Farshad Hashemi Rezvani

Robustness assessment of a generic steel fire-protected moment-resisting frame under travelling fire

Due to their vulnerability to high temperatures, steel structures are typically protected against fire by insulation materials, as recommended by fire codes. The fire resistance rating of a protected member is determined based on a standard fire test, which simulates post-flashover fire conditions. Empirical evidence has shown that fires in large open-plan compartments do not burn uniformly, and a method called “traveling fire” has been introduced in previous research to simulate the effects of spreading fires. An investigation is performed here to examine the robustness of a generic four-story moment-resisting steel structure with one-hour fire resistance rating subjected to travelling fire. Various fire sizes are considered: 12.5, 25, 50 and 100% of the floor area. The results show that while no collapse occurs during the 12.5, 50 and 100% fire sizes, the structure collapses under the 25% fire size at 85 min due to the inability of the system to transfer the load after one of the columns buckles. This seems to be in contradiction with traditional belief that a fire covering a larger portion of the floor plan would reduce the safety margin. The investigation performed underlines that the fire protection of structures based on the standard time-temperature curve does not necessarily provide adequate resistance under travelling fires. With the lack of adequate fire regulations codified for large compartments, more research is required before arriving at a better understanding of the application of travelling fire to large compartments.

Structural evaluation of tall steel moment-resisting structures in simulated horizontally traveling postearthquake fire

AbstractPostearthquake fire (PEF) can result in a catastrophe even worse than the earthquake itself. Investigating the effect of PEF loads on structures is thus of paramount importance. On the other hand, because of the complexity of fire, the simulation to account for the thermal loads exerted on structures has always been an issue. Although there are two main methods for simulating real fires—standard fires and natural fires—observations have shown that both of these are more correct for small/medium-size structures. For large open-plan structures, alternative methods (such as traveling fires) should be used. Here, an investigation is performed on a tall seven-story steel moment-resisting structure with a plan area of 900  m2. The structure is first pushed to arrive at a target displacement corresponding to the life safety level of performance according to FEMA code. The damaged structure is then subjected to different traveling fire sizes of 16.7, 50, and 100%. To draw a comparison between the results,...

Approximations in Alternate Load Path Capacity of Welded Unreinforced Flange-Bolted Web Connections

In the event of sudden column removal, beam-to-column connections play a significant role in transferring the load carried by the damaged parts to the intact ones. This role involves experiencing a magnified bending moment and axial load which ultimately produces catenary action in the beam. This mechanism can eventually replace the flexural action in carrying the gravitational loading. Therefore, the beam and the connection must be able to develop and maintain a sufficiently large axial tension which explains the important role that connections play in mitigating progressive collapse. However, the strength and ductility of the out-of-plane members and connections affect the total load carrying capacity of structures employing moment connections. In this study, using component-based modelling, the effect of modelling these structural elements in the final result is investigated. Analysis results of a generic structure revealed that ignoring the out-of-plane members and connection can lead to overestimate the load carrying capacity of the affected span by 72%.

Failure progression resistance of a generic steel moment-resisting frame under beam-removal scenarios

Purpose The purpose of this paper is to determine the failure progression resistance of the steel moment-resisting frames subjected to various beam-removal scenarios after application of the design earthquake pertinent to the structure by investigating a generic eight-story building. Design/methodology/approach The structure is first pushed to arrive at a target roof displacement corresponding to life safety level of performance. To simulate the post-earthquake beam-removal scenario, one of the beam elements is suddenly removed from the structure at a number of different positions. The structural response is then evaluated by using nonlinear static and dynamic analyses. Findings The results show that while no failure is observed in all of the scenarios, the vulnerability of the upper stories is much greater than that of the lower stories. In the next step, the structural resistance to such scenarios is determined. The results confirm that for the case study structure, at most, the resistance to failure progression in upper stories is 58 percent more than that of lower stories. Originality/value Failure and fracture of beam-to-column connections resulting in removal of beam elements may lead to a chain of subsequent failures in other structural members and eventually lead to progressive collapse in some cases. Deficiency in design or construction process of structures when combined by application of seismic loads may lead to such an event.

Effect of span length on alternate path capacity of welded unreinforced flange-bolted web connections

In this study, the effect of span length on progressive collapse resistance capacity of Welded Unreinforced Flange-Bolted web (WUF-B) connections was investigated by using the alternate path method as per Unified Facilities Criteria (UFC). Towards this aim, several nonlinear analyses were performed for three generic moment resisting frames with WUF-B connections considering various span lengths. In order to reduce the simulation time, a macro-element, or component-based model is introduced and verified against the NIST test results. The analysis results revealed that for a certain frame length, by decreasing the span length (increasing the number of spans) as an indirect approach, it is possible to enhance the alternate path capacity WUF-B connections and decrease the risk of progressive collapse occurrence. For example, for the cases studies here, it was shown that by decreasing the span length to half the failure overload factor of the frame increases more than twice.