Reinforced Concrete Design: Components, Load Combinations, and Slab Types

This article discusses the design of reinforced concrete structures, including the components of columns, beams, and slabs, load combinations, and different types of slabs. It also covers the factors that affect the durability of these structures and the methodologies used in their design. The subject is relevant to civil engineering courses and is applicable to various colleges and universities. Get study material, solved assignments, essays, and dissertations on Desklib.

Running Head: Reinforced concrete design
Reinforced Concrete Design
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Table of Contents

Reinforced concrete design 2
CHAPTER 1: INTRODUCTION.............................................................................................................2
CHAPTER 2: LITERATURE REVIEW.................................................................................................3
2.1 Reinforced concrete.........................................................................................................................3
2.2 Design of the Structural Components of a Reinforced Concrete Building..................................5
2.3 Load Combinations for the Building..............................................................................................5
2.4 The Slabs..........................................................................................................................................6
2.5 Columns..........................................................................................................................................10
2.6 Beams..............................................................................................................................................11
CHAPTER 3: METHODOLOGY AND ANALYSIS...........................................................................12
References.................................................................................................................................................21

Reinforced concrete design 3
CHAPTER 1: INTRODUCTION
Steel and concrete have been the most dominant construction materials. However, both are
susceptible to environmental factors with concrete segregating into its constituent elements while
steel susceptible to corrosion. Before the discovery of reinforced concrete, steel was the most
dominant construction material because of its numerous advantages. To begin with, steel is
comparatively lighter to the other materials and as such can be used in areas where the soil has a
lower bearing capacity. Secondly, the erection of steel structures is very fast because of the ease
of fabrication.
Concrete may be regarded as very durable because of its constituent sand, aggregate properties.
Furthermore, the material is very versatile. It is, however, heavier than steel and therefore not
suitable in areas where the soil has a lower bearing capacity. The need for reinforced concrete
arises from the individual weaknesses of concrete and steel. Concrete is strong in compression
but weak in tension while steel is strong in tension but weak in compression (Arya,
2009).Therefore, combining these two materials results to a much stronger construction material.

Reinforced concrete design 4
CHAPTER 2: LITERATURE REVIEW
2.1 Reinforced concrete
Reinforced concrete is suitable for the construction of various structures such as retaining walls,
single and multi-story buildings, containment structures and retaining walls among
others.Basically, there are three most dominant types of frames that are used in the reinforced
concrete design and are: The medium rise frame, the single-story portal and the multistory frame
(McGinley & Choo, 1990).The multistorey type may contain a central core which is meant to
ensure the building resists lateral as well as wind loads effectively (McGinley & Choo, 1990).
The design of a building mainly entails the different structural elements.These elements include
beams, columns, slabs, and walls: either load bearing or non-load bearing. To begin with, a
column is a structural member that is subjected to compressive forces and transfers the loads
from the beams to the foundation. Beams are structural members that support the slabs and
transfer the loads from the slabs to the columns. However, it is important to note that there are
primary as well as secondary beams. The secondary beams are located centrally while the
primary beams are located at the edges of the slab. Finally, the slab is the structural element that
supports the live as well as the dead load and transfers these loads to the beams.
There are two methodologies through which the building elements can be designed: ultimate
limit state and serviceability limit state. This is known as the limit state design and is opposed to
the plastic design used in designing of steel structures (McCormac & Brown, 2014).To begin
with, the limit state is mainly concerned with cracking and deflection and models the building
when it is in the working state (Arya, 2009). Therefore, when a building is subjected to excessive
bending and cracking, the serviceability limit state is used as a check. On the other hand, the

Reinforced concrete design 5
ultimate limit state is concerned with the building when it is subjected to a number of
mechanisms that might eventually lead to its failure. Some of these mechanisms may include
tension, the compression mechanism, shear, torsion among others. Therefore, the state shows the
potential for failure when the building is subjected to a couple of forces and mechanisms.
2.2 Design of the Structural Components of a Reinforced Concrete Building
As stated before, the most important components are the columns, beams, slabs and the
walls.Therefore, the aspect of the design is very important to every engineer. Nevertheless, some
important factors to consider before the design, factors that affect the durability of these concrete
buildings include the cover provided by the concrete to the reinforcement, the content of the
cement as well as the water and cement ratio, the type of cement and the strength of the concrete.
However, good engineering practices and the workmanship are equally as important when it
comes to the durability.
2.3 Load Combinations for the Building
In the determination of the load requirements of the building, it is essential for the designer to
determine the types of loads prevalent in the building. Basically, there are three types of loads
that the building may be subjected to wind, dead and live loads. By definition, the dead loads are
those that are permanent to the building and includes the self-weight of the structural
components of the building, the partition and any other structural concrete that may be needed.
The imposed loads are those that are temporary to the building and depends on the type of
occupancy. In this, the loads can either be snow loads, distributed loads, impact loads, inertia etc.
Finally, there are the wind loads that are attributed to wind factors such as the wind speed, force
coefficients, dynamic pressure and the pressure coefficients among others (McGinley & Choo,

Reinforced concrete design 6
1990).There are two forms of resistance provided to buildings because of wind loads: bracing
and unbracing. In braced structures, the resistance is provided by the shear walls, stairs, and the
lift shafts. On the other hand, resistance in unbraced structures is provided by bending in the
frame (McGinley & Choo, 1990).
The design of the structural elements is primarily influenced by the loading and as such, there are
various load combinations necessary. In this, the designer has to consider loadings at different
parts of the building while at the same time applying a factor of safety. Generally, the partial
factor of safety is integrated into the design to ensure that the most severe case is used in the
design ( Standard, 2000).
The building’s structural elements mainly comprise the slabs, columns, and beams. Though they
are separate elements, they serve the function of ensuring that the stability of the building is
maintained and that the loads are transferred effectively to the soil. The case of monolithic
casting is an exception where the members are assembled as a single package. The formwork for
slabs, beams, and columns ensures that the cured concrete results in an agglomeration of the
different elements.
2.4 The Slabs
The slabs are the surface elements on which the loading occurs. In this, the primary, secondary,
wind, imposed, superimposed all occur on these surface and therefore its design is very
important. According to Arya (2010), slabs can be made if the beams may be cast side by side,
therefore, ensuring that the loads between them are shared equally. The development of slabs is
recommendable because they have been used to form other structural elements such as walls,
stairs, floor systems etc.Furthermore, the design aspect of all these structural elements is similar

Reinforced concrete design 7
to that of beam since their modeling is around beams.Therefore, it is easy to see why beams and
slabs are easily intertwined in the design.
The difference between a slab and a beam is the critical design state. In slabs, the serviceability
limit state is usually critical than in beams. Slabs can easily fail under working loads in
comparison to beams. Furthermore, bending and shear is not as critical in slabs as in beams
(Arya, 2009).There are various factors that influence the choice of a slab and some of them
include the builder’s preference, workability, economy etc.. The choice of whether a ribbed slab,
cast in situ slabs, precast slab, solid slab, however, depends mostly on the span. In this, the short
spans require solid slabs while medium spans require flat slabs. However, the case of medium
spans and flat slabs is highly influenced by the fact that the slabs have a soffit that is flat in
nature. Therefore, construction using these types of slabs reduces the accompanying costs while
increasing the speed at which high rise buildings can be erected.Furthermore, this type of floor is
mainly employed in stories of a lower height and as such, there a few restrictions to the
partitioning as well as the fittings that may be used.
The window system, in this case, may even be installed between the two floors and thereby
providing the necessary light and ventilation at specific. On the downside to the use of such
types of slabs, excessive bending, as well as punching shear, are the major causes of failure
(Arya, 2009).Of the two failure mechanisms, bending may be reduced by increasing the
thicknesses of slabs, especially at the columns while punching shear may be reduced by
increasing the diameter or girth of the columns that support the slab. The punching shear is
mainly attributed to increased shear stresses, especially at the column-foundation interface,
arising from increased concentrated loads from the slabs. Furthermore, the problem may be
resolved by increasing the flaring of the columns thereby increasing the area over which the slab

Reinforced concrete design 8
loads are distributed in the foundation. All these are prescriptive measures with the use of shear
links and cages termed as more appropriate but rather very costly and labor consumptive.
The one way and two-way slabs
Slabs may define by the orientation of the supports which basically beam. Some slabs may be
supported by beams at opposite ends while some may be supported by beams at all four edges.
The latter definitely is more expensive because of the amount of concrete as well as the number
of steel used but it may be very appropriate in various circumstances because of the strength that
comes with this type of orientation. However, economic factors, just to mention one, may be a
limitation of its application
In the design of one-way slabs, the first design aspect is to consider their run as to be one meter.
This simplifies the calculations involved since they run for a bigger width. Therefore, the
designer can consider these types of slabs as beams of 1-meter width. Basically, the main
reinforcement runs in a direction that is parallel to this direction with the secondary
reinforcement running in directions that are transverse to the main direction. All this is necessary
to protect the slab from bending. However, it is the main reinforcement that should be located in
the outer layers with the main aim providing a greater lever arm.Furthermore, other factors such
as curtailing and bending of these steel should also be considered prior to the actual design.
There are two types of slabs spanning in one direction: the single slab and the continuous solid
slab. In the first case, the effective span is taken as to be the lesser of the clear distance between
two beams in addition to the beam depth or the clear distance between the supports.
On the other hand, the continuous slab requires the main reinforcement within the slab,
particularly at the bottom parts while the secondary reinforcement is contained within the

Reinforced concrete design 9
support. Furthermore, the difference in loading, as well as the indeterminacy of the structure,
gives the two types of single way slabs some difference. The main reason is that the continuous
type is statically indeterminate while the single is determinate and that there may be considerable
loading conditions that may have to be considered prior to the design of the continuous. In this,
the loading of this type of slab may be based on the moment distribution methodology, unlike the
latter case where the relatively easier methods of design may be used.
The two-way spanning slab is considered so because of the four supports accorded to it. By
providing the slab with four supports, the slab spans in all the four directions. The main
advantage over the single span slab is the fact that the flexural capability is increased and can,
therefore, sustain higher loads than the one-way slab. However, the bending will depend on the
difference in spans of the adjacent slabs and the support is given. Slabs which are square provide
equal spans for the loads in both directions while rectangular slabs have more loading in the
shorter direction than the longer one. As a matter of fact, the design aspect of a single spanning
slab may be considered for slabs where the loading in one direction is more than in the other
direction.The reinforcement details of these slabs mainly require the shorter slab to have the steel
bars placed further apart from the neutral axis than in the longer direction. By placing the bars
further away from the neutral axis, the effective depth of the slab is increased. Furthermore,
because of the difference in length between the two edges of the slab have the effect of ensuring
that the moments are unequally distributed. There are two types of slabs that span in two
directions: the restrained and the simply supported. To begin with, the simply supported will tend
to lift or curl up at the supports and therefore there is a need to prevent torsion and moments at
these supports. Moment factors are provided by different codes and the reinforcement is to be

Reinforced concrete design 10
provided in all directions. Some of the factors to be considered for the design of this type of two
way spanning slab include; the span to effective depth, the length of the span among others.
The slabs that are restrained at the supports effectively resist moments as well as torsion and
therefore the lifting, as well as the curling, is restricted. These might be expensive design aspects
but may be more durable than the former. In designing such a slab, the slab is provided with
reinforcement at the middle of the span as well as along the edges.However, the reinforcement in
the middle sections of the spans effectively resists moments both in the x and y directions. When
considering the discontinuous edges, there is the requirement for torsional resistance and as such,
some of the measures may include; provision of mats both at the top and bottom with the
reinforcement provided in both directions, code requirements of the edge extent. In addition,
discontinuous directions in both edges may require additional steel reinforcement in relation to
the mid-span, reinforcement details requirements at an edge discontinuous in one direction.
2.5 Columns
These are structural elements that are mainly subjected to compression forces. Furthermore, they
transfer the loads from the beams to the foundations but in other instances may transfer the loads
to the bottom slab, a slab that may be in direct contact with the ground. Furthermore, the
columns have to be designed in such a way that they are able to resist forces that may subject it
to bending. Therefore, the design analysis of a column is basically concerned with the limit state
design with cracking and bending as the most significant causes of failure.
There are two types of columns, according to the technique employed for the resistance of lateral
loads as well as the frame arrangement. The two types described therein are the braced and
unbraced columns. The braced column is used to restrict lateral loads by the use of braces while

Reinforced concrete design 11
the unbraced column resists lateral loads by the bending moments induced on the columns which
consequently means the bending of the column acts as a resistance measurement to these loads.
In trying to describe the braced column, axial and bending moments are mainly caused by the
types of loads with the dead and imposed loads having a significant effect. However, the
unbraced columns have lateral loads, in addition to the dead and imposed loads, imposed on
them. The other aspect to be considered in the arrangement of the two types of columns is that
the braced column has to consider the maximum axial force as well as the bending moment.
The two types of columns may be classified as either short or slender.The term is used to define
the width as well as the lengths along the XX and YY axes. In this respect, the short columns
will, under working conditions, fail by crushing while the slender types will fail by buckling.
Therefore, besides designing the column type and the lateral supports, the mode of failure should
be considered. However, the two types of columns can fail by any of the following methods: the
short columns may fail due to material properties regardless of the deflection which in most
cases may be considered negligible, and columns which have intermediate slenderness ratio may
also fail due to material failure.
To cap the needs of column design, the general requirement is that the design load is within the
load capacity of the column. This is a rather straightforward requirement with the material
properties playing a fundamental role in the capacity which thereafter prevents the column from
buckling or failing. Columns which have a higher slenderness ratio tend to have a carrying
capacity that is lower than that of a less slender column. This is a rule of thumb when it comes to
this phase of design.

Reinforced concrete design 12
2.6 Beams
Beams may fall into the following groups of classification: the reinforcement position, the cross-
section properties and the support conditions of the beam. It is therefore important to consider all
these factors prior to defining the beam. To begin with, beams may be classified as singly
reinforced or doubly reinforced depending on the location of the reinforcement. Reinforcement is
provided to ensure that the beam resists tensile or/and compressive forces. Beams that are
provided with steel to resist tensile forces are known as singly reinforced while those provided
with steel to resist tensile as well as compressive forces are known as doubly reinforced. Most
people and construction bodies prefer the double reinforced beam because it has a higher
moment carrying capacity meaning that there is an increase in the slenderness. In this regard, the
compression steel is located at the top while the tensile steel located at the bottom. The designer
has also to consider the possibility of a monolithic system of the beam and slab because of
failure of the concrete below the tension line. It will eventually result in a T or L beam with T
beams used in the internal parts of the slab while the L beams serving as the primary beams, at
the edges of the slab.
The other classification of the beam depends on the support conditions. The two types of beams,
in this case, the simply supported and the continuous beam. The most common type is the
continuous beam because of the increased design of continuous slabs. However, regardless of the
type of beam, the most important design considerations are the bending failure, deflection, and
shear forces.

Reinforced concrete design 13
CHAPTER 3: METHODOLOGY AND ANALYSIS
Building analysis
Each floor may be described by the following plan.
There are two aspects to be considered in the design: the one way and two way slab system.
Providing beams on all the four edges of the panels will result in a 2 way spanning slab but as
per the above diagram, the floor is made up of one way spanning slabs. In determining the type
of slab, the aspect ratio ly/lx must also be calculated with a two way slab system having the ratio
lesser than 2.
The following design is based on the two way span.

Reinforced concrete design 14
Design
As with our building, the design will have to consider the permanent, imposed and
superimposed loads. There are three types of combinations that may be present: dead load alone,
the dead load+ the superimposed load, the dead load +imposed loads. The common denominator
is the dead load because the weight of the structural should always be considered in the design.
The dead load of the building
For the beams, the maximum depth is 1000mm.Furthermore, our design calculations will be
based on a unit width of the element. The other variable in the calculation is the unit weight of
concrete.
Tthe design will be based on a unit strip of the element and as such, the beam
width=1m.Furthgermore, because the depth of the beam is 1m, the total load imposed per unit
strip=1*24=24kN/m2
The load form the slab: depth of the slab=0.2m per unit strip-therefore, the load by the
slab=0.2*24=4.8kN/m2 per unit strip of the depth
The total dead load=28.8kN/m2
The imposed and superimposed loads are given as 3.0kN/m2 and 1.0kN/m2
Because there are three load cases:
AS1170.0:2002 clause 4.2.2
Load case 1=dead load only=28.8*1.35*5=194.4kN/m
Load case 2: dead load +imposed load= (1.2*28.8+1.5*3.0)5= 195.3kN/m2

Reinforced concrete design 15
Load case 3: dead +superimposed= (1.2*28.8+1.5*0.4*1.0)5= 175.8KN/m2
This is a general definition of the load cases for the whole building.
The serviceability state and strength limit states for the slab, column and beam designs
The serviceability limit state, as described before, describes the building under working
conditions. The crack and deflection controls are done at this stage.
Slab: the maximum load that can be applied on the slab=195.3kN/m2
Therefore, the total load that can be applied over the whole span=195.3*5=976.5kN
Therefore, the shear force 976.5/2=488.25kN
AS3600:2009
Design shear= therefore=0.7*97.65= 68.355kN
This is the same shear strength as that of the s1ab and consequently transferred to the beam.
The bending capacity of th4e whole structure depends on the moment. The general rule for the
moment=wl2/12
The results are 203.4375 kN/m2
6.10.3.2: The moments in the slab can be calculated from: ly/lx=1 therefore, By=Bx=0.024(four
edges discontinuous)
Therefore, the moment=0.024*195.3*5=23.436 the bending moment is exceeded in both states

Reinforced concrete design 16
Bending moment and shear force distribution diagrams
In describing the moments:
Substituting G=28.8+1.0
Q=3.0
= 40.26kN/m2
But according to the design, each panel lx=ly=5m therefore, the ratio=1
From the drawings, there are two types of slabs: two adjacent edges discontinuous and one short
edge discontinuous. Nevertheless, the ratio=1
From table 6.10.3.2 A
One short edge discontinuous: = 0.028 0.028 therefore, 28.182
Two adjacent edges discontinuous: = 0.035, therefore, 35.2275
Furthermore, from clause 6.10.3.2, the above are the positive bending moments at midspan.
At the edges, the moment is 1.33 times the midspan value.
Therefore, the moment at the edges=one short edge discontinuous=37.41024kNm
Two adjacent edges discontinuous=46.852575kNm

Reinforced concrete design 17
Sketching our moment diagram based on the larger moment:
Calculating the shear force at the supports= =100.65kN

Reinforced concrete design 18
1 2 3 4 5 6 7
-40
-30
-20
-10
0
10
20
30
40
50
60
position
Series1 Series2
B E A M 1 MI DPO I N T B E A M 2 MI DPO I N T B E A M 3 MI DPO I N T B E A M 4
-150
-100
-50
0
50
100
150
shear force
\

Reinforced concrete design 19
Design of the structural elements of the building
The above elements may be described by the following simple design.
Slab design for a single floor
The slabs in this scenario have been described to be single spanning because of the number of
spans. In this, the design aspects will have to be in line with those of a one way slab.
The design of a slab:
From the above calculations, the maximum moment that the slab needs to withstand=46.86kNM
The effective depth of the slab can be derived from the formula=total depth-cover-0.5 the
diameter of steel used for construction. We may assume to be using the N20 types of steel and a
cover of 25mm
Therefore, the effective depth=165mm, using the 20mm bars
T= =165*500=82500
0.85d=140.25
Mu=11570625Nmm
Therefore, from the tables and using an area of 165mm2/m, steel reinforcement should be
provided as follows: N20 steel at 1000mm center to center. However, the quantity surveyor may
determine the suitability of the lesser grades steel.

Reinforced concrete design 20
Checking on the stress control,
Therefore,
Therefore, the reinforcement provided for shear is as follows: N10 at 350mm center to center
(Area=229mm2).Checking the suitability of the shear reinforcement (clause 8.2.7 AS 3600).
Which equals to 282.88kN
The design of beams applies the same principles as those used in slab design. The reinforcement
and the spacing used in the slab may also be used for the beam.

Reinforced concrete design 21
References
Standard, I., 2000. Plain and reinforced concrete–code of practice. New Delhi: s.n.
Anon., 2002. AS1170.0. s.l.:s.n.
Anon., 2009. AS3600. s.l.:s.n.
Arya, C., 2009. design of structural elements. s.l.:Taylor & Francis.
McCormack, J. C. & Brown, R. H., 2014. Design of reinforced concrete. s.l.: John Wiley and
sons.
McGinley, T. J. & Choo, B. S., 1990. Reinforced concrete: design theory and examples. Second
ed. London and New York spon press: Tylor and Francis group.