1849-1850 Royal Commission bridge safety factor study

The Commission asked Charles Fox and other bridge designers what safety factor to use; answers ranged from 3 to 7; the Commission chose 6. Used to anchor the 150-year safety-factor trajectory (6 → 1.67) at the section's pivot to modern practice.

In 1849, the Royals [Royal] Commission in Britain asked a bunch of bridge designers how big safety factors should be. They were designing bridges. They'd designed a girder; they hadn't really worked out all the formulas like we have now. They might just put the girder across two pieces and load it up with 10,000 pounds of something and see if it breaks. They asked some of the big names in the industry at the time — Charles Fox the engineer, Crystal Palace, others. One came up and said, "Well, I think three." Another said seven. The Commission came up and said, "We're going to use a safety factor of six." So in 1850 we were using structural safety factors of six.

The bridge designers, around 1850, were just beginning to be able to calculate the strength of a beam. If you go back, Galileo has a picture of a weight hanging off a beam — Galileo Galilei's way of doing things — trying to understand the strength of a beam. They started developing those equations in the mid-1800s, so they could calculate the strength of a bridge kind of like a sophomore mechanical engineer, and they found a factor of safety around six. About that time there was actually a commission in 1850 in Britain that determined they should have a factor of safety of six. Before that it was all, well, I built one like this and it didn't fall down for the last fifty years, so I'll build one similar. Then someone would come along with a different material or a different design to try to save some money or some time, and the factor of safety would go down and things would get worse.