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Structural Solutions for Blast Resistant Buildings
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 blast resistant building.
Nothing can be guaranteed to eliminate all risks; but if the following blast resistant design features were to be incorporated, many lives could be saved and many structures and businesses would survive.