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DESIGN OF SLABS By:Ir. Mohamad Salleh YassinFaculty of Civil EngineeringUniversiti Teknologi MalaysiaDesign ProcedureStepTaskStandard1Determine design life, Exposure class & Fire resistanceEN 1990 Table 2.1EN 1992-1-1: Table 4.1EN 1992-1-2: Sec. 5.62Determine material strengthBS 8500-1: Table A.3EN 206-1: Table F13Determine thickness of slab EN 1992-1-1: Table 7.4N EN 1992-1-2: Table 5.84Calculate min. cover for durability , fire and bond requirementsEN 1992-1-1: Sec. 4.4.15Estimate actions on slabsEN 1991-1-16Analyze structure to obtain critical moments and shear forcesEN 1992-1-1: Sec. 57Design flexural reinforcementEN 1992-1-1: Sec. 6.18Check shear EN 1992-1-1: Sec. 6.29Check deflectionEN 1992-1-1: Sec. 7.410Check crackingEN 1992-1-1: Sec. 9.311DetailingEN 1992-1-1: Sec.8 & 9.3Slab thicknessThe selection of slab thickness from structural viewpoint is often dictated by deflection control criteria. In practice, the overall depths of slabs are often fixed in relation to their spans. Span to overall depth ratios of 20 to 30 are generally found to be economical in the case of simply supported and continuous beams.

EN 1992-1-2: Table 5.8

Concrete CoverThe nominal cover can be assessed as follows:Cnom = Cmin + DCdevWhere Cmin shall be provided in order to ensure:The safe transmission of bond forcesThe protection of steel against corrosion (Durability)An adequate fire resistanceAnd DCdev is and allowance which should be made in the design for deviation from the minimum cover. It should be taken as 10 mm. It is permitted to reduce to 5 mm if the fabrication subjected to a quality assurance system. Minimum cover for bond EN 1992-1-1Arrangement of barsMinimum cover cmin,b*SeparatedDiameter of barBundleEquivalent diameter n = nb 55 mm Where nb is the number of bars in the bundle, which is limited to nb 4 for vertical bars in compression nb 3 for all other cases* If the nominal maximum aggregate size is > 32 mm, cmin,b should be increased by 5 mmMinimum cover for durabilityEN 1992-1-1Structural ClassExposure Class according to Table 4.1 EC 2X0XC1XC2/XC3XC4XD1/XS1XD2/XS2XD3/XS3S110101015202530S210101520253035S310102025303540S410152530354045S515203035404550S620253540455055

Minimum cover for fire resistance

Rather than giving a minimum cover, the tubular method based on nominal axis distance is used. This is the distance from the centre of the main reinforcement bar to the top or bottom surface of the member.a > Cnom + flink + fbar/2 AnalysisThe analysis of reinforced concrete slab to determine bending moment and shear force may be done either by using elastic analysis or by considering plastic collapse methods. Hence one of the following methods can be use.Elastic analysis Yield line methodHillerborg strip methodMoment and shear coefficient from Code of practice or handbook

DESIGN OF FLEXURAL REINFORCMENTThe design procedure for flexural design in Figure 2. The derived formula is based on the following simplified stress block.

EN 1992-1-1: Sec. 6.1

EN 1992-1-1: Sec. 9.3

SHEAR EN 1992-1-1: Sec. 6.2.2

DEFLECTIONEC 2 has two alternative methods for checking deflection, either a limiting span-to-depth ratio or the theoretical deflection calculation. The span-to- depth ratio should ensure that deflection is limited to span/250.

EN 1992-1-1: Sec. 7.4CRACKINGEN 1992-1-1: Sec. 9.3To resist cracking of the concrete slabs, EC2 specify details such as minimum area of reinforcement required in a section and limits to the maximum and minimum spacing of bar.

The minimum area of principal reinforcement is As,min = 0.26fctmbtd/fyk but not less than 0.0013btd, where bt is the mean width of the tension zone.

CRACKINGEN 1992-1-1: Sec. 7.3The minimum area of secondary reinforcement is 20% As,min. In areas near support, transverse reinforcement is not necessary where there is no transverse bending moment.The spacing of principal reinforcement bars should not exceed three times the overall depth of slab (3h) or 400 mm whichever is the lesser. For secondary reinforcement the spacing should not exceed 3.5h or 450 mm whichever the lesser. These rules apply for slabs not exceeding 200 mm thick.

DESIGN EXAMPLESee:Example 7.4Tutorial 6End of Lecture 6The calculations for flexural reinforcement follow a similar procedure to that use in beam design. 1. CalculateK =2. If K Kbal (= 0.167), compression reinforcement is not required, and

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