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Architectural Glazing PracticesPreventing problems with glazed curtain walls

By Robert G. Spindler

Aluminum curtain walls for commer-cial office buildings began to evolvein the early 1950's. Today these glassand metal building facades have pro-ven themselves cost-effective,aesthetical ly pleasing, energy efficientand capable of per{orming understringent environmental conditions.Architects continue to design uniquestructures with innovative concepts re-quiring, at times, the buildingmaterials to meet extraordinaryspecifications. With the advent of newmaterials and philosophies in ar-chitectural design concepts, the totalcurtain wall, including metal, glass,

sealants and gaskets, has had to per-form under a variety of conditions;namely, thermal environment,negative and positive wind loads, wallsystem dead loads, water penetration,and air infiltration. The design of aproperly functioning curtain wallsystem requires specific considerationof the glass retaining system or glaz-ing members.

high performance reflective glasses,

sealants, gaskets and other materialsused in the glazing channel are expos-ed to considerable temperature ex-tremes and high concentratíons ofultraviolet light. Temperatures ofmonolithic reflective glass in spandrelor nonvision areas have beenmeasured at 1900 F. This imposes con-siderable expansion and hightemperature resistance requirementson all materials that come in contactwith the glass. Depending on the typeof glass, significant amounts ofultraviolet light can be reflected fromthe glass into the glazing material. lf

the materials used in the glazing chan-nel are affected by ultraviolet light,these materials most likely willdegradg producing costly repairs onthe project.

Chosen glass materials must meetthearchitect's specifications for perfor-mance, which usually are:

. Glass thickness and type (an-

nealed, heat treated) to meetspecified and wind loadrequirements,o Thermal efficiency requirementsfor both summer and winterconditions,o Aesthetic requirements,. Class type to resist the potentialof breakage due to thermal stressconditions, ando Building codes.

GIass and Shading Devices

Draperies, blinds or other interiorshading devices must be hung so as toprovide space either at the top and bot-tom or one side and bottom to permitnatural air movement over the roomside of the glass, Figure 1 depicts the

Glass Considerations

With advent of heat absorbing and

thirdissue 89

volume 7 number 3 copyright Hoffmann Architects 1989

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criteria that should be metto avoid for-mation of a heat trap. The use of heattreated glass may be necessary to off-set the effects of a lack of adequatevent¡lation.

Glass

l-1r2 (37mm)MinimumClearance

Venetian Blind2'(50mm)MinimumClearance

Drapery or

Air Flow

Figure 1. indoor shading and heatduct Iocations

1. Minimum 1-Vz" clearance required top andbottom or one side and bottom betweenshading device and surrounding construction.2. Minimum 2" clearance between glass andshading device.3. Heating/cooling outlets must be to roomside of shading device.

tion at low temperatures may imposesignificant stresses on glass, sealantsand gaskets.

The metal framing system mustuniformly support the glass. The glassindustry has set guidelines that theframing needs to be within limits forbow, squareness, and corner offset asindicated:

o Bow - 1/16 inch in any four footlength of framing;. Squareness - 1/B inch differencein the lengths of the diagonals ofthe frame;o CornerOffseL-1/32 inch ateachcorner,

The glass industry has also setguidelines on glass framing memberdeflection under design loads. Thisguideline states that to minimize thepotential for glass breakagg the glasssurround should not impose any ben-ding or high concentrated com-pressive loads on the glass and shouldnot deflect more than 1/l75th of itsspan under loading or 3/4 inch,whichever is less. ln addition, the glaz-ing system chosen should isolate theglass from other parts of the wall.

Sealant and Gasket Considerations

Sealants and gaskets must withstandthe anticipated movement of memberswith which they come in contact, becompatible with other materials beingused, retain their properties after ex-posure to anticipated environmentalconditions and not impose any unduestress on the glass. Gaskets should bedesigned so that the section contactingthe outer glass face has a slope on it toprovide water runoff. Caskets shouldbe designed to give a uniform contactpressure of fourto six pounds per inchto make the seal watertight. Pressures

of over .10

pounds per inch should beavoided.

Glazing Guideline Dimensions

The glazing system should provide forminimum face clearances, edgeclearances, and nominal bite (Figure2). Adequate edge and face clearanceswill properly cushion the glass, ther-mally and mechanically isolate theglass framing members, and preventglass to metal contact. A nominal biteon the glass will provide adequateglass retention without excessive glasscoverage. Excessive glass coverage canincrease thermal stresses at the glassedge.

Setting Blocks

Glass lights should be set on two 80to 90 durometer neoprene settingblocks positioned at the quarterpoints. When this is impractical, the

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Roller Shade

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Metal Considerations

Because the coefficient of thermal ex-pansion for aluminum is approximately 13 x 1ùe inchedinch, thermal mor,ement of the curtain wall metal systemmust be properly considered. Expan-sion at high temperatures and contrac-

(b) (c)Blte Edge

(a)

FaceC eaËnce C ea6nce

G aasMonol¡thlc

lnsulallngGlas Unlt3

14' tla" elø"

sla" ¡ls"

l¿-1' 31rc" V2'

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l-1t2"

2

t-

1-1t2'

SEÍ|NG BLOCKS (2 per sill)

figure 2. glazing dimensions

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setting blocks can be installed towithin six inches of the vertical glassedge. The setting block should be 1/10

inch in length for each square foot ofglass area, but no less than four inchesin length. The setting block should be1/16 inch less than the full channelwidth and be of sufficient he¡ght toprovide the nominal recommendedbite and minimum glass edgeclearance (Figure 2).

Clearances, Shims, Drainage

Tolerances for the metal frame andglass must be taken into account sothat the glass is free to float in theopening and does not directly touchthe framing system (Figure 3). Edgeblocking should be used to limit lateralmovement or side walking of the glasscaused by horizontal expansion orcontraction and building vibration.Lack of edge blocking can permit glassto framing contact on one edge thatcould result in glass breakage. lf theglass is permitted to move to one sideof the glazing channel, air and waterinfiltration can result.

Water should not be permitted to re-main in the glazing rabbet. A weepsystem should incorporate enoughweep holes to ensure adequatedrainage; usual ly this consists of three% in. diameter holes or equivalent,equally spaced at the sill (Figure 2).

Glazing SystemPerformance Consideration

The glazing system must have thecapability of transferring wind and im-pact loads to the surrounding struc-ture while cushioning the glass. ltmust accommodate thermal expan-sion and contraction of the frame andglazing materials and accommodate

-'lCLEARANCE BETWEENCLASS AND FRAME

TOLERANCES FORSQUARENESS OF GLASSWITHIN MANUFACTURINGLIMITS

BITE IN ACCORDANCÊWITH MANUFACTURERSRECOMMENDATIONS

EDGE BLOCKSRECOMMENDED FORDRY GLAZING SYSTEMS

ALLOW FOR DBAINAGE@ JAMBS

SETTING BLOCK2 REOUIRED

WEEP HOLES 3 _ 3/8'' DIA.OR EOUIVALENT

figure 3. clearances, shims, drainage

temperature differentials caused byvarious adjoining material, shadingpatterns and shading devices. lt mustalso have the ability to prevent waterpenetration, prevent or minimize airinfiltration or exfiltration, and createthermal barriers to prevent heat lossthrough the frame and condensationon the frame.

At the same time, the system must alsopresent an appearance consistent withdesign goals and retain its appearanceand function over the anticipated lifespan of the building, given themaintenance program planned. Final-ly, the glazing system must match any

special performance required of therest of the glazing components.

Considerations that may influence thechoice of glazing systems include theinitial and replacement costs, and theworkmanship available, since wetsystems requi re better workmanship.The location of the glazing joints mayalso have an influence. These are,from least to most demanding: com-pletely interior, interior joint or ex-terior light, protected interior andexposed exterior.

The choice oÍ glazingsystem can alsobe influenced by the glazing or

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sealant materials used. Tinted andreflective coated glass can heatsealants to high temperatures. Reflec-tive glass also increases intensity ofultraviolet radiation on exterior capbeads. Polysulfides, polyurethanes,and silicones are generally well suitedfor severe conditions; acrylics andbuÇls can be used in less demandinglocations. Polysulfides, polyurethanes,and silicones can withstand morejoint movement than acrylic or butylbased sealants.

The location of sealant bead in glaz-ing joint is also important:

. Cap bead - expected to provideprimary weather seal, therefore itsadhesive strength, weather resis-tance, and movement capabilitywill be major properties toconsider;. Toe bead - applied on exteriorside of channel before glazingmaterial is installed; provides asecondary seal under cap bead or

preformed tape; ureather resistanceis less critical but adhesive andmovement capabilities remainimportant.. Heel bead - applied on interiorside after glazing material is install-ed; provides a seal at removablestop; same considerations as thosefor toe beads.

Solvent release butyls, acrylics andacetic acid liberating silicones shouldnot be used when they come in con-tact w¡th insulating glass units, as thesealant may not be compatible withthe sealing material used for the in-sulating glass units. The sealant supplier and the glass supplier should becontacted to determine specific com-patibi I ity recommendations.

lnstallation Considerations

Most sealants cannot be installedbelow 40 F. Silicones can be appliedat very low temperatures G20 F) withproper precautions. Acrylics may re-quire warming if temperatures isbelow 60 F. lf the sealant bead is in-stalled on a hot day, it will be in con-stant tension during cold weather. lfinstalled on a cold day, the bead willbe in constant compression duringhot weather. ldeally, the sealantshould be installed at a temperatureabout halfway between the extremes.Tack free time affects the timeavailable for tooling and clean up. lfthe new sealant bead must attain pro-perties quickly to withstand windloads or thermal movement, setting orfull cure time will be important andfast curing sealants should beconsidered.

The basic concept in glazing systemsto deal with the requirements cited isfloating the glazing material within itsframe þ, means of setting block placed

under the bottom edge of the glassand spacer shims to ensure properclearance between the face of theglazing material and the framingchannels.

Common Causes ofGlazing Failures

These are the most common causesof glazing failures:

¡ Installation at temperaturesbelow 40 F.

¡ Failure to properly seal miter andbutt joints.¡ Sash rabbet not clean and freeof contamination.¡ Lateral shifting or walking ofglass.o Failure to properly bed, cushionor center the glass.. lmproper glazing system used,one that is not suited to the sash

design or building conditions.¡ Setting blocks used incorrectlyor not at all.. Frames or surrounds out ofplane, improperly anchoredand/or out of square.. Lack of or improper positioningof spacers or edge blocks.¡ Damage to sash, rabbet or stops.o lncompatibility of sealants.

trt]Ttr

Excerpted from 'Architectural Glazing Prac-tices" which appeared in the September 1985issue of ASTM Standardization Nans, a publica-tion of the American Society for Testing andMaterials and usd with permission. Mr. RobettG. Spindler is Director of Sales and Marketingfor Cadinal lG.

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Glossary of Glazed Gurtann VMa[[ ]lernns

Acrylic - A group of water white thermoplastic resins formed bypolymerizing the esters of the acrylic acid.Adhesion - The ability ofa coating or sealant to stick or bond to thesurface to which it is applied.Adhesive Failure - Failure ofa compound by pulling away from thesurface with which it is in contact.Angle Bead - A bead of compound whose cross section is triangularshaped with the hypotenuse side exposed. Also called fillet.Annealed Glass - Normal glass that is basically free of residualinternal stresses so that it can freely cut. Annealed glass will frac-ture into jagged shards when broken.Annealing - A process ofslow cooling during curing which removesinternal stress inherent in the material.Anodize - To coat aluminum with an oxide coating by electrolyticaction. May include dye materials as colorants.Backer - A material placed into a joint primarily to control the depthofthe sealant.Bead - Sealant after application in ajoint irrespective ofthe methodof application, such as caulking bead, glazing bead.

BEDDING

Bed or Bedding - The bead ofcompound applied between light ofglass or panel and the stationary stop or sight bar ofthe sash or frame,and usually the first bead of compound to be applied when settingglass or panels.Bent Glass - FIat glass that has been shaped while hot into a cylin-drical or other curved shape.Bite - Amount of overlap between the stop and the panel or light.Block - A small piece of lead, neoprene, or other suitable materialused to position the glass in the frame.Bow and Warp - The departure from flatness normally inherent intempered glass from the manufacturing process. They are measuredwhile the glass is resting on a flat surface with the concave side down.Break Fattern - The resultant geometric pattem formed by the crackswithin an individual glass light when it is broken. (also calledfracture pattern)Butt Joint Glazing - Method of installing glazing where theglazing is held at the head and sill by conventional methods, and thevertical edges are sealed with a silicone sealant. No verticalframing members are used between lights.Buttering - Application of putty or sealant to the flat surface of a

member before placing the member in position, such as buttering a

removable stop before fastening the stop in place.Butyl - Synthetic rubber formed by the co-polymerization ofisobutylene with isoprene.Cap Bead - Bead of compound applied so as to have a slanted topsurface that will drain water away from the glass or panel. (also

called beveled bead)

BITE

Ceramic Frit - A colored reflective coating fused to a glass panel(usually spandrel panels) during the heat-treating process.Channel - A three-sided, U-shaped opening in sash or frame unitsin which the light or panel is retained by a removable stop. Contrastedwith a rabbet which is an L-shaped opening.Channel Depth - The measurement from the bottom of the channelto the top of the stop, or measurement from the sight line to the base

of the channel.Channel Glazing - The sealing of the joints around lighs or panelsset in a U-shaped channel employing removable stops.Channel Width - The measurement between stationary stops(or stationary stop and removable stop) in a U-shaped channel.Cladding - Material used as skin of curtain wall.Clips - Wire spring devices used to hold glass in rabbeted sash withoutstops, and face glazed.Cohesive Failure - Splitting and opening ofa compound resultingfrom over-extension of the compound.

CHANNEL DEPTH

Compatibility - The ability of two or more materials to exist in closeand permanent association for an indefinite period with no adverseeffect of the one on the other.

HEEL BEAD

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Compound - A formulation of ingredients, usually grouped as

a vehicle or polymer, pigment, and fillers, to produce caulkingcompound, elastomeric joint sealant, glazing putty, etc.

Compression - Pressure exerted on a material that tends to squeeze it.Curtain Wall - A non-bearing wall built between piers or columnsthat carries no structural loads except its own weight.Dry Glazing - Installing lighs using extruded rubber gaskets or closedcell sponge as one or both ofthe glazing seals.

Durometer - A device used to measure hardness. Also a word usedto describe the hardness of a material.Exterior Flush Glazing - Glazing installed outside the structuralframe of the building and attached on all four sides to the framingsystem by means of sealants. (also called structural glazing or s3opless

glazing.)Exterior Glazæd - Glass set from the outside of the building.Exterior Stop - The removable molding or bead that holds the lightor panel when it is on the exterior side of the light.Face Glazing - On a rabbeted sash without stops, the triangular bead

of compound applied with a glazing knife after bedding, setting andclippingt the light in place.

FILLET

Fillet - A bead of compound whose cross section is triangular withthe hypotenuse side exposed. (also called angle.)Float Glass - Glass composed of silica sand and added alkaline saltsmade by floating a continuous stream of molten glass onto a bed ofmolten tin.trTont Putty - The putty forming a triangular fillet between thesurface ofthe glass and the front edge ofthe rabbet.Gasket - Pre-formed shapes of rubber or rubberlike composition,used to fìll and seal ajoint or opening either alone or in conjunctionwith a supplemental application of a sealant.Glazing - The act of securing ofglass in prepared openings.Gun Consistency - Compound formulated in a degree of softnesssuitable for application through the nozzle of a caulking gun.Head - The top of a window or door.Heat Strengthened Glass - Glass strengthened by subjecting anneal-ed glass to a special heat treating process. Not considered a safetyglass because it fractures in the same patterns as regular annealedglass.Heat Tleated - Indicates that the glass has been treated to createcompression stresses in the surface.Heel Bead - Compound applied at the base of channel, after settingthe light or panel and before the removable stop is installed. One ofits purposes is to prevent leakage past the stop.

Insulating Glass Units - Multi-paned glass units consisting of twoor more panes of glass that enclose a hermetically sealed air space.Interior Glazed - Glass set from the interior of the building.

RABBET

FACE GI.AZINGSPACER SHIM

CLIP

SETTING BLOCK

Interior Stop - The removable molding or bead that holds the lightin place, when it is on the inside of the light.Jamb - The two vertical members of a window or door sash.See also Reveal.Kink - An abrupt deviation from a flat plane or normal bowcontour, most commonly found near an edge of a piece of temperedglass, A kink is always localized as compared with bow or warp.Knife Consistency - Compound formulated with a degree ofhardness suitable for application with a glazing knife.I¿minated Glass - Ttvo or more plies of glass sandwiched withclear or tinted polyvinyl butyl, a plastic, between them.Lateral Shims - Material used to center the light laterally withinthe glazing rabbet.Light - Another term for a pane of glass used in a window orcurtain wall.

HEAD

JAMB JAMB

SILL

Lock-Strip Gasket - A synthetic rubber gasket designed to engagethe edge of glass or other sheet material in a surrounding frameby forcing an interlocking filler strip into a pre-formed groove inthe face of the gasket. (also called zipper or structural gasket)Mastic - Heavy consistency compounds that may remain adhesiveand pliable with age.Methacrylate Lacquer - A temporary protective coating appliedto aluminum panels in the factory to protect against damage duringshipping.

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Mitred Joint - A 45 degree joint where horizontal members meetvertical members in a sash.Mullion - A vertical member that supPorts and holds two adjacentunits of sash, glass, panels or sections of curtain wall.Muntin - Bars that divide the window into smaller lights of glassor holds panes of glass within a window, window wall or glazed door.Neoprene - A synthetic rubber having physical properties closelyresembling those of natural rubbe¡ but not requiring sulphur forvulcanization.Panel System - A type of curtain wall system that is similar to a

unit system except that panels are homogeneous ståmped or castmetal, masonry, or concrete monoliths.Plate Glass - A high quality glass sheet of the same chemicalcomposition as sheet glass, but both faces are ground flat and paralleland then are polished.Polycarbonate - Plastic sheet used as glazing material, often inskylights.Polysulfide - A synthetic rubber used for sealing glazing joints.Folyurethane - A synthetic rubber used for sealing building joints.

MITRED JOINT

Pressure Wall - A wall assembly in which the units are all heldin place through pressure applied by tightening bols or clamps withgaskes.

þrolytic Deposition - A method of coating glass by applying themetallic oxide to hot glass in a heat strengthening oven or float glassline.Rabbet - A two-sided, L-shaped recess in sash or frame to receivelights or panels. When no stop or molding is added, such rabbetsare face glazed. Addition of a removable stop creates a channel.Racking - Movement or distortion of sash or frames due to lackof rigidity.Rainscreen - Waterproofing system that works by negating the waterdriving force of wind by equalizing air pressure on either side ofjoints.Rellective Glass - Clear or tinted glass coated with an extremelythin layer of metal or metallic oxide causing it to reflecta significant amount of the light and heat whicb strikes it.Reveal - The side of an opening in a wall for a window or door.Safety Glass - Glass so constructed, treated or combined with othermaterials as to reduce the likelihood of injuries to people when theglass cracks or breaks.Sash - The framework which holds the glass in a window or door.Screwon Bead or Stop - Stop, molding or bead fastened by machinescrews as compared to those that snap into position withoutadditional fastening.

Sealant - Compound used to fill and seal a joint or opening,usually by means of a caulking gun but may be pourable.Setting - Placement of lights or panels in a sash or frame. Also,action of a compound as it becomes firmer after application.

CAP BEAD

SHIM

SETTING BLOCK

Setting Block - Small block used to distribute the weight of theglass or panel to the strong points of the sash, aid in the centeringof glass or panel and to prevent glass to metal contact.Shading Patterns - Shadows thrown onto partial areas of installedglass which cause differential thermal stresses in the glass.Shim - Small blocks of composition, lead, neoprene, etc. placedunder the bottom edge of a light or panel to prevent its settling downonto the rabbet or channel, thus distorting the sealant.Shore Hardness - Measure of flrrmness of a compound by meansof a durometer hardness gauge.

Sight Line - Imaginary line along the perimeter of lights or panelscorresponding to the top edge of stationary or removable stops.

SIGHT LINEl

Silicone - A synthetic rubber used in sealing glazing joints andadhering exterior flush glazed panels and lights.Sill - A horizontal closure at the bottom ofa glazed unit or assembly.Sloped Glazing - Any glazing installed 15 degrees or more fromthe vertical.Spacers - Small blocks placed on each side oflighs or panels tocenter them in the channel and maintain uniform width of sealantbeads.Spandrel - The panel of a curtain wall bétween adjacent structuralcolumns and between the window sill and the window head belowit. Spandrels are usually used to obscure the floor line.Spandrel-and-Column-Cover - Custom designed curtain wallsystems where column cover panels are attached directly to the

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structural columns and spandrel panels connected to the floor slab,spanning the distance between columns.Spandrel Glass - Heat strengthened or tempered glass with aceramic frit color permanently fused to the interior surface. Nota vision light.Spline - A thin strip locking two panels together by being insertedinto a groove cut into the end of each panel.Squareness - Measurement of the accuracy of the glass dimensions.

CHANNEL

STATIONARY SÍOP

Stationary Stop - The permanent stop or lip ofa rabbet on the sideaway from the side on which lights or panels are set.Stick System - Tlpe ofcurtain wall system made up ofvertical andhorizontal members that assemble in place on the building.Stile - One of the upright structural members of a frame.Stop - The stationary lip on the back of a rabbet or the removablemolding at the front of the rabbet, either or both serving to holdlight or panel in sash or frame.

Tempered Glass - Glass strengthened by subjecting annealed glassto a special heat treating process that prestresses the glass makingit four times stronger than normal annealed glass. TÞmpered glasswill f¡acture into small pebble size fragments with rounded edges.Thermal Break - A gasket system that separates the outside andinside metal surfaces to reduce the heat flow by conductance throughthe metal.finted Glass - Glass made by adding various colorants to thenormal clear glass batch. The color reduces the solar transmittanceof the glass by frlæring out part of the color spectrum , cuts down glare,and may be used to control ultraviolet radiation and increase heatabso¡ption.Tlrc Bead - Compound applied at the base of a channel beforesetting the panel or light, its purpose being a secondary seal toprevent leakage past the exterior stop.

Ultraviolet - Short wave length light outside the visible spectrum atthe violet end.Unit - Term normally used to refer to one single light of insulatingglass.Unit System - Tlpe ofcurtain wall system that are prefabricated, shop

assembled, one story or multi-story units designed to interlockvertically and horizontally. Usually shop glazed.United Inches - Total of one width plus one height in inches.Unitized Stick System - Tlpe of curtain wall system that consists ofprefabricated vertical mullions and interlocking horizontal membersthat function like a unit system after installation.Vacuum lleposition - A method ofcoating glass by means ofvacuumchambers and electrical energy.Vent - Provide circulation of air between two partitions.Weatherstripping - A thin strip of material used to cover the jointbetween the sash and thejamb, casing or sill to keep out rain, air,dust, etc.

Wedge - An extruded resilient dry glazing component shaped so as

to develop a sloped, finished top.

WEEPHOLE

SECTION THROUGHHORIZONTAL MEMBER

PI.AN VIEW

Weep - A small opening in a wall or window to drain water to theexterior.Wet Chemical Deposition - A method of coating glass byprecipitating metal from a chemical solution by means of a reagent.Wet Glazing - Insølling lights by use of preformed tape, gunnablesealants, putty or glazing compounds.I{ired Glass - Glass that has a welded wire netting introduced intomolten glass just before rolling.

Glossary is publishd by Hoffmann Architects, specialists in in-vest¡gative and rchabi litatitrc arch¡tecturdengineering. Our offìceis locatd at 432 Washington Avenue, North Haven, CI 06473,Phone (203) 239-6660.

REMOVABLE STOP

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selvtces

Representative Projects

Specialists in the solution of roofing,masonry curtain wall and structuralproblems, Hoffmann Architectsrecently completed work on thefol lowing representative projects.

Curtain Wall and FacadeThe firm prepared constructiondocuments and will assist in the bid-ding process for roof and masonryrestoration at the Prentice-HallBuilding in Englewood Cliffs, NewJersey.

The Durst Organization, a New YorkCity real estate owner, retained Hoff-mann Architects to survey the condi-tion of the windows, masonry andcurtain wall of the Random HouseBuilding at 825 Third Avenue inManhattan.

For the Rockefeller Center Manage

825 lhird Avenue in Manhattan

ment Corporation, Hoffmann Ar-chitects prepared construct¡ondocuments and administered the con-struction contracts for curtain wallrehabilitation at the Warner Com-munications and Eastern Airlinesbuildings.

Another Rockefeller Center projectinvolves rehabilitation of thelimestone facade of the ExxonBuilding. Hoffmann Architects com-pleted construction documents andwill administer the construction con-tract for repair of the stonework,resealing of the joints and cleaning ofthe limestone. The building is ownedby Mitsui Fudosan (New York) lnc.

Hoffmann Architects is preparing con-struction documents and administer-ing the contract for an exteriorrehabilitation project involving re.pointing, sealing, painting and win-dow repair of a Southern NewEngland Telephone facility in NewHaven, Connecticut.

RoofingFor the Schering-Plough Corporation,Hoffmann Architects surveyed theroofs of 5 buildings in Bloomfield,New Jersey.

ln White Plains, New York, the firmsurveyed the condition of the roof ofa General Foods Corporation facility.

For Union Carbide Corporation,Hoffmann Architects sunq,ed the roofand is preparing constructiondocu ments for a facility inMoorestown, New Jersey.

Journal i s a publ icati on of Hoffman nArchitects, specialists in in-vestigative and rehabilitative ar-ch itecture/engi neeri ng, i ncl ud i ngthe analysis and solution of roofing,exterior wall, glazing and structuralproblems.

We welcome contributions to HNJfrom our clients and friends. Pleasesend news, technical information,address changes or requests for freesubscriptions to Ann Prokop,Hoffmann ArchitectlJournal at 432Washington Avenue, North Haven,Connecticut 06473.

W h i I e H offman n A rch i tectlJ ou rn alattempts to provide the most ac-curate information on general sub-jects, it is not intended to be asubstitute for professional arch itec-tural and engineering services. Westrongly urge you to consult aqualified rehabilitative architec-tural/engineering firm (ours) foranswers to specific questions. Formore information on the seruices weoffeç please call Brian Schafer at(20Ð 239-6660.

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Precautions inGlass

SpecityinS

By Donald Vild, PE

Low emittance (Low-E) coatings offermajor improvements in the thermalperformance of insulating glass units.But there is a potential pitfall that isoften overlooked.

Low emittance coatings for glass havebeen available for 40 years or more.Unfortunately, they tended to havedistinct, unpleasant colors by reflec-tion, often with severe variations.\Mthin the past decade, improrcmentsharre been made in the aesthetic quali-ty of the coatings. The color is subtle,often unnoticed, and uniform. Thishas caused a rapid growth in the useof Low-E coatings in insulating glassunits.

The coating is applied to one of theair space surfaces of the glass andreduces the heat transfer by radiationacross the air space of the insulatingglass unit. Thermally, the result issimilar to inserting a blanket of fiberglass in the air space. The outboardglass is closer to outdoor airtemperature and the inboard glasscloser to room temperature.

Nominally, a double (two panel) in-sulating glass unit with a Low-Ecoating will have the same insulatingvalue as a triple (three pane unit)without a Low-E coating. Equal in-sulating value is achieved at a markd-ly reduced cost. lnsulating glass with

a Lor¡¡-E coating is primarily a residen-tial product. lt is furnished on clearglass only, except in rare cases. Thecoating does little to reduce solar heatgain - the main concern in commer-cial buildings.

Regardless, its use is increasing incommercial buildings, principally inconjunction with reflective and heatabsorbing glasses. This is where theproblem starts. The effect of thecoating on glass thermal stresses isignored.

Based on analysis and practical fieldexperience, certain types of high per-formance insulating glass units havemost often been furnished with bothpanes of annealed glass. Typical areunits with gray, green, or bronze tintedheat absorbing glass as the outboardpane. The maximum edge thermalstress for these types of glass in a wallwith no major exterior shading isroughly 2050 psi. Breakage would notbe expected. The stress might increaseto 2400 to 27OO psi when there isshading from vertical or horizontalprojections. At these levels, breakagewould be minimized if the glass cut-ting is proper and the edges are notseriously damaged. Field experiencehas supported this.

The pitfall

However, when a Low-E coating is in-cluded, the outboard sunlit glass willreach a higher maximum temperature

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technical notesthan without the coating, all otherthings being equal. This increases theedge thermal stresses and the poten-tial for thermal stress breakage.

For insulating glass units with heat aþsorbing glass, the stress increases toroughly 2400 psi for a flush wall andto a range of 2800 to3200 for a wallwith shading from projections. Thisassumes that the coating adds no ab.sorptance to the glass. For most typesof Low-E coatings, their absorptancewill add 200 to 300 psi to the stresses.

It is generally agreed that an edgethermal stress of 3000 psi is the upper working limit for annealed glass.Above this value, thermal breakage ispossible. lf thermal breakage is to beprerrented, annealed glass may have tobe heat-strengthened when a Low-Ecoating is applied. Examples for manyreflective coated glasses used in com-bination with a Low-E coating lead tothe same conclusion.

Two basic precautions to follow:

a. Do not assume that glass productsthat have historically been fur-nished with annealed glass willnot be affected by a Low-Ecoating.

b. Require that a thermal stress

analysis be furnished by the glassmanufacturer or fabricator. Be cer-tain that the analysis addresses theshading patterns that may occuron the building. lnln

Reprintd from EXTERIORS magazine, Wnter1988. Used with permission. Donald Vild, PEis with Donald J. Vild Associates, Orcgon, Ohio.

W¡ndow Standards Alert

ln 1985, the American ArchitecturalMan ufaturers Association (AAMA) i n-troduced a voluntary standard foraluminum prime windows. ln 1988,AAMA updated those standards and asof June 30, 1989, all certification testswill be to those standards.

the specifier to select windows thatwill not have an objectionable amountof condensation based on outsidetemperature and indoor relativehumidity factors, also replacing Romannumeral desi gnations. A specificationfor "Dual Action" windows has beenadded. Dual Action windows are thosethat may be opened two different waysat the user's option.

Wind Velocity Pressure. ln some com-binations of wind speed and height,this may require changing the windowspecification to the next highest Pefor-mance Class designation compared tothe old standard. Several Area Factorsin the Class Table have been increas-ed. Area factors are factors by whichthe tabulated maximum glass area maybe multiplied to take into accountthestrength of various types of glass.

Thermal transm ittance and condensa-tion resistance have been added, butare optional. "U-Value Classes,, havereplaced the Roman numeral designa-tions formerly used by AAMA. ,,-RfClasses" have been added to enable

Wind load tables have also been revis-ed to reflect an upgrading of DesignPressure from 1.-l to 1.25 times the

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staff news

Staff News

fane B. Beaudry and Frank B. Scherr,associate members of AlA, are par-ticipating in the National Council ofArchitectural Registration Board's(NCARB) lntern DevelopmentProgram.

Brian W. Schafer has been appointedto the Membership Development TaskForce of BOMA New York.

Richard P. Kadlubowski and lane B.Beaudry recently completed CADDtraining for the firm's new CADVANCEcomputer aided design and draftingsystem.

Hoffmann Architects is pleased to an-nounce the addition of three ns,v staffmemben. They are Ernest E. SwanberyAlA, who joins us as senior architect,Scott D. Chamberlain, drafter/projectrepresentatiw, and Malik M. Nawaz,also d rafter/project representatir¡e.

GlazingSurfaces

Which glass surface in an insulatingglass unit is considered the No. 1? Forarchitectural glass materials, the glasssurfaces are counted from the outsidein. Therefore, the No. 1 suÉace istotheexterior of the building. This becomesextremely i mportant when specifyi ngcoated glass for multiple glazing applications, since where the coating islocated affects its usefulness.

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We've Moved!

Hoffmann Architects is pleased toannounce its move to new offices at432 Washington Avenue, North Harren,Connecticut O6473.Our new phone is(203) 239-6660.

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