Industrial Structures & Trusses

Industrial buildings

Industrial buildings — factories, workshops, warehouses, hangars, power houses — need large column-free floor areas, overhead cranes, good lighting and ventilation, and quick, economical construction. Steel suits them well.

Planning considerations

• Floor area and clear height for the process and crane hook height.
• Bay width (spacing of frames along the length): commonly 6–9 m; span (across the width): 15–40 m or more.
• Lighting: north-light roofs, monitors, translucent sheets; ventilation: ridge ventilators, louvres.
• Crane type, capacity and duty; future expansion (end walls designed to be removable).
• Drainage of large roofs (gutters, downpipes), insulation, fire safety.

Structural framing systems

Columns in crane buildings

Crane columns carry vertical loads, crane surge, wind and eccentric moments; effective lengths differ for the upper and lower parts and for in-plane and out-of-plane buckling.

Cladding and secondary members

• Purlins carry roof sheeting; side girts (sheeting rails) carry wall cladding and span between columns.
• Sag rods reduce purlin/girt spans in the weak direction.
• Eaves struts at the eaves carry longitudinal forces and support gutter and sheeting.
• Wind columns in gable ends reduce girt spans on end walls.

Components of a steel roof

• Roof covering — galvanised iron (GI) or pre-coated steel sheets, asbestos-cement (older), aluminium, polycarbonate for daylight.
• Purlins — beams spanning between trusses that carry the sheeting.
• Trusses — principal rafters (top chord), main tie (bottom chord), struts and ties (web members), gusset plates.
• Bracing — rafter bracing in the roof plane, bottom-chord bracing, vertical bracing between columns — resists wind and provides stability.
• Eaves girder / gable girder, ridge, bearing plates on columns or walls.

Types of trusses

Beyond about 40 m, space frames, portal frames or lattice girders are usually preferred.

Geometry

• Pitch $=\frac{\text{rise}}{\text{span}}$. Common pitch for GI sheeting: about 1/4 to 1/6 of span (slope roughly 18°–30°); flatter roofs with profiled sheets or built-up roofing.
• Spacing of trusses: commonly span/4 to span/5, about 3–4.5 m for small sheds, up to 6–9 m for larger ones with heavier purlins.
• Purlin spacing: decided by the sheeting's safe span, typically 1.2–1.8 m for GI sheets; purlins placed at panel points so the top chord carries no bending (if not, the rafter is designed for bending too).
• Economical truss depth at the centre commonly about span/4–span/5 for pitched trusses.

Loads on roof trusses

Dead load

Sheeting, fixtures, purlins, bracing and the truss self-weight. An empirical estimate of the self-weight of a steel truss per m² of plan area is often taken as

(or about 0.10–0.15 kN/m² for ordinary spans).

Imposed (live) load on roofs — IS 875 Part 2

Wind load — IS 875 Part 3

• $V_b$ = basic wind speed for the location (from the wind zone map, 33–55 m/s).
• $k_1$ = probability (risk) factor; $k_2$ = terrain roughness and height factor; $k_3$ = topography factor; $k_4$ = importance factor for cyclonic regions.
• $K_d$, $K_a$, $K_c$ = directionality, area averaging and combination factors.
• Force on a roof surface $F=(C_{pe}-C_{pi})A{p}_d$ — external pressure coefficients depend on roof slope and wind direction; internal pressure coefficients (commonly ±0.2 for buildings with low permeability, ±0.5 for medium) depend on openings.

For light roofs, wind suction (uplift) often governs, reversing member forces — ties may go into compression, so slenderness limits for reversal apply.

Load combinations

Typical: (DL + LL), (DL + WL), (DL + LL + WL) with IS 800 partial safety factors. For uplift, dead load is taken with factor 0.9.