Pre-engineered and composite structures

In short

Pre-engineered buildings (PEB) are factory-designed, bolt-together steel frames — usually tapered built-up portals with cold-formed purlins and metal sheeting — supplied as a kit for a specific span, bay and loading. Composite structures combine structural steel and concrete so that the two act as one section through mechanical shear connection: composite beams with a slab flange, composite slabs on profiled steel decking, and composite columns (encased or concrete-filled). Both are the standard answer in India for fast-track industrial sheds, warehouses, metro stations, airport terminals and multi-storey office floors, and both are examined in GATE (CE), university electives on advanced/steel-concrete composite design and in design-office interviews (Kirby, Tata BlueScope, Interarch, Everest, L&T, AFCONS, Pennar, Tata Steel). The governing documents are IS 800:2007 (limit state steel), IS 11384 (steel-concrete composite construction), IS 456:2000, IS 875 Part 3:2015 (wind), IS 1893 Part 1:2016 (seismic) and IRC:22 for composite road bridges.

Explained

What a pre-engineered building is and how it is made up

• Definition: a building system whose members are *designed and fabricated to the exact force demand at each section* in a controlled shop, shipped as marked pieces, and field-assembled with high-strength bolts — minimal site welding.
• Primary framing: clear-span or multi-span rigid frames of built-up I sections with tapered (non-prismatic) webs — deep at the knee where the moment is largest, shallow at the apex and base. Web plates are typically 5–6 mm, flanges 8–25 mm, plates to IS 2062 E250/E350.
• Secondary framing: cold-formed Z or C purlins on the roof and girts on walls (1.6–3.2 mm thick, galvanised or painted), with sag rods to control lateral/torsional sag and eave struts at the junction.
• Cladding: 0.47–0.60 mm pre-painted galvalume profiled sheets, standing-seam or trapezoidal, with glass-wool/PUF insulation, translucent panels, turbo-ventilators, ridge and gutter trims.
• Bracing system: frames resist transverse load by in-plane portal action; longitudinal load is taken by X-bracing (rod/angle/pipe) in the roof plane and side walls, or portal bracing where openings prevent diagonals.
• Accessories: mezzanine floors (often composite deck slabs), crane gantry girders on brackets or stepped columns, canopies, louvers, anchor bolts and base plates (usually treated as pinned bases to reduce foundation moments).
• Roof slope: flat-ish, commonly 1 in 10 (≈5.7°) to 1 in 20, which keeps the frame shallow and economical.

PEB design basis, codes and economics

• Load combinations: dead (sheeting + purlins ≈ 0.10–0.20 kN/m²), live/roof imposed per IS 875 Part 2 (0.75 kN/m² for inaccessible roofs, reducible for slope), wind per IS 875 Part 3:2015, seismic per IS 1893 Part 1:2016, crane loads with impact per IS 807/IS 3177, temperature, and collateral loads (services, sprinklers).
• Wind usually governs a light shed: net roof uplift exceeds the dead load, so purlin fixings, frame rafters, anchor bolts and footings are designed for suction, not gravity.
• Design method: limit state design to IS 800:2007 — partial safety factors γ_m0 = 1.10 (yielding), γ_m1 = 1.25 (ultimate/rupture); cold-formed members to IS 801/IS 811 (IS 801:1975 is in working-stress format and is dated — many designers use AISI/Eurocode 3-1-3 provisions with the manufacturer's section tables).
• Checks on a tapered member: flexure with lateral-torsional buckling of the compression flange between purlin/flange-brace points, web shear and web crippling at the knee, combined axial-bending interaction, bolted end-plate (moment) connections at knee and apex.
• Serviceability (IS 800 Table 6, indicative): purlins/girts about span/150–span/180, industrial building frames with elastic cladding about span/240 (span/300 brittle cladding), crane gantry girders span/500 (manual) to span/750–1000 (electric), lateral sway at column top about height/150 with elastic cladding (height/240 brittle, height/200 where a gantry is present).
• Seismic: light roofs mean small seismic mass, so wind dominates; response reduction factor R is taken for ordinary moment frames (R = 3 per IS 1893 Part 1:2016) unless ductile detailing to IS 800 Section 12 makes it a special moment frame (R = 5).
• Typical economy: steel consumption roughly 25–45 kg/m² against 55–80 kg/m² for an equivalent conventional truss-and-column shed; erection in weeks rather than months. Rates vary widely with steel price and specification, so quote them from current vendor offers rather than memory.
• Limitations: rigid geometry (future span/load changes are costly), thin cold-formed and sheeting elements need corrosion and fire protection, poor suitability for very heavy crane or process loads, dependence on one vendor's proprietary system and on accurate anchor-bolt setting.

Composite action: principle, interaction and construction sequence

• Idea: a concrete slab sits on a steel beam; without connection the two slip at the interface and bend about separate axes. Welding shear connectors to the top flange transfers the longitudinal shear, so the slab acts as the compression flange of a deep composite section.
• Benefits: 25–40% saving in steel weight, shallower floor depth, 2–4 times the stiffness of the bare steel beam (so better deflection and vibration behaviour), slab contributes fire and diaphragm action.
• Degrees of interaction: *no interaction* (free slip), full interaction (no slip, rigid connectors), partial interaction (flexible connectors, some slip — real behaviour).
• Degree of shear connection η = n/n_f (connectors provided ÷ connectors needed for full connection). Full connection develops the lesser of the slab and steel squash capacities; partial connection gives a reduced moment capacity, interpolated between the bare steel and full-composite capacities.
• Propped (shored) construction: props carry wet concrete, so the *whole* load acts on the composite section — lighter steel, but props obstruct work and shrinkage/creep effects are larger.
• Unpropped (unshored) construction: the bare steel beam alone carries self-weight + wet concrete + construction load; only the superimposed load acts on the composite section. Stresses from the two stages are added, and this staged check is the most commonly missed step in exams.
• Long-term effects: creep is handled by using an effective modular ratio (commonly 2m, i.e. half E_c) for sustained load; concrete shrinkage causes additional sagging deflection and tension in the slab; differential temperature matters in bridges.
• Continuous composite beams: at supports the slab is in tension (concrete cracked) and reinforcement acts as the tension flange, while the bottom steel flange is in compression — check lateral-distortional buckling and provide hogging reinforcement with crack control.

Shear connectors and transverse reinforcement

• Types: headed studs (16, 19, 22 mm dia, 75–150 mm long — the standard, stud-welded through the deck), channel connectors, angle/tee connectors, hoops, and bond/friction only for encased sections.
• Design force: for a simply supported beam the total longitudinal shear to be transferred from support to the point of maximum moment equals the smaller of the slab compressive capacity and the steel tensile capacity.
• Connectors are spaced uniformly when the section is plastic/compact (plastic redistribution) and in proportion to the shear flow (closer near supports) in elastic design.
• Detailing rules (typical): overall stud height ≥ 4d (so α = 1.0 and the "ductile" strength applies), minimum longitudinal spacing about 5d, transverse spacing about 4d, cover above the head ≥ 20–25 mm, maximum spacing limited (of the order of 600–800 mm or 4× slab thickness) to keep the slab attached against uplift; connectors also resist uplift, hence the head.
• Transverse reinforcement across the beam line is essential: it prevents longitudinal splitting of the slab along the connector line. Design it for the longitudinal shear per unit length on critical planes (through the slab depth and around the connector group).
• Failure modes to know: stud shear-off, concrete cone pull-out/crushing around the stud, longitudinal splitting of the slab, and (in composite slabs) shear-bond slip of the decking.

Composite slabs on profiled steel decking

• Deck does two jobs: permanent formwork + working platform during construction, and tension reinforcement in the finished slab — so bottom bars are usually not needed.
• Mechanical interlock comes from rolled embossments/indentations, re-entrant dovetail ribs, end anchorage by the through-deck welded studs, and friction.
• Typical build: 0.8–1.25 mm galvanised deck, 50–80 mm rib, overall slab 100–150 mm, spans 2.5–3.5 m unpropped, nominal welded mesh for shrinkage/crack control and fire.
• Limit states: construction stage (deck as a thin-walled beam — flexure, web crippling, deflection with ponding), composite stage flexure, longitudinal shear bond (verified by the empirical *m-k* test method or partial-connection method), vertical shear, punching shear at concentrated loads, and fire (deck sacrificial, reinforcement for fire resistance).
• Indian practice relies on IS 11384 together with manufacturer load-span tables derived from tests to BS 5950 Part 4 / Eurocode 4 Part 1-1 — slab shear-bond capacity cannot be calculated from first principles.

Composite columns, trusses and codes

• Types: fully encased steel section in concrete (best fire/corrosion protection), partially encased (concrete between the flanges), concrete-filled tubes (CFT) — circular or rectangular hollow sections filled with concrete, now common in high-rise cores and bridge piers.
• Why they work: concrete delays local buckling of the steel plate and carries axial load cheaply; the steel tube confines the concrete (strength and ductility gain, strongest in circular CFTs); the steel section makes a compact, erectable column with no formwork.
• Capacity: plastic squash load = steel + concrete + reinforcement contributions, reduced for slenderness by a buckling curve; the steel contribution ratio δ must fall between about 0.2 and 0.9 — below 0.2 design it as RC, above 0.9 as bare steel.
• Load introduction: shear studs, through-plates or bearing must transfer column load into the concrete core at connections; also check shear connection in the load introduction length.
• Other composite forms: composite trusses and open-web joists, composite castellated beams, composite box and plate girders in bridges, composite slabs on precast planks, and composite steel-concrete shear walls.
• Codes and standards to cite: IS 11384 — *Code of practice for composite construction in structural steel and concrete* (the long-standing 1985 edition was in working-stress format; IS 11384:2022, the first revision, superseded it and IS 3935 and is in limit state format in line with IS 800 and IS 456 — confirm which edition your exam/project follows); IS 800:2007; IS 456:2000; IS 875 Parts 1–5; IS 1893 Part 1:2016; IS 2062, IS 4923, IS 1161, IS 811 (sections); IS 813/IS 9595 (welding); IRC:22-2015 for composite road bridge superstructures and IRC:24 for steel bridges; Eurocode 4 and AISC 360 are used as reference for gaps (composite slabs, CFT).

Key points

• PEB - factory-designed, bolted steel building kit with tapered rigid frames, cold-formed secondaries and metal cladding.
• Tapered member - depth follows the bending-moment diagram; deepest at the knee of the portal.
• Purlin and girt - cold-formed Z/C secondary members spanning between frames, with sag rods for minor-axis support.
• Bracing - in-plane frame action transversely, roof and wall X-bracing longitudinally; without it the shed is a mechanism.
• Wind uplift governs light roofs - net suction (external −Cpe plus internal +Cpi) usually exceeds the dead load.
• Composite action - slab and steel beam act as one section only because shear connectors prevent interface slip.
• Full vs partial shear connection - η = n/n_f; partial connection gives a capacity between bare steel and full composite.
• Effective flange width - the limited slab width assumed fully effective, because longitudinal stress is not uniform across the slab.
• Modular ratio m = E_s/E_c - converts concrete area to equivalent steel in the transformed section; use ~2m for long-term (creep) load.
• Propped vs unpropped - unpropped means the bare steel carries wet concrete; stage stresses must be superposed.
• Headed stud - the standard connector; the head resists uplift, height ≥ 4d for ductile behaviour.
• Transverse reinforcement - prevents longitudinal splitting of the slab along the connector line; never omit it.
• Profiled deck slab - decking is formwork plus tension steel; shear-bond capacity comes from tests (m-k method), not calculation.
• Concrete-filled tube (CFT) - concrete stops local buckling, tube confines concrete; steel contribution ratio δ between 0.2 and 0.9.
• Hogging region of continuous composite beams - slab cracked, reinforcement in tension, bottom flange in compression (distortional buckling check).
• Stiffness gain - a composite beam is typically 2–4 times as stiff as the bare steel beam, so deflection and floor vibration improve sharply.
• Governing Indian codes - IS 11384 (composite), IS 800:2007 (steel), IS 456 (concrete), IRC:22 (composite bridges).

Formulas and examples

Example 1 — Elastic composite section, stresses and deflection. Simply supported composite beam, span L = 8 m, beams at 2.4 m c/c, slab 100 mm thick in M25, steel ISMB 400 (A_s = 7846 mm², I_s = 204.6 × 10⁶ mm⁴, depth 400 mm), propped construction, service moment 200 kN·m.

• b_e = least of L/4 = 2000 mm and spacing 2400 mm → b_e = 2000 mm.
• E_c = 5000√25 = 25,000 N/mm² → m = 200,000/25,000 = 8.
• Transformed slab area = 2000 × 100/8 = 25,000 mm²; A_tr = 25,000 + 7846 = 32,846 mm².
• ȳ from slab top = (25,000 × 50 + 7846 × 300)/32,846 = (1.25 × 10⁶ + 2.3538 × 10⁶)/32,846 = 109.7 mm (just inside the steel, so the whole slab is in compression).
• I_tr = (2000/8)(100³/12) + 25,000(59.7)² + 204.6 × 10⁶ + 7846(190.3)² = 20.8 × 10⁶ + 89.1 × 10⁶ + 204.6 × 10⁶ + 284.1 × 10⁶ = 598.7 × 10⁶ mm⁴ (2.93 × the bare steel value).
• Bottom-fibre steel stress: y = 500 − 109.7 = 390.3 mm → σ_s = 200 × 10⁶ × 390.3/598.7 × 10⁶ = 130.4 N/mm² (safe against 250/1.10 = 227).
• Top-fibre concrete stress: σ_c = (200 × 10⁶ × 109.7/598.7 × 10⁶)/8 = 36.6/8 = 4.6 N/mm².
• The same moment comes from w = 8M/L² = 8 × 200/64 = 25 kN/m. δ = 5 × 25 × 8000⁴/(384 × 2 × 10⁵ × 598.7 × 10⁶) = 11.1 mm = L/720; the bare steel beam would deflect 32.6 mm = L/245.

Example 2 — Plastic moment capacity, full shear connection. Same section, f_y = 250 N/mm², γ_m0 = 1.10.

• T = 7846 × 250/1.10 = 1783 kN.
• Slab capacity = 0.36 × 25 × 2000 × 100 = 1,800,000 N = 1800 kN > 1783 kN → PNA lies in the slab.
• x_u = 1,783,000/(0.36 × 25 × 2000) = 1,783,000/18,000 = 99.1 mm (< 100 mm, consistent).
• Lever arm = 300 − 0.42 × 99.1 = 300 − 41.6 = 258.4 mm.
• M_p = 1783 × 0.2584 = 460.6 kN·m. Bare steel (Z_p ≈ 1176 cm³): M_d = 1176 × 10³ × 250/1.10 = 267.3 kN·m — the slab adds 72% capacity for the cost of studs alone.

Example 3 — Number of shear connectors. 19 mm dia × 100 mm headed studs, f_u = 450 N/mm², M25 (E_c = 25,000), γ_v = 1.25, h/d = 5.26 ≥ 4 so α = 1.0.

• Steel failure: 0.8 × 450 × (π/4 × 19²)/1.25 = 0.8 × 450 × 283.5/1.25 = 102,060/1.25 = 81.6 kN.
• Concrete failure: 0.29 × 1.0 × 19² × √(25 × 25,000)/1.25 = 0.29 × 361 × 790.6/1.25 = 82,785/1.25 = 66.2 kN → governs, P_Rd = 66.2 kN.
• F_cf = 1783 kN (Example 2) → n = 1783/66.2 = 26.9 → 27 studs per half span (54 in total), uniformly at 4000/27 ≈ 148 mm c/c in a single row, which satisfies the 5d = 95 mm minimum spacing. Check IS 11384's own table of connector strengths for the edition in force; provide transverse reinforcement for the resulting longitudinal shear of 1783/4000 ≈ 446 N/mm run.

Example 4 — PEB roof uplift from wind. Industrial shed near Chennai: V_b = 50 m/s, terrain category 2, eaves height 10 m, cyclone-prone coastal location.

• k1 = 1.00, k2 = 1.00 (TC2, 10 m), k3 = 1.00 (flat), k4 = 1.15 (industrial structure in cyclone region) → V_z = 50 × 1.15 = 57.5 m/s.
• p_z = 0.6 × 57.5² = 0.6 × 3306.25 = 1984 N/m² = 1.98 kN/m².
• p_d = K_d·K_a·K_c·p_z = 1.00 × 0.80 × 1.00 × 1.98 = 1.59 kN/m² (K_d = 1.0, not 0.90, in cyclone-affected regions, IS 875-3 cl. 7.2.1; K_a = 0.8 for tributary area ≥ 100 m²).
• Roof with C_pe = −1.2 and C_pi = +0.5 → net coefficient −1.7 → uplift = 1.7 × 1.59 = 2.70 kN/m² (suction).
• Dead load of sheeting + purlins ≈ 0.15 kN/m² → net uplift ≈ 2.55 kN/m². Wind reversal, not gravity, sizes the purlin fixings, rafter bottom-flange bracing, anchor bolts and footing weight.

Example 5 — Concrete-filled tube column (short column squash load). SHS 300 × 300 × 8 mm (Yst 310 to IS 4923) filled with M30 concrete, no bars, corner radii ignored.

• A_a = 300² − 284² = 90,000 − 80,656 = 9344 mm²; A_c = 80,656 mm².
• Steel contribution = 9344 × 310/1.10 = 2633 kN.
• Concrete contribution = 0.45 × 30 × 80,656 = 1089 kN (Eurocode 4 with f_cd = f_ck/1.5 = 20 N/mm² would give 1613 kN — note the difference in partial factors).
• N = 2633 + 1089 = 3722 kN; δ = 2633/3722 = 0.71, within 0.2–0.9, so composite column rules apply. Filling adds ~41% capacity, needs no formwork, and the concrete suppresses local buckling of the 8 mm plates.

Where it is used

• Industrial and logistics construction: PEB warehouses, cold stores, e-commerce fulfilment centres, auto and textile plants — Tata BlueScope, Kirby, Interarch, Pennar, Everest, Smith Structures and L&T supply and erect these across India.
• Transport and public infrastructure: metro station roofs and concourses, airport terminal roofs, stadium canopies, bus depots; composite deck girders in metro viaducts and composite steel-concrete road bridges to IRC:22.
• Multi-storey buildings: composite floors on profiled decking with composite or CFT columns in IT parks and commercial towers (fast cycle time, shallow floors, services in the ribs).
• Job roles: PEB design engineer (frame optimisation, connection design, MBMA/IS 800 checks), detailer (Tekla/STAAD/SAP2000/ STAAD.Pro & proprietary PEB software), erection and QA/QC engineer, structural design engineer for composite floors in consultancies (AECOM, Sterling, Mott MacDonald, L&T, Tata Consulting Engineers).
• Retrofitting and mezzanines: adding composite mezzanine floors inside existing sheds, and strengthening existing steel beams by connecting them to a new concrete topping.
• Pre-fab and modular housing: light gauge steel framing and composite deck floors used under PMAY and Global Housing Technology Challenge demonstration projects.

Interview questions

• Q: Why are PEB frame members tapered? A: The bending moment in a portal varies along the member, so matching the depth to the moment removes unused material — steel is placed only where demand exists.
• Q: Why does wind uplift usually govern the design of a PEB roof? A: The roof is very light (0.10–0.20 kN/m²), while net suction from external negative pressure plus internal positive pressure can reach 2–3 kN/m² in coastal cyclone zones, so the load reverses and the connections and foundations must resist uplift.
• Q: What is the function of sag rods in purlins? A: They provide intermediate lateral support about the weak axis, reducing minor-axis bending and twisting of thin cold-formed Z/C purlins during and after erection.
• Q: What exactly makes a beam "composite"? A: Shear connectors that transfer the interface longitudinal shear, so the slab and steel section bend about a common neutral axis instead of slipping independently.
• Q: Why is the effective flange width less than the full slab width? A: Longitudinal compressive stress in the slab peaks over the beam and decays sideways through shear lag; the effective width is the equivalent uniform-stress width that gives the same force.
• Q: Difference between propped and unpropped construction? A: With props the composite section carries all the load; unpropped, the bare steel carries self-weight and wet concrete and only later load acts composite, so the stresses from the two stages must be added.
• Q: Why does a headed stud have a head? A: To resist uplift/separation of the slab from the steel flange, in addition to carrying horizontal shear.
• Q: What happens if transverse reinforcement is omitted across a composite beam? A: The slab can split longitudinally along the line of connectors, destroying composite action well before the calculated flexural capacity is reached.
• Q: Why is the capacity of a composite slab on profiled decking verified by testing? A: Its governing limit state is shear bond between the embossed deck and concrete, which depends on the proprietary profile geometry and cannot be computed from first principles — hence the m-k or partial-connection test method.
• Q: What does concrete do for a concrete-filled steel tube, and what does the tube do for the concrete? A: The concrete restrains inward local buckling of the thin tube walls and carries axial load cheaply; the tube confines the concrete, raising its strength and ductility, and acts as formwork.
• Q: What is the steel contribution ratio and why is it limited to 0.2–0.9? A: It is the share of the plastic squash load carried by the steel section; outside that range the member behaves essentially as a reinforced concrete or a bare steel column, and the composite design rules no longer apply.
• Q: Which Indian code covers steel-concrete composite construction in buildings, and for bridges? A: IS 11384 for buildings (used with IS 800:2007 and IS 456:2000); IRC:22 for composite road bridge superstructures.
• Q: How does creep of concrete affect a composite beam? A: Sustained compression makes the slab shorten over time; this is modelled by roughly doubling the modular ratio, which shifts load into the steel and increases long-term deflection.

Common mistakes

• Using the full slab width instead of the effective flange width, which overstates both capacity and stiffness.
• Forgetting the construction stage in unpropped work — designing only the composite section and missing the bare-steel check under wet concrete.
• Taking the slab's transformed width as b_e instead of b_e/m, or using the short-term modular ratio for sustained load.
• Assuming the plastic neutral axis is always inside the slab — it must be checked, and when the steel force exceeds the slab capacity the extra compression taken by the steel section changes the lever arm.
• Providing studs but no transverse reinforcement, or exceeding the maximum connector spacing so the slab is not restrained against uplift.
• Treating PEB roof loads as downward only, and so under-designing purlin fixings, rafter bottom-flange bracing, anchor bolts and footing self-weight for wind suction.
• Omitting the bracing system or assuming the cladding provides stability — a braced-bay layout in roof and side walls is what carries longitudinal load.
• Ignoring lateral-torsional/distortional buckling of tapered PEB rafters and of the bottom flange of continuous composite beams in hogging regions.
• Mixing units in composite calculations (kN with N, cm⁴ with mm⁴, m with mm) — the single largest source of wrong numerical answers in exams.

Related topics

Steel structures design (IS 800) · Reinforced concrete design (IS 456) · Bridge engineering · Tall buildings, shear walls and lateral loads · Structural analysis: determinate and indeterminate structures · Earthquake-resistant design