Section 18.7.1/Special Moment Frame Beams
ACI 318-19 Section 18.7 covers the design and detailing requirements for beams in special moment frames resisting earthquake forces, including flexural reinforcement limits, confinement hoops, and strong-column weak-beam checks.
Beams in special moment frames must satisfy dimensional limits and detailed reinforcement requirements. Section 18.7.2 limits the beam width to at least 10 inches and the width-to-depth ratio to at least 0.3. The clear span must be at least 4 times the effective depth. Section 18.7.4 limits the positive moment strength at any section to at least one-half the negative moment strength at the face of the joint, and neither positive nor negative moment strength at any section may be less than one- quarter of the maximum moment strength at either face of the joint. Section 18.7.5 requires hoops (closed stirrups with seismic hooks) within 2h of the column face at both ends of the beam, spaced at the smallest of d/4, 6 inches, or 6 longitudinal bar diameters. Outside the hoop region, stirrups must be spaced no more than d/2. Section 18.7.6 requires the strong-column weak-beam check: the sum of column moment strengths at a joint must be at least 1.2 times the sum of beam moment strengths at the same joint.
Why this section exists
Special moment frames are designed to dissipate earthquake energy through controlled yielding of beam reinforcement near beam-column joints. The reinforcement limits and confinement requirements ensure beams yield in a ductile flexural mode rather than failing in brittle shear. The strong-column weak-beam requirement forces plastic hinges to form in beams rather than columns, preventing story-mechanism (soft-story) collapse. The hoop spacing limits in the plastic hinge zone confine the concrete core and prevent bar buckling during the large inelastic rotations expected in design-level earthquakes. These provisions apply in Seismic Design Categories D, E, and F where special moment frames per ASCE 7 Table 12.2-1 are required or selected.
What plan reviewers look for
Plan reviewers check beam sections and schedules for compliance with dimensional limits (10-inch minimum width, 0.3 width/depth ratio). They verify hoop sizes, spacing, and extent in the plastic hinge zone (2h from column face). They check that hoops have seismic hooks (135-degree bends with 6-diameter extensions) rather than standard 90-degree hooks. They verify the flexural reinforcement limits (positive moment at least half the negative at the joint face). They check the strong-column weak-beam calculation at each joint. For special shear walls, parallel detailing requirements apply. They verify lap splices are not located in the plastic hinge zone.