Webinar
A Webinar: Design Braced Frames in Resilient Structures with Yield-Link Brace Connection in RAM Structural System was presented April 27, 2023, and is available on-demand. This wiki includes links to the webinar and to other useful resources on the topic. Several questions were submitted during the webinar, the Q & A is included below.
Presented by:
Tim Ellis, PMP, Market Segment Manager, Simpson Strong-Tie
Mary Nunneley, P.E., R&D Engineer, Simpson Strong-Tie
Allen Adams, P.E., S.E., Chief Structural Engineer, Bentley Systems, Inc.
Presented Live: April 27, 2023
Link to Webinar Recording, On-demand:
Link to Presentation PDF:
Technical Documentation
Time limitations on the webinar only allowed for an introductory presentation on the Yield-Link connection and its analysis and design in the RAM Structural System. Several resources are available for more comprehensive information on the Yield-Link braced frame connection and to assist in modeling, analyzing and designing the connection in the RAM Structural System.
For more information on the Yield-Link brace connection, see the Simpson Strong-Tie Yield-Link website:
https://www.strongtie.com/solutions/steel-construction/yield-link-brace-connection
For a detailed step-by-step guide to Yield-Link in the RAM Structural System, see this wiki, “Specifying and Designing Yield-Link Brace Connections in the RAM Structural System” on Bentley Communities:
RAM Structural System:
https://www.bentley.com/en/products/product-line/structural-analysis-software/ram-structural-system
Q&A
Can you design with the YLBC currently even though it is not referenced in AISC 341?
Yes, the YLBC system qualifies as a seismic-force resisting system in accordance with section 12.2.1.1 of ASCE 7-16. The evaluation and associated peer review performed on the YLBC are consistent with those requirements. In addition, the YLBC has been issued ICC-ES ESR-4342 recognizing qualification in accordance with ICC-ES AC494. Design and implementation of the YLBC shall be performed in accordance with ICC-ES ESR-4342. Included in the evaluation report are references to the applicable building codes, including AISC 360 and AISC 341, to be used for the design and detailing of the YLBC.
What R, Omega_0, and Cd factors should be used with the YLBC?
The YLBC has been issued ICC-ES ESR-4342 recognizing the following design coefficients: R = 8, Omega_0 = 2.5, Cd = 5.
If desired you may use the following design coefficients for a “Steel systems not specifically detailed for seismic resistance, excluding cantilever column systems” as long as the associated requirements and restrictions in ASCE7-16 Table 12.2-1 are met: R = 3, Omega_0 = 3, Cd = 3.
How much will the braced frame stiffness be modified with the Yield-Link?
The stiffness of the brace and fuse configuration assemblies can be viewed as 2 or 3 springs in series depending on whether there are fuses at one end or two ends of the brace respectively. The change in stiffness can be calculated using the equation in section 4.1.5 of ICC-ES ESR-4342 for the effective elastic stiffness of the assembly.
When would YLBC be specified for both ends of a brace vs one end of the brace?
Where structures are assigned a Seismic Design Category (SDC) of A, B, or C fuse configurations may be located on one end of a single brace member. Where structures are assigned a SDC of D, E, or F fuse configurations must be located at each of the brace unless the exceptions in ICC-ES ESR-4342 are met.
What types of requirements are still applicable to the gusset design (i.e., 2t offset, elliptical failure surface)?
Since yielding is concentrated in the YLBC fuses, gusset plates are sized for the maximum expected strength of the YLBC fuses and are not intended to form a plastic hinge like SCBF gusset design. There are dimensional requirements in the YLBC brace-to-gusset-connection to ensure that the slotted flange plates can move freely when the fuse plates deform in compression. Please refer to Figure 3 in ICC-ES ESR-4342 for more details on the requirements.
For the full-scale YLBC test assemblies, what beam-column connections were installed for the braced frames? Please state if AISC 358 prequalified moment connections were used (e.g., welded unreinforced flange connections) or if simple connections (with permissible rotation capacities per AISC 341) were used.
The YLBC qualification testing utilized simple connections where the gusset plate connected to both the beam and column as defined in Section F4.6b.a in AISC 341.
Related to the question above, which beam-column connection is recommended for the design of the frame?
We recommend using a simple connection where the gusset plate connects to both the beam and column as defined in Section F4.6b.a in AISC 341. This has a significant erection cost savings as field CJP is the most expensive process in steel construction. All of our testing and analytical work was performed with “simple” pinned connections validating that the YLBC has desired performance without the fixed beam-to-column connection. A fixed connection is still an option to the designer.
For the test assemblies, what column orientation was considered? Was there any difference in behavior noted for assemblies with weak-axis column orientations?
We tested with columns in the strong direction only. Previous testing outside of Simpson Strong-Tie has shown that columns oriented in the weak direction provides some stress relief on the gusset plate, especially at the base connection. The objective of our testing is to challenge the YLBC with all potential instability and limit states to ensure performance with those design elements, such as strong direction columns.
Were scenarios involving modified work-point locations considered in the development of the YLBC system? Do any of the analytical, design considerations change if a modified work-point is used?
Modified work-points are acceptable with eccentricities that do not exceed the beam depth per Section 3.2 of ICC-ES ESR-4342.
Has this system been tested for use in multi-tiered braced frames? Can it be used in MTBF?
Although we have not tested the Yield-Link Brace Connection in a multi-tiered brace frame configuration it is acceptable to do so in accordance with ICC-ES ESR-4342.
How much force imbalance is expected for chevron configurations?
The compression-to-tension force ratio for the YLBC is relatively small at approximately 5% which results in a minor unbalanced force in chevron configurations.
How is the YLBC typically detailed at the bottom of the frames where it must interface with concrete foundation elements? Does the connection terminate at the top of the slab-on-grade or does it terminate at the footing proper, which may be located 1’-0” or so below the slab-on-grade?
There are multiple options when detailing the base connection. One option is to anchor to the concrete foundation. Another option is to anchor to the foundation and provide concrete block out that allows for movement of the fuse and slotted flange plates. Another option is to anchor to the foundation and provide the dimensional clear distances shown in Figure 3 in ICC-ES ESR-4342 from the top of concrete slab.
Any thoughts about the potential foundation and associated anchorage savings with the YLBC compared to SCBF?
The YLBC permits the use of R=8 rather than R=6 so the seismic design forces are significantly reduced resulting in smaller foundations and anchorages.
Can you comment on the cost savings afforded by the Yield Link brace connection (YLBC) system when compared to Special Concentric Braced Frames (SCBF)?
The following factors combine to provide significant cost savings when compared to SCBF:
- The YLBC has higher R value when compared to SCBF, 8 versus 6, which alone reduced the base shear by 25%. The YLBC will typically have a longer period and can result in a combined reduction of the base shear of 50% when compared to SCBF. The reduction in force demand will see significant savings to the foundations, anchorages, and collector elements of the building.
- The expected strengths of the YLBC are substantially lower than SCBF, which means that beams, columns, steel connections, anchorage to the foundation are generally smaller.
- Opposing braces are not required as they are for SCBF because YLBC has balanced behavior in compression and tension. This potentially reduces the quantity of frames required.
- Large beams to address unbalanced loads in SCBF V and inverted-V configurations are avoided with YLBC due to the relatively balanced behavior in compression and tension
- Field welding are avoided with YLBC as it is an all-bolted connection.
Can you comment on the cost savings afforded by the Yield Link brace connection (YLBC) system when compared to Special Moment Frames (SMF)?
The systematic difference between the two is that braced frames resist lateral forces more effectively through the axial strength of the brace rather than flexure of the beam-to-column connection. As a result, braced frames are quite stiff relative to moment frames such that drift rarely governs the design.
How do we obtain specifications, general note information and drawing details for the YLBC systems? Are these available for download? If so, please provide the link to these documents.
Yes, the requested documents can be downloaded from our website at this address: https://www.strongtie.com/yieldlink_structuralsteel/ylbc_brace/p/yield-link-brace-connection#RelatedLiterature
Has there been any Canadian-equivalent coefficients developed for this system?
No. The Yield-Link brace connection launched in April of 2023 for the U.S. market and the release of design coefficients and factors for use with the Canadian Building Codes are pending at this time.
How is the Edge-Tie anchored into the slab?
(Note: This not part of the YLBC, it is a different item that was mentioned in the webinar) The Edge-Tie Channel is welded to the spandrel beam or outriggers. Additional anchorage can be provided using rebar or headed studs welded to the channel. Standard details can be found here https://www.strongtie.com/search?v=%3Arelevance&tab=drawing&keywordFilter=Edge-Tie. If you have any questions, please contact Simpson Strong-Tie.