The section is the visible half of the order

A steel frame drawing reads as a schedule of beams and columns, and the quotation that follows usually prices it the same way. The steel structure page describes the assembly more honestly: a structure built from steel beams, columns and trusses formed from section steel and steel plates, joined together and then given rust removal and an anti-rust system. Section steel is the part with a mill designation, a published weight per metre and a grade stamped on the web. Steel plate is the part with none of that, cut to whatever the connection needs. The same page lists what else the frame is made of beside the main sections, and the list is instructive: Q235B and Q355B welded H sections, Q235B C and Z purlins, Q235B circular pipe, round bar and angle steel. On a portal frame the plate content is base plates, gussets at every knee and apex, stiffeners where a point load lands on a web, and cleats for the purlins. None of it appears in the beam schedule, and all of it has to be bought, cut, drilled and coated.

Reading a section number before trusting it

The five dimensions on the H-shaped steel page are the ones worth copying into an enquiry: thickness 5 to 34 mm, flange width 50 to 400 mm, flange thickness 8 to 64 mm, web width 100 to 900 mm, web thickness 5 to 16 mm. Those numbers, not the nominal depth alone, decide bending capacity and whether a bolt and a spanner will fit between the flanges. A designation such as 50 by 150 on the 50x150 H-beam page is read as 50 mm of flange width against 150 mm of web height, a proportion chosen for load carrying rather than for stiffness in both directions. European designations behave differently. The IPE 450 page explains that IPE belongs to the I-beam system of EN 10025-2:2004, with the P marking parallel legs and the E a quality class, so an IPE 450 and a 450 mm deep H-section are not interchangeable at the same depth.

One section, several grades, several prices

Grade is the second number to fix, and it is where substitutions happen quietly. The same welded H-section is offered as Q235B or Q355B on the H-shaped steel page. The mild steel beam page puts Q235, Q235B, Q345, Q345B, S355J2, S235JR and A36 on one line, which is a lot of choice for a member that was designed at one yield strength. The SS400 page is clear that SS400 is the Japanese standard equivalent of Q235B and that Q345B is the low-alloy high-strength step above it. A swap from Q355B to Q235B is therefore a design change rather than a procurement convenience. The A36 universal beam and the A29M programme sit in the same bracket, and the latter names the tests behind the claim: chemical composition analysis, mechanical properties testing and metallographic structure inspection on every batch before dispatch.

Plate is where the joints are decided

Connection design converts force into plate area, bolt count and weld length, and the plate grade rarely matches the section grade. The Q235 plate page describes moderate tensile strength with excellent weldability and good toughness, which is why it turns up as base plates on frames whose sections are Q355. The Q345 plate trades some of that weldability margin for higher tensile and yield, and the A36 plate page notes the material can be welded, bolted or riveted, worth checking when a connection is specified as bolted only. Thickness runs from 0.3 to 200 mm in 3 to 18 tonne lots at regular widths of 1000, 1250 and 1500 mm, and thickness is the variable that decides whether a stiffener is a plate or a bar. The 45# plate and 16Mn plate pages cover the higher-carbon and higher-manganese ends, where a machined part rather than a welded one is the intended outcome.

The processing list is a drawing, not a service

Every hole, bevel and bend in a frame is a separate operation with its own tolerance. The cutting page sets out which process suits which thickness: mechanical cutting below 10 mm in a straight line, hydraulic shear above 10 mm, CNC turret punching for simple profiles in carbon plate under 2 mm, laser for complex patterns at higher cost, plasma for thick plate at speed, waterjet where a heat-affected zone is unacceptable. Shearing is the same decision viewed from the straight cut. The punching page adds the point that deburring or sanding after punching has to be specified rather than assumed. The bending page lists knife, two-knife and three-knife bends, 180 degree and double-layer work, the bending coefficient, the unfold calculation and the radius that forms on every edge, and it puts deburring, polishing and painting in a post-treatment step. Send a finished part drawing instead of a flat pattern and the unfold arithmetic becomes the fabricator's risk to price, which is usually more expensive than doing it yourself.

Coating, bolts and the last metre

Protection is decided after the geometry is fixed, and it is where a frame that fits in the workshop stops fitting in the container. The hot-dip galvanized H-beam page describes the zinc-iron alloy layer and the gloss it leaves; the galvanized structural page states the choice plainly, listing Q355B H-sections as alkyd painting or hot dip galvanized, C and Z purlins galvanized in Q235B, and bolts as either galvanized steel or stainless. That last line deserves its own clause, because a galvanized frame closed with carbon bolts is a corrosion cell at every connection. Length is the other constraint: the same page gives a 20 ft GP container 5,898 mm of internal length against 12,032 mm for a 40 ft, so a 12 m member is either spliced by design or shipped differently. Surface treatment before coating, whether sandblasting, shot blasting or pickling, is a priced operation rather than a default.

What to settle before the order

Five items close most of the gap. Fix the section by its five dimensions and its standard, not by nominal depth. Fix the grade and state which mechanical property the design used. Price the plate content separately from the sections, because that is where a cheap quotation hides its margin. Send the processing as a list with thickness against each operation, covering cutting, punching, bending and welding, and ask for custom processing to be quoted on the same sheet. Then state the coating system, the bolt material and the maximum delivered length. Where the frame includes pipe bracing or a stainless element, the square tube and stainless pipe ranges run 0.5 to 75 mm of wall and are quoted on a different basis again. A frame order carrying those five lines is quotable. One carrying a beam schedule and a note reading galvanized is where the arguments start.