The document provides an introduction to reinforced concrete design, including: 1) Defining reinforced concrete as a composite of concrete and reinforcement materials to improve tensile strength and ductility 2) Describing design purposes as resisting tensile stresses to prevent cracking or failure 3) Explaining limit states as the ultimate limit state of preventing collapse under loads and the serviceability limit state of preventing excessive deformation

1. A) INTRODUCTION TO DESIGN
i) Define the reinforced concrete design
Reinforced concrete (RC) is a composite material in which concrete's relatively
low tensile strength and ductility are counteracted by the inclusion of reinforcement
having higher tensile strength and/or ductility. The reinforcement is usually, though not
necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the
concrete before the concrete sets. Reinforcing schemes are generally designed to resist
tensile stresses in particular regions of the concrete that might cause unacceptable
cracking and/or structural failure. Modern reinforced concrete can contain varied
reinforcing materials made of steel, polymers or alternate composite material in
conjunction with rebar or not. Reinforced concrete may also be permanently stressed (in
compression), so as to improve the behavior of the final structure under working loads.
In the United States, the most common methods of doing this are known as pre-
tensioning and post-tensioning.
For a strong, ductile and durable construction the reinforcement needs to have the
following properties at least:
 High relative strength
 High toleration of tensile strain
 Good bond to the concrete, irrespective of pH, moisture, and similar factors
 Thermal compatibility, not causing unacceptable stresses in response to
changing temperatures.
 Durability in the concrete environment, irrespective of corrosion or sustained
stress for example

2. ii) Interpret The Design Purposes
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iii) Describe the characteristic of creep
Concrete creep is defined as deformation of structure under sustained
load.Basically,long term pressure or stress on concrete an make it change shape. This
deformation usually occurs in the direction the force is end applied. Like a concrete
column getting more compressed or a beam bending.Ceep does not necessarily cause
concrete to fail or real apart. Creep is faired in when concrete structure’s are designed.
Characteristics of creep:-
The final deformation of member can be 3 – 4 times the short term elastic
deformation
Deformation is roughly proportional to the intensity of loading and to the inverse
of concrete strength
If load is removed, only instantaneous elastic deformation will recover – the
plastic deformation will not
There is a redistribution of load between concrete and steel present

3. iv) State the influences that affect the concrete durability
Durability of concrete may be defined as the ability of concrete to resist weathering
action, chemical attack and abrasion while maintaining its desired engineering
properties. Concrete durability influenced by :
Exposure conditions
The cement type
The concrete quality
The cover to the reinforcements
The width of any cracks

4. Factors Related to Concrete Durability
High Humidity and Rain: With little to no organic content, concrete is resistant to
deterioration due to rot or rusting by in hot, humid climates. Moisture can only enter a
building through joints between concrete elements. Annual inspection and repair of
joints will minimize this potential. More importantly, if moisture does enter through joints,
it will not damage the concrete. Walls need to breathe or concrete will dry out if not
covered by impermeable membranes.
Portland cement plaster (stucco) should not be confused with exterior insulation and
finish systems (EIFS) or synthetic stucco systems that may have performance
problems, including moisture damage and low impact-resistance. Synthetic stucco is
generally a fraction of the thickness of Portland cement stucco, offering less impact
resistance. Due to its composition, it does not allow the inside of a wall to dry when
moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing,
and wood framing. It also corrodes metal framing and metal attachments. There have
been fewer problems with EIFS used over solid bases such as concrete or masonry
because these substrates are very stable and are not subject to rot or corrosion.
Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm
concrete. Using colored pigments in concrete retains the color in aesthetic elements
(walls or floors, for example) long after paints have faded due to the sun’s effects.
Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some
softer materials are inedible but still provide pathways for insects. Due to its hardness,
vermin and insects will not bore through concrete.
Moderate to Severe Exposure Conditions for Concrete: The following are important
exposure conditions and deterioration mechanisms in concrete. Concrete can withstand
these effects when properly designed. The Specifier’s Guide for Durable
Concrete, EB221 and Design and Control of Concrete Mixtures, EB001.15 are intended

5. to provide sufficient information to allow the practitioner to select materials and mix
design parameters to achieve durable concrete in a variety of environments.
Resistance to Freezing and Thawing: The most potentially destructive weathering
factor is freezing and thawing while the concrete is wet, particularly in the presence of
deicing chemicals. Deterioration is caused by the freezing of water and subsequent
expansion in the paste, the aggregate particles, or both.
When it has a proper system of microscopic air bubbles, obtained through the addition
of an air entraining admixture and thorough mixing, concrete is highly resistant to
freezing and thawing. These microscopic air bubbles within the concrete accommodate
the expansion of water into ice and thus relieve the internal pressure generated.
Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than
concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete
with a low water-cementitious ratio and an air content of 5 to 8 percent of properly
distributed air voids will withstand a great number of cycles of freezing and thawing
without distress.

6. Chemical Resistance: Concrete is resistant to most natural environments and many
chemicals. Concrete is regularly used for the construction of waste water transportation
and treatment facilities because of its ability to resist corrosion caused by the highly
aggressive contaminants in the wastewater stream as well as the chemicals added to
treat these waste products.
However, concrete is sometimes exposed to substances that can attack and cause
deterioration. Concrete in chemical manufacturing and storage facilities is especially
prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids
attack concrete by dissolving the cement paste and calcium-based aggregates. In
addition to using concrete with a low permeability, surface treatments can be used to
keep aggressive substances from coming in contact with concrete. Effects of
Substances on Concrete and Guide to Protective Treatments, IS001, discusses the
effects of hundreds of chemicals on concrete and provides a list of treatments to help
control chemical attack. Read more on acid resistance.
Resistance to Sulfate Attack: High amounts of sulfates in soil or water can attack and
destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate,
sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated
compounds in the hardened cement paste. These reactions can induce sufficient
pressure to slowly cause disintegration of the concrete.
Like natural rock such as limestone, porous concrete (generally with a high water-
cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples
of salts known to cause weathering of concrete include sodium carbonate and sodium

7. sulfate.
Sulfate attack and salt crystallization are more severe at locations where the concrete is
exposed to wetting and drying cycles, than continuously wet cycles. For the best
defense against external sulfate attack, concrete with a low water to cementitious
material ratio (w/cm) (less than 0.45 for moderate sulfate environments and less than
0.40 for more severe environments) should be used along with cements or cementitious
material combinations specially formulated for sulfate environments
Seawater Exposure: Concrete has been used in seawater exposures for decades with
excellent performance. However, special care in mix design and material selection is
necessary for these severe environments. A structure exposed to seawater or seawater
spray is most vulnerable in the tidal or splash zone where there are repeated cycles of
wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater
require the use of low permeability concrete to minimize steel corrosion and sulfate
attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over
reinforcing steel must be provided, and the water-cementitious ratio should not exceed
0.40.
Chloride Resistance and Steel Corrosion: Chlorides present in plain concrete (that
which does not contain reinforcing steel) is generally not a durability concern. In
reinforced, the paste protects embedded steel from corrosion through its highly alkaline
nature. The high pH environment in concrete (usually (greater than 12.5) causes a
passive protective oxide film to form on steel. However, the presence of chloride ions
from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion
threshold is reached, an electrochemical current is formed along the steel or between
steel bars and the process of corrosion begins.
The resistance of concrete to chloride is good; however, for severe environments such
as bridge decks, it can be increased by using a low water-cementitious ratio (about

8. 0.40), at least seven days of moist curing, and supplementary cementitious materials
such as silica fume, to reduce permeability. Increasing the concrete cover over the steel
also helps slow down the migration of chlorides. Other methods of reducing steel
corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing
steel, surface treatments, concrete overlays, and cathodic protection.
Resistance to Alkali-Silica Reaction (ASR): Alkali-Silica Reaction (ASR) is an
expansive reaction between certain forms of silica in aggregates and potassium and
sodium alkalis in cement paste. The reactivity is potentially harmful only when it
produces significant expansion. Indications of the presence of alkali-aggregate reactivity
may be a network of cracks, closed or spalling joints, or movement of portions of a
structure. Alkali-silica reaction can be controlled through proper aggregate selection
and/or the use of supplementary cementitious materials (such as fly ash or slag cement)
or blended cements proven by testing to control the reaction. With some reactive
aggregates, controlling the concrete alkali level has been successful. Lithium-based
admixtures have also been shown to prevent deleterious expansion due to
ASR. Standard Guide for Reducing the Risk of Deleteerious Alkali-Aggregate Reaction
in Concrete, ASTM C1778, provides thorough guidance.

9. Abrasion Resistance: Concrete is resistant to the abrasive effects of ordinary weather.
Examples of severe abrasion and erosion are particles in rapidly moving water, floating
ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly
related to the strength of the concrete. For areas with severe abrasion, studies show
that concrete with compressive strengths of 12,000 to 19,000 pounds per square inch
(psi) work well.
v) Explain the specification of materials
• The selection is governed by the strength required – depends on the intensity of
loading and the form and size of structural member
• Concrete strength: measured by the crushing strength of cubes or cylinders of
concrete made from the mix
• Identified by its class. Ie: C25/30 – characteristic cylinder crushing strength (fck)
of 25N/mm2 and cube strength of 30N/mm2
B) THE PRINCIPLE OF LIMIT STATE DESIGN
i) Explain the principles and give an example of ultimate limit state (ULS),serviceability
limit state (SLS) and other limit state.
Limit state can be classified in three group :-
 Ultimate limit state: the whole structure or its element should not collapse,
overturn or buckle when subjected to the design loads.

10.  Serviceability limit state: the whole structure should not become unfit for use
due to excessive, deflection, cracking or vibration.
 Other Limit States :May be reached including considerations of durability,
vibration, and fire resistance of structures
Ultimate Limit State Design
A clear distinction is made between the Ultimate State (US) and the Ultimate Limit State
(ULS). The US is a physical situation that involves either excessive deformations
leading and approaching collapse of the component under consideration or the structure
as a whole, as relevant, or deformations exceeding pre agreed values. It involves of
course considerable inelastic (plastic) behavior of the structural scheme and residual
deformations. While the ULS is not a physical situation but rather an agreed
computational condition that must be fulfilled, among other additional criteria, in order to
comply with the engineering demands for strength and stability under design loads. The
ULS condition is computationally checked at a certain point along the behavior function
of the structural scheme, located at the upper part of its elastic zone at approximately
15% lower than the elastic limit. That means that the ULS is a purely elastic condition,
located on the behavior function far below the real Ultimate point, which is located
deeply within the plastic zone. The rationale for choosing the ULS at the upper part of
the elastic zone is that as long as the ULS design criteria is fulfilled, the structure will
behave in the same way under repetitive loadings, and as long as it keeps this way, it
proves that the level of safety and reliability assumed as the basis for this design is
properly maintained and justified, (following the probabilistic safety approach). A
structure is deemed to satisfy the ultimate limit state criterion if all
factored bending, shear and tensile or compressive stresses are below the factored
resistances calculated for the section under consideration. The factored stresses
referred to are found by applying Magnification Factors to the loads on the section.
Reduction Factors are applied to determine the various factored resistances of the
section.

11. The limit state criteria can also be set in terms of load rather than stress: using this
approach the structural element being analyses (i.e. a beam or a column or other load
bearing element, such as walls) is shown to be safe when the "Magnified" loads are less
than the relevant "Reduced" resistances.
Complying with the design criteria of the ULS is considered as the minimum
requirement (among other additional demands) to provide the proper structural safety.
Serviceability Limit State Design
The structural design criteria used for the SLS design of steel-plated structures are nor-
mally based on the limits of deflections or vibration for normal use. In reality, excessive
deformation of a structure may also be indicative of excessive vibration or noise, and
so, certain interrelationships may exist among the design criteria being defined and
used separately for convenience. The SLS criteria are normally defined by the operator
of a structure, or by established practice, the primary aim being efficient and economical
in-service performance without excessive routine maintenance or down-time. The
acceptable limits necessarily depend on the type, mission and arrangement of
structures. Further, in defining such limits, other disciplines such as machinery
designers must also be consulted

12. Other Limit State
Limit state design (LSD), also known as load and resistance factor design (LRFD),
refers to a design method used in structural engineering. A limit state is a condition of a
structure beyond which it no longer fulfills the relevant design criteria.[1] The condition
may refer to a degree of loading or other actions on the structure, while the criteria refer
to structural integrity, fitness for use, durability or other design requirements. A structure
designed by LSD is proportioned to sustain all actions likely to occur during its design
life, and to remain fit for use, with an appropriate level of reliability for each limit state.
Building codes based on LSD implicitly define the appropriate levels of reliability by their
prescriptions.

13. ii) The Characteristic material strength, actions and partial factor of safety for
material and action.
Action
EC2 terminology for loads and imposed deformations.EC2 defines and action (F) as a
force or load applied to structure. Characteristic actions used in design and defined in
EC2 are as follows:
Characteristic permanent action Gk – Self weight of structure, finishing weight
etc.
Characteristic variable action Qk – people, furniture, equipment etc.
Design Action
Design value of an action is obtained by multiplying the characteristic actions Fk
by partial safety factor for actions γf
Fd = Fk x γf
γf accounts for possible increases in load, inaccurate assessment of the effect of
loads, inaccurate modelling of the load
Values of γf are given in EN 1990: Annex A1
Combination Action
Permanent and Variable actions will occur in different combinations. All must be
considered to determine the most critical design situation
Partial factors of safety
Other possible variations such as constructional tolerances are followed for by partial

14. C) REALIZE THE MODE OF FAILURE IN SECTION DURING LOADING
• Reinforcing steel can sustain very high tensile strains, due to the ductile behavior
of steel
• Concrete can accommodate compressive strains which is much lower in
comparison
• The final collapse of a normal beam at ULS is usually cause by crushing of
concrete in compression
• Depending on the amount of RS provided, flexural failure may occur in 3 ways:-
1) Under reinforced design
Reinforced concrete beam sections in which the steel reaches yield strain
at loads lower than the load at which the concrete reaches failure strain
are called under-reinforced sections. Every singly reinforced beam should
be designed as under-reinforced sections because this section gives
enough warning before failure. Yielding of steel in the section does not
mean the structure has failed, as when steel yields, excessive deflection
and cracking in beam will occur before failure which gives enough time to
occupants to escape before the section fails. The failure in under-
reinforced section is due to the concrete reaching its ultimate failure strain
of 0.0035 before the steel reaches its failure strain which is much higher
0.20 to 0.25

15. 2) In balance reinforced design
As given in assumption 2 above that the reinforced concrete
section in bending is assumed to fail when the compression strain in
concrete reaches the failure strain in bending compression equal to
0.0035. Reinforced concrete beam sections in which the tension steel also
reaches yield strain simultaneously as the concrete reaches the failure
strain in bending are called balanced sections.
.

16. 3) Over Reinforced design
Reinforced concrete beam sections in which the failure strain in
concrete is reached earlier than the yield strain of steel is reached, are
called over-reinforced sections. If such beam is designed and loaded to
full capacity then the steel in tension zone will not yield much before the
concrete reaches its ultimate strain of 0.0035. This due to little yielding of
steel the deflection and cracking of beam does not occur and does not
give enough warning prior to failure. Failures in such sections are all of a
sudden. This type of design is not recommended in practice of beam
design

17. a) Structural elements
1. Simplysupportedbeam
2. Continuousbeam
Simplysupported
beam
ContinuousBeam

18. 3. Concrete slab
4. Column
Concrete slab
Column

19. 5. Staircase
6. Wall
Staircase
Wall

20. b) BuildingParts
1. Fire resistance door
2. Fasciaboard
Fire resistance
door
Fasciaboard

21. 3. Rafters
4. Gutter
Rafters
Gutter

22. 5. Downpipe
6. Roof awning
Downpipe
Roof awning

23. 7. Valleyroofing
8. Hip roofing
Hip roofing
Valleyroofing

24. 9. Parameterdrain
10. Mildsteel (m.s) grating
ParameterDrain
Mildsteel grating

25. 11. Drainage sump
12. Top hug window
Draibage sump
Top hug window

26. 13. Casementwindow
14. Adjustable louveredwindow
Casementwindow
Adjustable louvered
window

27. 15. Staircase handrail
16. Roof tiles
Staircase handrail
Roof tiles

28. 17. TilesFinishes
18. CementRenderfinishes
Tilesfinishes

29. 19. Nosingtiles
20. Buildingapron
Nosingtiles
Buildingapron

30. 21. Road bump
22. Road kerb
Road bump
Road kerb