A bridge drawing reads like a building drawing and is not one

The plates, bolts and mill certificates are the same documents on both jobs. The load history is not. A warehouse frame is designed around a load that arrives once and stays; a bridge is designed around one that arrives again and again. Twenty thousand vehicles a day over a hundred years is 730 million passes, and if one in ten is heavy, the stiffener end sees near 73 million cycles. Steel does not care much about the peak. It cares about the range, repeated, and about what sits in the water at the bottom of it.

The stress range picks the detail, and the detail picks the grade

Fatigue does not start where stress is highest. It starts where geometry changes: a weld toe, a bolt hole, a cover plate end, a flame-cut edge left as cut. A bad detail cracks at 90 MPa of range; a good one survives 150 MPa in the same steel. So the first decision is not the grade, it is whether the splice can sit where the range is low. Only then does the grade answer its two questions: toughness at service temperature, and what corrosion does while the load cycles.

The 2205 duplex plate page is direct about the second. It lists high corrosion and erosion fatigue among the alloy's properties and says applications subject to both an aggressive corrosive environment and cycle loading can benefit, at yield about twice the austenitics. On a coastal crossing that is the splash zone and bearing shelf, not the deck. 254 SMO names fatigue corrosion among the four modes it resists and quotes nearly twice the 300 series strength, which buys a thinner section where weight governs. 625, with excellent fatigue strength and a range from cryogenic to 1093 C, turns up on expansion joint anchors rather than in the girder.

Machined parts are another argument. 416HT is the grade a machinist reaches for, at about 85 per cent of the machinability of free-machining carbon steel, and its application list names bridges. The same page carries the caveat: high-sulphur grades are not for marine or chloride service, and weldability is poor. A pin in a dry bearing housing is one thing; the same pin on a salted crossing is another.

Plate width, not thickness, decides where the splice lands

The Q345 plate page quotes 0.3 to 200 mm at stock widths of 1000, 1250 and 1500 mm, against 6,000 tonnes of monthly inventory and coil weights of 3 to 18 tonnes. A 3,000 mm bottom flange cannot be rolled in one piece at those widths, so it becomes two plates and one longitudinal weld down the middle of the most heavily stressed fibre.

Three honest answers: move the splice where the range is low and detail it properly; buy wider plate as a special roll and accept the lead time; or change the section so the flange fits inside 1500 mm. Ordering 1500 mm plate and letting the fabricator decide does not work: the weld lands where the plate ends, and the plate ends in the worst place.

Thickness carries its own arithmetic. A 20 mm plate at 1500 by 12 m weighs 2.83 tonnes, inside the coil range. At 40 mm it is 5.65 tonnes. At 200 mm it is 28.3 tonnes, which no coil line will produce, so it ships as a discrete plate on its own cradle. A36 plate and Q235 plate follow the same geometry at lower yield, and the structural plate grade summary says which belongs where.

The secondary steel is a separate order and arrives late

Diaphragms, stiffeners, cross-frames and walkway brackets are bought after the plates, from a section table rather than a drawing. The A36 universal beam listing runs 5 to 34 mm thick, flanges 50 to 400 mm at 8 to 64 mm, webs 100 to 900 mm at 5 to 16 mm, in 6 to 12 m lengths at plus or minus 1 per cent, from one tonne. The Q235 H-beam page adds the sizes a cross-frame uses, a 25 tonne minimum and Q345, ST52, Q420 and S355JR in the same table. Where the member is small, the 50 x 150 section is quoted down to 5 mm.

Length is where these orders go wrong. Twelve metres is the mill standard and also the practical limit of a 40 foot container once packing is added, so a 12 m cross-frame either ships break-bulk or is spliced on site. Ask for 11.8 m, or accept the splice and design it.

Salt, drainage and a coating inspected from a basket

The steel structure page says it plainly: steel structures are prone to corrosion and generally need derusting, galvanizing or painting. On a bridge the corrosion is not uniform. It concentrates where the deck drains, where a joint leaks, on the top face of the bottom flange, and inside a box sealed with water still in it. Those are also the surfaces nobody can reach, which is why a repaint costs more than the steel and why the maintenance engineer writes the coating spec.

The surface treatment page sets out the sequence: rust removal by blasting, shot blast or acid pickle, then a coating that isolates air and moisture, or galvanizing for items that will never be repainted. For that group, galvanized plate and Z275 coil cover parapet supports, access covers and cable trays, and the processing line behind them turns coil into the finished part.

Rope arrives on a reel, and mostly it is temporary

On a bridge job the rope is rarely the structure. It is the sling, the tie-down and the lashing that holds a plate on a cradle. The wire rope listing covers 0.8 to 21.4 mm in 6x19, 6x37, 1x7 and 7x7, grades 1770 to 2160, to GB/T 20118, API Spec 9A and ISO 10425, on wooden reels from one tonne. Where rope stays as handrail or netting on a coastal crossing, 304 and 316 rope is the version that needs no repainting.

What to send with the enquiry

Six lines carry most of it. Grade and standard, as a standard plus a grade. Plate thickness and width with the tolerance the design assumes. Splices permitted, and where they may sit. A section list with lengths and whether 12 m is acceptable. The coating system with the surface preparation grade it assumes. And the certificate class, because coil and plate are routinely issued to EN 10204 3.1 and occasionally to 3.2, and the two are not interchangeable at the witness point.

Add the processing and the order stops being a material order. Cutting starts at mechanical shear below 10 mm and moves to hydraulic shear above it, with CNC turret punching under 2 mm, laser for profile work and waterjet where a heat-affected edge is unacceptable. Bending covers knife, two-knife and three-knife work, 180 degree returns and double-layer folds. Punching, plate welding and surface treatment turn a plate list into a delivered member, and the enquiry that says so comes back with one price and one date.