Hi Mosh

I think the 5 years is a rule of thumb possibly mentioned in a British Standard and often specified by insurers.

However, like most frequencies, particularly since the level of prescription set out in e.g. the Factories Act 1961 was set aside, this will ultimately come down to a risk assessment preferably informed by a structural engineer or building surveyor.

The 5 years is not universal. As example, UK national highways authorities' standards say that a structure such as a bridge should usually have a "principal inspection" at intervals of 6 years but this is also subject to being adjusted if the case is made for principal inspections to be more or less often.

So, you need to consider the specific structure - location and purpose. I have never investigated a staircase collapse but for parallel I have investigated accidents involving partial failure of metal walkways - one (internal) suffered corrosion due to the industrial fumes, the other (external) as a result of exposure to the "elements" including sea water. 

The greater the traffic, then the likelihood that you need to do more frequent inspections, with the exception of emergency routes that may be used very rarely but when used will have very high volumes.