Performance Requirements
In designing glazed barriers, four essential criteria should be considered and satisfied; these are as follows:
- Glass barrier to resist design loads with an appropriate factor of safety
- Displacement of the barrier, under the action of design loads, should be within acceptable limits that do not cause human/users discomfort.
- The glass barrier should have good resistance to accidental impact.
- Post-failure behavior should be safe.
In addition to the above main criteria, some other considerations, which are project-specific, should be taken into account:
- Corrosion resistance and coating quality: Due to the project’s location and its proximity to the shoreline, corrosion resistance importance elevates with such conditions.
- Load transition to glass balustrades due to the contact with any other facade element. For example, balcony fascia cladding.
- Demarcation of spaces – where glass balustrades are installed- that they are more likely to be crowded.
Risk factors
The regulations, codes, standards, and relevant tests provide the needed risk mitigation measures to avoid falling from heights hazard through balconies balustrades. However, these measures are categorized based on function and probability of occurrence. The designer should consider, in-depth, factors affecting the categorization of each case where a balustrade would be assessed, and accordingly, the Risk magnitude (a function of event likelihood and impact) should be the governor.
Risk magnitude = Risk likelihood × Risk impact
Sample of risk factors related to a project:
- Large balconies increase the likelihood of people to congregate at such areas.
- Glass balustrades are frameless, although the glass is Toughened Laminated, if breakage (failure) occurred, there is a doubt that the glass pane will provide the necessary containment (refer to BS 6180 the requirement for containment given above applies to ‘barriers with glass infills or freestanding balustrades)
- Large balconies areas may increase the probability (likelihood) of soft body impact and hard body impact (if furniture exists within 500mm from the barrier)
- Glass Balustrades relationship with other facade elements which may induce additional loads on the barriers.
All these factors and performance requirements should formulate the criteria based on which Glazed barriers should be designed.
1- Design Loads
Static loads on barriers
Static loads are usually specified in three forms:
- Uniformly distributed line load (Horizontal line load at a specific height from floor level
- Area-based uniformly distributed horizontal load (Pressure)
- Horizontal point load – occur anywhere on the barrier
Above mentioned loads are considered individual, and they are not additive.
For the Balconies barrier, they will also be exposed to wind load.
Barriers and wind loads
BS 6180 states that wind load should be considered a separate load case and not combined with the imposed load. However, as mentioned in Risk Factors, the large balconies increase the likelihood of people congregating at these large balconies. Accordingly, this should be reconsidered by the designer/s and suppliers in handrail calculations to mitigate this risk.
Maintenance Loads
Maintenance is likely to be suspended when high wind load is forecast, accordingly it is unlikely to carry out maintenance or cleaning activities concurrently with the maximum wind load.
2- Displacement and occupants comfort
Deflection limits should be tested and calculated. The calculation should include the effect of both curvature of the barrier and the rotation at the base relative to the floor (as shown in the figure). The rotation effect may contribute to the total displacement more than the curvature.
Although there is no limit for displacement at partial load, many free-standing glass barriers appear to be more flexible at low loads when people first lean against the barrier.
Recommendation
ARKS designs recommends that a limit of 10mm under half the design line load for a domestic barrier be adopted. Under this load, any movement due to ‘slack’ in the fixings is likely to occur.
The maximum allowed deflection at the full load in the same category should not exceed L/65 or 25mm, whichever is smaller anywhere in the barrier.
The Client may discuss tighter limits to deflection considering the New Alamein Towers are prestigious projects, and a higher level of tenants’ comfort should be achieved.
The deflection of barriers under static loads can be a significant issue. Many barriers move relatively easily when first pressed against, although once the initial slack has been taken up, the barrier will tend to become much stiffer; due to this, previous recommendations regarding the allowable deflection limits under half load should be considered.
3- Impact Performance
In addition to static loads, glass barriers are subject to impact. Glass performance under impacts is assessed by testing in accordance to BS EN 12600 or ASTM 62353-16
The test “BS EN 12600” is carried out with three impact energy levels so that α and Φ may take the value 1, 2, 3, or 0. Class 1 is the highest level of performance, and the impact energy decreases from 1 to 3. Zero “0” indicates the glass failed to give a satisfactory performance at any level of impact energy.
Where the free path is less than 1.5m, the glass is required to provide containment under a class 3 impact, and where the free path is more significant than 1.5m, the glass is required to provide containment under a class 1 impact.
4- Post-Failure
Laminated glass is usually considered safety glass. Laminated glass in which one sheet of glass remains unbroken is unlikely to fall from place in the short term.
If the laminated glass is a laminated toughened – toughened construction and if all the sheets of glass are fractured, the laminated glass will have little residual strength and may fall like a blanket at any time.
Glass Selection and Recommendations
The laminated glass should have sufficient strength to resist the full design load with one ply of the laminate broken; the deflection can exceed the limits in BS 6180, but deflection should not be sufficient to allow a person to fall through an opening in the barrier or drop an object.
The barrier should also remain in place with some residual strength if both laminate plies are broken. BS 6180 requires glass balustrades to provide containment post-failure.
These requirements can be satisfied when:
- Using of a structural interlayer; (such as SentryGlas), or
- The use of heat-strengthened glass (as one of the two panes) in the laminated pane will also give greater residual strength than toughened glass.
- The use of top handrail can withstand design loads in case of full failure of the glass panes. (BS 6180 recommends that a handrail is used with freestanding glass balustrades that are protecting a change in level.).This requires the adjacent glass panes to be able to withstand the additional load from the handrail.
- Use of a glazing type that provides containment under the standard BS EN 12600 test does not ensure that the glass will provide containment when used in the barrier. Under test conditions, the glass is four-edge supported, in the contrary, the Glazed balustrade is freestanding one edge supported. It is recommended that a project-specific actual size mockup test take place to ensure containment post-failure; test might be conducted as per ASTM E2353-16 “Standard Test Methods for Performance of Glazing in Permanent Railing Systems, Guards, and Balustrades.”
- Where a handrail is not used for outdoor balustrades, ARKS recommends that the top edge of the glass is protected by a capping (min.15mm). This can prevent both panes from breaking simultaneously under impact and will also protect the top of a laminated glass pane from standing water.
ARKS has profound, knowledge-based expertise in facade engineering, structural design, and the practical experience of developing bespoke and standard building envelopes solutions. We can provide expert advice on all aspects of the facade systems selection, envelope risks assessment, tests, specifications, and structural performance. For more information, send your query to contact@arksdesigns.com.
References:
- BS EN 1990 Eurocode 0: Basis of structural design
- EN 1991 Eurocode 1: Actions on structures.
- NA to BS EN 1991-1-1 UK National Annex to Eurocode1: Actions on structures – General actions
- PD 6688-1-1 Recommendations for the design of structures to BS EN 1991-1-1
- BS 6180:2011 British standards: Barriers in and about buildings
- BS EN 12600:2002 Impact test method and classification for flat glass
- ASTM E2353-16 Standard Test Methods for Performance of Glazing in Permanent Railing Systems, Guards, and Balustrades
- CWCT, Guidance on the actions on non-loadbearing building envelopes
- GANA GLASS MANUAL
- NGA-GANA LAMINATED GLASS REFERENCE MANUAL
