Permanent loads

The weight of the building itself. This lesson builds up the permanent load of floors, walls, beams and roofs from the densities of SANS 10160-2, sorts partitions into permanent and imposed, and uses the STATIX quick tools and masonry calculator to check the numbers.

1. Learning outcomes

After this lesson you can:

find a layer's weight per square metre from its density and thickness;
build up the permanent load g_k of a floor: slab, screed, finishes, services and ceiling;
turn a wall into a load per square metre of face and per metre run;
decide whether a partition is a permanent or an imposed load;
add the self-weight of concrete beams and steel members without counting anything twice.

2. Before you start

Lesson 1: G, Q and the combinations.
Your copy of SANS 10160-2 (Annex A and clause 9.3.1.9), the free quick tools and the masonry wall calculator.
A pocket calculator and about 40 minutes.

3. What counts as permanent

Permanent loads are everything fixed in place for the building's life: the structure itself, screeds, floor and wall finishes, ceilings, services and fixed walls. SANS 10160-2 gives the nominal densities to work them out.

4. Densities and layers

The basic rule. A layer of material γ kN/m³ heavy and t metres thick weighs γ t kN on every square metre. Add the layers and you have the floor's permanent load in kPa.

Densities. Annex A of SANS 10160-2 lists nominal densities of building materials. Where it gives a range, take the value that fits the product (STATIX takes the upper 23.0 for screed). For masonry use the density of the actual units; the STATIX masonry calculator starts at 19 kN/m³ and lets you change it.

The build-up. Most of a concrete floor's permanent load is the slab itself. The screed, finishes, services and ceiling add less each, but together they are often a third of the slab's weight and must not be left out.

a layer: g = γ t (kN/m³ × m = kPa) a floor: g_k = Σ γ t + finishes + services and ceiling

5. Walls and partitions

Walls. A wall weighs γ t per square metre of its face, and γ t h per metre run. Add the plaster on each face. That line load goes onto whatever the wall stands on: a slab, a beam or a footing.

A wall on a beam becomes a uniform line load on the beam, added to the beam's own weight and any slab reaction. (A deep wall can arch over openings and change this; that is beyond this lesson.)

Partitions. A wall built to stay is a permanent load where it stands. Light partitions that can be moved are an imposed load in SANS 10160-2 (9.3.1.9): on floors that can spread the load (categories B and C) they become an extra uniform imposed load, by their weight per metre of wall. Heavier ones are placed where they stand.

a wall: γ t per m² of face; γ t h per metre run movable partitions (9.3.1.9, floors B and C): + 0.5 / 0.8 / 1.2 kPa imposed for up to 1 / 2 / 3 kN/m of wall

6. Beams, steel members and roofs

Beam self-weight. Where a beam is cast with the slab, the part within the slab depth is already in the slab's g_k. Add only the downstand below it, so the concrete is not counted twice.

Steel members. A rolled section's mass per metre is part of its designation (the 60 in 406×178×60 UB) and of the section table. Multiply by g to get kN/m. It is usually small next to the floor it carries.

Roofs. A light roof's permanent load is the sheeting, insulation and purlins, plus a ceiling and services if they hang from it. Take these from the products' data. It is small, which is exactly why wind uplift can exceed it (lesson 1, the 0.9 G case).

7. In STATIX

In STATIX: the quick tools build up a floor's g_k layer by layer with the Annex A densities and show each row; the masonry wall calculator takes the wall's unit weight and adds its self-weight to the load it carries down.