IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 5 Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com Parametric Investigation on the Seismic Response of Voided and Solid Flat Slab Systems

Jerry Paul Varghese2 and Manju George2

1 Civil Engineering Department, APJ Abdul Kalam Technological University, Mar Baselios Institute of Technology and Science Nellimatam, Ernakulam, India

2Civil Engineering Department, APJ Abdul Kalam Technological University, Mar Baselios Institute of Technology and Science Nellimatam, Ernakulam, India

Abstract early 1900s so that we can reduce the weight-to- Demand for safer and lighter floor system and to stiffness/strength ratios. improve the span limit has always troubled with the high weight- The inclusion of the voids is intuitively expected to to-stiffness/strength ratios of solid reinforced . It led compromise the stiffness and structural capacity of the slab. to the emergence of spherically voided biaxial slab (SVBS). A However, the extent of the compromise is not explicitly square and a rectangular slab models with a thickness of 28 cm were developed as solid and voided, simulated under uniform known. Therefore, in this paper, the seismic responses load including seismic force to compare the deflection responses exhibited by different configurations of voided slabs are and structural capacities of these slabs by conducting a pushover investigated in comparison to same configurations of solid analysis in ANSYS 16.0. Directional and the Equivalent stress slabs with equal dimensions. 3D non-linear finite element results are found to be slightly higher for voided slab that of solid analyses software, namely ANSYS 16.0 has been chosen to slab. Static Structural and Response spectrum results exhibit a conduct the investigation. similar pattern in the percentage variance of deformation and stress of voided over solid slab. The Load carrying capacity of SVBS is very close to that of solid slab, it even gets closer as the 2. Methodology span gets longer up to certain limit. Spherical voids around the middle region of the slabs do not significantly compromise the The Following procedure was adopted to compare the load capacity and deflection behavior of slabs and it provides Seismic response of Voided and Solid Flat slab systems. considerable material savings. 1. A square and a rectangular slab models with a thickness Keywords: Voided slab, Seismic Response, Equivalent Stress, Deformation, Static Structural, Response Spectrum of 28 cm were developed as solid and voided in ANSYS. a) Simulated under uniform load including seismic force to compare the deflection responses and structural capacities 1. Introduction of these slabs. b) Static Structural analysis is done. Flat slab systems give immense freedom to architects and c) Response Spectrum Analysis is done. engineers in the luxury of designing. In a traditional 2. The results were compared in terms of Equivalent stress slab span, the bottom portion will be in and Directional deformation. tension the top portion will be in compression, and the 3. Loading:-The live load and floor finish on slab panel are middle portion will effectively work only as a bridge 2kN/m2 and 1kN/m2 respectively. The design Seismic load holding the top and bottom portions together. Since the is calculated based on the zone factor of 0.16, medium soil, lateral load resistance of the slab column connection importance factor of 1 and response reduction factor 5 as system is small, flat slabs are often designed only for per IS: 1893-2002. gravity loads, while the seismic force is resisted by shear 4. Nonlinear dynamic analysis can be done by direct walls. Even though slabs and columns are not required to integration of the equations of motion by step by step share the lateral forces, these deforms with rest of the procedures. An earthquake accelerogram (time dependent structure under seismic excitation. The concern is that forcing function) is applied and the corresponding under such deformations, the slab column system should response-history of the structure during the earthquake is not lose its vertical load capacity. In order to ensure this, computed. Fig 1 shows the acceleration-time history of El the idea of voided concrete has been investigated since the Centro Imperial valley 1940 Ground motion records.

255 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 5 Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com each node translations in the nodal x, y, and z directions. The element is capable of plastic deformation, cracking in three orthogonal directions, and crushing. The crushing feature is based on the compressive strength of the concrete (f’c) and the cracking feature is based on an ultimate tensile stress (fr). Generally, reinforcing steel can be modeled either as discrete element, embedded reinforcement that assumes perfect bond between the Fig. 1 Acceleration time history of El Centro Imperial reinforcement and the concrete or as smeared concrete valley 1940 Ground motion records. element, in which a weighted average property of steel and concrete (Table 1) is used (Barbosa and Ribeiro 1998)[15]. For slab sizes to be considered in this study, large amounts of memory and CPU time will be required for the discrete 2. Voided Slab Systems element approach; hence, only the smeared approached is used [5]. Equal reinforcing steel is assumed to be provided Various types of hollow slab systems now existing in the in the longitudinal and transverse directions at both top and world are the Airdeck concept, U-boot®, Cobiax system bottom of the slabs. The ratio of steel to concrete in the and BubbleDeck. If the void formers are assembled in such smeared steel concrete layers is 6.47% by volume. a way that the requirements are met, such hollow core For a mechanical model system, such as the one used in slabs can be designed according to the common design this analysis, the mechanical physics preference is to be codes. The main effect of the voided slab system is to selected. The preferred meshers for mechanical analysis decrease the overall weight by as much as 30% to 35% are the patch conforming meshers (Patch Conforming when compared to a solid slab of the same capacity[3]. Due Tetrahedrons and Sweeping) for solid bodies. Relevance of to reduction in self-weight of the voided flat plate slab, the 20 with medium sizing is to be chosen. In general, When moment is reduced from 7 to 10 % to that of solid flat slab the imported geometry represents a structural region (or considering the same loading configuration. Bubble deck solid part), or when the imported geometry represents a slab is conceived to omit a significant volume of concrete fluid region (or fluid flow volume), use part-based meshing (compared to a solid slab) in the central core where the to create a mesh. We can also use part-based meshing to slab is principally un-stressed in flexure. In slabs, the depth create separate meshes for fluid regions and structural of compressed concrete is usually a small proportion of the regions when importing multiple geometry files. In slab depth and this means that it almost always involves general, boundary conditions for structural physics include only the concrete between the ball and the surface so there Externally applied forces, pressures, and moments, is no sensible difference between the behavior of a solid Supports, Steady-state inertial forces, Zero and nonzero slab and Bubble Deck. The only elements working are the displacements, Temperature Conditions (for thermal outer 'shell' of concrete on the compression side and the strain). The phenomenological approach is based on steel on the tension side. In terms of flexural strength, the conducting experiments on test specimens whose moments of resistance are the same as for solid slabs dimensions are large compared to the representative provided on this compression [7] while the depth is checked volume element (RVE) to determine the major responses during design so that it does not encroach significantly into (Fig. 3) the ball. Table 1: Material Properties

Fig. 2 C/S of Voided slab system.

3. Structural Model and Material property

A Solid65 element is used to model the concrete. This element has eight nodes with three degrees of freedom at

256 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 5 Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com

Varying the span length enables the investigation of the presumed advantage of SVBS over solid slab. The thickness and the span length used for the long span slab represent one of the practically used SVBS configurations. Table 2. Geometric parameters of Solid and Voided Slab

Fig 5. Equivalent stress (von-mises) in square Solid and SVBS concrete slab panel (SSV, SSD)

Fig 6. Directional deformation in rectangular solid and SVBS concrete slab panel (RSD1, RSV1)

Fig. 3 Representative volume element extraction from plain concrete slab 4. Finite Element Analysis Depending on the type of physics involved (static structural, modal structural, harmonic, random vibration, Fig 7. C/S view of directional deformation in rectangular response spectrum) provides several means to control the solution of the physics simulation. Deformation and stress SVBS concrete slab panel (RSV1) is measured at mid-span of the slab from load-deformation diagrams of the solid and voided slabs by analytical results. There may be several effects that can cause higher stiffness in the finite element models. On the other hand, the finite element models do not include the micro-cracks. Static Structural analysis determines the displacements, stresses, strains, and forces in structures or components caused by loads that do not induce significant inertia and Fig 8. Equivalent stress (von-mises) in rectangular solid damping effects. Steady loading and response conditions and SVBS concrete slab panel (RSD1, RSV1) are assumed; that is, the loads and the structure's response are assumed to vary slowly with respect to time.

Fig 9. Directional deformation in rectangular solid and Fig 4. Directional deformation in square Solid and SVBS SVBS concrete slab panel (RSD2, RSV2) concrete slab panel (SSV, SSD)

257 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 5 Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com

Fig 10. Equivalent stress (von-mises) in rectangular solid and SVBS concrete slab panel (RSD2, RSV2)

Fig 11. Percentage variance of Directional deformation The von Mises yield criterion (also known as the maximum distortion energy criterion) suggests that yielding of a ductile material begins when the second deviatory stress invariant reaches a critical value. It is part of plasticity theory that applies best to ductile materials (Table 3).

Table 3. Tabular Data of Static Structural analysis results

Fig 12. Percentage variance of Equivalent Stress

Response spectrum analyses are widely used in civil 4. Conclusions structure designs, for example, high-rise buildings under wind loads. Another prime application is for nuclear power The Static Structural and Response spectrum results plant designs under seismic loads. A Response Spectrum exhibit a similar pattern in the percentage variance of analysis has similarities to a Random Vibration Analysis. deformation and stress of voided over solid slab. The load However, unlike a Random Vibration analysis, responses carrying capacity of SVBS under uniform load is very from a Response Spectrum analysis are deterministic close to that of solid slabs, it gets even closer as the span maxima. For a given excitation, the maximum response is gets longer. Providing spherical voids around the middle calculated based upon the input Response Spectrum and region of the slabs do not significantly alter deflection the method used to combine the modal responses. An behavior of slabs under seismic effect also, it provides earthquake data most commonly taken is El Centro ground considerable material savings. motion (Fig. 1) for conducting a Seismic analysis.

Table 4. Tabular Data of Response Spectrum analysis results Acknowledgments

I express my deepest sense of gratitude to Mrs. Manju George, Asst. Professor, MBITS, Nellimattom, my esteemed guide and my cordial thanks for her warm encouragement, thoughtful guidance, insightful decision, critical comments and correction of the thesis. I also express my sincere thanks to all the faculty members and students of the Civil Engineering Department of Mar Baselios Institute of Technology and Science for their co- operation and support.

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