Occupant evacuation: routes, exits and sizing (SI 3)

Occupancy is not what the developer expects, it is what the section's table assigns. Each use and each type of zone has a regulatory occupancy density, expressed in square metres per person, and the calculation consists of applying that density to the usable areas of each zone. As well-known references, dwelling areas count at one person per 20 usable square metres and office use at one per 10, with much more intense densities in public areas of retail and assembly uses, where the table reaches values of a few square metres, even fractions, per person.

Two application nuances matter. First: occupancy is calculated by zones and accumulated towards the exits, so the same building yields different occupancies at each point of its evacuation scheme. Second: zones of zero occupancy, plant rooms for example, are excluded from the count but not from route requirements when they are visitable.

The classic error, calculating low to ease the sizing, does not travel far: municipal technical services systematically recalculate occupancy, and an occupancy corrected upwards at permit stage drags exits, widths and stairs along in cascade. Occupancy is therefore best calculated from the table from the outset.

The escape route is the real path, measured along corridor axes and passable lines, from any occupiable point to a floor or building exit. The section sets maximum lengths depending on the number of available exits: as a general reference, routes to a single exit may not exceed 25 metres, a limit extended to 50 when the occupant has more than one alternative exit, with variants by use and with extensions where an automatic extinguishing installation exists.

The tracing has its fine rules. Routes may not cross zones that aggravate the risk, a restaurant's kitchens for example, nor depend on elements that may be locked or occupied. Points within a room are measured from its interior, not from its door. And alternative routes must be genuinely independent: two adjacent exits converging into the same corridor are no alternative.

With requirement SUA 9 of the code, evacuation also incorporates the accessibility dimension: accessible routes and refuge areas for people with disabilities are coordinated with the escape routes, a crossover developed in the article on DB-SUA 9 accessibility. Both sets of routes are drawn on the same plan and at the same time.

The number of exits required depends on occupancy, route lengths and use. Floors or rooms with reduced occupancy and short routes can be resolved with a single exit; beyond the thresholds of the corresponding table, two or more sufficiently independent exits are needed. The operational rule for the designer is to look at the thresholds early: the jump from one to two exits, or the need for a second staircase, is among those that redraw entire floor plans, and discovering it with the design advanced is among the most painful design cost overruns there are.

With occupancy assigned to each evacuation element, sizing applies simple formulas. The general reference for doors and openings is a width equal to or greater than the assigned occupancy divided by 200, with an absolute minimum of 0.80 metres for doors; corridors follow an analogous logic with their own minimums, and stairs apply specific divisors depending on whether they are protected or not and on the direction of evacuation. The blockage hypothesis completes the method: where there are several exits, sizing must be checked assuming one of them is unusable, with the occupancy redistributed among the rest.

Stairs deserve their own mention. Above certain evacuation heights, depending on use, staircases must be protected or specially protected: enclosures with their own fire resistance, fire doors and, where applicable, independence lobbies and specific ventilation. They are vertical fire compartments, and in budget terms they are among the most expensive pieces of the section, because they combine compartmentation, fire-rated joinery and often smoke control or pressurisation systems.

Evacuation is completed by its guidance elements: direction and exit signs conforming to their standard, and emergency lighting guaranteeing minimum illumination levels along the routes, the latter required by section SUA 4 of the code in coordination with the DB-SI. They are minor items in amount and major ones in inspection: they rank among the most systematic checks in activity licences and periodic reviews.

The section's economic conclusion is that its cost is played out at preliminary design. An evacuation scheme well laid out from the start barely adds to the building's cost: it consumes just the necessary widths, places the stairs where they serve both evacuation and composition, and avoids threshold jumps. The same building with evacuation resolved late pays for added staircases, widened corridors and lost square metres. Section SI 3 is the perfect example of the general principle of the article on the cost of CTE compliance: cheap compliance is compliance designed early.

The first, already mentioned: occupancies calculated low that the permit corrects with a domino effect. The municipal correction arrives once the design is already closed.

The second, measuring routes in a straight line on the drawing instead of along the real passable path, an error that surfaces at municipal review with routes that suddenly do not comply. The route is measured along the passable path, with its turns and obstacles.

The third, forgetting the blockage hypothesis when sizing multiple exits. The hypothesis is applied by disabling each exit in turn.

The fourth, resolving evacuation while ignoring its coexistence with accessibility: doors that meet evacuation width but not accessible route width, or refuges planned without coordinating with the cores. Widths are checked against both criteria at once.

The fifth, treating signage and emergency lighting as site finishing touches rather than designed items, with the usual result of routes signed at the installer's discretion. Both are designed and measured like any other installation.