Week 2: Prestressed RC Flashcards
What is prestressed RC?
Compressive stress is applied to concrete prior to service
What are the benefits of prestressed RC?
- Eliminates tensile stresses
- No cracked zone so section is fully operative (material better put to use)
- Cross section can be reduced, improving efficiency
- If no tension allowed, composite section remains elastic in service
- Deflection can be substantially reduced
- Full exploitation of material properties
- Can bridge longer spans than RC
Cables are used instead of bars in prestressed RC. What are the three different options depending on section size?
- Cold drawn wires (For smallest sections)
- 3 or 7 wire strands
- Multi-strand tendons (For largest sections, made up of multiple strands)
How do we create prestressed concrete? (Clue: Steel must be ‘pretensioned’)
- Strands are stressed (tensioned)
- Concrete is cast
- Concrete sets and hardens
- Strands stressed for 1-3 days
- Concrete has gained enough compressive strength
- Steam curing can be used to accelerate hardening
- The strands are then cut
- Prestress force is transferred
- Jacks are removed
Why is eccentric prestressing preferred over axial prestressing?
Eccentric is more efficient than axial prestressing because:
- For a given M service we can reduce the prestressing force P
- This means it’s more economical
- For a given P, maximum M service is increased
- Therefore more capacity
How do you calculate maximum eccentricity?
emax = yb - c
Where:
yb = Distance between bottom fibre and centroid
c = Min concrete cover in order for reinforcement to be in concrete
What is the equation for minimum axial prestressing force required to have all of the section in compression?
What is the equation for minimum eccentric prestressing force required to have all of the section in compression?
What needs to be controlled at transfer during eccentric prestress?
Beam cambers during eccentric prestress
This must be controlled
At transfer, the top fibre can be partially de-compressed or even put in tension
What is the second moment of area of a rectangular section?
I = bd^3/12
In sectional analysis at transfer, what is the equation to calculate elastic modulus of concrete at the time of transfer?
In sectional analysis at transfer, what is the equation to calculate the total prestress force applied?
IMPORTANT: Stress from jack considers the reduction factor due to the loss in nominal strength
Where Ap = No. of strands * Area of one strand
In sectional analysis, and given the following information, how do you calculate the stress induced from jack?
σp,jack = 0.8 * fpk
Where:
fpk = Rupture stress
0.8 = loss due to nominal strength
IMPORTANT: If not told, assume loss due to nominal strength to be 5% / 0.95
In sectional analysis, and given the following information, how do you calculate the strands initial strain?
In sectional analysis, how do you calculate the strands stress before transfer? (Clue: Trick question)
THERE IS NO STRESS OR STRAIN IN CONCRETE BEFORE TRANSFER
In sectional analysis, what is the equation to calculate stresses at bottom fibre?
In sectional analysis, what is the equation to calculate stresses at top fibre?
How can you identify the depth of the neutral axis? (Clue: Ratios, equations required)
Where:
500 is the depth of the section
In sectional analysis, and with the following information, what is the equation to calculate strains in the concrete at the depth of the strands? Ec (t) was previously calculated to be 33000 N/mm^2.
Where 390mm is the depth to the neutral axis.
The final value is supposed to be NEGATIVE
In sectional analysis, how what is the equation to calculate strands strain immediately after transfer?
Where 0.0076 is strain before transfer
And -0.000188 is the strain in concrete at depth of strands
In sectional analysis, how can you calculate the prestress force loss due to the elastic deformation of concrete?
