ONE-WAY SLABS
8.1 TYPES OF SLABS
Structural concrete slabs are constructed to provide flat surfaces, usually horizontal, in
building floors, roofs, bridges, and other types of structures. The slab may be supported by
walls, by reinforced concrete beams usually cast monolithically with the slab, by structural
steel beams, by columns, or by the ground. The depth of a slab is usually very small
compared to its span.
Structural concrete slabs in buildings may be classified as follows:
1. One-way stabs: If a slab is supported on two opposite sides only, it will bend or
deflect in a direction perpendicular to the supported edges. The structural action is one way,
and the loads are carried by the slab in the deflected short direction. This type of slab is
called a one-way slab.
1
If the slab is supported on four sides and the ratio
of the long side to the short side is equal to or
greater than ( ), most of
the load (about or more) is carried in the
short direction, and one-way action is considered
for all practical purposes. If the slab is made of
reinforced concrete with no voids, then it is called
a one-way solid slab.
2
The following limitations are specified by the ACI code:
Atypical imaginary strip 1m wide is assumed.
The minimum thickness of one-way slabs using grade 420 steel can be defined
according to the ACI Code, 9.5.2.1, Table 9.5a, for solid slabs and for beams or ribbed
one-way slabs .
ACI 9.5.2.1 – Minimum thickness stipulated in Table 9.5(a) shall apply for one-way
construction not supporting or attached to partitions or other construction likely to be
damaged by large deflections, unless computation of deflection indicates a lesser thickness
can be used without adverse effects.
It is preferable to choose slab depth to the nearest .
Shear should be checked, although it does not usually control.
Concrete cover in slabs shall not be less than at surfaces not exposed to
weather or ground. In this case, .
In structural slabs of uniform thickness, the minimum amount of reinforcement in
the direction of the span shall not be less than that required for shrinkage and
temperature reinforcement (ACI Code, Section 7.12).
Straight-bar systems may be used in both tops and bottoms of continuous slabs. An
alternative bar system of straight and bent (trussed) bars placed alternately may also
be used.
In addition to main reinforcement, steel bars at right angles to the main must be pro-
vided. This additional steel is called secondary, distribution, shrinkage, or
temperature reinforcement.
3
8.4 MINIMUM REINFORCEMENT RATIO.
For structural slabs and of uniform thickness, in the direction of the span shall be the
same as that required by 7.12.2.1 for temperature and shrinkage reinforcement (section 8.5).
Maximum spacing of this reinforcement shall not exceed three times the thickness, nor
.
To limit the widths of flexural cracks in beams and slabs, ACI Code Section 10.6.4 defines
upper limit on the center-to-center spacing between bars in the layer of reinforcement
closest to the tension face of a member. The spacing limit is:
( )
but
( )
where is the least distance from
surface of reinforcement to the tension
face. It shall be permitted to take as
.
8.5 TEMPERATURE AND SHRINKAGE REINFORCEMENT.
Concrete shrinks as the cement paste hardens, and a certain amount of shrinkage is usually
anticipated. If a slab is left to move freely on its supports, it can contract to accommodate
the shrinkage. However, slabs and other members are joined rigidly to other parts of the
structure, causing a certain degree of restraint at the ends. This results in tension stresses
known as shrinkage stresses. A decrease in temperature and shrinkage stresses is likely to
cause hairline cracks. Reinforcement is placed in the slab to counteract contraction and
distribute the cracks uniformly. As the concrete shrinks, the steel bars are subjected to
compression.
Reinforcement for shrinkage and temperature stresses normal to the principal
reinforcement should be provided in a structural slab in which the principal reinforcement
extends in one direction only.
The ACI Code, Section 7.12.2, specifies that: area of shrinkage and temperature
reinforcement shall provide at least the following ratios of reinforcement area to gross
concrete area, but not less than :
For slabs in which grade 280 ( ) or 350 ( ) deformed bars
are used, .
For slabs in which grade 420 ( ) deformed bars or welded bars or
welded wire fabric are used, .
For Slabs where reinforcement with yield stress exceeding measured at a
×
yield strain of percent is used, 𝜌
𝑓𝑦
4
For temperature and shrinkage reinforcement, the whole concrete depth exposed to
shrinkage shall be used to calculate the steel area.
8.6 REINFORCEMENT DETAILS.
In continuous one-way slabs, the steel area of the main reinforcement is calculated for all
critical sections, at midspans, and at supports. The choice of bar diameter and detailing
depends mainly on the steel areas, spacing requirements, and development length. Two bar
systems may be adopted.
In the straight-bar system: straight bars are used for top and bottom reinforcement
in all spans. The time and cost to produce straight bars is less than that required to
produce bent bars; thus, the straight-bar system is widely used in construction.
In the bent-bar, or trussed, system: straight and bent bars are placed alternately in
the floor slab. The location of bent points should be checked for flexural, shear, and
development length requirements. For normal loading in buildings, the bar details at
the end and interior spans of one-way solid slabs may be adopted as shown in
figures.
5
Example:
Design a simply supported one-way solid slab, span of , subjected to service live load
( ). Dead Load – own weight only. and .
𝑚
𝑚 𝑠𝑡𝑟𝑖𝑝
Solution:
Minimum thickness (deflection requirements). For simply supported one-way solid
slab:
Assume bar diameter for main reinforcement.
Loads calculation:
𝑚𝑚
𝑚𝑚
s 𝑚𝑚
Cover
( √ ) ( √ )
Provide
Use then
6
Take .
Step ( ) is the smallest of:
1. ×
2.
( ) ( )
( ) ( )
( )
Take .
Step ( ) is the smallest of:
1. ×
2.
–
𝑚𝑚
𝑚𝑚
