In steel the structure is not built, it is assembled. The real work happened in the shop, and three things left on site decide the outcome — the level of the holding-down bolts, the temporary bracing and the bolt torque.
A steel frame arrives on site already finished: profiles cut, drilled, welded and painted, marked to match the erection drawing. Assembly is therefore a job of geometric precision rather than construction, and the sequence is dictated by stability: until the permanent bracing is in place, the frame stands only thanks to temporary props and guys, which are to all intents part of the design. The floor is completed with profiled steel decking and a pour that is not a floor but the compression part of the beam, made composite by the shear studs. What remains is the chapter most often underestimated: bare steel loses strength quickly in a fire, and fire protection must be designed as part of the structure, not added at the end.
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The detail of the craftthe base plate levelled on grout
Setting the holding-down bolts with a template before the pad foundations are cast, then seating the base plates on shims and levelling them.
A template costs half a day. One misplaced bolt costs a week and an engineer's report.site notebook — editorial synthesis
The curve that explains everything
Steel does not burn: it yields. Its strength falls continuously with temperature, and around five hundred and fifty degrees roughly half is left — the half that, in the fire load combination, is roughly what it is actually carrying. On a bare profile that temperature arrives in minutes. Protecting it means buying time: the time people need to get out and the fire service needs to get in.
Qualitative curve, drawn to explain the principle. The reduction factors to be used in calculation are those of the structural Eurocodes for fire conditions: they are not reproduced here, only the reason they exist is explained.
Two materials, one beam
Rest a slab on a beam and you have two elements that bend together but slide over one another: each works on its own. Prevent that slip and the two become a single section, much deeper and therefore much stiffer. The stud is the piece that prevents the slip, and the difference between the two cases is not a detail: it is the reason composite floors exist.
Schematic. It is also why a composite floor cannot be rescued by adding topping thickness: without connection, extra concrete is only weight.
Rarely by cost per kilo, almost always by time and span. Steel wins when you need long spans, shallow beams, a short programme, or a structure that will be altered later. Concrete wins on stiffness, thermal mass, intrinsic fire resistance and maintenance cost. In many projects the answer is hybrid: a concrete core for stability and a steel frame for the floors.
Hot-dip galvanising is bulk protection: it lasts decades with no maintenance and is the obvious choice for external steelwork, humid environments or elements that are hard to reach. A paint system is more flexible on colour and can be touched up, but it has to be redone. In the harshest cases a duplex system is used — galvanising plus paint — which lasts longer than the sum of the two, because the paint protects the zinc and the zinc protects the steel wherever the paint is scratched.
Not without a check. Web openings are possible, but position, diameter and distance from the supports change the shear capacity in ways that are anything but intuitive, and the zone near supports is the most delicate. The site rule is simple: no hole that is not drawn. If one is needed, ask for the calculation and, if required, reinforcement with plates welded in the shop or certified on site.
Because in the shop each profile was cut for one precise position, with holes and plates that suit only that joint. The piece mark ties the member to the erection drawing and to its qualification records. Swapping two apparently identical pieces almost always means misaligned holes and, at worst, a profile of a different grade in the wrong place.