Strong-Wall® High-Strength Wood Shearwalls: Anchor Reinforcement Solutions on Grade Beams
Simpson Strong-Tie® provides grade beam anchorage solutions for the Strong-Wall high-strength wood shearwalls, which have been calculated to conform to ACI 318-14. Through funding from the Structural Engineers Association of Northern California, initial testing at Scientific Construction Laboratories Inc. confirmed the need to comply with ACI 318 requirements to prevent plastic hinging at anchor locations. Follow-up testing at the Simpson Strong-Tie Tyrell Gilb Research Laboratory was then used to confirm these findings and validate performance. The testing consisted of specimens with closed tie anchor reinforcement, non-closed u-stirrups and control specimens without anchor reinforcement. Flexural and shear reinforcement were designed to resist amplified anchorage forces and compared to test beams designed for non-amplified strength level forces. The test program has proven the performance of the anchor reinforcement details developed by Simpson Strong-Tie.
Signifcant Findings from Testing
Grade beam flexural and shear capacity is critical to anchor performance and must be designed to exceed the demands created by the attached structure. In wind load applications, this demand includes the factored demand from the Strong-Wall high-strength wood shearwalls (WSHW). In seismic applications, testing and analysis have shown that in order to achieve the anchor performance expected by ACI 318 anchorage design methodologies, the concrete member design strength needs to resist the amplifed anchor design demand from ACI 318-14 Section 22.214.171.124.3 and ACI 318-11 Appendix D Section D.126.96.36.199. To help designers achieve this, Simpson Strong-Tie recommends designers apply the seismic design moment listed in the table below at the WSWH location when evaluating the grade beam design strength under seismic loads. The tabulated moment correlates to the lowest of the anchor tension design limits defined in the sections listed above as they relate to each WSWH model.
Closed tie anchor reinforcement is critical to maintain the integrity of the reinforced core where the anchor is located. Testing with u-stirrups that did not include complete closed ties showed premature splitting failure of the grade beam.
Grade-Beam Anchorage Solutions
- Anchor reinforcement conforms to ACI 318-14 Section 188.8.131.52 and ACI 318-11 Section D.5.2.9. Full-scale testing was used to validate anchor reinforcement configuration and placement.
- Minimum concrete compressive strength, f'c = 2,500 psi.
- Closed-tie anchor reinforcement to be ASTM A615 Grade 60 (min.) #4 rebar.
- Grade-beam longitudinal and tie reinforcement shall be specified by the designer for flexure and shear loading. Design should consider project-specific design loads and allowable soil pressure.
- Simpson Strong-Tie recommends using the tabulated minimum amplified LRFD applied seismic design moment to ensure grade-beam design flexure and shear strength is adequate to prevent plastic hinge formation under demands associated with anchorage forces corresponding to ACI 318-14 Section 184.108.40.206.3 and ACI 318-11 Section D.220.127.116.11.
- Designer may use reduced moment due to applied WSWH lateral load. Minimum moment shall be the lesser of the tabulated moment or the amplified LRFD design moment for seismic: (ASD design demand shear/0.7) x Ωo x WSWH wall height for grade-beam design.
- Minimum grade-beam design moment for wind and seismic in Seismic Design Category A and B and detached one- and two-family dwellings in SDC C: (ASD design demand shear/0.6) x WSWH wall height.
- Closed tie may be single-piece hoop or two-piece assembly with a u-stirrup with standard 135° hooks and a top cross-tie cap. See detail 6/WSWH1.1.
- See details for grade-beam anchor reinforcement placement, installation and spacing requirements. Closed tie anchor reinforcement quantity is per wall for the 12" wall model, and per anchor for the 18" and 24" models.