Long-span trussed roofs

How does Long-span trussed roofs work, step by step?

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   Step 1: Design the roof and the erection sequence together

   On a long-span roof the sequence is part of the design. The designer sets the truss depth, the member sizes, the connections and the bracing, and the temporary works designer establishes how the structure will be stable at every stage of erection. Splice positions are chosen so that pieces can be transported and lifted, connection types are chosen for what can realistically be done at height, and camber is set so the finished roof arrives at the right level once it is loaded. Deciding all of this after the steel is ordered is how long-span projects lose months.

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   Step 2: Plan the site for craneage, laydown and airside access

   The crane positions, their standings, their radii and their lifting capacities are established, and the ground beneath them is designed and certified as a working platform. Laydown space is allocated for truss sections, which are large and awkward and cannot simply be stacked. Where the terminal is on or beside an operational apron, the crane oversail, the height limits and any restrictions the airport operator imposes on lifting near live aircraft movements are agreed in advance, and they frequently limit when lifting can happen at all.

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   Step 3: Fabricate off site and trial assemble where it is warranted

   Trusses are fabricated in a shop, where welding and dimensional control are far better than anything achievable at height. Sections are marked, and on complex geometry a trial assembly in the fabrication yard proves the fit before anything reaches site. Bolt holes, splice plates and bearing details are checked against the survey of the supporting structure as built, not against the drawing, because the supports will have been constructed with their own tolerances.

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   Step 4: Complete and survey the supporting structure

   The columns, cores or supporting frames that the trusses will land on are completed and surveyed before delivery. Bearing levels, positions and the setting out of holding down details are checked against the steelwork, and any deviation is resolved with the designer before the lift rather than being taken up by force at height. Where the trusses bear on movement or sliding details, those details are installed and checked, and any temporary locking arrangement is recorded so it can be released at the right stage.

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   Step 5: Assemble at ground level as far as the sequence allows

   Work done on the ground is safer, faster and better than the same work done at height. Truss sections are bolted or welded together in the laydown area into the largest assembly the crane can lift and the site can move, with lifting points, temporary stiffening and rigging fitted while access is easy. Fall protection anchorages, walkways and lifelines are also fitted at ground level so that the crew has protection the moment they reach the steel.

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   Step 6: Lift, land and immediately restrain

   The lift is executed to the plan, with the rigging arrangement, the crane configuration and the wind limits fixed in advance and enforced on the day. Once landed, the truss is secured at its bearings and restrained laterally at once - by temporary props, by ties back to the previous truss, or by guys - because a single truss standing free is unstable out of plane. Nothing is released from the crane until the temporary restraint is in place and checked by a competent person.

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   Step 7: Erect the bracing and secondary steel to complete the stable structure

   The permanent bracing, purlins and secondary members are erected as the trusses go in, converting a line of individual trusses into a braced three-dimensional structure. The order in which this happens follows the temporary works design, not convenience. Temporary props, guys and ties are removed only when the permanent bracing that replaces them is complete and signed off, and that release is a controlled step with a named person authorising it.

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   Step 8: Load the roof, coordinate the services and survey the result

   Roof decking, insulation and covering are installed, and the services in the roof zone are installed in the coordinated sequence agreed beforehand, since access between trusses becomes progressively harder as the roof closes. Deflections are surveyed as load comes on and compared with what the designer predicted, which confirms both the structure and the camber. The as-built survey and the record of when temporary works were released form part of the handover to the airport operator.