Wind lab: watch a glass pane bend

How the lab works

Nothing here is calculated a second time. The lab calls glasscheck.js, the copy of the thickness calculator's core that the schedule checker also uses and that an automatic check compares with the calculator figure by figure; the lab itself is compared with the calculator, over a grid of panes, in the same way. Small-deflection plate theory for a rectangular pane simply supported on four edges, b the short span:

α and β come from the calculator's table, interpolated on the aspect ratio. A laminated pane is checked, as there, with no shear transfer between the plies (the conservative bound); the full-shear bound is given in the note.

How much the drawing exaggerates

Real deflections are a few millimetres on a pane of two metres: drawn true, nothing would move. The drawing multiplies the deflection by the factor printed under it, and that factor depends only on the deflection limit you choose (×10 for b/60 and b/65, ×20 for b/100 and b/175), never on the result. So a pane exactly at its deflection limit is always drawn bent by between a ninth and a fifth of its short side, and you can read the bend as the deflection check. The shape across the pane, sin(πx/a)·sin(πy/b), is there for the picture; the centre value is the calculator's.

What the limit on the gauges means

The gauges measure against the design values the calculator uses. For stress, the design bending strength f_g;d = k_mod·k_sp·f_g;k/γ_M;A + k_v·(f_b;k − f_g;k)/γ_M;v, in the form of EN 16612, with k_mod = 1.0 for a wind gust: 25.0 N/mm² annealed, 45.8 N/mm² heat-strengthened and 87.5 N/mm² fully tempered. They come from the characteristic strengths f_b;k of the calculator's table (45, 70 and 120 N/mm²) through the partial factors γ_M;A = 1.8 and γ_M;v = 1.2. For deflection, the limit you choose, which EN 16612 does not set.

So 100 % is where the method stops accepting the pane. It is not the pressure at which the glass breaks, and the lab does not tell you that pressure: glass strength scatters, depends on the edges, the surface damage and how long the load lasts, and the safety factors exist precisely for that.

What the lab does not do

• It does not compute wind pressure. You give the already-factored design pressure, as in the calculator.
• Four-edge support, single pane, linear theory — the calculator's limits, word for word: two or three edges, point fixings and insulating units are different calculations, and past half the thickness of deflection the real pane is stiffer than shown (you get the calculator's warning).
• Metric only. For inches and psf, open the pane in the calculator and switch the units there.
• No safety, containment or post-breakage checks, no thermal stress, no barrier or impact loading.

Is the limit in the wind lab the breaking load of the glass?

No. 100 % is the design limit of the method the glass thickness calculator uses: the EN 16612-form design strength, after its safety factors, for stress, and the deflection limit you choose. Past it the method no longer accepts the pane; it does not mean the pane breaks at that pressure.

How much is the deflection exaggerated in the drawing?

By the factor printed under the drawing: ×10 for a limit of b/60 or b/65 and ×20 for b/100 or b/175. It depends only on the limit, so the bend grows in proportion as you drag the pressure, and a pane at its deflection limit is always drawn bent by a ninth to a fifth of its short side.