08.04 Framing materials

Categories: Glass & Glazing

Introduction
Glass in windows is usually framed, although there are examples of frameless opening lights and of course doors.  Occasionally glass is built directly into a wall but most examples of this are historic.  Direct installation of glass in this way makes the glass prone to fracture due to building movement and thermal movement of the glass.  A frameless appearance can be created by concealing the frame behind the glass.  The glass is then fixed to the frame using structural silicone, Section  10.03.

Details of frame construction are described in Section 08.05.  This Section describes the different materials, and material combinations, that may be used to frame and support glass and other infill materials.
 

Material requirements
Framing materials have to be sufficiently:

• Durable
• Robust

and when used in combination or separately must be sufficiently:

• Strong
• Stiff
• Thermally insulated

Framing materials must also be workable and the resulting products should be capable of easy installation / assembly on site.
 

Durability
Framing materials have to resist corrosion, rot and infestation to give the required life to the glazing product or assembly.  Materials may be treated or coated to improve durability, see Package 13.
 

Robustness
Framing materials and the joints used to form them into frames have to be robust against accidental damage and a level of vandalism.  The principal requirement here is the ability to resist localised hard impacts and to give robustness to the complete frame, Section 08.05.  Robustness of the assembled frame depends on the strength, and stiffness, of the joints and the strength of fixings into the frame that secure the window hardware.
 

Strength
In most facade applications the selection of materials and sizing of sections is determined more by stiffness than strength.  The limiting condition being the allowable deflection rather than the allowable stress.  Where strength is an issue steel and aluminium are used in preference to plastics.  Strength is primarily limited by the allowable stress in the material.
 

Stiffness
The stiffness of a framing member is important for the overall robustness of the window, to support window hardare and to limit the deflection of framing members.  This is important if they are to fully support the glazing materials and infill panels.

Glazing frames have to be stiff in bending, resist curvature, if they are to support the glazing units and to prevent the disengagement of bolts and other locking gear.

Glazing frames have to be stiff in torsion, resist twisting, if they are to support hardware and prevent the disengagement of locking gear and variable geometry hinges.

Glazing frames also gain rigidity from the glazing they contain,  Section 08.05.

Stiffness of a framing member depends on the material it is made from and its shape.

The material property that controls the stiffness of a framing member is the Young's modulus or modulus of elasticity.

In general a framing member of a given material and weight will be stiffer in bending if it is a hollow section.  The shape property that governs the bending stifness of a framing member is the I value or second moment of area. This principle is applied to extruded profiles of aluminium and pvc-u.  Torsional stiffness is also increased for a hollow section provided it is a closed hollow section, i.e. one with no lengthwise opening in it.  The section property that governs the torsional stiffness of a framing member is the J value, or polar moment of area.
 

Thermal conduction
Windows and walls are normally required to provide a degree of thermal isolation between the outside climate and the internal environment.  This is to reduce energy use and provide a more comfortable internal environment.  The reduction of condensation formation is also a concern in many cases.  A fuller explanation of the thermal performance of windows is given in Package 05.

In general metals are good conductors of heat and plastics are good insulators.  For this reason metal frames may have thermal breaks made of plastic.  Metal may also be used as part of a composite window profile that has a plastic or timber insulating element.

The material property that governs heat flow through a material is the k value or thermal conductance.  A material with a lower value is a better thermal insulator (thermal break).

The thermal performance of a framing profile depends on the geometry of the profile and the presence of an thermal breaks.  These may be within the profile or a sheathiing on the surface of the profile.  A full description of this aspect of profile design / selection is given in Package 05.
 

Materials
Materials used to form glazing frames include:

• Timber
• Steel
• Bronze
• Aluminium
• pvc-u

Windows have traditionally been made of timber.  Steel was introduced as a framing member in the 19th Century with the use of hot rolled framing members.  More recently steel frames have been made from cold formed steel profiles.  Aluminium has been in use since the middle of the 20th Century and pvc-u was first introduced 30 years ago.  All of these materials are, or have been, widely used and have at some time been the dominant material in a particular market.  Currently pvc-u is dominant in the UK housing market whilst aluminium is dominant in the commercial market.  In some countries, particularly those with indigenous timber industries, timber frames predominate.  Bronze windows are generally used as part of a high quality facade.

Recent times have seen the introduction of thermally broken aluminium frames and composite frames comprising components of plastic, metal and timber.
 

Material properties
The material properties for the four principal frame materials are given below.
 

Timber window frames
Historically there have been many styles of timber window.  Today timber can be used to form most types of window frame subject to some size limitations when considering commercial buildings.

There are many different profiles in use and the cost of setting up to produce a different profile is only a few pounds.

There are no timber systems as such but there have been standardised designs.  In these standard profiles are machined in any joinery workshop.  The most recent example in the UK was the EJMA (English Joinery Manufacturers' Association) window of the 1960's.  Timber is machined as a profile from hard or softwood.  Softwoods are generally pressure treated to resist rot and infestation.  Timber is joined by tenon joints and finger joints to produce glazing frames.

Traditionally windows were single glazed.  Glass was secured by pins into a simple rebate and fully bedded in a putty to provide a seal.  With the use of insulating glazing units it is necessary to keep the edge of the glazing unit dry.  This can be done in a timber window by using dry glazing techniques and using a glazing bead that allows drainage.

Hardware is fastened to the solid timber framing members by screwing and attachment of hardware seldom causes any problems.

Finishes to timber windows are applied after assembly of the frame.

The construction of timber windows is covered by BS 644 Part 1 1989 - Wood windows: specification for factory assembled windows of various types.
 

Steel window frames
Hot rolled profiles have been used for the manufacture of window frames.  The high modulus and strength of steel means that slender solid pofiles can be used and these give a fine and light appearence to the window.  They offer excellent robustness and are used where this is required.  For instance schools and penetentiaries.  Steel frames are also used for the construction of fire resistant windows, Section 08.10.

Steel frames are formed by welding profiles together and hardware is frequently attached by welding although it may be fixed by screws.  Traditionally windows have been made using the W20 profile and these are still available for the manufacture of replacement windows. Single glazing was retained in the frame rebate and fully bedded in a putty to form a seal. The need to use insulating glazing units and have a drained glazing rebate has lead to the introduction of the W40 suite of profiles that are today used for windows in new buildings.  These comprise a main framing profile and a metal bead that retains the glass.

Steel frames suffer from the good thermal conductance of steel but the slender profiles give a low projected area of frame as a proportion of the complete window.  This helps to reduce the heat loss through the frame and also gives a larger area of glass to allow heat gain.  A full description of thermal performance is given in Package 05.

Finishes to steel windows are applied after the assembly of the frame.

The construction of steel windows from hot rolled profiles is covered by BS 6510 1984 - Specification for steel windows, sills, window boards and doors.

More recently cold formed steel profiles have been introduced.  Cold formed profiles are manufactured from sheet steel that is folded by rolls to form a hollow profile.  They offer the strength and robustness of steel but produce a wider frame than  hot rolled steel steel profiles.
 

Aluminium window frames
Aluminium can be extruded to form profiles of complex shape.  These are frequently hollow profiles to provide stiffness and produce a light economical frame.  Aluminium has a lower elastic modulus than steel and the profiles are of greater width.

Aluminium profiles are formed into window frames by the use of mechnical joints.  Window frames comprise a main framing member that provides the strength and stiffness and an extruded glazing bead that generally clips into place to retain the glazing in a drained glazing rebate.  Glazing may be installed from the inside or outside.

Aluminium profiles are used for curtain walling, principally as mullions and transoms in a stick system.  These profiles comprise a structural box to the rear and a pressure cap to the front that retains the glass.

The use of aluminium frames in curtain walling is dealt with in greater detail in Section 04.05.

Hardware is attached to the frame by screwing into the aluminium.  Aluminium frames may be reinforced with a steel inner member particularly when a fire resistant frame is required.

Aluminium is the best heat conductor of all the framing materials and is normally used as a thermally broken section.  The exception is the construction of canopies and screens in the open.  Thermal breaks can be formed from a poured resin or a polyamide thermal break.

Resin thermal breaks are formed by extruding a single profile with a channel into which the resin is poured.  After the resin has cured the bridging aluminium is cut away to leave two independent aluminium sections bonded onto the resin break.  This form of frame is suitable for windows but there is insufficient bond between the resin and the aluminium to give a structural joint and it is not suitable for curtain wall frames.

Polyamide thermal breaks are used to join two separate aluminium profiles to form a single framing member.  The polyamide is crimped into each of the aluminium profiles by a rolling process to form a connection that will carry shear.  Framing members formed in this way are suitable for curtain walling.  The use of two separate aluminium extrusions to form the frame allows the production of frames with different coloured finishes to the inside and outside.

Finishes to aluminium frames are applied to the profile in bar lengths that are subsequently cut and machined.  Care is needed to avoid damage durimg machining of the aluminium.  Cut ends at mitre joints in windows should be sealed with a small joint sealant to reduce the risk of corrosion.

The construction of aluminium windows is covered by BS 4873 1986 - Specification for aluminium alloy windows.
 

pvc-u window frames
pvc-u can be extruded to form a wide variety of profiles.  pvc-u has the lowest elastic modulus of all the framing materials and has to be reinforced with steel or aluminium inner profiles to give it sufficient stiffness. Reinforcement is provided by either aluminium extrusions or cold formed steel sections.  These may be either closed or open hollow sections and should be in accordance with the system manufacturers guidance.

Reinforcement may be provided locally to accept the fixing screws for hardware attachment.  Frame members are normally reinforced along their whole length to give bending and torional stiffness.  However, metal reinforcing components cannot extend into the corners of the frame where the plastic frame is heat welded.  A 'fully' reinforced frame will have metal reinforcement that extends almost to the corners and joints of the frame in every frame member.  For a reinforcing profile to be effective it must be secured to the surrounding pvc-u profile and this is done by screwing through the pvc-u into the reinforcement.  Full guidance on reinforcement requirements are provided by the system manufacturer and guidance is also given in the BPF Code of Practice for the reinforcement of high impact modified pvc-u windows and door sets (BPF ref 323/1).

pvc-u framing members are formed into window frames either by heat welding the members at mitred joints or by mechanical joints.  Heat welded joints are most common and provide a clear seal that keeps water out of the frame.  Some early pvc-u windows suffered from failure of the welded joints and a test for the strength of these is included in BS 7413 1991 - Specification for white pvc-u extruded hollow profiles with heat welded corner joints for plastic windows: materials type A.  Window frames comprise a main framing member that provides the strength and stiffness and an extruded glazing bead that generally clips into place to retain the glazing in a drained glazing rebate.  Glazing may be installed from the inside or outside.

pvc-u is a good thermal insulator but the steel or aluminium reinforcement acts as a conductor.  Multi-chamber profiles are used to to keep the metal component remote from the inner and outer surfaces.

Construction of pvc-u windows is covered by BS 7412 1991 - Specification for plastics windows made from pvc-u extruded hollow profiles.
 

Composite frames
Framing members can be made from more than one material to give the benefits of each in combination giving an improved frame profile.  The main innovations have been:

• The use of aluminium on the outer face allows the use of highly durable finishes in many colours.
• The use of timber as the main or central element of a composite member can provide stiffness and strength along with thermal isolation.
• The use of pvc-u as the main or central element or as an inner cover gives thermal isolation.

Other combinations used have been the use of drawn bronze with aluminium to create what is essentially an aluminium window pretending to be a bronze window.  The bronze can be drawn onto the aluminium or a separate profile can be joined to an aluminium profile using polyamide thermal breaks.

Frame construction depends largely on the material of the main or central element.  For instance a timber window is made and then clad with metal and plastic.  The jointing technology has to take account of the materials to be joined and the presence of different materials and is generally more complex than for non-composite frames.

Thermal performance of a composite frame depends on the materials used and profile shape.

There is no British Standard covering construction of composite frames.  It is normal to refer to those window standards that are appropriate to the individual materials used.  Window performance should still meet or exceed the requirements of BS6375 Parts 1 and 2.