A building of 200 m (650 ft) is not a building of 20 m repeated ten times. It moves as one thing, and the movement has to go somewhere.
The movement is mostly thermal: a steel roof over a long span changes length between a cold night and a hot afternoon by an amount you can measure. It is also the movement of a frame settling, of a slab shrinking, and of foundations behaving differently at one end than the other. Left undirected, that movement ends up in the connections that are least able to take it — usually the ones at the ends.
1. How long is long: the working range
There is no universal trigger, and anyone who quotes one is guessing. What matters is the accumulated movement, and the point at which a frame can no longer absorb it.
As a working figure, a steel-framed building in a climate with a wide temperature range starts to need a deliberate movement strategy somewhere in the order of 60 to 100 m (200 to 330 ft) in one continuous length. Below that, bracing and connection ductility usually handle it. Above that, and certainly beyond 150 m (500 ft), a building without a defined joint is a building that is deciding the question by default.
The other trigger is not length but restraint, and restraint is often what actually decides it. A long building attached to a rigid element — a concrete core, a silo, a heavy gable — concentrates movement at that attachment regardless of overall length, and the joint may be needed there even in a short building.
2. Where the joint goes, and what it must not cross
A movement joint is cheapest where it crosses least.
The best position is at a natural break in the plan: between two bays, at a change of width, at the edge of a phase. The worst position is through a wet area, a fire compartment boundary, or the middle of a production line, because the joint then has to be sealed, drained and maintained in a place nobody wants to touch.
Two rules help. Keep the joint out of the roof valley, where water will find it. And keep it out of the floor slab if the floor has to be continuous for traffic — which is where a sliding floor plate or a proprietary joint becomes necessary, and that item is far easier to design in than to add.
3. Sliding connections: what has to be allowed to move
A joint only works if the structure on one side is free to slide, and that freedom has to be designed at every member that crosses the line.
In a steel frame, that means elongated holes or sliding bearings at the columns' base and at the rafters, and it means the bracing on each side must be independent, so the two halves are stable on their own. It also means the cladding has to accommodate the movement: a wall that runs continuously past a joint will tear at the fixings within a season.
The detail that fails most often is the column base. A base plate with ordinary holes will resist movement by friction even when the design intended it to slide, and the resistance is enough to crack the slab around it. Slotted holes with a low-friction bearing strip, or a properly detailed sliding shoe, are what make the analysis true.
4. The roof: movement, water and wind at the same line
The joint in the roof is the hardest detail on the building, because it has to do three jobs at once.
It has to allow movement along the length. It has to keep water out, which on a low-pitched roof means a raised upstand and a cover that overlaps in the direction of flow. And it has to resist wind uplift, which at a gap in the roof is higher than anywhere else because the wind gets under the cover.
The practical answer is a proprietary expansion assembly set on a raised kerb, with the kerb detailed as part of the roof structure rather than as a flashing added later. A joint made of two sheets lapped over each other will work for a year and then leak at the first storm with the wind behind it.
5. Floors, rails and everything that crosses the line
Anything continuous that crosses a movement joint has to be broken or made to accommodate it: crane rails, conveyor supports, racking, drainage channels, cable trays, and the floor itself.
Crane rails are the most consequential. A runway that runs the full length of a building with one joint in the middle needs the rail to be cut and spliced with a gap, and the crane's wheels will feel that gap every pass. Two separate runways stop at the joint, or a splice detailed to allow movement without a step. Either is fine; what is not fine is a continuous rail bolted through a joint that then moves.
Racking is the quiet one. A rack run that crosses a joint will be pushed by the floor on one side, and the rack is not designed for horizontal load. Breaking the run at the joint costs a little floor area and prevents a collapse.
6. What I want before drawings
- The building's total length, and the temperature range it will see.
- Whether it will be built in phases, and where the phase line is.
- Anything rigid it attaches to, and where.
- Whether the floor must be continuous for traffic.
- Rail, conveyor or rack runs that cross the length.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the joint's own uplift detail — the cover over a movement joint is the most wind-exposed element on the roof, and it is where a failure starts.
A long building does not need to be divided into many buildings. It needs one place where it is allowed to move, detailed so that everything crossing that place expects it. Decide that early, and the joint disappears from the argument.