Re: iron or steel

The bridge material is wrought iron and not "phosphorised mild steel."
Based on:
1. Carbon is 0.02% for bridge vs. 0.15-0.25% for spec A-36 and SAE mild steels.
2. Manganese is 0.03% for bridge vs. 0.30-0.90% for SAE mild steels.
3. Phosphorus is 0.11-0.17% for bridge vs. 0.04 max for A-36 and most SAE.

From the ORourke Eng. Handbook Sec.4 Par 25 Impurities in Steel:

Phosphorus unites with iron to form Fe3P, which in turn forms a eutectic with iron. The eutectic has a lower melting point than the remainder of the steel. During the freezing of steel the eutectic is rejected to the grain boundaries, producing planes of weakness which make the steel particularly likely to fail under impact loading. This characteristic of a metal­ increasing brittleness at ordinary temperatures-is called cold-shortness.

High phosphorus in "steel" does cause it to be brittle at low temperature, but is a required component in "wrought iron".

Byer WI Chapter 6:

In well made wrought iron the phosphorus content ordinarily ranges from 0.10% to 0.15 %.

As the MTL analysis shows.