Steel frame and reinforced concrete

Reinforced concrete and steel frames are designed by calculation, by a trained engineer, to a published code: the loads each floor will carry, the size of every member and every bar. Free codes a reader can hold offline include India's IS 456 for concrete and IS 13920 for earthquake detailing.

• A building permit fixes the number of storeys and the use. Use changes nothing without a new check: a floor of offices is not a floor of looms.
• An inspector checks the work at each stage that will be hidden: footings, bars before each pour, and the finished frame.
• A crack in a column, beam or wall that was not there yesterday stops work and empties the building until an engineer has seen it.

Less water makes stronger, denser concrete. IS 456 (Table 5) sets limits for reinforced concrete by exposure:

• Mild (protected indoors): at least 300 kg of cement per m³, free water/cement ratio at most 0.55, grade at least M20.
• Moderate (sheltered from severe rain, or buried in non-aggressive soil): 300 kg, 0.50, M25.
• Severe (severe rain, wetting and drying, coastal air): 320 kg, 0.45, M30.
• Very severe (sea spray, corrosive fumes): 340 kg, 0.45, M35; extreme (tidal zone, aggressive ground): 360 kg, 0.40, M40.

The grade number is the strength in N/mm² of a 150 mm cube at 28 days: M25 crushes at 25. Use clean, graded aggregate and drinkable water; batch by weight, not by shovel.

The slump test measures workability: fill the standard cone (300 mm high, 200 mm across the base, 100 mm at the top) in four equal layers, tamp each with 25 strokes of a 16 mm rod, lift the cone and measure how far the concrete sinks (IS 1199). IS 456 asks for:

• 25–75 mm for lightly reinforced slabs, beams, walls and columns, floors and strip footings.
• 50–100 mm for heavily reinforced sections.
• 75–100 mm for pumped concrete.

Never add water to a batch that is too stiff: reject it or have the plant adjust the next one. Place the concrete within the time the mix allows, in layers, and compact each with a poker vibrator until air stops rising; do not drop it from a height that separates the stone.

Concrete gains strength only while it stays damp. Keep exposed surfaces continuously wet, under ponded water or wet sacking, for at least 7 days after placing when made with ordinary Portland cement (IS 456, 13.5.1); longer in hot, dry or windy weather.

Cast cubes from the same batches: 150 mm steel moulds, cured in water, crushed at 7 and 28 days. The 28-day result must reach the grade. A low result means coring the member or loading it only after an engineer's check.

Use deformed bars of a known grade (Fe 415 or Fe 500 in IS 456: yield strength 415 or 500 N/mm²), clean of loose rust and oil, bent cold to the drawing.

Cover — the concrete between the bar and the surface — keeps out water, air and chloride (IS 456, Table 16):

• 20 mm mild, 30 mm moderate, 45 mm severe, 50 mm very severe, 75 mm extreme exposure; at least 50 mm in footings.
• Hold it with concrete or plastic spacers and chairs, never stones or offcuts of wood.

Laps: a bar in flexural tension laps its development length and at least 30 bar diameters; in direct tension, twice the development length; in compression, at least 24 diameters. Stirrups in beams no farther apart than 0.75 d (d = effective depth) or 300 mm; column ties no farther apart than the column's least width, 16 bar diameters or 300 mm.

In an earthquake the joints are where frames fail, so the steel there is closer and better anchored (IS 13920, 1993; its 2016 revision and other current codes are at least as strict):

• Hooks: every hoop and tie ends in a 135° hook with an extension of 10 bar diameters, and at least 75 mm, that wraps into the core. A 90° end opens under shaking.
• Beam ends: over twice the depth from each column face, hoops at no more than d/4 or 8 bar diameters (but not closer than 100 mm), the first within 50 mm of the face.
• Columns: from each joint, over the greatest of the column's larger dimension, a sixth of its clear height and 450 mm, hoops at no more than a quarter of its least width, 75–100 mm apart, continuing 300 mm into the footing. Lap column bars only in the middle half of the height.
• Minimum column width 200 mm, and 300 mm where beams span more than 5 m or columns stand more than 4 m unbraced.
• No soft storey: a storey much more open or flexible than the one above, such as an open ground floor under walled flats, gets confining steel over the full height of its columns, and an engineer's check.

For one- and two-storey buildings, confined masonry — walls laid first, then small reinforced-concrete tie-columns and tie-beams cast against them — performs well in earthquakes with far less steel than a frame (EERI and IAEE guide, 2011):

• Tie-columns at every wall end and intersection, no more than 4.5 m apart in regions of high seismicity and 6 m in moderate.
• A tie-beam along the top of every wall, at most 3 m up.
• Ties of at least 6 mm bar with 135° hooks, no more than 200 mm apart, halved near the tie-column ends in high seismicity; longitudinal laps at least 40 bar diameters; 20 mm cover to the ties.
• Walls at least 110 mm thick and no more than 3 m high.
• Enough wall in each direction: buildings that collapsed had walls of less than 1 % of the floor area in one direction; the guide's table gives the minimum for each hazard, soil and storey count.

Use hot-rolled sections of a stated grade, such as S275 or S355 (the number is the yield strength in N/mm²), joined by graded bolts tightened to the design. A steel frame is quick and light, and it loses strength in fire: loss begins at about 300 °C, quickens above 400 °C, and by 550 °C steel keeps about 60 % of its room-temperature yield strength and 45 % of its stiffness (SteelConstruction.info).

• Protect every load-bearing member to the fire resistance the building code asks: concrete or masonry encasement, fire boards or intumescent paint.
• After a fire, have every member and connection assessed; bolts heated above about 450 °C lose strength, and replacing them is the cautious course.

Concrete hides its mistakes, so inspect at the stages that cannot be seen later:

1. Excavation and footing bars before the footing is cast.
2. Column and wall cages, and beam and slab bars, before every pour: bar sizes, count, cover, laps, hooks and spacers against the drawing.
3. Formwork and props before the pour, and striking only when the concrete has the strength.
4. The finished frame before walls and finishes cover it.

Write each inspection down and sign it. Afterwards, the building's own record — its design loads, storeys and use — goes with it to every owner; any change goes back to an engineer.