There is an increasing need in the construction industry to modify tall buildings to such a level that they can withstand extreme damage without such damage leading to complete collapse of the building. In the construction industry, there is a widely held view that this is impossible to achieve in a cost effective and space efficient manner.

Where severe damage occurs to an intermediate level of a multiple storey building, the situation can arise whereby the damaged structure is unable to support the section of the building above the damaged area. As a consequence, this upper undamaged section of the building collapses onto the lower undamaged section of the building. The increased loading on this lower section is then too large to be maintained by the skeleton framework of the building and further collapse ensues. This chain of collapse can continue unchecked until the entire building is completely destroyed, effectively collapsing in a chain reaction.

The present invention seeks to prevent this form of collapse.

According to the present invention there is provided a method for controlling the collapse of a building having multiple floors, the method comprising the steps of: providing at least one load-bearing strut between at least one set of adjacent floors and attached to the load- bearing structure of the building, the strut being constructed such that, under building collapse conditions, it absorbs enough of the energy released during collapse of the building to control the rate of collapse of the building to reduce damage thereto.

According to the present invention there is further provided a collapse control system, for a building having multiple floors, the system comprising at least one load-

bearing strut located between at least one set of adjacent floors of the building, the strut being attached to the load bearing structure of the building, the strut being constructed such that, under collapse conditions, it absorbs enough of the energy released during collapse of the building to control the rate of collapse of the building to reduce damage thereto.

The strut may comprise an outer housing, this housing may be telescopic such that it reduces in length under particular loading conditions. Alternatively the housing may be designed to buckle in a controlled manner or to form an inversion tube under load conditions.

The struts may be incorporated into the original structure of a building forming an integral part of the framework or they may be added retrospectively to an older building. They may be designed such that, upon collapse of the building, a survival space is maintained between adjacent floors of the building. This survival space may be, for example, approximately half of the height of the floor spacing of the original, undamaged building.

The struts are preferably hollow in configuration and include a further means for absorbing energy. The struts may include a crushable core material which may be formed from an open or a closed cell filler material. The structure of such a filler may be one of the group of foam, honeycomb, eggbox or a number of individual crushable elements may be used, such as buckling tubes. A homogeneous porous material may also provide a suitable crushable medium. Suitable materials may be metal (such as copper, aluminium, steel), polymer or ceramic (such as concrete), or a combination thereof.

The strut may further contain water which has a two- fold benefit, firstly to improve the thermal transfer properties of the housing material and secondly, the fire mitigation properties of the water can be utilised upon failure of the housing material. Valves may be incorporated in the strut housing to assist water

distribution from the core of the struts. Alternatively, a water jacket could be incorporated into the strut to provide similar benefits.

Energy absorption means for the core of the strut may be provided by a mechanism which utilises a set of wires to be stretched or by manipulation of metal rods around a series of rollers within the strut.

The system may comprise plural struts, and in such a case stability of the collapse control system may be enhanced by interconnecting liquid filled regions of the struts such that multiple distributed struts are reduced in length at the same rate.

The housing may further comprise mechanical stops at regular intervals which support the static loading but fail under the increased dynamic loading associated with collapse. Internal column dividers may also be introduced to separate the core material into cells, this not only helps to prevent transmission of stress waves through the entire length of a strut but also eliminates the possibility of significant creep in the core material over the life of the building.

An example of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows partial views of a building using the present invention; Figure 2 illustrates alternative configurations for a telescoping strut for use in the present invention; Figure 3 illustrates alternative energy absorbing mechanisms of a strut for use in the present invention; Figure 4 illustrates further alternative energy absorption mechanisms for use in the present invention; Figure 5 illustrates operation of struts for use in the present invention involving the use of water; Figure 6 illustrates interconnected hydraulic systems according to the invention;

Figure 7 shows a partial cross-sectional view of a building having an integral strut for use in the present invention; Figure 8 is a side cross-sectional view of a system according to the present invention that is formed integral with a building; Figure 9 is a side cross-sectional view of a further system according to the present invention which is formed integral with a building; Figures 10 to 12 are schematic views of example components for employment in a system according to the present invention which employs active control; Figure 13 is a schematic side cross-sectional view, of a system according to the present invention employing plural struts; and Figure 14 is a schematic side cross-sectional view of a building employing struts according to the invention for additional earthquake or vibration compensation.

Figure la shows a partial view of a multi storey building which implements one example of the present invention. A standard load-bearing strut 1 is shown and floor trusses 4 are hung off this load-bearing strut 1 at regular intervals to form the individual storeys of the building. A telescopic energy absorbing strut 2 is located approximately parallel to the load-bearing strut 1 and attached thereto at one end. The other end of the energy absorbing strut 2 is attached to the floor 4 and, therefore, indirectly to the load-bearing strut 1.

Figure 1b shows an alternative attachment method in which the load bearing strut 1, the energy absorbing strut 2 and the floor trusses 4 are connected by a single bracket. In this example, since each end of the energy absorbing strut 2 is attached to adjacent floor trusses 4 and, therefore, indirectly linked to the load-bearing strut 1, any lateral displacement of the energy absorbing strut 2 may be prevented. As the load-bearing strut 1 fails and buckles, the telescoping mechanism 3 causes the two ends of

the energy absorbing strut 2 to move together until the full travel of the telescoping mechanism is taken up. An impulsive load is then transmitted to the load-bearing strut beneath the current one which, in turn, buckles thus the process continues until all of the energy has been absorbed. Figure 9 shows a similar configuration to that of figure 1B, but is one in which the energy absorbing struts 2 are interconnected coupling members 21 and also provide the load bearing function of the building. Again, these can support the individual floors 4.

In Figure 2 the energy absorbing strut 2 comprises a hollow, load-bearing housing 5 which is filled with a crushable core 6. As the housing 5 reduces in length, through the telescopic mechanism 3 as described above, the load is transmitted to the core 6. Under this loading the core 6 will be crushed, thus absorbing huge amounts of the energy being released from the collapsing building.

It is. an intention of the present invention that not only is the rate of collapse brought under control but that the extent of collapse be limited. In other words, after collapse of the damaged storeys, and potentially those immediately below the damaged sections as required to arrest the collapse phenomena, a survival space should be maintained on those levels where the energy absorbing struts 2 have been brought into play/activated. This survival space may, for example, be approximately head height or half of the original undamaged floor spacing distance. Such a clearance will allow any remaining personnel on the storey in question to escape being crushed by their local ceiling 4 (and floors above).

The telescopic mechanism 3 is shown in greater detail in Figure 2. In its simplest form, in Figure 2a, the mechanism comprises two main sections allowing a particular amount of compression before the integrity of the housing 5 is compromised. Alternatively, as shown in Figure 2b, an increased number of telescopic sections 3 may be

implemented to allow a greater level of compression within the telescopic travel of the housing 5.

Energy absorption in the examples detailed so far is provided by compression of the crushable core 6.

Additional energy can readily be absorbed by designing the housing 5 such that it fails predictably under particular loading conditions. Such modes of failure are illustrated in Figure 3. Figure 3a represents an axially buckling tube and Figure 3b represents a device known as an inversion tube.

Axial buckling of the housing 5 of the energy absorbing strut 2, rather than lateral buckling associated with the load-bearing strut 1, can be controlled by designing the struts 2 to the correct dimensions and introducing grooves or other geometrical changes into the strut housing 5 to initiate the buckling mechanism when a particular load is experienced. This buckling phenomena will then propagate along the length of the strut 2, wrinkling the housing 5, as a consequence significant levels of energy are absorbed.

Tube inversion, illustrated in Figure 3b, involves not only deformation of the impact end of the strut housing 5, but continual metal deformation as the inverted section increases in length until ultimately the strut 2 is entirely doubled over (and approximately halved in length).

This deformation process requires large amounts of energy which are, therefore, extracted from the system thus reducing the energy levels transmitted to the storeys below.

Figure 8 shows a similar system to that of figure 3B but is one which is incorporated into a building as part of its initial construction. In this case, anvil guides 20 are provided to support individual struts 2 with filler 6.

The anvils 20 can also provide support for individual floors 4. The anvils 20 have curve sections which enable the required inversion.

Energy absorbing struts 2 designed as in either of these examples could easily make use of the additional benefit of being filled with a crushable core 6, as in the telescopic example above, to further enhance the energy absorption. properties of the device.

Suitable fillers may be formed from open cell structures or closed cell structures such as foam, honeycomb, egg box shaped layers or even porous materials.

These fillers may be formed from metals, polymers or ceramics, the latter being formed, for example, by incorporation of soft foam beads into the wet concrete such that these form cavities in the set concrete which allow the material to be crushed, in this case the core will be pulverised, thus absorbing large amounts of energy.

Energy absorption may be performed through alternative means as illustrated in Figure 4. Figure 4A shows how a series of wires 7 may be utilised, under loading of the telescopic housing 5 these wires 7 can be stretched, thus absorbing energy from the system. In Figure 4B wire. rods 8 are drawn through a series of rollers/pulleys 9. The work done in this wire drawing process, involving constant bending and manipulation of the rods 8, extracts large amounts of energy from the system.

Rather than using one of the above fillers, the strut housing 5, as shown in figure 5, may be simply filled with water 10 which, whilst absorbing large quantities of energy when discharged through valves, will have the added advantage of mitigating potential fire hazards. Valves 11 may be incorporated into the housing 5, these valves 11 being set to activate when the surrounding material experiences a certain level of loading. In this way, the heat transfer properties of the strut walls will be improved prior to failure as the water 10 in the core 6 maintains a higher temperature gradient through the thickness of the wall 5. Additionally, water 10 can be sprayed onto the surrounding area to mitigate fire once failure of the strut housing 5 has occurred. Furthermore,

if an open cell filler core material 6 is used the struts 2 may be additionally filled with water 10 to achieve the same results. Alternatively a water jacket 12 may be located around the strut 2 or the core 6 as illustrated in Figure 5a, or the strut may consist of a piston and glider filled with liquid as in Figure 5b.

A situation may arise where the damage to the building is local to only a few load bearing struts 1. This could, potentially, result in the upper storeys toppling to one side as asymmetric failure occurs. Such a scenario could be avoided where several of the energy absorbing struts 2 are evenly distributed over the plan form of the building and are hydraulically linked 13 to each other (see Figures 6a and 6b). In other words, as a strut 2a is compressed on one side of the building the hydraulic linkage 13 could force the corresponding strut 2b on the other side to compress by the same amount, thus retaining a level reduction in height over the surface area of a floor 4 of the building. Figure 6a illustrates an example of the present invention where the volume of the region from which the fluid is being displaced must be the same as the region to where this fluid is being forced. Figure 6b illustrates an alternative example where relief valves 14 are incorporated into the design such that the flexibility of the design is enhanced, for example, where evaporation of liquid over time may be an issue if the liquid filled unit is not completely sealed a discrepancy in the relative volumes may occur such that the respective volumes are no longer identical. Pressure control valves 40 in the linkages 13 control the rate of response and provide static load carrying capacity.

Such an approach could be considered to be a passive system and is also shown generally in figure 13, in which plural struts 2 are interlinked by passageways 13. In this case, if one of the struts 2 at one side fails then fluid leaks from other struts and the building sinks but remains vertical. In this case it may well be that valves (not

shown) are provided at appropriate positions in the system to ensure static stability of the structure.

As an alternative to the above compensation arrangements, active compensation may be provided, and examples of active control are shown in figures 10 to 12.

In this example, when one of the struts 2 fails a sensor first detects failure and a processor 31 sends an actuation signal to a strut on the opposite of the building to initiate a controlled collapse on that side. If plural struts are used and distributed around the building then struts can be actuated at different times to control the collapse and prevent skew of that collapse.

Figures 11 and 12 show two example struts that could be employed in such an arrangement to enable controlled actuation. In the strut 2 of figure 11 a hydraulic connection 13 is provided to a fluid filled chamber 26.

The connection 13 has a valve 33 which can be actuated by the processor 31 to be opened. Fluid is then expelled from the strut and it collapses in a controlled manner.

In the example of figure 12 the strut 2 has a collapsible core 6 and a member 35 which is supported in a load-bearing arrangement by bolts 34. The bolts 34 are fitted with an explosive charge that can be actuated by the processor 31 to remove them and, again, initiate collapse of the strut 2. For struts of the type shown in figures 11 and 12 the energy absorbing core 6 will absorb energy from a collapse, whether or not the strut has been activated by valve 33 or bolts 34.

In the above arrangement the struts 2 are disposed in a generally vertical manner to control collapse in a downward direction. In the example system of figure 15, however, struts 2 are placed across vertical members in the building in order to provide energy compensation from horizontal vibrations that may be caused by an impact to the building or an actual event such as an earthquake.

Again, the struts may employ either passive or active control.

The examples described so far have generally been focussed on secondary devices mounted along side the main load bearing struts 1 of the building, however, the present invention may also be used as an integrated feature within new builds as described above in relation to figures 8 and 9. As mentioned above the energy absorbing struts 2 can be combined with the load bearing struts 1, a further such system is illustrated in Figure 7. This example uses a telescopic mechanism 3 filled with a crushable core 6 with floor trusses 4 hanging directly from housing 5.

In this example under normal conditions, relative motion of the telescopic sections 3 is prevented by mechanical stops 15. These stops 15 are designed to sustain the static loading of the building but to shear under the increased dynamic loading associated with collapse of the building. The failure of these stops 15 reduces the length of the housing 5 such that the load is transmitted to the core material 6. Under this loading the core will be crushed, thus absorbing energy being released from the collapsing building. As described above, water 10 may be additionally filled into the hollow housing 5 to achieve improved thermal properties and introduce a level of fire mitigation.

In any of the above examples the energy absorption properties of the crushable core 6 may be improved by installing column dividers 16 such that the core is split into separate cells. This has the added benefit of preventing any significant amount of creep in the core filler material 6 over the life of the building.