A hand picks the rail; a load picks the post

On a stair drawing the balustrade is one line with a note that says stainless. By the time it reaches a fabricator the line has become two different tubes. The top rail is chosen because a hand closes around it: 42.4, 48.3 or 50.8 mm outside diameter, in a wall thin enough that the rail does not feel like a scaffold. The posts are chosen to stop a person who falls against them, which is a bending problem and has nothing to do with how the tube feels. Orders go wrong when one section is asked to do both jobs.

The arithmetic separates them quickly. A 50 x 50 x 1.5 mm square stainless steel pipe spanning 1.2 m sags about 1.6 mm at mid-span under a 1 kN point load. A 42.4 x 2.0 mm round tube carrying almost the same mass per metre, 1.99 kg against 2.28, sags 3.5 mm, because at that diameter the round section has under half the second moment of area. The rail that looks sturdier is the softer of the two.

The post is a cantilever, and it sets the price

A guard post is not a beam. It is fixed at the floor and free at the top, so its deflection grows with the cube of its height, and a 1.1 m post pushed at the top moves far more than any rail. In 50 x 50 x 2.0 mm the top travels about 15 mm under 1 kN. In 60 x 60 x 2.0 mm it travels 8.5 mm, for 21 per cent more steel, 3.01 kg per metre against 3.64. On a public stair the first number reads as sway; the second does not.

Grade does not rescue a soft post. Austenitic and duplex grades all sit near 200 GPa, so moving from 304 to 316L, or on to 2205 duplex, changes corrosion resistance and yield strength but not stiffness. A 2205 post holds twice the load before yielding and deflects exactly as much. Where the post is too flexible the answer is a larger section, a thicker wall or a top fixing into the wall; the alloy decision is a separate conversation.

The base plate is where the drawing goes quiet

Most balustrade failures start at the floor line. A 100 x 100 x 8 mm plate with two M12 anchors into a 150 mm slab is a common detail, and the three numbers that matter are edge distance, anchor type and drainage. A chemical anchor close to a slab edge splits the concrete before the post bends; a cast-in ferrule set 60 mm back does not. Slotted holes let the erector line the posts up, but they also remove the friction that was carrying part of the moment, so they need a plate washer and a grout bed rather than a shim at each corner.

Water is the second detail. A post welded to a plate with no drain hole holds a cup of rainwater at the weld, which is the one place the passive layer was destroyed. Drill the plate, leave a gap under it, or seal the joint completely. The third is the galvanic pair: a stainless post on a galvanized Q235 base plate puts a small zinc anode against a large stainless cathode, and the plate disappears around the weld. A stainless plate costs more and removes the argument.

Grade follows the air around the building

Indoors, dry and washed, 304 is correct and cheapest. Within a kilometre of the coast, on a sea-facing elevation, or anywhere a contractor spreads de-icing salt, 316L earns its molybdenum; the grade carries 2 to 3 per cent and the L suffix matters because every post is welded. On a pier, a pool deck or a kitchen washdown line the chloride is concentrated and persistent, and the ladder moves to 317LMN at 4 to 5 per cent molybdenum with 0.10 to 0.20 per cent nitrogen, to 2507 at 25 per cent chromium and 4 per cent molybdenum, or to 254 SMO at 6.0 to 6.5 per cent molybdenum, where strength is nearly twice the 300 series.

Tea staining is the usual complaint, and it is a surface condition rather than a loss of section. It appears at welds, at cut edges and under protective film left on too long, and it comes off with pickling and passivation, not a scouring pad. On a coastal job the cheapest prevention is 316L with a properly finished weld; inland, a clean 304 weld will outlast the paint.

The infill sets the gap and half the load

A guard is judged by what passes through it. A 100 mm sphere is the usual test, which puts vertical bars on roughly 110 mm centres and puts mesh in a different category. Where the design wants transparency, the stainless wire rope net range runs from 80 x 140 mm mesh on 1.6 mm rope at 2.17 kN breaking force to 120 x 208 mm on 3.2 mm rope at 7.38 kN, in 304, 316 and 316L, and it needs a tensioned frame rather than a series of point fixings. A net tensioned by eye sags, and the sag is what the client sees.

Infill also changes the post load. A glass panel on four clamps feeds wind and impact into the base as a couple; a rope net on a stainless frame transfers almost nothing until someone leans on it. The post that was adequate for bars at 110 mm centres is often not adequate for glass of the same width, even though the drawing shows the same line.

Finish is a sequence the site can undo

A satin finish is a direction as much as a roughness. No.4 and HL surfaces are polished one way, and a weld dressed across that direction shows as a patch under grazing light. The sequence is: dress the weld flush, grind with progressively finer discs, polish to match the surrounding grain, then pickle and rinse. A carbon steel disc used anywhere near the work leaves free iron that rusts within a week, which is why stainless shops keep separate tools. Surface treatment at the works is worth more than any amount of site polishing.

Protective film is the other trap: it is for transport, not for the building's first year, and adhesive baked on in summer needs solvent and patience. On the maintenance side, bleach and hydrochloric descaler attack 304 and pit 316L, and a balustrade cleaned with either shows it at the bottom of the posts first.

What goes on the order

Four lines cover most guard orders: rail size and wall, post section and wall, grade with the L suffix where welded, and finish with the face it applies to. Add the cut list, because cutting to length at the mill removes a saw cut on site, and bending and welding details where the post is fabricated.

The tube is the visible part of the job. The post, the plate and the anchor are what the guard is actually made of.