Chapter 32 Exterior Wall Cladding-IV (Wall Systems in Glass) (1)

32

CHAPTER

Exterior Wall Cladding–IV (Wall Systems in Glass)

CHAPTER OUTLINE 32.1 GLASS-ALUMINUM CURTAIN WALLS 32.2 ANCHORAGE OF A STICK-BUILT GLASS CURTAIN WALL TO A STRUCTURE 32.3 STICK-BUILT GLASS CURTAIN WALL DETAILS

32.6 ENVIRONMENTAL PERFORMANCE CRITERIA FOR A GLASS CURTAIN WALL 32.7 OTHER GLASS-ALUMINUM WALL SYSTEMS 32.8 NONTRADITIONAL GLASS WALLS

32.4 UNITIZED GLASS CURTAIN WALL 32.5 STRUCTURAL PERFORMANCE OF A GLASSALUMINUM WALL

Transparency, luminosity, and elegance are the reasons for the popularity of glass walls in modern architecture. Most glass walls are constructed with aluminum sections to support the glass. In other words, the glass panes (also called lites ) are held within vertical and horizontal aluminum framing members. Therefore, they share some of the characteristics of their smaller counterparts—the aluminum windows, discussed in Chapter 31. 31 . However, there are many differences between the two: scale, aesthetic character, performance performance properties, design, detailing, and installation. Three commonly used glass-aluminum wall system systems are • Glass-aluminum curtain walls • Punched and strip glazing systems • Storefront systems The vast majority of contemporary buildings include one or mo re of these systems in the same building. The reasons include the unparalleled opportunity provided by them to obtain the maximum amount of daylight and view, the cost savings compared with other exterior wall cladding systems, and the recent technological advances in the thermal and structural performance of glass wall systems. Of the three systems listed above, the most frequently used and the most complex is the glass-aluminum curtain wall system, which is presented here in detail. The other two systems (strip system and storefront system) are discussed to the extent that they differ from the curtain wall system. Finally, the chapter deals with nontraditional glass wall systems— systems that do not include aluminum sections to support the glass. 777

Part 2

32.1 GLASS-ALUMINUM CURTAIN WALLS

Materials and Systems of Construction

Because of their common use, glass-aluminum curtain walls (or simply  glass curtain walls ) are constantly evolving in their design and performance. Therefore, a succinct classification that includes all contemporary glass curtain walls is impossible. The American Architectural Manufacturers Association (AAMA), an association of the manufacturers of windows and curtain walls, however, classifies glass curtain wall systems into five types based on their anatomy: • • • • •

Stick-built (or, simply, stick) systems Unitized systems Unit and mullion systems Panel systems Column cover and spandrel systems

These systems are illustrated in Figure 32.1. 32.1 . The stick system is the oldest and the most  widely used system. The remaining four systems are different from the stick system because they consist of prefabricated wall units similar to the (opaque) curtain wall panels.

S TANDARD

AN D  C USTOM  C URTAIN  W AL L  S YSTEMS

Most major glass curtain wall manufacturers have their own facility for extruding the aluminum sections. Walls constructed from a manufacturer’s commonly used and pretested aluminum sections are referred to as standard walls . Custom curtain walls utilize cross-sectional shapes extruded specifically for a project in response to an architect’s design. Because the cost of dies and other equipment required to extrude custom cross sections can be recovered from just one fair-size project, custom curtain walls are fairly common. Custom walls should, however, be tested for performance before they are used in a project. Performance Pe rformance data for standard walls are available from the manufacturers.  Wallss made from stan  Wall standard dard comp componen onents ts are obvi obviously ously more eco economi nomical. cal. Howe However, ver, this does not imply that the standard components yield only one type of wall design. In fact, the components are generally quite adaptable, and manufacturers can provide a few custom components for a standard system, so that the facade expressions obtained from the use of standard components can be numerous. If the number of custom components in a wall becomes excessive, the cost of a standard wall may approach that of a custom wall.

(a) STICK SYSTEM

Mullion expansion splice Anchor Spandrel beam Vertical member (mullion) Horizontal member (rail)

Mullion expansion splice (see also Figure 32.3)

In the stick system, the curtain wall is installed piece by piece at the site. Generally, the mullions are installed first, followed by b y the rails. Subsequently Subsequently  the glass panes are are installed within the mullion-rail  framework. The anchorage of the wall to the structural  frame is through through the mullions. mullions. The mullions may span from floor to floor or over two floors. Thermal expansion and contraction of mullions are accommodated by expansion joints in mullions. The system components are shop-fabricated and shipped to the construction site in a knockeddown (KD) version. Therefore, the system has relatively relativ ely low shipping costs and also permits a greater degree of on-site adjusment as compared  to the other systems. systems. Its disadvantages include longer on-site assembly time and more on-site labor than the other systems.

FIGURE 32.1 Types of glass curtain walls—the stick system. (Illustration adapted from AAMA, Curtain Wall Design Guide, 1996, with

permission) 778

(b) UNITIZED SYSTEM

Anchor

A unitized system consists of framed wall units that are shop fabricated, preassembled, and generally preglazed. The units are designed so that the vertical and horizontal members in adjacent units interlock to form common mullions and rails. The units may be one or two stories high. They are anchored to the building’s structural Spandrel  frame in essentially the same way as the mullions in the stick system. beam The advantage of this system is its greater degree of quality control resulting from shop fabrication. Its disadvantages are the greater shipping cost because of the added bulk from assembled Preassembled units, the need of a greater degree of protection of units during unit  transporation, and a lower degree of field adjustment. Anchor

Mullion Preassembled unit

Spandrel beam

Anchor

Preassembled unit

Spandrel beam

(c) UNIT AND MULLION SYSTEM

(d) PANEL SYSTEM

The unit and mullion system combines the advantages of both the stick system as well as the unitized system. It is constructed by  first installing the mullions; subsequently,  factory-assembled units are placed between  the mullions. Because the system is a compromise between the stick and unitized systems, it has the advantages and disadvantages of both, i.e., its transportation cost is lower than that of the unitized system but greater than that of the stick system. A greater degree of site adjustability is available in the unit and mullion system, but it is less that that of the stick system.

The panel system consists of preassembled (and sometimes preglazed) homogeneous sheet metal panels with glass infills that generally span from floor to floor. The curtain wall’s appearance is more integrated and comprehensive rather that a grid pattern of horizontal and vertical elements. The panels can be formed by stamping or casting. The casting system is economical only where a large number of identical panels are needed.

FIGURE 32.1 (continued ) Types of glass curtain walls—unitized system, unit and mullion system, and panel system. (Illustrations adapted from AAMA, Curtain Wall Design Guide, 1996, with permission)

32.2 ANCHORAGE OF A STICK-BUILT GLASS CURTAIN WALL TO A STRUCTURE Like other curtain walls, a glass curtain wall must be spaced away from the building’s structural frame to account for the small dimensional variations (within the allowed tolerances) in the structural frame. A 2-in. space is generally the minimum requirement. A wider space may be required for tall buildings. 779

Column cover Spandrel beam

(e) COLUMN COVER AND SPANDREL SYSTEM This system, though not a true glass curtain wall system, consists of separate column covers connected to spandrel covers that generally span  from column to column. Infill glazing units may either be preassembled or assembled at the site like those of a stick-built system. The system provides an independent expression of the structural system rather than concealing it behind a (more homogeneous) wall.

Spandrel panel Glazing infill

FIGURE 32.1 (continued ) Types of glass curtain walls—column cover and spandrel system. (Illustration adapted from AAMA, Curtain Wall Design Guide, 1996, with permission)

D EAD -L OA D  A NCHORS

AN D  E XPANSION  A NCHORS

 As shown in Figure 32.1(a), a stick-built glass curtain wall consists of vertical members (mullions ) and horizontal members (rails ). The profiles of both mullions and rails are almost identical and are tubular in cross section. The wall is anchored to the building’s structural frame through the mullions. All mullions in a wall are installed first; then the rails are inserted between them. Three rails are commonly used per floor to create two separat e areas of glass at each floor—vision glass and spandrel glass. In a building (or part of a building) where there is no vision glass, such as in a multistory parking garage, intermediate rails are needed only to reduce the size of glass panes. Two rails per floor are commonly used in that situation, Figure 32.2. The center-to-center spacing between mullions is generally 4 to 6 ft, depending on the lateral load intensity and the desired appearance of the facade.

Office  floors

Parking  floors After the framing of the curtain wall (mullions and rails) for the parking floors is complete, its glazing has started, while  the framing for the office floors is yet to begin. Because there are no vision areas on the parking floors, the glazing on each  floor has been divided into two parts by one intermediate rail. The purpose of the rail is merely to reduce the size of the glass.

FIGURE 32.2 (a)

The progress in the installation of a stick-built glass curtain wall on an office building in which the lower floors are parking floors and the upper floors are office floors. See also Figure 32.2(b).

780

In this photograph, the glazing of the curtain wall on the parking floors of the building shown in Figure 32.2(a) is almost complete. Now the framing for the curtain wall on the office floors has begun. As shown here, the mullions are installed first,  followed by the rails. This (part of the) photograph shows the progress in the installation of rails on the office floors. Because an office floor has separate spandrel and vision glass areas,  there will be three rails per floor.

Office  floors

Parking  floors

FIGURE 32.2 (b)

The progress in the installation of a stick-built glass curtain wall on an office building in which the lower floors are parking floors and the upper floors are office floors; see also Figure 32.2(a).

To allow for the expansion and contraction of mullions caused by temperature changes, each mullion must be provided with expansion joints. Thus, the mullions consist of short lengths (one or two floors tall) that terminate in expansion joints at both ends, Figure 32.3.  An expansion joint also absorbs the creep in concrete columns and the live-load deflection of the spandrel beam to which the mullions are anchored. Therefore, the expansion  joint width must be determined on a project-by-project basis. Note that an expansion joint allows movement in the vertical direction only. Because all loads on a wall are transferred to the structural frame through the mullions, each mullion is provided with a dead-load support anchor (or, simply, a DL anchor ) designed to carry the weight of the respective portion of the curtain wall.  A DL anchor fully restrains the movement of a mullion; that is, the mullion is immobile in all three principal directions at a dead-load support. Therefore, a DL anchor transfers both the dead loads and the lateral loads on a mullion to the building’s structural frame. Two types of mullion spans are generally used in a stick-built glass curtain wall, Figure 32.4: • Single-span mullion systems • Twin-span mullion systems In a single-span mullion system, each mullion extends only over one floor. DL anchors are, therefore, required at every floor, except at the ground floor, wher e the building’s foundation provides dead-load support to the first mullion length, Figure 32.4(a). In a twin-span mullion system, the mullions extend over two floors. Because a mullion can have only one dead-load support, DL anchors are provided at alternate floors, Figure 32.4(b). Another difference between a single-span and a twin-span system is that in a twin-span system, expansion anchors (or, simply, EX anchors ) are also required at alternate floors. In a single-span system, an EX anchor is required only at the second floor of the building. DL anchors and EX anchors are steel (or aluminum) members to which the mullions are bolted. As shown in Figure 32.4(b), they are almost identical. The only difference between them is that in a DL anchor, the upper pair of holes is round, and in an EX anchor, the upper pair of holes has vertically slotted holes that allow vertical movement.

781

I-shaped expansion splice is inserted into the tubular part of the lower mullion and fastened to it. The tubular part of  the upper mullion length slides freely over  the splice with a snug fit. A gap is left between the two mullions for movement, as shown in the lower photograph. See also Figures 32.4, 32.5(b) and (c). Lower mullion length

Mullion anchor; see Figure 32.5(a)

Plastic shim separates the steel washer  from the aluminum mullion to prevent a galvanic reaction between the steel and aluminum. Top of spandrel beam

Upper mullion length

Expansion joint between upper and lower mullion lengths. The width of this joint must be determined based on the creep in columns (if any), deflection of the spandrel beam, and the thermal movement of the mullion length. Face of spandrel beam Lower mullion length

FIGURE 32.3 A typical expansion joint between two mullion lengths.

782

Upper mullion Expansion joint   n   a   p   s   n   o    i    l    l   u    M

Expansion joint

Expansion splice DL ANCHOR

EX ANCHOR Lower mullion

Spandrel beam

Expansion joint   n   a   p   s   n   o    i    l    l   u    M

EX ANCHOR DETAIL See also Figures 32.5(b), (c) and (d)

DL ANCHOR

DL ANCHOR

Expansion joint

Expansion joint   n   a   p   s   n   o    i    l    l   u    M

  n   a   p   s   n   o    i    l    l   u    M

  n   a   p   s   n   o    i    l    l   u    M

Upper mullion DL ANCHOR Expansion splice

EX ANCHOR

Expansion joint   n   a   p   s   n   o    i    l    l   u    M

Lower mullion

Expansion joint   n   a   p   s   n   o    i    l    l   u    M

Spandrel beam

DL ANCHOR

DL ANCHOR DETAIL See also Figures 32.5(b), (c) and (d) EX ANCHOR Dead-load support

  n   a   p   s   n   o    i    l    l   u    M

DL ANCHOR

Expansion joint   n   a   p   s   n   o    i    l    l   u    M

EX ANCHOR Dead-load support

(a) Single-span mullion support system

(b) Twin-span mullion support system

Each mullion length spans from floor to floor and is provided with one dead-load anchor support at the top  from which the mullion is hung. The first mullion length, however, begins with a dead-load support at the foundation and is anchored to an EX anchor at the top. The tubular part of the second mullion length slides freely over the expansion splice in the lower mullion (see Figure 32.3).

Each mullion length spans two floors and is provided with a DL anchor support at alternate floors. Each mullion slides freely at both ends over the expansion splices. At these ends, the mullion is anchored to EX  anchors.

FIGURE 32.4

Support systems for single-span and twin-span curtain wall mullions. Observe that each mullion has only one dead-load

support.

783

A NCHORING

Part 2 Materials and Systems of Construction

A  M ULLION TO A  DL OR  E X

A NCHOR

Figure 32.5 shows the anchorage details of a mullion to a DL anchor and an EX anchor.  Anchoring a mullion to a DL anchor (or an EX anchor) is a two-step process. The first step includes providing a temporary connection between the mullion and the anchor, Figure 32.5(b). After all mullion lengths are correctly aligned, a permanent connection between the mullion and the anchor is made.  A permanent connection requires field drilling into the mullion through predrilled holes in the anchors, Figure 32.5(c), (d), and (e). Predrilled holes in anchors provide for field adjustment to cater to the (allowed) dimensional variations in the structural frame of the building.  As with other curtain walls (precast concrete, GFRC, natural stone, etc.), the anchorage system of a glass curtain wall to the building’s structure is typically provided by the curtain  wall’s manufacturer. The installation of the wall is generally done by the manufacturer’s own installation crew or by an approved third-party installer. For some simple curtain walls that utilize a manufacturer’s standard sections, an installer may provide all detailing assistance to the architect.

Nut and bolt sleeve embedded in spandrel beam allows horizontal adjustment for anchor location

(a) Connection of a DL or EX  anchor to spandrel beam

Spandrel beam

Anchor comprises two steel angles welded to a steel plate Slotted hole in anchor provides vertical adjustment of anchor location

Aluminum expansion splice  fastened to mullion Horizontal slotted hole in the anchor and vertical slotted hole in the mullion provide adjustability in the connection of mullion to anchor. This is a temporary connection. After this connection is made, the (permanent) dead-load support connection of the mullion to the anchor is made through one of the  two round holes above; see also Figure 32.5(c).

Two holes in anchor for DEAD-LOAD support of mullion; see also Figure 32.5(c) Vertical face of spandrel beam Plate provides permanent connection of anchor to spandrel beam. After the anchor is in the desired location, the plate (with a hole that just fits the bolt diameter) is welded to the anchor.

(Vertical) slotted hole in mullion (Hortizontal) slotted hole in anchor

(b) Temporary connection of mullion to anchor

FIGURE 32.5 Typical anchorage details of a mullion to a spandrel beam. 784

Bolt and washer for  temporary connection. After the mullion has been permanently anchored, this bolt is removed, as shown in Figure 32.5(c).

Two same-size round holes in anchor  for DEAD-LOAD support of mullion. Erector field drills into the mullion  through the more suitable of the two holes for permanent connection of   the mullion to the anchor.

Slotted hole in mullion Slotted hole in anchor Temporary connection bolt removed after making permanent connection

This bolt is removed after making permanent connection

(c) Dead-load anchor support of mullion to a reinforced-concrete spandrel beam Two slotted holes in the anchor for EXPANSION ANCHORAGE support of the mullion. Erector field drills into the mullion  through the more suitable of the two slotted holes for permanent expansion anchorage of the mullion. DL anchor Pour stop engineered  to support the loads

(d) Dead-load support of mullion to a steel spandrel beam FIGURE 32.5 (continued )

(e) Expansion-anchor support of mullion to a reinforced-concrete spandrel beam

Typical anchorage details of a mullion to a spandrel beam.

785

PRACTICE

QUIZ

Each question has only one correct answer. Select the choice that best answers the question. 1. A stick-built glass curtain wall consists of a. mullions and glass. b. rails and glass. c. mullions, rails, and glass. d. mullions, rails, and preassembled units. e. preassembled units and glass.

the wall. 8. In a twin-span mullion system for a glass curtain wall, a. one dead-load anchor is provided at every floor level. b. two dead-load anchors are provided at every floor level. c. one dead-load anchor is provided at every alternate floor level. d. two dead-load anchors are provided at every alternate floor level. e. only one dead-load anchor is provided for the entire height of

2. A unitized glass curtain wall consists of a. mullions and glass. b. rails and glass. c. mullions, rails, and glass. d. mullions, rails, and preassembled units. e. preassembled units and glass.

the wall.

3. Glass curtain walls in which aluminum framing sections are specially

profiled for a particular project are a. rare because of the prohibitive cost of manufacturing custom profiles. b. uncommon because of the extremely high cost of manufacturing custom profiles. c. not uncommon because the cost of custom profiles can be recovered from a few repeat mid-sized to large projects. d. fairly common because the cost of custom profiles can be recovered from one large project. 4. In a stick-built glass curtain wall, the mullions are typically spaced at a. 2 ft to 4 ft on center. b. 4 ft to 6 ft on center. c. 6 ft to 10 ft on center. d. 10 ft to 15 ft on center. e. as needed for the project. 5. In a stick-built glass curtain wall, the rails are typically spaced at a. 2 ft to 4 ft o.c. b. 4 ft to 6 ft o.c. c. 6 ft to 10 ft o.c. d. 10 ft to 15 ft o.c. e. as needed for the project. 6. A stick-built glass curtain wall is anchored to the building’s structure

through a. mullions. c. both mullions and rails.

7. In a single-span mullion system for a glass curtain wall, a. one dead-load anchor is provided at every floor level. b. two dead-load anchors are provided at every floor level. c. one dead-load anchor is provided at every alternate floor level. d. two dead-load anchors are provided at every alternate floor level. e. only one dead-load anchor is provided for the entire height of

b. rails. d. none of the above.

9. In a single-span mullion system for a glass curtain wall, a. one expansion anchor is provided at every floor level. b. two expansion anchors are provided at every floor level. c. one expansion anchor is provided at every alternate floor level. d. two expansion anchors are provided at every alternate floor level. e. only one expansion anchor is provided for the entire height of

the wall. 10. In a twin-span mullion system for a glass curtain wall, a. one expansion anchor is provided at every floor level. b. two expansion anchors are provided at every floor level. c. one expansion anchor is provided at every alternate floor level. d. two expansion anchors are provided at every alternate floor level. e. only one expansion anchor is provided for the entire height of

the wall. 11. The width of an expansion joint between adjacent mullion lengths in

a typical stick-built glass curtain wall a. is generally 1 in. standard. b. is generally 12 in. standard. c. is generally 14 in. standard. 1 d. is generally 16 in. standard. e. must be determined on a project-by-project basis.

32.3 STICK-BUILT GLASS CURTAIN WALL DETAILS  After the mullions have been anchored to the structural frame, the remaining items in the  wall’s erection require • Connection of the rails to the mullions • Installation of the glass.

R AI L - TO -M ULLION  C ONNECTION Manufacturers use various methods to connect the rails to the mullions. A commonly used method involves short aluminum extrusions, called shear blocks, which are fastened to the mullions with screws. Subsequently, the rails are snapped over the shear blocks, one shear block at each end of a rail, Figure 32.6. Thus, no fasteners are used between the rail and the shear blocks. Because the length of each rail is small (4 ft to 6 ft is typical), a fairly small space is 1 required for the expansion or contraction of a rail. In general, the length of a rail is 16 in. less than the clear distance between mullions.

O UTSIDE -G LAZED

AN D  I NSIDE -G LAZED  C URTAIN  W ALLS

One of the factors that determines the cross-sectional shapes of mullions and rails i s whether the glass in the wall is to be installed from the outside or the inside of the building, referred to, respectively, as • Outside-glazed curtain walls • Inside-glazed curtain walls 786

Mullion Rail snapped over shear blocks Sealant Shear block, one on each end of a rail Screw spline in shear block to  fasten shear block to mullion

Mullion Screw spline in shear block

Shear block

Rail Length of rail is 1/16 in. shorter  than clear distance between mullions to allow thermal expansion and contraction of rail FIGURE 32.6

Bottom cover is snapped onto the rail after the rail has been snapped to shear blocks

Typical connection between rails and mullions of a stick-built glass curtain wall.

In an outside-glazed wall, the glass panes are installed from the outside of the building by  workers standing on a scaffold or staging. This method of installing glass is less efficient and more expensive due to the cost of scaffolding or stagi ng. It is generally used for low- to midrise buildings. The glass in an outside-glazed wall can be secured in two ways: • Pressure plate–captured glass (Figures 32.7 to 32.9) • Structural silicone sealant–adhered glass (Figure 32.10) In an inside-glazed wall, the glass is installed by workers standing on the appropriate floor of the building. The system is more efficie nt because it does not require scaffolding or staging. It is the system of choice for high-rise buildings. However, the cross-sectional shapes of mullions and rails for the inside-glazed system are more complex than the corresponding shapes for the outside-glazed system.

O UTSIDE -G LAZED  W ALLS  (P RESSURE  P LATE –C APTURED  G LASS ) In an outside-glazed curtain wall, the glass is held by horizontal and vertical pressure plates,  which are fastened to the mullions and rails with screws. A plastic insert is used between the pressure plate and the mullion (or the rail), which functions as a thermal separator. The pressure plates are finally covered with snap-on covers, Figure 32.7 . Because the covers are the only externally visible part of the curtain wall frame , they have a major influence on the curtain wall’s appearance. The covers can be profiled into various shapes, Figure 32.8. The exterior and interior gaskets should prevent water from leaking through the wall. However, a curtain wall system typically includes accommodations for the drainage of  water, should it penetrate beyond the gaskets. This is accomplished through drainage weep holes in the pressure plates and the covers. Thus, in a typical curtain wall, each glass-pane frame is drained independently. Figure 32.9 shows typical sections through a pressure plate–captured outside-glazed curtain wall. 787

Gasket

Pressure plate Snap-on cover

Mullion

Insulating glass unit in vision area of  curtain wall (1 in. thick typical) Rail

Gasket

Pressure plate fastened to rail; see also Figure 32.9 Snap-on cover—aesthetically the most important component of a curtain wall. Manufacturers provide covers in anodized finish or painted finish in various colors. Custom cross-sectional cover profiles can also be obtained; see Figure 32.8. Monolithic glass (typically 1/4 in. thick heat-strengthended glass) in spandrel area of curtain wall. The interior surface of glass contains fired-on ceramic frit opacifier or a polyester film opacifier toward the interior so that  this glass approximates the IGU in appearance. An IGU (the same as in the vision area) may also be used in the spandrel area in place of a monolithic glass; see also Chapter 30, Figure 30.12. FIGURE 32.7 (a)

788

Anatomy of an outside-glazed glass curtain wall (pressure plate–captured glass).

Chapter 32 Exterior Wall Cladding–IV (Wall Systems in Glass)

Small length of  pressure plate

Full-length pressure plate

FIGURE 32.7 (b)

In fastening the pressure plates to curtain wall framing members, the glass is temporarily held by small pressure plate members. After several glass panes are in position, the temporary pressure plates are removed and replaced by full-length pressure plates.

Custom mullion

Rail

Standard cover Snap-on custom cover

FIGURE 32.8

A custom cover and custom mullion for an outside-glazed glass curtain wall. (Photo courtesy of Vistawall Architectural Products)

789

Vision glass

Q

P Spandrel glass

Mullion

Vision glass

Spandrel glass Inside tape

R Vision glass

Vision glass

Insulating glass unit Gasket (also  functions as  thermal separator)

Thermal separator

Detail P

Pressure plate  fastened to mullion Snap-on cover

FIGURE 32.9 (a)

Typical details of an outside-glazed glass curtain wall (pressure plate–captured glass). Aluminum sections used in the details are by Vistawall Architectural Products. Other manufacturers provide similar sections.

790

Insulating glass unit (1 in. thick  typical) Gasket (also functions as  thermal separator)

Inside glazing tape Rail

WEEP HOLES in pressure plate here Thermal separator Snap-on cover Pressure plate fastened to rail WEEP HOLES in cover here Adapter for spandrel glass Spandrel glass; see also Figure 32.7 

DETAIL Q Spandrel glass; see also Figure 32.7  Adapter for spandrel glass (unnecessary if 1-in.-thick stone spandrel or insulating glass is used)

Rail

Setting blocks (2 per glass pane) WEEP HOLES in pressure plate here Snap-on cover Pressure plate fastened to rail

Inside glazing tape

WEEP HOLES in cover Insulating glass unit (1 in.  thick typical)

DETAIL R Typical details of an outside-glazed glass curtain wall (pressure plate–captured glass). Aluminum sections used in the details are by Vistawall Architectural Products. Other manufacturers provide similar sections. FIGURE 32.9 (b)

791

O UTSIDE -G LAZED  W ALLS  (S TRUCTURAL  S ILICONE S EALANT –A DHERED  G LASS )

Part 2 Materials and Systems of Construction

 Another version of an outside-glazed curtain wall is one in which the glass is held by structural silicone sealant. In this type of system, the vertical edges of a glass pane are adhered to the mullions with beads of structural silicone sealant. The mullions in this wall are similar to those of an outside-glazed wall without the mullion nose. The horizontal edges of the glass are supported on rails and anchored to them through standard pressure plates, Figure 32.10. The absence of vertical pressure plates in the system accentuates the horizontality of the covers.

I NSIDE -G LAZED  W ALLS In an inside-glazed wall, pressure plates are not used. Therefore, the aluminum curtain wall sections are different from those used for the outside-glazed wall. These sections include glazing pockets—in both mullions and the bottom rail—of an opening. The top rail of the opening is open and has no glazing pocket. The openness allows the glass to be inserted in the opening. After the glass is inserted, a glazing stop is snapped on the top rail of the opening from the inside. This secures the glass in the opening, Figure 32.11 (b). Figure 32.11(c) shows a plan view of the process of inserting the glass. Other details of an inside-glazed wall are shown in Figure 32.11 (a) and (d). An important point to note is that in the inside-glazed wall, the mullion and rail covers must be installed before inserting the glass, Figure 32.12.

D ETAILING A  M ULTISTORY  G LASS  C URTAIN  W AL L Figure 32.13 shows the details of a typical multistory (outside-glazed) glass curtain wall at the floor, sill, and ceiling levels. At the sill level, an aluminum stool provides the interior finish. It is snapped to a continuous clip on one side, and its vertical leg is fastened to a treated wood nailer on the other side. Usually a heat-strengthened glass pane (with a fired-on ceramic frit opacification or a polyester film opacification on the interior surface of the glass to prevent seeing through it)

Vision glass

S Q

Spandrel glass

R Mullion

Structural silicone sealant Polyurethane spacer Insulating glass unit

DETAIL S

Details Q and R similar to Details Q and R in Figure 32.9(b)

Backer rod and sealant weather seal

FIGURE 32.10 Typical details of an outside-glazed curtain wall (structural silicone sealant–adhered glass). Aluminum section used in the

detail is by Vistawall Architectural Products. Other manufacturers provide a similar section. 792

Spandrel glass; see also Figure 32.7  Gasket

Thermal separator

Mullion

Rail

Setting block Weep holes here Snap-on cover Weep holes here Snap-on glazing stop

(b)

(a)

DETAIL at ceiling level

DETAIL at mullion Insulating glass unit Mullion

Thermal separator

Gasket

Rail

Cover Setting block Weep holes here Deep glazing pockets in mullions allow the glass  to be inserted from within the building. The open rail at the top of the opening facilitates  the insertion. The opening is closed with a glazing stop after the glass is in position; see (b) DETAIL at ceiling level.

(c)

DETAIL of glass insertion in opening

Adapter for spandrel glass Weep holes in cover here

(d)

DETAIL at sill level

FIGURE 32.11 Typical details of an inside-glazed curtain wall. Aluminum sections used in the details are by Vistawall Architectural Prod-

ucts. Other manufacturers provide similar sections.

is used in the spandrel area (see Figure 32.7(a)). A fire-containment assembly is generally required to prevent the passage of fire and smoke between the adjacent floors of the building. This assembly consists of semirigid mineral wool insulation pressure-fitted (and supported on metal clips) in the space between the curtain wall and the spandrel beam. To obtain a good seal, the insulation is topped with a liquid-applied, fire-resistive sealant. In addition to the fire-containment assembly, the entire spandrel area o f the curtain wall is provided with mineral wool insulation placed behind the spandrel glass. To prevent condensation between the glass and the insulation, a vapor retarder is provided in this area. This generally consists of an aluminum foil lamination on the insulation’s interior face.  Where a high degree of condensation potential exists, a metal panel (referred to as a metal back pan ) should be specified in place of an aluminum foil lamination. 793

Part 2 Materials and Systems of Construction

FIGURE 32.12 In an inside-glazed

curtain wall, the covers are installed before the glass. In this photo, an installer is installing the snap-on cover on the mullion from an upper floor, and is helped by an installer at the lower floor. In an outside-glazed curtain wall, the covers are generally installed after installation of the glass and pressure plates.

Semirigid mineral wood insulation and vapor retarder in spandrel area; see also Chapter 30, Figure 30.12

Spandrel glass; see also Figure 32.7 

Perimeter fire containment cover over aluminum  framing in spandrel area

Rail Snap-on closure

Ceiling

IGU in vision area

DETAIL at ceiling level Aluminum clip to provide snap-on connection to stool

IGU in vision area

Aluminum stool Screw fasten aluminum stool here Treated wood nailer

Spandrel glass; see also Figure 3.27 

Gypsum board Light-gauge steel stud wall with insulation

DETAIL at sill level

Perimeter fire containment cover over aluminum framing

Space between glass curtain wall and structural  frame

Floor slab

DETAIL at floor level

Semirigid mineral wool insulation and vapor retarder in spandrel area; see also Chapter 30, Figure 30.12

Fire-stop assembly in space between curtain wall and spandrel beam comprising compression  fit mineral wool and liquid fire-rated sealant. This assembly is known as a perimeter  fire-containment assembly.

FIGURE 32.13 Typical floor-level, sill-level, and ceiling-level details of an outside-glazed glass curtain wall. Note the fire-stopping in the

space between the spandrel beam and curtain wall. 794

PRACTICE Each question has only one correct answer. Select the choice that best answers the question. 12. The width of an expansion joint in the rails of a typical stick-built

glass curtain wall a. is 1 in. standard. b. is 12 in. standard. 1 1 c. is 4 in. standard. d. is 32 in. standard. e. must be determined on project-by-project basis. 13. In a stick-built glass curtain wall, the mullions are erected first and

the rails are inserted between them. a. True b. False 14. A shear block is used a. at a mullion expansion joint to allow the mullion to move. b. at the dead-load anchor of a mullion. c. at the expansion anchor of a mullion. d. to connect a rail to adjacent mullions. e. none of the above. 15. A glass curtain wall in which the glass is pressure plate captured is

glazed from the a. building’s interior. c. either (a) or (b).

b. building’s exterior.

QUIZ

16. As shown in this text, a glass curtain wall in which the glass is

structural silicone adhered is glazed from the a. building’s interior. b. building’s exterior. c. (a) or (b). 17. In a glass curtain wall with pressure plate–captured glass, there are a. no covers. b. covers in both horizontal and vertical directions. c. covers only in the horizontal direction. d. covers only in the vertical direction. 18. As shown in the text, a glass curtain wall with structural silicone–

adhered glass a. has no exterior covers. b. has exterior covers in both horizontal and vertical directions. c. has exterior covers only in the horizontal direction. d. has exterior covers only in the vertical direction. 19. A typical glass curtain wall is provided with weep holes to drain

infiltrating water even though it is sealed from both the inside and the outside with gaskets or tapes. a. True b. False

32.4 UNITIZED GLASS CURTAIN WALL  As shown in Figure 32.1(b), in a unitized glass curtain wall, the wall units are preassembled (and generally preglazed) in a fabrication shop and brought to the site for installation, so that the wall is assembled at the site, unit by unit, instead of assembling sticks of mullions and rails. Figure 32.14(a) to (d) show the important steps in the installation of a typical unitized wall to a building structure. The units are designed to mate with the adjacent units at the mullions, and at the top and bottom rails. The bottom rail of the upper unit connects to the top rail of the lower unit. As shown in Figure 32.14(b), the splices projecting from the top rail of the lower unit fit snugly into the void in the bottom rail of the upper unit. This detail provides the lateral-load resistance and is similar to the detail used in a stick-built wall (see Figure 32.3). The dead-load resistance of the unit is provided through its anchorage to the floor, Figure 32.14(d). Two adjacent units generally share the same dead-load anchorage.

32.5 STRUCTURAL PERFORMANCE OF A GLASS-ALUMINUM WALL The most important structural requirement of a glass-aluminum wall is its ability to resist lateral loads (particularly wind loads), including missile-impact resistance in hurricaneprone regions. Just as the design of a glass-aluminum wall’s anchorage to the structure is accomplished by the wall manufacturer (or installer, see Section 32.2), its lateral-loadresistance design is also provided by the manufacturer (or installer) based on the lateral-load intensities provided by the project architect or structural engineer. Manufacturers generally have several standard sections designed to suit various lateralload intensities. For high lateral-load intensities, a strategy often used is to enclose structural steel (or aluminum) sections within the mullions, Figure 32.15 . The enclosed steel sections and the mullions are fastened together to produce a composite action between them. Structural C- or I-sections are commonly used as enclosed sections. Channels provide the advantage of nesting, so that two or three channels may be used within the same mullion. The enclosed steel sections are suitably coated to prevent galvanic action between the aluminum and steel.  An alternative to enclosed steel sections is to anchor the mullions to an independent steel structural frame, Figure 32.16. This strategy is generally used in a tall glass-aluminum wall  where the mullions do not have intermediate supports to reduce their span, such as those provided by the floor structure in a multistory curtain wall. 795

(a) Using a crane, unitized curtain wall elements are carried up from the delivery truck to  the building facade

(b) The new unit is installed by placing both of its mullions over the (projecting) splices in the lower unit. The splices  function as expansion splices (see Figure 32.3) and also provide lateral-load support to the unit. The lower unit is anchored to the floor structure through a dead-load support to the unit.

Projecting splices (one on each side of the lower unit)  function as expansion splices and also as lif ting elements. Observe holes in splices to which lifting cables are attached. Unit already installed Dead-load anchor for the unit here; see (d) below 

(c) The new unit being forced over the (projecting) splice FIGURE 32.14

796

Installation of a unitized curtain wall element.

(d) Anchorage of unit to the floor structure, providing dead-load support

Chapter 32

Steel (or stainless steel) channel. Two or three channels can be nested for added strength.

Exterior Wall Cladding–IV

Aluminum mullion

(Wall Systems in Glass)

FIGURE 32.15

One of the ways to increase the lateral-load resistance of aluminum mullions is to enclose structural steel sections within them.

In this glass-aluminum wall, the supporting structure for aluminum curtain wall sections consists of steel pipe verticals. In a very tall curtain wall, vertical steel trusses and horizontal steel members or a steel space frame may be used. Aluminum curtain wall section

Steel pipe to provide lateral load support to curtain wall

FIGURE 32.16

A tall glass wall with standard curtain wall sections anchored to an interior structural steel vertical member to provide lateralload support to aluminum mullions. In this building, a steel pipe support is used. In taller walls, vertical steel trusses are common.

32.6 ENVIRONMENTAL PERFORMANCE CRITERIA FOR A GLASS CURTAIN WALL The nonstructural performance of a glass curtain wall is just as important as its structural performance. Among the important nonstructural design criteria for a glass curtain wall are • • • •

 Air-infiltration control Rainwater- and meltwater-penetration control U-value Solar heat gain 797

Part 2 Materials and Systems of Construction

• • • • • • •

Condensation resistance Vapor diffusion Sound transmission Hurricane resistance Seismic resistance Thermal and structural movement Glass-cleaning-equipment load

For standard curtain walls, manufacturers provide the values for these criteria based on the tests conducted by recognized third-party laboratories. For custom walls, technical design support is generally available from t he manufacturers. For a complicated wall design, the architect may need additional help from a curtain wall design consultant and a specialized testing laboratory to determine the wall’s performance.

A IR -I NFILTRATION  C ONTROL

NOTE cfm is an acronym for cubic feet per minute.

In the United States, the maximum air infiltration allowed through a glass curtain wall is typically 0.06 cfm/ft 2 under an inside-outside air pressure difference of 1.57 psf. In Canada, the requirement is three times more stringent—that is, 0.02 cfm/ft 2 under an air pressure difference of 1.57 psf.  Where lower air infiltration is required, curtain wall systems, which provide a rate of up to 0.01 cfm/ft2 under an inside-outside air pressure difference of 6.24 psf, are available. (Note that a 1.57-psf air pressure difference is equivalent t o that exerted by a wind speed of 25 mph. Similarly a 6.24-psf air pressure difference is equivalent to that created by a 50-mph wind speed; see Chapter 3.  Air-infiltration control not only conserves energy but also reduces ice buildup on the exterior of curtain wall components. Ice buildup is caused by the condensation of water vapor that escapes from the building’s interior along wi th air. When the ice melts, the melt water may leak into the building’s interior. Therefore, a more stringent air-infiltrationcontrol criterion is generally needed in colder climates.

R AINWATER -

NOTE AAMA and Water Penetration The American Architectural Manufacturers Association (AAMA) defines water penetration as the appearance of uncontrolled water other than condensation on the interior face of any part of the curtain wall.

AN D  M ELTWATER -P ENETRATION  C ONTROL

 Water-penetration control (of both rainwater and meltwater) is perhaps the most important nonstructural performance requirement of a glass curtain wall. Glass curtain wall systems are generally designed to ensure no water penetration when tested under a static air pressure difference (between the inside and the outside) that is at least 20% of the inward structural design  wind load on the curtain wall. Thus, if the inward structural design wind load on the wall is 50 psf, the system is tested for water penetration under a static air pressure difference of at least 10 psf. A more stringent water-penetration criterion is required for buildings located in areas sub jected to frequent and intense wind-driven rain. In addition to conforming to the static pressure criterion, a glass curtain wall is required to conform to dynamic pressure test criterion.  Water-penetration control is accomplished in different ways by system manufacturers; it typically includes adequate drainage in the aluminum joinery and glazing pockets. For example, in the stick-built glass curtain wall described earlier, weep holes are provided in the pressure plates and snap-on covers that drain the water to the outside. The architect’s details must also ensure the management of water entering the curtain wall from a nonglass facade that is above the curtain wall, where such a facade exists.

U-V ALUE , S OLAR  H EAT  G AI N ,

AN D  C ONDENSATION  R ESISTANCE

These three interrelated criteria are a function of the type of glass, the type of aluminum framing (thermally improved or not), and the center-to-center spacing of framing members, as explained in Chapter 30. The architect must specify their values in consultation  with the HVAC consultant (who also needs these values to design the building’s HVAC system and to meet the energy code requirements).

V APOR  D IFFUSION  A NALYSIS  As previously stated, interior water vapor may result in ice buildup on curtain wall framing and condensation of water vapor in the building’s interior. These issues are particularly critical in cold climates, where a vapor analysis of the curtain wall system is generally required.

S OUND  T RANSMISSION Glass curtain walls in buildings located in areas where high levels of exterior noise are present (e.g., near airports or busy highways) may need a higher sound-transmission-loss specification than other areas. 798

H URRICANE  I MPACT  R ESISTANCE

AN D  S EISMIC  R ESISTANCE

Because of Florida’s experience with exte nsive wind damage to glass curtain walls from hurricanes, several coastal cities in the United States are requiring that glass curtain walls be missile-impact resistant, particularly in the lower floors of the building. Similarly, because of California’s experience with earthquake damage to glass curtain walls, resistance to shaking, glass drifting, and horizontal movement of components is required for buildings in seismically active areas.

T HERMAL

Chapter 32 Exterior Wall Cladding–IV (Wall Systems in Glass)

AN D  S TRUCTURAL  M OVEMENT

 Aluminum-framing members and glass expand and contract due to temperature changes and the sudden cooling effects of precipitation. Glass curtain walls require sufficient expansion and contraction control built into them to allow thermal movement and the movement of spandrel beams due to live-load deflection. The expansion splice required in a stick-built curtain wall (Figure 32.3) accounts for this requirement.

G LASS -C LEANING -E QUIPMENT  L OA D High-rise curtain walls include provisions for peri odic cleaning. This means that they must include anchorage points for the staging of cleaning equipment that is lowered from the roof to the front of the curtain wall. These anchors add point loads on the wall, and this information needs to be communicated by the architect to the system manufacturer.

32.7 OTHER GLASS-ALUMINUM WALL SYSTEMS In addition to curtain walls, two additional glass-aluminum wall systems commonly used are • Punched and strip glazing • Storefront system

P UNCHED

AN D  S TRIP  G LAZING

Punched glazing is similar to a punched window (see the window terminology in Chapter 31), except that the glass in punched glazing is generally fixed and site installed due to its large size. By contrast, a punched window is generally shop glazed and may contain operable sashes. The frame for punched glazing may either be shop assembled or stick-built on site. The frame is anchored to the opening (jambs, head, and sill) instead o f being anchored to the building’s structural frame (as in a curtain wall). Strip (also called ribbon ) glazing is similar to punched glazing, with several glazing units placed in a linear alignment. Strip glazing is also anchored to the head and sill of an opening.

S TOREFRONT  S YSTEM  A storefront is a large glass-aluminum wall that is generally one story tall and extends from the ground to the second floor of the building. Three major differences distinguish a curtain wall from a storefront. They are: (a) The storefront wall lies under the second-floor structure of the building and (unlike

a curtain wall), it is not spaced away from the structural frame of the building. Often, a storefront is protected by an overhang to control water leakage through the system. (b) The glazing system’s performance for structural and nonstructural criteria (air infiltration, water penetration, CRF, etc.) is lower than that of a curtain wall. (c) Rainwater that enters a curtain wall is drained by the (horizontal) rail support of each individual lite. In a storefront system, the water entering through a lite must travel vertically down the mullion and be drained at the weep holes at the ground level.

32.8 NONTRADITIONAL GLASS WALLS The glass-aluminum wall types discussed so far are traditional types that have evolved over a long period. Three recently introduced systems that do not rely on aluminum sections for support are • Glass wall supported by cable trusses • Glass wall supported by a pretensioned cable net • Double-skin glass walls 799

G LASS  W AL L  S UPPORTED

Part 2 Materials and Systems of Construction

BY  C ABLE  T RUSSES

 A sophisticated and unique version of a glass wall that is relatively uncommon (due to its cost) is the mullionless glass wall developed by the Pilkington Company. In this system, each glass pane is suspended at four corners by stainless steel spider-shaped connectors . The connectors are held by a horizontal truss consisting of stainless steel tension cables and compression struts. Several such trusses are anchored to vertical steel frames, Figure 32.17. The glass used in the system is generally a high-R-value, insulating glass unit with laminated and heat-soaked tempered glass (see Section 30.2). Only elastomeric, silicone sealant separates a glass pane from the adjacent panes.

G LASS  W AL L  S UPPORTED

BY A  P RETENSIONED  C ABLE  N ET

 Another recently introduced glass wall system is one that uses a grid of pretensioned (stretched) cables to support the glass. Conceptually, the support system resembles a two-directional grid of strings in a tennis racket. Because the strings in a tennis racket are highly stretched, they create a stiff plane. When held vertically, the stringed plane of the racket resembles a stiff wall that can resist lateral loads and can be used to suspend planar elements from it. Similar to the stringed plane of a tennis racket, a cable-net-supported glass wall consists of pretensioned (approximately 1-in.-diameter) stainless steel cables arrayed in both the horizontal and vertical directions. The vertical cables are stretched between the top and bottom of an opening—generally between the spandrel beams at the upper and lower floors. The horizontal cables are stretched between the sides of the opening—generally

Cable-and-strut truss Strut

Vertical steel  truss

A typical spider connector. Each connector holds one corner of all four insulating glass units that meet at the connector. Elastomeric silicone sealant seals the edges between the glass units.

This glass wall is supported by a two-directional (consisting of  horizontal and vertical elements) backup structure. The horizontal elements of the backup structure consist of trusses with tension cables and compression struts. The vertical elements c onsist of   trusses made of steel pipes. The glass is held by spider connectors, which are connected to the compression struts. At a level close to  the floor where tension cables and compression struts cannot be used, glass fins have been used to provide lateral support to the glass. Pilkington’s Planar Glass Wall System used in the Rose Center for Earth and Space, New York City (see also Chapter 30, Figure 30.4). Architects: Polshek Partnership. FIGURE 32.17

800

between the columns or walls supporting the building. Two examples of cable-netsupported glass walls are shown in Figure 32.18(a) and (b). The horizontal and vertical cables are held together at intersections through special stainless steel connectors, which also serve as points for securing the glass to the grid. A typical connector is shown in Figure 32.18(c). The glass panels generally consist of i nsulating glass units with laminated and heat-soaked tempered glass. Because the cables are highly stressed, they impose a large load on the boundary elements of the opening—the two spandrel beams and the columns (or walls)—which must be designed to resist this additional load.

Chapter 32 Exterior Wall Cladding–IV (Wall Systems in Glass)

(a) Cable-net glass wall in the entrance lobby of the One North Wacker Building, Chicago, Architects: Goettsch Partners.

(b) Cable-net glass in the atrium of the Time Warner Center, New York City. Architects: Skidmore, Owing and Merril.

(c) Typical connector at a node—the point of  intersection between cables. The connector connects the cables together and also provides suppor to the glass. Four glass corners meet at each connector.

FIGURE 32.18

Two examples of cable-net-supported glass walls. Note that all photographs have been taken from inside the buildings. 801

D OUBLE -S KI N  G LASS  W ALLS

Part 2 Materials and Systems of Construction

PRACTICE

 Another innovative glass wall system is the double-skin wall system, also referred to as a bioclimatic glass wall. In this system, two glass walls, separated by 1 ft to 5 ft of air space, are used. The air space serves as a buffer between the two skins, tempering the outdo or air, and also serves as a plenum for the building’s HVAC system. The outer skin may include computer-controlled operable glazing and solar shading devices.  Although the primary benefit of a double-skin system is energy conservation, it also provides effective water-penetration control, better air-infiltration control, higher sound insulation, and so on. The proponents of the system, which has been used extensively in Europe, claim it to be more sustainable under the present energy prices in Europe, which are considerably higher than those in the United States.  As smart glazing systems with environment-adaptive technologies (e.g., electrochromic glass) and energy-generating capabilities (e.g., photovoltaic glass) evolve further, doubleskin glass wall systems may become more popular. In that scenario, the outer glass skin may not only help conserve energy but also generate some energy to power the building.

QUIZ

Each question has only one correct answer. Select the choice that best answers the question. 20. The air infiltration rate through a glass curtain wall is specified in

terms of a. pounds per square feet. b. cubic feet per minute. c. cubic feet under a given inside-outside air pressure difference. d. cubic feet per minute under a given inside-outside air pressure difference. e. none of the above. 21. In a glass wall supported by cable trusses, the glass a. bears on (horizontal) rails. b. is connected to (vertical) mullions.

c. both (a) and (b) above. d. none of the above. 22. In a cable-net-supported glass wall, the glass a. bears on (horizontal) rails. b. is connected to (vertical) mullions. c. both (a) and (b) above. d. is supported by a spider-shaped connector. e. none of the above. 23. The environmental performance requirements for a storefront are

generally much higher than those for a glass curtain wall. a. True b. False

REVIEW QUESTIONS 1. Using sketches and notes, explain the differences between a single-span mullion support system and a twin-span mullion support system for a glass curtain wall. 2. Use three-dimensional sketches to illustrate each of the following: a. A typical dead-load anchor used in a glass curtain wall b. A typical expansion anchor used in a glass curtain wall 3. Sketch in three dimensions a typical spider-shaped connector. 4. List the major differences between a glass curtain wall and a storefront. 5. Explain the purpose of the following items and state where they are used: (a) shear block, (b) pressure plate, and (c) adapter for spandrel glass. 6. Using sketches and notes, explain how we increase the lateral load-bearing capacity of a standard glass-aluminum wall.

802