A long span is a transport problem before it is a structural one

A stadium roof, an aircraft hangar, a distribution centre with 40 m clear across it. The drawing shows a truss. The purchase order shows pieces of a fixed length, because mills roll and stock H-sections in 6 to 12 m lengths and a truck carries about the same. That one fact decides how many pieces a 36 m chord arrives in, how many site splices the erector has to make, and how many crane picks the programme can afford.

None of it is fixed by changing grade. Depth, piece length, splice location and the secondary package are settled before anyone argues Q235 against Q345, and a frame detailed without them is re-detailed after the first delivery.

What the mill will actually put on the truck

Standard ASTM A36 H-beam is quoted from 5 to 34 mm thick, with a 50 to 400 mm flange at 8 to 64 mm, a 100 to 900 mm web at 5 to 16 mm, supplied in lengths from 6 to 12 m and held to plus or minus 1 per cent. The A29M hot-rolled H-section sits in the same window, tested for chemistry, mechanical properties and metallographic structure batch by batch. IPE 450 gives 450 mm of depth in 6 to 12 m, and 50 x 150 sections cover the small end of the range from 5 to 36.5 mm.

So a 36 m chord arrives in three pieces and carries two field splices. A 48 m span needs four and three. Each splice is a pair of A36 gusset plates, a bolt group or a site weld, a temporary support while it is made, and an inspection record - and on a roof at 20 m, a splice is the most expensive metre of steel in the building.

Depth is what the span buys

Bending moment rises with the square of the span, chord force is moment divided by depth. Keep the depth and double the span and the chord force goes up roughly four times, not two. The depth has to grow with it, and that is where a rolled section order turns into something else.

The rolled sections listed here stop at a 900 mm web. A 3 m deep truss cannot be rolled, so it is fabricated: chords cut from Q345 plate 0.3 to 200 mm thick, in 1000, 1250 or 1500 mm widths, joints prepared and closed under a qualified plate welding procedure, edges squared on a cutting line, holes punched before assembly on a punching station and the stiffeners and gussets formed where they need a fold on a press brake. From that point the order is plate by the sheet plus a weld map, not sections by the tonne.

Half the tonnage is secondary steel that nobody draws

The Steel Structure specification lists the package as it is really bought: purlin in Q235B C and Z, knee brace in Q235B angle, tie bar in Q235B round steel, bracing in round bar, and vertical and horizontal support in angle, bar or pipe, with the base plates and stiffeners cut from Q235 plate. A 60 x 40 m roof at 1.5 m purlin centres runs about 27 lines across the 40 m, so roughly 1,600 m of purlin section for a building most people would describe as a single steel frame.

That 1,600 m is not ordered as a section. C and Z purlins are cold-formed from coil, which makes them a Z275 galvanized coil order at 0.12 to 2.0 mm plus a roll-forming slot, cut to length and punched on the same line. The galvanized processing chain - laser cutting, bending, welding, zinc, assembly - is where the purlin, the edge trim and the bracket all come from, and it is usually a different supplier from the one rolling the main frame.

The envelope runs on someone else's schedule

Roof and wall are single colour steel sheet at 0.5, 0.6 or 0.7 mm, or a sandwich panel with EPS, PU, fibreglass or rock wool core. Edge cover in PPGI, gutter in galvanized or stainless, downpipe in PVC or metal, ventilation in stainless, doors sliding or rolling, windows aluminium or PVC. None of that sits in the frame contract, and all of it hangs off the purlin line and the setting-out of the first bay.

A TDC51D prepainted coil at 0.12 to 2 mm and 600 to 1250 mm wide arrives in a different container from 30 mm plate, and a colour match between PPGL roof and an edge trim bought six weeks later is not guaranteed. Corrugated galvanized sheet, stainless roof panel for the aggressive end of the market and square stainless tube for canopies and mullions are quoted on their own lead times, and they are the first items to arrive late because nobody put them on the critical path.

Paint, dip, or both, but decide it with the splice

On the same specification the main frame is Q355B, alkyd painted or hot dip galvanized, and the purlin is galvanized as standard. The choice is normally made on access: a frame that will be closed in and never seen again is painted, and a frame that will be rained on, or that sits over a wet process, is dipped. Galvanized H-beam and shop surface treatment both exist for that reason.

The trap is that coating and splice are one decision. A galvanized member cannot be welded on site without stripping zinc back from the joint and re-protecting it afterwards, which costs more than the galvanizing saved. If the design needs field welds, the frame gets painted; if it gets dipped, the splices move to bolted connections with galvanized assemblies, and the bolt grade and hole tolerance follow.

Erection is the real constraint on piece length

Crane capacity falls with radius, so a 12 m segment at 6 t and a 24 m assembly at 14 t are different lifts on different days. Most erectors will build two or three segments on the ground, brace them, and pick the assembly - which means the splice that was designed for transport becomes a splice made at 1 m above the slab, and the temporary guys are wire rope that nobody priced.

Inside the building there may be another frame order hiding: an overhead crane of 5 to 20 t needs a runway beam, brackets and a rail, all of it bought with the structure and installed before the roof goes on.

The enquiry sheet

Span, bay spacing, roof load and clear height under the tie. Then piece length, splice type and location, bolt grade, purlin spacing and gauge, cladding system with colour and core, gutter and downpipe material, coating system with dry film thickness or the galvanizing standard, and the crane capacity that will be on site. Send the erection drawing with the enquiry rather than the span alone.

A long span is never bought as a truss. It is bought as 12 m pieces, a purlin run measured in metres and a cladding schedule, and the failures happen at the joints between those three orders.