A conveyor gallery is a long, light, narrow structure carried on a series of towers. The instinct when detailing it is to make every connection to every tower the same, because consistency looks like good practice.
That instinct is what cracks the welds at the ends of the run. A gallery pinned at every tower has no idea where to put the movement that its own length generates, so it puts it into the connections, and the connections at the two ends carry the most.
1. One tower holds the run, the rest let it move
The structural arrangement that works is simple to state: one tower is the fixed point, and it takes the longitudinal loads of the whole run. Every other tower allows the gallery to slide.
What the fixed tower resists: belt starting and stopping forces, the longitudinal component of belt tension at a drive or a turn, and thermal movement of the gallery's full length. What the other towers do is carry vertical load and allow the gallery to move along its own axis.
Once that is drawn, the rest of the detailing follows. The fixed tower gets a connection designed for horizontal force and a foundation sized for it. The sliding towers get bearings, guides and a specified movement range.
2. Why pinning everything looks reasonable and is not
On a short run — three or four towers, a hundred metres — the movement is small and a fully pinned gallery appears to perform. The failures show up on long runs, on sites with a wide temperature swing, and on belts that start loaded.
When every tower is fixed, the gallery cannot expand, so it locks in a stress that was never analysed. The visible symptom is usually a torn base plate, a cracked weld at a diaphragm, or a tower that has been pushed out of plumb by a few millimetres per year. By the time it is visible the cause is several years back in the detail.
The other problem is load sharing. With every tower pinned, nobody knows which tower actually takes the longitudinal force. In practice it is the stiffest one, which may be the one with the smallest foundation.
3. Sliding is not the same as free
A sliding connection is a designed item, not a loose bolt hole.
It needs a specified direction — along the gallery axis, restrained across it, because a gallery that can move sideways on its tower has no lateral stability.
It needs a specified range, derived from the length to the fixed point and the temperature swing, plus the construction tolerance.
It needs a guide that keeps the gallery on the tower while allowing the movement, and a stopper that limits it if the range is exceeded.
And it needs to be inspectable. A sliding bearing that nobody can see or grease becomes a seized bearing, and once it seizes the tower behaves as if it were fixed — which is how a design that allows for movement quietly becomes one that does not.
4. The forces arrive at one point, so design for that point
Because the longitudinal loads concentrate at the fixed tower, that tower is not the same structure as its neighbours. It carries horizontal force at gallery level, which means it needs bracing down to its foundation and a foundation that can take both the vertical load and the overturning from the horizontal one.
That asymmetry is worth marking on the drawing, because a tower fabricated to the same details as the others will be erected as though it were the same, and the braced one may end up wherever the crane happened to place it.
The related decision is where the fixed tower goes in the run. Putting it near the middle halves the movement at each end; putting it at one end doubles it at the other. Middle is usually better, unless the middle tower stands over something that cannot take the horizontal load.
5. Access is part of the same detail
A gallery with a fixed tower in the middle and sliding towers elsewhere still needs people to reach every transfer point and every idler.
That means an access tower with a stair, not a ladder to the roof; a walkway alongside the belt with headroom and a handrail; and a stopping place at each transfer where tools can be set down. These are not finishes — a transfer point that cannot be reached safely is a plant that stops for a day to clear a blockage.
Where the gallery is elevated, the same access tower usually doubles as a support tower, which is efficient but concentrates even more load into one place. That has to be a decision, not a coincidence.
6. What I want before drawings
- The full route: every crossing, and the clearance each crossing requires.
- The drive and tension stations, their belt tensions, and which direction the horizontal forces act.
- The temperature range at the site, because it sets the movement to be accommodated.
- Which tower can be the fixed point, and what is under it.
- How maintenance reaches every transfer point, and whether access towers coincide with support towers.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the gallery's open-side detail — a long, narrow, open-sided gallery on tall towers is the most wind-sensitive element on a bulk site, and the fixings at the towers take the load first.
Restraint is a decision that has to be made once, deliberately, and drawn. Made by accident — every tower the same, every bolt tight — it becomes a structure that fights its own length for twenty years.