CE 315: Design of Concrete Structures I
Dr. Tahsin Reza Hossain
Professor, Room No-649
Email: tahsin@ce.buet.ac.bd
Syllabus
• Fundamental behaviour of reinforced concrete
• Introduction to WSD and USD methods
• Analysis and design of singly reinforced,
doubly reinforced and T-beams according to
WSD and USD methods
• Shear and Diagonal tension
• Bond and anchorage according to WSD and
USD methods
• One-way slab
Books
• Design of Concrete Structures
– Nilson, Darwin, Dolan
14th Ed
• Structural Concrete- Theory and Design
– Hassoun, Al-Manaseer 4th Ed
• Reinforced Concrete- Mechanics & Design
– Wight & McGregor 5th Ed
Many more……..
Concrete, Reinforced Concrete (RC),
Prestressed Concrete (PC)
• What is concrete? Constituents?
– Stone like material, cement, coarse and fine aggregate,
water, admixture
• A bit of history
• Advantages, disadvantages
– Easy to make, relatively low-cost, formabilty, weather and
fire resistant, good comp strength
– Weak in tension
• Reinforced concrete-mild steel
• Where to place the reinforcement-examples
• Prestressed concrete
Roman Pantheon, unreinforced concrete
dome, diameter 43.3m, 25BC, 125AD
Structural forms: buildings
•Beam
•Column
•Slab
Loads
•Dead load
attached
•Live load
not attached
•Environmental load
•Wind
•Earthquake
•Snow, soil pressure, temperature
•Building codes- ACI, BNBC, IS, Eurocode
Wind
Load
Earthquake
Loads
Serviceability, Strength and Structural Safety
• To serve its purpose, a structure must be safe
against collapse and serviceable in use
• Strength of the structure be adequate for all
loads
• Serviceability – deflection small, hairline
cracks, minimum vibration
Strength and safety
• If loads and moments,
shears, axial force can be
predicted accurately,
safety can be ensured by
providing a carrying
capacity just barely in
excess of the known
demand.
• Capacity= Demand
Uncertainity
• There are a number of sources of uncertainty
in Analysis, Design and Construction
• Read 7 points
• Consideration given to consequence of failure
• Nature of failure is also important
Variability of Loads,
Strength, safety
Load can be considered as
random variable
Form of distribution curve
(probability density function) can
be determined from large scale
load survey
Probability of occurrence
Area under curve is probability of
occurrence
Qd design load
Sd Design strength
M is also a random variable
Beta between 3 and 4
corresponds to a probability of
failure of 1:100,000
Partial safety factor
• Strength reduction factor X
Nominal Strength >
Load Factor X Design Load
Why partial factors are
different
Concrete
Steel
Design Basis
• Strength Design
• Load factoredhypothetical overload
stage
• Material stress level
–
–
–
–
Nonlinear inelastic
Concrete fc’
Steel reaches fy
Both or one
• USD
– Ultimate Strength Design
• Service load design
• Load unfactored
– Service load
• Material stress level
– At allowable stresses
– Half of fc’
– Half of fy
• WSD
– Working Stress Design
Design Codes and Specifications
• International Building Code- consensus code
• American Concrete Institute ACI Code- Building
Code requirement for Structural Concrete -3182008
• AASHTO- American Association of State Highway
and Transportation Officials- for bridges
• American Railway Engineering and Maintenance
of Way Association –AREMA-Manual of Railway
Engineering
Bangladesh National Building Code
• BNBC
• First in 1993
• Up-gradation is in progress
Safety provision of ACI/BNBC Code
Load factors
Probability of overload 1/1000
Strength reduction factor
Probability of understrength 1/100
• Probability of Structural failure
1/100,000
Fundamental Assumption for RC Behavior
1.
2.
3.
4.
5.
Equilibrium
Strain in steel=Strain in surrounding concrete
Plane cross section remain plane
Concrete does not resist any tension
The theory is based on the actual stressstrain relationship of concrete and steel or
some simplified equivalent.
Read last para
Behaviour of members subject to Axial Loads
• Fundamental behaviour illustrated
• Axial Compression
– Economical to make concrete carry most loads
– Steel reinforcement is always provided
• Bending may exist
• Cross section reduced
RC Column
Square, tied column
Tie
• Hold longitudinal bar
during construction
• Prevent bucking under load
Circular spirally reinforced
column
Spiral
• same
• confinement to concrete
fc’=4,000 psi
fy= 60,000psi
•Slow loading
•Fast loading
•0.85fc’
Elastic behaviour
• Up to fc’/2, concrete behave elastic
• Also stress and strain proportional
• Range extends to a strain of 0.0005
• Steel is elastic nearly to yield 60 ksi, strain 0.002
Valid up to 50 to 60 percent of fc’
Inelastic range
Strength
Strength
Axial Tension
• If tension is small, both steel and concrete are
elastic
• Larger load than that cracks concrete
• At steel yields