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Unformatted text preview: Module 8 Reinforced Concrete Slabs Version 2 CE IIT, Kharagpur Lesson 19 Twoway Slabs Version 2 CE IIT, Kharagpur Instructional Objectives: At the end of this lesson, the student should be able to: • determine the shear force of twoway slabs subjected to uniformly distributed loads, • state the two types of twoway slabs mentioning the differences between them, • determine the preliminary depth of twoway slabs of different support conditions from the span to effective depth ratios as given in IS 456, • explain the provisions of torsion reinforcing bars at two types of corners of a restrained twoway slab, • design the two types of twoway slabs applying the different methods explained in this lesson and draw the detailing of reinforcing bars. 8.19.1 Introduction Lesson 18 explains the various types of slabs with different support conditions, plan forms, horizontal/inclined etc. Moreover, sec. 8.18.2 of Lesson 18 illustrates the sharing of uniformly distributed loads to the supporting beams of both one and twoway slabs including the profiles of deflection (Figs.8.18.4a and b). It is, thus, understood that twoway slabs span in both directions having the aspect ratio of l y /l x up to 2, considering l x as the shorter span. This lesson presents the different aspects of analysis and design of twoway slabs. Many of the stipulations of IS 456 are the same as those of oneway slabs. While mentioning the common stipulations with their respective section in Lesson 18, this lesson presents other relevant requirements regarding the analysis, design and detailing of twoway slabs. Numerical problems are also solved to illustrate the applications of the theory in the design of twoway slabs. 8.19.2 Twoway Slabs Twoway slabs subjected mostly to uniformly distributed loads resist them primarily by bending about both the axis. However, as in the oneway slab, the depth of the twoway slabs should also be checked for the shear stresses to avoid any reinforcement for shear. Moreover, these slabs should have sufficient depth for the control deflection. Thus, strength and deflection are the requirements of design of twoway slabs. Version 2 CE IIT, Kharagpur 8.19.3 Design Shear Strength of Concrete Design shear strength of concrete in twoway slabs is to be determined incorporating the multiplying factor k from Table 8.1 of Lesson 18 in the same manner as discussed in sec. 8.18.3 of Lesson 18. 8.19.4 Structural Analysis 8.19.4.1 Computation of shear force Shear forces are computed following the procedure stated below with reference to Fig.8.19.1. The twoway slab of Fig. 8.19.1 is divided into two trapezoidal and two triangular zones by drawing lines from each corner at an angle of 45 o . The loads of triangular segment A will be transferred to beam 12 and the same of trapezoidal segment B will be beam 23. The shear forces per unit width of the strips aa and bb are highest at the ends of strips. Moreover, the length of half the strip bb is equal to the length of the strip aa. Thus, the shear forces in both strips strip bb is equal to the length of the strip aa....
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 Summer '01
 Prof.J.N.Bandopadhyay

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