Also known aslohairon nailsteel
When a person with a debilitated Shani (Saturn) builds, small pieces of shami wood are placed in all four corners first and the iron (steel) is laid on top of them.
All four corners of the structure, beneath the iron or steel
Steel reinforcement raises the electromagnetic current in modern buildings; a copper pyramid of a particular angle placed at the bottom of every column nullifies this negative energy.
Bottom of every column
If stairs must be built in the North, leave some space from the North side and make them of wood, which also represents Mercury; a cement structure needs even more space left.
North zone
In Vedic vastu each direction represents a planet: East is the Sun (gold and copper), South-East is Venus (white metal), South is Mars (copper), South-West is Rahu-Ketu (panchdhatu), West is Saturn (iron), North-West is the Moon (silver), North is Mercury (kansa and brass) and North-East is Jupiter (gold and brass). The planets cannot be held in a fist, but their energy can be deposited in the home: the material representing a direction's planet captures that planet's energy there, multiplies it, raises the power of the place and nullifies negative energy.
Stairs should not be built in the North at all; if they must be, use wood. Many showrooms with wooden double-height staircases are found to do very well, because wood represents Mercury just as stairs and business do, so wooden stairs boost intelligence and keep business running.
No. Direction rules for metals apply above the ground, where the cosmic energy of the planet's location acts, not underground. If they applied underground, silver would be found only in the North-West, gold only in the East and iron only in the West. Iron rods are laid through the whole slab anyway, so cutting a zone with an iron strip means nothing; learn vastu with logic and science.
The wooden door is the main door. Iron is acceptable in the West, and an iron door in the boundary wall is acceptable. If the inner main door is on an unfavourable pada, correct it with the direction-wise bandhanwars and symbols taught in this class plus some door power. Moving it to S4 suits only if the male head of the house has Kark, Makar or Kanya rashi, because Vedic vastu judges this from the male head of the house alone, not from anyone else such as the wife.
Yes, if Saturn is exalted in your chart. The same thing that harms one person can benefit another.