On May 16th, 1968, a part of Ronan Point, a residential tower in London, collapsed. Due to a gas explosion, a façade panel was blown out of the 22 storey building. As a result, the upper façade panels lost their support and collapsed. The debris caused the collapse of the underlying floors and panels. This chain reaction, causing a damage much larger than the initial damage, is called progressive collapse. The Ronan Point collapse initiated research on preventing progressive collapse. One of the strategies is to design the structure in such a way, that it will not collapse if an element, for example a column, fails. The forces have to be carried by an alternate load path. In Eurocode 1-1-7 annex A, ties are proposed to create this alternate load path. In case of failure of a column, the floor is supposed to be suspended from these ties. In literature, however, many express their doubts whether these ties will work as they are supposed to do, in particular if a corner column in a precast concrete structure fails. This research aims to prove whether these doubts are legitimate, and if so, what ways there are to prevent progressive collapse in case a corner column fails. The effectiveness of the ties depends on the occurring yield line pattern. The wanted pattern, however, does not occur in most structures. Even if it would occur, the peripheral ties would not have the necessary strength to prevent collapse. Other alternate load paths are also investigated, but none of them appeared to be effective. It is shown that is possible to increase the robustness of precast concrete structures. This can be done by increasing the reinforcement in the top layer, to make a cantilevering -floor possible. In structures with façade panels, the façade can work as a cantilever. Tension forces are carried by the peripheral ties. With this solutions, extra care has to be taken regarding the contact area of at different times casted concrete. They might not be strong enough to resist the longitudinal shear stresses. Instead of increasing the reinforcement that is already there, one can also choose to add structural elements. In low structures with relatively small spans, applying a beam on the roof may prove beneficial. Underlying floors may be suspended from this beam. In bigger structures, infill-walls or diagonal bracing are a better option. This, however, introduces horizontal forces which need to be transferred to the adjacent bay. This requires extra attention when it comes to tension forces. It is, theoretically, possible to impose the 'right' yield line pattern to make the peripheral ties effective. This can be done by reinforcing the floor in a way that the preferred yield line becomes normative, or by designing the structure with two corners of 135 degrees instead of one of 90 degrees. This is, however, not recommended, as it is not yet proven whether or not other conditions, such as detailing and ductility, ar met. More research on this field is required. Moreover, it is not wise to depend on a certain failure mechanism if one is not totally certain it will occur. In all cases, it is beneficial to decrease the area of the floor. When is chosen to increase the top reinforcement of the floor to have it act as a contilever, the required reinforcement is about directly proportional to the floor area. If hollow core slabs are used, the forces will be mainly transferred in the longitudinal direction of the slab. As a result, the required reinforcement can, in some places, be around Ø16-100. Also, torsional moments will cause tension forces in the lower flange of the slab. In longitudinal direction this will not be a problem, as the slab is prestressed. In the transverse direction, however, reinforcement will have to be applied. Which type is preferred requires further investigation.
|Date of Award||31 Aug 2012|
|Supervisor||D.A. Hordijk (Supervisor 1), S.J. de Boer (Supervisor 2), Dik J. Hermes (External coach) & M.M. Stokla (External coach)|