Reinforced Concrete Exam 1

Different types of structural systems

Moment resisting Frame

Bearing wall system

Frame/shear-wall hybrid system

Gravity Load Path

Slab, beam, column, footing

One Way floor slab system

(long)/(short)>2

Two way floor slab system

(long)/(short)=1

Lateral Load Path

slab, shear wall, footing

In RC structures most beams and columns are ____members

continuous

The design and analysis of a single member
is dictated by the____

critical section (largest load action)

Simply supported beam

not common in practice but is used to check beams and girders

Fixed End Beam

Common, use when framing goes into a wall

Fixed end beam examples

Beam, Girders, Retaining Walls

Continuous Beam

very common, supports can be columns or intersecting girders

Cantilevered beam

Relatively common, supports can be walls or foundations

cantilevered beam examples

beams, girders, retaining walls, shear walls

moment frame

very common, moments are a function of relative stiffness between elements

shear wall

very common, similar to a cantilever beam but with axial load

Most to Least common members

1. Continuous beam

2. moment frame

3. shear wall

4. cantilevered beam

5. Fixed end beam

6. Simply supported beam

Design Considerations

• Structural safety and serviceability

• Functional requirements

• Economics

• Durability and environmental impact/footprint

Design Phase

• Concept Design

• Schematic Design

• Design Development

• Construction Documents

• Bidding and Permit

construction phase

• Shop Drawings (detailing of reinforcement)

• Inspections

• Installation of concrete formwork and shoring

• Placing reinforcement

• Placing concrete

• Curing the concrete

• Reshoring and special construction procedures

• Respond to RFI and Field Issues

• Field Observations

construction methods

• Cast-in-place Concrete

• Post-tensioned Concrete

• Precast Concrete

• Tilt-Wall

• Concrete Masonry
  

Design Process of RC Structures

1. Design Considerations

2. Design Phase

3. construction phase

4. construction methods

Loads produce load effects

Axial force
Shear
Moment
Torsion

Types of Loads

• Dead loads (constant magnitude and position)

• Live loads (change in magnitude and position)

• Environmental loads (snow, rain, wind, seismic etc.)

• Accidental loads

Working Stress Design (WSD)/ Allowable Stress Design (ASD) disadvantages

• Little knowledge on SF magnitudes against collapse

• Dead and live loads require different safety factors

• Does not account for variations in resistance and loads

• Does not account for the possibility that as loads increase, not all increase at the same rate

Working Stress Design (WSD)/ Allowable Stress Design (ASD)

• Actual loads are used to determine stresses

• Allowable stress is reduced by uniform safety factor

Ultimate Strength Design (USD) or Strength Design (SD)

• SD is a Load Resistance Factor Design (LRFD) method

• More rational approach that WSD

• More realistic consideration of safety

• Member capacities are more accurate than WSD

• Accounts for the stress-strain nonlinear relationship

• Results in a more uniform safety factor against collapse throughout the structure

• Provides with more economical designs

Strength Design (SD-LRFD)

• Load factors are applied to loads based on load type

• Strength is reduced by strength reduction factors, φ,
  depending on type of action

ACI 318-19, Section 5.3.1:

• Provides load combinations to be used in reinforced concrete design

• ACI load combinations involve load effects, not loads

• Load factors for dead loads are much smaller than the ones used for live and environmental loads
  

concrete

a mixture of fine aggregate (sand), coarse aggregate
(e.g., gravel, limestone), cement, water, air and admixtures

Admixtures

materials added to concrete to change certain
characteristics, such as:
– Workability
– Durability
– Hardening time
– Cost

concrete is a rocklike substance and thus has high ___ strength but very low ____strength

compressive

tensile

Reinforced concrete

a combination of concrete and reinforcement
steel

Why are concrete and steel compatible

bond together well

protects from corrosion and fire

similar thermal expansion

Types of Portland cement

Type 1

Type 2

Type 3

Type 4

Type5

Type 1

common, all-purpose cement

must cure about two weeks to achieve sufficient strength to permit removal of forms and application of small loads

Type 2

low heat of hydration and some resistance to sulfates

reaches design strength in about 28 days

Type 3

high, early strength; high heat of hydration

reaches design strength in three to seven days

Type 4

low heat of hydration, used for very large concrete structures

produces high heat of hydration; more likely to cause cracking

Type 5

used for concrete with exposure to high concentration of
sulfates

Aggregates Occupy about ___% of the concrete volume

75

Aggregates relatively____&____

inexpensive and economical

aggregate that passes a No 4 sieve (wires spaced
1/4 𝑖𝑛 on centers)

Fine

aggregate that don’t passes a No 4 sieve

coarse

concrete admixtures

• Air-entraining admixtures
• Accelerating admixtures
• Retarding admixtures
• Superplasticizers
• Waterproofing materials

Properties of Concrete

• Compressive strength
• Static modulus of Elasticity
• Dynamic modulus of Elasticity
• Poisson’s ratio
• Shrinkage
• Creep
• Tensile strength
• Shear strength

Specified compressive strength of concrete

f’c

Ordinary applications, 𝑓′ is ____𝑝𝑠𝑖 to ____𝑝𝑠𝑖

3000, 8,000

Pre-stressed concrete applications, 𝑓′ is ____𝑝𝑠𝑖 to___ 𝑝𝑠𝑖

5,000, 6,000

High strength applications, 𝑓′ is ____𝑝𝑠𝑖 to _____𝑝𝑠𝑖

10,000, 20,000

Maximum strength is at about ___strain

0.002

Ultimate strain is about

0.003 to 0.004 (assume
0.003)

____strength concrete achieves____
ultimate strains

Lower, higher

Concrete has a single modulus of elasticity

true

Dynamic modulus of Elasticity: Generally ___% to___ % higher than the static modulus

20, 40

Poisson’s ratio is important for 

arch dams, tunnels

shrinkage occurs when

concrete cures, water not used in hydration begins to evaporate

___% of shrinkage occurs within the first year

90

___% of creep occurs during the first year

75

creep can cause of ___15%-25%

concrete strength reduction

Tensile strength has a significant impact on

Deflections
– Bond strength
– Shear strength
– Torsional strength

Tensile strength of concrete is about 8% to 15% of ___

f’c

reinforcing steel used for

• Bars or welded wire reinforcement (WWR)

• Bars can be plain or deformed

• Deformed bars come in these sizes: #3 to #11, #14 and #18 #14 and #18 bars are rarely used in normal practice

• For bars up to #8, the diameter of the bar (units of
  in.) is the bar’s number divided by 8

Welded wire reinforcement (WWR):

• Often used in slabs, pavements and shells
  • Easily placed with excellent bond with the concrete
  • Spacing of wires is well controlled
  • W stands for smooth wire and D stands for deformed wire
  • Area of wire follows W or D: ex. W4 area is 0.04𝑖𝑛2
  

ASTM reinforcing steel standards:

• ASTM A615: deformed or plain billet steel. Must be marked with the
letter S (most widely used).
• ASTM A706: low alloy deformed or plain bars. Must be marked with the
letter W. Enhanced weldability or bendability.
• ASTM A996: deformed rail steel or axle steel bars. Must be marked
with the letter R (very limited availability)

Primary Considerations When Designing Reinforced
Concrete Beams

safety

deflection and serviceability

control of cracking

Tensile strength of concrete can be ____ 

neglected

Stages of cracking

Stage 1: uncracked

Stage 2: cracked -elastic stress

Stage 3: cracked- ultimate strength

when concrete is cracked neutral axis ___

moves up

ultimate or nominal flexural moments assume steel yields___

before concrete crushes at the compressive side

ultimate or nominal flexural moments replace nonlinear stress variation with a ____

uniform rectangular one

When concrete crushes at 𝜀𝑐 = 0.003 = 0.3%, then depending on steel strain members are categorized (for 60 ksi rebar)

– Compression-controlled members 𝜀𝑠 < 𝜀𝑦
– Balanced section members 𝜀𝑠 = 𝜀𝑦
– Transition zone members 𝜀𝑦 < 𝜀𝑠 < 0.00507
– Tension-controlled members 𝜀𝑠 > 0.00507

Tension-controlled members are preferred because they have
____behavior

ductile

Strength reduction factor 𝜑 accounts for

– Uncertainties of material strengths
– Inaccuracies in design equations
– Analysis approximations
– Variations in member as-built dimensions
– Variations in steel reinforcement placement
– Member importance

different types of strength reduction occurs

– 0.90 for tension-controlled members
– 0.75 for shear and torsion in beams
– 0.75 for columns with spiral reinforcement
– 0.75 for columns with tied reinforcement
– 0.65 for bearing on concrete