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Practical Tips on Doubler & Gusset Design


In this tip we review and compare some common doubler and gusset designs in terms of both the geometry and the weld detailing. We also rank them in order of best practise and performance for fatigue life and offer some comments on why this is the case.

Realistic analysis of welded structures takes a lot of expertise; particularly when it comes to fatigue strength. If you would like some peace of mind about some welded components you manufacture, please give us a call; we’ve got the tools and experience to help.

Doubler Design

Here are a couple of options to improve the fatigue strength of doubler plates with simple changes to weld callouts and geometry. To indicate the relative performance of each type a Fatigue Strength Reduction factor is used. The higher the factor, the higher the fatigue strength of the welded joint

Fatigue Strength Reduction Factor

Fatigue Strength Reduction Factor

Weld Detailing
Doubler weld 0 Doubler weld 1 (1)
OK – no weld across the end means a sudden change in stiffness between the doubler and section. Weld toe in location of high stress concentration BETTER – WHY? Smoother transition from the doubler into the section
Geometry Detailing
Doubler weld 1 Doubler geometry 2
OK BETTER – WHY? Gradual change in section. No stop/start of weld in location of high stress
Note that the underlying geometry also plays an important part with doubler design; i.e. an optimised doubler on an UB may perform poorly on a RHS box section.

 

Gusset Design

Weld Detailing
Gusset Weld 1 Gusset Weld
OK BETTER – WHY? Weld toe moved away from the sudden change in stiffness
Geometry Detailing
Gusset geometry 1 Gusset geometry 2
OK – but there is still a sudden change of stiffness at Point A BETTER – WHY? Modified gusset design gradually distributes stiffness

 

[All images sourced from: D Radaj and C M Sonsino: Fatigue Assessment of welded joints by local approaches. Woodhead Publishing Ltd. 1998]