Glass Deflection & Stress

Glass Deflection and Stress

A very important factor concerning the thickness of glass suitable for any application is whether the glass has 4 edges or 2 edge support. As an example a 1250x2500 6mm 4 edge supported panel under a load of 1500N/m² has a deflection of 17.8mm. A similar size 12mm 2 edge supported panel under a load of 800N/m² has a deflection of 43.5mm.

Even at this deflection the glass would be well within its allowable stress rating, but may be outside allowable deflection limits depending on position and building usage. This is especially true for atrium situations if the glass is used as a full height barrier. For general office use using 4 edge support, i.e. a standard stud & track partition, a 1200mm wide panel can be safely glazed up to 3000mm in 6mm toughened or 6.4 laminated glass. For a frameless glazed (2 edge supported) system the following tables give an indication of recommended sizes.

 

Table G1

2 Edge supported glass, Standard Office use. Non- protecting drop. Standard Medium duty loadings.

Glass Thickness (mm) - Recommended Maximum Height (mm)

10 - 2700

12 - 3000

15 - 3600

19 - 4200

 

Where a drop or atrium is being protected, or where a building has public access, such as a school or government building, it may be necessary to use the heavy duty loadings as shown in Table 3 below. Under those circumstances, the next thickness of glass up (i.e. 2mm to 15mm) will be required for the recommended maximum glass height.

 

An Indication of Deflection under constant load and panel width, but with change of glass thickness is given in Table 2 below.

 

Table G2

2 Edge supported glass

 

Glass Thickness 12mm

Panel Height - Deflection

1000 - 1.1mm

2000 - 17.8mm

3000 - 90.3mm

 

Glass Thickness 15mm

Panel Height - Deflection

1000 - 0.6mm

2000 - 9.4mm

3000 - 47.5mm

Maximum permitted deflection can be generally assumed to be 25mm at 1100mm above floor level. Free standing barriers are subject to different stresses, and deflection should be constrained to 20mm.

 

A rule of thumb is that moving up to the next standard glass thickness, halves the deflection under the same load conditions. These figures are themselves subject to variation.  If for instance a run of 3 panels was set into a solid opening, then the end two panels would have 3 edge support, the glass being slotted into an abutment track.

This would considerable stiffen the run (assuming the middle panel was dry or silicon jointed to the outer panels) OR Clamping at the bottom edge of the glass, between 100x100x12mm angles can halve the deflection, allowing the use of a thinner glass for any specific height, i.e. 12mm can be used in place of 15mm. This is mostly employed for free standing glass screens where the angles can be set beneath raised floors or under the screed, but can also be used at atrium bases where it can be made a feature.

Depending on building usage, screen design and construction can be adjusted.

The sizes of the glass used, must, as previously mentioned, also take into account the building usage, and the following table gives the British Standards for loads against building usage.

 

Table G3

 

Usage - Office

Line Load (N l/m) - 740

UDL (N/m²) - 1000

Point Load (N) - 500

Usage - Retail
School
University

Line Load (N l/m) - 1500

UDL (N/m²) - 1500

Point Load (N) - 1500

Usage - Place Of Assembly
Theatre or
 Shopping Mall

Line Load (N l/m) - 3000

UDL (N/m²) - 1500

Point Load (N) - 1500

 

In addition for atria and barriers, glass must:

  1. i) Conform to BS6206, and BS6180
  2. ii) Resist the usage design loads

iii) Be within deflection limits

  1. iv) Glass must not break under specified impact test
  2. v) Have impact performance to BS6262 & Building Regs Doc
  3. vi) Glass for atriums must have a minimum of 15mm edge cover at the head or glass thickness x1.5. This may also be necessary at the head to incorporate slab deflection, where a deeper head channel can be fitted. Slab movement can be positive and negative.

 

Where atria are concerned care must be taken if there are additional requirements other than barrier protection, or If the loading calculations indicate a larger thickness of glass than is desired or can be handled. In these cases it may be necessary to incorporate a separate handrail barrier independently secured to the slab at 800-1100mm ht sufficiently strong enough to take the required loadings.

As an alternative support posts can be secured to the slab and bolted to the glass panels preferably at 1100mm ht. There are prescribed hole sizes and clamping plates for this which are covered by the various British Standards This would obviously require the glass to be pre-drilled and is really only suitable for toughened glass not laminated. This may cause problems if there is a requirement for the containment of any glass in the event of breakage.

Safety films can be applied to toughened glass to contain any breakage, but in the case of bolted through systems, the film will need to be applied prior to installation. Obviously barriers and atriums are not necessarily all glass and the loadings applicable vary according to their position on the screen.

 

Important point to remember when specifying atrium glazing with 2 edge support

  1. Take loadings to be heavy duty unless contractor/architect states otherwise
  2. The smaller the aspect ratio of the panels (height divided by width) the better. This applies to point loads only and does not affect line loads.
  3. Large panels require specialist installation, and have cost and safety implications.
  4. If the system can be switched to framed, almost certainly a cost saving can be made.

 

From these various tables it can be taken that as a general rule, for 2 edge supported glass barriers, 10mm Toughened glass is not suitable for use in any atrium situations, unless the following conditions are met:

  1. There is an independently mounted handrail or dwarf wall/partition protecting the glass at dado rail height(1100mm approx) sufficiently strong to withstand the site loads.
  2. The glass ht is 2000mm or less, and the line loads do not exceed 740N/lm
  3. Glass can be clamped at 1100mm ht by post mounted, approved single point glazing clamps.