Course · lesson 4 of 6

Masonry walls under vertical load

How a loadbearing masonry wall is checked to SANS 10164-1: unit and mortar strength and the characteristic strength, slenderness and the capacity reduction, eccentricity from floors, concentrated loads, cavity walls and openings, with the STATIX masonry wall calculator.

1. Learning outcomes

After this lesson you can:

find f_k from the unit, its shape and the mortar class;
set the effective height and thickness and check the slenderness;
find the eccentricity from a floor and the capacity reduction factor β;
check a single-leaf and a cavity wall per metre;
handle piers beside openings and concentrated loads by hand where the calculator stops.

2. Before you start

Loads 1 (permanent and imposed loads, the take-down).
The free masonry wall calculator.
A pocket calculator and about 50 minutes.

3. The material

Units and mortar. Masonry is units (bricks or blocks) bedded in mortar. Its strength depends on the unit strength f_u stated by the maker, the shape of the unit, and the mortar class: SANS 10164-1 Table 1 sets classes I, II and III by mix, with laboratory strengths of 14.5, 7 and 2 MPa.

The characteristic strength f_k. SANS 10164-1 4.2.2.1 takes f_k from Table 3, read from the unit strength and the mortar class; there is no power formula on this route. Blocks use the line set by the unit height over its least horizontal dimension. The calculator interpolates as the table footnotes allow, and flags values outside the table.

Adjustments to f_k. Short piers and small columns are weaker (a); a half-brick wall of standard bricks is stronger (b); 90 × 90 modular bricks get their own factor (c). The calculator applies (b), and applies (c) only when you choose “Yes — 90 × 90 modular units” and set the unit to 90 × 90. It works per metre of wall and does not apply (a).

Enter the real height and width of your unit in the calculator. It opens at a standard brick (73 × 106 mm), and the modular factor (×1.10 or ×1.25) is an explicit choice, “90 × 90 modular brick”, set to No by default. Choose Yes only when the units really are 90 × 90 modular bricks.

4. Slenderness and the capacity reduction

Effective height. h_ef = ρ × the clear height. A concrete floor bearing on the wall at both ends (at least t/2 and 90 mm, SANS 10164-1 5.1.2.1) gives enhanced resistance, ρ 0.75 (5.1.2.2); a timber floor or a roof tied on gives simple resistance, 1.0; a wall free at the top is taken as 2.0 on the page. Use 1.0 if in doubt.

Slenderness. The slenderness ratio SR is the effective height over the effective thickness. A slender wall bows under load and carries less; beyond the limits of 5.1.1 it may not be used at all. The calculator checks the limit with the number of storeys you give.

The capacity reduction factor β. Table 8 of SANS 10164-1 gives β from the slenderness ratio and the eccentricity at the top: it falls as the wall gets more slender and as the load moves off centre. Some cells are blank (very slender walls with large eccentricity): there is no β, and the calculator fails the wall.

5. Eccentricity and the resistance

Eccentricity from floors. SANS 10164-1 5.3.1 a) puts the load from a concrete floor at one third of the bearing depth from the loaded face; the wall above is axial. The calculator turns that into e_x = N_floor e / N_Ed when you choose slab bearing and give the floor reaction, so you need not work it out.

The vertical resistance. Per metre of wall, N_Rd is β times the thickness times f_k over the material factor γ_m from Table 5, which depends on how well the units are made and the wall is built. The calculator can add the factored self-weight of the wall down to mid-height.

6. Concentrated loads, cavity walls and openings

Concentrated loads. Under a beam or lintel end the wall is checked locally (1.25 or 1.5 f_k/γ_m by the bearing type of Figure 2) and again lower down, where the load has spread at 45°. The masonry wall calculator does not check concentrated loads; the lintel calculator checks the bearing at lintel ends (lesson 6). For other beams it is a hand check to 5.4.3.

Cavity walls. Two leaves tied across a cavity of 50 to 150 mm. The ties make the leaves help each other against buckling, so the slenderness uses t_ef from both; but only the loaded leaf carries the load (5.2.1). The calculator has one leaf loaded or both; a collar-jointed wall is treated as one leaf loaded.

Openings. Windows and doors send the load above them to the piers beside them, so a narrow pier carries much more per metre than the wall around it, and may be small enough for 4.2.2.2(a). The calculator works per metre and takes openings only in its lateral check (lesson 5); work the pier load by hand first. This lesson uses the SANS 10164-1 route throughout; the page also has BS 5628-1 and EN 1996-1-1, but run one route at a time.