Design of RCC Rectangular Slabs as per IS 456-2000
1. GENERAL :
The design of Two way rectangular slabs supported on all four sides without provision for Torsion at corners is carried out as per Table 27, Indian Standard (IS) 456 App. D. Check for deflection is as per the provsions in Cl. 23.2 of the code.
2. Simply Supported Two Way Slab Design as per IS 456 :
i) The Input data for the slab design is :
* Effective Short Span Lex, Effective Long Span Ley, Grade of Concrete fck, Floor Finish Loads and Imposed Loads.
* Clear cover for the slab would be as per Table16 for Durability criteria and as per Table 16A for Fire resistance criteria. For a fire resistance of 0.5 hour 20mm cover is provided, based on the same eff. cover for short span would be d'x = ( D - short span bar dia. / 2) and for long span it would be d'y = ( D - shortspan bar dia. - long span bar dia /2). Generally speaking to start with d'x = 25mm and d'y = 35mm would be assumed considering 10mm bar dia.
* Main variable in Slab Design is assumption of slab thickness. It should provide eff. depth more than required as per Cl. 23.2. d,reqd = Lex / (20 * MF). But MF = Modification Factor depends on Area of Steel provided. So the problem is iterative, assume a suitable slab thickness D as Lex/25 to Lex/30 depending on edge conditions and magnitude of loads.
ii) Calculate total load incl. self weight of slab. Determine BM coefficient from Table 27 for given edge conditions and Ley/Lex ratio of spans by interpolation. Then after finding BM at critical sections 'Ast' at each critical location is calculated. Minimum area of steel Ast,min (mm2/m) = 1.2 * D(mm)i.e. 0.12%. For Modification Factor, 'Ast' for Short Span positive BM (span moment) is considered.
iii) If the check for deflection is not satisfied by a narrow margin, MF can be increased max. up to 2 by providing Ast more than the designed value for positive short span steel. This will reduce actual stress in steel and increase MF. But even after that If d,provided < d,reqd, slab thickness is to be increased and check for deflection is to be satisfied.
iv) Detrmine Sapcing of bars at critical section based on designed value of Ast by selecting appropriate bar dia. The spacing < 300mm and <3*d as per Cl.26.3.3.
3. TWO WAY SLAB DESIGN SPREADSHEET:
* The formulation adopted is for Limit State of Collapse as per IS 456. The BM Coefficients are from Table 27, Based on Ley / Lex ratio and the design Case adopted. The interpolation for BM Coefficient is automated.
* Concrete Grade M20 to M40, Steel Grade Fe 415 is considered.
* The Spreadsheet calculates area of steel for Limit State of Collapse in Flexure at all relevant sections. If the Check for Deflection is not satisfied it will automatically calculate increased area of steel for positive short span BM to satisfy deflection criteria. In case this can not be achieved, recommends the user to increase Slab Thickness.
* The spacing calculation is provided for a no. of bar dia. the designer would select the most appropriate one.
* Only singly reinforced section is considered in design. In case it becomes doubly reinforced, a message to revise thickness appears.
4. DESIGN PROCESS :
Give Input Data in white space, with red font. See the Instructions on the SLBDSN worksheet.
* TO BE USED BY QUALIFIED STRUCTURAL ENGINEER FOR DESIGN AS PER OF IS - 456. CHECK BEFORE USE.
* NO LIABILITY OF DEVELOPER OF ANY TYPE UNTILL A LICENCE AGREEMENT IS SIGNED BETWEEN USER AND DEVELOPER, AND THAT THE USER IS FORMALLY TRAINED FOR USE OF THIS SPREAD SHEET, INCL. ITS ADVANTAGES AND ITS LIMITATIONS.
Reinforce Concrete Design
Indian Standard 456
07 Sep 2012
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