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ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

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Wall sections: Curtain walls and glazing systems

"Curtain wall" refers to any cladding system that is "hung" from the structural frame. See my curtain wall essay for more details. On the other hand, it is most often associated with gridded systems of metal mullions and rails (mostly aluminum) and glass.

Glass

Glass "lites" as big as 3' x 4' were produced during classical Roman times; the 10th century production of glass was centered in Murano, Italy, and was based on the creation of flat panes from cylinders or spheres (the latter producing "crown" glass). By the 17th century, glass was "cast" in the form of plate glass, especially for fine mirrors, but this process required much grinding and polishing, and was not particularly economical. Joseph Paxton's Crystal Palace in London (1851) actually used a high-tech (for its time) version of the cylinder process.

Crystal Palace under construction
Crystal Palace (left) and under construction (right)

Kimmel Center for the Performing Arts 2001
Rafael Viñoly: Kimmel Center for the Performing Arts, Philadelphia, 2001

Modern glazing had it's start only in 1959 with the invention of the "float" glass process by Pilkington. Float glass is "cast" in a continuous manufacturing process onto a bed of molten tin, creating smooth surfaces that do not need further treatment. Such glass is annealed (slowly cooled), and is the primary glass product used in building.

float glass production
Float glass production process (image source)

Materials

Glass consists of SiO2 (silicon dioxide) + soda ash, lime, alumina, and potassium oxide. The primary ingredient, sand, is readily available. Glass is, according to legend, not a solid, but rather a "super-cooled liquid" with an open noncrystaline microstructure. Actually, it is neither a liquid — supercooled or otherwise — nor a solid. It is an amorphous solid — a state somewhere between those two states of matter. And yet glass's liquid-like properties are not enough to explain thicker-bottomed windows, because glass atoms move too slowly for any changes to be visible.

Glass comes in traditionally-defined thicknesses as follows:

Processes

Ordinary float glass is "annealed" (slowly cooled)

Tempered glass starts off as ordinary annealed glass, but is reheated to 1200 degrees F, then cooled rapidly to introduce — on purpose — residual stresses that bring the surfaces into compression. This makes the glass stronger (4x as strong as ordinary annealed glass) because the induced compression prevents brittle tensile fracture, the "achilles heel" of ordinary glass. Note that tempered glass must be cut before it is tempered, since once tempered, it cannot be modified.

shattered tempered glass
Tempered glass when broken (image source)

Tempered glass, if broken, will shatter into small, smooth fragments, so it is actually safer than ordinary glass, which will break into larger, sharper pieces (shards). It is therefore often specified for "safety glazing" applications.

Heat-strengthened glass is similar to tempered glass, but is not reheated to the same extent. It is about 2x stronger than ordinary annealed glass, and is useful in situations where the glazing is subject to thermal stresses.

Laminated glass consists of two or more layers of glass (often just annealed glass) with a vinyl interlayer between. This interlayer adheres to the glazing, so that if fractured, the glass "sticks" to the interlayer, and doesn't fall apart as a sharp, dangerous fragment. With multiple layers, such glass can be used in security applications (bullet-proof, blast resistant), or as safety glass.

laminated glass
Laminated glass diagram (left) and custom-produced piece (right) (image sources here and here)

Wired glass is used in applications like fire doors. The wire grid, cast into the glass during the manufacturing process, holds the glass in place even after it may crack when exposed to high heat. Wired glass is a form of rolled glass.

wired glass
Laminated glass diagram (left) and custom-produced piece (right) (image source)

Spandrel glass is just ordinary glass with an opaque coating on the protected side. It is used in curtain walls to hide the structural and mechanical stuff that would otherwise be visible between floors.

spandrel glass, Lever House
Spandrel glass at the Lever House (S.O.M.)

Where is safety glazing needed (per 2012 IBC, Section 2406)?

Answer — in hazardous locations, defined as follows (without all the fine print):

Solar gain issues

There are several strategies to make glazing more effective at resisting heat loss or heat gain:

See Brian Stephens, Window and Storefront thermal Performance, Construction Specifier, Nov. 2013 (click on "read more" to access the article).

Some other definitions that describe the light and heat transmitting properties of glass are as follows:

Type of glass Visible light transmission (VLT) Solar heat gain coefficient (SHGC) Winter Night U-value Light-to-solar gain (LSG) ratio
Dual-pane tiinted glass (blue-green) 69% 0.49 0.47 1.41
Double-silver MSVD solar-control low-e glass (clear) 70% 0.38 0.29 1.84
Tinted MSVD solar-control low-e glass (blue-green) 51% 0.31 0.29 1.66
Passive low-e with spectrally selective tinted glass (blue green) 64% 0.45 0.35 1.42
Triple-silver MSVD solar-control low-e lass (clear) 62% 0.27 0.29 2.30
Table adapted from Construction Specifier, October 2008 (click on Oct. 2008 issue here)

 

Definitions that describe advanced glass characteristics:

Curtain walls

Halladie Building
Halladie Building, San Francisco, Willis Polk (1917): Earliest glass curtain wall

Mies project for glass skyscraper
Mies: Glass skyscraper project (1920-21) (image source)

Bauhaus - Gropius 1926
Gropius: Bauhaus, 1926 (image source)

Hugh Ferriss rendering of UN Secretariat Building
Rendering of UN complex by Hugh Ferriss

lever house curtain wall section
Lever house curtain wall section (left) and generic mullion installation (right)

Gannett curtain wall installation at Cornell
Curtain wall mullions and rails installed at Gannett addition, Cornell University (left); glazing inserted (right). Photos by J. Ochshorn, Nov. 2015

Most curtain wall systems consist of vertical mullions which span from slab to slab, and horizontal rails which span from mullion to mullion. Within the grid thus created, glass panels are inserted, although other materials can also be used within such a grid (especially at the "spandrel" condition). The typical mullion or rail section has the following characteristics:
typical curtain wall mullion/rail

mullion detail section
Recommended aluminum curtain wall mullion (Insulated Glass Manufacturers of Canada, Ontario, Canada, 1986)

The mullions and rails are typically extruded aluminum sections.
mullion detail section
Typical aluminum extrusions and fastening strategies

When assembled on site, they are known as "stick construction;" when pre-assembled into rectangular panels, they are known as "unitized construction."

unitized vs. stick curtainwalls

Air Loop Curtain Walls vs. Pressure Equalized Unitized Curtain Walls

A May, 2012, article in the Construction Specifier titled Cause and Effect by Raymond Ting proposes "air loop" curtain walls (ALCW) instead of the pressure-equalized unitized curtain walls (PEUCW) described above. He points out that the PEUCW, while an improvement over non-pressure-equalized systems, still contains "critical seals" that can fail, and can result in staining on the exterior wall surface "due to the exposed drainage holes and the delayed drainage after diminishing of the positive wind pressure." (p. 38) One of the primary advantages cited for the ALCW systems is the uniformity of the panel unit design, whether at a typical vertical mullion position, at an inside corner, or at an outside corner: the same sealing detail is used for all conditions. This is accomplished by separating the structural mullion from the actual unitized glazed panel.

curtain wall details from Raymond Ting, May 2012 Construction Specifier
Details comparing pressure-equalized unitized curtain wall (PEUCW - left) with air-loop curtain wall (ALCW - right) taken from Raymond Ting, "Cause and Effect," Construction Specifier, May 2012.

Ting proposed two strategies; he calls them "functional isolation concept" (FIC) and "unit design concept" (UDC). The former means that wall components should perform a single function, rather than multiple functions; the latter means that simplifying designs so that a single design can satisfy multiple conditions will eliminate numerous errors and problems in the field.

Ting also proposes to eliminate what he calls "critical seals" which are defined as follows: seals against both water and air at the same location; or a water-only seal having a path leading to an air seal.

Water leakage in a curtain wall may come about when any of these three things are present:

  1. Water on the surface of the wall.
  2. Positive wind pressure causing a force on a critical seal, or pressure on the seal caused by water-head, or water brought into contact with a seal by capillary action.
  3. An actual failure of a critical seal.

It is unrealistic to expect to solve this problem by providing a perfect (critical) seal on the entire curtain wall face, even using modern sealants. Having internal gutters is an improvement, but still requires numerous critical seals applied in the field (e.g., where the gutters are spliced together). Having a pressure-equalized unitized wall is a further improvement, but is still problematic.

Like other forms of cladding attached to the structure by spanning from floor to floor, curtain wall mullions must be attached to resist gravity loads at one floor only. At the bottom (typically), the mullion slides over the mullion below it, allowing it to move vertically, but restraining it laterally.

Formal strategies

There are three primary formal intentions manifested in metal-glass curtain wall design:
  1. "Woven" grid of metal, with glass and/or metal infill.

    Mies van der Rohe (with Philip Johnson), Seagram Building, 1958 Adler & Sullivan, Prudential (Guananty) Building, Buffalo, NY, 1894
    Mies van der Rohe,with Philip Johnson, Seagram Building, 1958 (left); and Adler & Sullivan, Prudential (Guaranty) Building, Buffalo, NY, 1894 (right)

    Emery Roth, Raleigh Capital Bank Building, 1965
    Emery Roth & Sons, Raleigh Capital Bank Building, 1965

  2. Glass as literally transparent; as void.

    Mies van der Rohe, Farnsworth House, 1950
    Mies van der Rohe, Farnsworth House, 1950

    Bohlin Cywinski Jackson, Apple Store, 5th Avenue, NYC
    Bohlin Cywinski Jackson, Apple Store, 5th Avenue, NYC

    Bohlin Cywinski Jackson, Apple Store, 5th Avenue, NYC - broken window
    Apple Store window breaks after coming into contact with a snow plow (source)

  3. Glass as a sculptural (sculpted) solid.

    Cesar Pelli, Pacific Design Center, LA
    Cesar Pelli, Pacific Design Center, LA
    Cesar Pelli, Pacific Design Center, LA (both images)

    I.M.Pei & Partners, Hancock Building, Boston (Harry Cobb)
    I.M.Pei & Partners, Hancock Building, Boston (Harry Cobb): Curtain wall failure due to adherance of lead solder to reflective coating, and telegraphing of solder (or lead spacer) fatigue cracking into glass, through strong bond of solder to reflective coating.

Fastening mullions to the structural slab or deck

Fastening curtain wall mullions to structure
Fastening curtain wall mullions to structure

Fastening curtain wall mullions to structure
Fastening curtain wall mullions: details

Seagram Building under construction
Mies, Seagram Building curtain wall under construction

Seagram Building corner detail
Mies, Seagram Building curtain wall corner detail: steel column, concrete fireproofing, bronze cladding.

Structural spacer glazing systems, and variants

Structural spacer glazing
Structural spacer glazing system, Spectrum Glass Products, Tremco, Inc.

Structural spacer glazing examples
Structural spacer glazing examples, including Rhodes Hall stair towers, Cornell (Gwathmey Siegel & Associates)


Milstein Hall curtain wall (video link or index of all construction videos by J. Ochshorn)

Gaskets

Rubberized gaskets can also be used to support glazing, often attached to steel plates, as shown in the image below.

use of gaskets to support glazing Uris Hall. Cornell, S.O.M.
use of gaskets to support glazing

Structural glazing

The idea that certain glazing systems are "structural" while others are not is misleading. Even so, the name, "structural glazing" has been assigned to certain systems that avoid or minimize the use of mullions and rails. The most "primitive" of such systems uses silicone sealant as a "glue" to attach glass lites to internal mullions.

A more sophisticated system, pioneered by Pilkington, supports each glass panel only at the corners, using special stainless steel fittings which are then held in place by some independent structural system. Glass "mullions" can also be used to transmit the loads of (and on) the glazing to the building structure. Joints between the glass panels are filled with silicone sealant.

Structural glazing system: Pilkington
Structural glazing system: Pilkington

Diller Scofidio + Renfro, Institute of Contemporary Art, Boston, photo by J. Ochshorn 2007 Diller Scofidio + Renfro, Institute of Contemporary Art, Boston, photo by J. Ochshorn 2007 Diller Scofidio + Renfro, Institute of Contemporary Art, Boston, photo by J. Ochshorn 2007
Diller Scofidio + Renfro, Institute of Contemporary Art, Boston (photos by J. Ochshorn 2007: right-click for larger images)

Norman Foster, Willis Faber and Dumas
Norman Foster, Willis Faber and Dumas, Ipswich, England, 1975 (photo copyright credit)

Nicholas Grimshaw, Western Morning News Headquarters, Plymouth
Nicholas Grimshaw, Western Morning News Headquarters, Plymouth, England

New developments

Electronically tintable glazing
Electronically tintable glazing (electrochromic, or EC, glass)

Electrochromic (EC) glass
In-plane zoning with electrochromic (EC) glass within a single insulated glass unit (above a clear bottom pane). In the top pane, the top zone is also tinted to 1% to control glare. Source: Construction Specifier, Nov. 2016 (See article here)

La Defense Almere, The Netherlands, UNStudio Architects
Dichroic film: La Defense Almere, The Netherlands, UNStudio Architects, van Berkel, 2004. "This chromatic virtuosity, the key concept in the project, is made possible by a dichroic film capable of changing colour depending on the source of light with the aid of special crystals, designed by 3M to coat perfume bottles." (photo/quote credit)

Frank Gehry, IAC Building, NYC and fritted glass pattern by S.O.M.
Fritted glass: Frank Gehry, IAC Building, NYC (left); fritted glass pattern by S.O.M. (right)

digital printing with ceramic-based inks
"Digital printing processes generally rely on organic ultraviolet (UV)-cured, inorganic ceramic inks that are fired to the glass during the heat-treating process, or specialized inks for printing on interlayers." (Brian Savage, "Expanding the Glass Canvas with Digital Printing," Construction Specifier, August 2013)

digital printing with ceramic-based inks
"The versatility and resolution capability of ceramic ink means various types of images can be printed, including photorealistic images, text, variable graphics, and patterns. Additionally, textures similar to wood grain, granite, and marble can be produced with excellent results." (Brian Savage, "Expanding the Glass Canvas with Digital Printing," Construction Specifier, August 2013)

Plastics

Various types of plastics are used as glazing, in particular acrylics (e.g., Lucite, Plexiglas) and polycarbonates (e.g., Lexan). They tend to be softer than glass, so more easily scratched. They have a larger coefficient of thermal expansion than glass; and are more resilient (less brittle).

Polycarbonate shapes and details
Polycarbonate shapes and details

FTL Design Engineering Studio: NYC Battery Park all-translucent polycarbonate building
Polycarbonate: Robert Rhodes Assoc., Tennis Courts, Englewood, NJ

Robert Rhodes Assoc., Tennis Courts, Englewood, NJ
FTL Design Engineering Studio: NYC Battery Park all-translucent polycarbonate building