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Queensferry

Module 06

Serviceability: cracking, deflection and durability

What the structure will actually be like to own. Unfactored loads, a cracked elastic section, and the two effects that decide whether a concrete floor is a success.

What this module covers

  • Analyse a cracked section elastically and find its real stiffness
  • Calculate the cracking moment and say what changes when it is passed
  • Explain crack width as spacing times strain difference
  • Use the span-to-depth rule, and say honestly what it is and is not
  • Explain creep, and why it doubles or trebles a deflection
  • Select cover from exposure and bond requirements

Lessons

  1. 6.1

    The cracked elastic section

    The state a beam spends its life in — and a different calculation entirely from the ultimate one.

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  2. 6.2

    Crack width and deflection

    The two checks that decide whether a floor is a success — and the empirical rule that usually replaces one of them.

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Module checkpoint

Check what you have taken in

6 questions

  1. Question 1

    Which load combination should be used for a deflection check?

  2. Question 2

    A 250 × 450 mm C30/37 beam. What is its cracking moment, in kNm? Take fctm = 2.90 N/mm².

  3. Question 3

    Increasing cover from 30 mm to 50 mm for durability. What happens to the calculated crack width, all else equal?

  4. Question 4

    A beam with Ig = 3.125 × 10⁹ mm⁴ has a cracked second moment of area of 1.13 × 10⁹ mm⁴. By what percentage does its flexural stiffness fall on cracking? Give the answer as a percentage.

  5. Question 5

    An interior span of a continuous beam has d = 550 mm and a basic span/depth ratio of 14. What is its maximum span, in metres? Take K = 1.5.

  6. Question 6

    What does a creep coefficient of φ = 2 mean physically?