Design of two way slab

AHSANULLAH UNIVERSITY OF
SCIENCE & TECHNOLOGY
DEPT. OF CIVIL ENGINEERING
TITLE : DESIGN OF TWO
WAY SLAB
NAME : SARANI REZA
YEAR : 4TH YEAR 2ND SEMESTER ROLL : 110
SECTION : B

What is two way Slab?
 When the ratio of long span to short span is less

than 2, then the slab is known as a slab
spanning in two directions or two-way slab.

Types of Two-way Slabs:

FLAT SLAB WIT DROP
FLAT SLAB WITH DROP
PANELS
PANELS

TWO-WAY SLAB WITH
TWO-WAY BEAMS
SLAB WITH
BEAMS

Types of Two-Way Slabs:

FLAT PLATE
FLAT PLATE

WAFFLE
WAFFLE SLAB
SLAB

BASIC STEPS OF TWO WAY SLAB DESIGN :

 1. Choose layout and type of slab. Type of slab is strongly

affected by architectural and construction considerations.
2. Choose slab thickness to control deflection. Also, check if
thickness is adequate for shear.
3. Choose design method.
Equivalent frame method- use elastic frame analysis to
compute positive and negative moments.
Direct design method - uses coefficients to compute positive
and negative slab moments
4. Calculate positive and negative moments in the slab.
5. Determine distribution of moments across the width of the
slab. Based on geometry and beam stiffness.
6. Assign a portion of moment to beams, if present.
7. Design reinforcement for moments from steps 5 and 6. Steps
3-7 need to be done for both principal directions.
8. Check shear strengths at the columns.

DETERMINE MEXIMUM BENDING
MOMENT :

 Ws=WS/3 For Short Span.
 Wl=wS/3x(3-m2/2) For Long Span. To determine the

maximum moment in the supporting beams, we use
the uniform load per foot of beam.

Slab Reinforcement of 2 way
Slab:

Reinforcement Details Loads
After all percentages of the static moments in the
column and middle strip are determined, the steel
reinforcement can be calculated for negative and
positive moments in each strip.
 Maximum Spacing of Reinforcement

 At points of max. +/- M:

s≤2t

(ACI 13.3.2)

and

s≤45cm

(ACI

7.12.3)
 Spacing of Reinforcement at critical section shall

not exceed two times the slab thickness except
for portions of slab area of ribbed Construction.

Min Reinforcement Requirements
As

min

As

T&S

from ACI 7.12

ACI 13.3.1

Area of reinforcement in each
direction for two way slab systems
shall be determined from moments at
critical sections .But shall not be less
than required by shrinkage and
temperature reinforcement.

Methods of Design:
 1. Direct Design Methods (DDM):
Limited to slab systems with uniformly distributed loads
and supported on equally spaced columns. Method
uses a set of Co-efficents to determine the design
moments in critical sections. Two- way slab systems do
not meet the limitations of the ACI code 13.6.1 must be
analyze causing more accurate procedures.

2. Equivalent Frame Methods( EFM):
A three dimensional building is divided in to a series of
two dimensional frames by cutting the buildings alone
lines midway between columns. The resulting frames
are considered separately in the longitudinal and
transverse direction of the building and treated floor by
floor.

 Conclusion:

In the case of two-way slab it has been found
in practice that a huge moment develops at
the support i.e. at the support negative
moment is huge. In this case steel must be
provided to withstand this moment. In most of
the cases extra top bars are provided.

Design of two way slab

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