In a maintenance workshop the building exists to serve a crane, and the crane exists to serve equipment that has not been bought yet.
That is the whole problem in one sentence. The crane is usually selected from a catalogue, the building is quoted from a span and a height, and the machine it will lift five years from now has not been specified by anyone. So the design gets based on the smallest crane that fits today's budget, and the first time a haul truck differential or a shovel component needs lifting, the answer is a bigger crane and a taller building — which means a new building.
The bay is the one part of a workshop you cannot upgrade incrementally. Everything else can be extended.
1. The crane is bought before the building, or the building is wrong
I ask for the crane first, and if it does not exist yet, I ask for the heaviest thing that will ever be lifted.
Those are not the same question, and the second one is the useful one. A crane's rated capacity is not the same as its ability to lift a specific awkward object: the hook height, the headroom above the hook, the approach of the hook to the columns and the speed of the long-travel all matter as much as the tonnage.
Two figures should be written into the brief before anything is drawn: the maximum lift, and the maximum lift height above finished floor. Everything structural follows from those two numbers.
2. Rail level: the number that cannot be raised later
The crane rail level is set from the top down, not from the floor up.
Start at the highest point the load has to reach — usually the top of a component sitting on a stand or on a truck bed — add the sling height, add the hook block depth, add the crane's own headroom, and you have a rail level. Then check the column for the resulting eccentric load and the roof for the clearance above the crane.
Every one of those additions is a place where a project under cost pressure removes 300 mm (12 in). Cumulatively that becomes a metre, and a metre is the difference between lifting a component over a machine and dismantling the machine to get at it.
The rail level also decides the eaves height, because the roof has to clear the crane's top. Adding 500 mm (20 in) to a rail level after the columns are fabricated is not a design change; it is a rebuild.
3. Span and hook approach: the two numbers that trap you
A crane's span is quoted between rail centres, and its hook approach is how close the hook can get to each rail. The difference between those two numbers is the dead zone beside each column that the crane can never reach.
In a maintenance bay that dead zone matters more than in a warehouse, because the things being lifted are long, awkward and often need to be set down against a wall. A 20 m (65 ft) span crane with a 1.5 m (5 ft) hook approach gives you a usable width closer to 17 m (56 ft), and that is the number the workshop layout has to be drawn around.
The practical check is simple: draw the largest component at the position it must be set down, and confirm the hook can be there. If it cannot, the bay is the wrong size regardless of what the crane's capacity plate says.
4. The bay beside the crane: laydown, turning and the doors
A crane bay is not a single space. It is a lifting zone plus a laydown zone plus a route in and out.
Laydown is the one that gets squeezed. A component that has been lifted out of a machine has to sit somewhere while it is inspected, and if that somewhere is the lifting zone, the crane is occupied by a stationary object. Allocating a dedicated laydown area — roughly a third of the bay area for a general maintenance workshop — keeps the crane productive.
The doors follow. A door wide enough for the largest vehicle, tall enough for the tallest load on it, and positioned so a truck can reverse in without a banksman standing in the crane's working area. Door height is usually set by the load, not the vehicle, and that is another number that is fixed too low.
5. The runway beam and the column that carries it
The runway is where a crane building differs from a shed, and it is where the steel cost concentrates.
A loaded crane braking puts a horizontal force into the runway at rail level, and that force has to go somewhere: into the column, into the bracing, and eventually into the foundation. A column designed only for vertical load and wind will be undersized for this, and the symptom is not collapse — it is a column that moves enough over a few years that the crane starts to bind at one end of the bay.
That is why the bracing layout matters as much as the section sizes. In a long bay, one braced line at each end and a properly connected runway is a different structure from a frame that relies on the cladding for stability. The second one works until somebody removes a panel for a service opening.
6. What I want before drawings
- The heaviest single lift, and its shape — long, awkward loads behave differently from compact ones.
- The height the load must reach, measured from finished floor.
- The largest vehicle that enters, and the height of its load.
- Whether a second crane, or a future larger crane, is conceivable in this bay.
- Whether the workshop will ever handle components that arrive by road already assembled.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the roof edge and door detail — a large open door on a windy site changes the internal pressure, and that is a cladding and fixing question before it is a frame question.
A crane bay is decided by four numbers: capacity, hook height, span and hook approach. Get those on paper before the first column is drawn, and the rest of the workshop can be changed later without touching the structure.