Wind — multi-span roofs
The modification factors applied to the individual span coefficients — EN 1991-1-4 §7.2.7, Figure 7.10.
§7.2.7 gives no tables of its own: you start from the coefficient of a single bay, computed per §7.2.4 (monopitch) or §7.2.5 (duopitch), on the wind page.
From §7.2.5, for α < 0 per Figure 7.10c and d.▸ Fill in the data on the left and press Calculate
Tool information
What this page computes
External pressure coefficients for multi-span roofs — industrial halls with sawtooth profiles or repeated bays — to EN 1991-1-4 §7.2.7, Figure 7.10.
§7.2.7 gives no tables of its own
That is the first thing to understand. The section has no coefficient table like §7.2.4 or §7.2.5. It gives modification factors applied on top of the coefficient of a single bay, computed separately.
So the workflow is: go to the wind page, obtain \(c_{pe}\) for a monopitch or duopitch roof, then come here and find what each bay receives.
The cascade: 1.0 → 0.8 → 0.6
The first bay sees undisturbed wind. The following ones sit in the aerodynamic shadow of the one before, so they receive less:
| bay | factor |
|---|---|
| 1 | 1.0 |
| 2 | 0.8 |
| 3 and onwards | 0.6 |
It settles at 0.6 and drops no further. The flow has "settled" after the third bay — from there on, a hall with 5 or with 15 bays has the same values on the rear bays.
The exception that is not a factor
In configuration (b), when the first bay has a positive \(c_{pe}\) (pressure), the following bays do not receive a fraction of it. They receive an absolute value:
\(c_{pe} = -0.4\)
Nothing is multiplied. It is the only place in §7.2.7 where the rule shifts from multiplication to substitution, and it is easy to miss when reading the figure quickly.
It makes physical sense: if the first slope is pressurised, those behind it fall into suction — and that suction does not depend on how strong the pressure in front was. The page marks it in a different colour precisely because it is a different rule.
NOTE 1 to Figure 7.10 requires both cases to be considered, by the sign of \(c_{pe}\) on the first bay. Do not pick just one.
The second trap: the base changes, not just the factor
NOTE 2 to the same figure, in configuration (c): the first bay uses the \(c_{pe}\) of a monopitch roof, while the second and all the others use that of a duopitch roof.
So it is not only the factor that changes between bays — the number you start from changes too. The page shows on each row which base it used, precisely so that this stays visible.
And one more difference from configuration (d): there the second bay takes a factor of 0.8, here it takes 1.0. The same position, different factors, because the geometry in front differs.
What remains with the designer
§7.2.7(2): zones F, G and J are considered only for the windward slope. Zones H and I apply to every bay.
The page applies the factors; choosing the zone remains yours, because it depends on where you are checking. A roof panel near the ridge and one at mid-bay are not in the same zone.
The reference height is \(z_e = h\), the height of the structure — not of each bay, §7.2.7(3).
What it does not cover
- The individual span coefficient — taken from the wind page, §7.2.4 or §7.2.5.
- Choosing the zone F/G/H/I/J for the point being checked.
- §7.2.8 — cylindrical roofs and domes. The values there are given as charts (Figures 7.11 and 7.12), not as tables, and reading them off a scan would not be trustworthy. We prefer them missing to approximated.
- Wind directions other than 0°, 90° and 180°, for which §7.2.7(1) is written.
The factors and the two notes are transcribed from EN 1991-1-4:2006, pages 44–45.
Related calculations
- Wind — the coefficients for monopitch and duopitch roofs, where the base comes from.
- Snow — special roofs — the other climatic action on the same geometry, with valleys.
- EN 1990 combinations — the combination the result feeds.