FIRE RESISTANCE OF EARTHQUAKE DAMAGED REINFORCED CONCRETE WALLS

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1 1NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 214 Anchorage, Alaska FIRE RESISTANCE OF EARTHQUAKE DAMAGED REINFORCED CONCRETE WALLS S. Ni 1 and A. C. Birely 2 ABSTRACT Fire following earthquakes is a concern to the performance of reinforced concrete structures because the potential loss of cover concrete due to earthquake demands can lead to reduced fire resistance, while the duration of a fire may be extensive due to damaged fire suppression systems and/or the inability of emergency responders to access the structure. Reduced fire resistance of structural walls following an earthquake is a potential performance issue because walls may carry a significant portion of the gravity load and/or are located at stairs and elevators which serve as the primary means of access and egress. As a preliminary investigation into the effects of earthquake damage on the fire resistance of reinforced concrete walls, a series of numerical simulations was conducted. The basis of the simulations was a series of experimental tests documented in the literature that subjected reinforced concrete walls to high-temperature loads. The walls were first modeled without damage to determine the baseline fire resistance of the walls using the standard ASTM E119 time-temperature curve and with a set of generic timetemperature curves intended to represent realistic fires. These analyses were then repeated using increasing levels of damage to determine the reduction in the fire resistance of the walls as a function of the damage. 1 Graduate Student Research Assistant, Zachry Dept. of Civil Engineering, Texas A&M University, College Station, TX Assistant Professor, Zachry Dept. of Civil Engineering, Texas A&M University, College Station, TX Ni S, Birely AC. Fire resistance of earthquake damaged reinforced concrete walls. Proceedings of the 1 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 214.

2 Fire Resistance of Earthquake Damaged Reinforced Concrete Walls S. Ni 1 and A. C. Birely 2 ABSTRACT Fire following earthquakes is a concern to the performance of reinforced concrete structures because the potential loss of cover concrete due to earthquake demands can lead to reduced fire resistance, while the duration of a fire may be extensive due to damaged fire suppression systems and/or the inability of emergency responders to access the structure. Reduced fire resistance of structural walls following an earthquake is a potential performance issue because walls may carry a significant portion of the gravity load and/or are located at stairs and elevators which serve as the primary means of access and egress. As a preliminary investigation into the effects of earthquake damage on the fire resistance of reinforced concrete walls, a series of numerical simulations was conducted. The basis of the simulations was a series of experimental tests documented in the literature that subjected reinforced concrete walls to high-temperature loads. The walls were first modeled without damage to determine the baseline fire resistance of the walls using the standard ASTM E119 time-temperature curve and with a set of generic time-temperature curves intended to represent realistic fires. These analyses were then repeated using increasing levels of damage observed in the experimental tests to determine the reduction in the fire resistance of the walls as a function of the damage. Introduction Losses resulting from post-earthquake fires is cited as the largest single-source of earthquake related losses in the United States and Japan [1]. Key factors impacting these losses include simultaneous ignitions, blocked roads and hindered communication systems, damage to water supply systems [2], and damage to passive fire defense systems such as fire walls. Efforts to model the effect of post-earthquake fire and provide resources disaster planning have increased in recent years and are summarized by Lee et al. [1]. Lee et al. indicate that one factor yet to be explicitly incorporated into models for the effects of post-earthquake fire is the impact of building damage. For reinforced concrete structures, fire performance is generally not considered to be a major concern due to the non-combustibility and the low thermal conductivity of concrete; however, when the concrete is damaged by an earthquake, the fire resistance may be decreased significantly. Reduced fire resistance of damaged reinforced concrete structural walls following an earthquake is a potential performance issue because such walls i) may be expected to serve as fire walls to suppress the spread of fire, ii) may be located at stairs and/or elevator cores which 1 Graduate Student Research Assistant, Zachry Dept. of Civil Engineering, Texas A&M University, College Station, TX Assistant Professor, Zachry Dept. of Civil Engineering, Texas A&M University, College Station, TX Ni S, Birely AC. Fire resistance of earthquake damaged reinforced concrete walls. Proceedings of the 1 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 214.

3 serve as the primary means of access and egress, and iii) may carry a significant portion of the gravity load. The impact of earthquake damage on the fire resistance of reinforced concrete walls has not been previously studied. This paper presents a preliminary investigation into the fire resistance of earthquake-damaged structural walls. Numerical simulations are used to provide a baseline understanding of the impact of earthquake damage on the fire resistance of RC walls. Models are validated using experimental data for reinforced concrete walls subjected to fire loading. Artificial damage is added to the walls to investigate the impact of the damage characteristics to the fire resistance of the walls. The investigation of the post-earthquake performance of RC structural walls does not consider either seismic or fire hazard analysis; that is, it looks at fire resistance only. Efforts are currently underway to expand modeling efforts to investigate the reduction in fire resistance for experimental tests of structural walls subjected to simulated earthquake loads for which realistic damage data is available. Fire Resistance of RC Walls Overview of Previous Research Limited research is available on the fire resistance of reinforced concrete walls. Crozier and Sanjayan [3] tested eighteen large-scale slender reinforced concrete walls under standard fire conditions, investigating the impact of i) axial and out-of-plane loading, ii) wall thickness, iii) reinforcement ratios, and iv) concrete compressive strength. The walls were tested under the Australian Standard for fire testing (similar to ASTM E119) until collapse of the specimens occurred or excessive out-of-place deformations occurred. Observations from the tests included i) the occurrence of fire induced spalling was reduced in specimens with significant flexural cracking, ii) higher-strength concretes deflect less but have a limited impact on the load capacity of the walls, and iii) walls with centrally reinforced walls perform better than walls with two curtains of longitudinal reinforcement. Ta et al. [4] investigated the performance of high-strength concrete (HSC) walls subjected to standard and hydrocarbon fires. Results indicated that the damage due to spalling was significantly greater in HSC walls than normal-strength concrete (NSC) walls. McGinnis et al. [5,6] conducted numerical and experimental investigations of fullscale reinforced concrete load-bearing walls subjected to extreme temperatures. The structural behavior of the walls was found to be significantly impacted by the boundary conditions, particularly the restraint provided by adjacent structural members such as floor slabs. Post-Earthquake Fire Resistance of RC Structures Research about the performance of reinforced concrete structures subject to post-earthquake fires is less available than research about the performance of steel structures with earthquake damage. Sharma et al. [7] conducted an experimental test of the post-earthquake fire resistance of a fullscale reinforced frame. The full-scale frame was first loaded by cyclic lateral loading, resulting in simulated earthquake damage to the columns. The damaged structure was then subjected to a fire and was able to sustain a one-hour exposure to the fire without collapse. Wu and Xiong [8] conducted numerical simulations of undamaged and damaged reinforced concrete columns subjected to standard fire test curves. Damage to the columns was assumed to consist of loss of cover concrete, the length of which was estimated using an

4 empirically developed model for the length (along the height of the column) of the damaged concrete. Results of the analyses determined that the fire resistance of damaged columns was reduced to as little of 3% of fire resistance of the undamaged columns. Mostafaei and Kabeyasawa [9] conducted a 3D simulation of the fire resistance of a experimental test of six-story reinforced concrete structure subjected to the 1995 Kobe ground motion on a shake-table. The effects of material degradation and heat penetration were found to significantly decrease the fire resistance of the structure. It was found that heat penetration due to cracks was one of the key factors reducing the fire resistance. Behnam and Ronagh [1] conducted fire analysis on two RC frames with different height to length ratios for damage levels corresponding to Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP) damage levels. Damage corresponding to the IO performance level was found to have limited impact on the fire resistance of the structure, while damage corresponding to the LS and CP performance levels reduced the fire resistance. Numerical Models The literature provides limited resources for understanding the performance of RC structural walls under fire loading and the performance of earthquake damaged RC structures to fire loading; no studies focused on the post-earthquake fire performance of RC structural walls is found in the current literature. To provide a preliminary understanding of the performance of structural walls subjected to post-earthquake fire, a numerical study was conducted in ABAQUS. Numerical Models for Fire Resistance of RC Walls Initial numerical models were developed for three specimens tested by Crozier and Sanjayan [3]. The three specimens selected had similar concrete compressive strengths and wall thicknesses of 75, 1, and 15 mm. The 15 mm thick specimen (IL ) had two curtains of longitudinal reinforcement, while the 1 (IL1-48) and 75 (IL75-48) mm thick specimens each had one curtain of longitudinal reinforcement. The walls had a length of 12mm and a height of 36mm. The standard Australian fire temperature-vs-time curve in AS153.4 [11] was used to apply fire loading to one face of the walls, with another face of the wall insulated from the environment by draping a ceramic-fiber insulation blanket. The walls are simply supported at the ends along the height of the wall. Loads were applied to include the effects of self-weight and out-of-plane loading on the walls. ABAQUS models of the experimental tests consisted of uncoupled thermal-mechanical analysis of the walls, with the full thermal analysis of the walls conducted prior to conducting the mechanical analysis of the walls. Uncoupled modeling of the thermal and mechanical response of the walls significantly reduces the computational demands of the analysis and are consistent with the Mousavi et al. [12] recommendations for conducting post-earthquake fire analysis of structures. Thermal analysis of the walls consists of heat transfer analysis to determine the temperature distribution of the walls as a function of time. 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. Heat transfer between the concrete and reinforcement was

5 simulated using tie connections. The thermal properties of the concrete and steel were modeled in accordance with EC2-2 [13]. EC2-2 provides two models for thermal expansion coefficient based on the type of material properties, both of which were considered in the initial model validation. It is assumed that the two models provided serve as upper and lower-bound values for the response of the concrete used in the Crozier and Sanjayan experiments, with the model for siliceous concrete most closely representing that used in the tests. The variability of concrete material properties to extreme temperatures is considered to be widely varied and not well accounted for in material models, as documented by Kodur et al. [14], thus, some variation between experimental and numerical results is expected. The bottom face of the wall is exposed to the fire with film coefficient 4.74W/m 2 /K [15] and emissivity.8. The top face of the wall is insulated with film coefficient and emissivity both zero. The ABAQUS models for mechanical analysis used 8-node linear brick elements for concrete and 2-node linear 3-D truss elements for reinforcing bars. Reinforcement is embedded in the concrete with the assumption of perfect bond. The mechanical analysis is unable to account for the impact of cracking and spalling induced by thermal response of the concrete. Mechanical analyses were conducted for the full duration of the thermal analysis or until the models fail to converge. Preliminary investigation of convergence failures indicate that the models simulate a loss of load bearing capacity of the walls. Table 1 provides a comparison of the experimental and simulated temperature distribution at 3 and 6 minutes of fire loading of Specimen IL The numerical results provide a reasonable comparison to the experimental results, which indicates that the heat transfer analysis with well-defined properties of concrete and steel can predict the temperature distribution of the shear walls well. The discrepancies between the simulation and experiments are mainly caused by i) the furnace temperature is not exactly the same with the fire temperaturevs-time curve in AS153.4; ii) a lack of reported environmental temperature at test which may have varied from the room temperature of 2 o C assumed for the simulation models. Table 1. Comparison of experimental and simulated temperature distribution of Specimen IL15-52 tested by Crozier and Sanjayan [3]. Time, min Temperature ( C) Exposed Face Midheight Unexposed Face Experiment Simulation Figure 1 provides a comparison of the experimental and numerical out-of-plane deflection response of the three specimens as a function of time. The lateral deflection is measured at midheight of the wall, located half-way between the simply supported ends. The experimental deflection curves fall between the two simulated deflection curves that result from using the two

6 thermal expansion coefficients provided by EC2-2. During the tests, Specimens IL75-48 and IL1-48 were considered to have failed due to excessive deformations while Specimen IL15-52 failed due to fracture of the cross-section at mid-span. For Specimen IL75-48, excessive deformations occurred after approximately 3 minutes. Simulation results based on the thermal expansion model for siliceous concrete produce approximately the same deformations at 3 minutes. For Specimen IL1-48, excessive deformations occurred at approximately 9 minutes, with simulations indicating similar deflections at approximately 8 minutes, but failing to capture a rapid increase in the deflection of the wall after increased temperature increase. For Specimen IL , failure due to fracture of the concrete section occurred at approximately 6 minutes. The simulation model is unable to account directly for the fracture of the concrete. Postprocessing of the simulation results based on thermal expansion model for siliceous concrete indicated that the full cross-section of the wall has maximum principal stresses that are tensile (shown in Fig.2(a)), and maximum principal strain along 7% of the height of the middle cross section exceeds the total limit for strain due to thermal strain and stress-related strain, shown in Fig.2(b). Such response of the walls was not seen in the thinner wall specimens modeled IL75-48-Test IL Calcareous(EC2-2) IL75-48-Siliceous(EC2-2) 25 2 IL1-48-Test IL1-48-Calacareous(EC2-2) IL1-48-Siliceous(EC2-2) 1 Deflection(mm) 8 6 Deflection(mm) (a) Specimen IL (b) Specimen IL IL15-52-Test IL15-52-Calcareous(EC2-2) IL15-52-Siliceous(EC2-2) 1 Deflection(mm) (c) Specimen IL15-52 Figure 1. Experimental and simulated deflection-vs-time for walls tested by Crozier and Sanjayan [3].

7 (Top) Tensile strain limite+theramal strain Maximum principal strain Height(mm) Bottom strain (a) Distribution of maximum principal (b) Comparison of maximum principal strain strain and the tensile strain limit. Figure 2. Maximum principal stresses at the center cross-section of Specimen IL tested by Crozier and Sanjayan [3]. Numerical Models for Fire Resistance of Earthquake Damaged RC Walls To investigate the impact of earthquake damage on the fire resistance of reinforced concrete structural walls, the boundary and load conditions in the numerical model of Specimen IL were modified to be similar to those of experimental tests of reinforced concrete structural walls subjected to quasi-static cyclic loading to simulate the effects of earthquake loading (e.g. Lowes et al. [16]). Specimen IL was selected for further analyses as the thickness of the wall (15 mm; 6 in) is comparable to the thickness of many experimental test specimens investigating the seismic response of reinforced concrete walls and can be assumed to correspond to approximately 1/3 to 1/2 scale of walls in mid- to high-rise buildings. Specimen IL was modeled as a cantilever with a fully fixed base. Axial loads of 2.5% and 5% of the gross axial capacity were applied to the wall. Uncoupled thermal-mechanical analyses were conducted to establish a baseline fire resistance for the undamaged cantilever walls. Thermal loads were applied to the lower half (18 mm (5.9 ft)) of the wall. Out-of-plane deformations and the axial deformations were evaluated to determine the response of the wall. As a preliminary investigation into the effects of earthquake damage on the fire resistance of walls, damage was artificially added to the modified model of Specimen IL Typical damage to RC structural walls ranges from flexure and shear cracks, to spalling of cover concrete, crushing of confined concrete core, and buckling or fracture of longitudinal reinforcing bars. In this study, only the spalling of the cover concrete is considered, as this is expected to have the largest impact on the fire resistance of the wall due to the decreased heat transfer protection of the reinforcing bars and the increase of the temperature of the concrete at the damage region. Spalling was considered to happen in the lower toe of the wall, with varied dimensions of the damaged region. The damaged region was considered to be 1/4, 1/6, and 1/8 of the full height of the wall and 1/3 and 1/4 of the wall length. The region of damage due to spalling can be simulated using two different methods. The first is by removing the damaged region of the concrete and was used by Wu and Xiong [8] to model the fire resistance of earthquake-damaged RC columns. The second method, used in this study, modifies the material properties of the damaged region to significantly reduce the thermal

8 and mechanical properties. The thermal properties were modified to have an extremely small specific heat capacity, large thermal conductivity and small density, all of which would allow heat transfer similar to there being no concrete present. Mechanical properties were modeled to have no compressive strength and an extremely small modulus of elasticity. Heat Transfer Analysis Impact of Post-Earthquake Fire The temperature fields of the reinforcement and concrete of the damaged and undamaged wall after 36 minutes of heating are shown in Figure 3. The primary impact of the damaged concrete is on the temperature of the reinforcing bar and the temperature of the concrete at the damaged region. Fig. 4(a) shows the maximum temperature-time for reinforcing bars at the damaged region and Fig. 4(b) shows the temperature-time for the concrete at point a (shown in Fig. 3). (a) Undamaged wall (b) Damaged wall Figure 3. Comparison of temperature distribution in the steel and concrete for undamaged and damaged cantilever walls No damage H/6xL/4 7 6 No damage H/6xL/4 Temperature(deg.C) Temperature(deg.C) (a)reinforcement (b) Concrete Figure 4. Temperature-time curves in the toe of the damaged and undamaged walls.

9 Mechanical Analysis Figure 5 shows changes in the out-of-plane deformation profiles of the wall throughout the time history of the test. Significant reversals of curvatures occur at several points throughout the tests; such curvature reversals are consistent with observations made by McGinnis et al. [6] Figure 5. Displacement profiles for response to fire located on the right side of the wall. The out-of-plane deflection and axial deformation of walls with 5% axial load are shown in Figure 6. The fire resistance of the damaged walls is significantly reduced relative to the fire resistance of the undamaged wall. Although six different combinations of heights and widths of the damaged regions were considered, there were only two distinct fire resistances of the damaged walls, measured both in terms of axial deformation and out-of-plane deformation. Walls with length of the damage equal to 1/3L had fire resistance approximately 35% less than that of the undamaged wall and walls with length of damage equal to 1/4L had fire resistance approximately 21% less than that of the undamaged wall. The height of the damaged region had minimal impact on the fire resistance of the walls. This observation is inconsistent with the observations by Wu and Xiong that the size of damage along the height of a column impacted the fire resistance of earthquake damaged reinforced concrete columns. Axial loads of 2.5% and 5% of the gross-capacity were considered for all damage levels considered. Figure 7 shows, for damage levels with width equal to ¼ of the full wall length, the out-of-plane deflection and axial deformation under the two levels of axial loads. The fire resistance of the walls, as determined by failure of the analytical model, is not impacted by an increase in the axial load. The axial deformation and out-of-plane deflections are impacted by an increase in axial load. Thus, if deflection criteria are considered, axial load may be a critical factor in the fire resistance of damaged walls.

10 4 1 Out-of-deflection(mm) No damage 1/4Hx1/4L 1/4Hx1/4L 1/4Hx1/4L 1/4Hx1/3L 1/4Hx1/3L 1/4Hx1/3L Axial deformation(mm) No damage 1/4Hx1/4L 1/6Hx1/4L 1/8Hx1/4L 1/4Hx1/3L 1/6Hx1/3L 1/8Hx1/3L Times(minutes) (a) Out-of-plane deflection (b) Axial deformation Figure 6. Comparison of fire response of walls without damage and with varied level of damage to cover concrete for the cantilever wall with 5% axial load. 3 1 Out-of-plane deflection(mm) Average FR= minutes 1/4Hx1/4L(2.5%) 1/6Hx1/4L(2.5%) 1/8Hx1/4L(2.5%) 1/4Hx1/4L(5%) 1/6Hx1/4L(5%) 1/8Hx1/4L(5%) Average FR= minutes Axial deformation(mm) /4Hx1/4L(2.5%) 1/6Hx1/4L(2.5%) 1/8Hx1/4L(2.5%) 1/4Hx1/4L(5%) 1/6Hx1/4L(5%) 1/8Hx1/4L(5%) Average FR= minutes Average FR= minutes (a) Out-of-plane deflection (b) Axial deformation Figure 7. Comparison of fire response of damaged walls with varied levels of axial load. Conclusion Preliminary results from numerical analysis are presented to quantify the impact of earthquakedamage on the fire resistance of reinforced concrete walls. Models were developed by validation against experimental tests of reinforced walls subjected to fire loads. Boundary conditions of the numerical models were then modified to be similar to those typically tested for investigation of the earthquake performance of structural walls. Assumed regions of damaged concrete were then added to include the effects of spalled cover concrete. Loss of the fire protection properties of the concrete resulted in a more rapid increase in the temperature of the reinforcing steel and decreased the fire resistance of the walls. The fire resistance of cantilever walls was found to be impacted more by the size of the damaged region relative to the length of the wall than by the size of the damage relative to the height of the wall. As the axial load ratio increases, the fire resistance of damaged walls, as determined by failure of the analytical model, is not impacted. Results of models with out-of-plane restraint are currently under investigation. Future analyses

11 are planned to (i) incorporate the impact of concrete cracking on the heat transfer of damaged walls and (ii) investigate the fire resistance of experimental tests of walls originally tested to investigate the seismic performance of walls. References 1. Lee S, Davidson R, Ohnishi N, Scawthorn C. Fire Following Earthquake Reviewing the State-of-the-Art of Modeling. Earthquake Spectra 28; 24 (4): Scawthorn C, O Rourke TD, Blackburn FT. The 196 San Francisco Earthquake and Fire Enduring Lessons for Fire Protection and Water Supply. Earthquake Spectra 26; 22 (S2): S135-S Crozier DA, Sanjayan JG. Tests of load-bearing slender reinforced concrete walls in fire. ACI Structural Journal 2; 97 (2): Ta TB, Ngo T, Mendis P, Haritos N. Performance of high strength concrete walls subjected to fire. Incorporating Sustainable Practice in Mechanics of Structures and Materials McGinnis MJ, Mueller KA, Kurama YC, Graham KP. RC bearing walls subjected to elevated temperatures. Proceedings of ASCE Structures Congress McGinnis MJ, Mueller KA, Lisk MW, Kurama YC. Out-of-plan full-field strain and curvature behavior of two RC bearing walls under fire. Proceedings of ASCE Structures Congress Sharma UK, Kumar V, Singh B, Bhargava P et al., Testing of full-scale frame under simulated fire following earthquake. Proceedings of 7th International Conference on Structures in Fires 212; Zurich, Switzerland Wu B, Xiong W. Behaviours of seismic damaged RC columns in post-earthquake fire. Proceedings of 7 th International Conference on Structures in Fire 212; Zurich, Switzerland. 9. Mostafaei H. and Kabeyasawa T. Performance of a six-story reinforced concrete structure in post-earthquake fire, in Proceedings of the 9th Canadian Conference on Earthquake Engineering, Toronto, Canada, June, 212: Ronagh H. R., and Behnam B., Resistance of reinforced concrete portal frame, International Journal of Concrete Structures and Materials 212,6(4): AS153.4: Methods for Fire Tests on Building Materials, Components and Structures: Part 4 Fire Resistance Tests of Elements of Building Construction, Standards Association of Australia, Mousavi S, Bagchi A, Kodur VKR. Review of post-earthquake fire hazard to building structures. Canadian Journal of Civil Engineering 28; 35: Eurocode 2: Design of concrete structures. pren part 1.2: General rules Structural fire design, European Committee for Standardization, Brussels, Kodur VKR, Dwaikat MMS, Dwaikat MB. High-temperature properties of concrete for fire resistance modeling of structures. ACI Materials Journal (5): Zhang J., Liu Z. and Liu W, Experimental research on natural convective coefficient of concrete surface, Journal of Sichuan Building Science, 27, 33(5): Lowes LN, Lehman DE, Birely AC, Kuchma DA, Marley KP, Hart CR. Earthquake response of slender planar concrete walls with modern detailing. Engineering Structures 212; 43: