Retrofitting Existing Masonry Buildings to Resist Explosions

by: Stephen P Ward
Eur Ing CEng MICE MIExpE

This article is reproduced by kind permission of ASCE American Society of Civil Engineers
Journal of Performance of Constructed Facilities  May 2004  Volume 18, Number 2

ABSTRACT

In recent years, many buildings in the UK have had their windows protected or strengthened to resist the effects of explosions in an attempt to reduce the level of casualties associated with terrorist bomb attacks. Whilst the windows form the most vulnerable parts of a building, occasionally it becomes necessary to strengthen the walls as well, particularly in older and weaker historical structures or where the blast loads are high. This article examines a number of different techniques available to the structural engineer and security specialist that can be employed to make existing masonry walls stronger and more capable of resisting safely the effects of explosions.

DOI: 10.1061/(A5CE)0887-3828(2004)18:2(95)

CE Database subject headings: Retrofitting; Masonry; Explosions; Windows; Terrorism; Structural safety

For nearly 20 years, Cintec™ International Ltd based in Newport, South Wales and its sister company in the USA, Cintec™ America Inc has manufactured specialist anchors for reinforcing, strengthening and repairing all types of existing masonry structures around the world. Retrofitted reinforced masonry support anchors are comprised of stainless steel sections, a grouting sock and an engineered grout. Installation is by precisely drilled holes using wet or dry diamond coring technology. The company motto “Securing the Past for the Future” is a good indication of the philosophy behind the Cintec™ anchor system. More recently, Cintec™ anchors have been used to improve the load carrying capacity of masonry arch bridges, the impact resistance of parapets and provide seismic protection in multi-storey masonry structures.

RETROFITTING EXISTING MASONRY BUILDINGS TO RESIST EXPLOSIONS

1. Introduction. The tragic events of 11 September, have served to highlight the vulnerability of existing structures to terrorist attack. All western democracies are now acutely aware of the apocalyptic consequences of a well-orchestrated attack on high-profile government facilities and other related targets. Many of these buildings are historical, ornate, listed and constructed using traditional techniques with masonry elevations. Used in such structures, many of the modern retrofitted reinforcement techniques necessary to strengthen the existing building against terrorist attacks are unsightly, inelegant, intrusive and inappropriate. However, security specialists are well aware that whilst there might be little that can be done to defend a building against an aircraft attack, there is plenty of scope to defeat the more traditional car bomb and bullet. This article will focus attention on some of the methods available to the structural engineer to strengthen existing masonry structures to resist the effects of a blast attack.
2. Limitations. For space reasons it is not possible to consider the full spectrum of possible terrorist attacks (e.g. biological, chemical and nuclear) or methods (e.g. ballistic and vehicle impact). Therefore, this article will only consider defence against blast attacks delivered using vehicle borne IED¹.
3. Considerations. In strengthening an existing masonry building to resist the effects from blast, structural engineers have to consider a number of conflicting requirements. Some of these include:

1. Stand-Off. Stand-off (or to use the US term ‘set-back’) is the pre-requisite for all blast mitigating solutions and is often the cheapest. It can take many forms; open areas, car parks, pedestrian only zones or even sacrificial buildings to name a few. It may be also possible to create sufficient stand-off using low level walls, perimeter fences or barriers. However, in locations where adequate stand-off cannot be achieved, it will be necessary to either reduce the size of the threat (e.g. by restricting the size of the delivery vehicle) or providing some measure of structural protection².
2. Prevent the Blast Wave Entering the Building. Apart from the direct effects of an explosion on the structure, much of the damage caused is due to the effects of the blast wave entering the internal parts of the building. The relatively fragile components of modern offices offer little resistance to high-energy blast waves and yet are critical to the efficient functioning of the workplace (Figure 1). Partitions, false ceilings, lighting, heating and ventilation ductwork and trunking are very vulnerable not to mention computer systems, telecommunications and security apparatus. By keeping the blast wave out of the building, damage to the internal fabric and equipment is minimised, and recovery accelerated.
3. Windows. The most vulnerable parts of any building are the windows. Considerable research and development has taken place around the world to determine the best methods of protecting these vital parts of a structure. For many years, one of the most expedient measures has been to apply ASF³ combined with BBNC⁴. However, manufacturers only guarantee ASF for ten years and BBNC requires regular cleaning and obstructs the view. Removal of ASF is time consuming and labour intensive and studies have shown that once two cycles of ASF and BBNC has been applied and removed it may have been more cost effective to install blast resistant glazing from the outset.

Figure 1 – Widespread damage to conventional glazing

4. Protect Occupants. The most important function of structural protection is to safeguard the people who work and live in the building. Every building owner has a duty of care therefore, in addition to the requirements imposed by building regulations; there is a need to make the place safe from terrorist attack. This can take several forms depending of a whole spectrum of parameters. Smaller buildings with robust exterior walls can be strengthened to resist attack. For large buildings it may be more economical to establish ‘safe havens’ within the structure instead of strengthening the whole of a vulnerable facades. Properly trained security personnel, appropriate surveillance systems and well-rehearsed emergency procedures will all help to protect occupants in the event of a crisis.
5. Prevent Structural Failure. Regrettably, there have been many explosive incidents in which the victims have survived the initial attack only to lose their lives when the building subsequently fell down. Two of the most important factors structural engineers have to consider are robustness and redundancy. Robustness is a measure of the buildings ability to cope with hazards in an acceptable way. Redundancy is a condition relating to the ability of a structure to transfer loads into alternate areas. Buildings that are robust and structurally redundant are capable of surviving blast loads well; buildings that are not tend to suffer badly (Figure 2).

Figure 2 – Failure of the connections in a precast concrete frame

4. Reinforcing Existing Masonry Walls. The philosophy behind reinforcing existing masonry walls is to provide increased strength along with improved ductility and /or ‘catcher’ (restraint) systems wherever possible. There are several ways of achieving this depending on the size of the threat, the type of wall (load-bearing or infill) and degree of fenestration.

1. Steel Column and Plate. This is a particularly robust form of retrofit technique in which a number of steel columns are secured behind the wall and connected into the building frame at the floor and ceiling level (Figure 3). Steel plates connect the flanges of the columns together producing an in-situ tensile membrane capable of resisting loads of up to 50psi. Ideally suited where load-bearing walls must give support to the floor above, the internal surface preparation is minimal. However, the engineering is demanding, the installation process intense particularly as each connecting weld must be sound and construction details can be problematic. The technique is therefore relatively expensive.

 
Figure 3 – Steel column and plate

2. Steel Stud Partition. Steel studs (as opposed to timber studs) are used in many forms of modern building construction and this technique capitalises on their use. Vertical steel studs are fixed between floors and support reinforced gypsum board or laminated glass (Figure 4). This partition is then placed at least 300mm inside the existing non load-bearing wall to act as a catcher screen. The system is easy to install, requiring no surface preparation but can only be used for relatively light blast loads.

Figure 4 – Steel stud partition wall and window

3. Elastomeric Spray. Elastomeric spray is a relatively new concept and uses a urea or polyurea based coating up to 15mm thick applied directly to the rear face of an existing masonry wall. Once dry, the coating forms a tensile membrane enhancing the flexural capacity of the masonry significantly reducing spalling. The coating is relatively inexpensive but the wall must be prepared very thoroughly and considerable attention paid to the cleanliness of the masonry surface (Figure 5). The system has been exposed to blast pressures up to 35psi and impulses of 215psi-ms successfully reducing spalling, but cannot be used on load bearing walls without the support of another load bearing system.

Figure 5 – Elastomeric spray applied to hollow concrete block (CMU) wall

4. Geotextiles. Using technology developed in the geotechniques industry for the stabilisation of weak soils, tests where geotextiles have been secured to the rear of masonry walls and subjected to blast loads, have been conducted. The fabrics⁵ have either been mechanically attached to the floors above and below or glued to the internal face of the masonry wall. In so doing, they act as a ‘catcher system’ (Figure 6) restraining spalled and broken masonry from entering the building envelope. Whilst effective, considerable attention must be paid to securing fabric top and bottom or ensuring there is an effective bond between the fabric and the masonry. Further, special arrangements must be made for load bearing walls and for walls with windows.

Figure 6 – Anchored geotextiles secured to building frame acting as a ‘catcher system’.

5. Retrofitted Reinforced Masonry. Reinforced masonry is stronger and more ductile than unreinforced and is capable of resisting relatively high out-of-plane loads⁶ depending on the level of reinforcement. Retrofitted reinforced masonry⁷ uses techniques developed in the building restoration industry where existing structural masonry is diamond core drilled from the roof to the foundation and specially designed grout inflated masonry anchors are installed and allowed to cure. The system has been tested to 125psi and 284psi-ms (440lbs at 41ft) and can also be used to secure blast proof windows within masonry walls combining window security with masonry strengthening. Research has also shown that masonry walls with high levels of internal vertical loads (e.g. in multi-storey buildings) resist spalling better than those that are not load bearing (e.g. infill panels or single story construction). Retrofitted reinforced masonry can also be post-tensioned after installation to increase the internal vertical stress and maximise spalling protection in low-level masonry structures. The anchors are easily installed even in occupied buildings within the plane of the wall and are not visible once installation is complete. Further, retrofitted reinforced masonry can also be used in areas of high seismic risk where dynamic loads due to ground movement have to be resisted.

Figure 7 – Rear view of the retrofitted reinforced masonry hollow
block wall after loading with a blast wave from 200kg TNT at 12.5m.

6. Internal Concrete Skin. There are certain situations where the blast load is so large that it is not possible to provide the required level of protection using the conventional retrofitted techniques described above. In such cases, the only solution is to retrofit the building with an internal concrete skin. This is an effective but expensive solution. A full structural analysis is required to determine whether it is necessary to underpin the foundations to resist the additional dead loads. Further structural reinforcement may be required to strengthen the building frame to resist the huge dynamic loads likely to arise and prevent building collapse. Further, there will be loss of space inside the building equivalent to the thickness of the concrete skin and the necessary ‘air gap’ behind the existing wall⁸.
7. Durisol Block. A proprietary product known in the USA as Durisol Block provides a variation on the internal concrete skin. Made in Switzerland since the 1940s and in Canada since the 1950s, Durisol is basically a hollow concrete block made of mineralised wood shavings as the aggregate, instead of sand and stone. This mixture is used to make stay-in-place wall forms for concrete structures, freestanding sound barriers, and other products. Subject to the limitations above, Durisol Block provides an expedient solution to the problem of retrofitting masonry structures to resist the effects of explosions.

Figure 8 – Durisol Block places an internal concrete skin inside a modular formwork

5. Summary. Existing masonry walls can be strengthened in a variety of ways to resist the effects of explosions. Comparisons between each method are summarised in Table 1. Often it is possible to combine two or more systems into a hybrid scheme capable of resisting loads greater than sum of the individual systems – a ‘united we stand, divided we fall’ approach. Fundamentally, there is no substitute for stand-off and security arrangements must be tested against the ease with which it possible to deliver and detonate an explosive device. At this stage that it is often possible to mitigate against the effects of explosions by adopting changes to established procedures without making alterations to the building structure. However, if all those procedures are circumvented there is no substitute and existing masonry buildings must be retrofitted in some way.

Acknowledgements:

Figures 1 and 2 are taken from Blast Effects on Buildings, Thomas Telford, London, 1995 and are reproduced with permission from the editors GC Mays and PD Smith. The author also wishes to acknowledge the assistance provided by Mr Ed Conrath of the Corps of Engineers Protective Design Center, Nebraska, USA during the preparation of this article.

Table 1 – Comparative Advantages and Disadvantages of Existing Masonry Wall Retrofit Systems