Continuous bearing on all four edges, and why it matters
A walk-on unit is designed on an assumption about how it is held, and if the support does not match that assumption the calculation does not describe the thing that got built. Continuous bearing on all four edges is the usual assumption, and it is the detail most often compromised on site.
What continuous bearing means
The glass sits on a support that runs the whole length of each of its four edges, without gaps, at a consistent level, with the load spread evenly along the bearing.
Not resting on four corners. Not on a support that runs along two edges and stops. Not on a bearing that is proud in some places and hollow in others. Continuous, all four, level.
It is a simple requirement and it is surprisingly easy to miss by a small amount, and small amounts matter on glass.
Why glass in particular
Because a point load on a brittle plate is where cracks start.
A timber joist bearing unevenly settles and redistributes. Steel yields locally. Glass does neither: a high spot in a bearing concentrates stress at that point, and the stress does not relieve itself. It sits there for the life of the installation, and it is exactly the condition that turns a surface flaw into a crack.
A great many glass failures that get reported as spontaneous are a bearing that was never right.
Two edge and four edge support
| Two edges | Four edges | |
|---|---|---|
| The panel acts as | A beam, spanning one way | A plate, spanning both ways |
| Stress for the same load | Higher | Lower |
| Deflection | Greater | Less |
| Glass needed | Thicker | Thinner |
| Where used | Where a support cannot run all round | The normal arrangement |
The consequence that matters: a build-up calculated for four edge support and then installed on two is being asked to do something it was not designed for, and the margin it had is gone.
That is not a theoretical distinction. It is the difference between a designed unit and an overloaded one, and it happens when a support gets interrupted for a service run or a structural member.
How much bearing width
The support has to be wide enough for the load to spread into it and for the glass to have somewhere to sit with tolerance in both directions.
Too narrow and the load concentrates at the very edge of the glass, which is the most vulnerable part of the panel because that is where edge damage from handling lives. Too narrow also leaves no tolerance for a panel that is a millimetre off.
The figure comes from the engineer with the build-up, and it is worth knowing that it is a specified dimension rather than whatever the frame happens to offer.
Setting blocks and bedding
What actually sits between the glass and the support, and it is not nothing.
Glass never bears directly on metal or on masonry. It sits on a resilient bearing: setting blocks at defined positions, a continuous bearing gasket, or a structural bedding compound, depending on the system.
That layer does three jobs. It spreads the load rather than concentrating it on high spots. It accommodates small irregularities in the support. And it isolates the glass from a hard material that would otherwise be in point contact with it.
Glass set directly onto a steel angle, or onto a concrete upstand, is glass with a stress concentration wherever the two touch.
Where setting blocks go
Not anywhere, and not in the corners.
Setting blocks are positioned in from the corners by a defined distance, because the corner is the stiffest part of the panel and loading it there produces exactly the stress concentration the blocks exist to avoid.
The positions come from the system and they are worth checking rather than assuming the installer knows. It is the kind of detail that is invisible once the unit is in and decisive over thirty years.
Level, and the tolerance that matters
The support has to be level and flat within a tight tolerance, and tighter than most construction works to.
A support with a high spot in the middle of one edge lifts the glass off the bearing either side of it, which converts continuous bearing into point bearing at that spot. A support that dishes leaves the glass bridging, which loads the corners.
Which means the support is surveyed before the glass arrives, not after. Correcting a support with a heavy glass panel sitting on it is not a practical proposition.
Packing, and doing it properly
Real supports are not perfect and packing is how the difference is taken up.
Done properly that means continuous packing of a suitable material, taking up the whole gap along the affected length, so the bearing remains continuous. Done improperly it means a few shims at the low points, which converts an uneven continuous bearing into a set of point supports.
The test is whether the glass, once set, is in contact with its bearing along the whole of each edge. Where it is not, packing has made it worse rather than better.
Why point fixings are a different product
Bolted point fixings exist in structural glazing and they are not a substitute for edge bearing on a floor.
A point fixed panel is designed for that arrangement from the outset, with the holes formed before toughening, local reinforcement in the build-up, and a calculation that accounts for the stress around each fixing. It is an engineered solution to a specific requirement, usually about appearance.
What is not acceptable is drilling or bolting a panel designed for edge bearing. Holes cannot be made in toughened glass after treatment, and a panel designed for continuous support does not have the local capacity for a point fixing.
The support has to be stiff too
An aspect people miss: it is not enough for the support to be level at the moment of installation.
If the support itself deflects under load, the glass loses its bearing where the support has dropped and gains it where it has not. A glass panel bearing on a beam that sags is a glass panel whose support conditions change every time somebody walks on it.
Which means the supporting structure has its own deflection limit, generally tighter than it would be for a conventional floor, precisely because it is carrying something brittle.
Thermal movement
Glass and its support expand at different rates, and the detail has to allow for it.
An aluminium frame in direct sun moves more than the glass it holds. Steel moves differently again. If the glass is bedded rigidly and restrained on all four edges with no allowance, thermal movement puts the panel into compression or tension it was never designed for.
The resilient bearing and the perimeter clearance are what accommodate that, which is another reason those are specified dimensions rather than a matter of getting the panel to sit down.
What goes wrong on site
The recurring list, and every one of them is a support problem rather than a glass problem.
- A support interrupted for a service run, turning four edge support into three.
- Shims at the low points instead of continuous packing.
- Glass set directly onto steel or masonry with no resilient bearing.
- Setting blocks omitted, or placed in the corners.
- The support not surveyed for level before the panel arrived.
- A supporting beam that deflects more than the glass detail assumed.
Surveying the support before the glass arrives
- Level along each of the four bearings, and across the diagonals.
- Flatness along each bearing, not just at the ends.
- Bearing width available on each edge, against the specification.
- Squareness of the opening, since the panel does not adjust.
- Continuity: nothing interrupting any of the four edges.
- The condition and fixing of the support itself.
That is half an hour before a heavy, expensive, made to size panel arrives, and it is the last opportunity to correct anything.
Restraint, and lifting off the bearing
Bearing carries the panel down. Something has to hold it in place against everything else.
Wind uplift on a roof unit, somebody catching an edge, thermal movement, and on a flush unit the possibility of a panel being lifted deliberately for access all mean the glass needs restraint as well as support. That is usually a perimeter clamping arrangement or a retaining trim rather than anything through the glass.
What matters is that the restraint does not become an additional bearing. A trim screwed down hard onto the top surface of the glass at intervals is a set of point loads applied from above, which is the same error as point bearing from below, upside down.
The corners are the vulnerable part
Worth understanding because it explains several of the rules above.
On a plate supported on four edges, the corners are where the panel is stiffest and where it resists lifting rather than bending. They are also the part of a glass panel most likely to carry edge damage from handling, since a corner takes the knocks.
Which is why setting blocks are kept away from corners, why edge condition is inspected before installation, and why a chipped corner on a panel about to be installed is a reason to stop rather than a cosmetic matter.
Once it is in
The support is now inaccessible, which is the reason all of the above happens beforehand.
A glass panel sitting on a bearing cannot be lifted to adjust a shim without the lifting equipment that put it there. Which means a bearing defect discovered afterwards is either lived with or is a repeat of the whole installation.
It is the least forgiving detail on any product we fit, and it is entirely a matter of preparation.
Handling damage before it is a bearing problem
Edge condition and bearing are the same subject, because the edge is what sits on the support.
A panel that has been knocked on a corner, chipped along an edge in transit, or set down on a hard surface without protection has damage exactly where the stress is highest and where the bearing acts. On a brittle material that is not cosmetic.
Which is why panels are inspected on delivery before anything is opened up, handled with the edges protected, and never rested directly on a hard surface. A panel with edge damage is refused rather than installed and watched.
Recording the bearing before it disappears
Photographs of the survey, the packing, the setting block positions and the panel in place before any trim goes on.
Every one of those becomes inaccessible the moment the installation is complete, and they are exactly what anybody investigating a problem in ten years would want to see. On a structural element that people stand on, that record is worth more than on any other product we fit.
It goes with the calculations, the load category and the specification in the paperwork handed over.
What the survey covers on walk on glass
We survey the support for level, flatness, width, square and continuity before the panel is delivered, and correct anything before it arrives rather than after. We use continuous packing rather than shims, set the panel on the specified resilient bearing with setting blocks at the specified positions, and check that all four edges are genuinely in contact.
The engineer calculated a panel on continuous four edge bearing. The installation either provides that or it provides something the calculation does not describe.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.