IS 11384 : 1985Code of Practice for Composite Construction in Structural Steel and Concrete

IS 11384:1985 is the code of practice for composite construction in structural steel and concrete — the design code for members where a steel section and a concrete slab act together through shear connectors, principally composite beams (steel beam + RCC slab) and composite floor decks. It is what you use to design the economical long-span steel-framed floors common in commercial and industrial buildings.

It is read with the steel–concrete stack:

• IS 800 — general construction in steel (the steel section, connections, construction-stage design)
• IS 456 — the concrete slab (flexure, the concrete in the composite section)
• IS 875 Part 1/2 — dead and imposed loads (construction stage vs composite stage)
• IS 875 / IS 1893 — lateral loads where the floor acts as a diaphragm

In a composite beam the concrete slab resists compression and the steel section resists tension, connected by shear studs/connectors so they act as one deep section — far stiffer and stronger than the steel beam alone, for little extra steel.

Key design themes in IS 11384:

• Two load stages: the construction stage (wet concrete + workers carried by the *bare steel beam*, often propped or unpropped) and the composite stage (superimposed + live load on the *composite section*). Both must be checked — unpropped construction often governs the steel beam.
• Effective width of the slab acting with the beam
• Shear connection — number, spacing and capacity of studs to transfer the horizontal shear (full vs partial shear connection)
• Longitudinal shear in the slab and transverse reinforcement across the connection
• Deflection and vibration — composite floors are stiff but long and light; serviceability (deflection, and increasingly floor *vibration*) frequently governs the design

Brief: a steel floor beam composite with a 125 mm RCC slab, unpropped construction.

Step 1 — construction stage: the bare steel section alone carries slab self-weight + construction live load; check its bending, deflection and lateral-torsional buckling (slab not yet acting) — this often sizes the steel beam.

Step 2 — effective width: compute the slab effective width acting with the beam per IS 11384.

Step 3 — composite moment capacity: plastic/elastic composite section (concrete in compression, steel in tension) resists the composite-stage moment from superimposed dead + live load; verify capacity ≥ demand.

Step 4 — shear connectors: size and space studs for the horizontal shear between steel and slab (full shear connection for the worked case); add transverse slab reinforcement for longitudinal shear.

Step 5 — serviceability: composite-stage deflection within limits and a floor-vibration check for the long span — for office/commercial floors vibration, not strength, is often the binding criterion.

1. Designing only the composite stage. Unpropped, the bare steel beam carries wet concrete — skip the construction-stage check and the beam fails before it ever acts compositely.

2. Under-providing shear connectors. Without adequate, correctly-spaced studs the steel and slab don't act together — you get a steel beam plus a cracked slab, not a composite section.

3. Ignoring longitudinal shear / transverse steel in the slab. The slab can split along the connector line without transverse reinforcement.

4. Strength-only design of long floors. Composite floors are governed by deflection and vibration for typical office spans — a strength-only design feels 'lively' and gets rejected by occupants.

5. Wrong propping assumption. Propped vs unpropped completely changes which stage governs and the locked-in stresses — fix the construction method in the design, and on the drawings.

• IS 800 — general construction in steel (steel design, connections, LTB)
• IS 456 — reinforced concrete (the slab)
• IS 875 Part 1 & Part 2 — dead and imposed loads (stage loading)
• IS 1893 Part 1 — seismic (floor diaphragm action)
• IS 11384 companion: shear-stud/connector references · IS 6533 (industrial)
• IS 2062 — structural steel material

IS 11384:1985 is old and working-stress-flavoured, while international composite practice (Eurocode 4, AISC 360 composite provisions) is limit-state and far more developed on partial shear connection, profiled-deck composite slabs and floor-vibration serviceability. On Indian projects designers commonly use IS 11384 as the referenced code while doing the actual design (especially floor vibration and metal-deck composite slabs) to Eurocode 4/AISC and documenting it — acceptable and usually necessary, because IS 11384 barely addresses modern composite-deck floors and vibration.

The enduring engineering messages are unchanged: composite action is economical but contingent — it exists only if the shear connectors and construction sequence are right, and long composite floors are serviceability-governed (deflection + vibration), not strength-governed. Always design both load stages, lock the propping assumption on the drawings, and run a vibration check for any commercial floor span. A BIS update aligning with limit-state composite design has long been due; until then, treat IS 11384 as the baseline and supplement deliberately.