Defining the design code: prEN 16612 and laminated glass balustrades
By Steve Brock, Director of Clear Structures.
Recently, free standing glass balustrades have become extremely popular in all types of building from private residences through to public transport hubs and stadia because they offer and attractive, sleek appearance and minimise visual obstructions.
We are all aware that the primary purpose of the balustrade is to prevent persons from falling, and this should not be compromised by any aesthetic considerations. It must be able to resist loads that may be applied by the building occupants.
For safety reasons, free standing balustrades are generally made with laminated glass so if one of the glass panes breaks, the other leaf will keep the panel in place and maintain a barrier.
Fundamentally, the thickness of the laminated glass is usually determined by the requirement to ensure the deflection under load does not exceed 25mm.
To assess the deflection of the glass by structural analysis, the effects of the interlayer material on the panel stiffness needs to be known. Until recently there was little statutory guidance on the structural design of laminated glass and reference was made to various technical publications and industry guidelines, sometimes leading to disagreement between designers.
In 2013 a draft Eurocode prEN 16612 was published titled ‘Glass in Buildings – Determination of the Lateral Load Resistance of Glass Panes by Calculation’.
The guideline gives parameters for the structural design of glass including the assessment of laminates. The stiffness of a laminated panel depends on the thickness of the glass and interlayer, the type of interlayer material, the duration of the loading, and the temperature. The draft groups the interlayer materials used in building into three families:
0 Interlayers which are assumed not to provide a shear connection between the glass leaves
1 General PVB interlayers
2 Structural interlayers
The document also provides a simplified calculation method of design which allows the use of an ‘effective’ thickness for the panel based on the above variables. Using this method, we have generally found that the analyses do not give the same deflection values as are published by the various tested base clamp systems, nor do they correspond with the result of in situ load tests on balustrades we have carried out ourselves. The simplified method gives larger deflection values than are found under test.
Part of this difference will be due to temperature. The code requires the properties of the laminate for a barrier to be assessed at a temperature of 30ºC. The tests are likely to have been carried out at a temperature closer to 20ºC and consequently the interlayer would have been much stiffer giving lower deflections values. A further reason is that the analysis method used in prEN16612 is based on the Wölfel-Bennison method which assumes a simply supported beam under uniform load. Tip loaded cantilevers may give different results.
The code allows for a more rigorous analysis of the panel properties by using the shear modulus values of the interlayer material being used, and this gives more accurate results. The shear modulus data for various temperatures and load durations are readily available for the family 2 structural interlayers but is difficult to obtain for the family 1 PVB interlayers.
It is likely that the main difference between the tested deflection values and the simplified calculated values is that the PVB is stiffer than the base value taken for a family 1 interlayer. Consequently, when designing PVB laminated glass balustrades using the simplified method, thicker panels are required than historically have been found satisfactory. The more accurate analysis method is not generally available due to the absence of detailed data on the family 1 PVB interlayers.
prEN 16612 is only a draft standard. It should not be used as a British Standard, and there is no requirement to actually comply with the draft document. Increasingly, however, compliance with the draft is a requirement written into many project specifications. This is because there is no other statutory design guidance that addresses the complex behaviour of laminated glass.
The industry needs this code of practice to be adopted soon to establish a consistent design method. Before this can happen, the interlayer industry needs to provide the detailed properties of their products over the specified ranges of temperature and load durations. This will ultimately ensure an accurate assessment of their performance can be made.