Studies on the Impact from a Failed Floor System

Vlassis et al. [43] reported that the impact from a failed floor is one of the most prevailing progressive collapse initiation mechanisms and can be considered as the limit state requirement in the progressive collapse analysis. In the event where a building is subjected to an internal explosion either accidentally or intentionally, all structural members within the vicinity of the explosion become vulnerable to severe damage, including the slabs.

The authors admitted that it would be a great challenge to design a floor system capable of arresting progressive collapse due to a failed floor impact in view of the significant kinetic energy acquired by one or more floors following failure. However, under specific circumstances (number of failed floors, energy loss upon impact and the strength of the lower floor) it is still possible for the lower structure to contribute in preventing progressive collapse. In arresting the progressive collapse, the ability of an impacted floor is directly linked to its energy absorption characteristic. The authors then proposed a new design- oriented methodology for the progressive collapse assessment of floor systems within multi- storey buildings subject to impact from an above failed floor.

This text box is where the unabridged thesis included the following third party copyrighted material:

McCann DM, Smith SJ. Resistant design of reinforced concrete structures. STRUCTURE Magazine. 2007; 22-26.

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Using a seven-storey steel frame grillage model and the impact of a failed floor as an independent event, they established the pseudo-static response of the floor based on the estimated energy transfer associated with the specific characteristic of the impact event. The analytical procedure started with calculating the anticipated range of kinetic energy transferred from an impacting floor to the floor below based on the nonlinear static response approach. Static load-deformation curves and dynamic demand for the impacted floor was established at this stage. The least demanding impact scenario was taken as the impact from a falling floor that carried half of the impacted floor gravity load and only 20% kinetic energy being transferred. Later, the linear static load-deformation response was modified using the pseudo-static response to account for the effect of the initial deformations of the lower floor under the gravity load. The capacity of the impacted floor based on the calculated kinetic energy transfer was then established. The results from the analytical simulation showed that within all the impact scenarios being considered, the ratio of the impacted floor capacity/demand never exceeded one. They concluded that a floor system within a steel- framed composite building has limited opportunity to arrest the impact from an upper floor even in the least demanding impact scenario where the capacity only marginally exceeds half its dynamic demand.

Kaewkulchai and Williamson [2] used a computational planar framework that accounts for the change of structural properties and configurations of a damaged structure to investigate the potential of progressive collapse initiated by an impact of a failed member. They highlighted that when a building is subjected to an abnormal loading condition such as blast, other structural members such as beams could fail and move independently from the main structure. This beam can fall down and impact another beam below with a large dynamic force and likely to be one of the key reasons causing the collapse of the building as shown in Figure 2.3 [2].

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Figure 2.3: Impact of a failed member on a structure due to blast load [2].

They used a five storey 2-D frame model as shown in Figure 2.4 (a). In the simulation, a non- impact, impact with load only and impact with load as well as velocity were considered. The two possible impact scenarios are shown in Figure 2.4 (b) and (c). The beams and columns were assigned with the appropriate sizes and respective yield moments as well as maximum force capacities. The failure was set when the response exceeded these maximum values.

(a) (b) (c)

Figure 2.4: Computational planar framework for assessing progressive collapse showing (a) five-storey 2-D model under consideration; (b) one end of a failed beam impacting on another

beam; (c) a beam member failed at both ends impacting on another beam [2].

Column A

Beam A

Column B

This text box is where the unabridged thesis included the following third party copyrighted material:

Kaewkulchai G, Williamson EB. Modeling the impact of failed members for progressive collapse analysis of frame structures. Journal of Performance of Constructed Facilities. 2006;20(4):375-383.

http://dx.doi.org/10.1061/(ASCE)0887-3828(2006)20:4(375)

This text box is where the unabridged thesis included the following third party copyrighted material:

Kaewkulchai G, Williamson EB. Modeling the impact of failed members for progressive collapse analysis of frame structures. Journal of Performance of Constructed Facilities. 2006;20(4):375-383.

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By plotting the moment histories for the impacted beam, they showed that the internal moment for Beam A was magnified eleven times higher than the yield moment when both the load and velocity were taken into account resulting in progressive collapse. A non-impact or impact with load only showed insignificant increase in the internal moment. However, the effect of impact considering the load and velocity did not affect the internal force of Column B significantly as the impact energy was reported to have dissipated through the plastic deformation of the failing beams. They concluded that the velocity of a failed member impacted onto an intact member played the most important role in the collapse process.

Strassosek [44] proposed the methods to avoid disproportionate collapse of tall buildings with tube structure based on five general approaches namely non-structural protective measures, specific local resistance, alternative paths, isolation of collapsing sections and prescriptive design rules. In the specific local resistance approach, the author considered the tube structure as the primary load transfer system and provided several recommendations on the detailing of the tube structure with regards to the minimum thickness, reinforcement type, opening, location and operational safety. The floors cantilevering from the tube were considered as the secondary load transfer system and the failure of these floors can lead to a pancake-type progressive collapse. As shown in Figure 2.5 [44], in the case where a local failure of a floor occurred, the impacted floor must be able to arrest a progressive failure by transferring the impact energy as little as possible to another floor below. According to the author, this mechanism can be achieved by designing the plastic hinge of the impacted floor with sufficient rotational capacity. As such, the impacted floor will deform until it touches the tip of the floor below and reduce the impact energy.

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Figure 2.5: Assumed damaged and admissible deformation in secondary load transfer of one floor to another floor below [44].

2.2.5 Summary

Current design methods mainly focus on the performance of the key elements such as columns and load bearing walls to arrest progressive collapse. However interest has also been shown by other researchers in investigating the contribution and performance from slab structures for the same purposes. This concern was raised due to the fact that an impact from a failed floor member onto another floor system is able to cause a devastating effect and has been highlighted in a number of studies in the open literature.

This text box is where the unabridged thesis included the following third party copyrighted material:

Starossek U. Avoiding disproportionate collapse of tall buildings. Structural Engineering International. 2008;18(3): 238-246.

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