Thomas Telford - Blast Effect on Buildings
Structural Engineering | GSA
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. 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. 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 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. Properly trained security personnel, appropriate surveillance systems and well-rehearsed emergency procedures will all help to protect occupants in the event of a crisis.
Retrofitting Existing Masonry Buildings to Resist Explosions
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 Reinforcing Existing Masonry Walls.
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. 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.
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The technique is therefore relatively expensive. Figure 3 — Steel column and plate 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 mm 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 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.
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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 psi-ms successfully reducing spalling, but cannot be used on load bearing walls without the support of another load bearing system. 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 fabrics 5 have either been mechanically attached to the floors above and below or glued to the internal face of the masonry wall. 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.
Retrofitted Reinforced Masonry. Reinforced masonry is stronger and more ductile than unreinforced and is capable of resisting relatively high out-of-plane loads 6 depending on the level of reinforcement. Retrofitted reinforced masonry 7 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 psi and psi-ms lbs 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. 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. 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.
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