Imposed loads

The loads that come with use: people, furniture, stores, vehicles and maintenance on roofs. This lesson reads the categories of SANS 10160-2, adds movable partitions, finds roof loads by loaded area, and uses the STATIX quick tools for every number.

1. Learning outcomes

After this lesson you can:

choose the category of use and read q_k and Q_k from SANS 10160-2;
add the movable-partition allowance where it applies;
find the imposed load on storage floors and inaccessible roofs;
keep the concentrated load as a separate, local check;
know what the area reduction and ψ do, and what the STATIX tools apply.

2. Before you start

Lesson 1 (combinations) and lesson 2 (permanent loads and partitions).
Your copy of SANS 10160-2 (Tables 1, 2, 4 and 5, clauses 9.3.1.9 and 9.3.1.10) and the free quick tools.
A pocket calculator and about 40 minutes.

3. Categories of use

Imposed loads come from the use of a building: people, furniture, equipment, stored goods, vehicles, and people on a roof for maintenance. SANS 10160-2 sorts uses into categories (Tables 1, 2, 4 and 5) and gives each a characteristic load.

Some characteristic distributed loads. Homes carry the least; crowds, shops and industry the most. The full tables have many more categories (bedrooms, balconies, kitchens, laboratories, stairs and escape routes); the quick tools list them all.

Routes and edges. Stairs, corridors, escape routes and balconies have their own rows in Table 1, usually higher than the rooms they serve, because crowds gather on them. The quick tools note that the escape routes of offices and shops are designed as category C5.

4. Distributed and concentrated loads

Two separate checks. The distributed load q_k is for the overall design; the concentrated load Q_k, on a small square (0.1 × 0.1 m for floors), is for local effects. They are not added together.

Vehicles (Table 4). Parking decks carry a modest distributed load, but the wheel loads are large and concentrated. For thin slabs and toppings the axle load, not the q_k, often governs.

5. Partitions and storage

The partition allowance. Light movable partitions are spread over the floor as an extra imposed load, by their weight per metre of wall. The quick tools add it for categories B and C only, as the table's note gives it, and warn if you pick it for another category.

Storage. Shelves of files or goods load the floor by their height, so the stack categories give q_k per metre of stack height, with a minimum. Tall archives can need far more than an office floor.

6. Roofs

Inaccessible roofs (Table 5, H). The load depends on the area A that the member carries: a small area (one sheet, one purlin bay) may be loaded by a person and their tools, a large one is unlikely to be fully loaded. Between 3 and 15 m² the footnote formula applies; outside it, the end values.

H2 (maintenance): q_k = 0.25 + (15 − A) / 48 A limited to 3 to 15 m² H1 (construction): q_k = 0.25 + (15 − A) / 24 J (accessible flat roof): a fixed value, like a floor

7. Reductions and ψ

Area reductions. Because a big floor is rarely full everywhere, SANS 10160-2 (9.3.1.10) lets a member that carries a large area use a reduced imposed load. STATIX does not apply it, so its imposed loads are never lower than the table values.

The category also sets ψ. When the imposed load accompanies a leading wind load, it is taken as ψ Q (lesson 1). Offices and homes are rarely full during a design storm; stores stay full, so their ψ is higher.

8. In STATIX

The STATIX quick tools: pick the category, give the loaded area (roofs), the stack height (storage) and the partitions; the tool returns q_k, Q_k, the total on the area and a note on the clause it used.