ROOF SLAB FOR ROOMS:
Ly = 218 feet = 66.272 M
Lx = 22.20 feet = 6.75 M
Sides ratio of the slab
Length/breath ratio of the slab =ly / lx
= 66.272 M / 6.75 M = 9.81
Since the side ratio is greater than 2, the slab is to designed as one way slab.
PROPERTIES OF SECTION:
For M20 grade concrete and Fe 500
Moment of resistance MR = 2.672 bd2
DEPTH REQUIRED FOR STIFFNESS:
Basic value of l/d ratio for simply support slab = 20
Assume a modification Factor of 1.7.
Clear span of the slab = 6.75 = 675 mm
Approximate effective depth required for stiffness
Minimum nominal cover for reinforcement for mild exposure is sales = 20mm.
Assuring the 12mm dia bars are provided a tension reinforcement
Total depth required
= 198.529 + 20 + 12/2
= 224.529 mm.
Provide a total depth of 230mm with and effective depth 204mm. Effective span:
Effective span of a simply supported slab will be the least of clear span + effective depth and centre distance of supports.
Clear span + effective depth
= 6.75 + 0.204
Centre to centre distance of }
= 6.75 + .274
Effective span = l = 6.954m
Imposed load on roof
= 1.500 kn/6.75
When acces is provided)
Weight of weathering course
Self weight of slab
Considering 1M width of slab
Load per meter length of span, W
Design load, Wu
8.25 X 1.5
Design bending B.M at mid span Nu=wul2 / 8
12.375 X 6.9542 = 74.8039 KNM
74.803 X 106N.MM
Maximum S.F at support, Vu = wul
12.375 X 6.954
= 43.027 KN
Depth required for strength:-
Equating the moment of resistance of the section to the maximum bending moment
74.803X102.672X10006 = 167.31mm
Effective depth provided
204 > 167.3
0.87 X 500 Ast [ 204 500Ast20x1000]= 74.8039x106 0.87 X 500 Ast [204-0.025 Ast] = 74.8.39 X 106
435 Ast [204 – 0.025 Ast] = 74.8039 X 106
88740 Ast – 10.875 Ast2 = 74.8039X106
-10.875 Ast2 + 88740 Ast = 74.8039 x 106
-Ast2 + 8160 Ast = 6.8785 X106
Ast = 954.64mm2
Minimum area of steel to be provided
x 1000 X 230 = 345 mm2
Spacing of 8mm dia bars
S = 50x1000Ast = 50x1000954.64100 = 52.375
Say => 50mm
Maximum permitted spacing for main reinforcement
=Least of 3d and 300
=3 X 204 = 612 mm
Provide 8mm dia M.s bars at 50 mm c/c
Now, actual area of steel provided.
= 50X100050 = 1000mm2
Percentage tension steel
Area of steel required (0.15%) = 345 mm2
Spacing of 8mm bars = 50X1000345= 144.95
Maximum permitted spacing for distributors
=least of 5d and 450mm.
= 5 X 204 = 1020mm.
Provident 8mm dis distributor at 140 mm c/c
Curtailment of main reinforcement bars.
Alternative bars may be cranked at distance equal to 0.1l = 0.1 X 6954 = 695.4mm Say
ROOF SLAB FOR BATH ROOM
Lx = 22.20 feet
Ly = 31.30 feet
Side ratio of the slab
Ly / lx =
Since the side ratio is greater than 2. The slab is to be designed as one way slab.
Properties of section:-
For m25 grade concrete and fe500 moment of resistance , MR = 3.325 bd2 Depth required for stiffness:
Basic value of l/d ratio of simply supported slab = 20
Assume a modification factpr
Clear span of the slab
= 6.75020X1.7= 198.529mm
Minimum nominal cover for reinforcement for mild exposure is slab = 20mm. Assuming 12mm dia bar are provided as tension reinforcement.
Total depth required
198.529 = 20 = 12 / 2
Provided a total depth of 230mm with an eddective depth of 204mm.
Clear span = effective depth
6.75 = 0.204
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