Analysis and Design of Slabs Two Way

Notes

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. Analysis and Design of Slabs Two Way

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 Analysis and Design of Slabs Two Way

Solution:

Panel Edge Conditions Analysis and Design of Slabs Two Way

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 Analysis and Design of Slabs Two Way