Complex sentence: Lugged steel construction uses standard cylindrical steel tubes which are connected with lugs, external fittings made of pieces of steel (sometimes stainless steel) which fit over the ends of the tubing.
Split into simpler sentences: Lugged steel construction uses standard cylindrical steel tubes which are connected with lugs. Lugs are external fittings made of pieces of steel (sometimes stainless steel) which fit over the ends of the tubing.