BS 5639 specifies the type of steel that must be used. Typically, this involves high-yield steel bars or cold-drawn steel wire. The steel must possess specific characteristics regarding ductility and tensile strength to ensure that if the manhole is overloaded, it bends rather than shatters (a failure mode known as "ductility").
Use BS 5639 isolators for severe environments. Use rubber for low-cost, controlled environments. bs 5639