the maximum spacing between two parallel main reinforcing bars in case of RCC Slab shall be 3d or 300 mm or whichever is less. Therefore, approximate analyses are generally used. Two-way action occurs when a slab is supported on all four sides. For example, for a one-way solid slab with #5 temperature-shrinkage bars, high chairs are used at 4 feet on center; for #4 bars, high chairs would be placed 3 feet on center. While a beam is generically a one-dimensional element, a plate is a two-dimensional element. He is a member of the Nigerian Society of Engineers. The minimum cover for bond should not be less than the bar diameter. There's no generally-accepted rule of thumb as to how to space rebar in a concrete slab. (b) Minimum tension steelThe main moment steel spans between supports and over the interior supports of a continuous slab. It possesses two free (unsupported) and sensibly parallel edges. Hence no further checks for shear will be done on the slab. A The table below gives bar spacing data in the form of areas of steel per metre width for various bar diameters and spacings. Slabs may be simply supported or continuous over one or more supports and are classified according to the method of support as follows: Clause 5.3.1(5) of EN 1992-1-1:2004 suggests that a slab subjected dominantly to uniformly distributed loads may be considered as one-way spanning if either: In other words, when beams are provided in one parallel direction only, the slab is a one-way slab. For indoor exposure, 1.5 inch is typical for beams and columns, 0.75 inch is typical for slabs, and for concrete cast against soil, 3 inch minimum is required. For two way slabs, the precise amount and distribution of the load taken by each support, and consequently the magnitude of the bending moments on the slab, are not easily calculated if assumptions resembling real conditions are made. of Bars; Compute the Total wt of steel required for slab reinforcement. For a patio you can use a grid spacing of 24 inches. Table 1: Area of reinforcement per metre width of spacing. These beams are not fixed together with a slab as are superstructure beams; consequently concrete has to be poured down the beam’s narrow top opening and reach the bottom of the beam. slabs are plate elements used to form the floors of buildings. In clause 5.1.3 of Eurocode 2, the following two loading arrangements are recommended for buildings. Other considerations It is further stated that in monolithic construction, where partial fixity occurs along an edge of a slab but is not taken into account, the top reinforcement should be capable of resisting at least 25 percent of the maximum moment in the adjacent span and this reinforcement should extend at least 0.2 times the length of the adjacent span measured from the face of the support. With some limitations, reinforcement for slabs can be sized using the Subgrade Drag Theory in order to increase the spacing of control or construction joints. For a slab of a given shape and support conditions, the distribution of shear forces, bending, and twisting moments in the slab due to externally applied loads cannot be determined easily. Disable spacing conditions. The effective span of a simply supported slab shall be taken as the lesser of … For deemed to satisfy basic span/effective depth (limiting to depth/250);Actual L/d of the slab must be ≤ Limiting L/d × βsThe limiting basic span/ effective depth ratio is given by;L/d = K [11 + 1.5√(fck)ρ0/ρ + 3.2√(fck) (ρ0/ρ – 1)1.5] if ρ ≤ ρ0L/d = K [11 + 1.5√(fck) ρ0/(ρ – ρ’) + 1/12 √(fck) (ρ0/ρ)0.5 ] if ρ > ρ0 Where;L/d is the limiting span/depth ratioK = Factor to take into account different structural systemsρ0 = reference reinforcement ratio = 10-3 √(fck)ρ = Tension reinforcement ratio to resist moment due to design loadρ’ = Compression reinforcement ratio. One-way slabs may be simply supported or continuous. the reinforcement and concrete, and finishing and curing the slab. Two-way spanning slabs; Cantilever slabs; In general, there are similar requirements for one-way, and two ways slab when slab reinforcement detail drawings are prepared. The value of K depends on the structural configuration of the member and relates the basic span/depth ratio of reinforced concrete members. Cover of concrete over/under the reinforcement must be provided to protect the steel from corrosion. This is actually an expensive operation (see cost of fixing reinforcement in Nigeria). Rebar is often installed in a grid pattern, and to estimate the amount needed it will be important to first identify the size and spacing is appropriate for your project. The maximum diameter of bar used in slab should not exceed 1/8 of the total thickness of slab. The minimum area of main reinforcement has to satisfy clause 9.2.1.1(1) of Eurocode 2; As,min = (0.26fctm/fyk)btd but not less than 0.0013bdwhere bt = width (for slab design 1000 mm), d = effective depth. To check the shear capacity of slabs, the shear stress (vEd = VEd/bd) must be checked against the shear capacity of an unreinforced section (VRc,d). A rectangular reinforced concrete slab is simply- supported on two masonry walls 250 mm thick and 3.75 m apart. The installation of the rebar is critical for the success of the overall project. Furthermore, if the longer side of a slab panel exceeds twice the shorter side, the slab is generally designed as a one-way slab, but that does not mean that the slab is transmitting load in one direction only. the maximum spacing between two secondary parallel bars in case of RCC Slab shall be 5d or 450 mm or whichever is less. The method applicable in any particular case depends on the shape of the slab panel, the conditions of restraint at the supports and the type of load. the maximum spacing between two secondary parallel bars in case of RCC Slab shall be 5d or 450 mm or whichever is less. Total Length Total Width Bar Spacing Exact Spacing Equal Spacing … Let’s take an example the below two-way slab diagram Main bars … If ln is the clear span (distance between faces of supports), the effective span leff is given by; One-way slabs should be designed to resist the most unfavourable arrangement of loads. Spacing of bar supports depends upon the size of the reinforcing bar being supported. Above and below: Properly placed/supported reinforcement will result in properly located reinforcement in the slab. Excessive deflection of slabs can cause cracking to partitions and finishes. Let’s discussed the design code requirement specified for slab reinforcement details. Therefore under normal circumstances, a concrete cover of 25 mm is usually adequate for floor slabs. The analysis must take into account the support conditions which can be idealised as fixed, hinged, or free. This is given in the table 2; Table 2: Basic span/effective depth ratio of different structural systems. The coefficients can be obtained from Table 3.14 of BS 8110-1:1997. The minimum-reinforcement percentage, which is between 0.18 and 0.20%, does not normally control cracks to within generally acceptable design limits. © (2020) Structville Integrated Services Limited. According to ACI Code 7.12.2, the lateral spacing of temperature and shrinkage reinforcement should not be placed farther apart than 5 times the slab thickness or 18 inches. 6.4—Maximum joint spacing Chapter 7—Design of slabs reinforced for crack-width control, p. 360R-32 7.1—Introduction 7.2—Thickness design methods 7.3—Reinforcement for crack-width control only 7.4—Reinforcement for moment capacity 7.5—Reinforcement location Chapter 8—Design of shrinkage-compensating concrete slabs, p. 360R-32 In design, a slab is considered to be two-way if the ratio of the longer side to the shorter side is less than two. The minimum reinforcement will be generated as 1,2 times the area required in ULS verification. Floor slabs are usually subjected to uniformly distributed loads, partially distributed loads, line loads, or concentrated loads in the transverse direction. For vertical stirrups, the highest spacing of shear reinforcement should remain under 0.75d and for inclined stirrups d is taken as 450. The effective span for one-way slabs is the same as that of beams. Another way to call out the reinforcement is to specify uniform top and bottom reinforcement across the whole slab, and to indicate any additional reinforcement that is required at the critical sections in the column and middle strips on the structural drawings; the spacing of these additional bars should be a multiple of that provided for the main bars. For slabs spanning in both directions published tables and charts should be used to find bending moment and shear per unit width of slab. Be sure to follow Rebar Engineering Specifications. If you've benefited from this free service please consider supporting: Calculate Rebar Spacing, Layout and Weight in a Concrete Slab. A slab should not deflect excessively under service load. Calculate the Value of ‘D’ (Depth of slab- Top cover- Bottom cover) Find out the No. The table below gives bar spacing data in the form of areas of steel per metre width for various bar diameters and spacings. It all depends on the size of the slab and whether the slab is on grade. The minimum reinforcement will be generated without limiting the spacing of bars. However, clause 9.3.1.2(1) requires that half the calculated span reinforcement must continue up to support. ALSO READ – Types Of Admixtures LIKED THIS ARTICLE? Reinforcement in slabs consists of a large number bars both ways which need to be tied together to form a mat. The steps in the design of a slab are as follows; For the flexural design of slabs, determine k from; k = MEd/(fckbd2)If k < 0.167, no compression reinforcement is required, and you can calculate the lever arm;z = d[0.5 + √(0.25 – 0.882k)]The area of reinforcement required is given by;As1 = MEd/(0.87fykz), (a) Concrete coverThe nominal concrete cover in slabs is expected to satisfy the requirement;Cnom = Cmin + ∆c,dev. Modification factor βs = 310/σs σs = (310fykAs,req)/(500As,prov) = (310 × 460 × 133.688)/(500 × 452) = 84.341 N/mm2 βs = 310/84.342 = 3.67 > 2.0 (take 2.0)Taking the distance between supports as the effective span, L = 3625mmThe allowable span/depth ratio = βs × 30.838 = 2.0 × 190.327 = 280.645Actual deflection L/d = 3625/119 = 30.462Since 280.645< 30.462 Therefore, deflection is ok. Shear VerificationMaximum shear force coefficient for slab(β) = 0.44VEd = βnlxUltimate shear force VEd = 0.44 × 10.9575 × 3.625 = 17.477 kN/mVRd,c = [CRd,c.k. The highest limit of spacing is 300 mm. Built something using our calculators? Thickness of the slab is decided based on span to depth ratio specified in IS456-2000. Rebar should be placed evenly throughout the project. VRd,c = [0.12 × 2 × (100 × 0.003798 × 25)1/3] × 1000 × 119 = 60476.947 N = 60.476 kN Since VRd,c (60.476 kN) > VEd (17.477 kN), no shear reinforcement is required.Note that shear reinforcements are rarely required in solid slabs supported by beams. Hence, k = Ly/Lx = 3.825/3.625 = 1.055 (say 1.1, however it is more appropriate to interpolate from the table). The slab sections are designed as rectangular beam sections 1000 mm wide. Disable SLS conditions. (f) Curtailment of bars in slabsCurtailment of bars is done according to the moment envelope. For vertical stirrups, the maximum spacing of shear reinforcement should not cross 0.75d. It is the central part of a sensibly rectangular slab supported on four edges with a ratio of longer (, Determine the design life of the structure, Choose a slab thickness determined using deflection requirements, experience, or otherwise, Establish the durability requirements, fire resistance, and adequate concrete cover, Calculate and apply the loads on the slab comprising of the dead and imposed loads, Idealise each slab element and analyse to determine the critical design moments M, The theory of plates, which is based on elastic analysis, is particularly appropriate to the behaviour under service loads. For one-way slabs supported on two opposite sides, the bending moments are calculated in the same way as for beams. Slab Reinforcement Details: The details of slab reinforcement are given below. Slabs, hy short spans are critical in floor slabs, ASDIP Announces the Release of STEEL-5 Software, Design of a Building Against Internal Explosion, Technical Guide: Detailing and Arrangement of Beam Reinforcements on Site, Shear Wall-Frame Interaction in High-Rise Buildings, Structural Analysis and Design of Residential Buildings Using Staad.Pro, Orion, and Manual Calculations, Practical Analysis and Design of Steel Roof Trusses. Can you identify the cause of failure of this building? The maximum area of steel provided should not exceed 0.04Ac. (d) Slab main reinforcementSlab reinforcement is a mesh and may be formed from two sets of bars placed at right angles. As,min = 0.26 × fctm/fyk × b × d = 0.26 × 2.5649/460 × 1000 × 144 = 208.76 mm2/mCheck if As,min < 0.0013 × b × d (187.2 mm2/m)Since, As,min = 187.2 mm2, the provided reinforcement is adequate. Moment coefficient for two adjacent edges discontinuous, Short Span Mid-span = 0.042 Continuous edge = 0.056, Long SpanMid-span = 0.034Continuous edge = 0.045, Design of short spanMid spanMEd = αnlx2 = 0.042 × 10.9575 × 3.6252 = 6.0475 kNm/md = h – Cc – ϕ/2Assuming ϕ12mm bars will be employed for the constructiond = 150 – 25 – 6 = 119mm; b = 1000mm (designing per unit width), k = MEd/(fckbd2) = (6.0475 × 106)/(25 × 1000 × 1192) = 0.0171, Since k < 0.167 No compression reinforcement requiredz = d[0.5 + √(0.25 – 0.882k)] z = d[0.5 + √(0.25 – 0.882 × 0.0273)] = 0.95d, As1 = MEd/(0.87fykz) = (6.0475 × 106)/(0.87 × 460 × 0.95 × 119) = 133.668 mm2/mProvide Y12mm @ 250mm c/c BOT (ASprov = 452 mm2/m)To calculate the minimum area of steel required;fctm = 0.3 × fck2⁄3 = 0.3 × 252⁄3 = 2.5649 N/mm2 (Table 3.1 EC2). Continuity across a beam is treated as fixed support. The minimum reinforcement required to control cracking will be generated without taking the SLS conditions into account. For a flat slab, such as a driveway, consider placing the rebar in an 18 inch grid spacing, keeping the edge rebar at the same distance from each side. Design of short spanContinuous edgeMEd = αnlx2 = 0.056 × 10.9575 × 3.6252 = 8.0633 kNm/mUsing the procedure described above;k = 0.0228; La/d = 0.95; As1 = 178.2236 mm2; As,min = 172.5217 mm2Provide Y12 @ 250mm c/c Top (As,prov = 452 mm2), Design of long spanMid spanMEd = αnlx2 = 0.034 × 10.9575 × 3.6252 = 4.895 kNm/mk = 0.0138; La/d = 0.95; As1 = 108.1945 mm2; As,min = 172.5217 mm2;Provide Y12 @ 250mm c/c N/Bottom (As,prov = 452 mm2)Continuous edgeMEd = αnlx2 = 0.045 × 10.9575 × 3.6252 = 6.479 kNm/mk = 0.0183; La/d = 0.95; As1 = 143.2057 mm2; As,min = 172.5217 mm2;Provide Y12 @ 250mm c/c Top (As,prov= 452 mm2). A concrete slab reinforced with rebar or welded wire fabric should have a minimum 1 1/2 inches of clear cover between the reinforcing and the top of the slab. In normal cases, a strip of slab 1 m wide is checked against span-to-effective depth ratios. Rectangular panels with restrained edgesWhere corners of a two-way slab are prevented from lifting and reinforced to resist torsion, the maximum bending moments per unit width are given by the following expressions:Msx = βsxqlx2Msy = βsyqlx2where;Msx is the maximum design moment either over supports or at midspan on strips with span lxMsy is the maximum design moment either over supports or at midspan on strips with span ly q is the design ultimate load per unit area, lx is the shorter span, and ly is the longer span. according to ACI code 7.6.5 The lateral spacing of temperature and shrinkage reinforcement should not be placed farther apart than 5 times the slab thickness or 18 inch according to ACI code 7.12.2 4. Effective Span of Slab. The general arrangement of the floor plan of a building is shown below. The spacing must not exceed 18 in., nor three times the slab thickness, in any case. Care should be taken to locate any reinforcement bars away from any anchor positions of the specific lift. For a one-way slab to have a fire rating of one hour (REI 60), the minimum thickness should be 80 mm and the concrete cover (from the surface to the centre of bar) should be minimum of 20 mm. At Structville, we stop at nothing in giving you new dimensions to the profession of civil engineering. The provision of adequate reinforcement, slab thickness, and proper detailing to satisfy ultimate and serviceability limit state requirements forms the basis of the design of reinforced concrete (R.C.) Rebar Calculator - Slab, Driveway or Patio Calculate the amount of rebar reinforcement needed for slabs and drive ways. Note that distribution steel is required at the top parallel to the supports of continuous slabs. These coefficients are based on elastic analysis from thin plate theory. the maximum spacing between two parallel main reinforcing bars in case of RCC Slab shall be 3d or 300 mm or whichever is less.

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