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40 LICENSED ARCHITECT • VOL 13 NO. 1 • SPRING 2009

TYPES of CURTAINWALLIn strictest architectural parlance, a "curtainwall" is

any non-load-bearing exterior wall that hangs (like acurtain) from the face of floor slabs, regardless ofconstruction or cladding material. However, incommon usage, the term curtainwall usually refersto aluminum-framed systems carrying glass, panels,louvers, or occasionally, granite or marble. Thedistinctions between the system types discussedbelow are blurred, and it’s often difficult to clearlydifferentiate between one system and another.

"Storefronts" are non-load-bearing glazedsystems that occur on the ground floor, whichtypically include aluminum entrances, and areinstalled between floor slabs, or between a floor slaband building structure above.

"Stick" curtainwall systems are shipped inpieces for field-fabrication and/or -assembly. Thesecan be furnished by the manufacturer as "stocklengths" to be cut, machined, assembled, andsealed in the field, or "knocked down" parts pre-machined in the factory, for field-assembly and -sealing only. All stick curtainwalls are field-glazed.

Frame assembly requires the use of either, a)"shear blocks" to connect vertical and horizontalframing elements, or b) "screw-spline" construction,in which assembly fasteners feed through holes ininterlocking vertical stacking mullions into extrudedraces in horizontals.

Many stick curtainwalls are called "pressurewalls," because exterior extruded aluminum platesare screw-applied to compress glass betweeninterior and exterior bedding gaskets. A snap-oncover or "beauty cap" is then used to concealpressure plate fasteners.

Once very popular, "I-beam" walls have seenmarket penetration decrease. "I" or "H" shaped,structural, vertical back members are set intoopenings in the field, with horizontals then clipped toverticals. After glazing, extruded aluminum interiortrim is cut and snapped into place at vision areas.Since unexposed spandrel areas receive no interiortrim, savings in material and finish (painting oranodizing) can result, partially offset by added fieldlabor. Of course, maintaining vapor retardant

OMNI HOTEL in San Diego Photography: Mark Long

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Licensed Architects

Continuing Education

Special to Licensed Architect

CURTAINWALL DESIGN: Products, Performance and Practicalityby Steve Gille, architectural services manager, Wausau Window and Wall Systems

Learning Objectives:1. Learn different types of curtainwall systems and their applications in commercial

and institutional buildings.2. The anchorage and structural considerations associated with curtainwall systems.3. The thermal and other performance criteria, and sustainable design characteristics

of curtainwall systems.

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LICENSED ARCHITECT • VOL 13 NO. 1 • SPRING 2009 41

(Continued on page 42)

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continuity at interior trim joints can be challenging ifany positive building pressure is present.

Performance of any field-assembled or field-glazedcurtainwall is only as good as field workmanshipallows, limited by variables such as weather, access,and job site dirt and dust. Many critical seals arenecessary, even in systems that are designed to drainor "weep" rain penetration from the system back tothe exterior.

To accomplish as many of these critical seals aspossible in controlled factory conditions, and minimizedependence on scarce and expensive field labor,"unitized" curtainwall systems have beendeveloped. Unitized curtainwalls are factory-assembledand -glazed, then shipped to the job site in units thatare typically one lite wide by one floor tall. Only oneunit-to-unit splice (usually a silicone sheet or patch)needs to be field-sealed, and only one anchor perjamb needs to be attached to the face of the floor slab.

Interlocking unitized curtainwall frame members areweather-stripped to seal to one another, bothhorizontally and vertically. This accommodates thermalexpansion and contraction, inter-story differentialmovement, concrete creep, column foreshortening,and/or seismic movement.

Most curtainwall systems are installed in asequential manner around each floor level, movingfrom the bottom to the top of the building.

"Window wall" systems span from the top of onefloor slab to the underside of the slab above. Basically,window wall assemblies are large, side-stackingwindow units, contained in head and sill receptors,known as "starters" that accommodate movementand drainage. Slab covers can clad the face of theslab and can be fabricated from aluminum extrusions,

sheet, or panels, or even glass. Window walls easily acceptoperable windows, and unlike curtainwalls, can easily be installedin any sequence.

Of course, creative designers constantly push the bounds ofconventional system types. One such innovation is a "hybrid," or"composite" between window wall and curtainwall. Likeconventional, unitized curtainwalls, hybrid systems are factory-assembled and -glazed, and shipped to the job site in units thatare one lite wide by one floor tall. In lieu of attaching these to theface of floor slabs, they are instead, anchored at each unit cornerto the top and underside of each slab. Units cantilever past theface of slab, and interlock with the unit above.

These hybrid, or composite wall types have many of theadvantages of unitized curtainwall, while eliminating the need forseparate starters and slab covers, and allow the entire installationcrew to remain on a single floor. Sequencing is similar to unitizedcurtainwall.

ANCHORAGECurtainwall anchorage must be designed for each individual

project’s conditions, due to almost unlimited combinations of loads,tolerances, movements, and substrates. However, there are basicanchor types and design principles that are applicable to wide rangeof conditions.

All curtainwall anchors must carry the dead load weight of thecurtainwall. This is transferred from horizontal framing members tovertical mullions, and down to anchor points, where it is transferredto the building structure. Wind loads act normal to the plane of the

wall in both directions.One of the most economical ways to anchor

curtainwall is through the use of manufacturer-specific, custom-designed, three-wayadjustable anchors. These anchors allow for in-and-out, up-and-down, as well as side-to-sideadjustment during installation, and feature a "jackscrew" for fine adjustment. The jack screw stopsthe movement of a "saddle plate" attached to theside of the mullion, thus allowing the hoist tounhook and pick another unit, while thecurtainwall is being set to its final position. Thissaves field labor by utilizing hoist "travel time" forconcurrent, fine adjustments.

In a more standard anchorage method,"double angle" mullion anchors straddle bothsides of the vertical mullion, and are secured witha through-bolt and pipe spacer. The pipe spacer

allows for thermal and side-to-side building movement of mullions,even when anchor bolts are securely tightened. The doubleangles typically are attached to the face of the slab using insertweld plates, channel-shaped embeds, or expansion bolts drilledinto the floor slab. If embeds are used, it usually is recommendedthat the curtainwall manufacturer supply the embed layout

SYRACUSE UNIVERSITY Photography: Steve Santori


• In design, do not expect perfect visual alignment - A 1/2-inchreveal that varies +/- 1/2-inch looks awful, but a 1-1/2-inchreveal that varies the same +/- 1/2-inch is more forgiving.

STRUCTURAL ISSUESDead load calculations are based on materials, equipment, or

other elements of weight supported in, on, or by a structuralelement, including its own weight, that are to remainpermanently in place. Since curtainwalls are by definition, non-bearing walls, only their own weight usually needs to beconsidered.

Buildings and their components are designed to withstandcode-specified wind loads. Calculating wind loads is importantin design of wind force-resisting system, sliding, overturning,and uplift actions. Wind loads are often quantified usingAmerican Society of Civil Engineers’ "ASCE 7" publication andthe International Building Code model building code.Determining wind loads is the job of the building design team’sengineer of record, not the curtainwall manufacturer. Criteriashould be listed on the first sheet of the structural drawings.Differing interpretation of corner zones, insurer mandates, andlocal code peculiarities could result in costly re-design if windload determination is left ambiguous in bid documents.

Designed to withstand wind loads and provide adequate glassedge flexural support, curtainwall can be, a) "simplysupported," with curtainwall members anchored only at theirends; b) "twin span," with members spanning two floors andanchoring at the intermediate floor or other structure; or c)"continuous span," with the system’s vertical mullions splicedat points of zero moment (inflection points). These are listed inorder of increasing structural efficiency, but before deciding onan appropriate strategy, movements and the ability of thestructure to support dead loads must be considered.

The ideal splice location for a horizontal stacking unit wall istypically 20-22% of its span, to occur at the zero moment point,where flexural stresses reverse from compression to tension atmullion flanges. This can be important in locating interior stools,interior finishes, shadow boxes, or spandrel areas. The zeromoment point will be the most economical location for thesplice.

Designers need to be aware of the maximum deflections forboth vertical and horizontal frame members. The typical unit wallis hung from the top, therefore drywall and other interior finishesonly can be attached in a manner allowing freedom of verticalmovement with the floor above, and freedom of horizontalmovement with the upper unit mullion.

Live load movements result from all occupants, materials,equipment, construction or other elements of weight supportedin, on, or by structural element that will or are likely to be movedor changed during the life of the structure. Live loads can beupward or downward. For example, a downward live load on afloor below can result in disengagement of improperly designedcurtainwall anchors on a floor above that remains static, whileresulting in a crushing load on a floor below.


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drawings, eliminating excess coordination time andcostly errors.

Other practical anchorage design considerations areworthy of mention.

• When planning to field-drill into floor slabs or otherconcrete structures, it is necessary to considerwhere the rebar or post-tensioning cables are tobe located. This requires close coordinationbetween architectural and structural disciplinesduring design and installation.

• A building will move during the daily temperatureand use cycle. Care must be taken in the design ofthe wall and its anchorage to accommodate thefull range of movements.

• The construction process is not one of perfection.A design may look precise on paper, but it will notbe exact in the field. Industry specifications cansometimes allow floor pours to be +/-1-inch out oftolerance in any direction. Anchorage andanchorage flexibility is important. If the anchoragecannot accommodate specified buildingtolerances, time and money is lost.

JOHNS HOPKINS UNIVERSITY’S BROADWAYRESEARCH CENTER

Photography: Wausau Window and Wall Systems



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SHADOW BOX SPANDRELSIncorporating a shadow box into the building design

can be aesthetically pleasing, giving spandrel areas acertain transparency. However, if not properlydesigned and installed, shadow boxes can createinternal condensation issues. The shadow box’s aircavity must be sealed nearly air tight relative to interiorair in cold climates, since condensation forms whenmoist warm air comes into contact with cold surfaces.Recommendations for venting the shadow box cavityvary with different applications and locations

In all cases, the use of mineral wool insulation isrecommended because it contains no organic materials,and is resistant to the formation of mold and mildew.

THERMAL PERFORMANCEThere are three basic thermal performance

parameters for wall systems. Expectations forcurtainwall performance are listed below.

Thermal Transmittance: A measure of heat flowper unit time, area and temperature difference. U-Value (or U-Factor) is measured in Inch Pound Unitsas BTU/hr.sqft.°F, and is the reciprocal of the morefamiliar R-value.

GRAND VALLEY STATE UNIVERSITY’SCOOK-DEVOS CENTER FOR

HEALTH SCIENCESPhotography: Wes Thompson

Condensation Resistance Factor (CRF): A dimensionless ratioof surface temperature to ambient temperature difference. (Note:CRF is useful in comparing design options, but less useful inpredicting field condensation.)

Solar Heat Gain Coefficient (SHGC): A dimensionless ratio ofthe total visible, infrared and ultraviolet energy flowing throughglazing, divided by incident energy.

Finite element thermal modeling software is widely used topredict thermal performance of untested or custom systems, or toestimate the performance of specific frame-glass combinations.Framing, edge of glass, and center of glass performance ismodeled, and such modeling forms the basis of NationalFenestration Ratings Council (NFRC) labeling programs, usingpublic domain software.

The use of aluminum sun shades is a growing trend inarchitectural design on buildings of all types. Just like any otherbuilding design element, architects are exercising their creativitywith sun shades using louvers, blades, catwalk grids, and solidpanels to accomplish their aesthetic and daylight control goals. Inthe most innovative designs, sun shades are combined withinterior light shelves to control glare while maximizing daylightpenetration depth. Sun shades present some engineeringchallenges in wind loading, snow loading, ice build-up, and loadsimparted by maintenance operations.

OTHER PERFORMANCE CRITERIAIn addition to basic air, water, structural, and thermal

performance, certain sites and occupancies require that curtainwallsystems have other performance characteristics.

When adjacent to highways, rail tracks, airports, or other noisesources, acoustic design of curtainwall systems is vital. While atypical curtainwall glazed with 1-inch insulating glass exhibits aSound Transmission Class (STC) of 30 to 34, this may or may notbe sufficient for its intended use.

Unfortunately, in many government and institutional buildings,blast hazard mitigation is a vital design criterion. Always contactan experienced manufacturer or blast consultant to discuss designrequirements. These requirements evolve as various agencies fine-tune their standards.

Specifications for projects in many areas of the U.S. dictateminimum movements and/or clearances determined by analysis ofseismic design criteria. One such criterion, "drift," is the amountof inter-story building movement anticipated during a seismic

U = 1.20 BTU/hr.sqft.°F Non-thermal,single glazed

U = 0.60 BTU/hr.sqft.°F Standard,uncoated,insulating glass

U = 0.20 BTU/hr.sqft.°F Highestperformingcurtainwalls

CRF 29 Non-thermal, single glazedCRF 52 Standard, uncoated, insulating

glassCRF 80 Highest performing curtainwalls

SHGC = 0.87 1/8-inch clear glass SHGC = 0.50 Grey, tinted,

insulating glassSHGC = 0.29 Grey, reflective glass

(Continued on page 44)

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event. The structural grid assumes a parallelogramshape, and glazing must provide sufficient spacebetween glass and frame to ensure that contact doesnot occur.

The following Table is reprinted from "ASCE 7-05.”Table 9.5.2.8 Allowable Story Drift, ▲▲a

In the Table above, "h" is story height. Obviously,seismic movement can be significant.

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What is the most important, seismic design criterion? Makesure curtainwall and surrounding materials move at the samelocations, or if not, that differential movements are considered inthe design of the wall.

SUSTAINABLE DESIGNEnvironmentally responsible, sustainable building design and

operation is a top-of-mind issue for anyone in architecture,construction and real estate. Buildings represent about one-thirdof the energy consumption in the U.S., along with thecorresponding amount of greenhouse gas emissions.

Since its inception in 2000, U.S. Green Building Council’svoluntary, consensus-based LEED® (Leadership in Energy andEnvironmental Design) Rating System™ has emerged as the leadingsustainable building "scorecard." In addition to prerequisiteconstruction, performance, and environmental practices, the LEEDscorecard tallies points in six credit categories:

• Sustainable Sites (SS)• Water Efficiency (WE)• Energy and Atmosphere (EA)• Materials and Resources (MR)• Indoor Environmental Quality (EQ)• Innovation in Design (ID)

There are both environmental and financial benefits to earningLEED certification. These include:

•Lowering operating cost and increased asset value• Reducing waste sent to landfills• Conserving energy and water• Increasing health and safety for occupants•Reducing harmful greenhouse gas emissions•Qualifying for tax rebates, zoning allowances and other

incentives in hundreds of cities•Demonstrating an owner’s commitment to environmental

stewardship and social responsibilityUp to 10 LEED credits of the 58 total credits in the above

categories may be affected directly by window and curtainwallselection and many more points may be influenced indirectly bythese decisions.

A FINAL WORD…For buildings using curtainwall systems as design elements, it

is important to consult with an experienced manufacturer early inthe process. Teamed with a reputable, local glazing sub-contractor, manufacturers can provide design input, budgetpricing, sequencing, and schedule information that will proveinvaluable to the design team.■

Seismic Use GroupStructure I II III

Structures, other thanmasonry shear wall ormasonry wall frame

structures, four stories orless with interior walls,partitions, ceilings and

exterior wall systems thathave been designed to

accommodate the story drift.Masonry cantilever shear

wall structures 0.010h 0.010h 0.010hOther masonry shear

wall structures 0.007h 0.007h 0.007hMasonry wall

frame structures 0.013h 0.013h 0.010hAll other structures 0.020h 0.015h 0.010h

0.025h 0.020h 0.015h

TIGER WOODS LEARNING CENTER’S BIPVCURTAINWALL

Photography: Mark Long

About Wausau Window and Wall SystemsNationally recognized for its innovative expertise, Wausau Window and Wall Systems is an industry leader in engineeringwindow and curtainwall systems for commercial and institutional construction applications. For more than 50 years, Wausauhas worked closely with architects, building owners and contractors to realize their vision for aesthetic beauty, sustainabilityand lasting value, while striving to maintain the highest level of customer service, communication and overall satisfaction.Learn more at http://www.WausauWindow.com or call 877-678-2983.



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Program Title:“CURTAINWALL DESIGN: Products,

Performance and Practicality”

ALA/CEP Credit: This article qualifies for 1.0LU’s of State Required Learning Units and mayqualify for other LU requirements. (Validthrough Feb. 2010.)

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QUIZ QUESTIONS

1. Place these beam support schemes inorder of structural efficiency:a. Simple span________ b. Most efficientb. Continuous span ___ c. Moderately

efficientc. Twin span__________ a.Least efficient

2. True or False: Curtainwall systems typicallysupport the weight of other materials suchas roof deck and floor slabs.

3. A typical curtainwall glazed with 1"insulating glass exhibits a SoundTransmission Class (STC) of:a. 30 to 34b. 20 dBc. Class Bd. It depends on the noise source

4. True or False: Building envelope designhas little impact on a building’s LEED®scorecard.

5. The most important seismic designcriterion for curtainwalls is:A) Make sure curtainwall and surrounding

materials move at the same locationsB) Make sure the curtainwall and

surrounding materials move atseparate locations

C) Make sure the curtainwall’s glass andframe are in constant contact

D) Make sure the curtainwall’ssurrounding materials are rigid

6. Unitized curtainwall systemsaccommodate movement by:a. Bending the glass or infill materialb. Sliding the glass in the glazing pocket

c. Inter-locking weather-stripped framemembers

d. All of the above

7. Inter-story lateral movement induced by aseismic event is commonly referred to as:a. Droopb. Driftc. Foreshorteningd. Creep

8. The acronym "NFRC" standards for:A) National Fire Rating CouncilB) National Fire Rated CertificationC) National Fenestration Rating CouncilD) National Fenestration Ranked

Contractors

9. True or False: "Stick" curtainwalls areshipped in pieces for field-fabricationand/or –assembly.

10. Unitized curtainwalls are usually installedin what sequence:a. Floor-by-floor from the bottom to the

top of the buildingb. From the top of the building

downwardc. In random order, starting anywhere on

the buildingd. Any of the above

Learning Objectives:

After taking this course, the reader will have a better understanding of:1. The different types of curtainwall systems and their applications in commercial and institutional buildings.2. The anchorage and structural considerations associated with curtainwall systems.3. The thermal and other performance criteria, and sustainable design characteristics of curtainwall systems.


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