How to make Buildings Robust / How to make
Some Structural Solutions
Some Cladding Solutions
Defence Structures Design
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.
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.
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.
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.
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.
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.
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.
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
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.