Corbelled masonry
A corbel of masonry courses stepped out from a wall: as a detail, by the prescriptive limits of TMS 402-22 5.6.2, or as a bearing ledge that carries a line load, designed by allowable stress for unreinforced masonry (5.6.1, Chapter 8). The corbel itself and the concrete masonry wall below it are checked. US and metric units.
Edition. TMS 402-22 is not on hand. The corbel provisions were read in TMS 402-16 5.5 (pp. C-59 to C-61, Figures CC-5.5-1 and CC-5.5-2); the TMS cross reference list renumbers them 5.6 in 402-22 with no change noted. The Chapter 8 values come from the shared masonry library (402-13 text with 402-22 numbering where confirmed); the allowable tension parallel to the bed joints was added to it from knowledge and is to be confirmed against the code text.
Inputs of the corbel #
- Corbel: load-bearing or not; the number of corbelled courses; the projection of each course; the nominal unit height and thickness; whether the units are solid or filled with mortar or grout; the wall kind (single wythe, hollow, cavity or veneer: half the wythe; multiwythe bonded: half the wall) and the nominal thickness the limit uses; whether the back surface stays within 1 in. of plane.
- Loads (load-bearing): dead and live load on the corbel per foot, its position from the wall face (the centre of the bearing), the bearing width across the corbel, and the dead and live load on the wall above, which helps against tension in the wall below.
- Wall below (load-bearing): concrete masonry thickness, grouted solid or hollow, f′m, mortar, and the height between its supports.
The method switch is hidden: allowable stress design only, load combinations D and D + L (ASCE 7-22 2.4.1).
Auto-design varies the number of courses, the wall thickness and its grouting. Objective: the fewest courses, then the lightest wall. The projection per course, the units, the loads, f′m and the mortar are kept.
Checks performed #
| Check | Basis |
|---|---|
| Total projection, projection of one unit (height and thickness), solid or filled units, back surface | 5.6.2 (a) to (d): code checks for a non-load-bearing corbel, guidance for a load-bearing one |
| Bearing on the corbel | 0.25 f′m (8.1.5 in the library, to confirm) |
| Corbel flexural tension parallel to the bed joints | The corbel as a cantilever from the wall face, its courses acting together; Table 8.2.4.2, running bond (engineer's model) |
| Corbel shear at the wall face | 1.5 V / A against min(1.5 √f′m, 120 psi, 37 psi), no precompression on the vertical plane (8.2.6.2) |
| Wall below: axial and flexural compression | fa/Fa + fb/Fb ≤ 1, the load at e + t/2 from the wall centre (8.2.4.1) |
| Wall below: buckling | P ≤ Pe/4, Pe = π² Em I / h² (1 − 0.577 e/r)³ |
| Wall below: net tension | Table 8.2.4.2 normal to the bed joints, under the dead-load combination |
The drawing is a section through the wall and the corbel with each course's projection and the load position.
Limitations #
- Unreinforced, allowable stress only; reinforced or strength-designed corbels are not covered.
- The wall below is concrete masonry; brick or stone walls below a corbel are not covered.
- The ledger or angle on the corbel and its anchorage are not designed (the masonry ledger and masonry anchor types).
- The wall's own wind or seismic load is not added: check the wall with the masonry wall or unreinforced masonry type.
- Pe goes to zero when the eccentricity reaches 1.73 r, as the equation is written: heavy ledges far from the wall fail the buckling check by design.
Verification examples #
Verification status: 10 values compared in 3 examples (0 published, 3 hand calculations from the cited equations): 10 within tolerance, 0 recorded deviations. No published example: every value is a hand calculation. That checks the program against this reading of the code, not the reading itself. Code basis: not yet recorded for this module.
No published corbel example is on hand: hand calculations of the 5.6.2 limits, the bearing, the corbel's tension and shear, and the wall below (axial and flexural compression, buckling load, net tension), from the cited equations.