How a reinforced concrete corbel and a continuous nib are designed with the STATIX corbel calculator on the SANS 10100-1 route: the strut-and-tie model, the main tie and its anchorage, links, bearing, the horizontal force, nibs and the detailing.
1. Learning outcomes
After this lesson you can:
recognise a corbel and set up its strut-and-tie model;
design the main tie and anchor it at the front and in the column;
place the horizontal links and check shear and bearing;
handle a horizontal force at the bearing;
design a continuous nib and read the bar shapes the page schedules.
When a bracket is a corbel. A concrete bracket on a column face is a corbel when the load is closer to the face than the effective depth, av < d (SANS 10100-1 6.2.4.2.1). Such a short member does not bend like a beam: the load runs straight down a strut into the column. The corbel clause sits in the precast section of the code, 6.2.4.2.The strut-and-tie model. The load is carried by an inclined concrete strut from the bearing down to the column face and by a steel tie along the top, which holds the top of the strut. Moments about the strut’s lower node give the tie force Fv av/z, plus a share of any horizontal force.The strut. SANS 10100-1 asks for a simple strut-and-tie system but gives no strut stress. The calculator uses the textbook SANS / BS form: the rectangular stress block of 4.3.3 seen on the inclined strut, with the neutral axis limited to d/2, and says so in the row.Shape. The front of a corbel must be at least half as deep as its root at the column face, and the shear at the face is limited to the lesser of 0.7√fcu and 4 MPa. Shear near a support is enhanced by 2d/av (4.3.4.2.1), which a short corbel makes large.
4. Steel and bearing
The main tie. At least 0.4 % of the section at the face (6.2.4.2.3), anchored at the front by welding to a transverse bar of equal strength or by a loop, with the bearing kept within the straight part of the bars. At the back it is anchored in the column, usually with a bend down at the far face.Horizontal links. Closed links spread over the upper two thirds of the depth at the face, with at least half the area of the main tie, control the splitting of the strut and add to the shear strength. With av < d horizontal links work better than vertical ones (4.3.4.2.2).Bearing. The pad under the beam end is checked for its stress, which depends on what it sits on (6.2.4.4.4), over an effective length that never counts more than half the pad plus 100 mm or 600 mm, and for its net width. It must also stay clear of the spalling zone at the corbel front (6.2.4.5.1).Horizontal force. A precast beam on a corbel shrinks, creeps and moves with temperature, and pulls on its bearing. SANS 10100-1 gives no minimum, so the calculator asks for Fh and warns that BS 8110 asks for at least half the vertical load. A stated Fh must be carried entirely by extra bars welded to the bearing plate and anchored in the column; without them the page says NOT CHECKED.
5. Nibs and detailing
Nibs. A continuous nib along a beam, less than 300 mm deep, is designed per metre as a short cantilever slab: the load at the outer edge of the bearing, bars near the front anchored by a welded bar or a 180° loop (bars of 12 mm or less), at least 60 mm of overlap with the supported unit’s bars, and the beam’s links taking the load up to its compression zone.Detailing. The calculator lays out the tie, the cross bar, the links and any Fh bars, and the calc sheet (with an account) lists them on a SANS 282 bending schedule with these shapes; nib bars use the same family. Detailing 1 explains the shape codes.Routes and options. This lesson works the SANS 10100-1 route. The Eurocode routes design corbels by EN 1992-1-1 Annex J.3 with their own strut limits; they have no nib clause, so a nib there reads NOT CHECKED. The page also takes several loads on one corbel and corbels on both faces of a column.
6. Example A: a corbel for a precast beam
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7. Example B: a nib for hollowcore units
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8. Do it in STATIX
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9. Common mistakes
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10. Exercises
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11. Self-check quiz
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12. Lesson 4 on one page
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Next: Lesson 5, staircasesWaist-slab flights, landings and the bar list with the staircase calculator. Go to lesson 5
A worked example for learning, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.
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