Chapter # 7 Design of Two-Way Slab Reinforced Concrete Slabs Reinforced Concrete Slabs are large flat plates that are supported by reinforced concrete beams, walls, or columns, by masonry walls, by structural steel beams or columns, or by the ground. If a slab is supported on two opposite sides only, it is referred to as a One-Way Slab because the bending is in one direction only – that is, perpendicular to the supported edges If a slab is supported by beams on all four edges, it is referred to as Two-Way Slab because the bending is in both direction Dr. Roz-Ud-Din Nassar CIEN 422 2 Reinforced Concrete Slabs A one-way slab shows a cylindrical failure, while a two-way slab fails in bowl action. In a slab, if the ratio of the long side to the short side is 2 or more than 2, it is referred to as a one-way slab, otherwise it is a called a two-way slab A Two-way slab carries the load in the two perpendicular directions The main flexural reinforcement is therefore placed in the two perpendicular directions Dr. Roz-Ud-Din Nassar CIEN 422 3 Reinforced Concrete Slabs Dr. Roz-Ud-Din Nassar CIEN 422 4 The Two Way Slabs ๏ฎ In two way slab, the load is transmitted in two directions hence the system is called two way slab Dr. Roz-Ud-Din Nassar CIEN 422 5 Types of Two-way Slabs Flat Plate Flat Slab Dr. Roz-Ud-Din Nassar CIEN 422 6 Types of Two-way Slabs Slab with Beams Waffle Slab Dr. Roz-Ud-Din Nassar CIEN 422 7 Slab Curvatures and Moments Dr. Roz-Ud-Din Nassar CIEN 422 8 Statical Equilibrium of Two-Way Slabs Consider a floor made up of simply supported planks supported by simply supported beams. Dr. Roz-Ud-Din Nassar CIEN 422 9 Statical Equilibrium of Two-Way Slabs The floor carries a load of w lb/ft2. The moment per foot of width in the planks at section A-A is : ๐ค๐12 ๐= ๐๐ก − ๐๐๐๐ /๐๐ก 8 The total moment in the entire width of the floor is: ๐ = ๐ค๐2 ๐ค๐12 ๐๐ก − ๐๐๐๐ 8 Which is the familiar equation for the maximum moment in a simply supported floor of width ๐๐ and span ๐๐ Dr. Roz-Ud-Din Nassar CIEN 422 10 Statical Equilibrium of Two-Way Slabs Moments in a Plank-and-Beam Floor Dr. Roz-Ud-Din Nassar CIEN 422 11 Statical Equilibrium of Two-Way Slabs The planks apply following uniform load on each beam: ๐ค๐1 /๐๐ก 2 The moment at section B-B in one beam is thus: ๐1๐ ๐ค๐1 ๐22 = ๐๐ก − ๐๐๐๐ 2 8 The total moment in both beams is: ๐ = ๐ค๐1 Dr. Roz-Ud-Din Nassar ๐22 ๐๐ก − ๐๐๐๐ 8 CIEN 422 12 Transfer of Moments in Two-Way Slab It is important to note that full load was transferred East and West by the planks, causing a moment equivalent to ๐๐๐เต ๐ in the planks, and then was transferred North and South by the beams, causing a similar moment in the beams Same thing happens in the two-way slab. The total moments required along sections A-A and B-B respectively, are: ๐ = ๐ค๐2 Dr. Roz-Ud-Din Nassar ๐12 8 and CIEN 422 ๐ = ๐ค๐1 ๐22 8 13 Transfer of Moments in Two-Way Slab Moments in a Two-Way Slab Dr. Roz-Ud-Din Nassar CIEN 422 14 Transfer of Moments in Two-Way Slab In this case again, the full load was transferred East and West, and then the full load was transferred North and South, this time by the slab in both cases Such mechanism should be true regardless of whether the structure has one-way slabs and beams, two-way slabs or some other system Dr. Roz-Ud-Din Nassar CIEN 422 15 Effect of Beam Stiffness on Moments in Slabs CIEN 422 16 Design Methods of Two Way Slabs • Direct Design Method • Equivalent Frame Method • Elastic Yield Line Theory Dr. Roz-Ud-Din Nassar CIEN 422 17 Two-Way Slab: Direct Design Method Steps in Slab Design 1) Choose the layout and type of slab to be used. The choice of type of slab is strongly affected by architectural and construction calculations 2) Choose the slab thickness. Generally, the slab thickness is chosen to prevent excessive deflection in service. Equally important is to satisfy the shear requirements at both exterior and interior columns 3) Choose the method for computing the design moments (direct design OR equivalent frame) 4) Compute the positive and negative moments in the slab Dr. Roz-Ud-Din Nassar CIEN 422 18 Two-Way Slab: Direct Design Method Steps in Slab Design 5) Calculate the distribution of moments across the width of the slab. The lateral distribution of moments within a panel depends on the geometry of the slab and the stiffness of the beams (if any) 6) If there are beam, a portion of the moment must be assigned to the beams 7) Reinforcement is designed for the moments from steps 5 and 6 8) The shear strengths at the columns are checked Dr. Roz-Ud-Din Nassar CIEN 422 19 Limitations on the Use of Direct Design Method The limitations given by ACI Code Section 13.6.1 are as under: 1) There must be a minimum of three continuous spans in each direction. Thus a nine-panel structure (3 by 3) is the smallest that can be considered 2) Rectangular panels must have a long-span / short-span ratio not greater than 2 3) Successive span lengths in each direction shall not differ by more than one-third of the longer span 4) Columns may be offset from the basic rectangular grid of the building by up to 0.1 times the span parallel to the offset Dr. Roz-Ud-Din Nassar CIEN 422 20 Limitations on the Use of Direct Design Method 5) All the loads must be due to gravity only. The direct design method cannot be used for unbraced laterally loaded frames, foundations mats, or prestressed slabs 6) The service live load shall not exceed two times the service dead load 7) For a panel with beams between supports on all sides the relative stiffness of the beams in the two perpendicular directions given by: ๐ผ1 ๐22 ๐ผ2 ๐12 (where ๐ผ is beam-to-slab stiffness ratio) The above ratio shall not be less than 0.2 or greater than 5 Dr. Roz-Ud-Din Nassar CIEN 422 21 Beam-to-Slab Stiffness Ratio ๐ผ Slabs are frequently built with beams spanning from column to column around the perimeter of the building. These beams act to stiffen the edge of the slab and help to reduce the reduce the deflections of the exterior panels of the slab. In the ACI code, the effects of beam stiffness on deflections and the distribution of moments are expressed as a function of ๐ผ, defined as the flexural stiffness 4๐ธ๐ผ Τ๐, of the beam divided by the flexural stiffness of a width of slab bounded laterally by the centerlines of the adjacent panels on each side of the beam: ๐๐ฌ๐๐ ๐ฐ๐ ๐ ๐ถ๐ = ๐๐ฌ๐๐ ๐ฐ๐ ๐ Dr. Roz-Ud-Din Nassar CIEN 422 22 Beam-to-Slab Stiffness Ratio ๐ผ Since the lengths "๐” of the beam and slab are equal, the equation reduces to: ๐๐ฌ๐๐ ๐ฐ๐ ๐ถ๐ = ๐๐ฌ๐๐ ๐ฐ๐ Where ๐ธ๐๐ ๐๐๐ ๐ธ๐๐ are the moduli of elasticity of beam concrete and slab concrete, respectively, and ๐ผ๐ ๐๐๐ ๐ผ๐ are the moments of inertia of uncracked beams and slabs. The sections considered in computing ๐ผ๐ ๐๐๐ ๐ผ๐ are shown in figures on next slide. Dr. Roz-Ud-Din Nassar CIEN 422 23 Beam-to-Slab Stiffness Ratio ๐ผ ๏ฎ Definition of terms Ecb = Modulus of elasticity of beam concrete Ecs = Modulus of elasticity of slab concrete Ib = Moment of inertia of uncracked beam section Is = Moment of inertia of uncracked slab section Dr. Roz-Ud-Din Nassar CIEN 422 24 Beam-to-Slab Stiffness Ratio ๐ผ๐ Dr. Roz-Ud-Din Nassar CIEN 422 25 Beam-to-Slab Stiffness Ratio ๐ผ๐ Cross Section of Beams as Defined in ACI Code 13.2.4 Dr. Roz-Ud-Din Nassar CIEN 422 26 Problem # 1 An 8-in. thick slab is provided with an edge beam that has a total depth of 16 in. and a width of 12 in., as shown in figure. The slab and the beam are cast monolithically and have the same concrete strength and the same Ec. Compute “๐ถ๐ ” in this case. Dr. Roz-Ud-Din Nassar CIEN 422 27 Problem # 1 20 ๐๐. Find the centroid of the L-section 8 ๐๐. 8 ๐๐. 12 ๐๐. Dr. Roz-Ud-Din Nassar CIEN 422 8 ๐๐. 28 Problem # 1 - Solution Solution ๐๐ฌ๐๐ ๐ฐ๐ ๐ถ๐ = ๐๐ฌ๐๐ ๐ฐ๐ Since: ′ ๐๐๐ ′ = ๐๐๐ and ๐ธ๐๐ = ๐ธ๐๐ ๐ฐ๐ → ๐ถ๐ = ๐ฐ๐ Dr. Roz-Ud-Din Nassar CIEN 422 29 Problem # 1 - Solution 3 163 8 ๐ผ๐ = 12 × + 12 × 16 × 12 + 8 × + 8 × 8 × 32 12 12 ๐ผ๐ = 5205 ๐๐.4 83 ๐ผ๐ = 126 × = 5376 ๐๐.4 12 ๐ผ๐ = 5376 ๐๐.4 Dr. Roz-Ud-Din Nassar CIEN 422 30 Problem # 1 - Solution ๐๐๐๐ ๐ถ๐ = = ๐. ๐๐๐ ๐๐๐๐ Alternatively: ๐ฌ๐๐ ๐ ๐ ๐ถ๐ = ๐ฌ๐๐ ๐ ๐ ๐ ๐ From graphs: ๐ โ Dr. Roz-Ud-Din Nassar = 2 ๐๐๐ ๐ โ = 12 8 = 1.5, we get ๐ = 1.27 CIEN 422 31 Problem # 1 - Solution ๐ฌ๐๐ ๐ ๐ ๐ถ๐ = ๐ฌ๐๐ ๐ ๐ 12 16 ๐ถ๐ = 126 8 Dr. Roz-Ud-Din Nassar ๐ ๐ 3 CIEN 422 × 1.27 = 0.968 32 Division of Slab into Frames for Design For design, the slab is considered to be series of frames in the two directions These frames extend to the middle of the panels on each side of the column lines In each span of each of the frames, it is necessary to compute the total statical moment, Mo ๐ค๐ข ๐2 ๐๐2 ๐๐ = 8 Dr. Roz-Ud-Din Nassar CIEN 422 33 Division of Slab into Frames for Design ๐ค๐ข ๐2 ๐๐2 ๐๐ = 8 ๐ค๐ข = factored load per unit area ๐2 = transverse width of the strip ๐๐ = clear span between the columns In computing ln, circular columns or column capitals of diameter dc are replaced by equivalent square columns with side lengths 0.886dc. Dr. Roz-Ud-Din Nassar CIEN 422 34 Division of Slab into Frames for Design Dr. Roz-Ud-Din Nassar CIEN 422 35 Division of Slab into Frames for Design Dr. Roz-Ud-Din Nassar CIEN 422 36 Problem # 2 Compute the statical moment in the slab panels shown in figure below. Slab has a thickness of 8 in. and supports a live load of 100 psf. Dr. Roz-Ud-Din Nassar CIEN 422 37 Problem # 2 - Solution Solution: 1. Compute the factored uniform loads 8 ๐ค๐ข = 1.2 × 0.150 ๐๐ ๐ + 1.6 0.100 ๐๐ ๐ 12 ๐ค๐ข = 0.280 ๐๐ ๐ Note: Live load reduction must be carried out if allowed by the code Dr. Roz-Ud-Din Nassar CIEN 422 38 Problem # 2 - Solution 2. Consider panel A spanning from column 1 to column 2 ๐ค๐ข ๐2 ๐๐2 ๐๐ = 8 ๐๐ = ๐๐๐๐๐ ๐ ๐๐๐ ๐๐ ๐ ๐๐๐ ๐๐๐๐๐ 1 20 1 24 ๐๐ = 22 ๐๐ก − ๐๐ก − ๐๐ก 2 12 2 12 ๐๐ = 20.17 ๐๐ก Dr. Roz-Ud-Din Nassar CIEN 422 39 Problem # 2 - Solution ๐2 = ๐ค๐๐๐กโ ๐๐ ๐ ๐๐๐ ๐๐๐๐๐ 21 20 ๐2 = ๐๐ก + ๐๐ก 2 2 Hence: → ๐2 = 20.50 ๐๐ก ๐ค๐ข ๐2 ๐๐2 ๐๐ = 8 0.280 ๐๐ ๐ × 20.5 ๐๐ก 20.17 ๐๐ก → ๐๐ = 8 2 = 292 ๐๐ก − ๐๐๐๐ This moment will be used to design the reinforcement running parallel to line 1-2 in this panel Dr. Roz-Ud-Din Nassar CIEN 422 40 Problem # 2 - Solution 3. Consider panel B spanning from column 1 to column 4 Dr. Roz-Ud-Din Nassar CIEN 422 41 Problem # 2 - Solution Section through slab panel B For calculation of ๐๐ the circular supports are replaced by equivalent square columns having side length ๐1 Dr. Roz-Ud-Din Nassar = ๐๐ CIEN 422 ๐ 2 ๐๐ 0.886๐๐ 42 Problem # 2 - Solution 3. Consider panel B spanning from column 1 to column 4 ๐๐ = ๐๐๐๐๐ ๐ ๐๐๐ ๐๐ ๐ ๐๐๐ ๐๐๐๐๐ 1 12 1 24 ๐๐ = 20๐๐ก − ๐๐ก − 0.886 × ๐๐ก 2 12 2 12 ๐๐ = 18.61 ๐๐ก ๐2 = 19 22 ๐๐ก + ๐๐ก 2 2 → ๐2 = 20.50 ๐๐ก Dr. Roz-Ud-Din Nassar CIEN 422 43 Problem # 2 - Solution ๐ค๐ข ๐2 ๐๐2 ๐๐ = 8 0.280 ๐๐ ๐ × 20.5 ๐๐ก × 18.16 ๐๐ก → ๐๐ = 8 2 = 248 ๐๐ก − ๐๐๐๐ This moment will be used to design the reinforcement running parallel to line 1-4 in this panel Dr. Roz-Ud-Din Nassar CIEN 422 44 Definition of Column Strip and Middle Strip The moments vary continuously across the width of the slab panel To add in steel placement, the design moments are averaged over the width of column strips over the columns and middle strips between the column strips as shown in figures on next slide The width of these strips are defined in ACI Section 13.2.1 and 13.2.2 and are illustrated in figures on slides 46 & 47 Dr. Roz-Ud-Din Nassar CIEN 422 45 Definition of Column Strip and Middle Strip Middle strips Column strips Dr. Roz-Ud-Din Nassar CIEN 422 46 Definition of Column Strip and Middle Strip Column strips Middle strips Column strips Dr. Roz-Ud-Din Nassar CIEN 422 47 Column Strip and Middle Strip Four edges fixed and non-supported on non-deflecting beams Dr. Roz-Ud-Din Nassar Four edges fixed on flexible beams CIEN 422 48 Positive and Negative Moments in Interior Span In the direct design method, the total factored moment Mo is divided into positive and negative factored moments according to rules given in ACI Section 13.6.3 In interior spans, 65 percent of Mo is assigned to the negativemoment region and 35 percent to the positive-moment regions The exterior end of an exterior span has considerable less fixity than the end at the interior support. The division of Mo in an end span into positive and negative-moment regions is given in Table on slide 51 Dr. Roz-Ud-Din Nassar CIEN 422 49 Positive and Negative Moments in Interior Span Exterior Span Interior Span Assignment of positive and negative moment regions Dr. Roz-Ud-Din Nassar CIEN 422 50 Distribution of Mo in an Exterior Span Dr. Roz-Ud-Din Nassar CIEN 422 51 Distribution of Moments between Column Strips and Middle Strips ACI code 13.6.4 defines the fraction of negative and positive moments assigned to column strips The remaining amount of negative and positive moments is assigned to adjacent half-middle strips The division is a function of ๐ผ๐1 × ๐2 Τ๐1 which depend upon the aspect ratio of the panel ๐2 Τ๐1 and the relative stiffness ๐ผ๐1 of the beams (if any) spanning parallel to and within the column strip Dr. Roz-Ud-Din Nassar CIEN 422 52 Percentage Distribution of Interior Negative Factored Moment to Column Strip Dr. Roz-Ud-Din Nassar CIEN 422 53 Percentage Distribution of Midspan Positive Factored Moment to Column Strip Dr. Roz-Ud-Din Nassar CIEN 422 54 Percentage Distribution of Exterior Negative Factored Moment to Column Strip Dr. Roz-Ud-Din Nassar CIEN 422 55 Torsional Stiffness Ratio ๐ฝ๐ก ๐ธ๐๐ ๐ถ ๐ฝ๐ก = ๐ธ๐๐ ๐ผ๐ ๐ถ=เท ๐ฅ ๐ฅ 3๐ฆ 1 − 0.63 ๐ฆ 3 Where: ๐ธ๐๐ = Modulus of Elasticity of beam concrete ๐ธ๐๐ = Modulus of Elasticity of slab concrete ๐ผ๐ = Moment of inertia of uncracked slab Dr. Roz-Ud-Din Nassar CIEN 422 56 Percentage Distribution of Exterior Negative Factored Moment to Column Strip Division of edge members for calculation of torsion constant “C” Dr. Roz-Ud-Din Nassar CIEN 422 57 Minimum Thickness of Two-way Slab ACI Section 9.5.3 defines minimum thicknesses that generally are sufficient to limit slab deflections to acceptable values Thinner slabs can be used if it can be shown that the computed slab deflections will not be excessive For computation of thickness, slabs have been divided into two categories: • Slabs without Beams between Interior Columns • Slabs with Beams between Interior Supports Dr. Roz-Ud-Din Nassar CIEN 422 58 Thickness of Slabs Without Beams Between Interior Columns Dr. Roz-Ud-Din Nassar CIEN 422 59 Thickness of Slabs With Beams Between Interior Supports For slabs with beams between interior supports, ACI Code Section 9.5.3.3 gives the following minimum thicknesses: A) For ๐ผ๐๐ ≤ 0.2 , the minimum thickness in Table 13-1 shall apply B) For 0.2 < ๐ผ๐๐ < 2.0 the thickness shall not be less than: โ= ๐๐ 0.8 + ๐๐ฆ Τ20,000 36 + 5๐ฝ ๐ผ๐๐ − 0.2 but not less than 5 in. C) For ๐ผ๐๐ > 2.0 the thickness shall not be less than: ๐๐ 0.8 + ๐๐ฆ Τ20,000 โ= 36 + 9๐ฝ Dr. Roz-Ud-Din Nassar CIEN 422 but not less than 3.5 in. 60 Thickness of Slabs With Beams Between Interior Supports D) At discontinuous edges, either an edge beam with a stiffness ratio ๐ผ๐ not less than 0.8 shall be provided or slab thickness shall be increased by at least 10 percent in the edge panel In A, B, C, and D above: โ = overall thickness ๐๐ = clear span of the slab panel under consideration, measured in the longer direction ๐ผ๐๐ = the average of the values of ๐ผ๐ for the four sides of the panel ๐ฝ = longer clear span/shorter clear span of the panel Dr. Roz-Ud-Din Nassar CIEN 422 61 Problem # 3 Figure on slide # 63 shows an interior panel of the flatplate floor in an apartment building. The slab thickness is 5.5 in. The slab supports a design live load of 50 psf and superimposed dead load of 25 psf for partitions. The columns and slab have the same strength of concrete. The story height is 9 ft. Compute the column-strip and middle-strip moments in the shown direction of the panel. Dr. Roz-Ud-Din Nassar CIEN 422 62 Problem # 3 Dr. Roz-Ud-Din Nassar CIEN 422 63 Problem # 3 - Solution Solution: 1) Compute the factored loads 5.5 ๐๐ข = 1.2 × 150 + 25 + 1.6 50 = 193 ๐๐ ๐ 12 Note: Live load reduction must be carried out if local building code allows it (Use KLL = 1.0 for two-way slab panels) 2) Compute moments in the shorter span of the slab a) Compute ๐๐ and ๐๐ and divide the slab into column and middle strips Dr. Roz-Ud-Din Nassar CIEN 422 64 Problem # 3 - Solution 10 ๐๐ = 13.17 ๐๐ก − = 12.33 ๐๐ก 12 ๐2 = 14.15 ๐๐ก The column strip extends the smaller of ๐2 Τ4 or ๐1 Τ4 on each side of the column centerline, as shown in figure. Thus column strip extends 13.17Τ4 ๐๐ก = 3.29 ๐๐ก on each side of the column centerline. Dr. Roz-Ud-Din Nassar CIEN 422 65 Problem # 3 - Solution The total width of the column strip is 6.58 ft Each half middle strip extends from the edge of the column strip to the centerline of the panel The total width of the two halfmiddle strips is 14.5 – 6.58 = 7.92 ft Dr. Roz-Ud-Din Nassar CIEN 422 66 Problem # 3 - Solution b) Compute ๐๐ ๐๐ข ๐2 ๐๐2 ๐๐ = 8 0.193 × 14.5 × 12.332 → ๐๐ = = 53.2 ๐๐๐ − ๐๐ก 8 c) Divide MO into negative and positive moments From ACI code Section 8.10.4.1 ๐๐๐๐๐ก๐๐ฃ๐ ๐๐๐๐๐๐ก = −0.65๐๐ = −0.65 × 53.2 = −34.6 ๐๐๐ − ๐๐ก ๐๐๐ ๐๐ก๐๐ฃ๐ ๐๐๐๐๐๐ก = 0.35๐๐ = 0.35 × 53.2 = 18.6 ๐๐๐ − ๐๐ก Dr. Roz-Ud-Din Nassar CIEN 422 67 Problem # 3 - Solution d) Divide the moments between the column and middle strips From Table 13-3 (slide # 53) for ๐ผ๐1 ๐2 Τ๐1 = 0 (๐ผ๐1 = 0 because there are no beams between columns A and B in this panel) Column − strip negative moment = 0.75 × −34.6 = −26.0 ๐๐๐ − ๐๐ก Middle − strip negative moment = 0.25 × −34.6 = −8.66 ๐๐๐ − ๐๐ก Dr. Roz-Ud-Din Nassar CIEN 422 68 Problem # 3 - Solution Process of calculating slab moments Dr. Roz-Ud-Din Nassar CIEN 422 69 Problem # 3 - Solution Division of MO into positive and negative moments Total moments in column and middle strips Dr. Roz-Ud-Din Nassar CIEN 422 70 Problem # 3 - Solution Division of MO into positive and negative moments Total moments in column and middle strips Dr. Roz-Ud-Din Nassar Mapping of moments with strips CIEN 422 71 Problem # 3 - Solution Calculation of moments Dr. Roz-Ud-Din Nassar CIEN 422 72