BUILDING CONSTRUCTION SUBMISSION 2

SIDDHARTH AGARWAL 32/11

The skeleton construction which has increasingly been used since the turn of the century has inevitably given rise to new possibilities for the facade. The size, shape and number of windows were no longer limited by structural requirements following the introduction of curtain facades, since the loads were now primarily transmitted by posts and columns.

DESIGN

Todays modern facades are characterized by external wall elements equal to one floor in height and insertedbetween the respective structural floors.Non-supporting metal facades suspended in front of the building have increasingly become established for economic reasons, particularly in high-rise construction.The scope for design is enlarged by coloured or mirrored window panels and linings of natural stone, ceramic tiles or brick. Almost any desired appearance can be produced.

TECHNICAL PROPERTIES

The panes are made of high-grade glass filled with noble gases or with a surface coating that reflects infraredlight. On the inside, modern facades are highly impermeable to water and water vapour in order to prevent damage due to moisture.Modern facades also require a sophisticated ventilation and cooling system. The air-conditioned or twin facade is a case in point. Here an additional facade of laminated glass is arranged in front of the conventional facade, thus creating a space through which air can circulate. More complex ventilation concepts for routing air into and out of the building may be realized by including additional vertical and horizontal bulkheads. Individually controlled ventilation flaps are capable of providing a more natural and far less complex exchange of air.

PRODUCTION AND ASSEMBLY

The frames, glazing, parapet lining, sunshades and anti-glare finish, as well as thermal insulation and sealing are all assembled into single-storey facade elements in the manufacturers plant.

In many cases, such technical equipment parts as radiators, air outlets and the ducting for electrical and electronic equipment are also already integrated at this stage.

In the meantime, fixing elements can be mounted on the shell of the high-rise building. These elements can usually be displaced in three planes to compensate the dimensional tolerances occurring in the shell.

The facade elements as such are fitted without the help of scaffolding, thus greatly reducing the time required for this work.

The frame profiles are assembled with labyrinthine indentations to compensate the deformation arising in the building as a result of wind and live loads, as well as temperature differences. Permanently elastic rubber profiles ensure that the facade remains impermeable to air and water.

TYPES OF FACADE

UNITISED FACADE SYSTEMS

Whereas the majority of curtain wall systems are constructed on site as stick systems with linear framing components and planar infills, unitised systems entail factory fabrication and assembly to large panels and may include glazing. These completed units are mounted to the primary structure of the building.

The benefits are fabrication in controlled interior environments and faster on-site installation times. A disadvantage can be greater use of materials and greater logistical effort. Today, large buildings or areas with high labour costs benefit the most economically.

STRUCTURAL GLAZING

Structural -Glazing (SG) or Structural-Sealant-Glazing (SSG) describes a curtain wall system in which the glass panes are fixed to the frame by a glue or sealant. Pressure plates and screwed connections that press the glass panes onto the rebate gaskets are needed.

The self-weight of the glass panes is typically transferred to the frame by hidden mechanical devices. Glass panes can even be used to stiffen the faadestructure.

DOUBLE FACADE

A double faade or double-skin facade is a faade system that consists of two skins placed in such a way that air flows in the intermediate cavity. Typically, insulated glass units form the inner skin and the outer skin are made of single glass layers but other constellations are possible.

The ventilation of the cavity can be natural, fan supported or mechanical. Apart from the type of ventilation inside the cavity, the origin and destination of the air can differ depending mostly on climatic conditions, the use, the location, the occupational hours of the building and the HVAC strategy.

FRAMELESS FACADE

A frameless faade consists mainly of single glass panes or insulated glass units without framing elements. Instead, the necessary joints between the glass panes are closed by backfill elements and silicone sealants.

The glass panes are directly connected to structural load bearing components on the inside or the outside.

POLYVALENT WALL

The Polyvalent Wall is a vision for a new faade type. It was intended to be built up from different layers on top of a glass layer to act as absorber, radiator, reflector, filter and transfer device at the same time, and needed to operate at a molecular level rather than at a mechanical level.

It also includes sensing nodes and a local micro brain, connected to a central processor, to assure that the faade reacts to permanently changing external and internal conditions.

Historic curtain walls

Rolled-steel construction: The Palm House

Detail of connection between colum and main structure Side elevation of the Palm Houset

The main structure of the faade consists of I-shaped, bent wrought-iron beams with a height of 22.8cm and a length of 3.6m bolted together to form beams of an overall length of 12.8m. The distance of this main grid is 3.81m.

It is subdivided by wroughtironand cold bent mullions that follow the entire length of the quadrant shaped envelope.

Sketch of mullion, Palm House

Detailed function structure of the faade of the Palm House

Standard steel profiles: Crown Hall

The column-free space of 66m by 36m was created by a construction suspended from a 18m grid of 36m spanning beams.

The building shows a metal-glass faade. It is not a curtain wall because it forms an integral part of the external load-bearing structure of the building.

Its functionality is very basic compared to modern curtain walling systems. It acts as a weather shield with a high level of transparency. The insulation capacity is very limited and sun-shading is only provided by internal sun screens; actually separated from the faade.

The detail shows that it is composed of standard steel profiles and glass sheets that are fixed by putty and steel glazing beads.

The entire faade is assembled on-site and the different parts are joined by welding and screwing.

Detailed exterior and interior views show welded connections. Only glass beads are fixed by screws

FAADE DETAIL

The main structure is composed of I-beams, L-profiles and flat irons. These form an integral structure.

The putty serves several functions: It allows movement, helps fixing the glass panes and seals the system. The glazing beads are decoupled by being screwed into place; they allow the glass panes to be exchanged in case of damage.

Contemporary curtain wall faades and components

In terms of the structural concept) defines three main areas of construction for curtain walls:

Primary structure forming the main load-bearing structure of the building Secondary structure, which is the load-bearing structure for the faade (curtain wall) and constitutes the connecting element between levels one and three Infill elements

The primary structure takes on the load-bearing function of the entire building and transfers the loads from the faade to the foundation.

The secondary structure comprises the load-bearing structure of the faade. It transfers its loads onto the primary structure. At this interface to the interior the differing movements of the structure of the building and the faade need to be balanced.

The structure of the building usually falls under the subcontract for concrete work whereas the faade is assigned to the metal subcontract.

At the same time, infill elements such as glazing, panels etc. are mounted on the secondary structure.

General layout of curtain walls, primary, secondary structure, infills and interfaces between them

CURTAIN WALL SYSTEM

Perspective of a typical cross joint of a curtain wall

Horizontal detail section of curtain wall

Assembling a curtain wall on-site

Curtain wall with pre-assembled ladder`

UNITISED SYSTEMS

Unitised faade systems belong to the family of curtain walls but follow a slightly different strategy. In order to be able to manufacture the faade in the workshop, it is built in components.

Therefore, a sectional interface needs to be introduced that allows the connection of the components on-site.

Assembling a unitised system

The unitised approach results in a different constructional strategy:

Instead of a mullion, a more complex frame system is needed. The size of the combined frames is usually bigger than that of a single mullion, and more material is needed to stiffen the units during transport.

But still, especially for large and complex projects unitised systems can result in overall cost savings.

FRAMELESS SYSTEMS

With the curtain walling system being complex and, in terms of thermal insulation, the weakest spot in faades, frameless structures made from insulated glass units seem a logical step. Glass panes come in limited sizes; which is a disadvantage but is also useful in terms of handling the parts.

They need to be joined with water, wind and moisture tight connections. These connections should also have appropriate thermal qualities. The glass panes have to be connected to a load-bearing structure. This connection has to allow tolerances and movement.

One type of frameless systems is the so called Frog hand or Spider faade.

Several systems exist on the market, which include the construction of the structural spiderand the point fixing.

The connection between spider and point fixing has to handle tolerances and movement, and the connection is a typical slot interface. After being bolted to the inner or inner and outer glass pane, the point fixings are integrally glued to the glass.

DOUBLE FAADES

The traditional arrangement of functions within the physical faade space is either side-by-side, layered or a combination of both. The insulated glass unit, uses a strategy of layering on the element level - double faades on the level of building parts.

With double faade, the construction is extended by a second glass layer that acts like a rain and wind shield to protect the sun-shading system. This can be very important for high rise buildings.

Outside sun-shading systems are very efficient and will still be in operation under high wind loads.

Double faades are also used to block traffic noise or to allow natural ventilation of buildings. In winter, they can provide an additional thermal layer.

However, the concept brings with it several downfalls: The cavity works like a greenhouse and needs controlled ventilation to prevent overheating. Condensation on the inner side of the outer glass pane can occur, if windows are opened in the inner, thermally insulated layer during cold outside conditions. Cleaning costs are high.

This is why double faades only make sense in noisy locations or for high rise buildings where the investment is justified.

There are different climatic strategies for double faades:

The cavity can be separated horizontally in extension of the floor slabs .

Each faade unit of the RWE Tower is individually ventilated (box faade).

The second skin of the Ramboll building forms one cavity (chimney faade) and is not unitised.

But alternating concepts are conceivable as well. They have boxed windows in combination with single layer areas that have direct operable parts.

View A showing horizontal and vertical wall joints and splicing layout

CONNECTION DETAILS BETWEEN LOAD-BEARING FAADES

DETAIL A DETAIL B

Connection Details between Window Unit with Supporting Wall1. PC window wall with beam DETAIL C 2. PC wall 3. In-situ concrete joint 4. Rebars from PC wall

DETAIL D1. PC beam for bay window2. PC wall3. In-situ concrete joint4. Rebars from PC wall

CONNECTIONS BETWEEN PRECAST FAADES/ WALL PANELS AND FLOOR SLAB

The connections at the horizontal joints between precast faades/ wall panels are often connected by means of dowel connection, particularly for load bearing wall.

Core holes within the wall panel are formed using proprietary splice sleeve or corrugated pipe sleeves.

These holes together with the vertical continuity bars are filled with grout after wall installation.

As for the connection between the precast faade/ wall with floor slab, the use of starter bars for bridging and continuity is commonly adopted as illustrated in the following figures.

Connections between Precast Slab Panels and with Other Supporting Structures Connection between precast full slab panels

1. PC full Slab2. Bottom rebars from PC slab3. In-situ joint4. Top rebars from PC slab5. Rebars placed on site

Connection between precast full slab panels using cast in-situ pour strip

1. PC full slab2 Rebars from PC slab3. In-situ pour strip4. Rebars placed on site