Beam-column joint
The shear in a reinforced concrete beam-column joint and its detailing, by ACI 318-19 Chapter 15 or, for a special moment frame, 18.8: Aj and the table coefficient, column shear, strong column, joint depth for bars passing through, hooks and hoops. One direction of the frame.
Inputs of the joint #
- Frame: special moment frame (bars at 1.25 fy, φ 0.85, Table 18.8.4.3), or other (bars at fy, φ 0.75, Table 15.4.2.3).
- Joint: interior, a beam each side; or exterior, one beam.
- Column continues, beam continues, confined: the three questions of the table. Yes where the column meets 15.2.6, the beam 15.2.7, and two transverse beams meet 15.2.8. These are your reading of the frame.
- Column: h along the beams, b, the distance between its mid-heights. The inflection points are taken there.
- Sum of the columns' nominal strengths: from your column calculation, at the axial force giving the least strength (18.7.3.2).
- Beams: bw, h, d, the width for the top moment, the offset from the column axis, and top and bottom bars on each side. Add slab bars within the flange to the top bars: SP-17 warns that ignoring the slab is unconservative for the strong column check.
- Column shear: statics from the beam bars with no beam shear; or from entered moments and beam shears (Mpr and Ve for a special frame, 18.8.4.1); or entered.
- Joint hoops: bar, legs along b, spacing, cover, hx, the smallest column bar, Pu.
- Beams on four sides: with each beam at least 3/4 of the column's width, halves the hoops and allows 6 in. (18.8.3.2).
Auto-design varies the column depth and width and the hoop spacing. Materials, beams, axial force and the frame's choices are kept.
Checks performed #
| Check | Basis |
|---|---|
| Joint shear Vu ≤ φ Vn | 18.8.2.1, 18.8.4 or 15.4; Aj by 15.4.2.4 |
| Strong column, weak beam | 18.7.3.2, special frames |
| Joint depth for bars through | 18.8.2.3, interior joints of special frames |
| Hooked bars | 18.8.5.1 or 25.4.3, exterior joints |
| Joint hoops, amount | 18.8.3, Table 18.7.5.4 |
| Joint hoops, spacing | 18.7.5.3, 18.8.3.2 |
| Joint hoops, other frames | 15.3.1.3, 15.3.1.4: at least two layers, at most 8 in. Skipped where confined; 15.3.1.1 also needs the joint to be outside the seismic system and outside SDC D, E, F, which you confirm |
Limitations #
- One direction. Enter a second calculation for a joint framed both ways.
- A beam moment computed here is a singly reinforced rectangle: enter it from your beam calculation when compression bars or a T section matter.
- The column, the beams, wide beams with bars outside the core (18.8.3.3), headed bars and splices are out. So is a joint of a column on a wall or a transfer.
Verification examples #
Verification status: 47 values compared in 23 examples (7 published, 16 hand calculations from the cited equations): 42 within tolerance, 5 recorded deviations. Code basis: the provisions it uses were read in the target edition.
| Example | Source | Values |
|---|---|---|
| Intermediate moment frame | ACI SP-17(14) Column Example 3, p. 380 | T 252, C 108, Vj 308 kips, Aj 576 in² |
| Special moment frame | ACI SP-17(14) Column Example 4, p. 392 | T 315, C 180, Vj 424 kips, Vn 611 kips |
| Vn, all cells | ACI 318-19 Tables 15.4.2.3, 18.8.4.3 | Hand calculation |
SP-17(14) is to 318-14, the only published example on disk: ACI MNL-17(21) would be to 318-19. Two values differ, both documented. Under 318-19 Example 3's joint takes 20 in place of 15 in Vn. Its column shear of 52 kips does not follow from the moments and shears it prints, which give 56 kips; the source's number is kept when the shear is entered, and statics when you enter moments. Hoops, strong column and hooks are hand calculations. The auto-design is a simple size search, untested against a published design.