Equipment Selection for a Bridged Opening in a Heritage Building
A heritage building refurbishment needs a scaffold across a 6 m wide opening at first-floor level, with the scaffold continuing above to 18 m. The ground floor below the opening is a protected stone floor that cannot be loaded. The designer proposes a bridged section using two beams spanning the opening, with the upper scaffold carried on the beams. The available beams are 203 mm universal beams with a stated capacity that the contractor has not verified for this span and loading. The upper scaffold carries three working lifts and a fan at 12 m. The site is in a sheltered courtyard.
1. Identify the structural checks needed before the bridged section can be accepted.
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The bridged section is a load path change that must be verified, not assumed. Checks needed:
- Beam capacity for the actual span and load. The 203 mm beams must be checked for bending, shear, and deflection over the 6 m span under the upper scaffold's dead and live loads. The contractor's 'stated capacity' must be replaced by a calculation for this configuration.
- Load take-down. The upper scaffold's standards must bear on the beams at defined points, and the beam reactions must be carried into the building structure at each end. The supports must be checked for the concentrated reactions.
- Deflection limits. Deflection of the beams changes the geometry of the upper scaffold and can induce sway and load redistribution. The designer must set and check a deflection limit.
- Lateral stability of the beams. The beams need restraint against lateral-torsional buckling and against rolling, and the upper scaffold must be tied or braced to prevent the bridged section from swaying.
- Wind and fan loading. The fan at 12 m adds wind area and eccentric load. Even in a sheltered courtyard, the designer must confirm the design wind assumption and the fan's fixing.
- Protected floor. The support points must not load the stone floor; the reactions must be taken into the building frame or onto independent foundations.
Only after these checks should the bridged section be accepted.
2. The contractor wants to substitute two shorter beams joined with a splice plate to make the 6 m span. Evaluate this.
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A spliced beam is a new structural element and cannot be treated as equivalent to a single beam without design.
Issues:
- Splice capacity. A splice at mid-span, where bending moment is highest, must be designed to carry the moment and shear at that location. A simple plate splice may carry shear but not the full moment unless specifically designed.
- Connection design. The splice needs bolts or welds sized for the forces, with edge distances and bearing checked. Field-fabricated splices are a common source of weakness.
- Deflection. A spliced beam will deflect more than a continuous beam, affecting the upper scaffold's geometry.
- Lateral stability. The splice may interrupt the beam's restraint, increasing buckling risk.
- Verification. The substitution requires calculation and sign-off by the designer, plus inspection of the fabricated splice before loading.
The contractor's proposal should be rejected unless the designer provides a splice design and the completed splice is inspected and accepted. The judgement is that 'it makes the span' is not the same as 'it carries the load.'