Overhead Crane Workshop: Four Checks Before Steel Detailing

An overhead crane decides more of a steel frame factory building than the wall panel or the roof sheet ever will. In our drawing office, four checks come before any member is detailed: the clear height measured to the right datum, the runway beam and its bracket, the load path that carries crane surge into the ground, and the tolerance split between the civil contractor and the steel erector. When a client sends a single-section drawing with an eaves height and a crane capacity, those four points are usually where the delay starts. This is what I look at, in the order I look at it, before cutting begins.

Start with the crane, not the envelope

Most enquiries for a factory arrive as a floor plan with a crane noted in one bay. The plan is the right place to start and the wrong place to stop. A crane is a moving load on a frame designed as a static one, and it reaches into every column, bracket and brace in the bay it travels through. If the crane note is treated as an accessory, the frame gets detailed first and then re-detailed, and that second pass is where drawings and steel fall out of step. So I work backwards from the hook: tall enough for the hook to reach the load, wide enough for the crane to travel where the process needs it, stiff enough that the runway does not fight the roof bracing. Once those three are fixed, cladding, doors and mezzanine are ordinary decisions.

Inside a crane bay: the runway beam sits on a bracket welded to the column, and the hook travels the length of the bay over the finished floor.

Check one: clear height, measured to the correct datum

An eaves height is not a hook height, and the gap is exactly where projects lose a metre.

Three heights sit inside one crane bay, and clients often state only one:

• Eaves height — the top of the column where the roof line starts. A building dimension, not an operating one.
• Under-hook height — the working height under a raised hook, which is what a truck bed, a mould or a machine actually needs.
• Clear height under the girder — the underside of the runway beam, which limits anything lifted over a fixed obstruction.

The working height is the under-hook figure. It equals the clear height under the girder, minus the depth of the crane itself, minus the height of the lifted load and its rigging. If the drawing states an eaves height and a crane capacity but never states an under-hook figure, the crane has been sized without the load it is meant to lift. I ask for the height above finished floor at which the load must clear, and what that floor will finally be: a slab poured higher than assumed removes hook travel nobody allowed for.

What the height question actually changes

A shortfall in under-hook height is not fixed by adding purlin depth. It is fixed by raising the crane bay columns, which changes the roof geometry at the step, the gable framing and the downpipe runs. That is a layout conversation, and it belongs before detailing.

Check two: the runway beam and the bracket that holds it

The runway beam is the part of a crane workshop drawing set I read most carefully. It is where the crane and the building meet, and where two suppliers most often assume the other has checked something.

Four details decide whether the runway works:

• How the beam sits. A runway carried on a bracket welded to the column web behaves differently from one carried on a separate crane column. The bracket version loads the column in bending and torsion at a point the column was not necessarily checked for; the separate column version takes floor area and foundation work but keeps the building frame out of the crane load path.
• Where the runway stops. End stops, buffer plates and the last metre of rail are routinely left to site. They should be on the drawing, because the last metre of travel is usually the metre beside a door or a wall.
• What happens at a structural joint. If the crane bay has an expansion joint, the rail has to break there too, and the break has to be detailed. A rail drawn straight across a joint will be cut on site by someone without a drawing.
• How the rail is fixed down. Welded and clamped rail have different tolerances for the top surface, and the choice decides whether the rail can be adjusted after the columns are standing.

On an industrial crane building I also check that the crane supplier's wheelbase and end carriage dimensions match the runway we have detailed. Two correct drawings of two different cranes still produce a crane that stops short of the bay end.

Check three: the load path that carries crane surge to the ground

A crane stopping is a horizontal force, and a horizontal force at the top of a column has to go somewhere. In most frames it goes into the roof plane and down through vertical bracing in one or two bays. That is a structural decision and a process decision at once, because a braced bay cannot hold a door.

This check most often comes back after the layout is drawn: the bracing lands in the bay earmarked for the main entrance, or for the line that must run straight through. Moving it is possible but it changes the frame, so I raise it while the layout is still a sketch.

Two other things I look for: that tie members at the crane bracket level are drawn rather than assumed, and that brace connections are detailed for a reversing load. A static drawing treats those connections as adequate; a reversing one does not.

Check four: who holds which tolerance

Every factory has a seam between concrete and steel, and it sits at the base plate. Someone has to hold the anchor bolt setting-out, the level of the foundation top and the verticality of the erected column; if the drawing set does not say who, both parties assume it is the other.

On a crane bay the seam matters more than elsewhere, because rail alignment stacks on top of column alignment. A column that is within tolerance for a warehouse can still put the rail outside the adjustment range of its fixings. So I ask for the sequence: foundations surveyed and handed over against a stated datum, base plates set, columns plumbed, then the runway surveyed as its own item before the crane is commissioned. That sequence is not a formality — it is the only point at which a rail problem can still be corrected cheaply.

Parameter list I send to the drawing office

Before the first member is detailed, this is the list I want complete. It is deliberately short, and every line has been the cause of a revision at least once.

The three heights inside one crane bay: h1 at the eaves, h2 under the hook, h3 under the runway beam. Only h2 is an operating dimension.

What changes when the site is in Africa or South America

None of the four checks changes with geography, but the inputs do, and three of them catch out drawings that were fine on paper.

The first is wind. A figure in km/h copied from a previous project in another country is not a design input. Coastal and high-altitude inland sites do not share one value, and the number has to come from the local code for that site, stated in km/h. When a client cannot supply it, I list it as an open item rather than filling the gap with a default: a frame sized on an assumed value looks identical on the drawing to one sized correctly, which is exactly why the assumption is dangerous.

The second is the environment the steel will live in. Salt-laden air near a coast, and humid inland conditions with a long wet season, both put the coating specification ahead of paint colour. The environment also decides how members are detailed at contact points, because water sits in a connection long after the rain has stopped.

The third is transport and lifting. A member that cannot travel inside a container, or cannot be lifted by the crane at site, becomes an on-site splice, which is a new detail that has to be on the drawing. I ask for the container type and the site crane capacity at the same time as the building dimensions, because both of them reach back into the frame layout.

Our workshop works under ISO 9001 quality management, and the practical effect on this list is simple: the drawing check carries a signature before cutting starts, so a missing under-hook height stops the job at the drawing.

The four checks, summarised

If a crane bay drawing reaches me with one number per item, these are the four I test first: an under-hook height stated above a named floor level; a runway bracket drawn and its load path closed; a braced bay the layout can live with; and a stated owner for the base plate tolerance. Panel type, door schedule and gutter detail can wait for a normal revision; these four cannot, because each of them moves steel.

If you are preparing a steel frame factory building with a crane in it, the fastest thing you can send is not a finished drawing. It is the hook height, the surface it has to clear, the crane supplier's wheel loads, and the bay you cannot brace. Send those four and the contact form note can be answered with a frame layout instead of a list of questions.

One question to close: on your current drawing set, is the height written as an eaves height or as an under-hook height — and above which finished floor level is that hook meant to clear?

Images: an overhead travelling crane inside a steel frame factory bay; and the parameter list used in the drawing check, with the hook height and runway bracket marked.