Progressive collapse susceptibility of a long span suspension bridge

Long span bridges are complex structural systems, often having strategic roles in the network infrastructures; consequently, their susceptibility to a disproportionate response in case of local failures needs to be assessed. In particular, current regulations prescribe that the structural robustness should be maintained in case of an accidental hanger detachment. Local damages in bridges, which are characterized by an horizontal load transfer system, may progress along the deck or along the suspension system, as the dynamic overloading of the structural elements immediately adjacent to the failed ones may lead to subsequent failures. In suspension bridges, which are characterized by a relatively low continuity of the system, the damage of the deck may favor a collapse standstill, in case of an early detachment of the deck collapsing section. In the paper, a long span suspension bridge is taken as case study and the robustness of the system is investigated with respect to the entity and the location of an initial damage of the hangers. The analyses are carried out with the avail of a finite element model (FEM) of the bridge, which accounts for mechanical and geometrical nonlinearities as well as for the dynamic effects of the abrupt failures. Critical aspects of the design are highlighted and possible countermeasures for enhancing the structural response are suggested. © 2013 American Society of Civil Engineers

Fire-induced collapse of steel structures: Basic mechanism and countermeasures

Single-story steel buildings such as car parks and industrial halls are often characterised by stiff beams and flexible columns and may experience an outward (sway) collapse during a fire, endangering people and properties outside the building. It is therefore a current interest of the research to investigate the collapse behaviour of single-story steel frames and identify relevant structural characteristics that influence the collapse mode.In this paper, a parametric study on the collapse a steel beam-column assembly with beam hinged connection and fixed column support is carried out under the assumption of a protected column and a standard temperature-time curve on the beam. The study shows that sway collapse can be avoided by increasing either the restrain offered by the column or the load-to-resistance ratio of the beam. It seems possible to extend these results to multi-span frames with bracing system, in case of a fire located on one outer span –situation that represents the worst case for the risk of sway collapse.With respect to this type of frames, a methodology is proposed for the development of design tables that relate the profiles of the elements to the soliciting load on the beam. By means of those tables, a simple method for the assessment and the countermeasure of unsafe collapse mode of single-story steel buildings can be derived

Structural response of steel high rise buildings to fire:System characteristics and failure mechanisms

Due to the significant vertical elevation and complexity of the structural system, high rise buildings may suffer from the effects of fire more than other structures. For this reason, in addition to evacuation strategies and active fire protection, a careful consideration of structural response to fire is also very important. In this context, it is of interest to investigate the characteristics of the structural system that could possibly reduce local damages or mitigate the progression of failures in case of fire. In this paper, a steel high rise building is taken as case study and the response of the building is investigated up to the crisis of the structure with respect to a standard fire in a lower and in a higher storey: the comparison of the fire induced failures at the different height allows highlighting the role played in the resulting collapse mechanisms by the beam-column stiffness ratio and by the loading condition

MODELLING OF FIRE IN AN OPEN CAR PARK

Steel car parks exhibit high vulnerability to fire, as a consequence of the degradation of the steel mechanical properties at high temperatures and of the combustible type and amount. Real fire accidents in open car parks demonstrated a much faster and extended fire spread than predictions, assuming that a fire spread rate of 12 min and consider at most 3-4 vehicles on fire at the same time. Fire Dynamic Simulator (FDS) is applied in this current paper to study fire spread between cars. The outcomes of the investigations show that the fire spread is strongly influenced by the geometrical layout and that the distance between cars plays a determinant role on the fire spread rate and ignition of adjacent cars. In particular it was found that the fire spread can be faster than 12 minutes in the case of the cars parked 40 and 60 cm from each other

Fire resistance of extruded hollow-core slabs

Purpose Prefabricated extruded hollow-core slabs are preferred building components for floor structures in several countries. It is therefore important to be able to document the fire resistance of these slabs proving fulfilment of standard fire resistance requirements of 60 and 120 min found in most national building regulations. The paper aims to present a detailed analysis of the mechanisms responsible for the loss of load-bearing capacity of hollow-core slabs when exposed to fire. Design/methodology/approach Furthermore, it compares theoretical calculation and assessment according to the structural codes with data derived from a standard fire test and from a thorough examination of the comprehensive test documentation available on fire exposed hollow-core slabs. Findings Mechanisms for loss of load-bearing capacity are clarified, and evidence of the fire resistance is found. Originality value For the first time, the mechanisms responsible for loss of load-bearing capacity are identified, and test results and calculation approach are for the first time applied in accordance with each other for assessment of fire resistance of the structure. </jats:sec