Analysis of Flat Plate Structures Exposed to Fire

Posted: August 26th, 2021

Analysis of Flat Plate Structures Exposed to Fire

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July 27, 2020

Analysis of Flat Plate Structures Exposed to Fire

1. Literature Review

A flat plate structure is one that contains slabs supported by columns beams. In these structures, the slabs are constructed using reinforced concrete and uniform thickness (George & Tian, 2012). Since these structures are mainly for supporting low gravity loads, they are built with relatively thin slabs. This makes them susceptible to punching failure in the high-temperature conditions that can result from fires within the structure. Punching failure derives from the severe load redistributions that occur when the fire is applied beneath the slab.  Such failure can, in turn, cause the collapse of the structure.

Wolisinki sought to investigate flat slab structures concerning their robustness and vulnerability (2017). He explained a robust structure that is not sensitive to eternal disturbances and can withstand localized damage with proportionate damage. A vulnerable structure, on the other hand, has consequences disproportional to the responsible damage. From previous studies and analysis, Wolisinski learned that flat slab structures bear risks brought about by unseen extreme events such as a fire. These risks include punching failure on the flat slabs, gross human errors, and column failures. He concludes that risk assessment is the key to identifying structural weaknesses and analyzing if the precautions provided against excessive risks are effective.

1. Punching Shear in Flat Plate Structures

Different studies on the punching shear strength of slabs were reviewed by Ramana (2017). Among the different floor slabs, the flat-plate floor, and the beam-slab floor, the flat-plate floor slab is the most susceptible to punching failure. However, such floors are popular because they reduce the height of the building, and evidence quickly sets up formwork. From the various experiments and studies reviewed, Ramana determined that the strength and stiffness of concrete and the reinforcing steel are essential in the punching shear criterion for slab elements.

Figure 1: Structure After Punching Failure (Ramana, 2017)

Various factors affect the punching shear capacity of a concrete slab. These factors include the tensile strength of concrete, the compressive strength, the yield strength, and the amount of reinforcement in the slab (Ghoreishi, Bagchi & Sultan, 2014). Although the concrete within the slab is relatively fire-resistant, the reinforcing steel within it is sensitive to elevated temperatures.  The compressive strength of the concrete is seen to decline with increasing temperature.  Both chemical and physical changes are experienced in concrete exposed to fire. Spalling at the surface is experienced at around 240 ⁰C, and significant strength loss manifests at temperatures above 600 ⁰C. However, the structure’s failure load depends on several factors, including the size of the column, the compressive strength of the concrete, the tensile strength provided by the reinforcing bars, and the effective depth of the column. To adequately insure structural stability and prevent punching failure due to elevated temperature, the researchers recommend a minimum of 1% and a maximum of 2% flexural reinforcement in flat concrete slabs.

1. Effects of Fire on Flat Plate Structures

When exposed to fire conditions, a concrete element can be expected to fail within specific structural limits. Annerel et al. elaborate on experimental results from a Ghent University study investigating the behavior of flat slabs exposed to fire (2012). The researchers report that when a flat slab was subjected to a fire for 120 minutes, it experienced a punching shear capacity loss of about 40%. However, a safety margin of about 1.8 to 2.2 times the service load remained. This led to the eventual failure of the slab.

An allied investigative study was done to determine how elevated temperature and fire affect the punching shear behavior of flat concrete slabs (Ghoreishi, Bagchi & Sultan, 2014). They acknowledged that, despite the advantages of flat concrete slabs, such as flexibility of partition and ease of construction, these slabs yet evidence associated risks. The risks derived from the complex three-dimensional stresses exerted at the columnar connections to the two-way concrete slab. This complexity has the potential of causing a brittle punching shear failure, which can be disastrous. Punching failure can take the form of either punching shear or flexural punching.  It begins to manifest when a shear crack through the slab causes a reduction in the slab’s shear strength. The failure is inevitable when the shear strength reduces to the point at which it equals the tensile splitting strength of concrete. At this point, the failure occurs in the compression zone. The researchers’ study used a model based on this theory for a concrete slab-column connection in which the ultimate punching shear strength can be predicted. From the model and the various experimental studies reviewed, Ghoreishi, Bagchi & Sultan (2014) stated that flexural failure happens when the reinforcing steel yields before the punching occurs. On the other hand, shear failure comes about when small deflections occur in the slab, with attendant localized yielding of the tension reinforcement bars. Results from the model and experimentations were consistent, serving to validate the use of the model in estimating the ultimate punching shear strength.

When exposed to fire, concrete slabs tend to sag toward the fire. This increases the axial load at the connection between the column and the slab. The additional load is due to an indirect torque induced at the supports by the sagging (Annerel et al., 2013). A column in direct contact with the fire also experiences expansion, which, in turn, further increases the axial load. An increase of the loading beyond the allowable safety margin ultimately leads to structural failure. The figure below shows how a flat slab behaves in a fire condition.

Figure 2: Behavior of Flat slab in Fire (Annerel et al., 2013)

Concrete is known to lose strength due to extreme heat conditions. Arna’ot et al. attribute this loss to various factors, including the concrete’s initial strength and water-cement ratio (2017).  While they focused mostly on the effects of fire upon flat-plate column assemblies, their work is broadly extensible to other structural applications.  They state that, at 400 ⁰C, a loss of approximately 18% to 20% of the initial strength can be experienced. At temperatures between 540 ⁰C and 590 ⁰C, the concrete can be expected to lose up to one-half of its initial strength. This loss of strength is among several ancillary factors that collectively contribute to incipient failure.  Particularly within the context of joints composed of concentric flat plates, the effect of the unbalanced heating arising from differing lengths of adjacent spans is as noteworthy as that of the high heat itself upon the mechanical and thermal properties of the material.  In general, the parameters that most decidedly influence punching shear behavior were the temperature of the fire, the duration of exposure, the degree of surface exposure, and the preloading and stress history of the concrete infrastructure.  The researchers were able to derive the following relationship between time and punching resistance in the event of a fire.

Figure 3: Decrease in Punching Resistance over Time, 350-mm and 200-mm Slab Thickness (Arna’ot et al., 2017)

Arna’ot et al. further determined that concrete specimens of high compressive strength and high reinforcement ratio tended to degrade more severely than those of low compressive strength and reinforcement ratio (2017).  Notably, shear reinforcement evidenced minimal effect upon residual strength of slab column assemblies pursuant to the fire event.  Finally, sudden cooling after the thermal disturbance was more contributory to strength deterioration than was gradual cooling.

1. Fire Resistance of Flat-Plate Structures

Fire resistance refers to the ability of given material to withstand the impact of fire. According to the American Concrete Institute (American Concrete Institute, 2019; Fletcher et al., 2007), a flat-plate structure is regarded as fire-resistant when it can continue performing when exposed to fire. Equally, the structure should have elements that can confine the fire. Moreover, the physical and thermal properties of materials affect the structural capability of the structure to withstand fire. Notably, Fletcher et al. (2007) identified various thermal properties such as the free moisture in the structure, the aggregate type in the concrete, and the volume per square foot of the exposed area of the material affects the fire resistance of structure. Equally, stress level, the cover of the reinforcing bars, lateral resistance conditions, and aggregate tendency and free spalling are among the physical properties that influence resistance to fire (Fletcher et al., 2007). Therefore, in understanding fire resistance, an analysis should be done using particular standard methods to assess structural elements that enhance fire resistance properties in a flat-plate structure. The following subsections detail the variety of analytical, numerical, and hands-on methods that various authors describe helping understand fire resistance;

1. Analytical Methods

Moss et al. employed an analytical method to find out how multi-bay, two way reinforced concrete slabs behave in a fire. They subjected a three-bay-by-three-bay structure to fire over its entire surface area (2008). They used the SAFIR computer program to carry out analyses by simulating the concrete structure’s fire behavior. They discovered that the deflection at the corners was minimal, resulting from significant flexural stiffness at the edge beams, partially exposed to the fire. The most significant degree of deflection was observed at the central point of the slab. This was caused by the high tension in the top and high compression in the bottom layer, both of which resulted from exposure to extreme temperatures. The researchers concluded that the fire results in a thermal gradient, which causes the significant distribution of the bending torque. At temperatures above 300 ⁰C, the steel yield strength decreased, thereby reducing the bending strength of the RC and leading to sagging. These occurrences are collectively responsible for the ultimate failure of the structure.

1. Numerical Methods

Computer simulations have made it more facile analytically to model the effects of fire in multistory buildings. Lim et al. noted that finite shell elements in 3-D analyses provide the most accurate means for modeling RC concrete slabs in fire conditions (2004). Through this modeling method, in-plane membrane forces responsible for supporting the loading on the slab can be accounted for. Several finite-element software packages are available commercially and can effectively be used for modeling concrete slabs in fire conditions. These programs include Abaqus, Vulcan, and Adaptic. By way of contrast, the SAFIR software was specially developed for the analysis of structural behavior in fire, including finite element representation of trusses and beams. The SAFIR quadrilateral shell element is defined by a total of four corner nodes and a cubic membrane displacement shield. In this shell element, shear strains are assumed not to change. Apart from element formulation, SAFIR can also model biaxial concrete, analyzing reinforced concrete slabs possible. Based on a smeared model, modeling the reinforcing bars is also possible with the SAFIR shell element. The shell element is also readily amenable to thermal analytical techniques that can calculate temperature distribution within the concrete slab.

Lim et al. (2004) further validated the shell element programs’ abilities by modeling two-way slabs in fire conditions. They did so by modeling the exposure of three slabs to fire. They reported that none of the slabs collapsed after three hours of ISO fire exposure. After SAFIR analysis and experimental analysis, the researchers concluded that two-way slabs could effectively resist fire, assuming that deformations in the slabs are in double curvature and that the slabs are supported on all four edges.  This fire resistance can be attributed to the tensile membrane action developed in the early stages of fire exposure. The successful application of the shell model demonstrates that numerical modeling is effective and efficient for analyzing the behavior of concrete elements under fire conditions.

1. Hands-on Methods

Hands-on experimentation is an alternative technique for studying the behavior of structural slabs under fire conditions. A sample test structure is constructed and loaded to replicate the conditions evidenced in loaded structural members under field conditions. The test structure is then subjected to a furnace prepared with specific requirements and closely observed for a while.  Detailed data are collected and tabulated and can be used to validate computer simulations of structural fires.

Ghoreishi, Bagchi & Sultan, 2014 reviewed the experimental method in their journal article. They describe an experiment where six columns attached to slabs were tested, three in ambient conditions, and three under elevated temperatures. The specimens evidenced different reinforcement ratios despite having equal physical dimensions. They were constructed from the concrete of strength of 40 MPa, reinforced with hot-rolled bars of grade 400, 10 mm in diameter. The reinforcement in the columns and the slabs exhibited a concrete cover of the thickness of 15 mm. The reinforcement and dimensions of the column and slab arrangement were as shown below.

Figure 4: Arrangement of the Experimental Specimen (Ghoreishi, Bagchi & Sultan, 2014)

All of the columns were loaded and then exposed to increased temperatures. Before observing the columns’ behavior, a computer model was applied to predict how the columns might be expected to behave under the applied fire conditions. The researchers determined that, with a furnace heated to 600 ⁰C, the top surface of the slab could reach 319 ⁰C before failure. It took the surface 345 minutes’ worth of exposure to reach this temperature. The figure below shows how the temperature was distributed within the flat slab.

Figure 5: Temperature Distribution in Slab (Ghoreishi, Bagchi & slab’s Sultan, 2014

1. Representative Research Studies
   

A variety of studies have been performed on flat plate structures over the years, specifically investigating how they are affected by fires and how structural failure can be averted in case of a fire event. The following is a representative sample of studies published on the topic.

1. Ghoreishi, Bagchi & Sultan (2010)

The researchers sought to estimate how a flat slab system responds to exposure to fire. They made use of a 250 mm thick slab under a parametric fire curve. For their study, they exposed the finite element (FE) model to fire. They observed that concrete from the slab started spalling after twelve minutes of exposure.  Following the spalling, a displacement of up to 20 mm was observed, taking up to eighty minutes’ worth of exposure fully to develop.  The research team concluded that exposing concrete structures to high temperatures may cause them to fail suddenly.

1. Makate, Lohar, Shetti & Patil (2019)

This study was undertaken to investigate how thermal loading affects both RCC conventional slabs and flat slabs. The researchers prepared twelve mockup structures using both conventional and flat slab systems. The mockups were then exposed to different temperatures ranging from 15 ⁰C to 50 ⁰C.  The team observed that flat slabs experienced about 5.10% more displacement than the conventional RCC slabs when exposed to heat.  They attributed the difference to the presence of beams within the conventional slabs, which enabled them to resist most deflection and bending torques. By way of contrast, the absence of such members in flat slabs renders them more vulnerable to these agents’ effects.  The team concluded that temperature parameters must be duly accounted for when designing either conventional slab or flat slab construction systems.

1. Torelli et al. (2016)

Torelli et al. conducted a review of the effects of transient thermal conditions on concrete strains (2016). They specifically investigated how concrete behaves under high temperatures.  They focused on two single-axis test methods, the steady-state test, and the transient test. In the steady-state test, the test material is first heated uniformly to a specified temperature and then subjected to mechanical loading. For the transient test, the test material is first loaded mechanically and then subjected to uniform heating while the load is kept constant. The researchers determined that, when concrete is subjected to a constant compressive load under heating, its thermal strain significantly decreases. This is a result of the nonlinear relationship between temperature increase and concrete expansion.  The team determined that more experimentation under uni-axial loading conditions is necessary to develop a clearer understanding of the associated phenomena, albeit uni-axial analytical models appear to be more comprehensive than their multi-axial analogs.

1. Al Hamd et al. (2018)

The researchers sought to investigate how elevated temperatures affect load-induced thermal strain (LITS). In their study, they made use of a finite element software package called Abaqus.  They combined the software with a variety of established models used for investigating punching shear under ambient conditions, gradually extending the models to include the effects of elevated temperatures.  In the case of elevated temperature conditions, the modeling approach was subdivided into two phases.  First, a thermal heat transfer model was employed in order to derive a thermal profile.  This profile was then transferred to the concrete component of the mechanical model.

The researchers considered how various factors are responsible for the concrete’s behavior under the elevated temperature conditions.  These factors include Young’s modulus, thermal expansion, and the concrete’s tensile and compressive strengths.  The modeling was first performed without considering the effects of load-induced thermal strain.  The transient thermal strain was then added to the model to understand the effects of LITS. After modeling under conditions of both slow and rapid-fire, the researchers found that LITS plays a significant role in causing the deflections that result in the punching shear (Al Hamd et al., 2018). They thus determined that models must incorporate load-induced thermal strains accurately to replicate the effects of fire events in slab structures.

• Conclusion

Flat plate structures can be significantly affected by the high temperatures that result from exposure to fire.  The resultant temperatures induce load redistribution within the structure, causing additional bending torques to manifest and thereby result in punching failure of the structure.  One thereby determines that the consideration of temperature parameters is necessary when designing such structures.  At the same time, continued research in the field is recommended to bolster current understanding and increase the theoretical accuracy and, thus, the practical applicability of modeling techniques.

References

American Concrete Institute. (2019). Concrete.Org; American Concrete Institute. https://www.concrete.org/

Al Hamd, R. K. S., Gillie, M., Warren, H., Torelli, G., Stratford, T. & Wang, Y. (2018). The effect of load-induced thermal strain on flat slab behavior at elevated temperatures. Fire Safety Journal 97: 12–18.

Annerel, E., Taerwe, L., Merci, B., Jansen, D., Bamonte, P. & Felicetti, R. (2013). Thermo-mechanical analysis of an underground car park structure exposed to fire, Fire Safety Journal 57: 96-106.

Arna’ot, F.H., Abid, S.R., Özakça, M & Tayşi, N. (2017). Review of concrete flat plate-column assemblies under fire conditions. Fire Safety Journal 93: 39-52.

Chavan, D. R., Mohite, D. D., Pise, Pawar Y. P., Kadam, S. S., Deshmukh, C. M. (2016). The behavior of multi-storied flat slab building considering shear walls: A review. International Journal of Engineering Research and Application 6: 10-14.

Fletcher, I., Welch, S., Torero, J., Carvel, R., & Usmani, A. (2007). The behavior of concrete structures in fire. Thermal Science, 11(2), 37–52. https://doi.org/10.2298/tsci0702037f

George, S.J. & Tian Y. (2012).  Structural performance of reinforced concrete flat plate buildings subjected to fire, International Journal of Concrete Structures and Materials 6(2): 111–121.

Ghoreishi, M, Bagchi, A & Sultan, M.A.  (2010). Estimating the response of flat plate concrete slab systems to fire exposure.  Proc. Sixth International Conference, Structures in Fire: 86-293.

Ghoreishi, M., Bagchi, A. & Sultan, M. (2014). Punching Shear Behavior of Concrete Flat Slabs in Elevated Temperature and Fire. Advances in Structural Engineering 18(5): 659- 674.

Lim, L., Buchanan, A., Moss, P. & Franssen, J. (2004). Numerical modeling of two-way reinforced concrete slabs in the fire. Engineering Structures 26: 1081–1091.

Makate, J., Lohar, P., Shetti, R. & Patil P. (2019). Effects of thermal loads on RCC conventional slab and flat slab. International Research Journal of Engineering and Technology, 6(7).

Moss, P.J., Dhakal, R.P., Wang, G., Buchanan, A.H. (2008). The fire behavior of multi-bay, two-way reinforced concrete slabs. Engineering Structures 30: 3566–3573.

Mvogo, P., Mouangue, M., Zaida, J., Obounou, M & Fouda, H. (2019). Building fire: experimental and numerical studies on the behavior of flows at opening, Journal of Combustion.

Ramana, N. V. (2017). Review on punching shear strength of slabs. International Journal of Engineering Research and Development 13(10): 1-25.

Torelli, G., Mandal, P., Gillie, M., & Tran, V. (2016). Concrete strains under transient thermal conditions: A state-of-the-art review. Engineering Structures 127: 172–188.

Woliński, S. (2017). Robustness and vulnerability of flat slab structures. Procedia Engineering 193: 88 – 95.

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