Chapter 8 of 8

Industrial Structures & Trusses

In the APPSC AE Civil syllabus under Steel Structures · 2 parts

Industrial Buildings & Bracing Systems · Roof Trusses & Purlins

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📑 Contents (16 sections)

Part 1 of 2

Industrial Buildings & Bracing Systems

Last reviewed 16 Sept 2026 · 5 min read

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

System	Features	Suitable spans
Trusses on columns (column pinned to truss)	Knee braces or column fixity for lateral stability; simple analysis	Small–medium
Portal frames (rigid frames)	Columns rigidly connected to rafters; clear, unobstructed space; efficient under gravity and lateral loads; haunches at eaves	About 15–50 m
Pre-engineered buildings (PEB)	Tapered built-up members sized to moment demand, cold-formed Z/C purlins and girts, factory-fabricated	Wide range, fast erection
Lattice girders / space frames	Very long spans, hangars	40 m and beyond
Arched and shell roofs	Architectural spans	Large

Columns in crane buildings

Column	Description
Uniform column with bracket	Light cranes; gantry girder on a bracket
Stepped column	Upper (roof) part narrower; lower part wider to support the gantry directly on its inner leg
Laced / battened column	Two legs (roof leg and crane leg) connected by lacing — economical for heavy cranes
Separate (twin) columns	Independent crane column tied to the building column

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.

Lateral load path and bracing

Wind and crane forces must travel safely to the foundations in both directions.

Transverse direction (across the span): resisted by portal action (rigid frames) or by knee braces, fixed column bases, or trusses with column fixity.
Longitudinal direction (along the length): frames are usually pin-connected in this direction, so bracing is essential.

DefinitionTypes of bracing

Roof (rafter / top chord) bracing — cross bracing in the plane of the rafters in the end bays; transfers gable wind to the side walls and stabilises the top chords against buckling.
Bottom chord (lower chord) bracing — horizontal bracing at tie level; limits the slenderness of bottom chords under wind reversal and transfers longitudinal forces.
Vertical (column) bracing — X, K or portal bracing between columns in one or more bays; carries longitudinal wind and crane drag to the foundations.
Longitudinal ties / eaves struts — connect frames so that all frames share the bracing.
Knee braces — short diagonal members between column and truss, giving partial fixity for transverse stability.
Fly bracing — short members from purlins/girts to the inner flange of rafters or columns to restrain the compression flange against lateral–torsional buckling (important in portal frames under wind uplift).
Crane girder surge bracing — horizontal girders at crane level for heavy cranes.

Bracing members are often slender angles, rods or tubes designed mainly in tension (tension-only X-bracing, where the compression diagonal is assumed to buckle and the tension diagonal carries the load) with slenderness limits for reversal.

Portal frame design notes

Analyse for gravity, wind (pressure and suction, internal pressure), crane loads and notional horizontal loads.
Rigid eaves connections with haunches (deepened rafter near the column) increase the moment capacity where the moment is largest.
Plastic analysis is often used for compact rolled sections.
Check deflection at eaves (lateral) and apex (vertical) — excessive drift damages cladding and crane running.
Restrain the inner (compression) flanges with fly bracing, especially near eaves and under uplift.

Column bases in industrial buildings

Pinned bases (nominal anchor bolts) — smaller foundations, larger frame moments and drift.
Fixed bases — reduce frame moments and drift but need larger foundations with anchor bolts in tension; common for crane buildings.

Frequently tested points

Steel suits industrial buildings: large spans, cranes, fast erection.
Framing: trusses on columns, portal frames (clear space), PEB (tapered members).
Stepped and laced columns support heavy crane girders.
Longitudinal stability comes from vertical and roof bracing; transverse from portal action or knee braces.
Fly bracing restrains the inner flange of portal rafters and columns.
X-bracing is often designed as tension-only.

Common MistakeCommon mistakes

Omitting longitudinal bracing because the frames look strong in their own plane.
Designing bottom chords only for tension, ignoring wind reversal.
Forgetting gable-end wind columns and girts.

Revision SummaryChapter summary

Industrial buildings are planned around span, bay, crane and daylight needs.
Choose among trusses on columns, portal frames and pre-engineered systems.
Crane columns may be stepped or laced; secondary members carry cladding.
A complete bracing system gives stability and lateral load paths in both directions.

Part 2 of 2

Roof Trusses & Purlins

Last reviewed 16 Sept 2026 · 6 min read

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

Truss	Span range (typical)	Feature
King post	up to about 8 m	Simple, timber or steel
Queen post	about 8–12 m	Two vertical posts
Pratt	about 6–30 m	Diagonals in tension under gravity
Howe	about 6–30 m	Diagonals in compression under gravity
Fink (French)	about 10–40 m	Short compression members; economical for pitched roofs
Fan	about 10–15 m	Top chord divided into more panels
North-light (saw-tooth)	factories (bays)	Steep glazed face towards north for diffuse light
Quadrangular / Warren	large spans	Parallel chords
Bowstring, arched	30 m and above	Curved top chord

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

Geometry

Pitch $= \frac{rise}{span}$ . 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

w \approx \frac{L}{3} + 5 (kg/m^{2}) (L = span in m)

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

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

Code ProvisionIS 875 Part 2 — imposed load on sloping roofs (key values)

Flat, sloping or curved roofs with slopes up to 10°, with access: 1.5 kN/m²; without access (except maintenance): 0.75 kN/m².
Sloping roofs with slope greater than 10°: $0.75 - 0.02 (α - 10°)$ kN/m², but not less than 0.40 kN/m² ( $α$ = slope in degrees).
Members of roof sheeting and purlins are also checked for a concentrated load (for maintenance) as specified.

Wind load — IS 875 Part 3

V_{z} = V_{b} k_{1} k_{2} k_{3} k_{4}, p_{z} = 0.6 V_{z}^{2} (N/m^{2}), p_{d} = K_{d} K_{a} K_{c} p_{z}

$V_{b}$ = 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 $F = (C_{p e} - C_{p i}) A p_{d}$ — 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.

Analysis and member design

Convert roof loads to panel point loads (area tributary to each purlin line × spacing).
Analyse the truss (method of joints/sections or software) for each load case.
Design members:
- Rafters and struts as compression members — usually double angles back-to-back (buckling classes, effective length 0.7–1.0 of panel length).
- Ties as tension members — single or double angles; check reversal under wind (slenderness ≤ 350 when reversal is due to wind).
Design connections (bolts or welds) and gusset plates (thickness commonly 8–12 mm).
Check the end bearing — one end on a fixed support, the other allowing thermal movement for long spans.

Purlins

Purlins on a sloping roof carry the load at an angle to their principal axes, so they bend about both axes (normal component $w cos θ$ , tangential $w sin θ$ ). Design as continuous or simply supported beams:

Angle purlins — for small spans and light loads.
Channel / Z / cold-formed purlins — lighter and more efficient for larger spacing.
Sag rods at mid-span or third points reduce the span for the tangential component.

Exam TipSimplified angle purlins

For moderate roof slopes and uniformly distributed loads, Indian practice permits angle purlins to be proportioned by simple rules — leg perpendicular to the roof (depth) not less than about span/45 and the other leg not less than about span/60, with the bending moment taken as about WL/10 for purlins continuous over several trusses ( $W$ = total load on the purlin span). Designs outside these conditions are checked as beams in biaxial bending.

Worked example

Worked ExamplePanel loads on a roof truss

A Fink truss of 12 m span and 3 m rise (slope 26.57°) is spaced at 4 m. Purlins at 1.5 m along the slope. Sheeting + purlins + fixings = 0.20 kN/m² (slope area); truss self-weight 0.12 kN/m² (plan). Find the imposed load per m² of plan and the dead and imposed panel loads at an intermediate purlin point.

Solution. Imposed load: $0.75 - 0.02 (26.57 - 10) = 0.75 - 0.33 = 0.42$ kN/m² (> 0.40 ✔) on plan area.

Plan width tributary to one purlin $= 1.5 cos 26.57° = 1.342$ m.

Dead load per panel point:

Sheeting etc. on slope: $0.20 \times 1.5 \times 4 = 1.20$ kN
Truss self-weight on plan: $0.12 \times 1.342 \times 4 = 0.64$ kN
Total ≈ 1.84 kN

Imposed load per panel point: $0.42 \times 1.342 \times 4 = 2.25 kN$

Wind load panel forces would be computed from $p_{d}$ , $C_{p e}$ and $C_{p i}$ on the sloping area $1.5 \times 4 = 6$ m², acting normal to the roof.

Frequently tested points

Fink truss is economical for pitched roofs of moderate span; north-light trusses admit north light.
Truss spacing about span/4–span/5; purlin spacing set by sheeting (1.2–1.8 m for GI).
IS 875-2 roof imposed load: 0.75 kN/m² for slopes up to 10° (no access); reduce by 0.02 per degree above 10°, minimum 0.40.
Wind: $V_{z} = V_{b} k_{1} k_{2} k_{3} k_{4}$ , $p_{z} = 0.6 V_{z}^{2}$ ; uplift often governs light roofs.
Purlins bend about two axes; sag rods reduce tangential span.
Load reversal under wind changes ties into struts.

Common MistakeCommon mistakes

Adding imposed load on the sloping area instead of the plan area.
Ignoring internal pressure coefficients (they can add significantly to suction).
Placing purlins between panel points without designing the rafter for bending.

Revision SummaryChapter summary

Choose the truss type and geometry by span, pitch and use.
Dead, imposed (IS 875-2) and wind (IS 875-3) loads are converted to panel point loads.
Analyse for all combinations; wind uplift can reverse forces.
Design rafters and struts as compression members, ties as tension members, and purlins for biaxial bending.

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