ANNA C. BIRELY and SHUNA NI. ABSTRACT. The post-earthquake fire (PEF) behavior of reinforced concrete (RC) structural walls investigated using finite ...
On the Effect of Design Parameters and Boundary Conditions on the Post-Earthquake Fire Performance of RC Structural Walls ANNA C. BIRELY and SHUNA NI
ABSTRACT The post-earthquake fire (PEF) behavior of reinforced concrete (RC) structural walls investigated using finite element analysis. Damage due to an earthquake consists of damage to the concrete and steel at the heavily reinforced end regions of the wall. The loss of cover increases the spread of thermal damage through the wall, impacting the thermal fire resistance of the wall and damaging the material for load-bearing fire resistance. This paper investigates the influence of key wall characteristics (geometry, reinforcement ratios, and axial load demand) on the fire resistance. Results indicate that, for damaged RC walls, i) at low axial loads, the insulation criterion controls the fire resistance, ii) higher reinforcement ratios contribute to increased load-bearing fire resistance, and iii) longer boundary regions can have accelerate the rate of heat transfer through a damaged wall. INTRODUCTION Post-earthquake fire (PEF) can have a significant impact on the structural performance of a building due to the increased likelihood of fire ignition and extended fire duration resulting from damage to fire suppression systems and/or the inability of emergency responders to access the structure. The potential disaster resulting from extended fire duration times are compounded by the possible damage to the structure due to the earthquake as the damaged or missing concrete can enable a more rapid transfer of heat through the walls and there is less material available for the resistance of loads. Mousavi et al. [1] provide a detailed review of PEF, including the history, major factors, strategies for mitigation, and methods for evaluation of building performance. Evaluation of building performance requires two key steps: 1) seismic analysis and post-processing to determine effects on characteristics impacting fire resistance and 2) thermal and/or thermal-mechanical analysis to assess fire resistance. Mousavi et al. list a key research need as experimental and analytical studies to inform development of ________________________ Anna C Birely, 3136 TAMU, College Station, TX 77843 -3136, U.S.A. Shuna Ni, 3136 TAMU, College Station, TX 77843-3136, U.S.A.
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guidelines for assessing PEF performance of structures. The work presented in this paper is focused on numerical analysis of reinforced concrete (RC) structural walls. RC walls are critical components for the resistance of lateral loads generated by earthquakes and often serve the dual purpose of serving as fire barriers. Further, RC walls are very common lateral load resisting systems and can be found in both steel and concrete buildings. Consequently, the PEF performance of these components is critical to understanding the performance of many buildings. Damage to RC walls from an earthquake include cracks (horizontal and diagonal), loss of cover concrete, crushing of core concrete in boundary regions (heavily reinforced regions at the extreme compression/tension ends of the walls), and buckling/fracture of reinforcing bars. These physical changes to the wall can have a significant impact on the fire resistance of RC walls because they enable a more rapid transfer of heat through the walls and there is less material available for the resistance of lateral loads. In this research, finite element models are used to conduct uncoupled thermalmechanical analysis of reinforced concrete walls. The work builds on previous work by the authors in this area and specifically focus on boundary conditions for the walls and key wall characteristics (thickness, reinforcement amount and layout, and axial load ratio). The fire resistance is evaluated by both the insulation criterion (Criterion I) and the load-bearing criterion (Criterion R). PRELIMINARY INVESTIGATION Ni and Birely [2] conducted a preliminary investigation of the impact of fire resistance of earthquake damaged reinforced concrete walls using numerical models. The models were created using ABAQUS. For heat transfer analysis, 8-node linear heat transfer brick elements were used for concrete and 2-node heat transfer link elements were used for reinforcement. Tie connections were used for heat transfer between the steel and concrete. For mechanical analysis, 8-node linear brick elements were used to model the concrete and 2-node linear 3-D truss elements were used to model the reinforcing bars. Reinforcement was embedded with the assumption of perfect bond. The models were validated by comparing the results to experimental data of simply supported walls subjected to fire only (Crozier and Sanjayan [3]). The walls were 75, 100, and 150 mm thick, which is thinner than most lateral load resisting walls found in mid- to high-rise buildings on the west coast of the United States. Results of the heat transfer analysis were consistent with the experimental measurements. Minor discrepancies arose due to lack of environmental temperatures, inconsistencies in furnace temperatures relative to intended loading used in models, and uncertain material properties. Results of the mechanical analysis provided reasonable deformations in the walls. Discrepancies arose due to uncertainties in material properties and failure of the model to capture fracture of the concrete, a failure mode that controlled in some tests. Post-processing of the data to capture this was successful. In considering the damage due to earthquakes, the loss of cover is considered to be the most critical damage as it allows for a more rapid transfer of heat through the wall. To investigate PEF behavior of RC walls, Ni and Birely simulated the loss of cover concrete in the models by significantly reducing the thermal and mechanical properties to be sufficiently small that the impact of the concrete was similar to results for no
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concrete. This was chosen over the conducting a lateral load analysis using ABAQUS/STANDARD for two reasons. First, ABAQUS is not an ideal software for accurately capturing the response of walls subjected to cyclic lateral loads. Second, the aim of the investigation was to investigate the impact of different damage levels. By manually forcing these damage levels, the impact of damage characteristics can be directly evaluated. The sizes (length and height) of the concrete damage considered were selected based on damage characteristics of reverse-cyclic lateral load tests documented by Birely [4]. One of these experimental tests were used as the basis for investigating the PEF response of walls. The wall was considered to be a cantilever (fixed at bottom, free at top). Ni and Birely found that the fire resistance of cantilever walls was influenced more by the length of the damage relative to the length of the wall than it was by the height of the damage relative to the height of the wall. Increase in the axial load ratio from 2.5% to 5.0% was found to have a minimal influence on the fire resistance. In all cases, the fire resistance measured by the load bearing criteria was found to control over the thermal insulation criteria. Short comings of the investigation by Ni and Birely are i) the use of cantilever walls, when most walls in buildings are likely to have some degree of restraint, and ii) limited wall characteristics investigated. OVERVIEW OF PARAMETRIC STUDY To build on the findings of Ni and Birely [2], the work presented here investigates the PEF performance of RC structural walls with different boundary conditions and different wall design characteristics that are known to have influence on the fire and/or seismic response of walls. The RC walls investigated are based on a prototype walls considered to fall within a range of representative buildings characteristics and low- to mid-rise buildings. The prototype wall is four stories tall, with floor heights of 10 ft. The wall length is 10 ft. Classification of walls for lateral load resistance is based on one of two measures for the relative height to length. The first, aspect ratio (wall height to wall length ratio), is a function of geometry only. The second, shear span ratio is a function of the wall geometry and the assumed lateral load distribution. Walls with low aspect ratios or shear span ratios are considered to be squat walls and have response and failure dominated by shear. Walls with high aspect ratios or shear span ratios are considered to be slender walls and have response and failure dominated by flexure. The prototype wall in this study is at the low end of slender walls and earthquake damage would be expected to have the characteristics similar to the damage modeled. The thickness of the wall is a key property for both fire and lateral load resistance. For fire resistance, the thickness impacts heat transfer analysis and stability for axial load bearing resistance; walls are typically classified based on an out-of-plane aspect ratio of the unbraced height to the thickness of the wall. For lateral load resistance, the thickness impacts the cross-sectional aspect ratio (wall length divided by wall thickness). The cross-sectional aspect ratio has been shown to influence the ductility of the walls (Birely [4]). Two wall thicknesses are considered in this study: 8 inch (200 mm) and 12 inch (300 mm). A cross-section for one of the walls studied is shown in Figure 1. A typical design of walls is to have reinforcement concentrated near the extreme compression/tension fibers. These heavily reinforced regions, typically called boundary elements, have
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closely spaced confining reinforcement to improve the ductility of the walls. This confining reinforcement often also contains cross-ties between pairs of bars on opposite faces of the wall. The length of the boundary element relative to the length of the wall is a parameter that can impact the performance of a wall subjected to seismic loads, thus, the length is introduced as a variable in this study. Three boundary element lengths are considered, 10, 15, and 20% of the wall length. The area between the boundary elements is typically referred to as the web of the wall and contains code minimum longitudinal reinforcement. The total area of longitudinal reinforcement divided by the wall area is the reinforcement ratio, ; reinforcement ratios of 1.2% and 1.9% are considered in this study. Transverse reinforcement spanning the length of the wall contributes to the shear strength of the wall; impact of this variable is not considered in this study. The final parameter considered in this study is the axial load ratio of the wall. In most walls, an axial load ratio, N, of up to10% is reasonable for design and analysis, although larger axial loads up to 30% are possible. In this study, axial load ratios of 2.5%, 6.25%, 12%, and 25% are considered. Table I provides a summary of all walls considered in this study. The material properties were kept consistent for all walls considered in the parameter study. For concrete, a compressive strength of 6,000 psi (42 MPa) was used and for reinforcing steel, yield strength of 75 ksi (525 MPa) was used. The size of the expected damage was not varied in this study, as this was explored by Ni and Birely (2014). The damage was modeled on both ends of the wall and extended a length of one-quarter the wall length and one-tenth the wall height. The depth of the damage was considered to be the cover concrete only. This damage is a worst case scenario for the surface area extent of damage. 120 10-#4 bars
6-#8 bars
6-#8 bars 8
2 7
7
14
15
15
15
15
14
7
7 2
Figure 2. Cross-section of Wall 1 t8-be15%-1.2% walls. Dimensions shown in inches. TABLE I. MATRIX OF NUMERICAL ANALYSES.
Number Name tw, in LB/Lw 1(ref) 8 0.15 t8-be15-1.2-2.5 2 0.15 t12-be15-1.2-2.5 12 3 8 0.10 t8-be10-1.2-2.5 4 8 0.20 t8-be20-1.2-2.5 5 8 0.15 t8-be15-1.9-2.5 6 8 0.15 t8-be15-1.2-0.5 7 8 0.15 t8-be15-1.2-6.25 8 8 0.15 t8-be15-1.2-25
N 1.2% 2.5% 1.2% 2.5% 1.2% 2.5% 1.2% 2.5% 1.9% 2.5% 1.2% 0.5% 1.2% 6.25% 1.2% 25%
Uncoupled thermal-mechanical analysis of the walls was conducted in ABAQUS/Standard following the method used by Ni and Birely (2014), summarized earlier in this paper. The thermal properties of the concrete and steel were modeled in
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accordance with EC2-02. All models had one face of the bottom floor exposed to fire, with the other surfaces exposed to room temperature, as shown in Figure 2. For the fire-exposed surface, both heat radiation and heat convection were considered; for all other surfaces, only radiation was considered. The heat transfer parameters for the fireexposed side are: film coefficient = 25 W/m2/K and emissivity = 0.7; for other surfaces, emissivity=0.9. The fire exposed side is heated following the standard ASTM E119 time-temperature curve. For the mechanical analysis, two boundary conditions were considered: cantilever walls, similar to those used in previous studies, and a cantilever wall with a roller at the top. Realistically, some lateral restraint should be expected at each floor level, but use of a restraint only at the top provides an extreme on the potential load-bearing fire resistance while providing a more realistic restraint than a cantilever wall. The region of damage concrete was modeled by directly removing the concrete in that region.
Figure 3. Schematic of boundary conditions and fire location.
SUMMARY OF RESULTS Thermal Analysis The results of the heat transfer analysis are independent of the axial load applied to the wall and the boundary conditions used for the mechanical analysis, resulting in five unique models to consider. The fire resistances for the damaged and undamaged walls are presented in Table II. The fire resistance is based on the insulation criteria; that is the average temperature rise over the whole non-exposed surface is limited to 140 K (252oF) and the maximum temperature rise at any point of that surface does not exceed 180 K (324oF). Underlined numbers indicate the value (average or minimum) that controls the fire resistance; for all walls considered in this study, maximum temperature increase determines the fire resistance. As expected, walls with a larger thickness are less impacted by the earthquake damage. In part this is due to the depth of damage being the same for both thicknesses used. Unexpected results were the influence of the boundary element length on the damage. These differences are driven in part by the mesh sensitivity of the maximum temperature and the location of the maximum temperature in the walls with different reinforcing steel layout. However, it is clear that there is an influence of the amount of steel, which can increase the rate at which the heat is transferred to the unexposed
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face. This is consistent with the impact of reinforcement ratio on the decrease in fire resistance as a function of damage. TABLE II. FIRE RESISTANCE (INSULATION) OF WALLS.
Numbers
Name
1, 6-8 2 3 4 5
t8-be15-1.2 t12-be15-1.2 t8-be10-1.2 t8-be20-1.2 t8-be15-1.9
Undamaged Avg., Max, hrs hrs 6.46 6.19 15.32 14.43 6.47 5.95 6.46 5.26 6.18 5.63
Damaged Avg., Max., hrs hrs 5.73 2.53 14.30 8.27 6.15 4.02 5.79 3.11 5.45 2.65
% Decrease 59 43 32 41 53
Mechanical Analysis The results of the mechanical analysis for walls with the boundary conditions shown in Figure 2 are presented in Table III. The time to failure, determined by instability of the model and failure to support load, is provided for the damaged and undamaged walls. The impact of the boundary conditions used is significant. In the cantilever wall results presented by Ni and Birely, small axial load did not impact on the fire resistance of the wall, which was controlled by mechanical failure. In the walls considered in this study, small levels of axial have minor impact on the load bearing fire resistance of the walls. Although cantilever data is not provided in Table III, it is important to note that the boundary conditions have a significant impact on the fire resistance. For example, damaged Wall 1 has a fire resistance of approximately 3.5 hours for a cantilever boundary condition, controlling over the insulation fire resistance time. If damaged Wall 1 has a roller support at the top, the load-bearing fire resistance is approximately 5.6 hours and the insulation fire resistance controls. For Walls 1-7, the earthquake damage decreases the load-bearing fire resistance by no more than 20%, however, it is important to note that for all of these, the insulation fire resistance of the damaged wall controls the fire resistance. It is only with an increase in the axial load that the mechanical. For Wall 7, the axial load ratio is 6.25% and the load-bearing fire resistance comes close to controlling over the insulation fire resistance. At higher axial load ratios, it follows that the load-bearing criteria will control; this is the case for Wall 8 (25% axial load), which has a fire resistance that is controlled by the load-bearing criterion. Other variable considered in this study are the amount of longitudinal reinforcement and the length of the boundary element relative to the wall length. Both have a less significant influence on the reduction in load-bearing resistance of damaged walls. An increase in the total longitudinal reinforcement ratio for walls increases the fire resistance (Wall 5 compared to Wall 1). Conversely, the load resistance of the walls with longer boundary elements is decreased. It is believed that this is due to a larger volume of steel exposed to the fire loads (the confining reinforcement has smaller spacing than the transverse reinforcement in the wall web). This is consistent with the larger decrease observed in the insulation fire resistance for these walls. It is important to note that all other parameters varied were done so only at
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low axial load ratios and the impact at higher axial load levels may be more pronounced. TABLE III. FIRE RESISTANCE (MECHANICAL) OF WALLS.
Number 1 2 3 4 5 6 7 8
Name Undamaged, hrs 6.92 t8-be15-1.2-2.5 >8 t12-be15-1.2-2.5 6.83 t8-be10-1.2-2.5 6.98 t8-be20-1.2-2.5 >8 t8-be15-1.9-2.5 >8 t8-be15-1.2-0.5 3.91 t8-be15-1.2-6.25 1.64 t8-be15-1.2-25
Damaged, hrs 5.55 >8 6.67 5.33 >8 >8 3.13 1.25
% Decrease 20 3 24 20 24
CONCLUSIONS This study investigated the impact of wall boundary conditions (cantilever vs cantilever with roller support) and wall design and load characteristics, on the fire resistance of reinforced concrete structural walls damaged by earthquakes. Fire resistance was evaluated on the basis of the insulation and load-bearing criteria. Results indicate that typical wall axial loads (i.e. less than 10%), the fire resistance of damaged walls is controlled by the insulation criterion, with significant decreases in the fire resistance compared to undamaged walls. Key influences on the severity of the decrease were the wall thickness and the volume of reinforcing steel in the wall. At large axial loads, the load-bearing criteria controls for both damaged and undamaged walls, although the percentage decrease in the fire resistance due to the damage is not much more severe than it is for walls with low axial loads. For large axial load ratios, a more accurate representation of boundary conditions (i.e. restraint at all floors) may be informative in assessing the true risk to building exposed to post-earthquake fire. It is important to emphasize that this study investigate the impact of earthquake damage assumed to be the worst-case scenario for damage to cover concrete and does not represent a detailed exact representation of damage in a wall. That is, the impact of residual cracks and reduced mechanical properties of reinforcing steel and concrete are not considered. The results provide a preliminary examination of the impact of earthquake damage on the fire resistance of walls. REFERENCES 1. Mousavi, S., A. Bagchi, and V.K.R. Kodur. 2008. “Review of Post-Earthquake Fire Hazard to Building Structures,” Canadian Journal of Civil Engineering, 35: 689-698. 2. Ni, S. and Birely, A.C. 2014. presented at the 10th U.S. National Conference on Earthquake Engineering, July 21-25, 2014, Anchorage, Alaska, USA. 3. Crozier, D.A., and J.G. Sanjayan. 2000. “Tests of Load-Bearing Slender Reinforced Concrete Walls in Fire,” ACI Structural Journal, 97(2): 243-253. 4. Birely, A.C. 2012. “Seismic Performance of Slender Reinforced Concrete Walls,” Ph.D. Dissertation, University of Washington, Seattle, WA.
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