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CIVE3165 Structural Design 2 > How to analyse prestressed RC beam > Flashcards

How to analyse prestressed RC beam Flashcards

1
Q

What are the steps to analysing a prestressed RC beam?

A
  1. Consider the loads
  2. Calculate moments @ critical section (or stated position)
  3. Calculate the no. of strands
  4. Distribute strands in section
  5. Calculate A* and I* of transformed section
  6. Calculate stresses @ top and bottom fibres
  7. Calculate reinforcement index
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2
Q

What must be remembered when considering loads?

A
  • Loads may be in SDL form
  • Don’t forget SW
  • Break down into variable and permanent
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3
Q

What are the equations for calculating moments @ critical section for both UDLs and PLs?

A

(UDL*L^2) / 8 = BMmax

(PL * L) / 4 = BMmax

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4
Q

What are the partial factors of safety for permanent and variable loads?

A
  • For permanent: 1.35
  • For variable: 1.50
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5
Q

When calculating moment @ critical section right after transfer, what loads and safety factors do you include?

A
  • SW load only
  • Therefore just 1.35 as a safety factor
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6
Q

When calculating moment @ critical section in service, what loads and safety factors do you include?

A
  • ALL loads
  • 1.35 for permanent, 1.5 for variable
  • Variable load UDL must also be multiplied by 0.7 to convert characteristic variable action into representative one
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7
Q

How would you calculate the number of strands required in the section?

A
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8
Q

When distributing strands, what are the min spacing checks?

A

Cover must be >1.5 * diameter of strand where if <30mm, 30mm should be used.

Clear spacing >3 * diameter of strand where if <20mm, 20mm should be used

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9
Q

How do you calculate the max bars in bottom of section?
Clue: Equations

A
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10
Q

How do you calculate average eccentricity and therefore distribute strands?

A
  • Work from the equation and perform trial and error until average achieved
  • Average eccentricity < Target eccentricity
  • Make sure all minimum spacing checks are satisfied!!
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11
Q

During the process of calculating A* and I*, the first stage is to calculate the area of concrete. What is the equation to calculate Ac?

A

Ac = A - N*Ap(1)

Where:
A = Cross sectional area from Design Table
Ap(1) = Area of one prestressing steel strand

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12
Q

During the process of calculating A* and I*, the second stage is to calculate the elastic moduli ratios Emod. What is the equation to calculate np and what must we remember?

A

np = Ep / Ec

Ep = Youngs modulus of prestressing steel
Ec = Youngs modulus of concrete

  • Remember to do @ transfer and in service!
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13
Q

During the process of calculating A* and I, the third and final stage is to calculate the values of A and I*. What are the equations to do so and what must we remember?

A
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14
Q

When calculating the stresses @ top and bottom fibres, what do we need to remember?

A
  • Remember signs!!
  • If @ critical section, it will be on span therefore equation signs will be normal
  • If working @ supports, signs will change!
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15
Q

What are the equations for calculating the stresses @ top and bottom fibres @ critical sections @ TRANSFER?

A
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16
Q

What are the equations for calculating the stresses @ top and bottom fibres @ critical sections IN SERVICE?

A
17
Q

What are the checks when calculating the stresses @ top and bottom fibres @ critical sections IN SERVICE and @ TRANSFER?

A
18
Q

In terms of Magnel’s diagram, what should our average eccentricity ideally be for optimal sustainability both in terms of resource efficiency and economics?

A
  • Close to the emax but not equal to it
19
Q

How can we adjust designs to improve them?

A
  • Reduce or increase prestressing force
  • Change section size parameters
  • You can use Magnel’s diagram to see the feasibility region and therefore make adjustments accordingly
20
Q

What state are we considering when calculating the reinforcement index?

A
  • In service
21
Q

What is the equation to calculate Mcr, the first step of the reinforcement index process.

A
22
Q

What is the equation for the ultimate moment Mu in the reinforcement index calculation. Please also list the equations to calculate fpd, fcd, d.

A
23
Q

What is the reinforcement index check to determine whether the section fails by cracking (satisfactory and safe)?

A

Mu > 1.2 * Mcr

24
Q

What is the equation to determine the type of failure @ ULS and the associated check?

A

CIVE3165 Structural Design 2 (3 decks)

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