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This page describes the conclusions of technical issues that have been discussed within the CWCT Technical Committee.

Note: The conclusions reached were made based on the best available knowledge at the time. We welcome your comments and feedback on these issues. If you would like to get in touch, please email us.

Revised method of heat soaking toughened glass
Revised European curtain wall product standard BS EN 13830:2015
CE marking of curtain wall brackets
Deflection of glazing in windows and curtain walls

Following the tragic events at Grenfell Tower last month, focus is now turning to how the industry responds to the challenges ahead. This includes short term concerns such as ensuring that current buildings are safe, and longer term issues such as revisions to the Building Regulations.

An expert advisory panel has been created to provide independent advice to the Secretary of State on any immediate measures that may need to be put in place to make buildings safe for residents following the Grenfell Tower fire.

An industry response group has been formed consisting of the Construction Products Association (CPA), Build UK and the Construction Industry Council (CIC). The industry response group reports directly to the expert panel to ensure that the construction industry is adequately represented during this process. CWCT is part of a small group advising the CPA.

The current situation is uncertain and until such time as the Government publishes revised guidance will remain so. CWCT published Technical Note 98 in April 2017. This provides guidance on the requirements of the UK Building Regulations. The guidance within TN98 provides a conservative approach to compliance with Building Regulations, and remains valid.

If you have any comments on this subject, questions regarding materials and testing, concerns over the future direction of the Regulations and so on, please get in touch.

Added on 25.07.2017

Revised method of heat soaking toughened glass
A revision of BS EN 14179 Glass in building - Heat soaked thermally toughened soda lime silicate safety glass Part 1 Definition and description was issued in August 2016. The most significant change is to the temperature regime for the heat soaking process.

The holding temperature for the heat soak process is now 260+/-10C. During the heating phase the temperature of the glass is not permitted to exceed 290C and the time above 270C is to be minimised. The minimum duration of the holding phase remains at 2 hours. This change has been implemented as it is believed that conversion of some combinations of NiS occurs at a temperature of about 290C.

Other changes relate to tolerances and bring EN 14179 into line with the 2015 revision of EN 12150 Glass in building - Thermally toughened soda lime silicate safety glass and are as follows:

  • new terms and definitions have been included in Clause 3 eg air cushion process, edge lift, roller wave distortion (previously referred to a local bow),

  • additional nominal thicknesses have been included in Table 1,

  • tolerance on squareness has been revised and is now expressed as the difference in diagonals,

  • the section on flatness has been revised with different requirements for different production methods (horizontal, vertical and air cushion) and a reduced limit of 0.3mm for roller wave of uncoated float glass produced by the horizontal process (previously 0.5mm),

  • the flatness section now includes a method of measuring edge lift,

  • a new informative Annex dealing with an alternative method of measurement for roller wave distortion has been added,

  • the informative annex Curved heat soaked thermally toughened safety glass has been deleted.

Added on 26.09.2016

Revised European curtain wall product standard BS EN 13830:2015
Although this Standard has been published by BSI, it has not been approved by all the relevant European committees and therefore is not currently valid for CE marking.

The delay has in part been caused by a lack of resources within CEN (the European Committee for Standardisation), and in part due to the introduction of a number of new classes and thresholds for certain performance characteristics (such as allowable deflections of framing members under imposed wind loads).

The Chairman of the working group responsible for the standard has writen to the Commission in order to justify the changes that have been made. It is not known at present how long this process will take, and what the outcome will be.

Until the revised standard is cited in the Official Journal of the Europran Union, CE marking should be carried out in accordance with BS EN 13830:2003.

Added on 25.04.2016

CE marking of curtain wall brackets
CWCT has received enquiries about the need to CE mark curtain wall brackets in accordance with EN 1090 as they can be considered structural products.

CWCT's understanding is that curtain walling is excluded from the scope of EN 1090 and this is set out in document WG-6 0488-CPR 07/04/3 Cases where CE marking is not possible on the basis of EN 1090-1. The scope of curtain walling includes brackets so the brackets are not required to be CE marked separately as structural components.

Added on 25.04.2016

Deflection of glazing in windows and curtain walls
There are no established limits on the deflection of glass used in windows and curtain walls.

The only clear requirement to limit deflection of glass in British Standards is given in BS 6180, which limits the displacement of glass in barriers to 25mm. This displacement would include any movement of the support to the glass.

BS 6262 has design curves which appear to be based on stress limits and calculations suggest that storey height panes of toughened glass complying with BS 6262 could deflect by 40mm under full wind load.

BS 6262 contains a note to warn that glass having to withstand only low wind loads or glass used internally, might require to be increased in thickness to keep deflection within acceptable limits. No guidance is given on how this should be achieved.

Providing the glass is within allowable stress limits, the issue is not of safety, rather one of perception. If building users see glass undergoing fairly large displacements they perceive this to be unsafe and it causes concern.

This has proven to be a particularly contentious issue. We are aware of situations where glass used has been selected in accordance with the guidance given in BS 6262 which has subsequently been rejected by the client due to in their eyes excessive deflection. We would therefore encourage anyone with experience of this to get in contact with us.

Added on 25.04.2016

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