C SD Curtain Wall Aug 061

7/26/2019 C SD Curtain Wall Aug 061

1/3STRUCTURE magazine August 2006

discussions

on

design

issues

forstructuralengineers

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Dynamic Issues DriveCurtain Wall DesignBy Ken Brenden

Todays curtain wall is a complex,integrated system composed offraming members, sash, glazing,

spandrel panels, fasteners, and sealantsthat must accommodate a wide variety of

forces, both static and dynamic.The principal static force acting upon

a curtain wall is, of course, gravity.However, because of the relatively light

weight of materials used in curtain walls,this dead load is a force of secondarysignificance, rarely imposing any seriousdesign problems beyond ensuring thatthe horizontal members can support the

weight of large glazing infills withoutexcessive deflection or twisting. There-fore, in terms of structural design con-siderations, the requirements of stiffness

rather than strength usually govern.

Live Loads andProvision for Movement

Of primary concern are the dynamiclive loads, chief among them wind, ther-mal expansion and, in some areas, seis-mic movement.

The walls anchorage to the buildingframe must be designed with due regardto these dynamic forces, which can causesubstantial movements at many of the

joints between its parts. Joint design thus

becomes the key to accommodating thesemovements without jeopardizing the

walls integrity and weather tightness.Joint configurations used in curtain

wall design are typically of three types: Butt joint, in which the edges of two components meet in the same plane. Lap joint, which permits relative over-sliding movement of the joining parts. A combination of these two forms, such as the Surround or glazing type joint used at the periphery of

glass or panels. This is the most common type.

Wind Loading

Chief among the environmentally-imposed live loads are lateral wind forc-es. On the taller structures in particu-lar, the structural properties of framingmembers and panels, as well as thethickness of glass, are determined bymaximum wind loads. Winds also con-tribute to the movement of the wall, af-fecting joint seals and wall anchorage.

Designing to accommodate theseforces is a routine procedure, providedthat the nature and magnitude of the

wind loads are known. This is not a triv-ial task for high-rise structures, however,despite the straightforward methodol-ogy and handy availability of referencesfor wind loads such as ASCE-7-02 and

AAMA TIR-A11-04. That is becausethe nature and intensity of such loadsare affected by the height and geometryof the building, as well as by surround-ing structures or natural topography.

Often too little significance is at-tached to negative wind loading (suc-tion forces) acting on the wall, whichcan be augmented by internal buildingpressures due to air conditioning. Onhigh-rise buildings, these negative pres-sures usually reach a maximum near ex-terior corners, where they may be morethan twice as great as any positive load.This negative loading is why windowsare more likely to blow out of a high-risebuilding during an extreme wind event,rather than collapse inward.

ASCE 7-02 was developed basedon the effect of winds on rectangularbuildings; however, the vast majorityof buildings are not perfect rectangles.Especially for tall and/or irregularlyshaped buildings, it is best to have aboundary layer wind tunnel (BLWT)study performed on a scale model ofthe building.

Thermal Expansion

A representative maximum rangeof U.S. ambient temperature is about

120F. Metal surface temperatuhowever, vary over a much wider ra typically 150F and in some localemuch as 200F. This can translate ithermal expansion of3to 5/16inchea ten-foot length of aluminum. A shof glass alongside the aluminum wexpand by less than half as much dto aluminums relatively high coefficiof expansion.

This disparity causes relative moment that must be accommodated wout causing undue stress on glass, c

tain wall joints, anchors, joint sealsstructural elements.

Seismic Loads and Interstory Drift

Although wind is the principal acin dynamic loading, seismic loadinalso a major consideration in seveareas of the U.S. While both can cainterstory drift, or racking definas relative horizontal movement tween adjacent stories of a multi-stbuilding seismic action is by far greater concern.

Seismic events cause similar buildmovements, as do major wind evenIn the latter, the top of the framing wmove inward at the top of the storythe windward side of the building aoutward on the leeward side. It wmove laterally in the plane of the won sides parallel to the wind, tendingforce the framing out of square as building tries to lean with the wi(Note that some negative wind load

will also occur on the parallel sides.)seismic events, the same sort of mo



ments occur in relation to the direction ofground motion. System designs must takeinto account all of these motions.

Seismic design has evolved from a force-based approach (expected load multiplied byan appropriate safety factor) to one of calcu-lated deformations. The ultimate goal is tokeep the structure from collapsing in majorearthquakes, while accepting some damage asa result of different earthquake magnitudes.The designer must strike a balance with re-

spect to seismic design based on function,cost and probability of damage. Guidelinesare published in the 1997 NEHRP Recom-mended Provisions for Seismic Regulations forNew Buildings and Other Structures.

AAMA has published test methods 501.4-00, Recommended Static Test Method for Eval-uating Curtain Wall and Storefront SystemsSubjected to Seismic and Wind Induced Inter-story Drifts, and AAMA 501.6-01, Recom-mended Dynamic Test Method for Determiningthe Seismic Drift Causing Glass Fallout froma Wall System, that compliment one another

for use in laboratory evaluation of curtainwall performance when subjected to specifiedhorizontal displacements. AAMA 501.4 test-ing should, in general, be considered when-ever the calculated interstory drift in the pri-mary structural system exceeds 0.005 timesthe story height. AAMA 501.6 testing helpsdetermine the amount of drift that causesglass fallout from a curtain wall or storefront(indicated by

fallout).

Design Considerations toAccommodate Movement

Tolerances

Because curtain wall construction involvescovering a field-constructed skeleton witha factory-made skin, much of the detailingwork may be done by the manufacturer ratherthan by the architect/engineer. Also, the de-sign must consider that curtain wall systemsconnect to many other parts of the buildings,involving the work of numerous trades. Sig-nificant deviations from true alignment inthe building frame are generally not correct-able by field cutting, fitting and patching. Allof these factors converge to require carefulattention to manufacturing and installationvariances, with the resulting tolerance stack-ups still capable of providing the clearancesnecessary to accommodate building move-ment due to in-service live loads. Most suc-cessful curtain walls are the result of a teameffort, with the architect/engineer, contrac-tor, and fabricator pooling their knowledgeand talents to produce an attractive, efficientand practicable design.

Tolerances and clearances may be closelyrelated, but the two terms are often confused.

A tolerance is a permissible amount of devia-tion from a specified or nominal dimension.

A clearance is a space or distance purposelyprovided between adjacent parts (e.g., be-tween the building frame and the curtain

wall) to allow for movements or anticipatedsize variations, to provide working space, orfor other reasons.

Glass-holding members must be erected so

they provide glass openings that are withincertain tolerances for squareness, corneroffset and bow. For example, it is generallyrecommended that the difference betweenthe measured lengths of the diagonals shouldnot exceedc-inch.

Wall Anchorage

For the most part, rare instances of curtainwall failure or excessive deformations havebeen due to faulty anchorage details.

Anchors must, of course, have the hard-ness, yield and tensile strength to accommo-

date the weight of the wall itself, as well as thedynamic forces imposed by wind loads, whileaccommodating fabrication and constructiontolerances and allowing for thermal move-ment. Locking devices must be employed toprevent loosening or turning out due to ther-mal expansion and contraction cycles, build-ing movements or wind-induced vibration.

Exposure, metal type and coatings are im-portant considerations due to galvanic actionthat occurs when dissimilar metals become

wet from rain or condensation which cancause corrosion, especially in harsh envi-

ronments such as seacoast locations. Indus-try guidelines, such as AAMA TIR A9-91,specify the optimum metal combinations, as

well as the data necessary to select fastenersfor curtain wall framing members and com-ponents, and for anchoring the curtain wallto the building structure.

Joint Sealant

Sealants represent a very small part of thewall cost, but often play a major role in itsperformance.

Joint sealant performance is dependentthe design of the joints, the proper selectof sealant materials and proper applicati

Whether the joints are working joints, signed to accommodate movement, or n

working joints, secured by fasteners, sokind of seal is usually required. It is partilarly important that the size of the seal

joint take into account the maximum therexpansion and contraction, as well as buing movements that will affect the joint.

The most critical properties of a sealant its adhesive strength and cohesive strengUnless the sealant adheres securely acontinuously to the substrate, it will fail wsubjected to tensile stress. In many cadepending on the substrate material, a primmay be required to promote the bondaction. Clearly, cohesive strength is equimportant. A material that lacks the strento hold itself together under repeastress cannot provide a suitable seal. Aimportant are the sealants recovery abiafter deformation, modulus of elasticity,

durability under the effects of weatheriIn addition, sealants must be chemiccompatible with other compounds, substrand wall cleaning solutions.

The type of joint dictates much of sealant selection. In a butt joint (in whthe sealant is subjected to tensile and copressive stresses) the critical factor is jo

width. In lap joints (where the sealansubjected primarily to shear stress), it is thickness of the sealant that is critical.

Sealant compounds vary considerablythe amount of extension and compressmovement they can withstand before failu

The following performance classificatiare reflected in current federal specificatiand ASTM standards.

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AAMA References

The design professional will find the following material useful when approaching thedesign of a curtain wall system:

Aluminum Curtain Wall Design Guide Manual (AAMA CW-DG-1-96)

Metal Curtain Wall Manual (AAMA MCWM-1-89)

Joint Sealants (AAMA JS-91)

Glass and Glazing (AAMA GAG-1-97)

Structural Properties of Glass (AAMA CW-12-84)

Metal Curtain Wall Fasteners (AAMA TIR A9-91 and the 2000 Addendum)

Design Windloads for Buildings and Boundary Layer Wind Tunnel Testing (AAMA CW-11-85)

Maximum Allowable Deflection of Framing Systems for Building Cladding Components at design Wind Loads (AAMA TIR A11-04)

Installation of Aluminum Curtain Walls (AAMA CWG-1-89)

Recommended Static Test Method for Evaluating Curtain Wall and StorefrontSystems Subjected to Seismic and Wind Induced Interestory Drift (AAMA 501.4-00)and Recommended Dynamic Test Method for Determining the Seismic Drift CausingGlass Fallout from a Wall System (AAMA 501.6-01) (combined document)

These publications may be obtained onlineby visiting the Publication Store

at www.aamanet.org.

Low Performance Sealants or Caulks. These aresealants having minimum movement capabilitythrough their useful life in relatively stable joints.Rarely used in curtain walls, it was the failure of suchproducts when used insome of the early large curtain walls that, in fact,led to the development of the higherperformance sealants.

Medium Performance Sealants. Medium performancesealants are those having a predicted cyclic movementcapability of 5% to 12.5% throughtheir useful life. Their use is normally confined tonon-working joints and those working joints whererelatively small movement is expected.

High Performance Sealants. Class B Highperformance sealants are those which havea predicted cyclic movement capability of12.5% to 25% through- out their useful servicelife.Those with the higher capability, able toaccommodate 12.5% to 50% maximum cyclemovement, are designated as Class A.

Deflection of Glass-SupportingFrame Members

The rigidity of the framing membethat support the glass affects the amouof flexure the glass panel will experienunder load. The more the frame deflecunder the load, the more stress is placeon the glass and the greater the likelihooof breakage.

The typical design convention is limit frame member deflection to maxmum of L/175 of the unsupported (clea

span length (L), when subjected to desigloads. Certain sealants may require a lower deflection ratio than L/175, as will thneed to limit movement to accommodathe properties and location of interifinishes or to prevent disengagement applied snap covers or trim. Lower defletions usually require the use of heaviframe cross-sections or reinforcement

which has visual impact.

Ken Brenden possesses more than 30 yearof window and curtain wall experiencein manufacturing, testing, engineeringand designing. He serves as the Codeand Industry Affairs Manager for the

American Architectural ManufacturersAssociation (AAMA), a trade association

of approximately 275 manufacturers ofarchitectural, commercial and residentiafenestration products and components.

Documents

C SD Curtain Wall Aug 061