Columns of Special Moment Frames
Scope
This part of the code defines which columns the rest of the section applies to. It covers columns that belong to a building's seismic-force-resisting system and are designed to handle bending, shear, and axial load together. Older code editions let lightly loaded columns skip these rules and be treated more like beams; that loophole no longer exists. Today, every column in this category — no matter how much axial force it carries — must follow the full set of detailing rules.
- Applies to columns in the seismic-force-resisting system, proportioned for flexure, shear, and axial load.
- Applies regardless of axial load magnitude.
- Pre-2014 exception for low-axial-stress columns (beam-like detailing) has been removed.
Dimensional Limits
Before any reinforcement is placed, a column must satisfy two fundamental geometric constraints. These constraints establish a minimum cross-sectional width and limit the degree of section elongation, thereby precluding proportions that have exhibited inadequate performance under reversed cyclic seismic demand.
- Shortest cross-sectional dimension (through the centroid) must be ≥ 12 in.
- Ratio of shortest to perpendicular dimension must be ≥ 0.4.
Minimum Flexural Strength — Strong Column / Weak Beam
This clause codifies the strong-column/weak-beam principle, which governs the relative flexural strength of columns and beams at a joint. It first establishes applicability: columns are generally required to satisfy either the strength comparison that follows or an alternative provision covering the case where that comparison cannot be met, with a single exception, noted below. This exemption is grounded in the observation that a column terminating at that level and subjected to low axial demand exhibits inherently ductile behavior and is unlikely to precipitate a story-level failure mechanism, thereby rendering the strength comparison unnecessary in this limited circumstance.
- If a column doesn't continue above a given connection and its factored axial compressive force Pu (including earthquake effect E) stays below Agf′c/10.
The strength comparison itself is defined as follows:
Column flexural strength is computed using the factored axial force — consistent with the direction of the lateral forces under consideration — that yields the lowest attainable flexural strength, ensuring the comparison reflects a governing rather than a favorable condition. The summation convention requires that column moments oppose beam moments, and the inequality must be satisfied independently for beam moments acting in each direction within the vertical plane of the frame under consideration.
- Applicability: columns must generally satisfy either the strength comparison or the alternative provision; exempted only where the column is discontinuous above the connection and Pu (under load combinations including E) is less than Agf′c/10.
- Strength comparison: ΣMnc ≥ (6/5) ΣMnb, evaluated at the joint faces.
Longitudinal Reinforcement
These limits exist for practical reasons. A minimum amount of steel keeps the column from deforming too much over time and makes sure it cracks before it yields — not the other way around. A maximum amount keeps the bars from getting too crowded and keeps load transfer and shear stress at the column manageable. Splices also need care: the concrete cover near column ends tends to spall off first, so lap splices are safer placed near midheight and wrapped in transverse reinforcement for extra confinement.
This subsection sets the rules for the steel bars running the length of the column — how much steel is needed, how it must be arranged, and how splices are handled. The limits balance ductility and crack control against the practical risk of overcrowding the section with steel, while the splice rules account for the fact that concrete cover near column ends is prone to spalling during an earthquake.
- Steel ratio must fall between 1% and 6% of the gross cross-sectional area.
- Columns with circular hoops need at least 6 longitudinal bars.
- Over the column clear height, either (a) or (b) shall be satisfied:
- (a) Longitudinal reinforcement shall be selected such that 1.25ℓd ≤ ℓu/2.
- (b) Transverse reinforcement shall be selected such that Ktr ≥ 1.2db.
- Mechanical/welded splices follow standard code provisions elsewhere.
- Lap splices are restricted to the center half of the column length, must be designed as tension splices, and must be enclosed in transverse reinforcement.
Bond Resistance Over Clear Height
Under earthquake loading, longitudinal bars within a column's clear height can suffer a bond-splitting failure — the surrounding concrete splits along the length of a bar before the bar can fully develop its strength. This failure mode has been documented in research on beam-column behavior under seismic demand (Ichinose 1995; Sokoli and Ghannoum 2016), and it becomes more likely as the ratio of a bar's development length to the column's clear height grows.
Splitting is typically controlled through one of three measures: specifying smaller longitudinal bars, adding more transverse reinforcement, or using higher-strength concrete. Each of these, in effect, reduces the development length required of a bar relative to the column's clear height. Accordingly, the code permits satisfaction of this requirement through either of two alternative conditions, rather than a single prescribed rule: longitudinal reinforcement may be selected such that its development length remains within a specified fraction of the clear height, or transverse reinforcement may instead be selected to provide a level of confinement sufficient to satisfy the requirement in its place.
- Bond-splitting failure along longitudinal bars can occur within the column's clear height under earthquake demand.
- Risk is reduced by restricting bar size, adding transverse reinforcement, or increasing concrete strength — all of which shorten the required development length relative to clear height.
- Over the clear height, either condition (a) or condition (b) must be satisfied — not both.
- Condition (a): bar development length stays within a set fraction of the clear height (1.25ℓd ≤ ℓu/2).
- Condition (b): transverse reinforcement provides enough confinement relative to bar diameter (Ktr ≥ 1.2db).
Mechanical and Welded Splices
- Mechanical splices must comply with the following Section:
- Mechanical splices must be either Class G or Class S, with the following restrictions:
- (a) Splices must be classified as Class G or Class S.
- (b) Class S mechanical splices are permitted at any location, except as restricted by -. For beam reinforcement specifically, mechanical splices must be Class S and located no closer than h/2 (half the beam depth) from the joint face.
- (c) Class G mechanical splices in special moment frames are prohibited:
- Within the beam-column joint itself
- Within a distance equal to twice the member depth from the column or beam face
- Within twice the member depth of any critical section where reinforcement yielding is likely under lateral displacement beyond the linear range
- (d) Class G mechanical splices in special structural walls are prohibited:
- Anywhere lap splices are already prohibited in boundary regions [according to structural wall provisions by code]
- Within coupling beams
- Within twice the member depth of critical sections where yielding is likely under lateral displacement beyond the linear range
- Welded splices must comply with Section.
- Welded splices are not permitted at all in special moment frames or in special structural walls (including coupling beams).
- Welding stirrups, ties, inserts, or other similar elements onto longitudinal reinforcement required by design is not permitted.
Transverse Reinforcement
This is the most detailed part of the section, covering the hoops, crossties, and spirals that confine the concrete core and brace the longitudinal bars against buckling once the outer shell spalls away. The rules address where dense confinement is needed, how the ties must be arranged, how closely spaced they must be, and how much steel is required — with requirements tightening as axial load or concrete strength increases, reflecting how those columns tend to fail more abruptly if under-confined.
- Confinement zone (ℓo): required at each joint face and wherever yielding is expected; length equals the largest of the following:
- The column depth.
- One-sixth of the clear span.
- 18 in.
- Tie arrangement: transverse reinforcement must be configured as follows:
- (a) Transverse reinforcement shall comprise either single or overlapping spirals, circular hoops, or single or overlapping rectilinear hoops with or without crossties.
- (b) Bends of rectilinear hoops and crossties shall engage peripheral longitudinal reinforcing bars.
- (c) Crossties of the same or smaller bar size as the hoops shall be permitted, subject to the applicable code limitation on crosstie use. Consecutive crossties shall be alternated end for end along the longitudinal reinforcement and around the perimeter of the cross section.
- (d) Where rectilinear hoops or crossties are used, they shall provide lateral support to longitudinal reinforcement in accordance with the code's lateral-support requirements for reinforcement.
- (e) Reinforcement shall be arranged such that the spacing hx of longitudinal bars laterally supported by the corner of a crosstie or hoop leg shall not exceed 14 in. around the perimeter of the column.
- (f) Where Pu > 0.3Agf′c or f′c > 10,000 psi in columns with rectilinear hoops, every longitudinal bar or bundle of bars around the perimeter of the column core shall have lateral support provided by the corner of a hoop or by a seismic hook, and the value of hx shall not exceed 8 in. Pu shall be the largest value in compression consistent with factored load combinations including E.
- Tie spacing: the smallest of one-quarter of the minimum column dimension, a bar-diameter multiple tied to reinforcement grade, or a formula-based spacing capped between 4–6 in.
- Amount of confinement steel: set by a table with factors that increase requirements for higher-strength concrete and decrease them when bars are closely/effectively supported; requirements step up further at higher axial load or concrete strength.
- Beyond the critical zone: reduced but continuous transverse reinforcement is still required.
- Columns below discontinued stiff elements (e.g., walls): need full confinement detailing along the entire length below the discontinuity, with reinforcement extended into the member below.
- Thick cover (>4 in.): requires additional transverse reinforcement with tighter cover/spacing to prevent shell-spalling hazards.
Spacing of Transverse Reinforcement
Closely spaced ties are what keep the core confined and the bars from buckling, so the code caps their spacing. The spacing must not exceed the least of the four limits below, where the last limit depends on how widely the longitudinal bars are supported.
- Spacing must not exceed the least of the following:
- (a) One-fourth of the minimum column dimension.
- (b) For Grade 60, 6db of the smallest longitudinal bar.
- (c) For Grade 80, 5db of the smallest longitudinal bar.
- (d) so, as calculated by the equation below.
- The value of so shall not exceed 6 in. and need not be taken less than 4 in.
Amount of Transverse Reinforcement
The required amount of confinement steel is taken from the table below. It depends on the axial load level and concrete strength, and on whether the column uses rectilinear hoops or a spiral or circular hoop. Two modification factors enter the expressions: a concrete strength factor that raises the requirement for higher-strength concrete, and a confinement effectiveness factor that rewards columns whose longitudinal bars are well supported by hoop corners or seismic hooks.
| Transverse reinforcement | Conditions | Governing expression | Applicable expressions |
|---|---|---|---|
| Ash/sbc for rectilinear hoop | Pu ≤ 0.3Agf′c and f′c ≤ 10,000 psi | Greater of (a) and (b) |
(a) 0.3 (Ag/Ach − 1) f′c/fyt
(b) 0.09 f′c/fyt
|
| Pu > 0.3Agf′c or f′c > 10,000 psi | Greatest of (a), (b), and (c) |
(c) 0.2 kfkn Pu/(fytAch)
|
|
| ρs for spiral or circular hoop | Pu ≤ 0.3Agf′c and f′c ≤ 10,000 psi | Greater of (d) and (e) |
(d) 0.45 (Ag/Ach − 1) f′c/fyt
(e) 0.12 f′c/fyt
|
| Pu > 0.3Agf′c or f′c > 10,000 psi | Greatest of (d), (e), and (f) |
(f) 0.35 kf Pu/(fytAch)
|
Beyond the Confinement Length
Past the end zone where dense confinement is required, the column still needs continuous transverse reinforcement, only at a relaxed spacing. Damage during an earthquake does not always stay confined to the column ends, so the remaining height must still restrain the longitudinal bars and carry shear. The column may use spiral reinforcement, or hoops and crossties detailed under the standard rules for transverse reinforcement.
- Beyond the confinement length ℓo, the column shall contain spiral reinforcement, or hoop and crosstie reinforcement, meeting the standard detailing requirements.
- Spacing s shall not exceed the least of the following:
- 6 in.
- 6db of the smallest Grade 60 longitudinal column bar.
- 5db of the smallest Grade 80 longitudinal column bar.
- A greater amount applies where the lap splice or shear strength requirements demand it.
Spiral Reinforcement
Where spirals are used, they must be continuous and evenly spaced so the core is confined uniformly along the height. The clear spacing is limited at both ends: wide enough for concrete to flow around the bar, and narrow enough to confine the core. A minimum size is set for cast-in-place work, and a minimum volumetric ratio guarantees a reserve of confinement strength.
- Spirals shall consist of evenly spaced, continuous bar or wire, with clear spacing meeting both of the following:
- (a) At least the greater of 1 in. and (4/3)dagg.
- (b) Not greater than 3 in.
- For cast-in-place construction, spiral bar or wire diameter shall be at least 3/8 in.
- Except for transverse reinforcement in deep foundations, the volumetric spiral reinforcement ratio ρs shall satisfy the equation below, with fyt not taken greater than 100,000 psi.
- Spirals shall be anchored by 1-1/2 extra turns of spiral bar or wire at each end.
Columns Supporting Discontinued Stiff Members
A column that carries the reactions of a stiff member that stops above it, such as a wall, can be forced into large inelastic demand, because the discontinuity concentrates the earthquake load path into that column. These columns therefore receive full confinement over their entire height below the discontinuity, and the confining steel must be carried into the supported element so the load path is not interrupted at the transition.
- Transverse reinforcement shall be provided over the full height at all levels beneath the discontinuity if the factored axial compressive force in these columns, related to earthquake effect, exceeds Agf′c/10.
- Where design forces have been magnified to account for the overstrength of the vertical elements of the seismic-force-resisting system, the limit of Agf′c/10 shall be increased to Agf′c/4.
- Transverse reinforcement shall extend into the discontinued member at least ℓd of the largest longitudinal column bar, where ℓd is the development length.
- Where the lower end of the column terminates on a wall, the required transverse reinforcement shall extend into the wall at least ℓd of the largest longitudinal column bar at the point of termination.
- Where the column terminates on a footing or mat, the required transverse reinforcement shall extend at least 12 in. into the footing or mat.
Thick Concrete Cover
Where the concrete cover outside the confining reinforcement is thick, the unconfined shell becomes a large mass that can spall off during shaking. The code therefore calls for a lighter additional layer of transverse reinforcement within that cover to hold it in place.
- If the concrete cover outside the confining transverse reinforcement required elsewhere in this section exceeds 4 in., additional transverse reinforcement shall be provided.
- The additional reinforcement shall have cover not exceeding 4 in. and spacing not exceeding 12 in.
Shear Strength
Column shear design doesn't rely on ordinary elastic analysis alone. Instead, it uses a capacity-based approach: the design shear force is based on the maximum bending strength the column could realistically develop at each end, ensuring the column can survive full flexural yielding without a far more dangerous, brittle shear failure cutting that ductility short.
- The design shear force Ve shall be calculated by considering the maximum forces that can be generated at the faces of the joints at each end of the column.
- These joint forces shall be calculated using the maximum probable flexural strengths, Mpr, at each end of the column, associated with the range of factored axial forces, Pu, acting on the column.
- The column shears need not exceed those calculated from joint strengths based on Mpr of the beams framing into the joint.
- In no case shall Ve be less than the factored shear calculated by analysis of the structure.