Five Ways Out of a Design Corner
Factors of Safety, when it comes down to it, are just a formal mechanism for over-designing structures to provide capacity against the unknown.
I expect I’m not alone in wishing for a day where my design technique is so perfect I can just expel completed structures from my pencil onto the calculation pad. Until then, however, a major factor of any design is iteration; assumptions are proven wrong, the site investigation returns unexpected results, initial member sizes are under capacity… It all ends in revisiting and reiterating. This would be fine if design was separate from construction, but often program and pre-costing requirements fix the design before it is truly completed, and it’s easy to find yourself stuck into a corner. This is especially true for retro-fitted works, where an existing structure doesn’t quite make the grade, but the client can’t afford a new one.
Here are a few techniques I’ve picked up over my time, which can offer an escape when things just aren’t working out.
There’s always the “but would it really happen?” discussion. Engineering codes tend to be comprehensive, focusing on major Civils works and the many possible actions that might be-fall them. As a rule, standards should be followed, but when the design is tight it may be time to ask yourself if an over-weight articulated lorry really can get up the tiny access ramp? My favourite example of this is the new ‘service vehicle’ load (which weighs as much as a fire-engine) defined by EC1 for foot-ways, but if you’ve got a pedestrian footbridge with stairs on both sides, it’s probably safe (with client consent) to discard it, unless there’s likely to be significant changes to the access in the future.
Moment redistribution is a hard phenomenon to explain to the uninitiated (lit. someone who can say the phrase full-penetration butt-weld without laughing…). Essentially most construction materials do not fail immediately, but enter a stage of permanent deformation. At the point of deformation the material becomes weaker, which is where moment re-distribution comes in to play. Loads are attracted to stiffness like a bad taste simile to an inappropriate joke, and so the peak load that caused the local failure begins to spread, and the high moments start to become more forgiving. For the likes of reinforced concrete this can mean a more involved analysis, but it’s often the way to a more economic solution.
I once heard someone describe models as a complex way of repeating what you told the computer. This is somewhat true, and quite often the unexpected failure in the structure is simply a quirk of the model. Typically if an effect can’t be replicated by a hand calculation, there is good cause to be dubious. For example, Finite Element models have a tendency to develop peak stresses, which would normally result in relieving deflections: Make a judgement.
With complex loading scenarios it can often be worth consider “does it all happen at once?” For some materials (like pre-stressed concrete) this aspect is included as part of the normal design method, however where two structures are connected stress established before the interface (typically dead-loads) will not be transferred. By extension another consideration is “will this make it worse?” because, if a structure is well-established, works that change, but do not worsen, the loading conditions are unlikely to have a detrimental effect on the structure (although it’s a completely different story for geotechnical works!)
Reducing the Factor of Safety
Factors of Safety, when it comes down to it, are just a formal mechanism for over-designing structures to provide capacity against the unknown. By understanding what each partial factor is protecting against, and taking preventative action at another stage, you can begin the justify a reduction. Typically this needs stakeholder buy-in, but its often a way to avoid excessive repairs to existing structures. Examples include reducing the factor on surfacing by specifying the surface to be planed before re-laying, thus preventing the gradual material build-up the factor emulates.