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8/6/2019 Bright Future Article

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Contributed article as seen in The Construction Specifier Feb. 2008 Edition

A Return to SteelPerformance and aesthetics of steel framing curtain walls

By Jeff Razwick

The benefits of designing with natural light are well known in the building industry. From energy and operating cost savings, to

improved occupant productivity and enhanced aesthetics, facility designs incorporating daylight are increasingly popular. Naturally,

much of the discussion on how to effectively implement daylighting focuses on the glazing material itself such as the effect of

various coatings, films, and tints on translucency, reflectivity, and emissivity, or the proper placement of windows and skylights. Yet,

a critical factor often overlooked is the framing systems role in ensuring an appropriate amount of light reaches interior spaces.

According to Gregg D. Ander, FAIA, of the Southern California Edison Company, the simplest method to maximize daylight within a

space is to increase the glazing area.1 This observation relates directly to the framing system. In comparison to traditional alumi-

num assemblies, newer steel framing systems enable narrower frame profiles and larger areas of uninterrupted glass, while meet-

ing a range of other performance and design needs.

In the first half of the 1900s, steel was a popular material for window framing. It was readily available, strong, and well understood

and accepted by architects and designers due to its use as a structural element throughout buildings. Steel framing was used to de-

velop revolutionary designs, including the first application of a glazed curtain wall in the Hallidie Building in San Francisco in 1918.

Over time, steel fell out of favor as a window framing material because it was heavy, and in early applications, prone to rust. In the

middle of the century, aluminum became many designers framing material of choice because it was lightweight, easy to work with

and malleable enough to be readily formed into different mullion profiles.

However, steel is once again emerging as a pioneering material for glazed windows and doors, as well as curtain walls. European

manufacturing processes now allow the alloy to be formed into a wide range of framing profiles, matching the flexibility in profile

choice provided by aluminum framing systems. Architects and designers in Europe have used these advanced steel framing systems

for years, which are only now being introduced in North America.

The frames are well adapted for numerous design applications, including large and open straight walls, curved and serpentine

walls, and vaulted ceilings and atriums. Due to steels greater strength, larger spans of glass and minimal frame dimensions are

also possible, which allow in more natural light and can improve the buildings aesthetics.

Technical properties of steel

In comparison to assemblies made of traditional materials, steel has many physical properties that provide superior performance

for glazed curtain walls and window and door framing systems.

Modulus of elasticity

Steel has a modulus of elasticity (i.e. Youngs modulus) of about 207 million kPa (30 million psi) compared to aluminums approxi-

mate 69 million kPa (10 million psi). With three times the stiffness value, steel frames of a given profile or cross-section deflect less,

providing substantially greater load capacity (Figure 1 p2). As a result, for a given framing profile cross-section, steel can achieve

larger glazing spans than aluminum (Figure 2 p2). Or, for identical load and deflection requirements, narrower steel frame profiles

can be employed. In either case, steel enables larger areas of uninterrupted glass with less framing than aluminum.

The difference in frame profile sizes can be dramatic, with steel framing systems allowing much thinner profiles than a typical

aluminum assembly. Figure 3 (p2) depicts a standard two-story curtain wall span with a given set of design criteria. In this specific


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application, steel frames have a face width of only 45 mm (1.75 in.),

versus a typical 64 mm (2.5 in.) face width of aluminum, providing a

significantly reduced sightline. Steel also allows for a narrower frame

depth: 146 mm (5.75 in.) back mullion versus a required depth of

almost 200 mm (8 in.) for a typical aluminum curtain wall. Internal

reinforcements can further reduce required frame depths.

A general rule of thumb is that for the same size frame profile, steelallows for glass panes approximately three times the surface area as

possible with aluminum framing. For fixed design criteria (e.g. load),

the primary benefit of steel is a more visually appealing system with

thinner framing.

Thermal expansion coefficient

The thermal expansion coefficient of steel is approximately 7.3 in/

in. F x 10-6 which is of the same general order of magnitude as glass

at 5.0 and concrete at 8.0, compared to an expansion coefficient for

aluminum of approximately 12.3 (Figure 4). Steels lower susceptibil-ity to thermal expansion allows for greater design and installation

flexibility as temperature changes, the glass, steel framing, and

surrounding concrete all expand and contract together, requiring

fewer, if any, framing expansion joints.

Figure 2: Glass Size Comparison

The strength of steel framing allows for glass sizes

larger than possible with aluminum framing, for the

same framing profile cross section.

Figure 3: Mullion Section Comparison

This image illustrates typical steel and aluminum

curtainwall mullion sections for a given set of design

criteria.

Figure 1: Wind Load Capacity Comparison

With a higher modulus of elasticity, steel framing has three times

greater wind load capacity than typical aluminum framing systems.

ALUMINUM

wind load

3XX

STEEL

wind load

1000 1000

2000 2000

2000

2000

ALUMINUMSTEEL

mullion section

typical aluminum curtainwall system

13/4"

(45)Dlo Dlo

2

1/8"

(54.4

mm)

5

3/4"

(146mm)

7

7/8"

(200.4

mm)

21/2"

(63.5)Dlo Dlo

10

1/2

"

(266.7

m

m)

7

15/16"

(202.2

mm)

2

9/16"

(64.5

mm)

mullion section

steelbuilt curtainwall system

oVerall heiGht: 20' ( d )

oVerall wiDth: 9' 113/16"

Glass panel siZe: 36" x 797/16"

DesiGn pressure: 25 psF

DeFlection: l/240+1/4" l/175 < 13'6" l/240+1/4" > 13'6"

curtainwall illustration DesiGn criteria


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Heat transfer

Compared to aluminum framing, steel systems reduce heat trans-

fer in two ways. First, steel has one-fourth the thermal conductiv-

ity of aluminum (approximately 31 Btu per hour for steel versus

about 118 Btu per hour for aluminum). Second, the potential for

narrower frame profiles provides a smaller area of metal through

which heat can pass. Together, these characteristics result in

lower heat loss/gain and reduced likelihood of interior condensa-tion (i.e. sweating) on frames. Specifically, National Fenestration

Rating Council (NFRC) 100 computer simulations on steel curtain

wall systems combined with 25 mm (1 in.) insulating glass units

(IGUs) incorporating clear low-emissivity (low-E) glass provided U-

values of 0.31 far exceeding the thermal performance of many

aluminum curtain walls.

Anti-corrosion and durability

To address earlier steel framings propensity to

rust, modern systems are available with highanti-corrosion protection, such as double-sided

pre-galvanization. With todays technology,

liquid zinc can be applied to thicknesses of up

to 20 (0.001 in.), or 28 g (0.9 oz ) per 0.09 m2

(1 sf), which is on par with ASTM International G

90, Standard Practice for Performing Accelerat-

ed Outdoor Weathering of Nonmetallic Materi-

als Using Concentrated Natural Sunlight.

Steel frames are therefore suitable for either

interior or exterior use. After the framing com-

ponents are fabricated and before installation,

they can be either powder- or wet-coated to

match any desired color scheme. In addition,

numerous stainless steel options are available,

commonly desired for high-end storefronts and

coastal area applications.

With steel systems, galvanization occurs prior

to cutting, and the powder or wet coatings are

applied after to protect the cut ends. With theproper coating specified, painted steel should

perform similar to coated aluminum. Powder

coats are extremely durable, and if necessary, field touch-ups are possible by qualified personnel.

As a framing material, steel also offers unmatched long-term durability. With back mullions and cover caps made of steel, curtain

wall and window and door assemblies are significantly more resistant to scratches and dents over a buildings life. This quality is

especially beneficial in areas prone to abuse such as storefront entrances, as well as in door assemblies where steel can withstand

long-term hard use with lower potential for sagging or joint failure.

Figure 5: Structural Member Mounting Examples

Modular steel framing systems enhance design freedom by allowing attachment to a wide range

of structural members, including I-beams, wood, round steel tubes and stainless steel profiles.

Clamp isolator bush system

ensures accurate screw alignment

andcorrect pressure

Universal use

Small contact area and underside

ventilation

avoids contact corrosion

Protection against water penetratio

by full width EPDM gasket

Secure gasket guide

Figure 4: Thermal Expansion Comparison

Steel has a thermal expansion coefficient substantially lower than alumi-num, and is comparable to glass and concrete.

THERMAL EXPANSION COEFFICIENT

Glass

in/in.

Fx10-6

Concrete Steel Aluminum

5.05.0

0

10.0

15.0

8.0

7.3

12.3

by free choice of under construction

materials

Continuous screw fixing channel

variable fixing points of glazing beads


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Installation and costs

Steel framing systems are installed in a similar fashion to traditional aluminum assemblies. One system includes a plug-and-screw

connection for framing components, which allows the framing to be readily assembled without welding the result is crisp corner

joints without visible fasteners or weld beads.

The new generation of steel framing versus hollow metal or welded frames provides a clean finished look previously available

only with aluminum systems. The unique roll forming process of modern steel framing results in linear edges that are crisper than

the break-shape profiles of traditional hollow metal steel.

The framing is made watertight by a back seal, which covers the frame profile and is secured in grooves. Steel framing systems have

been tested for dynamic water penetration (American Architectural Manufacturers Association [AAMA] 501.1, Standard Test Method

for Water Penetration of Windows, Curtain Walls and Doors Using Dynamic Pressure) and static water penetration (ASTM E 331,

Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure

Difference). They have also been measured for air performance (ASTM E 283, Standard Test Method for Determining Rate of Air

Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen) and struc-

tural performance (ASTM E 330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain

Walls by Uniform Static Air Pressure Difference).

Modular steel framing system options are also available with back mullions customizable to almost any profile, including L-shaped

U-shaped, T-shaped, I-shaped, and hollow sections. Instead of limiting them to traditional square tube aluminum to support a

glazed curtain wall, the new modular steel systems allow designers to use almost any structural member as a curtain wall mul-

lion, including I-beams, wood glue-laminated (glulam) beams, round steel tubes, and stainless steel profiles (Figure 5). This choice

provides incredible design flexibility for curtain wall projects, in contrast to aluminum systems that do not offer the same range of

back mullion profiles.

With respect to first costs, steel from a raw material to a finished frame product will generally come at a higher price than alumi-

num, which is common for many building systems offering higher performance characteristics and product features. Nevertheless,

when considering cost, it is also important to keep in mind steel curtain walls are able to use fewer frame mullions than aluminum

(due to their higher strength and lower deflection) and that large glass lights and less framing can reduce system material costs and

labor and installation expenses since the number of system joints and complexity is potentially lower.

Steel framing for fire-rated applications

Steel framing is also available to meet fire ratings, and can be used for fire-rated curtain walls in areas such as large enclosed

stairwells, elevator cores, and exterior walls. Before the advent of modern fire-rated glass and framing systems, exit corridors and

other critical areas that divide a building into fire-blocking compartments required solid walls made of materials like masonry and

gypsum. Now, fire-rated glazing can be used throughout a building, allowing natural light to filter in and open the interior.

During the last two decades, the glazing industry has witnessed dramatic changes in the specialty field of fire-rated glass and

framing. Modern fire-rated glass looks like the glass of ordinary windows, outperforms wired glass in fire testing, and can provide

substantially better impact safety and thermal insulation. (See More Than a Pretty Frame: Putting Glass to the Test.) From an aes-thetic perspective, fire-rated glass is also available in large sizes, and a range of finishes such as beveled edges, etched surfaces,

and varied textures.

Fire-rated framing has also considerably improved in recent years, advancing beyond the functional, yet bulky wrap-around-style

hollow metal frames of the past. Those older style frames perform well at blocking fire and smoke, but do not necessarily accom-

modate many design considerations, especially those of glazed curtain walls.

New narrow profile steel frames offer a very different look than traditional hollow metal steel. Specialized manufacturing methods

create an extremely strong product that is thinner than traditional fire-rated framing.


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Advances in fire-rated framing have enabled the development of fire-rated curtain wall systems, allowing architects and designers

to stretch large expanses of fire-rated glass across multiple stories. Floor-to-ceiling fire-rated curtain walls are sometimes used to

divide interior spaces while offering security, daylighting, and aesthetic appeal, along with life safety. For example, fire-rated curtain

walls are especially suitable for lobbies and atriums as they provide uninterrupted views.

For exterior applications, most building codes do not require fire-rated materials for a buildings skin, unless it is close to a property

line. However, the outside of a building can significantly help prevent the spread of fire inside. An example that illustrates the im-

portance of the building envelope is the 1988 fire at the First Interstate Bank building in Los Angeles. In this case, the building en-

velope did not include fire-rated materials, allowing flames to spread to higher floors along the outside of the building. In contrast,fire-rated curtain wall can more effectively contain the spread of fire within a building, as well as to adjacent facilities.

Conclusion

Aluminum continues to be a popular framing material for glazing given its high strength-to-weight ratio. It is well suited for many

applications, including those requiring less stringent design criteria for glazing sizes, spans, and framing sightlines. However, for

higher performance needs, architects and designers may wish to consider the benefits of steel framing systems to meet their design

and challenges. When specifying steel, it is crucial to ensure the design takes advantage of this materials inherent strengths.

Notes1 See section D of Gregg D. Anders Daylighting. Visit www.wbdg.org/design/daylighting.php.

Additional Information

Abstract

The implementation of effective daylighting often focuses on glazing material and the proper placement of window glass and

skylights. However, when it comes to curtain walls, framing also plays a critical factor. Steel assemblies can enhance daylighting by

enabling much larger areas of uninterrupted glass and narrower frame profiles than their traditional counterparts, while meeting a

range of performance and design needs.

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