Braced steel frame with cellular beams

How does Braced steel frame with cellular beams work, step by step?

1. 1

   Step 1: Settle the grid and the stability system

   The frame starts with the grid. Commercial office grids are commonly in the order of 7.5-9 m, chosen so the floor plate suits the letting strategy, the ceiling grid and the car parking below, and the structural engineer then works within it. At the same time the stability strategy is fixed: braced bays, a concrete core, or a combination of the two. That decision reaches straight into the architecture, because a braced bay is a wall that cannot have a door in it. It also reaches into the programme, because a frame relying on a slipformed or jumpformed core cannot be completed until the core is far enough ahead of the steel. The engineer decides both; the contractor tests them against buildability and crane reach early, while a change is still just a drawing.

2. 2

   Step 2: Coordinate the services and freeze the cell layout

   Cellular beams only work if the services actually fit through the cells, so the mechanical and electrical routes are coordinated in a federated model before the steel is released for fabrication. Duct sizes, the sprinkler main, the primary containment and the crossings at the busiest points near the risers are all resolved and then locked. The structural engineer places and sizes the openings to suit the loads and the coordinated routes together. Once the beams are cut and welded, that layout is permanent. Anything the model missed becomes either a drop below the steel, which eats the floor zone that cellular beams were bought to save, or a formal design query for a new opening.

3. 3

   Step 3: Fabricate off site and deliver in sequence

   Beams and columns are fabricated in a controlled shop: sections cut, cells profiled and rewelded, end plates and fittings attached, holes drilled, and everything shot blasted and given its primer or protective treatment. Members are marked to the erection drawings and loaded onto wagons in reverse order so the first steel needed comes off the back of the first load. Fabricated cellular beams carry a longer lead time than plain rolled sections, and that lead time sits on the critical path from the moment the order is placed. Delivery slots, wagon turnaround and a hardstanding to unload on are planned with the same seriousness as the erection itself, because a steel gang with a crane and no steel is the most expensive idle plant on a commercial project.

4. 4

   Step 4: Erect in a planned sequence and plumb up

   The erection sequence is designed, not improvised. Columns go up first and are bolted to their holding-down bolts on the foundations, beams are landed and bolted, and the frame is brought into a self-supporting condition bay by bay before the crane is released. Temporary stability during erection is a temporary works matter and belongs to the temporary works designer working with the steelwork contractor, and no part of the frame is released from the crane or from temporary restraint until that designer's scheme says it can be. The frame is then plumbed and lined before the bolts are finally tightened, because a frame erected out of plumb stays out of plumb and every facade bracket downstream inherits the error. Erection is a permit-controlled operation with an exclusion zone below, worked to a lift plan by an appointed person.

5. 5

   Step 5: Deck the floors and give the followers a platform

   Metal decking is landed in bundles, laid, fixed down and edge-protected as the steel rises, which turns the frame into a series of safe working platforms far sooner than any wet trade could. The deck is normally the permanent formwork for a composite slab, so the frame and the floor construction are planned together. Holes for risers, service penetrations and lifting points are formed as the deck is laid, from the coordinated drawings, rather than cut afterwards. Edge protection, stair access and the safe means of getting the gangs up and down are installed with the deck and not after it, because the point at which a frame becomes genuinely useful to everybody else is the point at which it is safe to walk on.

6. 6

   Step 6: Bolt the connections to the engineer's design

   The connections are what make the frame a frame. Simple bolted connections - end plates, fin plates, cleats - transmit the forces the engineer has designed them for and nothing more, and their behaviour is assumed in the stability model. Bolt grade, size, count and tightening method all come from the design and the specification, and the site fits what has been detailed. Site welding is avoided where it can be, because it is slow, weather-dependent and needs its own inspection regime. Where a member does not fit, the answer is a query to the engineer and a fabricated remedy, never a torch and a bit of judgement. Bolt tightening records and the inspection of the completed connections are part of the handover paperwork for the frame.

7. 7

   Step 7: Fire protect the completed frame

   Bare steel loses strength as it heats, so the frame needs fire protection to meet the fire strategy for the building. On most commercial projects that is intumescent coating, sometimes applied off site to shorten the programme, sometimes sprayed or boarded on site. It is a follow-on trade, and it is the one most often squeezed. Applied on site it needs the steel clean, dry, within a temperature and humidity window, and reachable - which becomes progressively harder once ductwork is threaded through the cells and the ceiling grid is going in. Sequencing the protection early, and deciding what is done in the shop, saves a great deal of grief later. The required protection is set by the fire engineer and the specification, and its thickness and coverage are inspected and recorded rather than assumed.

8. 8

   Step 8: Hand over the frame to the facade and the fit-out

   A finished frame is a survey, not just a structure. The as-built positions of the columns, the floor levels and the perimeter line are surveyed and issued, because the facade is fabricated to those dimensions and the brackets have a limited range of adjustment. Any deviation outside the tolerances in the specification is reported and resolved with the designer before the cladding is made, not after it arrives on site. Holding-down bolt grout, deck edge details, riser edges and the fixings the following trades will hang from are all completed and signed off. From that point the frame becomes a fixed constraint on everything else, which is exactly why the surveying and the tolerances are worth the attention they get.