Structural Steel Design / module 17
Crane-supporting structures
A member with no fixed critical section, checked in positions rather than at a point — and then sized by a limit state that has not appeared anywhere else in this course.
What you will understand
- Find the absolute maximum moment from a group of moving wheel loads
- State and use the classical equidistant condition, and check a search against it
- Say what the dynamic factors represent and where they come from
- Explain why fatigue depends on stress RANGE and not on peak stress
- Apply the cube law, and say what doubling the life actually requires
- Say why improving a detail usually beats increasing a section
Lessons
Moving loads: a member with no critical section
Everything so far has had a worst point you could find and check. A crane girder does not — the worst point depends on where the crane is standing.
Fatigue: the limit state that sizes the girder
A crane girder is one of the few building structures where fatigue governs — and where the cheapest fix is a change of weld rather than a change of section.
Module summary
- A moving load has no fixed critical section — the maximum was 5.125 m from the support
- It occurs under a wheel, with that wheel and the resultant equidistant about mid-span
- 697 kNm against 955 for the same total at mid-span — the guess is 37% high
- Dynamic factors are inertial effects, not margins: φ₂ = 1.39 added 27%
- Fatigue is driven by RANGE: 160→100 and 60→0 are identical fatigue conditions
- Life goes with the CUBE of the range, so doubling it needs a 21% reduction, not 26%
- Strength picked a 123 kg/m girder at 54% utilisation; fatigue needed 384 kg/m
- Improving the detail from category 71 to 125 saved 133 kg/m — a third of the weight
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Practice
A crane has 150 kN self-weight and 400 kN hoist load, with φ₁ = 1.10 and φ₂ = 1.22. What is the dynamic vertical action, in kN?
Practice
A detail cycles between 210 and 130 N/mm². What is the stress range, in N/mm²?
Practice
The stress range at a detail falls from 120 to 80 N/mm². By what factor does the life increase? Take m = 3.
Practice
A category 90 detail with γMf = 1.35 is checked for 2 million cycles. What is the permissible stress range, in N/mm²?
Practice
A detail category improves from 71 to 125. By what percentage does the permissible stress range rise?
Practice
A moment of 547 kNm acts on a section with Wel = 9377 cm³. What is the stress, in N/mm²?
Check yourself
Why does a crane girder end up so much larger than its bending check requires?
Check yourself
Two details carry the same stress range. One cycles about a mean of 30 N/mm², the other about a mean of 200. Which has more fatigue damage?