This chapter shall apply to the design of nonprestressed, prestressed, and composite columns, including reinforced concrete pedestals.
Design properties for concrete shall be selected to be in accordance with Chapter 19.
Design properties for steel reinforcement and structural steel used in composite columns shall be selected to be in accordance with Chapter 20.
Materials, design, and detailing requirements for embedments in concrete shall be in accordance with 20.7.
If a structural steel shape, pipe, or tubing is used as longitudinal reinforcement, the column shall be designed as a composite column.
For columns with a square, octagonal, or other shaped cross section, it shall be permitted to base gross area considered, required reinforcement, and design strength on a circular section with a diameter equal to the least lateral dimension of the actual shape.
For columns with cross sections larger than required by considerations of loading, it shall be permitted to base gross area considered, required reinforcement, and design strength on a reduced effective area, not less than one-half the total area. This provision shall not apply to columns in special moment frames or columns not part of the seismic-force-resisting system required to be designed in accordance with Chapter 18.
For columns built monolithically with a concrete wall, the outer limits of the effective cross section of the column shall not be taken greater than 1.5 in. outside the transverse reinforcement.
for each face of width b
for circular sections of diameter hRequired strength shall be calculated in accordance with the factored load combinations in Chapter 5.
Required strength shall be calculated in accordance with the analysis procedures in Chapter 6.
Pu and Mu occurring simultaneously for each applicable factored load combination shall be considered.
For each applicable factored load combination, design strength at all sections shall satisfy ϕSn ≥ U, including (a) through (d). Interaction between load effects shall be considered:
(a) ϕPn ≥ Pu
(b) ϕMn ≥ Mu
(c) ϕVn ≥ Vu
(d) ϕTn ≥ Tu
For nonprestressed columns and for prestressed columns with average fpe < 225 psi, area of longitudinal reinforcement shall be at least 0.01Ag but shall not exceed 0.08Ag.
For composite columns with a structural steel core, area of longitudinal bars located within the transverse reinforcement shall be at least 0.01(Ag — Asx), but shall not exceed 0.08(Ag — Asx).
A minimum area of shear reinforcement, Av,min, shall be provided in all regions where Vu > 0.5ϕVc.
Lap splices, mechanical splices, butt-welded splices, and end-bearing splices shall be permitted.
Splices shall satisfy requirements for all factored load combinations.
Splices of deformed reinforcement shall be in accordance with 25.5 and shall satisfy the requirements of 10.7.5.2 for lap splices or 10.7.5.3 for end-bearing splices.
If the bar force due to factored loads is compressive, compression lap splices shall be permitted. It shall be permitted to decrease the compression lap splice length in accordance with (a) or (b), but the lap splice length shall be at least 12 in.
(a) For tied columns, where ties throughout the lap splice length have an effective area not less than 0.0015hs in both directions, lap splice length shall be permitted to be multiplied by 0.83. Tie legs perpendicular to dimension h shall be considered in calculating effective area.
If the bar force due to factored loads is tensile, tension lap splices shall be in accordance with Table 10.7.5.2.2.
Table 10.7.5.2.2—Tension lap splice class
| Tensile bar stress | Splice details | Splice type |
|---|---|---|
| ≤ 0.5fy | ≤ 50% bars spliced at any section and lap splices on adjacent bars staggered by at least ℓd | Class A |
| Other | Class B | |
| > 0.5fy | All cases | Class B |
If the bar force due to factored loads is compressive, end-bearing splices shall be permitted provided the splices are staggered or additional bars are provided at splice locations. The continuing bars in each face of the column shall have a tensile strength at least 0.25fy times the area of the vertical reinforcement along that face.
For composite columns, ends of structural steel cores shall be accurately finished to bear at end-bearing splices, with positive provision for alignment of one core above the other in concentric contact. Bearing shall be considered effective to transfer not greater than 50 percent of the total compressive force in the steel core.
Details of transverse reinforcement shall be in accordance with 25.7.2 for ties, 25.7.3 for spirals, or 25.7.4 for hoops.
For composite columns with a structural steel core, transverse ties or hoops shall have a minimum db of 0.02 times the greater side dimension of the composite column, but shall be at least No. 3 and need not be larger than No. 5. Spacing shall satisfy 25.7.2.1, but not exceed 0.5 times the least dimension of the composite column. Deformed wire or welded wire reinforcement of equivalent area shall be permitted.
Longitudinal reinforcement shall be laterally supported using ties or hoops in accordance with 10.7.6.2 or spirals in accordance with 10.7.6.3, unless tests and structural analyses demonstrate adequate strength and feasibility of construction.
If anchor bolts are placed in the top of a column or pedestal, the bolts shall be enclosed by transverse reinforcement that also surrounds at least four longitudinal bars within the column or pedestal. The transverse reinforcement shall be distributed within 5 in. of the top of the column or pedestal and shall consist of at least two No. 4 or three No. 3 bars.
In any story, the top tie or hoop shall be located not more than one-half the tie or hoop spacing below the lowest horizontal reinforcement in the slab, drop panel, or shear cap. If beams or brackets frame into all sides of the column, the top tie or hoop shall be located not more than 3 in. below the lowest horizontal reinforcement in the shallowest beam or bracket.
In any story, the bottom of the spiral shall be located at the top of footing or slab.
In any story, the top of the spiral shall be located in accordance with Table 10.7.6.3.2.
Table 10.7.6.3.2 —Spiral extension requirements at top of column
| Framing at column end | Extension requirements |
|---|---|
| Beams or brackets frame into all sides of the column | Extend to the level of the lowest horizontal reinforcement in members supported above. |
| Beams or brackets do not frame into all sides of the column | Extend to the level of the lowest horizontal reinforcement in members supported above. Additional column ties shall extend above termination of spiral to bottom of slab, drop panel, or shear cap. |
| Columns with capitals | Extend to the level at which the diameter or width of capital is twice that of the column. |
Where longitudinal bars are offset, horizontal support shall be provided by ties, hoops, spirals, or parts of the floor construction and shall be designed to resist 1.5 times the horizontal component of the calculated force in the inclined portion of the offset bar.
If transverse reinforcement is provided to resist forces that result from offset bends, ties, hoops, or spirals shall be placed not more than 6 in. from points of bend.
If required, shear reinforcement shall be provided using ties, hoops, or spirals.
Maximum spacing of shear reinforcement shall be in accordance with Table 10.7.6.5.2.
Table 10.7.6.5.2—Maximum spacing of shear reinforcement
| Vs | Maximum s, in. | ||
|---|---|---|---|
| Nonprestressed column | Prestressed column | ||
 | Lesser of: | d/2 | 3h/4 |
| 24 | |||
 | Lesser of: | d/4 | 3h/8 |
| 12 | |||