7.2 Columns, Ties, Spirals & Compression Member Detailing

7.2 Column Longitudinal Reinforcement, Ties & Spirals

Read the column mark, story and orientation on the approved schedule. Confirm cross section, longitudinal bar count and grade, transverse reinforcement, lap or coupler location, cage geometry and cover. Model IBC Table 1705.3 identifies reinforcement placement as a periodic special inspection. Special seismic confinement adds member-specific checks described in Section 7.3.

Longitudinal steel and confinement

Ties hold the bars in place and provide lateral restraint. A continuous spiral also confines the core as it expands in compression, improving deformation capacity. For the ordinary compression-controlled reinforced cases in ACI 318-19 Table 21.2.2, the strength reduction factor is 0.65 with other transverse reinforcement and 0.75 with spirals complying with Section 25.7.3. Table 22.4.2.1 separately limits maximum nominal axial compression to $0.80P_o$ for ties and $0.85P_o$ for qualifying spirals. These design factors explain why the specified confinement arrangement matters. A field inspection verifies the approved detail.

For a nonprestressed column, Section 10.6.1.1 generally sets $A_{st}$ between $0.01A_g$ and $0.08A_g$. Section 10.3.1.2 has a limited effective-area provision for columns larger than needed for loading. Section 18.7.4.1 sets 1% to 6% for special moment frame columns. In an 18-by-18-inch ordinary column with eight No. 9 bars, gross area is 324 square inches and longitudinal steel area is 8.00 square inches, about 2.47%. Check lap zones separately for constructability and the applicable splice detail.

For nonprestressed columns and the prestressed columns within its scope, Section 10.7.3.1 requires at least three bars within triangular ties, four within rectangular or circular ties and six enclosed by spirals or, for special moment frame columns, circular hoops.

Tie size, spacing and bar support

Section 25.7.2 requires a No. 3 minimum tie around No. 10 and smaller longitudinal bars and a No. 4 minimum around No. 11 and larger bars or bundled bars. Equivalent deformed wire or welded wire reinforcement is permitted subject to the applicable code conditions and approved detail.

Under Section 25.7.2.1, clear spacing between ties is at least four-thirds of nominal maximum aggregate size. Measure the maximum vertical spacing center to center. The least of these three limits governs.

• Sixteen longitudinal bar diameters.
• Forty-eight tie bar diameters.
• The smallest member dimension.

In the 18-inch square example with No. 9 bars and No. 3 ties, compare $16(1.128)=18.048$, $48(0.375)=18$ and 18 inches. The section and tie size both give an 18-inch limit. Verify any closer design spacing, including where shear or seismic details apply.

Under Section 10.7.6.2.1 the bottom tie or hoop is at most one-half the tie or hoop spacing above the footing or slab. Section 10.7.6.2.2 puts the top tie or hoop at most one-half the spacing below the lowest horizontal reinforcement in the slab, drop panel or shear cap. Where beams or brackets frame into all sides, the top tie or hoop is at most 3 inches below the lowest horizontal reinforcement in the shallowest beam or bracket. Identify the actual reference bar before measuring.

Under Section 25.7.2.3, a corner of a rectilinear tie with included angle at most 135 degrees must laterally support every corner and alternate bar. Any unsupported bar is at most 6 inches clear along the tie to a supported bar on each side. For three bars along a face, if the middle bar is 5 inches clear from one supported corner and 7 inches from the other, that side fails. Verify the approved crosstie or overlapping tie arrangement, including which bars its corners and hooks engage. Use Section 25.7.2.4 for the applicable circular-tie geometry.

Spirals

For cast-in-place construction, Section 25.7.3 specifies a minimum 3/8-inch spiral bar or wire and clear spacing of at least the greater of 1 inch and four-thirds nominal maximum aggregate size, up to 3 inches. For 1-1/2-inch aggregate the clear lower bound is 2 inches. With a 3/8-inch spiral and a 2-1/2-inch center-to-center pitch, the clear gap is 2-1/8 inches, within that spacing range.

For this column application under Section 25.7.3.3, the design minimum volumetric ratio is $0.45(A_g/A_{ch}-1)f'_c/f_{yt}$, where $A_{ch}$ is the core area measured to the outside edges of transverse reinforcement. Use the applicable design yield limit in Table 20.2.2.4(a). For $A_g/A_{ch}=1.5$, $f'_c=4000$ psi and $f_{yt}=60{,}000$ psi, the ratio is $0.015$, or 1.5%. Verify the spiral diameter and pitch on the approved detail.

Anchor the spiral with 1.5 extra turns of bar or wire at each end under Section 25.7.3.4. Sections 25.7.3.5 and .6 permit mechanical or welded splices meeting Section 25.5.7 or laps under the specified restrictions for $f_{yt}$ up to 60,000 psi. The lap minimum is the greater of 12 inches and the applicable Table 25.7.3.6 value. For uncoated deformed bars without hooks, use $48d_b$. The table gives different requirements for coated or plain reinforcement. Inspect support against displacement and the termination at the bottom and top of the story under Sections 10.7.6.3.1 and .2.

Offset transitions

Check the change in each column face across the story joint. Section 10.7.4.1 limits the slope of an allowed offset-bent bar to 1 horizontal in 6 vertical and requires the straight portions to be parallel to the column axis. A 2-inch bar shift needs at least 12 inches of vertical run. Separately, a centered 24-inch column transitioning to an 18-inch column has a 3-inch face offset on each side. Under Section 10.7.4.2 use separate dowels lap-spliced with the longitudinal bars adjacent to those faces.

Under Section 10.7.6.4, the design provides horizontal support for 1.5 times the horizontal component of calculated force in the inclined bar. The support can come from ties, hoops, spirals or parts of the floor construction. When transverse steel resists the offset force, locate it within 6 inches of the bend points. Compare the cage with the approved story-transition and splice details and document any discrepancy while the cage is accessible.