Defence Structures
structural civil engineering, designing shock resilient structures and blast resistant buildings

How to make Buildings Robust / How to make Structures Resilient

The Problem
Some Structural Solutions
Some Cladding Solutions
Defence Structures Design

The Problem

Explosions and blast can produce, in a very short time, an overload much greater than the design load of a building. Explosives or projectiles can cut or deform structural members with Chemical Energy or Kinetic Energy. In spite of this buildings can and do survive such effects without collapse, if correctly designed to do so. On the other hand structures which are not so designed can suffer rapid cumulative collapse, such as we have seen at Oklahoma City, the World Trade Centre, the Marine Base in Lebanon, Ronan Point, as well as countless collapses in Earthquake areas.

Cladding and glass can be detached and fly around, forming lethal weapons. Such debris is often the biggest cause of injury and death. Steps should be taken to maximise the distance from any attack using gates, barriers, chicanes and such like. Nothing can be guaranteed to eliminate all risks; but if the following design features were to be incorporated, many lives could be saved and many structures and businesses would survive.

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Some Structural Solutions

  1. Floors must be prevented from ‘falling off’ their supports. If pre-cast concrete planks are used they should have sufficient bearing; but they should not depend on bearing and gravity to stay in place: they should be made continuous with rebars between adjacent planks and preferably be made continuous with the supporting beams, using shear connectors. However a more robust detail is to pour continuous concrete slabs on to composite style decking which is itself continuous over 3 or so joists; such slabs should be poured so that they encapsulate the main beam to which the joists are fixed, and around the columns.

  2. Joists should be made continuous themselves, through every main beam and wherever they coincide with outer columns. The joints should exceed the plastic capacity of the joists so that, if they fail, it is by plastic hinge and not by joint failure. Where joists are attached into the webs of outer beams no moment resistance is possible but there should be sufficient bolts to make shear failure unlikely before plastic hinges form in the outer main beam.

  3. Main beams should be continuous across the structure and should have connections to the outer columns which exceed the plastic capacity of the main beam. This means that in the case of overload the beams deform, forming hinges, absorbing energy and taking time. Blast or shock loads will diminish in a very short time.

  4. The main outer columns should remain elastic and strong enough to carry likely loads even when main beams attached to them form plastic hinges. Care should be taken that the shear capacity of the column should not be exceeded within the moment connection zone by the moment in the beam: this almost always requires haunched beam-to-column connections.

  5. Very often the main beams will go through the internal columns, which will be bolted to the underside and top of the beams. These connections must be sufficiently strong to ensure full moment connection of the columns to the beams.

  6. The ground to first floor columns carry the heaviest loads. They are always more vulnerable to attack. They are almost always longer than columns on other floors. They often have less stability because of gaps between them. And they often have no continuity below, as they sit on 'pinned' feet. So special care has to be taken: they need to be stronger; to have barriers to protect them; to have continuity at footings level with ground beams or slabs.

If all this continuity is achieved, even if a column or two are cut or deformed, the grillage of beams and joists and slabs at each floor throughout the building will continue to carry the loads. They may well deform substantially, joists and beams may well bend and form plastic hinges or act as a catenary net to share loads; but it would be exceptionally difficult to demolish such a building. To do so would require long study, and the placing of numerous cutting charges all over the structure, with a planned firing sequence; something very unlikely in the event of a conventional attack.

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Some Cladding Solutions

  • With industrial cladding the solution is to make all the cladding double spanning; then the centre rail is made strong and continuous, and the connections to the centre rail are made strong; whereas the rails either side of the centre rail are made weak (though still continuous) and the fixings weak. In the vent of a blast the cladding will fail at either end but remain fixed to the centre rail around which it will bend. The sheet will not fly around, and the sheet folding will reduce the forces on the structure.

  • With commercial buildings the same principle applies to the regular cladding. Windows should be kept modest in size. Windows should all be laminated. They should be in sturdy frames. But the frames should be fixed firmly to a strong rail, at the top; or at the bottom; or one side or the other; and less firmly to the other three edges. They will thus resist normal climatic loads and reasonable accidental loads, but will hinge inward from the strong edge before they burst and scatter glass.

If these cladding rules are applied there should be a much reduced scattering of flying shrapnel as a result of an attack.

Defence Structures Design

All Defence Structures buildings have some of these features as a standard. Where there is a specific Seismic or Hurricane risk, this will be taken into account. Where there is a Security risk all of these features will be incorporated.

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