chapter 32 exterior wall cladding-iv (wall systems in glass) (1)

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

CHAPTER

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

32.4 UNITIZED GLASS CURTAIN WALL

32.5 STRUCTURAL PERFORMANCE OF A GLASS-ALUMINUM WALL

32.6 ENVIRONMENTAL PERFORMANCE CRITERIA FOR A GLASS CURTAIN WALL

32.7 OTHER GLASS-ALUMINUM WALL SYSTEMS

32.8 NONTRADITIONAL GLASS WALLS

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 hori-zontal aluminum framing members. Therefore, they share some of the characteristics of their smaller counterparts—the aluminum windows, discussed in Chapter 31 . However, there are many differences between the two: scale, aesthetic character, 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 more 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 sys-tems (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.

Part 2Materials and Systems of Construction

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32.1 GLASS-ALUMINUM CURTAIN WALLS

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 Architec-tural 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 . 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.

STANDARD AND CUSTOM CURTAIN WALL SYSTEMS Most major glass curtain wall manufacturers have their own facility for extruding the alu-minum 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 cur-tain walls are fairly common. Custom walls should, however, be tested for performance before they are used in a project. Performance data for standard walls are available from the manufacturers.

Walls made from standard components are obviously more economical. However, 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 com-ponents in a wall becomes excessive, the cost of a standard wall may approach that of a custom wall.

(a) STICK SYSTEMIn the stick system, the curtain wall is installedpiece by piece at the site. Generally, the mullionsare installed first, followed by the rails. Subsequentlythe glass panes are installed within the mullion-railframework. The anchorage of the wall to the structuralframe is through the mullions. The mullions mayspan from floor to floor or over two floors. Thermalexpansion and contraction of mullions areaccommodated by expansion joints in mullions. The system components are shop-fabricatedand shipped to the construction site in a knocked-down (KD) version. Therefore, the system hasrelatively low shipping costs and also permits a greater degree of on-site adjusment as comparedto the other systems. Its disadvantages include longer on-siteassembly time and more on-site labor than theother systems.

Mullion expansion splice

Anchor

Spandrel beam

Vertical member (mullion)

Horizontal member (rail)

Mullion expansion splice

(see also Figure 32.3)

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

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(b) UNITIZED SYSTEMA unitized system consists of framed wall units that are shop-fabricated, preassembled, and generally preglazed. The units aredesigned so that the vertical and horizontal members in adjacentunits interlock to form common mullions and rails. The units may beone or two stories high. They are anchored to the building’s structuralframe in essentially the same way as the mullions in the stick system. The advantage of this system is its greater degree of qualitycontrol resulting from shop fabrication. Its disadvantages are thegreater shipping cost because of the added bulk from assembled units, the need of a greater degree of protection of units duringtransporation, and a lower degree of field adjustment.

(d) PANEL SYSTEMThe 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 horizontaland vertical elements. The panels can be formed by stamping or casting. The casting systemis economical only where a large number of identical panels are needed.

(c) UNIT AND MULLION SYSTEMThe unit and mullion system combines theadvantages 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 compromisebetween 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.

Anchor

Spandrelbeam

Preassembledunit

Anchor

Spandrelbeam

Preassembledunit

Anchor

Mullion

Preassembledunit

Spandrelbeam

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 struc-tural 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.

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DEAD-LOAD ANCHORS AND EXPANSION ANCHORS 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 mul-lions in a wall are installed first; then the rails are inserted between them. Three rails are commonly used per floor to create two separate 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.

(e) COLUMN COVER AND SPANDREL SYSTEMThis system, though not a true glass curtain wall system, consists of separate column coversconnected to spandrel covers that generally spanfrom 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 expressionof the structural system rather than concealing itbehind a (more homogeneous) wall.

Column cover

Spandrelpanel

Glazinginfill

Spandrelbeam

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)

Officefloors

Parkingfloors

After the framing of the curtain wall(mullions and rails) for the parking floorsis complete, its glazing has started, whilethe framing for the office floors is yet tobegin. Because there are no vision areason the parking floors, the glazing on eachfloor has been divided into two parts by oneintermediate rail. The purpose of the railis 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).

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To allow for the expansion and contraction of mullions caused by temperature changes, each mullion must be provided with expansion joints. Thus, the mullions con-sist 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 deflec-tion 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 immo-bile 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 struc-tural 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, where the building’s foun-dation 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 mul-lion 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.

In this photograph, the glazing of the curtain wall on the parking floors of the building shown in Figure 32.2(a) is almostcomplete. 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 theprogress in the installation of rails on theoffice floors. Because an office floor hasseparate spandrel and vision glass areas,there will be three rails per floor.

Officefloors

Parkingfloors

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).

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

Lower mullion length

Plastic shim separates the steel washerfrom the aluminum mullion to prevent a galvanic reaction between the steel and aluminum.

Mullion anchor; seeFigure 32.5(a)

Top of spandrelbeam

Upper mullion length

Expansion joint between upper and lower mullionlengths. The width of this joint must bedetermined based on the creep in columns (if any),deflection of the spandrel beam, and the thermalmovement of the mullion length.

Face of spandrel beam

Lower mullion length

FIGURE 32.3 A typical expansion joint between two mullion lengths.

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DL ANCHOR

DL ANCHOR

DL ANCHOR

EX ANCHOR

Mu

llio

n s

pa

nM

ullio

n s

pa

nM

ullio

n s

pa

nM

ullio

n s

pa

nM

ullio

n s

pa

n

Expansion joint

Expansion joint

Expansion joint

Expansion joint

Expansion joint

Dead-load support

DL ANCHOR

Upper mullion

Expansionsplice

Lower mullion

Spandrelbeam

DL ANCHOR DETAILSee also Figures32.5(b), (c) and (d)

DL ANCHOR

EX ANCHOR

DL ANCHOR

EX ANCHOR

EX ANCHOR

EX ANCHOR DETAILSee also Figures32.5(b), (c) and (d)

Dead-load support

Expansion joint

Expansion joint

Expansion jointM

ullio

n s

pa

nM

ullio

n s

pa

nM

ullio

n s

pa

nM

ullio

n s

pa

n

Expansionsplice

Spandrelbeam

Upper mullion

Lowermullion

(a) Single-span mullion support system (b) Twin-span mullion support system

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

Each mullion length spans two floors and is providedwith a DL anchor support at alternate floors. Eachmullion slides freely at both ends over the expansionsplices. At these ends, the mullion is anchored to EXanchors.

FIGURE 32.4 Support systems for single-span and twin-span curtain wall mullions. Observe that each mullion has only one dead-load support.


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ANCHORING A MULLION TO A DL OR EX ANCHOR 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 assist-ance to the architect.

(a) Connection of a DL or EXanchor to spandrel beam

Nut and bolt sleeve embedded inspandrel beam allows horizontaladjustment for anchor location

Anchor comprises twosteel angles welded toa steel plate

Slotted hole in anchorprovides vertical adjustmentof anchor location

Spandrel beam

(b) Temporary connection of mullion to anchor

Aluminum expansion splicefastened to mullion

Horizontal slotted hole in the anchorand vertical slotted hole in the mullionprovide adjustability in the connection of mullion to anchor.This is a temporary connection.After this connection is made, the(permanent) dead-load supportconnection of the mullion to theanchor is made through one of thetwo round holes above; see alsoFigure 32.5(c).

(Vertical) slotted hole in mullion

(Hortizontal) slotted hole in anchor

Two holes in anchor for DEAD-LOADsupport of mullion; see also Figure 32.5(c)

Vertical face of spandrel beam

Plate provides permanentconnection of anchor to spandrel beam. After theanchor is in the desiredlocation, the plate (with a hole that just fits the boltdiameter) is welded to theanchor.

Bolt and washer fortemporary connection.After the mullion hasbeen permanently anchored, this bolt isremoved, as shown inFigure 32.5(c).

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

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

Slotted holein mullion

Slotted hole inanchor

This bolt isremoved aftermaking perma-nent connection

Temporary connectionbolt removed aftermaking permanentconnection

(c) Dead-load anchor support of mullion to a reinforced-concrete spandrel beam

Two slotted holes in the anchor for EXPANSION ANCHORAGEsupport of the mullion. Erector field drills into the mullionthrough the more suitable of the two slotted holes forpermanent expansion anchorage of the mullion.

DL anchor

Pour stop engineeredto support the loads

(d) Dead-load support of mullion to a steelspandrel beam

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

FIGURE 32.5 (continued) Typical anchorage details of a mullion to a spandrel beam.

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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.

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.

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. b. rails. c. both mullions and rails. d. none of the above.

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

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

the wall.

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. d. is generally 116 in. standard. e. must be determined on a project-by-project basis.

PRACTICE QUIZ

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.

RAIL-TO-MULLION CONNECTION 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 mul-lions 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 required for the expansion or contraction of a rail. In general, the length of a rail is 116 in. less than the clear distance between mullions.

OUTSIDE-GLAZED AND INSIDE-GLAZED CURTAIN WALLS One of the factors that determines the cross-sectional shapes of mullions and rails is 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

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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 staging. It is generally used for low- to mid-rise 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 efficient 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 corre-sponding shapes for the outside-glazed system.

OUTSIDE-GLAZED WALLS (PRESSURE PLATE–CAPTURED GLASS) 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.

Mullion

Sealant

Shear block, one oneach end of a rail

Screw spline in shear block tofasten shear block to mullion

RailLength of rail is 1/16 in. shorterthan clear distance betweenmullions to allow thermalexpansion and contraction of rail

Rail snapped over shear blocks

Mullion

Screw spline inshear block

Shear block

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

FIGURE 32.6 Typical connection between rails and mullions of a stick-built glass curtain wall.

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Mullion

Insulating glass unit in vision area ofcurtain wall (1 in. thick typical)

Rail

Gasket

Pressure plate fastened to rail; see alsoFigure 32.9

Snap-on cover—aesthetically the mostimportant component of a curtain wall.Manufacturers provide covers in anodized finishor painted finish in various colors. Customcross-sectional cover profiles can also beobtained; see Figure 32.8.

Monolithic glass (typically 1/4 in. thick heat-strength-ended glass) in spandrel area of curtain wall. The interiorsurface of glass contains fired-on ceramic frit opacifieror a polyester film opacifier toward the interior so thatthis glass approximates the IGU in appearance. An IGU(the same as in the vision area) may also be used in thespandrel area in place of a monolithic glass; see alsoChapter 30, Figure 30.12.

Gasket

Pressureplate

Snap-oncover

FIGURE 32.7 (a) Anatomy of an outside-glazed glass curtain wall (pressure plate–captured glass).

Chapter 32Exterior Wall Cladding–IV

(Wall Systems in Glass)

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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)

Full-length pressure plate

Small length ofpressure 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.

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Visionglass

Visionglass

Visionglass

Visionglass

Spandrelglass

Spandrelglass

P Q

R

Detail P

Mullion

Inside tape

Insulating glassunit

Gasket (alsofunctions asthermal separator)

Thermal separator

Pressure platefastened 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.

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Insulating glass unit (1 in. thicktypical)

Gasket (also functions asthermal separator)

WEEP HOLES in pressureplate here

Thermal separator

Snap-on cover

Pressure plate fastened torail

WEEP HOLES in cover here

Adapter for spandrel glass

Spandrel glass; see alsoFigure 32.7

DETAIL Q

DETAIL R

Spandrel glass; see also Figure32.7

Adapter for spandrel glass(unnecessary if 1-in.-thick stonespandrel or insulating glass is used)

Setting blocks (2 per glass pane)

WEEP HOLES in pressure plate here

Snap-on cover

Pressure plate fastened torail

WEEP HOLES in cover

Insulating glass unit (1 in. thick typical)

Inside glazing tape

Inside glazing tape

Rail

Rail

FIGURE 32.9 (b) 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.


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OUTSIDE-GLAZED WALLS (STRUCTURAL SILICONE SEALANT–ADHERED GLASS) Another version of an outside-glazed curtain wall is one in which the glass is held by struc-tural 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 hori-zontality of the covers.

INSIDE-GLAZED WALLS 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 open-ing 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 .

DETAILING A MULTISTORY GLASS CURTAIN WALL 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)

S

Q

R

Visionglass

Spandrelglass

Mullion

Structuralsilicone sealant

Polyurethanespacer

Insulating glassunit

Backer rod and sealantweather seal

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

DETAIL S

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.

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Deep glazing pockets in mullions allow the glassto be inserted from within the building. Theopen rail at the top of the opening facilitatesthe 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 (d) DETAIL at sill level

Mullion Thermal separator

Cover

Setting block

Weep holeshere

Weep holes incover here

Insulating glass unit

Gasket Rail

Adapter forspandrel glass

Mullion

Spandrel glass; see alsoFigure 32.7

Thermal separator

Settingblock

Weep holeshere

Snap-oncover

Weep holeshere

Gasket

Rail

Snap-on glazing stop

(b) DETAIL at ceiling level

(a) DETAIL at mullion

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 build-ing. This assembly consists of semirigid mineral wool insulation pressure-fitted (and sup-ported 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 of the curtain wall is provided with mineral wool insulation placed behind the spandrel glass. To prevent con-densation 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.


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Spandrelglass; seealso Figure32.7

IGU in visionarea

RailCeiling

Snap-on closure

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

Perimeter fire containment cover over aluminumframing in spandrel area

DETAIL at ceiling level

IGU in visionarea

Spandrel glass; seealso Figure3.27

Aluminum stool

Screw fasten aluminum stool here

Treated wood nailer

Gypsum board

Light-gauge steelstud wall with insulation

Perimeter fire containment cover over aluminum framing

Space between glass curtain walland structuralframe

Floor slab

Semirigid mineral wool insulationand vapor retarder in spandrelarea; see also Chapter 30, Figure 30.12

Fire-stop assembly in space between curtainwall and spandrel beam comprising compressionfit mineral wool and liquid fire-rated sealant. This assembly is known as a perimeterfire-containment assembly.

DETAIL at sill level

DETAIL at floor level

Aluminum clip to providesnap-on connection to stool

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.

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.

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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 resist-ance 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 hurricane-prone 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-load-resistance 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 lateral-load intensities. For high lateral-load intensities, a strategy often used is to enclose struc-tural 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 pro-vide 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.


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. c. is 14 in. standard. d. is 132 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. b. building’s exterior. c. either (a) or (b).

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

PRACTICE QUIZ

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(a) Using a crane, unitized curtain wallelements arecarried up from thedelivery truck to the building facade

(b) The new unit is installed by placingboth of its mullions over the (projecting)splices in the lower unit. The splicesfunction as expansion splices (see Figure32.3) and also provide lateral-loadsupport to the unit. The lower unit is anchored to the floor structure througha dead-load support to the unit.

Projecting splices (one oneach side of the lower unit)function as expansion splicesand also as lifting elements.Observe holes in splices towhich lifting cables are attached.

Unit already installed

Dead-load anchor for theunit here; see (d) below

(c) The new unit being forced over the (projecting) splice (d) Anchorage of unit to the floor structure, providingdead-load support

FIGURE 32.14 Installation of a unitized curtain wall element.



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

Aluminum mullion

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

Aluminum curtainwall section

Steel pipe to provide lateral loadsupport 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 lateral-load support to aluminum mullions. In this building, a steel pipe support is used. In taller walls, vertical steel trusses are common.

FIGURE 32.15 One of the ways to increase the lateral-load resistance of aluminum mullions is to enclose structural steel sections within them.

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


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• 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 the manufacturers. For a complicated wall design, the architect may need additional help from a curtain wall design consultant and a special-ized testing laboratory to determine the wall’s performance.

AIR-INFILTRATION CONTROL 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/ft 2 under an inside-outside air pressure difference of 6.24 psf, are available. (Note that a 1.57-psf air pressure difference is equivalent to 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 with air. When the ice melts, the melt-water may leak into the building’s interior. Therefore, a more stringent air-infiltration-control criterion is generally needed in colder climates.

RAINWATER- AND MELTWATER-PENETRATION CONTROL 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 differ-ence (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 pres-sure 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-VALUE, SOLAR HEAT GAIN, AND CONDENSATION RESISTANCE 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 mem-bers, 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).

VAPOR DIFFUSION ANALYSIS 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 criti-cal in cold climates, where a vapor analysis of the curtain wall system is generally required.

SOUND TRANSMISSION 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.

cfm is an acronym for cubic feet per minute.

NOTE

AAMA and Water Penetration

The American Architectural Manufacturers Association (AAMA) defines water penetra-tion as the appearance of uncontrolled water other than condensation on the interior face of any part of the cur-tain wall.

NOTE



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HURRICANE IMPACT RESISTANCE AND SEISMIC RESISTANCE Because of Florida’s experience with extensive wind damage to glass curtain walls from hur-ricanes, 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 seismi-cally active areas.

THERMAL AND STRUCTURAL MOVEMENT Aluminum-framing members and glass expand and contract due to temperature changes and the sudden cooling effects of precipitation. Glass curtain walls require sufficient expan-sion and contraction control built into them to allow thermal movement and the move-ment 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.

GLASS-CLEANING-EQUIPMENT LOAD High-rise curtain walls include provisions for periodic 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

PUNCHED AND STRIP GLAZING 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 opera-ble 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 of 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.

STOREFRONT SYSTEM 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 cur-tain 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 infil-tration, 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


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GLASS WALL SUPPORTED BY CABLE TRUSSES 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 lami-nated and heat-soaked tempered glass (see Section 30.2). Only elastomeric, silicone sealant separates a glass pane from the adjacent panes.

GLASS WALL SUPPORTED BY A PRETENSIONED CABLE NET 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

This glass wall is supported by a two-directional (consisting ofhorizontal and vertical elements) backup structure. The horizontalelements of the backup structure consist of trusses with tensioncables and compression struts. The vertical elements consist oftrusses made of steel pipes. The glass is held by spider connectors,which are connected to the compression struts. At a level close tothe floor where tension cables and compression struts cannot beused, glass fins have been used to provide lateral support to the glass.

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

Cable-and-strut truss

Strut

Verticalsteel truss

FIGURE 32.17 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.



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between the columns or walls supporting the building. Two examples of cable-net- supported 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 insulat-ing 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.

(a) Cable-net glass wall in the entrance lobby of the One NorthWacker 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 ofintersection between cables. The connectorconnects the cables together and also providessuppor to the glass. Four glass corners meetat each connector.

FIGURE 32.18 Two examples of cable-net-supported glass walls. Note that all photographs have been taken from inside the buildings.


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DOUBLE-SKIN GLASS WALLS 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 outdoor air, and also serves as a plenum for the building’s HVAC system. The outer skin may include com-puter-controlled operable glazing and solar shading devices.

Although the primary benefit of a double-skin system is energy conservation, it also pro-vides effective water-penetration control, better air-infiltration control, higher sound insu-lation, 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 con-siderably 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, double-skin 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.


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

PRACTICE QUIZ

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.

REVIEW QUESTIONS

chapter 32 exterior wall cladding-iv (wall systems in glass) (1)

Documents

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