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 this module covers
- 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
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.
Start lesson →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.
Start lesson →
Module checkpoint
Check what you have taken in
8 questions
Question 1
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?
Question 2
A detail cycles between 210 and 130 N/mm². What is the stress range, in N/mm²?
Question 3
The stress range at a detail falls from 120 to 80 N/mm². By what factor does the life increase? Take m = 3.
Question 4
A category 90 detail with γMf = 1.35 is checked for 2 million cycles. What is the permissible stress range, in N/mm²?
Question 5
A detail category improves from 71 to 125. By what percentage does the permissible stress range rise?
Question 6
A moment of 547 kNm acts on a section with Wel = 9377 cm³. What is the stress, in N/mm²?
Question 7
Why does a crane girder end up so much larger than its bending check requires?
Question 8
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?