Choosing a Frame for a Long-Span Building

A frame carries load through a small number of columns so that the plan below is free of walls that have to line up. That is the advantage, and it creates the problem: something else now has to stop the building leaning, and whatever that is occupies plan space and cannot be moved to suit a layout.

Reading the three entries side by side, the whole comparison starts there. Stability, the foundation grid and later insertions are three consequences of the same decision rather than three separate topics.

In a general steel frame the connections decide the behaviour. Joined so the beam ends can rotate, the frame leans on something else for stability; joined so that beam and column hold their angle, the frame resists sway itself. That is why a connection cannot be treated as a fixing detail and substituted for one that looks similar.

A portal frame concentrates the stiffness at the knee, where column meets rafter, and at the apex, commonly deepening the joint with a haunch so it is stiffer than the members it joins. A cast concrete frame provides stability through cores and shear walls cast integrally with the floors that feed them, which occupy fixed plan positions because the frame around them was designed on the basis that they were there.

In a portal frame building, the two directions are held by entirely different means. Across the building the frame stands because the knee and apex joints keep the angle between column and rafter, and nothing else is doing that work. Along the building the frames would fold like a row of cards without the bracing planes and the members tying them together.

That has a practical consequence owners meet constantly: a wide door is thought of as an opening in cladding when it may interrupt the only braced bay on that side, and removing a diagonal to make room for a door or a duct is an alteration to the building stability rather than to a wall.

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 while the bracing itself is unchanged.

In a cast frame the equivalent question is where an opening sits. The steel running through the top of a plate over a column is what holds it there, so an opening formed close to a column takes out exactly the reinforcement doing that work. An opening near a support and an opening in the middle of a plate are entirely different questions despite looking the same on a floor plan.

In a portal frame the same logic reaches the ground floor. Where the slab is relied on to tie the feet of opposing columns against outward push, it stops being a surface and becomes part of the structure.

Load arrives at points rather than along lines, so the substructure follows the column grid. In a steel frame 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.

A portal frame delivers horizontal thrust as well as vertical load at its bases, so the pads and any ties between them are resisting the frame spreading, and whether the base is arranged to resist rotation changes how the whole frame behaves. In a cast frame, cores in particular concentrate both weight and overturning, and beneath a core the ground is asked to do something different from what it is asked beneath a column carrying gravity alone.

This is where the choice is felt years afterwards. Adding a mezzanine is described as independent, though it loads existing columns and can alter how force reaches the bracing. A platform put into a clear volume either stands on its own supports or leans on the frame, and if it is tied to the columns it shares the building stability arrangement, which is a change to a structure designed without it.

In a portal frame the same applies to anything hung from the structure. A hoist, an inserted platform or plant sitting on the rafters all deliver their effects into members already working over their full length, and because the arrangement repeats, a change at one frame is seldom local: the bracing that served it may sit in another bay altogether.

Purlins and side rails are part of this too. They carry the covering but also restrain the members they are fixed to, so taking out a run to form an opening changes the member as well as the envelope.

A steel frame is made before it is erected, so the design has to be fixed early, access, craneage and delivery order become design constraints, and a change agreed on site is a change to something already fabricated and possibly already coated.

A cast frame is built in a sequence of pours, each depending on the support beneath it, and the information for each pour has to be complete before it begins. The frames also differ in what they need while going up: a steel frame standing on site may be relying on temporary arrangements until the floors are on, and temporary stability is designed as its own arrangement rather than assumed from the permanent one.

The questions that separate these frames are not about spans. They are about what is holding the building up sideways, where that element must remain, how the floor delivers force into it, what the ground beneath the stability elements is being asked to do, and what any later addition would change.