Lec01 Introduction to RC, Codes and Limit States (Reinforced Concrete Design I & Prof. Abdelhamid Charif)

24/2/2013
CE370 Prof. A.harif 1
CE 370
Reinforced Concrete Design-I
Introduction to Reinforced Concrete
and Building Codes
What is Reinforced Concrete ?
• Concrete constituents ?
• Concrete strength ?
• “Reinforced” = ?
• Why add steel bars ?
24/2/2013 February CE 370: Prof. A. Charif 2

24/2/2013
Concrete
 Rocklike Material
 Ingredients
– Portland Cement
– Coarse Aggregate
– Fine Aggregate
– Water
– Admixtures (optional)
Concrete and Reinforced Concrete
 Concrete has high compressive strength and low
tensile strength.
 Reinforced concrete is a combination of concrete
and steel. The reinforcing steel is used to resist
tension.
 Reinforcing steel can also be used to resist
compression (columns).

24/2/2013
All dimensions in mm
ø6@150
200
265
300
3-ø20
1-ø6
Reinforcement inside mould
Casting of RC beams
Casting of a Reinforced Concrete Beam Specimen in the Lab
RC Beam Testing
Beam section
Advantages of Reinforced Concrete
 High compressive strength relative to unit cost
 Resistance to effects of fire and water
 High stiffness
 Low maintenance cost
 A long service life
 Often the only economical material for footings, floor
slabs, basement walls and piers
 Architectural flexibility
 Uses local materials for aggregate
 Labor skills are not as high as in steel structures

24/2/2013
Concrete Properties
 Versatile
 Strong & Durable
 Does not Rust or Rot
 Resists Fire
 Does Not Need a Coating
Disadvantages of Reinforced Concrete
 Forms are required to hold the concrete until it
hardens. Formwork is expensive.
 Heavy. Concrete has relatively low strength when
compared to its unit weight.
 High unit weight translates into large dead load.
 Concrete members are relatively large, which
increases structural dimensions. Deep beams lead
to larger story heights and taller buildings.
 Quality control is difficult.

24/2/2013
Building Codes
• All reinforced concrete structures should conform to
certain minimum specifications and requirements,
imposed by building codes, with regard to design and
construction.
• Each nation, or group of nations, must have its own
code for reinforced concrete
• In the Kingdom of Saudi Arabia, Saudi Building Code
(SBC) 304 was issued in 2007.
• SBC 304 was inspired from the American ACI 318 code
Building Codes used in this course
• The Saudi Building Code (SBC 304-2007)
Concrete Structures
• American Concrete Institute, 2008 (ACI 318-
08). Building Code Requirements for
Structural Concrete
• The Saudi Building Code (SBC 301-2007)
Loading

24/2/2013
CE 370
Reinforced Concrete Design-I
Limit States and
Design Philosophy
Limit States
• Limit state: A condition at which a structure or some
part of a structure ceases to perform its intended
function.
• When a structure or part of it becomes unfit for its
intended use, it is said to have reached a limit state
• Violation of a limit state does not necessarily mean
that the structure has failed or collapsed. It implies
failure in the sense that a clearly defined limit state
of structural usefulness has been exceeded.
• There are three groups of limit states
24/2/2013 February C370: Prof. A. Charif 12

24/2/2013
Limit States
• There three groups of limit states:
1. Ultimate Limit States
2. Serviceability Limit States
3. Special Limit States
1 - Ultimate Limit States (ULS )
• Ultimate limit states (ULS) concern structural
safety against total or partial structural
collapse.
• Since this may lead to loss of life and major
financial losses, ULS must have a very low
probability of occurrence.

24/2/2013
1 - Ultimate Limit States (ULS )
Major Ultimate States are:
a) Loss of equilibrium (total or partial)
b) Rupture (total or partial)
c) Progressive collapse (successive member
failures, e.g. during explosions)
d) Formation of a plastic mechanism (yielding of
steel)
e) Instability (such as local or global buckling)
f) Fatigue (failure caused by cyclic loading)
2 - Serviceability Limit States
• Serviceability limits state (SLS) refer to the
performance of structures under normal service
loads, with use and occupancy of structures.
• There is less risk of loss of life than in ULS. A
higher probability of occurrence is tolerated.
• To satisfy serviceability limit state, deflections,
cracking and vibration must not be excessive.
• Violation of serviceability limit state may disrupt
the use of structures but does not usually involve
collapse.

24/2/2013
3 - Special Limit States
• Special limit states refer to structural damage
or failure caused by abnormal or exceptional
loadings:
Extreme earthquakes
Fire, explosions, vehicular collisions
Effects of corrosion and deterioration
Limit State Design
• Serviceability limit state is usually more tolerated
than ultimate limit state as it is less dangerous
(no loss of life risk)
• Design is generally performed using the ultimate
limit state, and then serviceability limit state is
checked (deflections, cracks, vibrations)
• Exceptions: Water and liquid containers (no
cracking allowed, service limit state is more
important)

24/2/2013
Design Philosophy
• The object of reinforced concrete design is to
achieve a structure that will result in a safe and
economical solution.
• The structure resistance must always exceed the
applied loads effects:
• Resistance ≥ Load effects
• Variability in structure resistance and in applied
loads must be considered
• For safety, design loads must be increased and
design strength must be reduced
Design Philosophy
• For safety, design loads must be increased and
design strength must be reduced
• fRn ≥ α1L1 + α2L2 + ...
• Rn = Nominal strength = Real specific strength
• fRn = Design strength = Nominal strength
multiplied by a reduction factor (less than unity)
• f = Strength reduction factor (less than unity)
• L1 , L2 … : Various load cases (dead, live, …)
• α1 ,α2 … : Load factors (greater than unity)

24/2/2013
Design Philosophy
• For bending moments (using Dead and Live
loads):
• fMn ≥ αDMD + αLML + ...
• For shear forces and axial forces:
• fVn ≥ αDVD + αLVL + ...
• fPn ≥ αDPD + αLPL + ...
Design Philosophy
• fMn ≥ αDMD + αLML + ...
• fVn ≥ αDVD + αLVL + ...
• At ultimate state the, combined effect is called
“ultimate” (ultimate moment, ultimate shear…):
• fMn ≥ Mu = αDMD + αLML + ...
• fVn ≥ Vu = αDVD + αLVL + ...
• At serviceability state, the combined effect is
called “service” (service moment…)

24/2/2013
Design Procedures
• There are two main methods for the
design of reinforced concrete,
prestressed concrete, as well as steel
structures:
The working stress method
The ultimate (strength) load method
Working Stress Method
• The basis of working stress method is that the permissible
(allowable) stresses for concrete and steel are not exceeded
any where in the structure when it is subjected to the worst
combination of working loads.
• There is no load magnification but the strength is reduced to
allowable limits (dividing by safety factors greater than unity)
• The working (allowable) stress method can be expressed as:
FS = Factor of safety, greater than unity
R = Resistance
Rall = Allowable resistance
L = Working load effects
all
all
FS
R
RL
 

or

24/2/2013
Ultimate Limit State Method
• The object of design based on limit state method is to achieve
an acceptable probability that a structure will not reach a limit
state in its life time . Ultimate state limit is most used.
• This method of design takes into account the uncertainties in
the material properties and loads through strength reduction
factors and load magnifying factors.
• The ultimate limit state method can be expressed as:
f = Strength reduction factor, less than unity
R = Resistance
Li = Working load effects
αi = Load factors, greater than unity


n
i
ii LR
1
f
Ultimate Limit State Method
f = Strength reduction factor, less than unity
R = Resistance
Li = Working load effects
αi = Load factors, greater than unity
• The summation sign denotes the combination of load effects
from different load sources, such as dead load, live load, wind
or earthquake loads, etc…
• In the limit state concept of design of reinforced concrete
structures, f and αi are called partial safety factors and are
determined using probabilistic methods.


n
i
ii LR
1
f

24/2/2013
ULS method versus WS method
• With a well reduced allowable strength, the working
stress (WS) method uses linear elastic analysis.
• The WS uses a single factor of safety whereas the
limit state method uses various partial safety factors
which can be adapted to the various uncertainties
associated with strength and loadings.
• This is the major advantage of the limit state method
• The working stress method does not account
properly for the variability of strength and loads and
is therefore unable to deliver an objective estimation
of the level of safety.
Design strategy
• Usually members are designed using ultimate
limit state and serviceability limit state is then
checked (deflections, vibrations, cracks)
• Exceptions: Water tanks and similar liquid
containing structures

24/2/2013
Strength Reduction Factors f
[1] Axial Tension f = 0.90
[2] Flexure f = 0.90
[3] Axial Compression w or w/o flexure
(a) Member w/ spiral reinforcement f = 0.70
(b) Other reinforcement members f = 0.65
[4] Shear and Torsion f = 0.75
LOADING
• Accurate estimation of the loads that may be
applied on a structure during its life is very
important. It is a difficult task faced by the
structural designer.
• No load that is reasonably expected to occur
should be ignored.
• After loads are estimated, the next problem is
to decide the worst possible combinations of
these loads that might occur at one time.
30

24/2/2013
Loading
Structural loading includes various types
• Dead load
• Live load
• Environment load (Wind, Earthquake..)
• Loads are specified in SBC 301
Dead Loads
• Dead loads are loads of constant magnitude
that remain in one position. They include:
• Weight of the structure under consideration
such as beams, columns, frames, walls, floors,
ceilings, stairways, roofs etc.
• Any fixtures that are permanently attached to
the member or structure.
32

24/2/2013
Dead Loads
Weight of all permanent construction (includes
self weight SW + superimposed dead load SDL)
DL = SW + SDL
SDL = Weight of any material resting on
member
Dead load is constant in magnitude and
location
Live Loads
• Live loads can change in magnitude and position. They include
occupancy loads, warehouse materials, equipments, …
• Other types of Live loads:
• Traffic loads for bridges: Concentrated loads of varying
magnitude caused by groups of trucks or train wheels.
• Miscellaneous loads:
– Soil Pressure (Retaining walls)
– Hydrostatic pressures (Water pressure on dams, tanks …)
– Blast loads (caused by explosions, sonic bombs, …)
– Centrifugal forces (as on curved bridges)
34

24/2/2013
Live Loads
 Loads produced by use and occupancy of the
structure.
 Maximum loads likely to be produced by the
intended use must be considered.
 Not less than the minimum uniformly
distributed load given by SBC-301 Code.
Typical Live Loads in Buildings

24/2/2013
Environmental Loads
 Snow Loads (not in KSA)
 Earthquake
 Wind
 Soil Pressure
 Ponding of Rainwater
 Temperature Differentials
Snow or Ice loads
• Snow is a variable load, which may cover an
entire roof or only part of it.
• In the colder regions, snow and ice loads are
often quite important.
• The snow loads that are applied to a structure
are dependent upon many factors, including
geographic location, the pitch of the roof,
sheltering, and the shape of the roof.
• No snow load in KSA
38

24/2/2013
Rain loads - Ponding
• If water on a flat roof
accumulates faster than it runs
off, the result is called ponding
• Ponding causes the roof to
deflect into a dish shape that can
hold more water, which causes
greater deflections, and so on.
• The ponding process continues
until equilibrium is reached or
until collapse occurs.
• Ponding is a serious matter. A
Large number of flat-roof failures
occur due to ponding every year.
39
Wind
• The wind loads vary with the wind velocity as well as the
structural surfaces exposed to wind pressure.
• Each country has a “Wind Map” giving maximum wind
velocities
• Forces resulting from wind pressure are proportional to
exposed surfaces
• Wind action is dynamic in nature and generates inertia
forces
40

24/2/2013
Seismic or Earthquake Loads
• During earthquakes the ground is displaced (acceleration),
and because structures are connected to the ground, they
are also displaced and vibrated. As a result, various
deformations and stresses are caused throughout the
structures.
• Although both dynamic, seismic and wind effects are very
different (a heavy structure is desirable to resist wind
loads, but increases seismic forces)
• Seismic forces depend on distribution of the mass and
stiffness in the building
• Each seismic country has a “seismic map”
41
Lateral Wind and Seismic Forces
• Wind ad seismic loads act mainly in the lateral
(horizontal) direction
• They also have vertical components but of less
magnitude.
• As any lateral direction can be considered, it is
common in design to consider two perpendicular
(principal) axes.
• For each direction, positive and negative orientations
must be considered.
42

24/2/2013
Selection of Design Loads
• Building codes and specifications provide
conservative estimates of live-load magnitudes
for various situations.
• Commonly used specifications are:
• In KSA, SBC 301 define loads in buildings
• For highway bridges, American Association of
State Highway and Transportation Officials
(AASHTO).
43
Load Factors
• Load factors are numbers, almost always greater than
unity, which are used to increase the estimated loads
applied to structures.
• These factors account for the uncertainties involved in
estimating their magnitudes.
• They also account for possibilities of combining different
loads together
• Note: Load factors for dead loads are much smaller
than the ones used for live and environmental loads
because dead loads can be estimated more accurately
than live and environmental loads.
44

24/2/2013
SBC Load Factors and Combinations
• SBC defines the critical design load effect (ultimate) as resulting
from any of the following seven combinations :
45
loadearthHorizontal:6.10.19.0/7
loadEarthquakeloadeTemperatur6.16.19.0/6
loadRainloadLiveRoof0.10.12.1/5
loadWind:loadLive)or(5.00.16.12.1/4
loadFluidloadDead)8.0or0.1()or(6.12.1/3
)or(5.0)(7.1)(4.1/2
)(4.1/1
HHEDU
E:T:HWDU
R::LLEDU
WL:RLLWDU
F:D:WLRLDU
RLHLTFDU
FDU
r
r
r
r







Load factors less than unity result either from small probabilities of
combinations of some load cases, or consider indirectly the upward
vertical seismic / wind effects (by reducing dead load).
Options in combinations (2) to (4) and alternate orientations of
wind / seismic loads, result in more than seven different values.
SBC Load Factors and Combinations
46
loadearthHorizontal:6.10.19.0/7
loadEarthquakeloadeTemperatur6.16.19.0/6
loadRainloadLiveRoof0.10.12.1/5
loadWind:loadLive)or(5.00.16.12.1/4
loadFluidloadDead)8.0or0.1()or(6.12.1/3
)or(5.0)(7.1)(4.1/2
)(4.1/1
HHEDU
E:T:HWDU
R::LLEDU
WL:RLLWDU
F:D:WLRLDU
RLHLTFDU
FDU
r
r
r
r







For single independent load effects (beam bending, shear…), the
critical combination is the one giving the maximum value.
For dependent load effects (such as axial force and bending in
columns), the critical combination is in general not obvious. The
designer must check safety for all possible combinations.

24/2/2013
Example
47
The axial force values acting on a column have been determined:
• Dead load = 670 kN
• Live load from roof = 265 kN
• Live load from other floors = 1335 kN
• Wind compression = 310 kN Wind tension = 265 kN
• Seismic compression = 220 kN Seismic tension = 178 kN
• (Wind and Seismic loads acts in two opposite directions and cause
compression or tension forces on the column)
Determine the critical design (ultimate) axial force using SBC
Combinations (single load effect).
D = 670 kN, L = 1335 kN, Lr = 265 kN, W = 310 / - 265 kN
E = 220 / - 178 kN , F = T = R = H = 0
Solution
48
 
 
kN425)0(6.1)178(0.16709.0)(
kN823)0(6.12200.16709.0)(
6.10.19.0
kN17906.1)265(6.16709.0)(
kN109906.13106.16709.0)(
6.16.19.0
kN196113350.1)178(0.16702.1)(
kN235913350.12200.16702.1)(
0.10.12.1
kN5.1847)265(5.013350.1)265(6.16702.1)(
kN5.2767)265(5.013350.13106.16702.1)(
)or(5.00.16.12.1
kN1016))265(8.0()265(6.16702.1)(
kN1476)3108.0()265(6.16702.1)(
kN2563)13350.1()265(6.16702.1)(
)8.0or0.1()or(6.12.1
kN3340.065)2(5.0)01335(7.1)00670(4.1
0)(Note)or(5.0)(7.1)(4.1
kN938)0670(4.1)(4.1
7
7
7
6
6
6
5
5
5
4
4
4
3
3
3
3
2
2
11



















Ub
Ua
HEDU
Ub
Ua
HWDU
Ub
Ua
LEDU
Ub
Ua
RLLWDU
Uc
Ub
Ua
WLRLDU
U
RRLHLTFDU
UFDU
r
r
r
Largest value = U2 = 3340.0 kN (13 values from 7 combinations)

24/2/2013
Case of Dead and Live loads only
• In this course CE370, only dead and live loads are
considered, with the following SBC combinations:
• Ultimate combination: 1.4 D + 1.7 L
• Service combination: D + L
49
Thank you

Lec01 Introduction to RC, Codes and Limit States (Reinforced Concrete Design I & Prof. Abdelhamid Charif)

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