Design of concrete Structure 2

Mohd Sharique Ahmad
Assistant Professor
Civil Department
6/28/2017 Mohd Sharique Ahmad 1

Flat Slabs
 A flat slab is a two-way reinforced concrete slab that usually does not have
beams and girders, and the loads are transferred directly to the supporting
concrete columns.
 Flat slab is defined as one sided or two-sided support system with sheer load
of the slab being concentrated on the supporting columns and a square slab
called drop panels

Flat Slabs are considered suitable for most of the construction
and for asymmetrical column layouts like floors with curved
shapes and ramps etc. The advantages of applying flat slabs are
many like depth solution, flat soffit and flexibility in design
layout.

Types of Flat Slab
Following are the types of flab slab construction:
 Simple flat slab
 Flat slab with drop panels
 Flat slab with column heads
 Flat slab with both drop panels and column heads

Uses of Column Heads
It increase shear strength of slab
It reduce the moment in the slab by reducing the clear
or effective span
Uses of Drop Panels
It increase shear strength of slab
It increase negative moment capacity of slab
It stiffen the slab and hence reduce deflection

Advantages of Flat Slabs
It is recognized that Flat Slabs without drop panels can be built at a
very fast pace as the framework of structure is simplified and
diminished.
Flat slab construction can deeply reduce floor-to –floor height
especially in the absence of false ceiling as flat slab.
This can prove gainful in case of lower building height, decreased cl
Thickness of flat slab is another very attractive benefit because thin
slab provides the advantage of increased floor to ceiling height and
lower cladding cost for the owner.adding expense and pre-fabricated
services.

Types of Flat Slab Design
Multitudes of process and methods are involved in
designing flat slabs and evaluating these slabs in flexures.
Some of these methods are as following:
The empirical method
The sub-frame method
The yield line method
Finite –element analysis

Benefits of Using Flat Slab Construction
Method
Flexibility in room layout
Saving in building height
 Lower storey height will reduce building weight due to lower partitions and cladding to
façade
 Approximately saves 10% in vertical members
 Reduced foundation load
Shorter construction time
Ease of installation of M&E services
Use of prefabricated welded mesh
Buildable score

Design of Flat Slab
Considerations
 A flat slab is reinforced concrete slab directly supporting on column
 Flat slabs is divided into column strips & middle strips.
 Column strips is a design strip with a width on each side of a column centre
line equal to 0.25L1 or 0.25L2,whichever is less.
 A middle strip is a design strip bounded by 2 column strips
 A panel is bounded by column, beams, or wall centre lines on all sides

Design Method
 There must a minimum 3 continuous spans in each directions.
 Panels shall be rectangular with a ratio of longer to shorter spans ,centre to
centre of supports ,not greater than 2.
 Successive span lengths, centre-to-centre of supports, in each direction shall
not differ by more than 1/3 of the longer spans.
 Columns may be offsets a maximum of 10% of the span (in direction o
offset) from either axis between centre lines of successive columns.
 All loads shall be due to gravity only and uniformly distributed over entire
panels. the live loads shall not exceeds 2 times the dead load.

Design Procedure
 First analysis the column strips & middle strips using 0.25L1/0.25l2.
 Drop panel is used to reduce the amount of negative moment
reinforcement over the column of the flat slab, the size of drop panel
shall be 1/6 of the span length measured from centre–to-centre of
support in that direction.
 Estimate the depth of flat slabs from clauses 14.2.5 & 3.3.2.2.(b)
Assume fy=300MPA
Fy (MPA) Exterior Panels Interior Panels
300 Ln/36 Ln/40
400 Ln/32 Ln/35

Design Contd..
 The absolute sum for the span shall be determined in a strip bounded
laterally by the center line of the panel on each side of centre of the supports.
 The absolute sum of positive and average negative moments in each
direction at the ultimate limit state shall be not less than:
Mo=WuL2Ln²/8
Negative & positive design moments
In an interior spans
 Negative moments—0.65
Positive moments---0.35

Design Contd..
In End Spans
Exterior edge
unrestrained
Slab with
beams
between all
supports
Slabs without beams between
interior supports
Exterior edge
fully
restrained
Without edge
beams
With edge beams
Interior – ve
moments
0.75 0.70 0.70 0.70 0.65
Positive
moments
0.63 0.57 0.52 0.50 0.35
Exterior – ve
moments
0 0.16 0.26 0.30 0.65

SHEAR STRENGTH
 Design of cross section of member subjected to shear shall be based on
v´<=¢Vn.
Where v´=shear force at that section . Vn=nominal shear strength of the
section.
¢ =strength reduction factor.
 The nominal shear stress Vn shall not exceed 0.2fc,1.1√fc or 9MPA.
 Spacing limits for shear reinforcements shall be:
 0.5d in non-prestressed member
 0.75 h in prestressed member
 600mm

Shear Strength Contd..
 Design of slab for two way action shall be based on
 Vn=Vn/bod
 Where vn shall not be greater than Vc
 Vc=0.17(1+2βc)√fc
 βc=shorter side/long side of the concentrated load

UNIT 2
Footings

Types of Footing
Wall footings are used to
support structural walls that
carry loads for other floors
or to support nonstructural
walls.

Footing
Definition
Footings are structural members used to support columns and walls and to
transmit and distribute their loads to the soil in such a way that the load
bearing capacity of the soil is not exceeded, excessive settlement,
differential settlement,or rotation are prevented and adequate safety
against overturning or sliding is maintained.

Types of Footing
Isolated or single footings
are used to support single
columns. This is one of the
most economical types of
footings and is used when
columns are spaced at
relatively long distances.

Types of Footing
Combined footings
usually support two
columns, or three columns
not in a row. Combined
footings are used when tow
columns are so close that
single footings cannot be
used or when one column is
located at or near a property
line.

Types of Footing
Cantilever or strap
footings consist of two
single footings connected
with a beam or a strap and
support two single columns.
This type replaces a
combined footing and is
more economical.

Types of Footing
Continuous footings
support a row of three or
more columns. They have
limited width and continue
under all columns.

Types of Footing
Rafted or mat foundation
consists of one footing
usually placed under the
entire building area. They
are used, when soil bearing
capacity is low, column
loads are heavy single
footings cannot be used,
piles are not used and
differential settlement must
be reduced.

Types of Footing
Pile caps are thick slabs
used to tie a group of piles
together to support and
transmit column loads to
the piles.

Distribution of Soil Pressure
When the column load P is
applied on the centroid of the
footing, a uniform pressure is
assumed to develop on the soil
surface below the footing area.
However the actual distribution of
the soil is not uniform, but
depends on may factors especially
the composition of the soil and
degree of flexibility of the footing.

Distribution of Soil Pressure
Soil pressure distribution in
cohesionless soil.
Soil pressure distribution in
cohesive soil.

Design Considerations
Footings must be designed to carry the column loads and transmit them to the soil
safely while satisfying code limitations.
The area of the footing based on the allowable bearing
soil capacity
Two-way shear or punching shear.
One-way bearing
Bending moment and steel reinforcement required
*
*
*
*

Design Considerations
Footings must be designed to carry the column loads and transmit them to the soil
safely while satisfying code limitations.
Bearing capacity of columns at their base
Dowel requirements
Development length of bars
Differential settlement
*
*
*
*

Size of Footing
The area of footing can be determined from the actual external loads such
that the allowable soil pressure is not exceeded.
 
pressuresoilallowable
weight-selfincludingloadTotal
footingofArea 
footingofarea
u
u
P
q 
Strength design requirements

Two-Way Shear (Punching Shear)
For two-way shear in slabs (& footings) Vc is smallest of
long side/short side of column
concentrated load or reaction area<2
length of critical perimeter around the
column
where, bc =
b0 =
ACI 11-35
dbfV 0c
c
c
4
2









b
When b >2 the allowable Vc is reduced.

Design of two-way shear
Assume d.
Determine b0:
b0 = 4(c+d) for square columns
where one side = c
b0 = 2(c1+d) +2(c2+d) for
rectangular columns of sides c1
and c2.
1
2

The shear force Vu acts at a section
that has a length b0 =
4(c+d) or 2(c1+d) +2(c2+d) and a
depth d; the section is subjected
to a vertical downward load Pu
and vertical upward pressure qu.
3
 
   columnsrrectangulafor
columnssquarefor
21uuu
2
uuu
dcdcqPV
dcqPV



Allowable
Let Vu=fVc
4
dbfV 0cc 4ff 
0c
u
4 bf
V
d
f

If d is not close to the assumed d,
revise your assumptions

Design of one-way shear
For footings with bending action
in one direction the critical
section is located a distance d
from face of column
dbfV 0cc 2ff 

The ultimate shearing force at
section m-m can be calculated








 d
cL
bqV
22
uu
If no shear reinforcement is to
be used, then d can be checked

bf
V
d
2 c
u
f

If no shear reinforcement is to
be used, then d can be checked,
assuming Vu = fVc

Flexural Strength and Footing reinforcement
2
y
u
s










a
df
M
A
f
The bending moment in each
direction of the footing must
be checked and the appropriate
reinforcement must be
provided.

bf
Af
a
85.0 c
sy

Another approach is to
calculated Ru = Mu / bd2 and
determine the steel percentage
required r . Determine As then
check if assumed a is close to
calculated a

The minimum steel percentage
required in flexural members is
200/fy with minimum area and
maximum spacing of steel bars
in the direction of bending
shall be as required for
shrinkage temperature
reinforcement.

The reinforcement in one-way
footings and two-way footings
must be distributed across the
entire width of the footing.
1
2
directionshortinentreinforcemTotal
widthbandinentReinforcem


b
footingofsideshort
footingofsidelong
b
where

Bearing Capacity of Column at Base
The loads from the column act on the footing at the base of the column, on an area equal
to area of the column cross-section. Compressive forces are transferred to the footing
directly by bearing on the concrete. Tensile forces must be resisted by reinforcement,
neglecting any contribution by concrete.

Force acting on the concrete at the base of the column must not exceed the bearing
strength of the concrete
 1c1 85.0 AfN f
where f = 0.7 and
A1 =bearing area of column

The value of the bearing strength may be multiplied by a factor for bearing
on footing when the supporting surface is wider on all sides than the loaded area.
0.2/ 12 AA
The modified bearing strength
 
 1c2
121c2
85.02
/85.0
AfN
AAAfN
f
f



Dowels in Footings
A minimum steel ratio r = 0.005 of the column section as compared to r = 0.01 as
minimum reinforcement for the column itself. The number of dowel bars needed is
four these may be placed at the four corners of the column. The dowel bars are usually
extended into the footing, bent at the ends, and tied to the main footing
reinforcement. The dowel diameter shall not =exceed the diameter of the longitudinal
bars in the column by more than 0.15 in.

Development length of the Reinforcing Bars
The development length for compression bars was given
but not less than
Dowel bars must be checked for proper development length.
cbyd /02.0 fdfl 
in.8003.0 by df

Differential Settlement
Footing usually support the following loads
Dead loads from the substructure and superstructure
Live load resulting from material or occupancy
Weight of material used in backfilling
Wind loads

General Requirements for Footing Design
A site investigation is required to determine the chemical and physical properties
of the soil.
Determine the magnitude and distribution of loads form the superstructure.
Establish the criteria and the tolerance for the total and differential settlements of
the structure.
1
2
3

General Requirements for Footing Design
4. Determine the most suitable and economic type of foundation.
5. Determine the depth of the footings below the ground level and the method
of excavation.
6. Establish the allowable bearing pressure to be used in design.
7. Determine the pressure distribution beneath the footing based on its width
8. Perform a settlement analysis.

Example
Design a plain concrete footing to support a 16 in thick concrete wall. The load on
the wall consist of 16k/ft dead load (including the self-weight of wall) and a 10 k/ft
live load the base of the footing is 4 ft below final grade. fc = 3ksi and the allowable
soil pressure = 5k/ft2

Unit 3 Retaining Wall

RETAINING WALL
 Basic function – to retain soil
at a slope which is greater
than it would naturally
assume, usually at a vertical
or near vertical position

Design of concrete Structure 2

 Retaining wall failure at the Shin-Kang Dam

Design of retaining wall
 retaining walls have primary function of retaining soils at an angle in
excess of the soil’s nature angle of repose.
 Walls within the design height range are designed to provide the necessary
resistance by either their own mass or by the principles of leverage.
 Design consideration:
1. Overturning of the wall does not occur
2. Forward sliding does not occur
3. Materials used are suitable
4. The subsoil is not overloaded

Factors which designer need to take
account
 Nature and characteristics of the subsoil's
 Height of water table – the presence of water can create hydrostatic pressure,
affect bearing capacity of the subsoil together with its shear strength, reduce
the frictional resistance between the underside of the foundation
 Type of wall
 Materials to be used in the construction

 Failure of retaining wall (dam) due to water pressure..

Types of walls
 Mass retaining walls
 Cantilever walls
 Counterfort retaining walls
 Precast concrete retaining
walls
 Precast concrete crib-retaining
walls

Mass retaining walls
 Sometimes called gravity walls and rely upon their own mass together with
the friction on the underside of the base to overcome the tendency to slide or
overturn
 Generally only economic up to 1.8 m
 Mass walls can be constructed of semi-engineering quality bricks bedded in a
1:3 cement mortar or of mass concrete
 Natural stone is suitable for small walls up to 1m high but generally it is used
as a facing material for walls over 1 m

Typical example of mass retaining walls
BRICK MASS RETAINING WALL

Brick retaining
wall
Stone retaining wall

Typical example of mass retaining walls
MASS CONCRETE RETAINING WALL WITH
STONE FACINGS

Cantilever walls
 Usually of reinforced concrete and work on the principle of leverage where
the stem is designed as a cantilever fixed at the base and the base is designed
as a cantilever fixed at the stem
 Economic height range of 1.2 m to 6 m using pre-stressing techniques
 Any durable facing material can be applied to the surface to improve
appearance of the wall

 Two basic forms:-
 A base with a large heel
 A cantilever with a large toe
Cantilever LCantilever T

Counterfort retaining walls
 Can be constructed of reinforced or prestressed concrete
 Suitable for over 4.5 m
 Triangular beams placed at suitable centres behind the stem and above the
base to enable the stem and base to act as slab spanning horizontally over or
under the counterforts

Precast concrete retaining wall
 Manufactured from high-grade pre cast concrete on the cantilever principle.
 Can be erected on a foundation as permanent retaining wall or be free standing to act as
dividing wall between heaped materials which it can increase three times the storage volume
for any given area
 Other advantages- reduction in time by eliminating curing period, cost of formwork, time to
erect and dismantle the temporary forms
 Lifting holes are provided which can be utilized for fixing if required

Precast concrete retaining walls

Pre cast concrete crib-retaining walls
 Designed on the principle of mass retaining walls
 A system of pre cast concrete or treated timber components comprising
headers and stretchers which interlock to form a 3 dimensional framework or
crib of pre cast concrete timber units within which soil is retained
 Constructed with a face batter between 1:6 and 1:8
 Subsoil drainage is not required since the open face provides adequate
drainage.

Unit 4 Tanks

• To study the Analysis and Design of elevated water tanks.
• To study the guidelines for the design of liquid retaining
structure according to IS Code IS: 3370 part 2-2009 & IS:
456:2000.
• To know about the design philosophy for the safe and
economical design of water tanks.
81
OBJECTIVES

 For storage of large quantities of water, tanks are required.
INTRODUCTION
WATER TANK
BASED ON
PLACEMENT OF
TANK
BASED ON
SHAPE OF TANK
1. UNDER GROUND
2.RESTING ON GROUND
3. ELEVATED
1. RECTANGULAR
2. CIRCULAR
3. INTZE
4. SPHERICAL
5. CONICAL

Tanks used to supply water through gravity are termed as
Elevated water tanks.
Elevated tanks may have circular or rectangular section with flat
or Conical bottom.
For small Storage capacity, tanks square in plan are economical.
Where as for large capacities circular tanks prove economical.
They may be made of masonry, steel,
reinforced concrete and pre stressed
concrete.
RCC tanks are very popular as,
 Simplicity in construction and design
 Cheap
 monolithic in nature
 Can be made leak proof.

Intze
Circular
Spherical bottom
Domed bottom Conical bottom
Rectangular

85
COMPARISON OF TANKS
RECTANGULAR
TANK
CIRCULAR TANK INTZE TANK
PRESTRESSED
TANKS
For smaller
capacities
rectangular
(square) tanks
are economical.
For large capacities circular tanks
are economical.
For same capacity, circular tanks
requires less concrete than
rectangular tanks.
On account of circular shape, it
can be made water tight easily as
there are no sharp corners.
Intze tank is
constructed to
reduce the
project cost
because lower
dome in this
construction
resists horizontal
thrust
For bigger
tanks,
prestressing is
the superior
choice
resulting in a
saving of up to
20%.

Elevated tanks are supported on staging which may
consist of Masonry Shaft, R.C.C tower or R.C.C.
column braced together with tie beams.
SUPPORT SYSTEM FOR RECTANGULAR TANKS
•The simplest form is Rectangular
tanks supported by beams and
four columns.
•With increase in plan area,
Internal beams and columns are
introduced to limit the span.

SUPPORT SYSTEM FOR CIRCULAR TANKS
 Circular beam
Masonry tower
 4 beams mutually orthogonal
2 beams mutually orthogonal
Internal beams and Columns
Single Annular shaft

Design of Tank
• Cover slab or Top dome
• Top ring beam
• Side walls or Cylindrical wall
• Base slab
Design of Staging
Design of Foundation
DESIGN OF ELEVATED WATER TANKS

Unit 5 Prestress Concrete

Reinforced concrete:
 Concrete is strong in compression weak in tension.
 Steel in strong in tension
 Reinforced concrete uses concrete to resist compression
and to hold bars in position and uses steel to resist tension.
 Tensile strength of concrete is neglected (i.e. zero )
 R.C beams allows crack under service load.

Pre-stressed Concrete
 What is Pre-stressed Concrete?:
 Internal stresses are induced to
counteract external stresses.
 In 1904, Freyssinet attempted to
introduce permanent acting forces
in conc. to resist elastic forces
under loads and was named
“Pre stressing”.

 i . The concept of pre stressing was invented
invented years ago when metal brands were wound
 around wooden pieces to form barrels.

 ii . The metal brands
were tighten under tensile stress which
creates compression between the
staves allowing them to resist internal
liquid pressure.

Principle of pre-stressing:
 Pre-stressing is a method in which compression force is applied to the reinforced concrete
section.
 The effect of pre stressing is to reduce the tensile stress in the section to the point till the
tensile stress is below the cracking stress. Thus the concrete does not crack.
 It is then possible to treat concrete as a elastic material.
 The concrete can be visualized to have two compressive force
i . Internal pre-stressing force.
ii . External forces (d.l , l.l etc )
 These two forces must counteract each other.

Principle of Pre-stressing:
 Stress in concrete when pre stressing is applied at the c.g of the section

Principle of Pre-stressing:
 Stress in concrete when pre stressing is applied eccentrically with respect to the
c.g of the section .

Pre-stressed Concrete: Methods
 There are two basic methods of applying pre-stress to a concrete member
 Pre-tensioning – most often used in factory situations
 Post-tensioning – site use

I . Pre-tensioning
In Pre-tension, the tendons are tensioned against some abutments before the concrete is
place. After the concrete hardened, the tension force is released. The tendon tries to
shrink back to the initial length but the concrete resists it through the bond between
them, thus, compression force is induced in concrete. Pretension is usually done with
precast members

II . Post tensioning
 In Post tension, the tendons are tensioned after the
concrete has hardened. Commonly, metal or plastic
ducts are placed inside the concrete before casting.
After the concrete hardened and had enough strength,
the tendon was placed inside the duct, stressed, and
anchored against concrete. Grout may be injected into
the duct later. This can be done either as precast or
cast-in-place.

•Take full advantages of high strength concrete
and high strength steel
•Need less materials
•Smaller and lighter structure
•No cracks
•Use the entire section to resist the load
•Better corrosion resistance
•Good for water tanks and nuclear plant
•Very effective for deflection control
•Better shear resistance

Disadvantages compared to RC:
 Need higher quality materials
 More complex technically
 More expensive
 Harder to re-cycle

•Bridges
•Slabs in buildings
•Water Tank
•Concrete Pile
•Thin Shell Structures
•Offshore Platform
•Nuclear Power Plant
•Repair and Rehabilitations

Design of concrete Structure 2

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