Design of Slabs 1

Effective Span

First step is know the effective span for analysis and design Effective span needed for calculation of moments, shear forces , etc. IS 456 provides guidance to calculate effective span

Code Provisions for effective spanClause 22.2

For a Member which is not built integrally with its supports (simply supported case) then it can be taken, lesser of the following two:

1. Clear Span + Eff. Depth of slab or beam2. Center to center distance of supports For a continuous member, if the width of the support is less than

1/12 of the clear span, the eff. Span shall be as per above case. 1. For end span with one end fixed and other continuous 2. For intermediate spansEff. Depth shall be clear span between the supports

Continued

For end span with one end free and other continuous, eff. Depth shall be lesser of the following two:

1. Clear span + half the eff. Depth2. Clear span + half the width of discontinuous support. In the case of spans with roller or rocker bearings, the eff. Span

shall always be the distance between the centers of bearings.

Continued

The eff. Length of cantilever shall be taken as 1. Its length to the face of the support + Half the eff. depth.2. The length to the center of support where it forms the end of a

continuous beam. In the analysis of continuous frame, center to center distance

shall be used

Clause 22.5

Gives guidance on moment and shear coefficients For Substantially UDLs over three or more spans which do not

differ by more than 15% of the longest coefficients are given in table 12 and table 13 for different conditions, please refer IS 456 for this.

Moment = A * Load * Length where A – co eff. and load = UDL and Length = Eff, spanFor this spans and loads should not differ much Shear force = B * LoadWhere B – co eff. And Load - UDL

Clause 22.5.1 and 22.5.2

For moments at supports where two unequal spans meet or in case where the spans are not equally loaded, the average of the two values for the negative moment at the support may be taken for design

When a member is built into a masonry wall which develops only partial restraint, the member shall be designed to resist a negative moment at the face of the support of WL/24, where W is the total design load and L is the eff. Span OR such other restraining moment as may be shown to be applicable. For this case, shear coefficients at the end support (table 13) may be increased by 0.05

RCC Solid Slabs

RCC Solid Slabs

One Way Two Way Flat Slabs Flat Plates

One way and Two way slabs

L2 /L1 > 2 L2/L1 < 2

Loading on Slabs for Building (IS 875)

Self wt. : 25 kN/cum Finishes and partitions : 1.5 kN/sq m Imposed load : 1. Roofs: 1.5 kN/sq m with access and 0.75 kN/sq m

without access 2. Floors: 2.0 kN/sq m for residential buildings and 3.0 kN/sq m for office floors

Cover

Refer Table 16 of IS 456

Control of deflection Clause 23.2

Final deflection due to all loads should not normally exceed span/250

Deflection occurring after erection of partitions and the application of finishes should not exceed span/350 or 20 mm whichever is less

The vertical deflection limits may generally be assumed to be satisfied provided that span to eff. depth ratios are not greater than the values obtained below: (for spans upto 10 m) Values are basic values

Cantilever : 7Simply Supported: 20Continuous: 26

For spans above 10 m, basic values are multiplied by 10/span Depending on the area and stress for tension reinforcement, the basic

values shall be modified by multiplying with modification factor as per fig. 4 of IS 456

Depending upon the area of compression reinforcement, the value of span to depth ratio is further modified by multiplying with modification factor obtained as per fig. 5

For flanged beams the multiplying factor should be modified as per fig. 6