10.03 Structural sealant glazing

Categories: Structural Glazings

Introduction
In structural sealant glazing the glass is held in place by a structural sealant on the inside face of the glass, image, rather than a glazing bead or pressure plate on the front face allowing a smooth face to the glazing to be produced. Structural sealant glazing may be used on all four sides of the glass to create a smooth façade, image, or on two sides only with glazing beads on the remaining sides to produce smooth bands of glazing, image.

There are no British Standards covering the use of structural sealant glazing. Some guidance on structural glazing systems is given in ETAG 002 which gives performance requirements to be used in assessing structural sealant glazing systems. The requirements of ETAG 002 are not mandatory in the UK but use of a system complying with it may be accepted as evidence of fitness for purpose for Building Regulations approval.
 

Load transfer
In structural sealant glazing the glass is required to support its own weight and wind loads which it must transfer to the frame. The resulting stresses in the glass will be similar to conventional glazing with the glass supported in a frame rebate. The difference is in the way the stresses are transferred at the edge of the pane.

In structural sealant glazing, the sealant may be used to transfer all load to the frame or may transfer only the wind load with the dead load carried by mechanical means, image. The system may also include an alternative means of carrying the load in the event of failure of the sealant. These supports may take the form of metal clips, image. These clips must be carefully designed to prevent contact with the glass under normal circumstances which could interfere with the operation of the system and cause failure of the glass.

ETAG 002 gives four types of structural sealant glazing based on the above mechanisms of load transfer as follows
 
 

ETAG 002 does not permit the use of type 3 and 4 with insulating glazing units.
 

Joint design
The overall design of the joint at the edge of the glass must ensure watertightness of the structure and accommodate movement of the glass relative to its support in addition to transferring the loads to the frame. Watertightness may be achieved by face sealing using a site applied silicone weatherseal, image. Depending on the design of the joint, there may also be secondary seals between the framing members at the back of the joint. Sealing may also be achieved using gaskets.

Movement is accommodated by shear in the sealant as shown in image. To accommodate movement in this way the sealant must have adequate depth. The structural sealant must only be adhered to two opposing faces as adhesion to a third face will restrict the shear deformation in response to movements.

The size of the sealant bead will be determined by a combination of practical limits, the load to be carried and the amount of movement to be accommodated. The amount of movement that can be accommodated increases with the depth of the sealant however as the sealant bead should normally have a width about twice the depth the width will also increase. One manufacturer recommends that the width of the sealant (known as the bite) should be four times the expected movement. The load that can be carried by the sealant increases with the area in contact with the glass. The stresses in the sealant are relatively low as shown by the following examples.
 

• Wind Load

For a wind load of 2000 Pa and a pane size of 2 m x 2 m, the total wind load is 8 kN.
For a bite of 10 mm, this load is carried by a sealant area of
2 x ( 2000 + 2000 ) x 10 = 80000 mm²
The required tensile strength of the sealant is therefore 0.1 N/mm².
Structural silicone sealants are available with tensile strength and tensile bond strength to glass and aluminium of greater than 1 N/mm².
 
• Dead Load

The dead load of a 10 mm thick pane 2 m square is 2 x 2 x 0.01 x 25000 = 1000 N
Assuming this load is to be supported by the sealant, the stress in a sealant bead with a bite of 10mm around the full perimeter of the glass is
1000 / (2 x ( 2000 + 2000 ) x 10) = 0.013 N/mm²
Structural silicone sealants are available with shear strength and shear adhesion to glass and aluminium of greater than 1N/mm².

Details of the permissible stresses for a particular sealant should be obtained from the sealant manufacturer.
 

Additional Considerations
Where a standard insulating glazing unit is used, as shown in image, only the back pane can be fixed with structural sealant. The sealant at the edge of the unit is therefore required to act as a structural sealant to hold the outer pane in position. An alternative solution is to use a stepped unit where the outer pane is larger than the inner pane as shown in image. It is then possible to apply the structural sealant either to both panes or the outer pane only.

The surface to which the sealant is applied must be capable of carrying the loads imposed by the sealant. The suitability of any coating on the glass must be checked. When bonding to an aluminium frame, the bonding surfaces of the frame may be anodised but should not have any other coating.

Satisfactory performance of the structural sealant joint is dependent on workmanship. It is therefore normal to apply the sealant under factory conditions to fix the glass to a carrier frame which is subsequently fixed to the structure using mechanical means. Care is still required during the site operations to ensure that the performance of the system is not impaired by poor site practices.

The complete joint at the edge of the glass may include a large number of components including weather seals, spacers, setting blocks, insulating glazing unit seals, backing strips and structural silicone. All the materials used within the system must be compatible. Obtaining all materials from the same manufacturer should reduce the risk of incompatible materials being used.

Structural sealant glazing systems are expected to have a service life of 25 years based on laboratory studies and monitoring of structures where they have been used. However as these systems have only been in widespread use for a limited period and the only true test of durability is exposure to the actual service conditions, accurate predictions of life expectancy are difficult.