Course · lesson 5 of 6

Slabs, footings and walls

The members that are mostly mats of bars: slab layers and top steel, openings, two-way corner steel, flat slab strips, pad and strip footings with their starters, pile cap ties, and walls with two layers, end U-bars and laps.

1. Learning outcomes

After this lesson you can:

name and order slab layers and set the extent of top steel;
trim an opening and add corner steel to a two-way slab;
detail flat slab strips with the simplified curtailment;
detail pad and strip footings, starters and pile cap ties;
detail a wall: layers, end U-bars and laps.

2. Before you start

Lesson 3 (laps, anchorage, chairs) and lesson 4.
The free slab design, pad footing and RC wall calculators.
A pocket calculator and about 50 minutes.

3. Slabs

Slab mats and their layer names. B1 is the outer bottom layer and usually spans the short way, where the moment is larger, so it gets the bigger lever arm; B2 sits on it. At the top, T1 is the outer layer and T2 below it. The drawing gives each layer its bar, spacing and extent.

Extents of top steel. Over a continuous support STATIX runs the top bars 0.3 of the adjacent span each side (at least 200 mm, rounded to 50 mm); over a cantilever they run the whole cantilever and back into the span. Bottom bars run into the supports. At a simply supported edge that is built in, a few top bars control cracking.

Trimming an opening. The bars the opening cuts are replaced by trimmer bars beside it, in a band max(2h, 300 mm) wide: at least half the area of the cut bars each side (practice), or more where the plate analysis asks, and anchored past the corners. Top trimmers where the top bars are cut.

Corner steel in two-way slabs (SANS 10100-1 4.4.4.2.2 e) to g)). Where a corner is held down, torsion steel goes in a square of one-fifth of the shorter span: four layers, top and bottom both ways, each 0.75 of the mid-span steel when both edges are simply supported, half that when one is continuous, and none when both are. Main bars already there count.

Flat slab strips. The moments are shared between column strips over the column lines and middle strips between them, and the bars follow. The flat slab schedule uses the simplified curtailment (practice): top bars 0.3 l each side of a column line, two-thirds of the column-strip top bars in the central half of the strip, bottom bars the span plus 10 % lapped over the columns. Punching links come from the punching check.

4. Footings and pile caps

The bottom mat of a pad footing. The bars must anchor past the column face, where the moment is largest: the anchorage length (at the stress the bars actually work at) against the projection less cover. STATIX uses straight bars where that works and bars bent up at both ends (shape 38) where it does not. In a rectangular base the short-way bars are concentrated in a band under the column.

Starters and kickers. The column starters stand on the bottom mat with a foot (shape 37, at least 12φ and 300 mm, practice) and links round them inside the base to hold them during the pour. A kicker, a short upstand cast with the base, sets out the column; the lap with the column bars is above it.

Pile cap ties. A pile cap works as a truss: the column load goes down struts to the piles and the bottom bars tie the piles together. The ties sit in bands over the piles and must be anchored past the pile centres; where the straight length is short the ends are bent up. Closed side links and a nominal top mat are practice.

5. Walls

Walls: two layers, and which goes outside. With the horizontal bars outside (the STATIX wall page’s layout) they control shrinkage cracking at the face and the vertical bars are easy to fix; with the vertical bars outside they get the bigger lever arm for out-of-plane bending, as in a retaining or basement wall. The cover is to the outer layer either way.

The end of a wall. With two layers, U-bars (shape 38) at every horizontal level close the end and lap with the horizontal bars; their legs are one lap long. It is a common closing detail rather than a code rule, and it also ties the two layers together.

Wall laps. Vertical bars lap with starters at each floor; horizontal bars lap at staggered positions along the wall. Strip footings under walls carry their main bars across the strip and distribution bars along it, with L-bars and tee bars at corners and junctions (practice).