What is a One-way Slab?
One way slab is like a slab in which loads are shared only through the shorter direction of the slab. Here, the ratio from the longer length of the slab to the shorter span of the slab is more than 2.
Design of One-way Slab:
Preliminary Design:
Take Span/d = 25 for simply supported slab.
= 30 for continuous slab.
D ≥ Dmin = 100mm for normal design.
= 125mm for earthquake resistant design.
(ly/lx)=>2 Then it is considered one-way slab.
Analysis of Slab:
Analyze the sheet with 1 m wide beam and see the loads, maximum bending time for the design, and optimal shear force for the design.
Design of Slab:
Check for Depth of Slab
Mu = 0.133 X fckX b Xd2
d < provided slab thickness
∴ A slab is safe against a moment.
Calculation of Ast:
Mu= 0.87 X fy X Ast X d (1- (fyXAst)/(b X d X fck ) )
Ast = 0.12% of gross area
Check for Shear:
Nominal Shear Stress, ﺡv = Vu/bd
Permissible Shear Stress,pt = (100Ast)/bd
Provide min. Ast as distribution bars in the longer direction of slab.
What is a Two-way Slab?
Two-way slabs are such slabs in which the components are distributed in the shorter and longer direction of the slab. The ratio of the long span to the shorter span is less than or equal to 2. A two-way slab is supported on four sides the load will be carried in both the directions.
Design of Two-way Slab:
Preliminary Design:
Find the depth of the slab using deflection control criteria.
Take span/d = 28 for simply supported slab and 32 for the continuous slab.
D ≥ Dmin = 100mm for normal design and 125mm for earthquake resistant design.
(ly/lx)<2 Then it is considered a Two-way slab.
Analysis of Slab:
A slab is analyzed by the IS code method. (Ref. Annex D-1 and Annex D-2).
Design of Slab:
Check for Depth of Slab
Mu = 0.133 X fckX b Xd2
d < provided slab thickness
Determine ‘Ast’ and ‘Φ’ and the number of bars in the shorter direction of the slab.
The slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.
Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.
Check the slab for shear in a shorter direction.
Calculation of Ast:
Mu= 0.87 X fyX Ast X d (1- (fyXAst)/(b X d X fck ) )
Ast = 0.12% of gross area
Check for Shear:
Nominal Shear Stress, ﺡv = Vu/bd
Permissible Shear Stress,pt = (100Ast)/bd
Provide torsion reinforcement at the discontinuous edge of the slab as per code provision.
