Structural Steel Frame System

A steel frame is assembled rather than formed. Members are cut, drilled and fitted with plates and cleats in a fabrication shop, arrive with their connection preparation already in them, and are bolted together in position. The geometry of the building is therefore decided at fabrication, and what happens on site is largely the consequence of decisions taken before anything was delivered.

The connections decide the behaviour. The same columns and beams joined so that the beam ends can rotate give a frame that leans on something else for stability; joined so that the beam and column hold their angle, they give a frame that resists sway itself. This is why a connection cannot be treated as a fixing detail and substituted for one that looks similar.

The nearest relative is portal-frame-system, and the distinction is visible in a building. A portal frame gets its stability in its own plane from a small number of rigid frames repeated along the building, and the deep haunches where rafter meets column give it away at the eaves. A general steel frame carries a separately identifiable braced bay or a solid core, and the diagonals or the core are what you can point at. The test is whether the frame itself resists sway or something else does.

A steel frame is also not a complete structure until its floors are on. The floor plates are what collect horizontal force and take it back to the bracing, so the frame standing during erection and the frame in service are different structures. Temporary stability is designed as its own arrangement rather than assumed from the permanent one.

The connection type propagates through the whole frame. A joint detailed to let the beam end rotate transfers vertical reaction but deliberately does not hold the beam square to the column, so the structure has to lean on bracing or a core. Replace that joint with a stiffer one, or the reverse, and the forces travel differently everywhere, not only at the joint that was changed.

Bracing only works if the floors can feed it. Horizontal force arrives across a whole facade and has to be collected by the floor plate and delivered to the braced bay, so a large stair opening, a riser or an atrium cut through that plate interrupts the delivery route. The bracing is unchanged and the frame behaves differently.

Everything gathers at the base. A whole column line arrives on a plate over a small footprint, where the bedding beneath transfers it and the holding-down assembly resists lift and turning. Because the frame is fabricated to fixed dimensions, a base set out inaccurately cannot be absorbed later, and the reconciliation of the two tolerances is a design matter rather than a site one.

The envelope and the frame meet at a bracket, and that bracket has several duties at once. It carries cladding load back into a structure that moves under wind and load, it passes through whatever is controlling water and air, and it is a metal path across the insulated line. Deciding it settles a structural question, a weathering question and a thermal question in the same detail.

Protective treatments and connections are in tension with one another. Shop-applied protection is broken wherever a bolt is made up, a plate is welded or a member is trimmed, so the positions that most need protection are the ones the shop could not reach. What is done at those positions is specified rather than left to the erection sequence.