7.3 Seismic Detailing for Special Moment Frames and Shear Walls (ACI 318 Ch. 18)

7.3 Seismic Detailing of Special Moment Frames & Structural Walls

Read the approved notes to identify the seismic-force-resisting system and find the applicable details. The member type and system determine which part of ACI 318-19 Chapter 18 applies. Beam and column hoop layouts differ even at the same joint. The cage inspection should identify the member, governing detail, confinement zones, steel grade and actual measurements.

Seismic reinforcement materials

For longitudinal deformed reinforcement covered by Section 20.2.2.5, the ASTM A706 route permits Grades 60, 80 and 100 in special structural walls and Grades 60 and 80 in special moment frames, subject to the other design limits and the specified product edition. The A706-16 edition discussed in Section 4.1 lists Grades 60 and 80.

For the A615 Grade 60 route verify four properties in the report. The actual yield may exceed specified yield by at most 18 ksi. The measured tensile-to-yield ratio is at least 1.25. Minimum fracture elongation over 8 inches is 14% for No. 3–6, 12% for No. 7–11 and 10% for No. 14 and 18. Minimum uniform elongation is 9% for No. 3–10 and 6% for No. 11, 14 and 18. Match the rolled marks and bundle identification to the report.

Special moment frame beams

Section 18.6.3.1 calls for at least two continuous bars at both top and bottom faces. At a joint face the positive moment strength must be at least half the negative strength there. Along the member, both positive and negative strengths must each be at least one-quarter of the greatest moment strength furnished at either joint face. The approved top and bottom bars and their anchorage provide these load paths during reversals. Section 18.6.3.1 also caps the flexural reinforcement ratio at 0.025 for Grade 60 and 0.020 for Grade 80.

Section 18.6.4.1 places hoops for $2h$ from each supporting column face toward midspan and for $2h$ on each side of any other section where yielding is likely under inelastic lateral displacement. Section 18.6.4.4 puts the first end hoop within 2 inches of the column face. In the hoop zone use the least applicable limit: $d/4$, 6 inches, and $6d_b$ for Grade 60 or $5d_b$ for Grade 80. Use the smallest applicable primary flexural bar and exclude longitudinal skin steel. Section 18.6.4.2 limits the spacing of laterally supported flexural bars in the hoop zone to 14 inches. Outside required hoop zones, Section 18.6.4.6 calls for stirrups with seismic hooks at both ends at spacing no greater than $d/2$, subject to tighter applicable shear requirements.

For a 24-inch-deep beam with $d=21$ inches and a smallest primary Grade 80 No. 8 bar, mark a 48-inch zone at each end. The three spacing candidates are 5.25, 6 and 5 inches, so 5 inches governs. A first hoop 3 inches from the column still misses its separate limit even if every interval is 5 inches.

Lap splices of deformed longitudinal beam bars are prohibited within joints, within $2h$ from a joint face, and within $2h$ of critical sections where flexural yielding is likely under the stated inelastic displacement. Where allowed, hoops or spirals must enclose the entire lap length with spacing at most the lesser of $d/4$ and 4 inches. Compare both splice endpoints with the detailed limits.

Special moment frame columns and joints

Section 18.7.3.2 gives the design check $\sum M_{nc}\ge 1.2\sum M_{nb}$ for the column-strength route. Apply the conditions of Sections 18.7.3.1–.3 when reading the approved design. The inspector verifies the cage and the joint detail. Under Section 18.7.5.1, measure confinement length $\ell_o$ from each joint face and on both sides of other likely yielding sections. The minimum is the greatest of the column depth at that face or section, one-sixth of clear column span and 18 inches.

Inside that length, compare three limits under Section 18.7.5.3. The smallest applicable value governs.

• One-fourth of the minimum column dimension.
• $6d_b$ for the smallest Grade 60 longitudinal bar or $5d_b$ for the smallest Grade 80 longitudinal bar, as applicable.
• $s_o=4+(14-h_x)/3$ in inches with a 4-inch lower bound and a 6-inch upper bound for $s_o$.

Section 18.7.5.2 measures $h_x$ between longitudinal bars laterally supported by crosstie corners or hoop legs, with a 14-inch maximum. In the stated high-load or high-strength rectilinear-hoop case, every perimeter bar or bundle must have the prescribed lateral support and $h_x$ is limited to 8 inches. The trigger is $P_u>0.3A_gf'_c$ under factored combinations with earthquake effect or $f'_c>10{,}000$ psi.

For a 24-inch-square column with 120 inches clear span, the length candidates are 24, 20 and 18 inches. Use at least 24 inches from the joint face. For a smallest Grade 60 No. 8 longitudinal bar and $h_x=11$ inches in the ordinary rectilinear case, the spacing candidates are 6, 6 and 5 inches. The maximum from this check is 5 inches. Also verify the detailed confinement area and configuration.

A crosstie under Section 25.3.5 engages peripheral longitudinal bars at both ends. One end is a seismic hook with a tail of at least $6d_b$ and at least 3 inches pointing into the core. The other end has a standard hook of at least 90 degrees, using Table 25.3.2. Alternate successive 90-degree ends where the same longitudinal bars are engaged, subject to the applicable exceptions. In a two-piece beam hoop with a stirrup having seismic hooks at both ends, Section 18.6.4.3 puts the 90-degree crosstie ends on the confined side if a slab confines the engaged longitudinal bars on one side only.

Section 18.8.3.1 governs the reinforcement through the joint. Under Section 18.8.3.2, if beams frame into all four sides and each is at least three-quarters of the column width, the required confinement amount under Section 18.7.5.4 may be halved and spacing increased to 6 inches within the overall depth of the shallowest framing beam. For a 24-inch column in both directions, the widths must each be at least 18 inches. A 16-inch beam on one face rules out this exception. Check the joint detail for reinforcement and beam anchorage where bars continue through or terminate in the joint.

Special structural wall boundaries

The approved wall section locates the distributed web steel and the confined boundary elements. Section 18.10.6.2 applies its displacement approach to the specified wall or pier geometry, effective continuity and single-critical-section design. Its compression-zone criterion is $1.5\delta_u/h_{wcs}\ge \ell_w/(600c)$, with $\delta_u/h_{wcs}$ at least 0.005 for this check. Here $c$ is the largest applicable neutral-axis depth for the factored axial force and nominal moment strength consistent with the displacement direction. This criterion is one step of the design. For an eligible wall with $\ell_w=144$ inches and a ratio of 0.008, the threshold depth is $144/(600\cdot1.5\cdot0.008)=20$ inches. A calculated $c=24$ inches meets the criterion. Section 18.10.6.2(b) adds design and vertical extent requirements.

For walls outside that design approach, Section 18.10.6.3 uses maximum extreme-fiber compression under factored combinations including earthquake effects, with a linear elastic gross-section model. Provide a special boundary element where stress exceeds $0.20f'_c$ at the wall boundary or opening edge. The element can be discontinued where the calculated stress falls below $0.15f'_c$. With $f'_c=5000$ psi the two thresholds are 1000 and 750 psi. A location with 1100 psi exceeds the first. At exactly 750 psi the discontinuation condition is still unmet.

Section 18.10.6.4 sets the horizontal extent at least equal to the greater of $c-0.1\ell_w$ and $c/2$ from the extreme compression fiber. With $c=24$ and $\ell_w=144$ inches, compare 9.6 and 12 inches. In a displacement-based case under Section 18.10.6.2(b)(i), the boundary transverse reinforcement extends above and below the critical section for at least the greater of $\ell_w$ and $M_u/(4V_u)$, subject to Section 18.10.6.4(i). Verify the approved vertical transitions, confinement support into the foundation and boundary hoop details. Section 18.10.6.4(k) requires horizontal web bars to extend to within 6 inches of the wall end and develop yield in the confined core with hooks or heads unless the specified straight-development alternative is met.

Splice locations and inspection

Under Sections 18.2.7 and 25.5.7, Type 1 mechanical splices meet the 1.25 times specified yield requirement for the required force direction. Type 2 splices also develop the specified tensile strength of the bars. Under Section 18.2.7.2 the location exception for restricted zones is for Type 2 splices on Grade 60 bars. Check twice the member depth from the column or beam face in special moment frames and twice the member depth from applicable sections where yielding is likely. For precast special moment frames with ductile connections covered by Section 18.9.2.1(c), beam splices are also at least $h/2$ from the joint face. Welded splices for reinforcement resisting earthquake-induced forces are separately restricted by Section 18.2.8. The mechanical splice exception cannot qualify a welded splice.

Confirm the approved splice type, grade, permitted location and installation evidence. Measure confinement from the specified face and record the first hoop, subsequent spacing, bar support and hook engagement. Where a cage varies from the approved detail, identify the affected zone and track the disposition through the project inspection procedure.