Transverse Reinforcement in Seismic Beams
In a special moment frame, a beam is intended to yield in a controlled, ductile manner during a strong earthquake, rather than fail in shear before its flexural capacity can be developed. Section 18.6.4 establishes the detailing requirements that render this behavior possible, specifying the regions in which confinement must be provided and the extent to which conventional detailing governs elsewhere.
Contents
Where Hoops Are Required
Two locations within a seismic beam are treated as critical, as flexural yielding is expected to concentrate in these regions under strong ground motion. Both are governed by the same characteristic length, 2h, equal to twice the overall beam depth.
Beam Ends
- Measured 2h from the face of the supporting column
- Extends toward the beam midspan
- Present at both ends of every seismic beam
Plastic Hinge Regions
- 2h on both sides of any section where yielding is expected
- May occur away from the column face, for example beneath heavy concentrated loads
- Identified from the governing moment diagram rather than assumed by default
Maximum Hoop Spacing
Within the 2h regions, hoops must be spaced closely enough to confine the concrete core and to restrain longitudinal bars against buckling following spalling of the cover. The code establishes this limit as the smallest of four values, the most restrictive of which governs.
| Limit | Value | Reasoning |
|---|---|---|
| Effective depth ratio | d / 4 | Ties spacing to member size |
| Fixed limit | 6 in. (150 mm) | Keeps confinement tight regardless of depth |
| Bar diameter, Grade 60 | 6·db | Restrains buckling of the smallest primary bar |
| Bar diameter, Grade 80 | 5·db | Tighter, since higher-strength bars buckle at lower confining demand |
where d = effective beam depth, and db = diameter of the smallest primary longitudinal flexural bar.
Lateral Support of Longitudinal Bars
Within the hoop regions, every primary top and bottom longitudinal bar must be laterally supported, either by a hoop leg or by a crosstie, to prevent outward displacement following damage to the surrounding concrete. The transverse spacing between adjacent supported bars shall not exceed 14 in. (356 mm).
- Skin reinforcement, along the side faces of the beam, is exempt from this requirement and does not require lateral support.
- Beams frequently require one or more interior crossties solely to satisfy the 14 in. spacing limit.
Constructing a Closed Stirrup
A closed stirrup need not be fabricated from a single bent bar. The code permits construction from one or more U-stirrups with 135° seismic hooks, closed by a crosstie that completes the enclosure. This detailing option facilitates field placement in regions of congested longitudinal reinforcement.
- Where consecutive crossties engage the same longitudinal bar, their 90° hooks shall be placed on opposite sides of the beam, distributing the confining force rather than concentrating it on a single face.
- Where a slab frames into only one side of the beam, all 90° hooks shall be placed on the slab side, where the slab provides additional restraint against outward displacement of the hook.
Outside the Hoop Regions
Beyond the 2h zones, the beam is no longer expected to yield, and the detailing requirements are correspondingly relaxed, though not eliminated.
- Ordinary stirrups are provided in place of seismic hoops throughout the remaining beam length.
- Each stirrup is still required to have 135° seismic hooks at both ends.
- Maximum spacing is limited to d/2 throughout this region.
- This provision maintains adequate shear capacity and member integrity without imposing the stringent confinement required in the vicinity of a plastic hinge.
Summary of Key Requirements
The following summary consolidates the requirements presented in the order in which a detailer would typically apply them.
- Over a length equal to twice the beam depth (2h), measured from the face of the supporting column toward midspan, at both ends of the beam.
- Over lengths equal to 2h on both sides of any section where flexural yielding is likely to occur due to inelastic lateral displacement.
- In regions requiring hoops, primary longitudinal reinforcing bars nearest the tension and compression faces shall be laterally supported.
- The transverse spacing of supported flexural reinforcing bars shall not exceed 14 in. (356 mm).
- Skin reinforcement need not be laterally supported.
- Closed stirrups may consist of one or more U-stirrups with seismic hooks at both ends, closed by a crosstie.
- Consecutive crossties engaging the same longitudinal bar shall have their 90-degree hooks placed on opposite sides.
- Where longitudinal bars are confined by a slab on one side only, the 90-degree hooks of the crossties shall be placed on that side.
- The first hoop or closed stirrup shall be located not more than 2 in. (50 mm) from the face of the supporting column.
- Spacing of hoops or closed stirrups shall not exceed the least of: (a) d/4; (b) 6 in. (150 mm); (c) six bar diameters (6db) for Grade 60 primary flexural reinforcement, excluding skin reinforcement; (d) five bar diameters (5db) for Grade 80 primary flexural reinforcement, excluding skin reinforcement.
- Stirrups with seismic hooks at both ends shall be provided throughout the remaining length of the beam.
- The spacing of such stirrups shall not exceed d/2.