Civil Experts: Analysis and Design of “One Way” Slabs

Slabs

In reinforced concrete construction, slabs are used to flat, useful surfaces..
A reinforced slab is a broad, flat plate, usually horizontal, with top and bottom surfaces parallel or nearly so.
It may be supported by reinforced concrete beams (and is usually cast monolithically with such beams), by masonry or by reinforced concrete walls, by steel structural members, directly by columns, or continuously by ground.

One-Way Slab

“The slab which resists the entire/major part of applied load by bending only in one direction”

If slab is supported on all four sides and R = (Shorter side / Longer side) < 0.5 it behaves as one-way slab.
Slabs having supports on less than four sides can be designed as one-way.
Two edge supported slab is always one-way.
Cantilever slab is always one-way.
Main steel is only provided parallel to span
One-way slab is designed as singly reinforced rectangular section.
h(min) for the slab is different compared with the beams.

L = Effective Span
Lesser of the following:
L= Ln + h/2 + h/2
= Ln + h
h = depth of slab and
L = c/c distance between supports.

Examples of One-Way Slab

Shades in the roofing system (cantilever)
Slab of stairs
Cantilever retaining walls
Footings

Bar Spacing Cover For Slabs

3 x h (local practice is 2 x h)
450 mm (local practice is 300 mm)
(158300/fy) -2.5Cc
12600/fy

Cc = Clear Cover

Distribution, Temperature & Shrinkage Steel for Slabs (ACI-318-7.12)

Shrinkage and temperature reinforcement is required at right angle to main reinforcement to minimize cracking and to tie the structure together to ensure its acting as assumed in design
Top and bottom reinforcements are both effective in controlling the cracks

s(max) shall be lesser of following

5 x h (field practice is 2 x h)
450 mm (field practice is 2 x h)

Minimum Steel For Slabs

Same as the distribution steel

Design Procedure for One-Way Slab

Check whether the slab is one-way or two-way.
Calculate hmin and round it to higher 10mm multiple.
i.Not less than 110 mm for rooms
ii.Not less than 75 mm for sunshades.
Calculate dead load acting on the slab.
Dead Load = Load per unit area x 1m width.
Calculate live load acting on the slab.
Live load = Load per unit area x 1m width.
Calculate total factored load per unit strip (kN/m)
Calculate the moments either directly (simply supported) or by using coefficient for continuous slabs
Calculate effective depth.
d = h – (20 + (½)db)
db = 10, 13, 15 generally used
Check that d ≥ dmin
Calculate As required for 1m width
Calculate minimum/distribution/temperature & shrinkage steel
Select diameter and spacing for main steel
Check the spacing for max. and min. spacing smin ≈ 90mm
if spacing is less than minimum increase the diameter of bar
For continuous slabs, curtail or bent up the +ve steel. For -ve steel see how much steel is already available. Provide remaining amount of steel.
Calculate the amount of distribution steel. Decide its dia. & spacing like main steel.
Check the slab for shear.
ΦvVc ≥ Vu
Carry out detailing and show results on the drawings
Prepare bar bending schedule, if required

Approximate Steel for Estimate

Approximate amount of steel in slab
= 0.07 kg/mm/m2
If slab thickness = 100 mm
steel = 0.07 x 100 = 7kg /m2

Example

Design a cantilever projecting out from a room slab extending 1.0m and to be used as balcony (LL = 300 kg/m2). A brick wall of 250 mm thickness including plaster of 1.0m height is provided at the end of cantilever fc’ = 17.25 MPa, fy = 300 MPa, Slab thickness of room = 125 mm. Slab bottom steel in the direction of cantilever is # 13 @ 190 mm c/c

Solution

Slab Load

Self Weight of Slab = (125/1000)*2400 = 300kg/m2
75 mm brick ballast/ screed = (75/1000)*1800 = 135kg/m2
60 mm floor finishes = (60/1000)*2300 = 138kg/m2
Total dead load = 300+135+138 = 573kg/m2

Live Load = 300kg/m2

Factored Load = (1.2×573+1.6×300)x9.81/1000
= 11.46 KN/m2 = 11.46KN/m for a unit strip
P = 1.2x(0.25x1x1)x1930x9.81/1000
P = 5.65 KN