A tower is a plate order with a paint job

An onshore tower runs 80 to 120 m to the hub in three or four bolted sections. The base can is 4.2 to 4.5 m across, rolled from plate that starts near 40 mm at the foundation and steps down to 12 or 16 mm at the top. Above the foundation it is all fabrication: rolled plate, two or three longitudinal seams per can, and a machined flange ring at each end.

So the order is placed as plate and section, and the tower appears later. Four decisions set the price - grade, thickness steps, plate width, and how the inside is protected. The last one is the one buyers get wrong.

Q345 does the work, A36 does the fittings

Q345 is the default shell grade in Chinese fabrication, 345 MPa minimum yield. Q345 plate stock runs 0.3 to 200 mm thick at 1000, 1250 and 1500 mm wide, with 6000 t behind it - enough for a tower programme. 16Mn is the older name for the same family and still appears on older drawings.

A36 plate is for the parts that do not carry the tower: door frames, cable brackets, Q235 platform plate, 45# machined blocks, and the small fabrications that are galvanized and bolted on. At 36 ksi minimum yield it is not a shell material for a 100 m structure, and a quote that offers it as one is quoting a different tower.

Grade selection matters less than it looks. The wall is governed by buckling and by fatigue at the girth welds, so moving to a higher strength grade saves less thickness than the yield ratio suggests, and the saving disappears if the plate then needs a different welding procedure and a different consumable. Q345 coil feeds the cut-to-length line for small cans and internal parts.

The course height is decided by the plate width

At 4.2 m diameter the circumference is 13.2 m. A 12 m plate will not wrap it in one piece, so a can is normally three segments of about 4.4 m with three longitudinal seams, then a girth weld to the can above. With 1500 mm wide plate, the course is 1.5 m high.

Run that on a 25 m section: 17 courses, 51 longitudinal seams and 16 girth welds. Order the same section in 2000 mm plate and it becomes 13 courses, 39 seams and 12 girth welds - a quarter of the welding removed by a width on the purchase order. Every metre of weld carries inspection, distortion and rework, so wider plate usually pays for itself. The stock widths here are 1000, 1250 and 1500 mm; wider plate has to be asked for, and the question is worth asking before the course height is frozen.

The flange ring sets the schedule

Each section ends in a ring cut from plate 80 to 140 mm thick, bored and faced on a machine tool. The ring is the tolerance datum for the whole tower: if the two faces are not parallel, the section leans, and the lean accumulates up the stack. Plate to 200 mm covers the ring, but cutting a 4 m circle is not cutting a bracket - plasma or abrasive waterjet first, then machining, and shearing the blanks square before that.

The ring is also the long-lead item. It is ordered before the shell plate is rolled, and a late ring stops the section because the first course cannot be closed without it.

Welding: the seam is tested, the zone beside it is not

The longitudinal seam is submerged-arc welded, one pass inside and one outside on plate above about 12 mm. Q345 at 40 mm carries enough carbon equivalent to need preheat in cold weather, and the procedure is where the money sits: preheat, interpass control, dried consumables, and inspection of a sample rather than every metre. Plate welding is a commodity. A qualified procedure for 40 mm Q345 at minus 10 C is not.

Rolling comes before all of it. A can that is not round cannot be forced round by the girth weld without leaving a peak that no amount of weld will remove, and punched or drilled holes in the ring have to line up with the mating section, not with the drawing.

The kettle decides how the tower is protected

Here is the part that surprises buyers. A 4.2 m ring cannot be galvanized, because a zinc kettle is roughly 2 m deep, 3 to 4 m wide and 12 to 18 m long. The shell is therefore painted inside and out, and only what fits in the bath is dipped: ladders, platforms, cable trays, brackets, door frames, and the galvanized H-beam or A29M H-section under the internal platforms, which are a steel structure of their own.

That one constraint shapes the internal design. Galvanized processing - cut, drill, weld, then dip - has to finish before assembly, because a field weld burns the zinc off around the joint and a zinc-rich brush is a patch, not a coating. Internals should arrive as bolt-in modules sized to the kettle, not as parts welded in place on site. Z275 coil or hot-dipped coil covers cable trays and small covers, and galvanized plate the heavier brackets.

Inside is the corrosion side

The outside is washed by rain and inspected by everyone. The inside is dark, humid, and on a coastal site full of salt carried through the door and the vent. Condensation runs down that wall for twenty years, and the internal coating decides whether the tower reaches its design life. Specify the system and the dry film thickness, not the word "painted", and put surface preparation in the same clause.

Stainless is minor on a tower - door hardware, brackets, the square tube handrail - and it is there because those parts are handled most and repainted least. 304 sheet is enough inland; on the coast the chloride count decides between 304 and 316L.

Transport fixes the last dimension

Road permits cap a section at about 4.5 m diameter and 30 m length in most markets, which is why towers ship in three or four pieces with bolted flanges rather than being welded on site. The bolted joint is a tolerance stack: flange flatness, hole alignment, and a gap that must close at the specified torque. A ring that leaves the machine shop inside its diameter tolerance but out of flat by a millimetre will need more bolt tension to close, and that preload goes into the first course as bending stress.

The enquiry sheet

Grade and impact test temperature, thickness per course, plate width, flange outside diameter, bore, thickness and flatness, weld inspection level, internal and external paint systems with dry film thickness, the galvanizing standard for internals, and packing. Add the design life and the site minimum temperature; those two change the grade more often than the load case does.

A tower looks like a structure order. It is a plate order, a welding procedure and a coating system, priced in that order.