Effective reinforcement layout for skew slabs

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1 Effective reinforcement layout for skew slabs A Kabir*, Bangladesh University of Engineering & Technology, Bangladesh S M Nizamud-Doulah, Bangladesh nstitute of Technology, Bangladesh M Kamruzzaman, Bangladesh nstitute of Technology, Bangladesh 27th Conference on OUR WORLD N CONCRETE & STRUCTURES: 29-3 August 22, Singapore Article Online d: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete nstitute All Rights reserved for C Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of C Premier PTE LTD Visit Our Website for more information

2 2m Conference on OUR WORLD N CONCRETE & STRUCTURES: 29-3 August 22, Singapore Effective reinforcement layout for skew slabs A Kabir*, Bangladesh University of Engineering & Technology, Bangladesh S M Nizamud-Doulah, Bangladesh nstitute of Technology, Bangladesh M Kamruzzaman, Bangladesh nstitute of Technology, Bangladesh Abstract Both experimental and numerical study has been carried out to investigate the effects of reinforcement arrangements on the ultimate behaviour of skew slabs. A total of four skew slabs were experimentally tested in the laboratory. All the slabs were identical in dimension except the reinforcement arrangements. Three types of reinforcement style were used. The reinforcing bars for three slabs were hooked at the ends except in the case of the fourth slab. The main bars for this slab ending at the free edges were welded to an extra bar provided and laid parallel to the two free edges of the slab. The load displacement behaviour of these slabs were carefully studied both numerically and experimentally to determine effective reinforcement scheme for skew slabs. Finite element layered Mindlin plate formulation was used to study the numerical response of these slabs. Keywords: Skew slabs, reinforcement layout, reinforced concrete, experimental study. 1. ntroduction Reinforced concrete skew slabs are used in bridges and building floor systems. The objective of this study is to determine the effect of different arrangement of steel reinforcement on the behaviour of reinforced concrete skew slabs. For two-way rectangular slabs, a typical reinforcement pattern is parallel to the edges i, e., supports. Depending on the geometry and boundary conditions, this may not be the best reinforcement orientation, especially so in case of skew slabs. For skew slabs, the sides are not orthogonal and so it is a matter of interest to study the effect of different types of reinforcement schemes to arrive at the best arrangement. An experimental investigation was undertaken by Desayi and Prabhakara [1] to predict the load-deflection behaviour of restrained reinforced concrete skew slabs. Non-linear analysis of reinforced concrete skew slab bridges has been carried out by Johnarry [2], Cope and Rao (3], E-Hafez (4]. They have also performed experimental investigations in order to provide experimental data and to validate the numerical formulations. But none of the researchers studied the effect of reinforcement layout on the load displacement behaviour of reinforced concrete skew slabs. n order to get an idea in this regard, an investigation was undertaken to study the performance of different types of reinforcement layout. Three types of reinforcement scheme were used in this study. n type-1, main reinforcement is parallel to the free edge and the transverse reinforcement is parallel to the supported edge. n type 2, main reinforcement is perpendicular to the supported edge and the transverse is parallel to the supported edge. n type-3, main reinforcement is parallel to the free edge and the transverse is perpendicular to the free edge. Three slabs were reinforced with these three types of reinforcement and all the bars were hooked at the ends. The fourth slab was reinforced with type-2 reinforcement but the main bars for this slab ending at the free edges were welded with additional two bars provided at the free edges. The reinforcement types are shown in Fig

3 Type 1 Type 2 Type 3 Fig.1 Types of Reinforcement 2. Slab Designation The test slabs were designated as S1-51, S2-52, S3-53 and S9-52W. All the slabs were identical in dimension except the reinforcement arrangements. The slab S1-51 was reinforced with type-1 reinforcement, slab S3-53 with type-3 and slab S2-52 and S9-52W with type-2. The reinforcing bars for slabs S1-51, S2-52 and S3-53 were hooked at the ends and for slab S9-52W, the main bars ending at the free edges were welded with two additional bars provided at the free edges. Table 1 gives the details of slab dimension, angle of skew, type of reinforcement and thickness of slab. Skew angle is measured between the free edge and the line nonmal to the support. A single point load was applied from top at the centre point of the slabs. All the slabs had simple supports on two opposite edges. Steel ratio in longitudinal direction was.86 percent and that in the transverse direction was.48 percent for all the slabs. Table 1 Details of Test Slabs with Designation Slab Slab Dimension Angle of Type of Type of Slab Designation (mm) Skew Loading Reinforcement Thickness (degree) (mm) Span Width, S One Point Type S One Point Type S One Point Type _.-.--._ S9-52W One Point Type 2 75 i l\lote: For all slabs, clear cover d' =15 mm Aspect Ratios, r =1.2 for all slabs 3. Constituent Materials The constituent materials used for the concrete were ordinary Portland cement, sand and crushed stone. The fineness modulus of sand was 2.84 and its specific gravity and unit weight were 2.61 and 14.4 kn/m 3 (89.8 b/ft 3 ) respectively. The stone that passed through 19.1 mm (3/4") sieve and retained on No.4 sieve was used. The unit weight and the specific gravity of stone chips were 15.7 kn/m 3 (97 b/ft 3 ) and 2.7 respectively. Defonmed bar reinforcement of 8 mm in diameter were used. A number of randomly taken samples were tested in a tensile testing machine following the recommendations of ASTM A [5]. The average yield strength y was 461 N/mm2 and ultimate strength was 75 N/mm

4 4. Fabrication of Test Slabs The concrete mix proportion used was 1: 2.1: 2.5 by weight of cement, sand and crushed stone. The reinforcing mesh was assembled and properly positioned so that 15-mm clear concrete cover was maintained.. The concrete was then placed in the formwork and compacted with a nozzle vibrator. The control cylinder specimens were cast in a similar way simultaneously with the slab. The test slab thus prepared was cured under wet burlaps. The formwork was removed after seven days. Curing of the slabs and the control specimens were continued for about 5 weeks after casting. 5. Experimental Setup The simple support system consisted of two -joists placed 12 mm apart on centre. The slab along the support line was held down with the help of clamps specially made to prevent uplifting at the acute angled corners. For single point loading, the load was spread over a square loading area of 5 mm sides using 12-mm thick steel plates. Underneath the steel plate a 15-mm thick hard rubber pad was glued. On the top surface of the steel plate a hemispherical groove was formed right at its centre. A high strength steel ball of 35-mm diameter was placed into the groove to apply load directly from the piston of the jack to the slabs through the ball. The loading arrangement is shown in Fig.2. Deflections were recorded at some selected points by means of electrical displacement transducers positioned at the bottom surface of the test slabs. 6. Test Procedure The test slab was placed on its supports. All the strain gauges were connected to the data logger through a scanner junction box. The deflection transducers were placed at the proper grid point locations. The test was started applying the load at equal increment of half ton. Deflection and strain gauge readings were recorded and stored in the data logger for every load increment. The load initiating the first visible crack was recorded. The load increment continued and the corresponding data were recorded until the failure load was reached. The ultimate stage was assumed to have been reached when the deflection readings were found to increase without any change in the applied load. At the end of every slab test, the accompanying cylinders were tested for compressive and tensile strengths respectively. Fig. 2 Loading Arrangement 273

5 7. Test Results and Discussion The data recorded for each of the test slabs during experiment were (i) Deflections at some selected points, (ii) Cracking loads and (iii) Failure loads. The experimental records of deformations against progressive loading for all the slabs were used to plot the load versus deflection curves. The load at the instant of sighting the first crack tenrned as cracking load and the ultimate load at failure were recorded for each test slab and are furnished in Table 2. The representative compressive and tensile strength of concrete obtained from cylinder crushing strengths and the split cylinder tensile strengths respectively are provided in the same table. As the load increased, the first crack for all the test slabs was found to fonrn on the bottom surface under the load. The cracks were more or less confined in the bottom mid-span area and propagated gradually somewhat parallel to the support lines towards the free edges. The crack directions were observed to be largely independent of the reinforcement directions. The load-deflection behaviour at the centre point for all the slabs is shown in Fig.3. The finite element model developed by Nizamud-doulah [6] and also described elsewhere [7] was used to determine the numerical response of these slabs having three types of reinforcement. The experimental and the numerical results are compared in Figs.4, Sand 6. Table 2 Comparison of Cracking and Failure Loads of the Test Slabs r ,-.- Slab Concrete Strength Cracking Load Per Failure Load Pu Designation N/mm2 (kn) (kn) i Compressive Tensile Experimental Numerical Experimental S1-S S S2-S2 3.S _._----.., S3-S3 29.9, S S9-S2W \ 47.4 Numerical The four test slabs were ide ntical in all respect except that they differed in the reinforcement arrangements only. The experimental' results in the form of load-deflection response for these slabs are shown in Fig.3. t can be seen from the figure and the Table 2 that the failure load for the slabs S1-S1, S2-S2 and S3-S3 are fairly close. Slab S3-S3 with type-3 reinforcement has slightly higher ultimate load compared to the other two. This observation indicates that Type-1, Type-2 and Type-3 reinforcements with conventional hooks at the ends are perhaps equally effective and comparable from ultimate load consideration. n spite of identical dimension and reinforcement pattern, the failure load for slab S9-S2W is about 27 percent higher than the failure load of slab S2-S2. t is also interesting to see from Fig.6 that the expe~imental behaviour of the slab S9-S2W correlates better with the numerical response having Type-2 reinforcement compared to the experimental response of slab S2-S2. The main bars ending at the free edges for the slab S2-S2 were merely hooked while the main bars for slab S9-S2W were welded with an extra bar of same size provided along the edge. The hooked bars ending at the free edges of slab S2-S2 could not provide enough bond length to develop the required stress level. This has resulted in premature failure. Welding has provided sufficient bonding of the main steel of slab S9-S2W compared to slab S2-52 and hence the improved performance. The lack of bonding of the main bars of type-2 reinforcement at the free edges is also demonstrated comparing the load-deflection response of test slabs S2-S2 and S9-S2W. Response of the former (S2-S2) is flexible compared to the latter. Type-1 reinforcement arrangement is most widely used layout in practice and is a natural choice over the other two. This is because it is simple and can be fabricated easily. The test slab reinforced with type-2 reinforcement layout without any extra care to prevent slip of the main bars at the free edges perfonrns as good as the type-1 in respect of ultimate load carrying capacity. Type-3 reinforcement 274

6 Z 3.:Jt:. Z.:Jt:. 3 " C\l S1-51 -=-S (l., S <J- S9-52W " C\l EXP -=-NUM Deflection, mm Deflection, mm Fig.3 Load-Deflection Curves for Slabs at the Centre (Experimental) FigA Load-Deflection Curves for Slab S1-51 at the Centre 5 4 Z 3.:Jt:. " C\l EXP --NUM Deflection, mm 5 4 Z.:Jt:. 3 " C\l 2 1 o 1 - EXP-S2-52 -NUM U--Q' M EXP-S9-52W Deflection, mm 3 Fig.5 Load-Deflection Curves for Slab S3-53 at the Centre Fig.6 Load-Deflection Curves for Reinforcement Type-2 layout appears to be more effective compared to type-1 and type-2 without welding. However, the type-2 layout with provision for sufficient end anchorage of the main bars ending at the free edges is perhaps the most effective steel arrangement for skew slabs. The end anchorage may easily be achieved by welding the main bars ending at the free edge to another reinforcing bar or flat bar. Thus, if reinforcement pattern-2 (i.e., main reinforcement perpendicular to the support) is selected then sufficient end anchorage should be provided along the free edge lines in order to get the best result. 275

7 8. Conclusion From the present study of skew slabs with point loading, the following conclusions may be drawn. (i) (ii) (iii) (iv) Type-1 reinforcement layout is the most common and widely used steel layout in practice even for skew slabs. ts preference over the other two types is more because of its ease of fabrication than performance. Any of the three possible reinforcement layouts (including type-2 without proper anchorage of main steel at free edge lines) performs almost equally well for skew slabs from ultimate strength consideration. However, slab with type-3 reinforcement fails at slightly higher load and may be considered to have an edge over the other two. Type-2 reinforcement layout with the main bar tips welded to an extra bar at the free edges is perhaps the most effective and desirable reinforcement layout from both serviceability (load-deflection behavior) and ultimate strength requirement Reinforcement layout type-2, should always be used with provision for sufficient end anchorage of the main bars ending at the free edges to get the best performance of skew slabs subject to point loading system. An extra bar at the free edges, welded to the main bars ensures sufficient anchorage. References [1] Desayi, P. And Prabhakara, A., "Load Deflection Behaviour of Restrained Reinforced Concrete Skew Slabs", Journal of Structural Div., Proceedings of the ASCE, May 1981, Vol. 17, No. ST5, pp [2] Johnarry, T., "Elastic-Plastic Analysis of Concrete Structures Using Finite Elements", Ph.D. Thesis, University of Strathclyde, May [3] Cope, R. J. and Rao, P. V., "Nonlinear Response of Reinforced Concrete Skewed Slab Bridges", University of Liverpool, 1 Research Report, [4] E-Hafez, L.MA, "Direct Design of Reinforced Concrete Skew Slabs", Ph.D. Thesis, University of Glasgow, [5] ASTM A 37, "Methods and Definitions for Mechanical Testing of Steel Products", American Society of Testing Materials, Vo.1.4, Section-1, Philadelphia, 1988, pp [6] Nizamud-doulah, S. M., "Nonlinear Analysis of Reinforced Concrete Skew Slabs", Ph.D. Thesis, Dept. of Civil Engineering, Bangladesh University of Engineering & Technology, Dhaka, June 2. [7] Nizamud-doulah, S. M. and Kabir, A. "Analysis of Reinforced Concrete Skew Slabs Using Layered Mindlin Plate Element", Journal of the nstitution of Engineers, ndia, Civil Engineering Division, Vol. 78, November,