Every steel frame spends a period of its life standing up without being finished. During that period it is loaded in ways the finished building never sees, and it is held together by pieces that will later be removed. Those pieces are almost never drawn.
That is the failure I see most often on EPC packages: the permanent structure is thoroughly designed, the temporary condition is left to the erector, and the erector — who is paid to erect, not to analyse — props with whatever is on site and takes the props out when they get in the way.
1. A frame is not stable until a closed system exists
A pair of columns with a rafter on top is a mechanism, not a structure. It becomes stable when three things exist: a base that can take moment or a pin that is triangulated, a line of bracing in the plane of the frame, and a connection to something that will not move.
The sequence question is therefore not "when can we take the props out" but "when does the closed system first exist". On a portal frame that is usually after the first braced bay is complete and the longitudinal ties are in — which may be bay three, not bay one.
Until then, each frame needs to be held by something. That something has to be designed for the load it actually carries, which during erection includes wind on a partially clad or unclad frame, the temporary loads of the crew and their equipment, and the out-of-plumb forces of the frame beside it.
2. Wind on an open frame is worse than wind on a finished building
This is the point that catches projects out.
A finished building has cladding, and cladding distributes wind into the structure through many fixings. A frame standing without cladding has a fraction of the surface area but concentrates what it does catch on a few members, and the shape of the structure at that moment may be worse than the finished shape — a bare frame with one gable clad is a classic case where the wind load path exists on one side only.
So the temporary condition needs its own wind check, at the site's wind speed, in the states the frame will actually pass through. Not a reduction factor, not a rule of thumb: the same site wind, applied to the structure as it will exist next Tuesday.
3. Guy ropes: three things that go wrong
Guys are the default answer and they fail in three predictable ways.
The angle. A guy at a shallow angle to the ground carries an enormous tension for a small horizontal force, and the anchor gets a pull it was not designed for. The useful range is the one where the anchor load stays within what a cheap anchor can take — and that is a design decision, not a site one.
The anchor. A guy tied to a kerb, a fence post or a piece of plant is not an anchor; it is a load path into something that was never checked.
The removal. Guys get taken out when they obstruct a lift, and they get put back — or not — afterwards. If the drawing says a prop stays until bay four is bolted, that has to be written where the crew reads it, not in a method statement filed at head office.
4. What the drawing has to say, and usually does not
A temporary support is only real if three things are on paper:
Its location, dimensioned from a grid line, so it is not placed where it is convenient.
Its section and connection, so it is not a piece of scaffold tube bolted through a painted flange.
Its removal condition, written as a state of the permanent structure — "after bays A1–A3 are plumbed, bolted and the longitudinal ties installed" — not as a date.
When those three are missing, the erector is being asked to make a structural decision with no information, on the day the crane is waiting.
5. The interface with everything that is not steel
Temporary works also decide what happens at the boundaries: a slab that will be poured under a frame that is still propped, a precast wall that arrives after the roof, a silo foundation that needs the crane position that the frame needs.
On these interfaces the temporary condition and the permanent one compete for the same space, and the sequence decides which wins. That is why the erection plan and the general arrangement have to be reviewed together — not sequentially, with the erection plan arriving after the drawings are issued.
6. What I want before drawings
- The intended erection sequence, even as a sketch, and the crane positions it assumes.
- Which bays must be stable independently and which may rely on neighbours.
- The site wind speed from the applicable code, and the states the frame will pass through while it is open.
- Whether any part of the frame stands over an area that will be poured, paved or occupied before completion.
- Who owns temporary works on this package, and whether they are in our scope or the erector's.
- Any element that arrives late, because the sequence has to survive its absence.
A frame is at its most vulnerable on the day it is half built, and nobody is looking at it then except the crew. Put the props on the drawing, say what they carry and when they come out, and the most dangerous week of the project stops being the least documented one.