A Guide to Specifying Blast Resistance Requirements for Building Facades

Introduction

It is important to consider building security as early in the planning process as possible.  Guidance is available in the Security Overlay for the RIBA Plan of Work.¹ Blast-resistance measures that are identified early in the design process and installed during construction provide better value and are more effective than retrofitted solutions.

Risk owners should follow a Threat, Vulnerability and Risk Assessment (TVRA) process to determine and document the level of risk and the measures introduced to mitigate the risks to their building. A suitable methodology is the NPSA Operational Requirement process.² This document is intended to introduce users to NPSA’s recommendations for specifying the blast loading requirements for windows, doors, curtain walling and non-loadbearing walls installed in the exterior faces of buildings that are intended to mitigate the blast loads caused by external IEDs.  This will increase a building owner’s confidence that these elements will provide effective mitigation for the blast threat.

The steps needed to develop blast performance requirements for building facades are:

• Step 1 - Identify the design threats.
• Step 2 - Define the level of risk accepted by the risk owner.
• Step 3 - Specify the blast performance requirement.

Buildings may be in use for decades so, as well as current threats, the possibility of threats changing during the life of a building should be considered.

The risk owner may also wish to consider the requirements for business continuity in the event of an IED attack and for the security of the building during a recovery period.

Step 1 - Identify the design threats

The external explosive design threats impacting the external façade will comprise of three elements:

• The type of explosive device.  This is likely to be a PBIED or a VBIED.
• The explosive output. There are many explosives, and these may have widely different effectiveness, for clarity it is usual to use the equivalent amount of TNT which would provide the same blast load.
• The location. There may be several locations for each type of device and a plan of the site should be developed showing these locations.  The distance from the device to the face of the building to be protected is known as the stand-off distance.

For each type of IED and each location, the threat will be expressed as a charge mass (kg TNT equivalent) and a stand-off distance.  Police Services have Counter-Terrorist Security Advisers (CTSA) who can provide advice on current threats.

For some sites, regulators or an organisation’s policy may specify the threats to be considered.

Only stand-off enforced by suitable physical barriers or other access control measures will provide effective mitigation.

• For VBIEDs, this will usually be by Hostile Vehicle Mitigation (HVM) measures.  It is important that the HVM plan and blast mitigation are co-ordinated.
• For PBIEDs, fences, gates or access control may be used to manage access for uncontrolled personnel.

Step 2 - Define the level of risk accepted by the risk owner

Managing the blast risk

Ideally building facades should provide blast protection but there may be a limit to the level of blast-resistance that can be economically achieved.  In city centre locations for example, where the available stand-off is limited, blast-resistance measures may need to be supplemented by a policy of invacuation to protected spaces. NPSA guidance is available on the Protected Spaces webpage.

The risk owner should consider the risks to people outside the building as well as building occupants and visitors.  Testing has shown, large pieces of glazing have frequently been thrown away from the building at distances where people would be expected to survive the direct effects of the blast wave.  Particularly if falling from upper parts of the facade pieces of glazing can be a severe hazard to those outside.
The risk owner is responsible for deciding the tolerable level of risk and therefore the amount of blast-resistance that the design must provide.  There are two approaches:

• Blast Protection
• Blast Mitigation

Blast protection requires the façade to have been designed to resist the blast load from a specified threat with the resulting hazards to people being no worse than a selected hazard level.

Blast mitigation reduces the hazard from glazing and other fragments under blast loading by using practical, cost-effective measures.  However, the level of mitigation provided cannot be quantified.  For glazing, these measures may include:

• Using laminated glass for the innermost glass or applying anti-shatter film to reduce internal hazards from glass fragments.
• Installing secondary glazing incorporating laminated glass behind the windows or curtain walling.
• Mitigation may be further improved by the addition of catcher systems behind these measures or improving window framing or curtain walling systems to produce a balanced design in which the likelihood of premature failure of the system is reduced.

It should be noted that the BS EN Test standards consider the loading from the air blast from IEDs but do not consider the fragments from an IED.  Some fragments are likely to pass through most types of glazing unless very thick glass is used.  It is unlikely that it will be practical to install glass capable of stopping fragments other than in limited areas.  Where protection from primary fragmentation (bomb fragments) is required, further advice should be sought from suitably qualified and experienced personnel (SQEP).  Appropriate SQEP will be on the list of blast specialists maintained by the Register of Security Engineers and Specialists (RSES)  or able to demonstrate equivalent competence.

Blast testing standards

The revised BS EN standards³  present a comprehensive method for blast testing windows and curtain walling.  The standards provide a set of standard blast Loading Categories and a method for rating the Hazard Levels from glazing fragments caused by an explosion. NPSA considers that earlier test standards are not sufficiently rigorous.

Blast Loads

The BS EN standard provides fixed loading categories for VBIED sized threats and lists indicative charge masses and stand-off distances for each loading category (VXR).

Table 1. Indicative charge masses and stand-off distances for VBIEDs

The BS EN standards also provide a set of blast standard loading categories (PXR) for PBIED threats.

Table 2. Indicative charge masses and stand-off distances for PBIEDs

If no standard loading category is appropriate a user defined loading may be specified.  In this case the loading should be specified as VXRU or followed by the charge mass and stand-off distance e.g. VXRU (200 kg @ 25 m).

Hazard Levels

The BS EN Test standards defines both internal and external Hazard Ratings.  They are summarised below; the standard provides detailed definitions for both VBIED and PBIED sized charges. Hazard levels for doors and non-loadbearing walls are defined in an NPSA supplement to the BS EN Test standards.⁴

The internal hazard ratings refer to the height and distance that glass fragments can be expected to travel.  For example, C - Minimal Hazard means that the glazing is cracked but that only non-penetrating glass slivers and dust are created whilst F - High Hazard indicates that projected glass fragments will be expected to be at least 0.5 metres high when 3 metres behind the glazing.

Similarly, the external hazard ratings consider the size of fragments and the area outside the building in which they would be expected to fall.
 

Level of tolerable risk

The risk owner is responsible for deciding the level of risk (Hazard Rating) they will accept and therefore the amount of blast-resistance that the design must provide. 

The risk owner should consider the risks to people outside the building as well as building occupants and visitors should be considered as large pieces of glazing may be projected from the building at distances where people would be expected to survive the direct effects of the blast wave.  Particularly if falling from upper parts of the building, pieces of glazing can be a severe hazard to those outside.

Where the Protection option is adopted, NPSA considers that it may be desirable to specify an internal hazard level of C - Minimal Hazard or better and an external hazard level of Y - Limited Hazard or better. 

In some cases, especially for doors, hazard level C may be difficult to achieve and the additional risk will need to be agreed and accepted by the building owner. 

Where the Mitigation option is used, the risk owner must consider whether the higher risk for people inside or adjacent to the building is tolerable.

Step 3 - Specify the blast resistance requirement

The blast resistance requirements should be specified in accordance with the NPSA Test Standard to provide both the blast loading category and the tolerable hazard levels for each threat that has been identified. 

For example, for a threat from a car bomb containing 100 kg of explosive located 15 m from the building with a tolerable internal hazard rating of C – Minimal Hazard and an external rating of X – No Hazard, the required classification would be VXR5/C/X.

Similarly, for a PBIED containing 3 kg of explosive at a stand-off of 5 m with a tolerable internal hazard level of B – Cracked and an external rating of Y – Limited Hazard the required classification would be PXR/B/Y.

The requirement may be met by using:

• Products that have been tested successfully using the procedures in the BS EN Test standards..
• Test reports for products that have been tested against other standards must be examined by a blast specialist to ensure the data fully satisfies the requirements of the BS EN Test standards..
• Products which are supported by detailed design calculations that have been approved by independent blast engineers.

The blast load on a building caused by a VBIED is greatest at ground level and reduces as the blast travels up the face of the building.  For taller buildings, the design blast loads can be reduced for the upper storeys permitting lighter more economical components to be used.  In such cases it will be essential to seek advice from a specialist blast engineer.

Additional considerations

In addition to the blast threats, windows, curtain walling and doors may also need to meet other regulations and performance criteria, which include:

• Safety (e.g. BS EN 12600)
• Thermal Performance
• Acoustic Performance
• Fire Performance
• Physical security
• Ballistic protection

These aspects are not covered in this guide.

Specialist advice

Police Services have Counter-Terrorist Security Advisors (CTSAs) who can provide risk owners with advice on current threats.

The effects of blast and blast mitigation are complex subjects, and it is expected that many risk owners will wish to seek advice from suitably qualified and experienced personnel (SQEP). Appropriate SQEP will be on the list of blast specialists maintained by the Register of Security Engineers and Specialists (RSES)  or able to demonstrate equivalent competence.

Summary

The risk owner is responsible for deciding the degree of blast-resistance against external IEDs that the windows, doors, curtain walling and non-loadbearing walls in the building façade should provide for both building occupants and people outside the building.

Although blast protection is the preferred aim, in some cases it may not be economically practical and the building owner may opt to choose a blast mitigation option.  In both cases it is important to record the TVRA process and decisions.

The blast performance requirement requires decisions on:

• The IED threats to be considered and their location (the charge mass and stand-off distance for each threat).
• The tolerable level of risk of injuries to people i.e. hazard level, both inside and outside the building, due to blast damage to windows, curtain walling and doors.
• The blast performance requirement comprises the threat threat/blast load and the tolerable hazard level for each of the identified threats.

Footnotes

¹Security Overlay to the RIBA Plan of Work (2023)

²NPSA Guidance: Operational Requirements

³BS EN 13123-2:2025 – Windows, doors, shutters and curtain walling – Explosion resistance – Requirements and classification – Arena test
BS EN 13124-2:2025 – Windows, doors, shutters and curtain walling – Explosion resistance – Test method – Arena test

⁴NPSA Supplement to the BS EN Test Standards (2025)