Gantry Girders

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.