Discover insights from Simpson Strong-Tie engineer Emily Morris Frazier, P.E., as she examines the evolution of moment frame construction over the past 150 years. She traces the transition from early rivet and angle connections to the prevalent use of welded moment frames in seismic zones. Emily addresses the challenges that emerged after significant earthquakes, which spurred the development of prequalified connections to boost safety and performance. She highlights the ongoing advancements in design strategies that aim to enhance the resilience of modern structures.
The Earliest Moment Frame Connections
Structure Magazine, February 2019, “Welded Steel Moment Resisting Frames” by Ronald O. Hamburger, S.E., and James O. Mailey, S.E. https://www.structuremag.org/?p=14163
FEMA 354/November 2000, “A Policy Guide to Steel Moment-Frame Construction” by SAC Joint Venture (SEAOC, ATC, and CUREe) chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.nehrp.gov/pdf/fema354.pdf
Approximately 150 years ago, steel framing revolutionized how buildings were constructed. The proportionately strong material compared to its weight and size led to larger and taller “high-rise” buildings. Initially rivet and angle connections were used to achieve the beam-to-column attachment. Steel structures that used this system appeared to perform very well after earthquakes in the early 20th century in California (1906 San Fransisco, 1926 Santa Barbara, 1933 Long Beach, etc.). Advancements in connection design led to fewer pieces by incorporating bolts and welding. By the 1960s, welded moment frames gained popularity, and their use became more widespread especially in seismically active regions due to the engineering community’s belief that steel moment frames would behave in a ductile manner. But then, in the 1994 Northridge and 1995 Kobe earthquakes, the unanticipated damage due to fracturing of the welded joints between the moment frame beams and columns created a crisis of confidence around the structural engineering world. Often the fractured joints showed no outward damage, even though the building was visually out– of– plumb, leading to difficulty in identifying the fractured joints.
Why Are Prequalified Connections in the Code?
SteelWise article, November 2016, “What’s New with Prequalified Connections?” by Michael D. Engelhardt, Ph.D., and Margaret A. Matthew, P.E. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.aisc.org/globalassets/modern-steel/archives/2016/11/steelwise.pdf
After the 1994 Northridge earthquake, it was observed that welded steel moment frame connections were not performing as expected. Widespread damage prompted a closer review of the connection details used for both special moment frames (SMFs) and intermediate moment frames (IMFs). This review highlighted the need for drift (± 0.04 rad for SMFs, and ± 0.02 rad for IMFs) to occur without significant loss in strength in the frames; and in order to ensure consistency, this performance requirement must be demonstrated through large-scale qualifying cyclic tests. The first prequalified connections were introduced in the year 2000 via FEMA 350: Recommended Seismic Design Criteria for New Steel-Moment Buildings. In the early 2000s, AISC assembled a panel of knowledgeable people to create the Connection Prequalification Review Panel (CPRP), which had the responsibility of qualifying IMF and SMF connections. Connections that met these performance requirements were included in the first version of AISC 358 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, which was released in 2005. But how did we get here?
The First Code-Approved Moment Frame Connections
In the late 19th century, construction began to become more sophisticated as cities became more densely populated. The buildings that were once constructed of masonry and concrete began to grow vertically to use land more efficiently and still accommodate the growing population. The Home Insurance Building, constructed in Chicago in the 1880s, was the first steel moment frame building and is often considered the first skyscraper. In 1927 the first Uniform Building Code (UBC) was published to act as a guide and create consistency for the booming construction industry. While the 1927 UBC included steel moment frames, the code leaned heavily on the newly created American Institute of Steel Construction (AISC)., AISC was created in 1921 and released the first edition of the Specification for Structural Steel Buildings in 1923, but the design requirements were minuscule compared to today’s building code requirements.
As the design of steel moment frames matured, engineers in different areas of the country found success in different detailing strategies. For wind-governed regions, frames that were stiff and non-yielding were desired, with the focus on strength of the members and connections. In seismic regions, frames with higher ductility were desired to dissipate the sudden cyclic forces acting on the frame. The differences in detailing and design coefficients vary greatly between moment frames in wind regions (primarily ordinary moment frames, or OMFs) and moment frames in seismic regions (IMFs and SMFs).
Current Prequalified Connection Types and Brief Summary
FEMA 354/November 2000, “A Policy Guide to Steel Moment-Frame Construction” by SAC Joint Venture (SEAOC, ATC, and CUREe) chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.nehrp.gov/pdf/fema354.pdf
Prequalified connections come in a variety of configurations, each with its own set of advantages and disadvantages, and no one connection will be universally appropriate for all applications. Design engineers must evaluate the parameters and preferences most appropriate for each project, although it’s likely that they’ll develop general preferences based on common design considerations, cost, and fabrication and erection complexity. Prequalified moment connections can be simplified into weakened beam or stiffened beam connection types, often based on strong-column, weak-beam philosophy. AISC 328-22 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications includes the following weakened beam types:
- Reduced beam section (RBS)
- Welded unreinforced flange-welded web (WUF-W)
- ConXtech ConXL
- SlottedWeb (SW)
and the following stiffened beam connections:
- Simpson Strong-Tie® Yield-Link® moment connection (YLMC)
- Bolted unstiffened extended end plate (BUEEP)
- Bolted stiffened extended end plate (BSEEP)
- Bolted flange plate (BFP)
- Kaiser bolted bracket (KBB)
- SidePlate
- Double-Tee
- DuraFuse Frames
With the Simpson Strong-Tie Yield-Link moment connection (YLMC), the energy dissipation and yielding occurs in the connection allowing both the beams and columns to remain elastic throughout the seismic cycles.
Rise of Functional Recovery
Earthquake Engineering Research Institute (EERI) White Paper, December 2019, “Functional Recovery: A Conceptual Framework with Policy Options” ” chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.eeri.org/images/policy/EERI-Functional-Recovery-Conceptual-Framework-White-Paper-201912.pdf
The current International Building Code (IBC) meets life safety standards, meaning that after a major event (earthquake, windstorm, etc.) the building will not collapse, allowing the occupants to escape. For densely populated areas, this standard saves lives during and immediately following the major event. However, it has the potential of leaving thousands of people unhoused and without water. In high seismic regions, policymakers have been calling for “better than code,” with the aim of shortening or eliminating the time between the major seismic event and reoccupancy so functional recovery can begin almost immediately. Functional recovery for infrastructure systems is defined as restoration of the system’s services needed to allow users to resume most of their pre-earthquake activities. The Earthquake Engineering Research Institute (EERI) uses the following definition as spelled out in a white paper published in December 2019: Functional recovery is a post-earthquake state in which capacity is sufficiently maintained or restored to support pre-earthquake functionality.
The YLMC dissipates energy through the yielding section of the moment connection links, allowing the gravity connections between the beams and columns to remain in place. This maintains life safety while allowing for repairs to the lateral system that do not require shoring, shortening the timeline for reoccupancy after an earthquake.
As the bolted, yielding, replaceable Yield-Link moment connection demonstrates, Simpson Strong-Tie is continually developing connections and systems to aid in the construction of safer, stronger, more resilient structures.
