In a progressive collapse energy is continually fed into the event and failure is transmitted through the structure via the connections. An important option in preventing local collapse progressing disproportionately is to
ensure that locally damaged areas can be bridged over by the remaining structure and the resulting loads can be transmitted safely through alternative load paths. Although the anchor system was not designed to improve post failure
behaviour nor had their locations been chosen with this in mind, the test demonstrated two significant improvements in the post failure performance of the end strengthened with the anchors.
The roof structure was able to bridge the width of the building when its support had been removed and therefore its weight did not contribute to the energy input to the failure at first floor level.
The lateral tying delayed the buckling failure of the side walls until the impact of the debris loading from first floor forced the lower panels outwards.
At End B the buckling failure of the side walls occurred almost at the same time as the first floor failure started.
Conclusion
The anchors clearly have the potential to modify the mode of failure of a structure and therefore offer a valuable tool to structural engineers in designing against the collapse mechanisms in existing structures.
Correctly designed and installed this form of anchor can effectively induce a high degree of plate action in floors and roofs comprising of individual hollow precast concrete components, enabling them to span laterally.
The introduction of plate action can stiffen existing precast floors and roofs.
The ability of the strengthened floors and roofs to transmit vertical and in plane forces through plate action was maintained, even in the presence of extreme deformations.
Repeat loading did not indicate any degradation in the anchors performance.
The ability of the roof system to transmit horizontal loads to the front and rear edge beams was increased at least 2 3/4 times by the addition of the anchors; the limiting factor during the test being the capacity of the
test equipment.
Ultimately the total collapse of the end dwellings was initiated by lateral instability of the side walls. In the dwelling with the anchors cut this instability occurred almost immediately the first floor lost support, but
only occurred when the first floor had completely collapsed in the dwelling where the anchors were intact.
The strengthening system does not introduce additional stresses into the structure and requires small deformations before it becomes active.
The structural model indicated by the dynamic testing was confirmed by the subsequent static load tests.
