A history of the safety factors

In the world of computer calculations, a safety factor is a strange
thing: loads and stresses are calculated to 0.01% precision - and
then a 'safety margin' of 50-100% is added to create the final
design. In the past, the safety factor was usually a simple number,
included in the code permissible stresses for convenience, but
modern codes have complex systems of partial safety factors.
How much of this is science and how much is 'black art'? How
have the values of safety factors been decided and how have they
changed over the years?
The history of how the existing safety factors have evolved is
important background knowledge for those involved in developing
new codes of practice. It is also important for engineers involved in
appraising existing structures. There are many issues to consider
in such an appraisal1 but if a change of use is planned, the
engineer needs to understand the original design at an early stage
and estimate what floor loadings it was designed for. To do this,
the engineer needs to know the permissible stresses and design
safety factors that were in use at the time of the original design.
An assessment of all aspects of safety in structural design
would be long and complex. For simplicity, this paper focuses on
the safety factors for bending, tension and compression in steel
and concrete, as these usually govern the sizes of members in
design and quantifying bending, tension or compression
resistance of structural elements is usually the simplest way to
establish what loads it was originally designed for. This paper
considers how UK building code safety factors have changed from
the late 19th century through to the 21st century.