d third issue 89 Architectural Glazing Practices Preventing problems with glazed curtain walls By Robert G. Spindler Aluminum curtain walls for commer- high performance reflective glasses, the materials used in the glazing chan- cial office buildings began to evolve sealants, gaskets and other materials nel are affected by ultraviolet light, in the early 1950's. Today these glass used in the glazing channel are expos- these materials most likely will and metal building facades have pro- ed to considerable temperature ex- degradg producing costly repairs on ven themselves cost-effective, tremes and high concentratíons of the project. aesthetical ly pleasing, energy efficient ultraviolet light. Temperatures of and capable of per{orming under monolithic reflective glass in spandrel Chosen glass materials must meetthe stringent environmental conditions. or nonvision areas have been architect's specifications for perfor- Architects continue to design unique measured at 1900 F. This imposes con- mance, which usually are: structures with innovative concepts re- siderable expansion and high . Glass thickness and type (an- quiring, at times, the building temperature resistance requirements nealed, heat treated) to meet materials to meet extraordinary on all materials that come in contact specified and wind load specifications. With the advent of new with the glass. Depending on the type requirements, materials and philosophies in ar- of glass, significant amounts of o Thermal efficiency requirements chitectural design concepts, the total ultraviolet light can be reflected from for both summer and winter curtain wall, including metal, glass, the glass into the glazing material. lf conditions, sealants and gaskets, has had to per- o Aesthetic requirements, form under a variety of conditions; . Class type to resist the potential namely, thermal environment, of breakage due to thermal stress negative and positive wind loads, wall conditions, and system dead loads, water penetration, o Building codes. and air infiltration. The design of a properly functioning curtain wall system requires specific consideration GIass and Shading Devices of the glass retaining system or glaz- ing members. Draperies, blinds or other interior shading devices must be hung so as to provide space either at the top and bot- Glass Considerations tom or one side and bottom to permit natural air movement over the room With advent of heat absorbing and side of the glass, Figure 1 depicts the volume 7 number 3 copyright Hoffmann Architects 1989 I c .10 criteria that should be metto avoid for- tion at low temperatures may impose of over pounds per inch should be mation of a heat trap. The use of heat significant stresses on glass, sealants avoided. treated glass may be necessary to off- and gaskets. set the effects of a lack of adequate vent¡lation. The metal framing system must Glazing Guideline Dimensions uniformly support the glass. The glass industry has set guidelines that the The glazing system should provide for framing needs to be within limits for minimum face clearances, edge bow, squareness, and corner offset as clearances, and nominal bite (Figure indicated: 2). Adequate edge and face clearances will properly cushion the glass, ther- o Bow - 1/16 inch in any four foot mally and mechanically isolate the length of framing; glass framing members, and prevent . Squareness - 1/B inch difference glass to metal contact. A nominal bite l-1t2" in the lengths of the diagonals of on the glass will provide adequate l-1r2 (37mm) the frame; glass retention without excessive glass Minimum o CornerOffseL-1/32 inch ateach coverage. Excessive glass coverage can Glass Clearance corner, increase thermal stresses at the glass edge. The glass industry has also set t- guidelines on glass framing member Venetian Blind deflection under design loads. This Setting Blocks 2'(50mm) Drapery or 2 guideline states that Minimum Roller Shade to minimize the Clearance potential for glass breakagg the glass Glass lights should be set on two 80 surround should not impose any ben- to 90 durometer neoprene setting ding or high concentrated com- blocks positioned the quarter 1-1t2' at pressive loads on the glass and should points. When this is impractical, the not deflect more than 1/l75th of its span under loading or 3/4 inch, Air Flow whichever is less. ln addition, the glaz- ing system chosen should isolate the glass from other parts of the wall. à Figure 1. indoor shading and heat duct Iocations 1. Minimum 1-Vz" clearance required top and Sealant and Gasket Considerations bottom or one side and bottom between b shading device and surrounding construction. Sealants and gaskets must withstand 2. Minimum 2" clearance between glass and the anticipated shading device. movement of members 3. Heating/cooling outlets must be to room with which they come in contact, be SEÍ|NG BLOCKS (2 per sill) side of shading device. compatible with other materials being used, retain their properties after ex- WEEP YSTEIV{ {3 ær sil) posure to anticipated environmental (a) (b) (c) Metal Considerations conditions and not impose any undue OreEll Face Blte Edge Thlckness C eaËnce C ea6nce stress on the glass. Gaskets should be Because the coefficient of thermal ex- designed so Monol¡thlc that the section contacting G aas tla" elø" 14' V4 pansion for aluminum is approximate the outer glass face has a slope on it to ly 13 x 1ùe inchedinch, lnsulallng sla" ¡ls" thermal mor,e provide water runoff. Caskets should Glas Unlt3 l¿-1' 31rc" V2' 1A ment of the curtain wall metal system be designed to give a uniform contact must be properly considered. Expan- pressure of fourto six pounds per inch sion at high temperatures and contrac- to make the seal watertight. Pressures figure 2. glazing dimensions 2 I d setting blocks can be installed to within six inches of the vertical glass edge. The setting block should be 1/10 -'l inch in length for each square foot of glass area, but no less than four inches CLEARANCE BETWEEN in length. The setting block should be CLASS AND FRAME 1/16 inch less than the full channel TOLERANCES FOR width and be of sufficient he¡ght to SQUARENESS OF GLASS provide the nominal recommended WITHIN MANUFACTURING bite and minimum glass edge LIMITS clearance (Figure 2). BITE IN ACCORDANCÊ WITH MANUFACTURERS RECOMMENDATIONS Clearances, Shims, Drainage EDGE BLOCKS RECOMMENDED FOR Tolerances for the metal frame and DRY GLAZING SYSTEMS glass must be taken into account so that the glass is free to float in the '//) V//' ALLOW FOR DBAINAGE JAMBS opening and does not directly touch @ the framing system (Figure 3). Edge SETTING BLOCK blocking should be used to limit lateral 2 REOUIRED movement or side walking of the glass WEEP HOLES 3 _ 3/8'' DIA. caused by horizontal expansion or OR EOUIVALENT contraction and building vibration. Lack of edge blocking can permit glass to framing contact on one edge that could result in glass breakage. lf the glass is permitted to move to one side of the glazing channel, air and water infiltration can result. figure 3. clearances, shims, drainage Water should not be permitted to re- main in the glazing rabbet. A weep system should incorporate enough temperature differentials caused by special performance required of the weep holes to ensure adequate various adjoining material, shading rest of the glazing components. drainage; usual ly this consists of three patterns and shading devices. lt must % in. diameter holes or equivalent, also have the ability to prevent water Considerations that may influence the equally spaced at the sill (Figure 2). penetration, prevent or minimize air choice of glazing systems include the infiltration or exfiltration, and create initial and replacement costs, and the thermal barriers to prevent heat loss workmanship available, since wet Glazing System through the frame and condensation systems requi re better workmanship. Performance Consideration on the frame. The location of the glazing joints may also have an influence. These are, The glazing system must have the At the same time, the system must also from least to most demanding: com- capability of transferring wind and im- present an appearance consistent with pletely interior, interior joint or ex- pact loads to the surrounding struc- design goals and retain its appearance terior light, protected interior and ture while cushioning the glass. lt and function over the anticipated life exposed exterior. must accommodate thermal expan- span of the building, given the sion and contraction of the frame and maintenance program planned. Final- The choice oÍ glazingsystem can also glazing materials and accommodate ly, the glazing system must match any be influenced by the glazing or 3 ¿ preformed tape; ureather resistance under the bottom edge of the glass is less critical but adhesive and and spacer shims to ensure proper movement capabilities remain clearance between the face of the important. glazing material and the framing . Heel bead - applied on interior channels. side after glazing material is install- ed; provides a seal at removable stop; same considerations as those Common Causes of for toe beads. Glazing Failures Solvent release butyls, acrylics and These are the most common causes acetic acid liberating silicones should of glazing failures: not be used when they come in con- ¡ Installation at temperatures tact w¡th insulating glass units, as the below 40 F. sealant may not be compatible with ¡ Failure to properly seal miter and the sealing material used for the in- butt joints. sulating glass units. The sealant sup ¡ Sash rabbet not clean and free plier and the glass supplier should be of contamination. contacted to determine specific com- ¡ patibi I ity recommendations. Lateral shifting or walking of glass. o Failure to properly bed, cushion lnstallation Considerations or center the glass. sealant materials used. Tinted and . lmproper glazing system used, reflective coated glass can heat Most sealants cannot be installed one that is not suited to the sash sealants to high temperatures.