Gantry Girder & Roof Truss

What a gantry girder does

In industrial buildings, an electric overhead travelling (EOT) crane or a hand-operated crane runs along rails. The rails sit on gantry girders spanning between brackets or stepped columns. The crane consists of a crane girder (bridge) spanning across the building, carrying a crab (trolley) with the hook. The gantry girder therefore carries moving wheel loads that are dynamic, repeated and applied eccentrically and laterally — which makes its design different from an ordinary beam.

Loads on a gantry girder

1. Vertical wheel loads from the crane (self-weight of crane girder, crab and lifted load), with the crab at the nearest approach to the gantry to give maximum wheel reactions.
2. Impact allowance on vertical loads (sudden lifting, braking, rail irregularities).
3. Lateral (horizontal surge) force perpendicular to the rails, from acceleration/braking of the crab — applied at rail level.
4. Longitudinal (drag) force along the rails from acceleration/braking of the whole crane.
5. Self-weight of the gantry girder and rail.

Maximum wheel load

With crane span $L_c$, minimum hook approach $a$, crab + lifted load $W_c$ and crane girder self-weight $W_g$ (assumed shared equally by both end carriages, each with two wheels):

Position of wheels for maximum effects

• Maximum bending moment: for two equal wheel loads $W$ at spacing $c$ (wheel base) on a span $L$ (with $c<0.586L$), place them so that the centre of the span bisects the distance between one wheel and the resultant (Barré's rule, see Influence Lines). If $c\ge 0.586L$, a single wheel at mid-span gives the maximum ($WL/4$).
• Maximum shear: one wheel just at the support with the other on the span.
• Lateral moment: same wheel positions, with the lateral forces at rail level.

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.