Lee Cunningham

Post earthquake fire behaviour of composite steel framed structures

This paper utilises a 3D numerical model to simulate behaviour of composite steel frames in fire following earthquake. The aim of this study is to provide insight into the effect of prior earthquake damage on structural behaviour in fire. It compares structural behaviour of the frame in two different conditions, fire in normal conditions (F) and post-earthquake fire (PEF). The earthquake analysis is performed using pushover analysis and then is followed by fire analysis using the Eurocode parametric fire. Additionally the effect of fire insulation delamination during earthquake is also taken into account. The contrast in behaviour of the two conditions is presented and explained. earthquake are the same when performance of the frames during earthquake is still within the operational performance limit 8 . However, there is a reduction in fire resistance of the structure when the performance of the frame surpassed ultimate design capacity and reached nearcollapse levels. Moreover, Memari et al. 9 presented finite element simulation to study post-earthquake fire performance of steel moment resisting frames with reduced beam section (RBS) connections. The frames were subjected to a suite of ground motion records to simulate earthquake using nonlinear dynamic analysis. Thermal-mechanical analysis was then performed to simulate postearthquake fire. The frames were considered fireproofed. Thus, fire was applied only at location of the RBS connection considering that fireproofed delamination occurs at the RBS connection during earthquake. Global and local responses of the frames were investigated according to ASCE standard 41-06 8 performance limit. All of the aforementioned studies have focused on non-composite steel frames. Most of the structures have been analysed assuming 2D plane-frame behaviour, without considering the presence of a composite slab. Although some essential issues of fire behaviour can be captured, 2D frames fail to consider the load redistribution path in a realistic structure in particular the tensile membrane action of the concrete slab. Previous studies 10 have shown that the effects of 3-D behaviour on undamaged steel-composite structures under fire loads are significant and can offer great reserves of strength. Especially since the UK’s Cardington fire test 11 , research has been conducted to investigate and understand the behaviour of steel-framed structures in fire. It was confirmed that the composite slabs have an important role in the survival of the frame, through tensile membrane action. This paper presents a study conducted on a generic three dimensional composite structure under fire following earthquake. A series of analyses are carried out to simulate a postearthquake fire event. In this research, fire insulation delamination and residual displacement are applied to the structure to represent earthquake damage. This is followed by fire analysis using standard fire and natural fire. Fire analysis in the absence of earthquake is also performed to the undamaged structure as a reference to observe the effect of earthquake damage in the postearthquake fire analysis. 2 GENERIC BUILDING To represent a typical commercial office building, a generic five-storey composite steel-frame structure is analysed in this study. The frame is designed according to Eurocode 3 12 , 4 13 & 8 5 for high seismicity. The structure’s plan and elevation views are shown in Figure 1. The combined dead and live loading at fire is taken as 5.4 kN/m 2, (load factor of 1.0 dead load and 0.5 for live load). Seismic design of the building is undertaken for a moment resisting frame (MRF) with medium ductility. The MRF is designed to be flexible by satisfying deformation criteria under seismic loading or the limitation of P-Δ effects under design earthquake loading. The steel and concrete properties at ambient temperatures are presented in Table 1 and Table 2. Main beams and columns are assumed to be protected with lightweight insulating material 14 which has thermal conductivity of 0.2 W/mK, specific heat of 1100 J/kgK and density of 300 kg/m 3 . In order to utilise the tensile membrane action in the slab panels, which are surrounded by the primary beams, the secondary beams are left unprotected. The concrete and steel properties at elevated temperatures are adopted according to Eurocode 4 15 . Table 1. Ambient temperature material properties for steel Material Modulus of elasticity Poison ratio Thermal expansion Yield strength E (Gpa) ν α (°C) σy (Mpa) Mild Steel 210 0.3 1.35 x 10 -5 300 Rebar 210 0.3 1.35 x 10 -5 450 Table 2. Ambient temperature material properties for concrete Material Poison ratio Thermal expansion Compressive strength Ultimate strain ν α (C -1 ) σc (Mpa) εu (Mpa) Concrete 0.25 9 x 10 -6 30 0.002 Figure 1. Generic frame structure 3 NUMERICAL MODEL The finite element software ABAQUS v6.14 is used to model and analyze the structure. Steel columns and beams are discretized using 1-D line elements and concrete slabs are modelled using shell elements. A solid flat concrete slab is considered as an idealisation of the trapezoidal slab with metal decking. This was done to evaluate the slab effect but avoid the difficulties in using shell elements to simulate the ribbed composite slab. A tie constraint is applied to accommodate composite action between the steel beam and the concrete slab. For simplicity, it is assumed that the beam-to-column and secondary beam-to-primary beam connections behave as rigid and pinned, respectively. In this study, a three-step analysis procedure is performed. First, the building is subjected to gravity load. Second, nonlinear pushover analysis is performed to simulate the earthquake event. Earthquake damages are discussed in Section 4.1. Third, a thermal-mechanical analysis is conducted to investigate post-earthquake fire behaviour. Material temperature distributions are discussed in Section 4.3. 4 POST-EARTHQUAKE FIRE ANALYSIS 4.1 Earthquake simulation According to FEMA 356 16 , Earthquake damage levels of the building are categorised into three different performance levels, i.e. Immediate Occupancy (IO), Life Safety (LS) and Collapse Prevention (CP) that represent minor to major damage. The performance levels are tied to the inter-storey drift ratio (IDR) as an indication of global stability of the structure. The IDR value is less than 0.7%, 0.7-2.5% and 2.5-5% for performance level of IO, LS and CP, respectively. In this study, it is assumed that earthquake damages are conservative and the performance of the building subjected to worst case scenario of earthquake will be considered. Therefore, collapse prevention (CP) performance level is selected. This can be achieved by performing nonlinear static pushover analysis. In this analysis, the building is pushed using a specific lateral load to arrive at a target displacement represented by the roof displacement at the center of mass of the building. Load duration is not essential for this analysis since long-term effects such as creep are not taken account. It should be noted that no dynamic effects are considered in this study. Figure 2 shows base shear subjected to the building against roof displacement. As can be seen, the building was loaded up to a horizontal displacement of 0.6 m (IDR 3%) and was then unloaded. The result shows that there is a residual displacement of 0.37 m in the top of the building. Figure 2. Base shear against top displacement As mentioned above, the philosophy of seismic design permits a certain degree of damage in the structural elements. There is a possibility that the active fire protection system is compromised by ruptured water supply and delayed response of firefighting after the earthquake event. Thus, a passive fire protection system, such as sprayed fire resistive material (SFRM), may play an important role in the building response in fire. However, the role of SFRM can be also compromised if the SFRM gets detached from the steel structures. Braxtan and Pessiki 17 conducted experiments in SFRM on steel subjected to earthquake. Two common types of SFRM were used in the experiments: a dry-mix material and wet-mix material. The results showed that there is SFRM delamination concentrated in the beams where plastic hinges are formed at both ends. Results of the heat transfer analysis indicated that SFRM damage on the beam cause an increase in temperature in the adjacent column. Further study by Kodur and Arablouei 18 showed that fire insulation delamination can reduce the failure time of the beam. The above studies proved that passive fire protection system has a significant role in the vulnerability of steel structures subjected to fire following earthquake. Therefore, to determine the effect of fire insulation delamination, the steel structures at plastic regions in the beams are left unprotected. Delamination length is assumed to be 1.0 m from face of columns as shown in Figure 3. Figure 3. Fire insulation delamination at plastic hinge regions

Effect of earthquake damage on the behaviour of composite steel frames in fire

Fire loading following earthquake loading is possible in any building in a seismic-prone area. However, most design approaches do not consider fire following earthquake as a specific loading case. Moreover, seismic design philosophies allow a certain degree of damage in structural elements which make structures more vulnerable when subjected to post-earthquake fire. This study uses three-dimensional numerical models to investigate the effect of earthquake damage on the fire resistance of composite steel-frame office buildings. A total of two types of earthquake damage, fire insulation delamination and residual lateral frame deformation, are investigated. It is concluded that earthquake damage can significantly reduce the fire resistance of composite buildings, with delamination of fire protection having the greatest effect. The results of this study can be used by designers to improve the post-earthquake fire resistance of composite buildings.

New household waste recycling centre in Blackpool, UK

This paper describes the design and construction of Blackpool Council’s new household waste recycling centre. The scheme demonstrates the incorporation of sustainability considerations through design, construction and commissioning. Seeking to reduce the amount of household waste sent to landfill sites, the provision of a high-quality facility for recycling will encourage increased sustainability awareness and participation within the community. As a means of delivering a quality scheme, the challenge of ensuring high durability in combination with aesthetic and environmental enhancement has been met with innovation. Use of polymer fibre reinforcement in the heavy-duty apron slabs provides high durability under aggressive service loads. Elsewhere, incorporation of subsurface skips facilitates spatial optimisation and allows significant noise attenuation with minimal visual impact. Where possible, materials won from the demolition of existing time-expired structures were incorporated in the new scheme or d...