Two Way Edge Supported Slabs

Slab resting on walls or sufficiently deep and rigid beams on all sides. Other options are column supported slab e.g. Flat slab, waffle slab.

Two-way slabs have two way bending unlike one-way slab.

Design Methods

1.ACI co-efficient method

2.Direct design method

3.Equivalent frame method

4.Finite element method

Notes

1.In two-way slabs shorter direction strip carry greater %age of load.

2.Steel will be more in shorter direction.

Shorter direction steel will be placed near the outer edge to get more “d” means more lever arm to get more flexural capacity.

ACI Co-efficient Method

Unit width strip is taken in both directions. The strip is designed separately  for +ve and –ve moment

“C” depends upon the end conditions of slab and the aspect ratio
Three tables are available for “C”

•  Dead load positive moment
•  Live load positive moment
•  -ve moment

M+ coefficients are increased by 25 % and M- coefficients are reduced by 10 % to get the result more closer to accurate solution.

Minimum Depth of 2-Way Slab for Deflection Control

According to ACI-318-1963
hmin = (inner perimeter of slab panel)/180
≥ 90 mm

Example:

Design the 4 marked slab panels of an ordinary house. Use US customary bars. fc’= 17.25 MPa fy = 300 MPa

Solution:

Panel Edge Conditions


Panel # 1
Lx = 4.5m ,  Ly = 7.0m
m = 0.64 > 0.5, 2-way slab
Panel # 2
Lx = 6.0m ,  Ly = 7.0m
m = 0.86> 0.5, 2-way slab
Panel # 3
Lx = 3.5m ,  Ly = 6.0m
m = 0.58 > 0.5, 2-way slab
Panel # 4
Lx = 6.0m ,  Ly = 6.0m
m = 1 > 0.5, 2-way slab
Slab Thickness
Generally same depth is preferred for one monolithic slab Calculate hmin for all the panels and select the largest value