International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018
ESTIMATION OF FIRE LOAD IN A BUILDING AND PROCEDURE TO ASCERTAIN SAFETY OF STRUCTURAL ELEMENTS Sushant Gadilohar1, Ratnesh Kumar2 1
PG Student, Deptt. of Applied Mechanics, VNIT Nagpur, Nagpur,
[email protected] Assistant Professor, Deptt. of Applied Mechanics, VNIT Nagpur, Nagpur,
[email protected]
2
ABSTRACT In recent years, occurrence of building fire has become a more frequent event which highlights the need of incorporating effect of fire in structural designing and detailing. Most of the fire affected buildings are put into re-use by only providing cosmetic repairs instead of evaluating the actual damage in the structural members caused due to fire. Present study provides a simplified procedure for assessing the stability of fire affected buildings. In the study, a realistic public building is considered for calculation of fire load density. The maximum rise in temperature in various rooms of the building, the material properties at the elevated temperature and the reduced member capacity due to fire exposure have been obtained from analysis using Eurocode-2 method. Reduced member capacity due to expected fire have been compared with expected demand under fire to ascertain the failure of structural elements. The study also provides a method to calculate fire rating of the structural elements for particular building. Key Words: Building Fire, Fire Load Energy Density, Fire Rating, Member capacity, Fire safety
INTRODUCTION Occurrence of fire inside the building is an unexpected phenomenon which can cause loss of lives and damage to the property. As per the world fire statistics (2011) given by the Geneva Association (National Crime Records Bureau, NFPA 2010), India suffers most deaths due to fire per year across the world. Even though the occurrence of building fire is more frequent other than extreme events like earthquake and hurricanes etc., the subject has not received adequate importance in structural design and detailing. Effect of fire on material properties and on structural members are the two important parameters for proper evaluation of damage of a building due to fire. Experimental studies by many researchers show that the material strength degradation at higher temperature is a major concerning issue in a fire event. Arioz (2007), studied the effect of high temperature on compressive strength of concrete with different types of aggregates and found significant reduction in compressive strength of concrete above 800oC. Hager (2013), presented effect of high temperature on physical properties (colour change, thermal strain and thermal strain under load) of concrete and studied the colour change of concrete at various temperature range. Topcu (2008), investigated performance of S220 and S420 reinforcing steel rebars under fire and found significant reduction in tensile strength of rebars above 950oC. Youssef and Moftah (2007), proposed stress-strain model of concrete incorporating effect of temperature. Jau and Hang (2007), studied the effect of non-uniform fire on strength of 1
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 reinforced concrete column subjected to 2 hour and 4 hour of fire exposure along with axial loading and biaxial bending. Kodur and Dwaikat (2008), developed the mathematical model for predicting the behavior of reinforced concrete beams subjected to fire with the effect of spalling. Tan and Yao (2004), presented simplified approach for predicting fire resistance of column subjected to different thermal boundary conditions and developed strength reduction factors for steel and concrete subjected to 1-face, 2-face, 3-face and 4-face heating by using SAFIR software tool. Wickstrom (1986), developed simplified approach for estimating temperatures in fire exposed RC members. Stern-Gottfried et al. (2012), investigated vertical fire propagation time in a building and found that vertically upward fire travel time was 6-8 minutes and downward travel time was 30 minutes. Law and Gillie (2010), proposed new method for developing interaction diagrams of sections at any temperature with the use of the sectional tangent stiffness. Pham et al. (2015), used yield design theory to develop the procedure to determine PM- Interaction diagrams of RC sections in fire. Hadole (2017), performed non-linear static pushover analysis to assess seismic behavior of fire affected models. Author considered 44 different fire possibilities in order to find out worst fire scenario for building. Different national codes provide different approach to handle structural fire related issues. National Building Code of India (2016) provides simplified tables for different fire rating to safeguards structural elements. However, it does not provide method for assessing reduced capacity of fire affected structural elements. Eurocode-1(EN 1991-1-2:2002) and Eurocode2(EN 1992-1-2:2004) provides detailed procedure for assessment of fire affected members. Eurocode uses ISO: 834 curve (ISO, 1999a) as a standard fire curve. It also provides stressstrain properties of concrete and steel at elevated temperature (shown in Figure 1). Eurocode2 provides two simplified approaches for estimating capacity of fire affected members: (a) Zone Method and (b) 500oC Isothermal Method. When the fire occurs in a room/compartment, extent of damage to structural elements mainly depends on two factors: Amount of fuel load available in a room (Fire Load Energy Density) and temperature rise in a room. Hence, in the study maximum temperature rise in a compartment/room/building due to fire load is determined and reduction in member capacity of beams due to expected fire exposure is estimated.
DESCRIPTION OF BUILDING Constructional Details In the present study, an institutional building is considered for assessment of structural fire effects. The plan of building is regular with longitudinal arrangement of rooms along with parallel 2.1 m wide corridor (shown in Figure 2). Selected building is two storey (G+1) RC moment resisting frame with 280 mm thick unreinforced burnt clay brick infill (considering both side plaster). The floor to floor height of building is 4 m and provided with 150 mm thick slab. The building is situated on medium soil strata and located in seismic zone II (as per IS 1893: (Part 1) 2016) having peak ground acceleration of 0.10g. The building is constructed with M20 grade of concrete (i.e. characteristics strength = 20 MPa) and Fe 415 grade steel (i.e. yield stress = 415 MPa). The building is consisting of 6 frames along longitudinal direction(x-direction) and 20 frames along transverse direction(y-direction). There are no significant damages and cracks observed in structural and non-structural elements of the building. 2
20˚C
18 16 14 12 10 8 6 4 2 0
300˚C 600˚C 900˚C
0
0.01
0.02
0.03
0.04
Stress (MPa)
Stress (MPa)
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018
450 400 350 300 250 200 150 100 50 0
20˚C 300˚C 600˚C 900˚C
0
0.05
0.05
0.1
0.15
0.2
Strain
Strain
(b) Steel (Yield stress = 415 MPa) (a) Concrete (ƒ’c = 16 MPa) Figure 1 Stress-strain curve for different temperatures (EN 1992-1-2:2004) Analysis and Design
WC
3.4
Corridor
WC
4.1
3
4.3
3.3
2.9
4.1
7.5
4.6
11.2
Room 7 Room 8
Staircase
Room 4
13.5
15.8
6.5
Room 3
Room 2
Room 1
Room Room 5 6
D D
D
D
D
D
D
Corridor (2.1 m wide)
10.8
Structural modelling, analysis and design has been performed using structural analysis software SAP 2000. The considered existing building has been constructed in 1960 and designed by prevailing Indian standard codes. Columns are eccentrically placed to create offset in external walls (as shown in Figure 3).
All Dimensions in Metres
Figure 2 First Floor Plan of the considered building The reinforcement details are not available, hence building has been designed using configurational details (from actual measurements, Table 1) for gravity loads as per provisions of IS 456:2000 for 1.5 (Dead Load + Live Load). The building loads are considered as per provisions of IS 875-Part I and Part II: 1987. The analytical model of the considered building is as shown in the Figure 4. Slab has been modelled using thin shell area element. Table 1 Configurational Details Particulars
Beams
Type 1 Type 2 Type 3 Type 4 Type 5
Dimension (mm) 230 x 400 250 x 650 280 x 550 400 x 1000 550 x 425
Particulars
Columns
3
Type 1 Type 2 Type 3 Type 4 Type 5
Dimension (mm) 250 x 280 250 x 500 350 x 600 280 x 1000 1300 x 425
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 Type 6
-
14 @ 4.6 m c/c
4.3
B6 B1
B5
B4
4.3
B3
2.9
B2
3 @ 3.7 m c/c
4.3
-
6.8
3.1
1300 x 425
2.2 All Dimensions are in metres
Figure 3 Structural plan of the building
Figure 4 Analytical Model of the building Expected Demand under fire As the fire is an accidental event, loads present at the time of fire are much lower than the actual design loads for structural elements at ambient temperature. Table 2 shows expected fire demand suggested by different codes. Table 2 Fire Limit State Load Combinations Codes
Load Combinations
Eurocode (CEN, 2002a)
Lƒ* = Gk# + 0.5Qk+ or Lƒ = Gk + 0.9Qk
US Standards (ASCE, 2010)
Lƒ = 1.2Gk + 0.5Qk
Australia/New Zealand Standard (SA, 2002)
Lƒ = Gk + 0.4Qk or Lƒ = Gk + 0.6Qk
*Factored Load Combination for Fire, #Characteristic Dead Load, +Characteristic Live Load In the paper, the stability of beams under fire is assessed using fire limit state combination suggested by Eurocode i.e. (Dead Load (DL) + 0.5 (Live Load)).
STAGES IN FIRE ASSESSMENT OF BUILDING Estimation of Fire Load/ Fire Load Energy Density
4
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 Fire Load is an important parameter to decide the maximum temperature rise in a particular compartment/room. Amount of combustible material in a compartment/room is generally expressed in terms of Fire Load Energy Density. Fuel available in a room in the form of furniture, plastic, paper, lining materials and some part of structural elements. All these material release energy/heat as per their calorific value. Following are the steps to evaluate fire load in any compartment (Buchanan and Abu, 2017). (a) Determination of total energy contained in the fuel The quantity and calorific values of each item present in the compartment is obtained. Range of calorific values of various items are used as per given in Purkiss (2007). Similar calorific values are also present in National Building Code of India (2016). Total Energy contained in the fuel (E) = Mass (M) x Calorific Value (∆Hc) = M ∆Hc (b) Heat Release Rate = Q = E/tb where, tb = time of burning (c) Fuel load energy density = ef = E/Af where Af = Total Floor Area (d) Total fire load in the compartment is given as Qfi,k = Σ Mk,i Hu,i where, Mk,i and Hu,i are the mass and calorific values of the ith material within the compartment. In the present study, fire load in various rooms are calculated. Figure 5 shows various types of material present in room 1 of the considered building. The various inflammable materials in room 1 are classified into three categories i.e. wood, plastic and papers according to their calorific values. Further, the weights of chairs, stools, cupboards, tables, computers, printers, partitions and paper type materials are calculated. The typical calculation of estimation of fuel weights for room 1 is given in Table 3. The fire load of room 1 has been obtained from fuel weights and its calorific values. Similarly, fuel weights and fire load energy density of other rooms are calculated and presented in Table 4.
Room 1-(a)
Room 1-(b)
Figure 5 Types of Fire Load in Room 1 Table 3 Estimation of Weights of fuel available in Room 1 List of Items
Wood
Plastic
Chair Stools Cupboard Small Tables Large Tables Chair PC Printers
No.
Weight (kg)
20 10 7 3 6 8 3
5 5 20 120 1.5 5 5 5
Total Weight (kg) 100 50 140 360 9 40 15
Total (kg)
650
84
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018
Papers
Partition Books
-
20 400
20 400
400
Evaluation of Maximum Temperature Rise in various rooms For appropriate evaluation of damage in structural elements, calculation of the compartment temperature is necessary. Maximum temperature rise in the various rooms of the considered building has been calculated using Parametric Fire Curve Equations (BS EN 1991-1-2:2002). Table 5 shows that the maximum expected temperature in rooms of the considered building calculated by incorporating the effect of ventilation factor, fuel load and wall lining material properties. Estimation of Reduced Capacity of beams under expected fire exposure The simplified calculation methods are given in BS EN 1992-1-2:2004 for evaluating capacity of structural members at elevated temperature. The scope of present study is restricted up to capacity assessment of beams only as most of the columns are covered with lining materials. 500oC Isothermal Method has been used in the present paper to calculate reduced moment capacity of beams. Figure 6 explains steps involved in estimating capacity of beams exposed to fire. Table 4 Estimation of Fire Load in different compartments of building Fire load Calorific Energy Floor Mass energy Value Contained Area density Particulars Material ∆Hc Af ef = E / Af M(kg) E(MJ) (MJ/kg) (m2) (MJ/m2) Room 1 Room 2
Room 3
Room 4
Room 5
Room 6
Room 7 Room 8
Wood Plastic Papers Wood Plastic Papers Wood Plastic Papers Wood Plastic Papers Wood Plastic Papers Wood Plastic Papers Wood Plastic Papers Wood Plastic
17.5 25 20 17.5 25 20 17.5 25 20 17.5 25 20 17.5 25 20 17.5 25 20 17.5 25 20 17.5 25
650 84 400 2975 10 50 2020 42 150 970 5 5 600 60 400 75 10 50 75 10 50 500 5
8750 2125 8000 50750 250 1000 33600 750 3000 16625 125 100 10500 1500 8000 1312.5 250 1000 1312.5 250 1000 8750 125 6
70.24 70.24 70.24 171.06 171.06 171.06 146.23 146.23 146.23 122.06 122.06 122.06 48.5 48.5 48.5 15.73 15.73 15.73 18.23 18.23 18.23 81.20 81.20
124.57 30.25 113.89 296.69 1.46 5.85 241.74 7.18 20.52 136.21 1.03 0.9 216.78 30.96 165.1 83.4 15.9 63.6 72 13.72 54.86 107.8 1.54
Total Fire load (MJ/m2) 269
304
267.3
138.05
413
162.92
140.58 110.54
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 Papers
20
5
100
81.20
1.24
Table 5 Maximum Expected Temperature Total Fire Load (MJ/m2) 269 304 267.3 138.05 413 162.92 140.58 110.54
Particulars Room 1 Room 2 Room 3 Room 4 Room 5 Room 6 Room 7 Room 8
Determine reduced crosssection corresponding to 500oC Isotherm
Obtain material properties at elevated temperature
Tmax (oC) 870 890 862 765 975 784 813 687
Calculate rebar temperature using penetration due to fire
Determine reduced Moment Capacity
Figure 6 Prescriptive Approach for estimating capacity of beams exposed to fire In the present study, maximum temperature of 890oC and 975oC was found to be reached in room 2 and room 5 respectively. Hence, the reduced moment capacity has been obtained for some beams in room 2 and room 5. The positions of the beams are given in structural plan of the building. The moment capacity of beam B1 was found to 381 kN-m at 975oC and it was observed that beam capacity is reduced by approximately 42 %. Similarly, reduced moment capacities of all other beams are calculated as shown in Table 6. Table 6 Reduced Capacity of Beams Beam No.
B1 B2 B3 B4 B5 B6
Crosssection (mm)
250 x 650 280 x 550 400 x 1000 400 x 1000 280 x 550 230 x 400
Fire Temperature (oC)
Reduced C/S(mm)
975 975 975 890 890 890
180 x 615 210 x 515 330 x 965 350 x 975 230 x 525 180 x 375
Moment Capacity (Mc) kNm (Ambient) 658 262 1417 2024 417 390
Reduced % Moment Reduction Capacity(Rf) kN-m 381 179 979 1454 290 169.15
42 25 30.91 12 30.4 56
Further, the reduced moment capacity of beams are compared with the expected fire demand i.e. fire limit state load combination Dead Load (DL) + 0.5 Live Load (LL) as shown in Table 7 to ascertain the stability of beams when exposed to fire.
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International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 The results from Table 7 have shown that all the beams are having their reduced moment capacity at their respective temperature found to be greater than moment obtained using accidental load case of fire. Hence, beams are said to be stable under expected fire demand. Table 7 Comparison of capacity of fire affected section with expected fire demand Beam No. B1 B2 B3 B4 B5 B6
Cross-section (mm) 250 x 650 280 x 550 400 x 1000 400 x 1000 280 x 550 230 x 400
Mu 1.5(DL+LL) kN-m 342 60 573 573 172 97
Mu (DL+0.5LL) kN-m 184 33 239 328 95 52
Rf (Fire Case) kN-m 381 179 979 1454 290 169.15
Calculation of fire rating of beams As per National Building Code of India (2016), Fire Resistance Rating is defined as the time which structural members can withstand when subjected to standard fire tests. The analytical study is carried out to decide the fire rating of the beams in room 2 and room 5. These beams are checked for different hours of Standard fire exposure. Table 8 shows the calculated moment capacities of all considered beams at different hour of fire exposure. The expected demand for B1 was found to be 184 kN-m which lies between moment capacities of B1 at 2-hour of fire exposure and 2.5 –hour of fire exposure i.e. between 217 kN-m and 145 kN-m respectively. Hence, it can be given fire rating of 2-hour. Similarly, fire ratings for all the beams are calculated as shown in Table 8. From this calculation, it has been found that fire ratings of building elements varies from 2-hour to 3-hour. Table 8 Fire Resistance Rating of Beams Beam No.
B1 B2 B3 B4 B5 B6
Mu (DL+0. 5LL) kN-m 184 33 239 328 95 52
Mambient (kN-m)
M0.5hr (kNm)
M1hr (kNm)
M1.5hr (kNm)
M2hr (kNm)
M2.5hr (kNm)
M3hr (kNm)
658 262 1417 2024 417 390
516 236 1266 1674 339 279
421 201 1089 1418 282 177
301 140 771 1037 204 126
217 97 534 755 147 97
145 59 323 492 98 70
109 42 231 355 73 56
Remarks (Fire Ratinghours) 2-hr 3-hr 2.5-hr 3-hr 2.5-hr 3-hr
CONCLUSION Past fire accidents shown that occurrence of fire in a building can result in damage as well as collapse of the structure. Hence, proper assessment of fire safety shall be done for anticipating and determining the amount of damage and chances of collapse. In this paper, simplified method to ascertain fire loads and structural safety has been provided. For case study, small institutional building has been considered and its realistic fire load is calculated. Calculation of realistic fire load indicated that temperature in different rooms/compartments can vary from 690oC to 975oC under an isolated fire event. The 8
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 reduction in capacity of beams have been determined for respective temperature rise occurred due to realistic fire load. Further, the anticipated demand for fire safety load case i.e. (DL+0.5LL) compared with available reduced capacity and it was observed that for present fire load, structure will remain stable i.e. no collapse of any part of structure will occur. Furthermore, using reverse calculation fire rating of different rooms/compartments have been ascertained. This simplified procedure can be applied to various buildings for assessing the behavior under fire event.
REFERENCES 1. Arioz, O. (2007), “Effects of elevated temperatures on properties of concrete”, Fire Safety Journal, 42, 516–522. 2. ASCE (2010), “Minimum Design Loads for Buildings and Other Structures”, American Society of Civil Engineers, Reston, VA. 3. Buchanan, A. H. and Abu, A. K. (2017), “Structural Design for Fire Safety (Second Edition)”, John Wiley & Sons Ltd., pp 35-83. 4. CEN (2002a), “Eurocode 0: Basis of structural design”, EN 1990, European Committee for Standardization, Brussels, Belgium. 5. EN 1991-1-2:2002, “Eurocode 1: Actions on Structures, Part 1-2: General actions – Actions on structures exposed to fire”, European Committee for Standardization, Brussels, Belgium. 6. EN 1992-1-2:2004, “Eurocode 2: Design of Concrete Structures, Part 1-2: General Rules – Structural Fire Design”, European Committee for Standardization, Brussels, Belgium. 7. Hadole, C. (2017), “Assessment of fire affected reinforced concrete building”, M. Tech. Dissertation, VNIT Nagpur, India. 8. Hager, I. (2013), “Behaviour of cement concrete at high temperature”, Bulletin of the Polish Academy of Sciences Civil Engineering Technical Sciences, 61(1). 9. IS 875 (Part 1) (1987), “Code of practice for design loads (other than earthquake) for buildings and structures (second revision)”, BIS, New Delhi, India. 10. IS 875 (Part 2) (1987), “Code of practice for design loads (other than earthquake) for buildings and structures (second revision)”, BIS, New Delhi, India. 11. IS 456 (2000), “Plain and reinforced concrete code of practice (fourth revision)”, BIS, New Delhi, India. 12. IS 1893 (Part 1) (2016), “Criteria for Earthquake Resistant Design of Structures, Part 1, general provision and buildings (sixth revision)”, BIS, New Delhi, India. 13. ISO (1999a) ISO 834-1:1999, “Fire Resistance Tests- Elements of Building ConstructionPart 1: General Requirements”, International Organization for Standardization. 14. Jau, W. and Haung, K. (2007), “A study of reinforced concrete corner columns after fire”, Cement and Concrete Composites, 30, 622-638. 15. Kodur, V. and Dwaikat, M. (2008), “A numerical model for predicting the fire resistance of reinforced concrete beams”, Cement and Concrete Composites, 30, 431-443. 16. Law, A. and Gillie, M. (2010), “Interaction diagrams for ambient and heated concrete sections”, Engineering Structures, 32, 1641-1649. 17. National Building Code of India (Volume 1) (2016), BIS, New Delhi, India. 18. National Crime Records Bureau, NFPA 2010, The Geneva Association –World Fire Statistics, Avalon Consulting Analysis. 19. Pham, D.T., Buhan, P.D., Florence, C., Heck, J. V. and Nguyen, H. H. (2015), “Interaction diagrams of reinforced concrete sections in fire: A yield design approach”, Engineering Structures, 90, 38-47. 9
International Conference on Advances in Construction Materials and Structures (ACMS-2018) IIT Roorkee, Roorkee, Uttarakhand, India, March 7-8, 2018 20. Purkiss, J. A. (2007), “Fire Safety Engineering: Design of Structures (Second Edition)”, Butterworth-Heinemann, pp 43-76. 21. SA (2002), “Structural Design Actions”, AS/NZS 1170:2002, Standards Australia, Canberra, Standards New Zealand, Wellington. 22. SAP2000 (2014), “Integrated Solution for Structural Analysis and Design” , Computers and Structures Inc. (CSI), Berkeley, California, USA. 23. Stern-Gottfried, J. and Rein, G. (2012), “Travelling fires for structural design–Part I: Literature review”, Fire Safety Journal, 54, 74–85. 24. Tan, K. and Yao, Y. (2004), “Fire Resistance of Four-Face Heated Reinforced Concrete Columns”, Journal of Structural Engineering, 129(9), 1220-1229. 25. Topcu, I. B. and Karakurt, C. (2008), “Properties of Reinforced Concrete Steel Rebars Exposed to High Temperatures”, Material Science, 2008(814137), 1-4. 26. Wickstrom, U. (1986), “A very simple method for estimating temperatures in fire exposed structures”, Elsevier Applied Science, 186-194. 27. Youssef, M. A. and Moftah, M. (2007), “General stress–strain relationship for concrete at elevated temperatures”, Engineering Structure, 29, 2618–2634.
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