Curtain Walls Report

2011

CURTAIN WALLING & STRUCTURAL GLAZING

advanced construction-I Prof. incharge:- sarika bahadure

Ashwini, Sanjay, ajay Vll th sem. 8/10/2011

HISTORY AND DEVELOPMENT

Early Concepts of Curtain Wall Construction

1.Early 20thcentury –industrial buildings in Germany pioneer glass curtain wall construction – Fagus Shoe –Last Factory (1911). 2.Architects Walter Gropius, Adolf Meyer develop glass curtain wall in the Bauhaus complex, Dessau –Germany. 3.The glass wall has mullions and transoms. Uninterrupted span between floors and is nonstructural. 4.Structural frame is set back and glass wall is suspended from the structure. 5.Allows for interior spaces to have greater daylight penetration. 6.Architects fascinated with the concept of transparency and dematerialization of the building envelope. 7.From the late 1940‟s to early 1960‟s glass curtain wall construction is explored on a wide range of scales in various cities. 8.Most influential expression in three office buildings constructed in New York : United Nations Secretariat, Lever House and the Seagram Building.

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 Oriel chambers, liverpool, england,1864. the world's first glass curtain walled building. the stone mullions are decorative.

 Glass curtain wall of the bauhaus dessau

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CURTAIN WALLING CONSTRUCTION

Curtain wall is a term used to describe a building facade which does not carry any dead load from the building other than its own dead load. These loads are transferred to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, wind forces acting on the building, seismic forces (usually only those imposed by the inertia of the curtain wall), and its own dead load forces.

First curtain walls were made with steel mullions, and the plate glass was attached to the mullions with asbestos or fiberglass modified glazing compound. Eventually silicone sealants or glazing tape were substituted. Some designs included an outer cap to hold the glass in place and to protect the integrity of the seals. The first curtain wall installed in New York City was this type of construction. Earlier modernist examples are the Bauhaus in Dessau and the Hallidie Building in San Francisco. The 1970’s began the widespread use of aluminum extrusions for mullions. Aluminum offers the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease. Similarly, sealing methods and types have evolved over the years, and as a result, today’s curtain walls are high performance systems which require little maintenance.

Main types of Curtain wall construction 1.Stick System. 2.Unitised System. 3.Panellised System. 4.Spandrel panel ribbon glazing. 5.Structural sealant glazing. 6.Structural glazing.

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1. STICK SYSTEM •Horizontal and vertical framing members („sticks‟) are normally extruded aluminium protected by anodising or powder coating, but may be cold-rolled steel (for greater fire resistance) •Members are cut to length and machined in the factory prior to assembly on site as a kit of parts: vertical mullions, which are fixed to the floor slab, are erected first followed by horizontal transoms, which are fixed in-between mullions. •Mullions are typically spaced between 1.0 and 1.8m centres. •Into the framework are fitted infill units, which may comprise a mixture of fixed and opening glazing and insulated panels (which may have metal, glass or stone facings). •These units are typically sealed with gaskets and retained with a pressure plate, screw-fixed every 150-300 mm, although hammer-in structural gaskets are used for some stick systems. •The pressure plate is generally hidden with a snap-on cosmetic cover cap or overlapping gaskets.

2. UNITISED SYSTEM •Unitised systems comprise narrow, storey-height units of steel or aluminium framework, glazing and panels pre-assembled under controlled, factory conditions. •Mechanical handling is required to position, align and fix units onto prepositioned brackets attached to the concrete floor slab or the structural frame. •Unitised systems are more complex in terms of framing system, have higher direct costs and are less common than stick systems. •The smaller number of site-sealed joints in unitised curtain walling simplifies and hastens enclosure of the building, requires fewer site staff and can make such systems cost effective. •If construction joints interlock consideration must be given to how damaged units could be removed and replaced. •The reduced number of site-made joints compared with stick systems, generally leads to a reduction in air and water leakage resulting from poor installation

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3. PANELLISED SYSTEM

•Large prefabricated panels of bay width and storey height, which connect back to the primary structural columns or to the floor slabs close to the primary structure. •Fixing the panels close to the columns reduces problems due to deflection of the slab at mid span, which affect stick and unitised systems. •Panels may be of precast concrete or comprise a structural steel framework, which can be used to support most cladding materials (e.g. stone, metal and masonry). •Joints may comprise gasketted interlocking extrusions, gaskets between separate extrusions or wet applied sealant. •The advantages of using panellised systems stem from the high utilisation of factory prefabrication, which allows better control of quality and rapid installation with the minimum number of site-sealed joints. •However to be cost effective a large number of identical panels is required. •Panellised systems are less common and more expensive than unitised construction. •The size and weight of panels is limited by the practicalities of manufacture, handling, storage, transport and erection. 4. SPANDREL PANEL RIBBON GLAZING •Spandrel panel ribbon glazing is a long or continuous run of vision units fixed between

spandrel panels supported by vertical columns or the floor slabs. •Glazed areas may comprise: 1.Several standard windows fixed together on site by joining mullions, 2.Pre-glazed, bay width, factory-assembled frames, or 3.Individual framing sections and glass infill panels which are site assembled. •Ribbon glazing is often used in conjunction with spandrel panels, that is, horizontally spanning prefabricated or precast concrete units. •Ribbon glazing/spandrel panel construction generally results in building having a horizontal banded or strip appearance.

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5. STRUCTURAL SEALANT GLAZING •Can be applied to stick curtain wall systems and windows, particularly ribbon glazing.Also be used in unitised and panellised systems. •Instead of mechanical means (i.e. a pressure plate or structural gasket), the glass infill panels are attached with a factory-applied structural sealant (usually silicone) to metal carrier units which are then boltedinto the framing grid on site. •External joints are weather sealed with a wet-applied sealant or a gasket. •All structural silicone joints are now made in a factory. •All elements used in the construction must be compatible with the silicone sealant. •Generally, the glass is mechanically supported to reduce the size of the sealant bead. •Structural sealant glazing can be used to create a building exterior that is free from protrusions, but the framing system will be visible at night when backlit. •The framing members are often more widely spaced than for traditional stick systems. •Any of the previous types of curtain walling and ribbon glazing could incorporate structural silicone glazed elements. 6. a) STRUCTURAL GLAZING-Bolted Assembly •Sheets of toughened glass are assembled with special bolts and brackets and supported by a secondary structure, to create a near transparent facade or roof with a flush external surface. •The joints between adjacent panes/glass units are weather sealed on site with wet-applied sealant. 6. b) STRUCTURAL GLAZING-Suspended Assembly •Here the glass is fixed together with corner, rectangular, patch plates and the whole assembly is then either suspended from the top or stacked from the ground and wet-sealed on site. •Suspended glazing systems utilise the minimum amount of framing for a given glass area. •Glass fins may be used to brace the assembly. •In some designs a light truss stabilises the wall and transfers wind loading, while the weight of the glass is transferred through the corner plates and suspension system

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INFILLS  Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element.  Glass  Stone Veneer  Metal panels  louvers and operable windows or vents Glass  By far the most common glazing type, glass can be of an almost infinite combination of color, thickness, and opacity.  For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. Presently, 1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the rest of the building.  In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Larger thicknesses are typically employed for buildings or areas with higher thermal, relative humidity, or sound transmission requirements, such as laboratory areas or recording studios. Stone veneer  Thin blocks (3 to 4 inches (75-100 mm)) of stone can be inset within a curtain wall system to provide architectural flavor.  The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size & various Shades  Common stone types used are:  Arriscraft (calcium silicate);  Granite;  marble;  travertine; and  limestone

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Panels  Metal panels can take various forms including aluminum plate; thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; and panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel. Other opaque panel materials include fiberreinforced plastic (FRP), stainless steel, and terracotta. Terracotta curtain wall panels were first used in Europe, but only a few manufacturers produce high quality modern terracotta curtain wall panels

Design

Curtain wall systems must be designed to handle all loads imposed on it as well as keep air and water from penetrating the building envelope. Loads The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building. The building structure needs to be designed and account for these loads. Dead load Dead load is defined as the weight of structural elements and the permanent features on the structure. In the case of curtain walls, this load is made up of the weight of the mullions, anchors, and other structural components of the curtain wall, as well as the weight of the infill material. Additional dead loads imposed on the curtain wall, such as sunshades, must be accounted for in the design of the curtain wall components and anchors. Wind load Wind load acting on the building is the result of wind blowing on the building. This wind pressure must be resisted by the curtain wall system since it envelops and protects the building. Seismic load Seismic loads need to be addressed in the design of curtain wall components and anchors. In most situations, the curtain wall is able to naturally withstand seismic and wind induced building sway because of the space provided between the glazing infill and the mullion.

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Snow load Snow loads and live loads are not typically an issue in curtain walls, since curtain walls are designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so, these loads may need to be considered. Thermal load Thermal loads are induced in a curtain wall system because aluminum has a relatively high coefficient of thermal expansion. This expansion and contraction is accounted for by cutting horizontal mullions slightly short and allowing a space between the horizontal and vertical mullions. In unitized curtain wall, a gap is left between units, which is sealed from air and water penetration by wiper gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to account for movement. Incidentally, this slot also accounts for live load deflection and creep in the floor slabs of the building structure. Blast load Accidental explosions and terrorist threats have brought on increased concern for the fragility of a curtain wall system in relation to blast loads Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a bomb attack. As such, blast resistant curtain walls must be designed to withstand such forces without compromising the interior of the building to protect its occupants. Since blast loads are very high loads with short durations, the curtain wall response should be analyzed in a dynamic load analysis, with full-scale mock-up testing performed prior to design completion and installation. Infiltration Air infiltration is the air which passes through the curtain wall from the exterior to the interior of the building. The air is infiltrated through the gaskets, through imperfect joinery between the horizontal and vertical mullions, through weep holes, and through imperfect sealing. Water penetration is defined as any water passing from the exterior of the building through to the interior of the curtain wall system. Sometimes, depending on the building specifications, a small amount of controlled water on the interior is deemed acceptable. Deflection One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. This translates to three times more deflection in an aluminum mullion compared to the same steel section under a given a load. Also, if deflection of a wall is quite noticeable, public perception may raise undue concern that the wall is not strong enough. Deflection in mullions is controlled by different shapes and depths of curtain wall members Another way to limit deflections in a given section is to add steel reinforcement to the inside Curtain Walling and Structural Glazing

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tube of the mullion. Since steel deflects at 1/3 the rate of aluminum, the steel will resist much of the load at a lower cost or smaller depth. Strength Strength (or maximum useable stress) available to a particular material is not related to its material stiffness (the material property governing deflection); it is a separate criterion in curtain wall design and analysis. This often affects the selection of materials and sizes for design of the system.

Fire safety Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully tempered glass to allow full fracturing of the panel into small pieces and relatively safe removal from the opening.

Features 

Easy frame handling and assembly method.

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Fully weather-stripped for high performance.

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50mm, 75mm, 100mm, 125mm sections to meet most structural requirements.

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Multiple capping options to suit decorative requirements.

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Reinforcing available for large spans.

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Suitable for single or double-glazing.

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Wide choice of paint finishes.

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Dual colour options.

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Accommodates glazing from 6 mm single to 28mm double glazed units or panels.

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Can be used with other Sigma Windows and Doors ranges of Windows and Doors.

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U-value and I value calculations available on request.

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Cost  An Aluminium Composite panel normally costs Rs.60 to 240 per sq.ft  Good quality Glass 150 per sq.ft

Maintenance and repair  Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years. Removal and replacement of perimeter sealants require meticulous surface preparation and proper detailing.  Aluminum frames are generally painted or anodized. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating.

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