Review on Comparative Study of R.C.C. Waffle Slab Vis-À-Vis Prestressed Concrete Waffle Slab

Akshay S. Raut *

Final Year Student (M.E. Structure), PRMIT&R, Badnera, akshayraut05s@gmail.com

Prof. Riyaz Sameer Shah Asst. Professor (Department of Civil Engineering)

PRMIT&R, Badnera, riyazsameer1@gmail.com

Abstract:- This paper presents a comparative study of R.C.C. Waffle slab and Prestressed Waffle slab. When a large space within a building needs to be covered without hindrance and supports, architects often deploy waffle slab to construct floor and ceiling. Waffle slab are generally used for heavy loads. The work includes the analysis and design of R.C.C. Waffle slab and Prestressed Waffle slab for small span, medium span and long span ranging from 8m to 30m. For analysis purpose ETAB 15 (Integrated Analysis, Design and Drafting of Building System) and SAFE 14 (Integrated Design of Slabs, Mat and Footing) software results will be taken into consideration for R.C.C. Waffle slab and Prestressed waffle slab and manual calculation will be carried out for both. Programming in EXCEL will be done to obtain the results for various spans for different elements of waffle slab. The rectitude of the program was checked by first designing the manually designed slabs comparing the results. The idea is to reach a definite solution that for longer span more than 25m; Prestressed Waffle Slab will achieve economy.

Keyword- Waffle Slab, Ribs, Prestressed Waffle Slab

1.0 INTRODUCTION

floor is an integral part of practically every modern industrial, commercial or residential

building. As expectation of building user, who is in everyday contact with the floors, rise, so the performance of floor structure in day-to-day service is becoming increasingly important. The choice of type of slab for a particular floor depends on many factors. Economy of construction is obviously an important consideration, but this is a qualitative argument until specific cases are discussed, and is a geographical variable. The design loads, required spans, serviceability requirements, and strength requirements are all important. When a large space within a building needs to be covered without hindrance and supports, architects often deploy waffle slab to construct floor and ceiling. Waffle slab are generally used for heavy loads.

ETAB and SAFE software will be used for analysis and design purpose. ETAB and SAFE are founded by Computers and Structures, Inc. (CSI) which is recognized globally as the pioneering leader in software tools for structural and earthquake engineering. ETABS has been developed specifically for multi-story commercial and residential building structures, such as office

towers, apartments and hospitals. The SAFE System provides an efficient and powerful program for the analysis and design of concrete slabs and foundations, with or without post-tensioning.

Waffle slab structure are defined as the combination of a flat flange plate , or deck and a system of equally spaced parallel ribs, that may be arranged in either orthogonal or skew assembly with monolithic inter-section. They are also known as two way ribbed flat slab and it includes recesses between the ribs. The intermediate cross beams are cast on a regular square grid that gives the underside of the floor the appearance of a waffle, hence the name. Waffle slab have economical and constructional benefits. They are used for heavy loads and large span structures as they exhibit higher stiffness and smaller deflection. As a result, waffle slab has been widely used for office buildings, hotels and multi-storey building in many countries.

2.0 LITREATURE REVIEW

Current Status:

Although several research work have been conducted on the various elements of R.C.C.

A

* Corresponding Author

Waffle Slab vice versa on Prestressed Waffle Slab, there is a little work on the same. An exhaustive literature review reveled that a minimum amount of research work had been done in India.

1. J. Prasad [1], S. Chander [2], A.K. Ahuja [3] (2005) - This paper deals with the optimum design of waffle slab for medium size floors. In order to carry out the parametric investigation to arrive at optimum values of ribs number and dimension for a given floor size. The waffle slabs with square floor plan of size 6x6 m, 7x7 m and 8x8 m have been considered keeping the residential and office floors in mind.

The author concludes that-

For 6x6 m square floor plan, 5 ribs of 140mm depth is found to be structurally efficient for 3KN/m

2

For 7x7 m square floor plan, 9 ribs of 180mm depth is found to be structurally efficient for 3KN/ m

2.

For 8x8 m square floor plan, 9 ribs of 240mm depth is found to be structurally efficient for 3KN/ m

2.

2. Indrajit Chaudhary [1], Jitendra Singh [2] (2012) - This paper propose a semi analytical method for the analysis of waffle slab with any arbitrary boundary conditions; fixed, free and simply supported. To validate the results, selected cases are compared with finite element analysis.

The author concludes that-

Two boundary conditions were selected; FEM results were lower bound by 12% with respect to Timoshenko’s value for simply supported slab.

Displacements for the proposed method are almost same as Timoshenko’s analysis and shorter for analysis by FEM on ANSYS.

3. Anjaly Somshekhar [1], Preetha Prabhakaram [2] (2015) - This paper focused on the structural behavior of Waffle slab with and without opening and the effect of opening size and locations on the ultimate load. In this paper the parametric study was conducted using implicit nonlinear finite element software ANSYS.

The author concludes that-

It is concluded from the results that special measures have to be taken to improve the performance of waffle slab.

For the opening which is located in the area bounded by two column strips, a very small effect was found in ultimate load, if opening width is less than 10% the width of the column strip. Hence, opening in this zone must not exceed 10%.

For the opening which is located in the area bounded by middle strips, no significant change is observed in the ultimate load with different opening sizes.

4. Ibrahim Arman [1] (2014) - The paper focused on the analysis of two-way ribbed and waffle slabs with hidden beams. Four slabs are taken in this study to show the moments distribution in two-way ribbed slab with hidden beam system.

The author concludes that-

By direct design method, it is found that the beam resist about 20% of the column strip moments.

Waffle slab with hidden beams can be treated approximately as the slabs without beams i.e. Flat Slabs.

5. Dr. A.C. Galeb [1], T.E. Ibrahim [2] (2014) - This paper aims to find the optimum dimensions of post-tension concrete (two-way ribbed) waffle slabs using standard dome size. Equivalent frame design method is used for the structural analysis and design of slab.

The author concludes that-

Span to depth ratio 1/23 to1/25 give an economic slab cost.

Using minimum slab thickness give you minimum cost and weight.

Using 750x750 domes with 150mm rib giving minimum cost and weight.

6. Attila Puskas [1], Jacint Virag [2] (2014) - The paper presents a set of design criteria applied in the structural design of a 25000 sq.m, multistory concrete building. In the article a multi-criteria comparison of the structural solutions is presented using two different types of slabs i.e. Flat Slab and Waffle Slab.

The author concludes that-

For the studied building the waffle slab solution used with the combined mat, point and beam foundations represents not only the most cost efficient but also the most energy-efficient structural solution.

3.0 RATIONALE FOR THE REVIEW

With the literature review presented and codal provisions given later, the silent objective of the present

study have been identified as follows-

1. To study the structural behavior and optimum analysis and design of Waffle slab.

2. For analysis purpose ETAB 15 and SAFE 14 software results will be taken into consideration for R.C.C. Waffle slab and manual calculation will be carried out for both.

3. The paper aims to show that for longer span more than 25m, prestressing will achieve economy providing considerable depth of slab.

4. To compare the results that up to what span R.C.C. waffle slab is economic.

5. To check whether it is safe to provide minimum depth to slab.

4.0 METHODOLOGY

The methodology workout to achieve the above mentioned objective is as follows-

Several analysis methods, Timoshenko’s Analysis, Finite Element Method (FEM), and Direct Design Method (DDM) are available to predict the structural behavior of the structure. In the project we are working on one long span structure and analyses it with the help of various software, ETAB 15, SAFE 14 for Waffle Slab calculation and will be cross checked with manual calculation for the both.

From the study of analysis, the structural behavior and cost comparison should be done. The work includes the analysis and design of R.C.C. Waffle slab and Prestressed Waffle slab for small span, medium span and long span ranging from 8m to 30m. The rectitude of the program was checked by first designing the manually designed slabs by using the program & comparing the results.

5.0 CASE STUDY

1. Waffle slab has its origin in a thick solid-slab floor from which the bottom layer concrete in tension is partially replaced by their ribs along orthogonal directions. The tensile forces are resisted by reinforcement provided in ribs. The dimension and spacing of the ribs are decided such as to achieve better load bearing distribution without providing shear reinforcement. The present paper shows the results from the analysis carried out on waffle slab medium size floor system with a view to achieve the optimum dimensions of rib spacing, its depth and width. The waffle slab is considered to be monolithically constructed with beams.

2. In order to carry out parametric investigation to arrive at optimum values of rib number and dimensions for a given floor size, the waffle slabs with square floor plans of size 6×6 m, 7×7 m and 8×8 m have been considered keeping the residential and office floors in mind. Larger spans would need much higher floor thickness, which would be suitable for Grid floor structural system.

3. Top slab of the waffle slab floors may be kept at its minimum thickness from construction point of view. It has been kept as 65 mm for all the floors studied herein. Width of the ribs has been taken as 100 mm (Figure 3). The shape of ribs is considered as rectangular during analysis, although in actual practice it may be slightly tapered. The rib depth has been varied in the range from 130 mm to 260 mm with a regular increment of 10 or 20 mm. The overall floor depth, thus, varies from 195 mm to 325 mm.

4. The number of ribs has been taken as five at the minimum with an increment of two until all the structural requirements are adequately satisfied for the particular floor plan. The rib depths after a certain number of rather closely spaced ribs are found to be inadequate in meeting a particular structural requirement. The maximum number of ribs for the largest span is, therefore, taken to be nine. The live load values adopted are 3 kN/m2 and 5kN/m2. Floor finish load has been taken as 1.5 kN/m2 and unit-weight of R.C.C. as 25kN/m3 in order to evaluate dead loads due to top slab, ribs and band beams. The present study has been carried out using M-20 grade concrete and Fe-415 grade steel as the materials of

construction. However, behavior of 8×8 m size waffle floor has been studied using M-40 grade concrete also.

6.0 RESULTS AND DISCUSSION

Results of the analytical study on 6×6 m square waffle slab under a live load intensity of 3 kN/m2 are presented-

Table 1 Deflection, bending moment and shear force values for waffle slab floor of size 6×6 m with no. of ribs Nx = 5 and Ny = 5 and live load of 3 kN/m2

Table 2: Deflection, bending moment and shear force values for waffle slab floor of size 6×6 m with no. of ribs Nx = 7 and Ny = 7 and live load of 3 kN/m2

Table 3: Deflection, bending moment and shear force values for waffle slab floor of size 6×6 m with no. of ribs Nx = 9 and Ny = 9 and live load of 3 kN/m2

Results of the analytical study on 6×6 m square waffle slab under a live load intensity of 5 kN/m2 are presented- Table 4: Deflection, bending moment and shear force values for waffle slab floor of size 6×6 m

with no. of ribs Nx = 5 and Ny = 5 and live load of 5 kN/m2.

Table 5: Deflection, bending moment and shear force values for waffle slab floor of size 6×6 m with no. of ribs Nx = 9 and Ny = 9 and live load of 5 kN/m2

Table 6: Deflection, bending moment and shear force values for waffle slab floor of size 6×6 m with no. of ribs Nx = 9 and Ny = 9 and live load of 5 kN/m2

7.0 CONCLUSION OBTAINED FROM THE ABOVE RESULTS

Following conclusions are drawn from the study presented in this paper.

1. For 6×6 m square floor plan, 5 ribs of 140 mm depth (overall depth 205 mm) is found to be structurally most efficient for 3 kN/m2 live load intensity.

2. For 5 kN/m2 live load intensity on a 6×6 m square floor plan, a choice between 9 ribs of 150 mm depth and 7 ribs of 160 mm depth becomes available. Percentage of dead load is about 50 in both the cases.

3. For square floor plan of 7 × 7 m, the most efficient structural system is 9 ribs of 180 mm depth for a live load intensity of 3kN/m2.

4. For a live load intensity of 5kN/m2, the most efficient structural system is 9 ribs of 200 mm depth for 7 × 7 m square floor plan.

5. For square floor plan of 8 × 8 m, the most efficient structural system is 9 ribs of 240 mm depth for a live load intensity of 3kN/m2.

For square floor plan of 8 × 8 m, the most efficient structural system is 9 ribs of 220 mm depth using M-

40 grade concrete for a live load intensity of 5kN/m2.

8.0 CONCLUSION

From the above research from the literature paper, I have concluded that the use of Waffle Slab for long span and heavy loading is economical as well as having constructional benefits as they exhibit higher stiffness and smaller deflection.

The use of Waffle slab can reduce the cost of project up to 20%.

From the present study done on waffle slab, it is concluded that, the depth of rib increases as the length of span increases, therefore it is economical to use Prestressed Waffle slab.

refrences

1. J. Prasad[1], S. Chander[2], A.K. Ahuja[3]; “Optimum Dimensions of Waffle Slab for Medium Size Floors”, Asian Journal of Civil Engineering VOL. 6, No. 3(2005).

2. Chowdhury [1], J. P. Singh [2]; “Analysis and Design of Waffle Slab with different Boundary Conditions”, The Indian Concrete Journal (2015).

3. Somasekhar [1], P. Prabhakaran [2]; “Analysis of Concrete Waffle Slab with Opening”, International Journal of Emerging Technology and Advanced Engineering VOL. 5, Issue 9 (2015).

4. A. C. Galeb[1], Z. F. Atiyah[2]; “Optimum design of reinforced concrete waffle slabs”, International Journal of Civil And Structural Engineering Volume 1, No 4, (2011).

5. Ibrahim Arman [1]; “Analysis of two- way ribbed and waffle slabs with hidden beams”, International Journal for Computational Civil AND Structural Engineering · January (2014).

Codes-

1. IS: 456 (2000) - Plain And Reinforced Concrete Code of Practice

2. IS: 1893 (2001) - Criteria for Earthquake Resistant Design of Structure.

3. IS: 13920 (1993) – Ductile Detailing of Reinforcement Concrete Structure Subjected to Seismic Forces.

Books-

1. Timoshenko

2. T. J. McGinley – Reinforced Concrete

Websites-

1. www.sciencedirect.com

2. www.ijert.org

3. www.ijoer.in

4. www.scihub.in

5. www.google.com

Author’s Biography

Mr. Akshay S. Raut obtained his B.E. degree from Sant Gadge Baba Amravati University, Amravati. He is pursuing post-graduation in Structural Engineering.

Prof. Riyaz Sameer Shah obtained his B.E. degree from Sant Gadge Baba Amravati University, Amravati and post-graduation in Structural Engineering. He is working as a Assistant Professor in Prof. Ram

Meghe Institute of Technology and Research, Badnera. He has published two papers in national journals & conferences. He worked as a structural designer with a design consultancy firm.