PRC.11 · Construction Process

Steel Frame: Erecting the Structure

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.

Holding-down bolt tolerance

± 5 mm on plan

Erection rate

one floor every 5–10 days

Strength loss in fire

≈ 50 % at 550 °C

Recyclability

> 90 % at end of life

Overview

TAV. 00

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.

The site score

TAV. H

The detail of the craftthe base plate levelled on grout

01Days 1–4

Holding-down bolts and base plates

Setting the holding-down bolts with a template before the pad foundations are cast, then seating the base plates on shims and levelling them.

Why it is done this wayThe entire geometry of the building starts here, and it is the only point where an error cannot be recovered with a turnbuckle. Holding-down bolts must be cast in with a rigid template, not by eye: two centimetres out and the column will not fit, so someone ovalises the base plate holes — the quickest way to halve the joint's capacity. The plate itself must not bear directly on the concrete: it is levelled on shims and the void is filled with expanding grout, so contact is full and even. Partial bearing concentrates the whole load on one corner of the pad.

± 5 mm · bolt position tolerance25–50 mm · grout bed thickness0 vuoti · contact under the plate

In handsteel bolt templateoptical or rotating laser levelcalibrated shims and expanding grout

bedding grout not poured below 5 °C

The mistake that costs dearlyBase plate holes flame-cut oversize to make the column fit: the bolt no longer works in friction, the joint becomes a loose pin and the frame sways at the first gust.

Site supervision checkBolt position and plumb against the template; plate level checked with an optical level; expanding bedding grout continuous, with no voids; no holes enlarged on site.

A template costs half a day. One misplaced bolt costs a week and an engineer's report.site notebook — editorial synthesis

The beats — with site daysbeat 1 of 5

7–28 dayscomposite slab curing before the floor is loaded and the temporary props come out

click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work

The curve that explains everything

Why steel needs to be protected from fire

TAV. G1

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

What the shear stud actually does

TAV. G2

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.

How it ages (and what betrays it)

TAV. P

PIEDE CORROSOBase plate without continuous grout or with water ponding around it (phase 01): corrosion starts at the contact, invisible under the floor, and reduces the section exactly where the load is highest.

TELAIO SVERGOLATOTemporary bracing removed before the permanent bracing is in (phase 02): the frame racks under wind and the joints stay out of square even after realignment, with bolts already stressed for nothing.

SOLAIO CHE NON COLLABORAStuds badly welded or too few (phase 03): the slab becomes dead weight, deflection doubles and the floor vibrates when people walk on it.

FESSURE SUGLI APPOGGIRestrained shrinkage with no fibres and no continuity reinforcement (phase 04): the slab cracks over the beams, water reaches the deck and the galvanising is eaten from above.

CLASSE AL FUOCO INTERROTTAJoints, plates and bracing left bare (phase 05): the protected element holds, the connection does not, and the declared rating of the assembly does not exist.

Questions from the site

TAV. Q

Steel or reinforced concrete: how is it really decided?

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.

Galvanising or painting?

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.

Can a beam be drilled on site to run a service through it?

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.

Why does the steelwork arrive marked piece by piece?

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.

Materials involved

TAV. M

Metallurgical coating

Hot-Dip Galvanized Steel

MAT.26/49F

Fiber reinforced concrete (FRC)

Fiber Reinforced Concrete (FRC)