08.05 Frame construction
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Introduction
Glazing frames are required to:
- Support the glass or infill panels,
- Provide a means of attaching operating hardware including hinges and locks,
- Give a visually acceptable, robust and durable to the edge of the glass or infill panel.
Most frames comprise a profile or set of profiles that are assembled to form a frame with a glazing rebate that holds the glass or panel.
Flush glazed windows may be constructed using either:
- A concealed frame with glass fastened to the outer face of the frame using a structural sealant joint, Section 10.03,
- A plain sheet of glass to which hardware is fixed directly by bolting or bonding. This is mainly restricted to the construction of shop doors and similar uses. It may be used as part of a bolted glass facade.
Glass support
The glazing frame supports the glass. It transfers the weight of the glass and the wind load back to the wall or roof structure. To do this the frame and glass have to be correctly dimensioned and the glass has to be supported on blocks, gaskets or a suitable filler. Modern glazing techniques use glazing blocks or a structural sealant joint to support the weight of the glass. The wind loading is transferred by either;
- Glazing gaskets, see Section 01.06,
- Distance blocks and a wet applied seal,
- A structural sealant joint, see Section 10.03.
It is important that the interface between the glass and the frame distributes the load on the edge of the glass evenly. Any gasket or wet applied sealant has to be of sufficient depth to avoid any glass to fame contact or concentrated load on the glass.
Edge cover
This is the distance by which the glazing frame rebate overlaps the edge of the glass, image. Edge cover has to be sufficient to allow for;
- Expansion and contraction of the frame and infill due to temperature change,
- Shortening of the glazing or infill panel under bending. This is a particular problem with plastic glazing materials that can undergo large deflections without breaking.
There should at all times be a sufficient bearing area between the glass and the frame.
Edge clearance
The difference between the size of the glass pane and the tight size of the glazing rebate gives an edge clearance at each edge of the glass pane, image.
This is required to allow for expansion of the glass when warmed and contraction of the frame when cooled. PVC-u has a greater coefficient of thermal expansion than glass and will contract more than the glass when cooled. Plastic glazing materials will expand more than a steel or aluminium frame when warmed. Glass and aluminium have similar coefficients of thermal expansion.
Glazing frames move, particularly in shear, if the supporting structure deflects, Section 04.03. If insufficient edge clearance is provided then the glass or infill panel will come into contact with the glazing frame. Once this has happened the glass acts as part of the building structure and structural loads are inadvertently transferred to the glass. This will generally lead to failure of the glazing. This is a particular concern when glazing is supported on relatively flexible structural frames such as atria roof frames.
Construction and manufacturing tolerances, Section 03.08 and Section 03.07, mean that the pane size and the tight size of the opening will vary from their nominal dimensions. The correct edge clearance should be achieved allowing for tolerances and the as constructed edge clearance should meet the requirements of manufacturer's guidance, specification and standards.
Glazing blocks
Glazing blocks are placed in the glazing rebate between the frame and the glass. They are used to:
Glazing blocks include;
- Setting blocks
- Location blocks
- Distance blocks
Setting blocks
Setting blocks are used to carry the weight of the glass and should fully support the glass, image. In the case of an insulated glazing unit the setting block should support both panes of glass, image. When an insulated glazing unit is manufactured each pane of glass will be of a slightly different size. Glazing units are manufactured so that the individual panes of glass are aligned on two edges. The unit should be installed with one of these edges as the lower edge that bears on the setting blocks. If a stepped glazing unit is used then separate glazing blocks will be required for each pane of glass.
There should be two setting blocks beneath each glazing unit and they should be placed within 30mm of the corner of the glass, image, and 100mm of the corner of the glazing rebate, image. The use of a third, central setting block gives no further benefit as the deflection of the glazing frame and rigidity of the glass mean that it will carry none of the glass weight. Setting blocks should be correctly sized to carry the weight of glass. Each block should have a length of 30mm for each square metre of glazing opening and should be at least 25mm long.
Location blocks
Location blocks are used to prevent the glazing from moving horizontally in the plane of the wall. They are placed one in each glazing rebate between the vertical edge of the glass and the frame, image. The use of more than one block at each side will restrict the shear movement of the frame. For a curtain wall or slope glazing this is unacceptable as structural loads may be transferred to the glass and location blocks should be used as shown, image. For opening lights it is necessary to use location blocks in combination with setting blocks to stiffen the frame, see below.
Distance blocks
Distance blocks are used to position the glass during assembly and ensure that the correct back clearance is achieved, image. Distance blocks are not intended to transfer wind, snow, access or impact loads from the glass to the frame. They serve only to position the glass in the frame during assembly while a wet applied sealant is applied to make the glass to frame joint.
Setting of glazing
Glazing and infill panels have to be correctly set in the frame in order to stiffen the frame. This is achieved by using appropriate combinations of setting and location blocks. The correct configuration depends on the style of window.
Fixed lights generally require just two setting blocks to carry the weight of the glass, image. However, fixed lights that are glazed in the factory and not reglazed at site require additional setting blocks to give robustness during transportation and allow for the window to be stood upside down, image. Fixed lights in curtain walling and slope glazing also require location blocks, image, to prevent the glazing from 'walking' within the frame. These frames are subject to shear movements as the building structure moves and the glass will move to one side of the glazing opening under the effect of repeated reversals of shear movement.
Opening lights generally have to contain more setting and location blocks to give the required strength, stiffness and robustness. This may appear to over stiffen the window and risk structural loading of the glass. However, the gap between the opening frame and the fixed frame will accommodate any movement that has to take place.
Top hung lights may undergo shear movement as they are opened and closed. To prevent the glass moving sideways under the effect of these repeated reversals of movement location blocks are provided in addition to the setting blocks, image.
Side hung lights may drop under the weight of the glass. The weight acts at an off set from the hinged edge of the opening light and the frame can generally only support the weight of the glass if it is stiffened by the glass. To achieve this stiffening the glass is set with a setting block and a location block at each of two opposed corners so that the glass effectively acts as a strut, image.
Bottom hung lights are set in the same way as top hung lights and for the same reasons, image.
Tilt and turn windows move as if they are both a side hung and a bottom hung light. The position of setting and location blocks is therefore a combination of the positions required for those two styles, image.
Horizontal pivot lights rotate about a central horizontal access and turn far enough for the glass weight to transfer to the top framing member of the casement. They must therefore have setting blocks at both the bottom and top of the glazing frame. Because of the small but repeated rcking that occurs when the window is opened and closed it is necessary to place location blocks on either edge of the glazing, image.
Vertical pivot lights require a central setting block to transfer the weight of the glass to the pivot hinge without distorting the lower framing member of the casement. To prevent the glass tilting on this single support location blocks are provided at the top and bottom of both edges of the glazing, image.
Off-set vertical pivot lights are prone to racking as the weight of the glazing is off-set from the line of the pivots. This leads to racking of the casement frame unless the setting and location blocks are placed in similar positions as for a side hung light. Note that the setting block is directly above the pivot to transfer the weight of the glazing without distortion of the lower framing member of the casement, image.
Vertical sliding windows require setting blocks to support each pane or glazing unit. In addition location blocks are placed at the upper edge to stiffen the top framing member of each sash as these are frequently handled to open and close the window, image.
Horizontal sliding windows require setting blocks to support each pane or glazing unit. In addition location blocks are placed at both vertical edges of each glazing unit to stiffen the vertical framing member of each sash as these are frequently handled to open and close the window, image.
Drainage
Weathertightness and drainage of the glazing frame are described in greater detail in Section 01.05. It is important that the frame construction and glazing techniques do not restrict drainage of the glazing frame.
Setting blocks should not obstruct any drainage paths. This can be achieved by either, (image):
- Using a setting block that bridges over the drainage path,
- Providing a drainage rebate that ensures drainage can take place beneath the setting block.
Drainage may also be restricted by large frame deflections that cause water to run to the centre span of the framing member instead of the drainage slots. Large deflections of this type may occur if setting blocks are not positioned as shown above.