Course · lesson 3 of 7

RC walls and slender walls

How a reinforced concrete wall is designed with the STATIX RC wall calculator on the SANS 10100-1 route: braced and unbraced walls, effective height, short and slender walls with the additional moment, in-plane bending and shear, the end zones, and the steel limits.

1. Learning outcomes

After this lesson you can:

tell a wall from a column, and a braced wall from an unbraced one;
set the effective height and decide short or slender;
work the additional moment of a slender wall;
check in-plane bending, the end strip and in-plane shear;
check the minimum and maximum steel and when links are needed.

2. Before you start

RC Design 1, lesson 5 (columns: effective height, slenderness, N-M interaction).
The RC wall calculator.
A pocket calculator and about 60 minutes.

3. Walls and their slenderness

What a wall is. SANS 10100-1 4.8.1.1 calls a vertical member a wall when its length is more than four times its thickness. Below that it is a column, which RC Design 1 lesson 5 covers; the calculator refuses it and says so.

Braced and unbraced. A braced wall is held at top and bottom by floors that do not sway, because other walls or cores take the horizontal load; an unbraced wall must resist the sway itself. The effective height l_e is the clear height times a factor β, assessed as for a column; you choose β on the page.

Short and slender. A short wall is checked on its section alone; a slender one bows sideways under the load, and the bow adds a moment. SANS 10100-1 sets the dividing line (4.8.1.5/6) and an upper limit beyond which a wall may not be used at all (4.8.5.2).

The additional moment. A slender wall deflects by a_u at mid-height, and the load times that deflection is the additional moment M_add, worked as for a slender column with K taken as 1.0 (conservative). A wall with only one central layer of bars is less stiff, so SANS 10100-1 doubles it.

4. Bending and shear

Bending in the plane of the wall. The axial load and the in-plane moment from wind or other lateral load share out along the wall, more at one end. SANS 10100-1 4.8.4.4 a) asks for an elastic distribution with no tension in the concrete; where part of the wall goes into tension the calculator uses the cracked section with all its bars. The whole wall is also checked as one section (4.8.4.3).

The end zone. The most heavily loaded metre at the compressed end is checked like a short column strip: its axial load per metre with the out-of-plane moment, never less than the minimum eccentricity, plus the additional moment if the wall is slender. A wall with a large in-plane moment can pass as a whole and still crush at its end.

In-plane shear. SANS 10100-1 has no wall shear clause, so the calculator uses the beam method of 4.3.4 with the wall as a deep beam: d = 0.8 l_w, v_c from the vertical steel in the tension half, and the horizontal bars as the shear reinforcement. It says so in the row; the enhancement of v_c by axial compression is not used.

5. Steel, layers and routes

Minimum and maximum steel. Walls need a minimum of vertical and horizontal steel, a maximum vertical amount including the laps, and links to hold the vertical bars once they carry much compression. The calculator checks each and the bar spacing in each layer.

One layer or two. Two layers, one near each face, give the wall bending strength both ways and meet the crack-control rule of 4.8.7. One central layer is cheaper but doubles the additional moment of a slender wall and leaves crack control to be shown otherwise.

The SANS 51992-1-1 option. The page also designs walls to SANS 51992-1-1 (the SA Eurocode, with its 2023 draft annex). It judges slenderness by λ = l₀/i against a limit that depends on the axial load, adds an imperfection and the nominal-curvature moment, and uses its own steel limits. Work one route at a time.

What is left to you. The calculator designs one solid wall at one storey. Walls with openings are designed pier by pier, a plain (unreinforced) concrete wall is not designed, and the bracing system as a whole is yours. Fire and cover are checked when you give the rating and the exposure.