Typical construction errors and how to avoid them: Error #2

This is a continuation of my blog about frequent construction errors.

Error #2: Stiffness jumps

Nature shows us for example in trees, that stability and strength when loading is achieved by weight, wind, snow and rain through soft transitions.

Baum SteifigkeitssprüngeA tree is starts sprouting out from a solid base, uniformly getting slimmer. The areas with the highest loads have the largest cross sections, the light leaves are only supported by thin branches.

The load distribution in the soil is carried out through a network of roots, which usually have the same dimension as the crown.

There are no great leaps in stiffness, the transitions are rather fluid

As a result, the stress gradient in the trunk and the branches is uniform.

Stiffness jumps, as hard as they are to find in nature, lead to local stress peaks, which give rise to cracks caused by changing loads.

Design flaws which often occur in cases of damage, are thin parts which are connected to relatively thick components without soft transitions.

Typical design flaws are outlined below:

Welded constructions with sheetsBlech

Sheets which carry forces and are attached to other sheets via welded joints have stiffness jumps at the marked points and thus unnecessary stress peaks (circled in red).

This can be avoided by phasing out the joint sheets.

Stiffness jumps are and also can be found where massive structures are connected with relatively soft components and transfer loads.

Shafts and rotors with large diameter jumpsWellemessersprüngen

In the event of broken shafts and rotors, the cause of the damage is often due to the fact that the radii of curvature or if necessary, the shaft angle is too small.

This may be avoided through an adjustment of the diameter, possibly even an intermediate step and large fillets or recesses.

Conclusion: stiffness jumps cause high stress peaks at crossing points, of which cracks are caused by changing loads.

The ideal body from the perspective of strength has the same stress level everywhere at the load.

However, tensions at the edges of the press seats are rather uncritical, since these are principally compressive stresses. There will be a lamination on the edge which does not derogate the fatigue strength. As a result, the stress is uniformed.

Looking forward to your questions 🙂

Stefan Merkle

➞ Read part 3 of this article