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Lessons of Injaka

I read with interest but no pleasure your coverage of the Injaka Bridge collapse over the last few months, especially the latest news report identifying the cause of collapse. If there can be any positive aspect to a catastrophe resulting in such large loss of life, this can perhaps only be in the form of learning lessons for the future.

Equilibrium of parts of a beam is as much dependent on the adequacy of shear strength as on bending strength. Design for bending strength is usually very elaborate and failures often ductile, providing some signs of distress before collapse. Design for shear strength is generally rudimentary and often an afterthought, yet collapses due to shear strength deficiencies are often sudden and rarely show adequate signs of distress prior to collapse. This justifies Codes' cautious approach of incorporating somewhat larger factors of safety against this form of collapse.

We have come across a few instances of even large bridges where designers appeared to have overlooked the need for providing adequate shear resistances at the appropriate sections of the bridge.

In incrementally launched bridges, designs should have to cater for the reversals of not only the bending moments but also of shear forces as sections are advanced from support to mid-span positions during the launching process.

Inevitably in long span box section bridges, the equilibrium shear flow through the narrow webs causes relatively large shear stresses. Provided shear stresses do not exceed the 'web crushing' limits, these are resisted by the provision of reinforcement and prestress as appropriate. A deceptive aspect of shear stresses, however, is that the equivalent (complementary) shear stress simultaneously acts on the longitudinal plane of the structure. If this is not adequately catered for, it can result in 'sliding' failure in the web or at the web/flange interface, greatly reducing the stiffness and strength of the structure. Deficiency in this capacity appears to have been the root of this collapse.

The international trend of ever greater emphasis on acquainting with the 'soft' facets of engineering leaves little time or emphasis on properly understanding the numerous subtle and complex concepts forming the 'core' of our engineering discipline. The key to reducing the potential for such disasters is, in my view, to begin to redress this balance.

Arvind Kumar (F), Principal, Kumar Associates Consulting Engineers, 2 Penn Road, Beaconsfield, Bucks HP9 2PX

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