Lifting and Setting Heavy Glass

A Panel Schedule Is a Weights List Nobody Wrote Down

The fabricator sends back a table of widths, heights and make-up codes — 2440 by 1050, 6/16/6.8, twelve off — and there is not a kilogram anywhere on it. That is not an oversight. Glass is sold by area, and the make-up code describes an optical and thermal specification rather than a handling one. Everything you have to decide, though, is decided in kilograms: how many people meet the lorry, whether a unit goes up a stair or waits for a machine, which cups come out of the van, and whether the word crane belongs anywhere in the programme. That conversion has to happen while the delivery date is still negotiable.

The code is worth reading properly, because it tells you exactly where the mass sits. A 6/16/6.8 unit is a 6 mm outboard lite, a 16 mm sealed cavity, and a 6.8 mm laminated inboard made from two 3 mm plies bonded by a 0.76 mm interlayer. Two of those numbers are most of the weight, one is a thin polymer film that adds less than a kilogram to the square metre, and the cavity contributes essentially nothing — argon in a 16 mm gap comes to under thirty grams per square metre, against the thirty-odd kilograms the two glass sides come to between them. Thickness in the code is not the same thing as mass in the code, and the units that surprise crews are the ones where a fat cavity made a light panel look heavy or a thin laminate made a heavy one look light.

Two separate figures come out of the arithmetic and different people use them. Per-unit mass settles the handling method one unit at a time, and it is the number that ought to be printed on the crate label before the glass leaves the works. Aggregate mass settles the logistics: what the loaded stillage weighs, what the tail lift is rated to take, and what a suspended slab is being asked to hold while forty units stand on their edges waiting to be glazed.

Where Twenty-Three Kilograms Comes From, and Why Glass Never Earns It

The number sitting in most manual-handling conversations traces back to the load constant in the revised NIOSH lifting equation, published in the Applications Manual for the Revised NIOSH Lifting Equation, NIOSH Publication 94-110. Twenty-three kilograms is not a limit; it is the mass an ideal lift could take, and ideal means the load starts at knuckle height, sits against the body, is lifted without twisting, has proper handles, and is not repeated all shift. From there a series of multipliers cut the figure down to a recommended weight limit for the lift you are actually doing. The constant is the ceiling before any of the site is taken into account.

Glass loses on nearly every multiplier at once. There are no handles, so the coupling term is poor even with hand cups — and a hand cup is a handling aid, not rated lifting equipment, a distinction that matters the moment anyone suggests hanging one on a hook. The panel's own width forces the load away from the trunk, because you cannot bring a 1,200 mm lite close to your chest and still see past it, and its length has you carrying at arm's reach to keep it off your shins. The unit starts on a stillage at floor level and finishes at cill height or above. And the lift almost always ends with a twist through a reveal or a doorway. A 23 kg constant can come down to a fraction of itself before the maths is finished.

Other regimes reach for the same problem differently and are worth knowing by name. ISO 11228-1, Ergonomics — Manual handling — Lifting and carrying, works from a reference mass for the population doing the work, taking 25 kg for the general working population and lower figures where the group needs more protection. In the United Kingdom, the Manual Handling Operations Regulations 1992 and the HSE guidance published with them, Manual Handling: Guidance on Regulations (L23), set guideline filter figures that fall away sharply as the load moves out from the body or above shoulder height, and treat a two-person team as capable of about two-thirds of the sum of what the two could each handle alone — three people, about half the sum. Two glaziers are not two glaziers' worth of capacity, and that derating is the single most misunderstood point on any glass job. OSHA publishes no numeric lifting limit at all; handling injuries in the United States are pursued under Section 5(a)(1) of the Occupational Safety and Health Act, the general duty clause, which obliges the employer to deal with a recognised hazard without telling anyone what the number is.

None of which lets you off the decision. The threshold you work to is yours, and it has to be defensible from the crew you have, the route they walk, the grip they will get and how many times they will do it before the shift ends. Write it down, apply it to every line of the schedule, and let the failures sort themselves into the units that need a machine. A threshold chosen after a unit has been dropped is not a threshold.

A Large Lite Is a Sail Long Before It Is a Load

Glass is heavy for its thickness and light for its face area, and wind cares only about face area. Three metres by one and a half is four and a half square metres of flat plate presented to a gust, and a flat plate is a poor shape to be holding — the drag factor for a panel is not the general value used for a compact load, and the coefficient that governs comes from the standard the lift plan is written under or from the crane manufacturer's own guidance. The gust that counts is the one at the height the unit will hang, not the average beside the site cabin, and every machine has a maximum permissible wind speed printed for its configuration.

Rotation is the part that catches people. A panel presented edge-on to the wind is barely loaded until it is allowed to turn, and then it finds its face on its own. Control comes from tag lines attached to the lifting frame or the head, never to the glass, and from keeping the fall short so the pendulum has less to work with. On a hand-carried panel the same physics applies with nobody to help: a gust across an open podium takes the panel and the glazier with it, which is why an outdoor manual carry has a wind limit as surely as a crane pick does.

Mechanical handling inside the building brings its own arithmetic. Glazing robots and mini spider cranes work off a slab that was designed for occupancy rather than for a tracked machine and its counterweight, and the loads a structure sees during construction are the subject of ASCE/SEI 37, Design Loads on Structures During Construction. Outriggers on a finished floor need spreading and the floor needs a check by someone entitled to make it. The capacity chart falls away with reach exactly as any crane's does, so the unit at the far corner of the atrium is not the unit the machine was chosen for.

Setting the Unit: the Moment the Weight Changes Hands

The set is the transfer, and it is the part of the operation where the mass you calculated stops being carried by cups and starts being carried by two small blocks of rubber. Setting blocks take the entire dead weight of the unit for the rest of its life. ASTM C864, Standard Specification for Dense Elastomeric Compression Seal Gaskets, Setting Blocks, and Spacers, covers the material; the GANA Glazing Manual, published by the National Glass Association, covers where they go and how big they are: a pair standing on the sill, each set in from its own end by about a quarter of the width, with length, width and durometer chosen for the load that will stand on them for the next thirty years. Where the fabricator's detail differs, the detail wins.

On an insulating unit both lites have to be sitting on the block. A block that supports the inboard lite and misses the outboard one hangs the outer pane on nothing but the perimeter seal, which then carries in shear a load nobody designed it for — keeping the cavity dry is the whole of that seal's job. The unit that fogs in year three is very often the unit that was set on a block half a millimetre too narrow. Hardness follows the same logic in the other direction: too soft and the block creeps under a heavy unit until the glass finds the frame, too hard and it becomes a point that concentrates load into the edge.

Sequence the transfer so that nothing is held by hand at the moment it could pinch. The unit lands on the blocks, the crew confirms contact and edge clearance all round, and only then do the cups come off. Releasing while a hand is in the reveal is how the standard glazing injury happens, and it happens to experienced people because the panel behaves predictably right up until it does not. Toughened glass in particular gives no warning: it either holds or it becomes a heap, with nothing in between.

Blocks are also a counting problem, and a trivially solvable one. Two per opening over a whole floor — and on an insulating unit that one pair carries both panes, so it stays two and not four — is a number somebody should have ordered with the glass, in the right durometer, with spares. They are small, they get lost in packaging, and a set stops dead without them.

1. Confirm the block size, hardness and material against the fabricator's detail before the first unit is offered up, not after.
2. Check that each block bears across the full make-up thickness, outboard lite included, before any weight comes onto it.
3. Bring the unit down slowly and square, never sliding it sideways once it is bearing and never prising against the frame to nudge it over.
4. Check edge clearance and face clearance all round while the lifter still has the unit, because correcting position afterwards means picking it up again.
5. Release the cups deliberately, with hands out of the reveal, and confirm the unit is stable and captured before anyone steps away.
6. Log the unit as set, with anything noticed about its edges, so a later breakage has a history rather than a guess attached to it.