By Patrick McManus, PE, SE, Ph.D
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Developing an idea into a viable product or system can take many paths and is an arduous process that may require years of innovation, research and testing, qualification, and product development. Such was the case for a new steel braced-frame system that was released last year by Simpson Strong-Tie called the Yield-Link brace connection (YLBC).
Utilizing replaceable fuses as the primary connector between wide flange braces and gusset plates, the system is a complement to the Yield-Link moment connection (YLMC), which is prequalified by the American Institute of Steel Construction (AISC) per Chapter 12 of ANSI/AISC 358s2-20, “Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, including Supplements No. 1 and No. 2.” Both solutions isolate structural damage from a significant seismic event to bolted, easily replaceable components, while the remainder of the structure remains essentially elastic.
Constant across all approaches to developing a new system is a need for time and resources. The path of least personal risk to an inventor is to obtain funding from an outside source, such as a grant, investors, or an employer. Regardless of the type of investor, obtaining funding almost always requires a proposal that thoroughly outlines the product development plan, all associated costs, and the expected return on investment for the investor, even if the return is a benefit to the industry rather than monetary. In the case of a grant, particularly in the structural industry, the proposal is likely subject to a lengthy selection process in competition with many other proposals from experienced researchers. In the case of other investor sources, the process may be more fluid and expeditious but typically with the tradeoff of higher expectations for a return on the investment. In each of these scenarios the development process is influenced by the desires of the entity funding the project.
The YLBC originated with a concept to isolate axial brace deformations to a relatively compact component that would connect a brace member to the gusset plate of a brace connection. Bolted components of a size and weight that can be handled by one or two workers allow for easy replacement and repair of the lateral force-resisting system. Enhanced reparability was not an attribute of braced-frame systems prevalent in the building industry. The idea depended on arriving at component material and geometry that could provide needed strength and stiffness and also accommodate sufficient deformation without premature failure under low-cycle fatigue. Even if developing such a component was successful, there was no direct code path yet established to qualify a braced-frame system. Substantial analysis and testing efforts would be necessary at the local level to begin answering the questions of viability, with even greater efforts required at the global level to ultimately achieve some form of qualification and acceptance.
If the inventors, Patrick McManus and Jack Petersen, both of Martin/Martin Consulting Engineers, and Jay Puckett, then with the University of Wyoming, were to control the destiny of the concept, some level of risk acceptance was required. With starry eyes, the inventors chose to move forward, though it is questionable if they would have arrived at the same decision with full knowledge of the effort ultimately required. The decision was not made lightly as each inventor continued to work full time at their respective jobs. This warranted disclosure to their various employers and agreements as to how the inventors could proceed while maintaining employment.
Once the decision was made to invest in furthering the concept, protecting the intellectual property was paramount to provide any means to be compensated for the effort, whether this meant the potential for a profit or simply a reduction in the costs. Protection of intellectual property is a delicate dance between being the first to file and having performed enough research to ensure what is being patented is representative of what is ultimately being brought to market. Filing a provisional patent application sets the date the concept is first introduced to the patent office and creates precedent over similar concepts that others may subsequently file. However, filing of the provisional patent application also starts the clock ticking on the 12-month duration to file the non-provisional patent application, which is the detailed document that ultimately becomes the patent, assuming the claims made have not been found to already be in the public domain or present in another patent application with an earlier priority date.
Twelve months may seem like a long time, but it goes by quickly when testing, analysis, production methods, and market research activities are all necessary to vet a concept. As it happens, the inventors were acquainted with Robert Bowman, a structural engineer who switched careers and became a patent attorney. Bowman was engaged to assist in the patent prosecution process.
Before returning to Martin/Martin as a consulting engineer, McManus worked for steel fabricator Puma Steel. Puma was invaluable in the development of the YLBC technology. Test frames previously fabricated for McManus’s PhD research under Puckett were repurposed for component tests and full-scale braced-frame tests of the initial YLBC configurations at Puma’s fabrication facility.
The effort to arrive at a component with adequate performance was considerable. ANSI/AISC 341, “Seismic Provisions for Structural Steel Buildings,” provide specific connection and system performance requirements for steel intermediate moment frames and special moment frames, as well as a code path to qualify new moment connection concepts (proprietary or otherwise) within ANSI/AISC 358 through submission to AISC’s Connection Prequalification Review Panel. However, no corollary performance criteria exist for generically defined braced-frame systems, nor does there exist an associated prequalification path for braced-frame systems. Because no other system existed with a similar ductile yielding mechanism, the only potential path to acceptance was to follow the methodology of the Federal Emergency Management Association’s FEMA P695, “Quantification of Building Seismic Performance Factors.”
The FEMA P695 methodology requires experimental testing to establish load deformation characteristics and failure mechanisms for a given system. These characteristics are then used to design building frame archetypes that address a broad spectrum of frame configurations and critical design parameters. A powerful attribute of the methodology is the ability to adjust the acceptable collapse margin ratio based on the robustness and thoroughness of the analysis, testing and design provisions used for the assessment. A peer review panel is required to be a part of the evaluation, one duty of which is to verify appropriate levels of uncertainty are assumed in adjusting the acceptable collapse margin ratio.
The fundamental proportioning of the yielding elements for strength were based on relatively simple mechanics that could be calculated by hand. Finite element analysis verified the strength predictions and provided predictions for elastic and inelastic stiffness. These parameters could be used in building frame models for response history analysis. However, the component maximum deformation capacity under low cycle fatigue, simulated by a cyclic loading regime, was needed to define the failure characteristics of the ductile yielding mechanism within the models to properly simulate collapse. This limit is difficult to determine directly by finite element analysis without some experimental testing to calibrate fracture models.
Once finite element analysis had been taken as far as possible without fracture modeling, experimental component testing of various geometries was performed. The elastic and inelastic behaviors matched very closely; however, the experimental testing demonstrated substantially reduced maximum deformation capacity under a cyclic regimen as compared to monotonic loading. Although this was expected, the degree to which cyclic deformation capacity was reduced resulted in reconsideration of the component geometry, alternate cutting methods and even alternate materials. Ultimately, it was the concept of placing multiple components of similar geometry in series to spread deformations evenly across the components (accordion-like behavior) combined with nontraditional cutting methods that won the day.
With the prospect of a successful component, a strong peer review panel was engaged before moving into full-scale testing, development of design provisions and nonlinear analysis of archetype designs. Farzad Naeim, Michael Engelhardt, and Rafael Sabelli were each highly renowned for their contributions to performance-based design and steel analysis and design for seismic applications. The peer review panel members were compensated as suggested by FEMA P695 and graciously remained involved through the full process providing invaluable insight and examination of the system.
Experimental testing often comes with surprises, particularly exposure of unanticipated behavior and failure mechanisms—the very reason it is required to justify using new products and systems. Full-scale testing of YLBC proved to be no exception. Surprises were encountered that mandated additional testing to bring the system to resolution. Most notably, because the fuse components were used to connect the brace members to the gusset plates and behaved inelastically at large deformations, global out-of-plane instabilities of the gusset plate were prevalent at large deformations in early tests. Again, the inventors had to find a way to overcome the challenge or face the prospect of abandoning the endeavor altogether. Slotted plates attached to the beam flanges were introduced to stabilize the gusset plates. The solution was effective, and full-scale testing proceeded successfully.
The FEMA P695 process culminated with letters from the peer review panel members acknowledging their review of, and agreement with, the testing and analysis reports, design provisions, and material specifications for the YLBC system. Using the system on individual projects would then require acceptance of the peer-reviewed product information by the engineer of record and authority having jurisdiction under alternative systems’ provisions in the applicable building code. While building codes, such as the International Building Code (IBC), generally provide a path for alternative systems, the language is often generic to all building disciplines (not just structural components). As such, the requirements tend to lack specificity to any one system while still being appropriately stringent in the interest of life safety. This important requirement can make the introduction of any innovative product challenging, let alone something so comprehensive as a full lateral system.
As it happens, the American Institute of Steel Construction’s (AISC) Task Committee on Seismic Design was contemplating an improved path for alternate systems in AISC 341 as a means to better promote innovation. Several individuals on this committee were also involved in committee work for the American Society of Civil Engineer’s “Minimum Design Loads for Buildings and Other Structures” (ASCE 7). They recognized the issue was not material-specific and should be taken up at the ASCE 7 level. The culmination of this effort was enhanced provisions for alternative seismic systems in ASCE 7-16, including commentary that specifically cited the use of the FEMA P695 and P-795 methodologies as the intended mechanisms to qualify alternate seismic systems.
To close the code loop for simplified acceptance by engineers and the authorities having jurisdiction, Jim Harris, founder of J.R. Harris and Company, had been collaborating with the International Code Council (ICC) to develop AC494 – Acceptance Criteria for Qualification of Building Seismic Performance of Alternative Seismic Force-resisting Systems outlining parameters to use the FEMA P695 and P-795 methodologies to meet the requirements of ASCE 7, including the use of a qualified peer review panel. This process provided the YLBC inventors a mechanism to develop an annex to AC494 specific to braced-frame systems with fuse connections and use the work performed to obtain an Evaluation Service Report (ESR-4342).
YLBC was developed by intentionally addressing the concerns of each stakeholder; owner, engineer of record, building official, fabricator and erector. After qualifying a seismic system with an approved ICC ESR, the inventors recognized the effort to evaluate and put in place manufacturing methods, distribution, collaboration with software vendors, engineering support, and market awareness was potentially even greater than what had already been undertaken. They determined the best path forward would be to place the product in the hands of an entity with expertise and infrastructure already in place to serve these needs. Simpson Strong-Tie had already developed a replaceable fuse for seismic moment-frame systems and was seeking a similar solution for braced frames. With its recent advances in the structural steel industry, the company recognized YLBC as a perfect fit to enhance its suite of solutions.
Upon acquiring the YLBC technology, Simpson Strong-Tie began a research and product development effort using its in-house analysis and testing capabilities to double the capacity of the original fuse components. This endeavor culminated in a robust full-scale frame testing program rivaling that of any other seismic system.
Bringing the YLBC from concept to industry was a long and challenging journey. The endeavor was also highly rewarding to all those involved, and the invention will benefit many projects in the years to come. ■
About the Author
Patrick McManus, PE, SE, Ph.D, Novel Structures, has more than 20 years of experience working as a consulting engineer and as a fabricator’s specialty engineer focusing on analysis and design of steel structures, particularly in seismic applications. He serves as a member of the American Institute of Steel Construction’s committees on Seismic Design, Connections and the Connection Prequalification Review Panel. (patrick.s.mcmanus@novelstructures.com)
Jack Petersen, PE, SE, and Jay Puckett, PE, Ph.D of Novel Structures; Tim Ellis, Mary Nunneley, PE, and Priscilla Yata of Simpson Strong-Tie also contributed to this article.