Special Moment Frame — Anchorage Design
Moment Frame Design Requirements and Assumptions
Anchorage to Concrete
In addition to the steel frame design, Simpson Strong-Tie also offers anchorage design. We have two solutions for anchorage of the column bases to concrete:
- MFSL — The MSFL anchorage assembly comes with a preattached shear lug, so no field-bent ties or hairpins are required for shear capacity (see Figure D8.1).
- MFAB — The MFAB assembly requires field-installed ties or hairpins, but also provides higher shear capacity depending on the amount of reinforcing provided (see Figure D8.2).
Figure D8.1 — MFSL Anchorage Assembly
US Patent 8,336,267 B2
Figure D8.2 — MFAB Anchorage Assembly
Anchorage Design Notes
The steel-strength calculations for anchor shear and anchor tension are per ACI 318-11 (2012 IBC) and ACI 318-14 (2015 /2018 IBC). Tension and shear anchorage are designed as follows:
|Element||2012 IBC Code Section||2015 /2018 IBC Code Section|
|Anchor rod steel strength in tension||CI 318-11, D.5.1||ACI 318-14, 17.4.1|
|Anchor breakout strength in tension||ACI 318-11, D.5.2||ACI 318-14, 17.4.2|
|Anchor pullout strength in tension||ACI 318-11, D.5.3||ACI 318-14, 17.4.3|
|Anchor rod steel strength in shear||ACI 318-11, D.6.1||ACI 318-14, 17.5.1|
|Embedded plate bending strength||AISC Chapter F||AISC Chapter F|
|Concrete shear strength — shear lug||AISC Design Guide 1||AISC Design Guide 1|
|Concrete shear strength — tied anchorage||ACI 318, Chapter 10||ACI 318, Chapter 10|
Anchorage designs are based on LRFD loads. For designs under the 2012 IBC, tension anchorage for seismic loads complies with ACI 318 Appendix D. The design strength is governed by the maximum tension that can be transmitted to the anchors by the frame capacity or the maximum tension obtained from design loads combinations that include E, with E increased by ΩO. (Section D.188.8.131.52 with modifications contained in 2012 IBC section 1908.1.16.)
For designs under the 2015/ 2018 IBC, tension anchorage for seismic loads complies with ACI 318-14 Chapter 17. The design strength is governed by the maximum tension that can be transmitted to the anchors by the frame capacity or the maximum tension obtained from design loads combinations that include E, with E increased by ΩO. (Section 184.108.40.206.3 with modifications contained in 2015 IBC section 1908.1.16.)
For strength calculation, strength reduction factors in tension are based on:
- Seismic Design Category, φseismic = 0.75
- Crack/Uncrack Concrete factor, φconc = 0.70
Strength reduction factor in shear included:
- Grout pad reduction factor = 0.8
Simpson Strong-Tie Strong Frame Anchorage design calculates anchor bolt shear and tension interaction above the concrete using the AISC 360 bolt interaction equation. However, for capacity within the concrete, anchorage designs are based on anchor embedment into the foundation for tension, while shear design is based on the resistance within the curb or slab. The designer must consider shear and tension interaction of the concrete if failure surfaces overlap. If this failure mode occurs, we recommend providing supplemental reinforcing to transfer the shear forces into the concrete. Where a greater end distance is required, the designer should specify this on their plans. Additional studs can be specified to increase this end distance.
Calculations for the anchorage are provided and typically assume a cracked concrete design based on ACI 318 with no supplementary reinforcing and a centered square pad. Alternate design and detailing of anchorage can be specified by the designer as well.
Since the entire Simpson Strong-Tie® Strong Frame special moment frame is designed to be field bolted, no field welding is required. Welding for the frames is performed on the premises of a fabricator registered and approved in accordance with 2015/ 2018 IBC Section 1704.2.5. Special inspections prescribed in IBC Section 1704 are not required for approved fabricators. Nevertheless, all Strong Frame special moment frames are inspected by a certified welding inspector. Inspection is also provided for the pretensioned bolts between the Yield-Link® stem-to-beam flange connections on top of the code inspection requirements. Welding and bolting inspection documents as well as bolt preinstallation testing records can be obtained from Simpson Strong-Tie at the request of the project designer or by scanning the QR code on the frame at the jobsite.
Even though the Strong Frame can be field bolted and all field bolting is specified as snug tight, the latest IBC code references AISC 360 and AISC 341 for bolting inspection requirements. AISC requires inspection prior to, during and after bolting similar to welding inspections, although not much is required during snug-tight bolt installation.
In addition to field-bolting inspection, different building jurisdictions might have base plate grouting inspection requirements. Please consult with your project building jurisdiction about this requirement.
Simpson Strong-Tie Strong Frame special moment frames have had all required special inspections performed and are built in a factory environment under strict quality-control measures as required under AISC 341, AWS D1.1 and AWS D1.8.
All factory welds for the Strong Frame special moment frame are inspected and documented by a Certified Welding Inspector.
In addition to welding, structural ASTM A325 as well as F2280 twist-off type high-strength bolts are lot tested and stored under requirements of the Research Council on Structural Connections (RCSC). Bolting of the SMF Yield-Link structural fuse to the beam flanges (ASTM 3125 Grade F2280 [A490-TC] Bolts) are documented.
Special Moment Frame QR Code Label
Lot Inspection for Tension Controlled Bolts with DTI Washers
The structural fastener assembly lots are randomly sampled. The samples are tested to the preinstallation verification requirements for pretension bolts conforming with AISC Steel Construction Manual 14th Edition. Bolting and welding inspection reports and material certifications for any individual frame are available by contacting Simpson Strong-Tie with the work order number listed on the frame stickers or by scanning the QR code on the Strong Frame moment frames and entering the work order number.
Special Moment Frame Label
During the frame installation, some special inspections might be necessary depending on jurisdictional requirement; please contact your project’s building department for specific requirements. In the table below are some of the inspections that may be required: