Planning How the Facade Gets Cleaned and Maintained

The Machine Is Only Useful on Days the Wind Lets It Out

A cradle is a suspended load with a floor, a guard rail, a mesh or solid side and two people standing in it, and wind treats it exactly as it treats anything else hanging on a rope. Every suspended platform has an in-service wind limit on its plate and in its manual, set against the design criteria of EN 1808, the standard for suspended access equipment; above it the machine parks, and above the out-of-service figure the machine has to be secured rather than merely stopped. The reading that governs is at roof level and at the height the cradle is working, not at the anemometer somebody mounted beside the site cabin.

The number worth putting in front of a client is not the force but the offset. Take a modest cradle presenting five square metres to the wind, with two operatives and their kit coming to something like five hundred kilograms all in, hanging on sixty metres of rope in a twelve metre per second gust. The horizontal push is only a few hundred newtons — trivial next to the weight — but a load hangs at the angle whose tangent is that force over that weight, and six degrees of tilt at sixty metres of rope is more than six metres of horizontal travel. That is not a cleaning inconvenience. It is a cradle arriving somewhere on the elevation nobody intended, which is the whole reason restraint exists: sockets, buttons or a continuous guide track built into the curtain wall so the platform is held against the face rather than hanging free from it. Those restraint points are a facade item on a facade drawing, and they load the mullion, so they are decided with the curtain wall and not after it.

Two design consequences follow, and both are cheap now and impossible later. The first is that restraint provision has to be specified with the cladding package, because a socket every few floors on every drop line is a fabrication item and there is no retrofit. The second is a programme argument rather than an engineering one: the number of workable days a year is a function of the site's wind climate, and a coastal or exposed high-rise loses enough of them that the cleaning cycle the client is being promised may not be deliverable with one machine. Local meteorological data answers that, and answering it during design is how a second machine or a longer cycle gets agreed instead of discovered.

The Anchors Are the Part the Owner Keeps for Thirty Years

Machines get serviced, break down and are eventually replaced. The anchor provision on the roof is what carries the building in between, and it is also what everyone else uses: the roofer chasing a leak, the mechanical contractor changing a fan, the surveyor with a camera, the aerial rigger. Design it for that population rather than for the window cleaner alone, because it is the roofer at seven in the morning in December who actually meets the edge.

Start by deciding whether the roof is a restraint job or an arrest job, because they are different products and different geometry. Restraint means the line is short enough that the worker physically cannot reach a position from which they could fall, and it is always the better answer where the plan allows it — nothing deploys, nothing has to be rescued, and no clearance arithmetic is needed. Arrest means accepting the fall and catching it, which brings in energy absorbers, clearance below and a rescue plan with a time limit on it, all of which the edge protection guide works through in detail. On a roof where the parapet is high enough to serve as permanent guarding for the zone people actually work in, neither is needed for that zone at all, and that is a decision about parapet height taken at the same moment as the one about the track.

The anchor standards to write into a specification are named documents, so name them: EN 795 for anchor devices, with CEN/TS 16415 where more than one person can be attached to the same device at once, and BS 7883 for the design, selection, installation and periodic recertification of systems built from them. On a roof, EN 516 covers installations for roof access such as walkways and treads and EN 517 covers roof safety hooks; AS/NZS 1657 does comparable work for fixed platforms and walkways, and AS/NZS 4488 for industrial rope access systems. What matters as much as the standard is the register: an anchor that is not identified, recorded, tested and re-inspected is an anchor a competent contractor will refuse to clip to, and a roof full of those is a roof with no access system regardless of what is bolted to it.

Note what a horizontal lifeline is and is not. It is a single engineered system with a tested span, a defined sag and a maximum number of users, not a row of independent points, and its intermediate bracket spacing comes from the system manufacturer rather than from a general rule. Discrete anchors along a walk route are the simpler case and the one a layout estimate genuinely helps with — the route from the access hatch to the plant deck and out to the parapet, counted at the spacing the system states, with one more anchor than there are intervals.

Somebody Has to Get a Pane Out, and a Pane Back In

Cleaning is the easy half. A sealed unit is a manufactured assembly with a service life, not a permanent component: EN 1279 and ASTM E2190 exist to characterise how long the edge seal keeps the cavity dry, and units do fog, glass does get broken, and somebody will be replacing panes on this elevation for as long as the building stands. The awkward question for the roof plan is whether the access system that cleans the facade can also change it, because those are very different duties and a machine chosen for one will not do the other.

Consider what a reglaze from a cradle actually is. Two operatives stand on a platform that moves under them, with no floor to set anything down on, no second crew to hand to and nothing to lean the pane against. A modest unit — say one and a half metres by two, twelve millimetres of glass across both leaves — is already ninety kilograms. There is no threshold under which two people turn that in a cradle, which is the point: the pane goes on the hoist, or on a glazing manipulator arm, or it does not go at all. The install-day version of this arithmetic, done on a floor with a route and a stillage, is worked through in the glass lifting guide; in a cradle the honest threshold is far lower than anything you would accept at ground level, and running the check is how you demonstrate that rather than assert it.

Rate the Platform for the Reglaze, Not for the Squeegee

The platform safe working load is a single number in the machine specification and it is set now, in the design stage, by whoever writes the performance requirement. Get it from the worst load case the building will ever present rather than from the routine one, because upgrading the rating later means a different machine, a different track loading and a different roof structure. Work it as an aggregate: the heaviest replacement unit on any elevation, plus whatever carrier frame or manipulator holds it, plus two operatives, plus their tools and the suction gear.

Take a real corner unit on a commercial tower — one point six by three metres, with fifteen millimetres of glass once both leaves are counted. At the density soda-lime glass is specified to, that is a hundred and eighty kilograms of glass before the frame. Add a manipulator, two people and their kit and the platform is being asked for something near five hundred, which is a decision about which class of machine goes in the specification and is not a decision anybody makes cheaply after the roof steel is fabricated. The unit make-up arithmetic behind that figure — leaves, cavities, interlayers and what each contributes — belongs to the glass lifting guide and is not repeated here; what matters at this stage is the biggest number on the schedule and what it does to the machine.

Two things fall out of the same figure and both belong in the design record. The first is the machine class and therefore the track reactions, which the structural engineer needs before the roof is issued. The second is a plain statement, in the maintenance strategy, of the largest unit the access system can handle — because the day a designer specifies a feature pane larger than that, somebody has quietly committed the building to a mobile crane and a street closure every time it breaks.

The Machine Arrives After the Tower Crane Has Gone

A building maintenance unit is delivered in modules — base frame, mast or turntable, jib sections, hoist, cradle, counterweights — and it is installed near the end of the job, which on a great many programmes is after the tower crane has been dismantled. That leaves a mobile crane on a street that is now a finished public highway, with a road closure, a licence, a lift plan, and a night in which to complete a series of picks over a parapet onto a roof that already has its waterproofing on it. It is a lift that gets discovered rather than planned, and it belongs in the programme conversation at the same time as the machine selection.

The rigging has one feature worth flagging at design stage rather than at the yard. Hook height over a tall parapet is often the binding constraint: the crane has to clear the parapet with the load and still have headroom for the bridle, and where headroom is short the legs come in shallow, which raises the tension in every one of them. A jib section is also long and end-heavy, so the centre of gravity sits away from the middle of the lift points and one leg takes far more than its nominal share. The angle-factor mechanics and the rule about crediting only two legs of a four-leg bridle on a rigid load are set out in the multi-leg rigging guide; the design-stage consequence is simpler, which is that a taller parapet and a tighter street both push this lift toward a bigger machine.

There is a second, quieter consequence for the roof itself. The modules land on a finished roof and are then moved into position across it, so the route from the landing point to the track needs a temporary protection and load spreading strategy — and that strategy is the same problem again in miniature the day the machine is eventually replaced. Where the crane stands, what it stands on, and how its reactions reach competent ground is the ground bearing question the crane planning guide works through, and it is a different question from this one only in that it happens at street level.