the water contained in their physical or chemical composition, offering an excellent source of fire protection. Large amounts of heat can be stored in the mass of masonry, enabling it to withstand exposure to fire for a longer period of time before the opposite wall face temperature rises appreciably. 5 Panels and glazing must also withstand stresses caused by the building movement resulting from expansion or contraction, or lateral loading from earthquakes and hurricanes. This is much greater than the dead load of the panels which is usually much smaller in comparison. Nevertheless, structurally, the design of a curtain wall hinges on the effectiveness of the sealant and the connection to the structural framing. 4 5
Hunt, William Dudley. Hunt, William Dudley.
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The adhesive used to fix the glass of curtain walls in place is a silicone elastomeric sealant. Silicone sealants used in structural glazing vary as to type and usage, however the main properties are generally that they are little affected by ultraviolet light, can bond reliably to glass and aluminum and have a sufficient elasticity to accommodate thermal expansion and structural movements. The structural adhesive joint design requires engineering analysis, considering the strength of the sealant in tensile adhesion, shear adhesion, cohesion and movement capability, the geometry of the sealant in contact width and thickness and the imposed loads on the sealant. The silicone adhesive must be sized to carry all forces and accommodate all movements without over stressing it. Additionally, the system must drain water from the seals and should be capable of being retrofitted with new glazing for when the glass breaks. At the start of large scale curtain wall glazing construction, the use of sealant was an uncertain for many engineers since all of silicone’s material properties had not been fully researched. Safety factors of greater than six were not uncommon among designs. However, as the material science and research developed, creative and less overdesigned techniques were used in the construction using the sealant. Thus, developments such as structural glazing spacers mitigated the structural progressive collapse failure of the curtain wall. 6 All structures whether steel or concrete will transfer movement to the building façade due to thermal expansion and contraction of the materials supporting a building. Steel structures are more generally more flexible than concrete and will apply forces to the façade in its movement. However, concrete structures also impart movement to the exterior wall due to
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Donaldson, Barry.
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long term shrinkage. Therefore, wind, earthquake and other lateral loads imposed on buildings make it critical for engineers to carefully design exterior walls to handle these dynamic forces. There are several types of curtain wall systems that are currently employed in the construction of buildings. The most common include conventional masonry, panelized wall systems and aluminum framed curtain walls. However, there are many more and different types of exterior wall systems that are used in buildings because curtain walls by definition include any type of non‐bearing wall. The different techniques used to clad a building are usually architecturally driven, each giving a different look and feel to the building. Nevertheless, as the economy, energy and functionality of buildings becomes a dominating role in design, exterior systems are more engineering driven, with each type of façade having unique qualities that is advantageous for different design criteria. Conventional masonry exterior wall systems consist of a brick or stone façade, supported by concrete masonry unit backing. These types of wall systems are typically well suited for buildings that require various non‐uniform wall openings throughout the facade. Therefore its application is most resonant with residential design methods since residential buildings are typically not repetitive horizontally. The usage of the floor space in residential buildings varies much more than floor space in office buildings requiring varied window openings in the façade. Masonry is ideal for this type of building because its small elemental size can adapt easily, enabling these openings better than aluminum or glass systems.
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Jack Greenberg
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Panelized exterior wall systems consist of prefabricated or factory‐manufactured panels that form a structural envelope. 7 The building panels are best and economically manufactured uniformly; therefore, unlike the conventional masonry façade, they are unsuitable for small varied openings in the façade. However, panelized wall systems are typically well suited for large buildings with a repetitive façade design because they can be systematically installed for quick construction of a building. Similar to panelized systems, the aluminum framed curtain wall is a system which contains panels. In this scheme, the mullions are installed first, and then the panels, usually made of glass, are inserted into the mullion framing in the field. 8 This exterior cladding is well suited for large uniform buildings because it too is prefabricated and can be systematically installed into buildings. However, it is economically more flexible than panelized systems and is even suitable for smaller construction projects as well. In addition to the function of the building, other qualities of a building that are important in determining an appropriate façade include the structural framing of the building. Since the façade of a curtain wall is not a bearing member, it must be attached to the framing of the building for support. The connection between the façade and the structure can vary based on the type of façade and structural system used. Concrete structural systems generally lend themselves to simple connections between all types of curtain walls because of its customizable shapes and sizes; whereas, steel framing’s less customizable form can at times make it difficult to join with the exterior curtain wall. This is the most important implication of 7
NAHB Research Center. "Design, Fabrication, and Installation of Engineered Panelized Walls: Two Case Studies." Stuart, Matthew, P.E., S.E., F.ASCE. "Curtainwall Primer for Design Professionals ‐ An Online Course for Engineers and Architects."
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choosing an appropriate curtain wall because the detailing of the connection usually dictates an enormous portion of the cost of the curtain wall. 9 The general properties of structural system integration with the exterior wall are apparent in the construction of conventional masonry. Concrete structural systems typically integrate very well with conventional masonry facades. The attachment of the masonry wall to the concrete structure is easy because horizontal masonry ties can be fitted easily into dovetail slots that are embedded in concrete and can also easily be cast into concrete beams or columns. Additionally, concrete’s intrinsic ability to be formed into a variety of profiles makes it an ideal material for interaction with masonry cladding. Steel framing, on the other hand, is not ideal for conventional masonry cladding because the connection to the masonry wall is very difficult. However, panelized systems and aluminum framed curtain walls generally work well with either system. Panelized systems as compared with conventional masonry have a vastly different influence on the perimeter of the building structure. Generally spanning from column to column, the points of attachment to the building structure are limited with few connections necessary to the structure. This quality, though limiting the location of where the attachments must be made, actually makes the number of connections much more manageable and easily attachable to both concrete and steel framing alike. Similarly, aluminum framed curtain walls are easily attachable to both concrete and steel systems since the aluminum shape that holds
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Donaldson, Barry.
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the system in place can be manufactured into complex shapes that fit into the steel and concrete structure. 10 Another important consideration in determining the type of building envelope to use is the structural implications imposed on the building. Conventional masonry walls with concrete masonry backing usually distribute loads evenly over the beams at the edge of the building; however, they are usually much heavier than other types of curtain walls and can have a large influence in the sizes of these beam members. However, gravity loads of panel systems are generally much smaller than conventional systems and are generally self‐supporting. In addition, the panels usually work well with buildings that contain columns at the edge of the building. Attaching to the edge eliminates the need for the spandrel beam to resist the vertical loads of the wall. However, when the panel is required to be tied back to the lower flange or web of the spandrel beam, torsional loads must be factored into the structural design. 11 As opposed to the plethora of types of curtain walls, there are generally two methods of curtain wall installation, stick and unitized systems. Today, the vast majority of curtain walls employ the use of stick system installation. Framing members, the “sticks”, are either prefabricated or assembled on site and are first installed into the structural framing of the building. Glazing or panels are then attached to the stick faming, completing the construction of the wall. Although not as popular as the stick system, unitized system is an effective method of installation that is usually most effective for large building projects. The curtain walls are factory fabricated to include the glazing and all elements of the wall, installing only the wall in 10 11
Donaldson, Barry. Donaldson, Barry.
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one step into the structural framing of the buildings. Because of the control exercised in prefabrication as opposed to field construction, unitized systems generally provide a better quality building envelope. Although the fabrication costs are higher than stick system installation, this method entails a lower field installation cost and are usually less expensive in locations with high labor rates.
With the design and of construction of new buildings in the past century, curtain wall
engineering has extended to become its own field of engineering with many engineers starting specialty firms to address the complexity of curtain wall design. Nevertheless, curtain wall construction is still in its infancy as curtain wall systems are being developed to mitigate the effects of temperature on heating and cooling costs of the buildings. Similarly, the improvement of construction techniques and material installation will change the way curtain walls are design and erected in the future.
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Bibliography Donaldson, Barry. Exterior Wall Systems: Glass and Concrete Technology, Design, and Construction. Philadelphia, PA: ASTM, 1991. Print. Hunt, William Dudley. The Contemporary Curtain Wall. New York: Dodge, 1958. Print. Marshall, Richard D., and H. C. S. Thom. Proceedings of Technical Meeting Concerning Wind Loads on Buildings and Structures: Held at National Bureau of Standards, Gaithersburg, Maryland, January 27‐28, 1969. Washington, D.C.: U.S. Dept. of Commerce, National Bureau of Standards, 1970. Print. NAHB Research Center. "Design, Fabrication, and Installation of Engineered Panelized Walls: Two Case Studies." ToolBase Services by NAHB Research Center | Home Building Technical Information Resources Builders Construction Remodeling Innovations. U.S. Department of Housing and Urban Development Office of Policy Development and Research Washington, DC, Jan. 2002. Web. . Stuart, Matthew, P.E., S.E., F.ASCE. "Curtainwall Primer for Design Professionals ‐ An Online Course for Engineers and Architects." Engineering Continuing Education PDH CEU for PE LS AIA License Renewal. Web. . Zhou, Y. S. An Introduction to the Design of Curtain Walls, Aluminum Windows, Glass Walls, Skylights and Canopies. Hong Kong: Wilson Curtain Wall Consultant (HK), 2002. Print.
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