Iron-framed and multi-storey building
Raise buildings of three storeys and more on iron columns, iron or steel beams and brick walls, with every load traced to the ground, cast iron used only where it is squeezed, walls no more slender than the code allows, and the iron protected from fire.
Everything for this step
- Field guide
- Engineering → 7 sections
- Builds
- None yet.
- Reference sheets
- None.
- Sciences it rests on
- Engineering · Physics
- Track
- Dwellings, level 4 of 5 · next: Steel frame and reinforced concrete
Why this matters
The blast furnace makes iron cheap enough to hold up floors, and with it the factory, the warehouse and the tenement grow to several storeys. Iron comes in two kinds that must not be swapped: cast iron "has good compressive strength and low tensile strength" and is brittle, which is why builders "relegated [it] to columns", while wrought iron has "good tensile and compressive strengths" (STRUCTURE magazine). A cast-iron beam fails without warning, at the bottom, where it is pulled.
Height brings two more limits. A brick wall can buckle before it crushes, so its height is limited by its thickness (IS 1905). And iron is not fireproof: steel starts to lose strength at about 300 °C (SteelConstruction.info). A building of this size is calculated by someone trained to do it; this step teaches what the calculation must cover and what to check on site.
The shape of it
- Draw the load path before anything is ordered: every floor's load to a beam, a column or wall, and the footing, at 2.0 kN/m² for homes and 4.0–5.0 kN/m² for halls (IS 875).
- Use cast iron only in compression, for columns and bases; beams, lintels and ties are wrought iron or steel.
- Keep brick walls stocky: effective height at most 27 times the thickness in cement mortar, and 13 in lime mortar above two storeys (IS 1905).
- Tie every floor and the roof to the walls, and bond cross walls into the outer walls, so wind reaches the foundations.
- Encase the ironwork in brick, concrete or plaster, and give every storey above the ground two ways out.
- Test a sample of each delivery of beams under load before they go into the building.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Draw the load path and set the loads
Trace every load from the roof to the ground on the drawings before ordering any iron: roof and floors onto beams, beams onto columns or walls, columns and walls onto footings, footings onto ground that can carry them.
Design for the heaviest use the building could have, not today's. India's loading code, IS 875 part 2, gives the imposed load to add to each floor's own weight, uniformly spread:
- Dwellings: 2.0 kN/m² for rooms and kitchens.
- Offices: 2.5 kN/m² with separate storage, 4.0 without.
- Assembly halls: 4.0 kN/m² with fixed seats, 5.0 without.
A kilonewton is roughly the weight of 100 kg, so a 5.0 kN/m² floor is designed for about 500 kg on every square metre, on top of its own weight. Factories and stores need the code's higher figures and an engineer's calculation.
2 · Put cast iron where it is squeezed, and nowhere else
Cast iron is strong in compression and weak and brittle in tension, so it serves as columns, column bases and compression struts, never as a beam, lintel, cantilever or tie (STRUCTURE magazine).
- Columns: stand them plumb on bases bedded on the footing, with each floor's beams bearing on a cap or bracket cast for them.
- Beams and ties: wrought iron or rolled steel, which take tension and bend before they break.
- Joints: bolt beams to column caps.
- Inspect every casting before it goes up: blowholes, cracks or a column wall thinner on one side than the other mean it is rejected.
3 · Keep every brick wall within its slenderness limit
A wall's slenderness ratio is its effective height divided by its effective thickness. IS 1905 (Table 7) limits a load-bearing wall to:
- 27 in cement or Portland pozzolana cement mortar, at any number of storeys.
- 20 in lime mortar for buildings of up to 2 storeys, and 13 above.
Worked example: a wall one brick (230 mm) thick in lime mortar, in a building of three storeys, may have an effective height of at most 13 × 0.23 = 3.0 m between floors that brace it. In cement mortar the same wall could stand 6.2 m.
Where a wall is too slender, thicken it, add bonded cross walls or piers, or brace it with floors tied into it.
4 · Tie the floors and cross walls into the outer walls
Wind and earth tremors push walls sideways, and only the floors and cross walls stop them. IS 1905 (clause 4.2.2.2) deems a load-bearing building of up to four storeys stable when:
- its height is no more than twice its width;
- floors and roof bear on the cross walls or are anchored to them, so sideways loads pass through them to the foundations;
- cross walls are built with the bearing walls and bonded or toothed into them, or anchored to them.
Anchor every floor beam or joist into the wall with an iron strap or tie, not just a resting end.
5 · Protect the iron from fire and give two ways out
Bare iron does not burn but it softens. Steel starts to lose strength at about 300 °C; the loss quickens above 400 °C; at 550 °C steel keeps about 60 % of its yield strength and 45 % of its stiffness (SteelConstruction.info).
- Encase columns and beams in brick, concrete or thick plaster on lath, to the fire resistance the building code sets.
- Two ways out from every storey above the ground, and above 4.5 m an escape stair enclosed in fire-resisting walls down to an outside door (the code's figures).
- Fire walls divide large floors, carried up through the roof as the code's party walls are.
6 · Test the iron, inspect the work, and keep the use within the design
Iron varies from one delivery to the next. Load a sample beam from each delivery to its working load before it goes into the building, and measure its deflection; reject a delivery whose sample cracks or stays bent.
Inspect at each stage that will be hidden: footings before backfilling, columns plumb and bedded before floors go on, ties and anchors before walls are plastered. Record each inspection with its date and signature.
A change of use is a change of load. Turning dwellings into a warehouse or a mill can more than double the load on every floor; it goes back to the designer first. The reinforced concrete step shows what happens when it does not.