Special Moment Frame Connections with Sloped Columns
Welded moment connections in special moment frames are more susceptible to fracture when the connections are sloped. DuraFuse Frames connections can accommodate slope without changing the stress states in the critical elements.
ANSI/AISC 358 Chapter 15 DuraFuse Moment Frame CPRP Review Version
AISC Connection Prequalified Review Panel has recently reviewed an ANSI/AISC 358 Ch. 15 proposal incorporating DuraFuse Frames. The chapter is currently working its way through the consensus process. Check out the latest version embodying the design process of this cutting-edge connection system.
How to Use the DuraFuse Features in RAM Frame
RAM Frame software has the option to model moment frame connections as DuraFuse connections. The software accurately models the elastic stiffness of frames with DuraFuse connections and performs the seismic checks that are pertinent to the members and joints. Detailed connection design that is not required for stiffness calculations or member checks is performed outside of RAM.
Technical Bulletin 4
Quantifying Connection Rigidity for DuraFuse Frames
Full-scale experiments were performed to evaluate the stiffness of connections in DuraFuse Frames. Sub-assemblies with beam depths ranging from 20.8 to 37.1 inches were subjected to initial cycles within the elastic range. The connection stiffnesses determined from the experiments were 2.7 to 3.5 times greater than the minimum connection stiffness required in order for the connections to be considered fully rigid (FR). Connections in DuraFuse Frames can be modeled as fully rigid (FR) connections for gravity and seismic loads.
Backbone Curve for DuraFuse Connections
A general backbone curve for the DuraFuse Frames connection was developed for use in nonlinear analysis. The general backbone curve was derived based on nine experimental specimens subjected to cyclic loading. The backbone curve is defined by eight points calibrated from the test data. This bulletin summarizes the backbone curve, describes its development, and compares the general curve with the experimental data.
Continuity Plates and the DuraFuse Frames Connection
Continuity plates are used in welded moment connections to transfer forces to the column web and to make the stress distribution at the beam-to-column welds more uniform. The DuraFuse Frames connection has a different load path that does not require standard continuity plates to transfer loads to the column. The cover plates in a DuraFuse Frames connection transmit the beam forces to the column and prevent local column flange deformations.
How DuraFuse Frames are Represented in RAM
The software RAM Frame has the option to model moment frame connections as DuraFuse Frames connections. When this option is selected, the joints have stiffness parameters assigned to reflect the stiffness of DuraFuse Frames connections, as determined from analytical methods and experimental testing. The beam-to-column connection is considered to be fully restrained (FR) and the panel zone stiffness is modeled using the scissor method, following procedures recommended in the literature.
Modeling DuraFuse Frames Connections in ETABS
The software ETABS is sometimes used to model steel moment resisting frames. When DuraFuse Frames are used for the steel moment frame, the joints in the ETABS model should be defined to accurately reflect the behavior of the DuraFuse Frames connections. The beam-to-column connection at each joint is considered to be fully restrained (FR) and the panel zone stiffness is modeled using the scissor method, following procedures recommended in the literature.
DuraFuse Frames Code Approval IAPMO UES ER 610
Third-party code-approval reports are an important component of regulatory compliance in the construction industry, enabling innovation, problem solving and competitiveness. Their use is widespread in the realm of proprietary connection system and seismic technology, assuring compliance or equivalency with the governing building codes, such as IBC or CBC, or referenced standards, such ANSI/AISC 341 or ANSI/AISC 360. DuraFuse Frames technology is covered by the IAPMO UES ER 610.
Difference in Repair Cost for DuraFuse Frames vs. Other Steel Moment Frame Systems
Buildings that are designed with current codes are expected to be damaged during severe earthquakes. FEMA-P58 was developed to quantify expected building losses, based on the characteristics of the building and the seismic hazard. This bulletin summarizes a FEMA-P58 study that was used to compare the difference in expected building repair costs for DuraFuse Frames vs. a traditional steel moment frame system (RBS). DuraFuse Frames reduce structural repair costs by a factor of four, and residual drift demolition losses by a factor of ten.
Post-Earthquake Evaluation of a Building with DuraFuse Frames
The March 18, 2020, earthquake near Salt Lake City, Utah, caused strong shaking at a site where a DuraFuse Frames building was under construction. The building was the new Student Center for the Salt Lake Community College (SLCC) Jordan Campus. A post-earthquake evaluation of the steel frame confirmed perfect performance of the DuraFuse Frames DF360 system during the event, with no damage to the steel frames. DuraFuse Frames systems are the most resilient steel moment frames available.
Subassembly Stiffness Comparison for R=3 Steel Moment Frames
In low-seismic regions, proprietary moment frame connections may be used for R=3 steel moment frames to reduce beam and column sizes. A study was performed to compare the weight and stiffness of moment frames with R=3 DuraFuse Frames (DFF) and R=3 SidePlate (SP) connections. Finite element modeling of four subassemblies with each type of connection was performed with ANSYS. The analyses showed that the DFF subassemblies had greater stiffness and less connection weight than comparable SP subassemblies. For R=3 moment frames, DFF connections can be used in place of SP connections without changing beam and column sizes.
Technical Bulletin 13
DuraCore Combined System for Optimum Building Design
While most steel buildings in high seismic areas use either buckling-restrained braced frames (BRBFs) or special moment frames (SMFs), there are advantages to using combined systems in some cases. DuraCore (DuraFuse + CoreBrace) combines the best features of SMFs and BRBFs. DuraCore can reduce weights, as compared to SMFs, and can improve architectural flexibility and foundation demands as compared to BRBFs. DuraFuse engineers can help EORs explore possible benefits of a combined system on particular projects.
Using DuraFuse Frames in Seismic Retrofit Applications
Most existing special moment frames do not meet current code requirements. The DuraFuse Frames (DFF) Arrow retrofit introduces a fuse plate that mitigates common deficiencies including welds that are susceptible to brittle fracture, weak column panel zones, insufficient column flexural strength, insufficient frame stiffness, and insufficient beam width-thickness ratio.
Reducing Seismic Losses by Using DuraFuse Frames
Buildings that are designed with current codes are expected to be damaged during severe earthquakes. SP3 software was used to perform FEMA-P58 analyses to compare expected losses for DuraFuse Frames vs. other special moment frames.
Using BRBFs with Eccentric Configurations in Combination with DuraFuse Frames
One application of BRBFs that has been underappreciated is their use in eccentric configurations (BRBF-Es). Case studies were performed to compare the weight of BRBF-Es with SMFs in 1-and 3-story frames. BRBF-Es provided nearly the same amount of unobstructed bay space as SMFs with weight reductions of up to 58%. Another case study illustrates how some bays in an SMF can be converted to a BRBF-E, resulting in over 40% reduction in weight and elimination of grade beams without any architectural impact.
Stiffness of Panel Zones with Cover Plates
In steel moment frames, panel zone flexibility contributes to story drift, so cover plates are often used to stiffen the panel zones. Detailed finite element models were used to evaluate the effectiveness and efficiency of different styles of cover plates. The finite element analyses found that standard cover plates, as used in DuraFuse Frames, provide the same panel zone stiffness as extended cover plates, while requiring less plate weight. When engineers are comparing moment frame systems with cover plates, they should use the same panel zone assumptions regardless of whether standard or extended cover plates are used. When rigid panel zones are assumed for DuraFuse Frames, cover plates will be checked to ensure they meet the requirements for that assumption per ASCE 41 guidance.
Evaluating the Accuracy of Moment Frame Design Models
Structural models that are used for design must reasonably represent the strength and stiffness of the structure. The accuracy of typical ETABS and RAM models for special moment frame systems was evaluated. For reduced beam section (RBS) and DuraFuse Frames (DFF) systems, ETABS and RAM models produced results that were similar to those obtained from rigorous models where connection geometries were modeled explicitly (using ANSYS). For SidePlate (SP) frames, the ETABS and RAM models based on SP modeling recommendations, had stiffnesses that were more than 1.3 times greater than the upper-bound stiffness determined from rigorous models. The artificial stiffness observed in the SP ETABS and RAM models was caused by inaccurate modeling assumptions recommended by SP. Modeling assumptions for SP frames should be corrected so that SP ETABS and RAM models have reasonably accurate stiffness.
Beam Lateral Bracing for DuraFuse Frames
Traditional Special Moment Frame (SMF) beams are designed to form plastic hinges under seismic loading. AISC 341-16 specifies prescriptive beam lateral bracing requirements for those beams to mitigate instability caused by the plastic hinges. In contrast, DuraFuse Frames systems have fuse plates that eliminate plastic hinge formation in the beams. DuraFuse Frames systems have been tested without lateral bracing and are prequalified through ER-610 for use without bracing per AISC 341-16 D1.2b and E3.4b. To mitigate lateral torsional buckling unrelated to plastic hinges, DuraFuse Frames systems use beam lateral bracing per AISC 360 based on the maximum moment that can develop in the beam. This prequalified approach results in around 70% fewer beam lateral brace points than other SMF systems, saving time and money on projects. Case studies are presented that illustrate the savings on lateral bracing costs by using DuraFuse Frames.
Improving Functional Recovery by Using Replaceable Fuses
Current building codes rely on high ductility systems to resist earthquakes safely. Buildings designed in this manner may be impractical to repair following a severe earthquake. Efforts are being made to improve building codes to address re-occupancy and functional recovery. One option is to design more structures as Risk Category IV. This would be costly and may backfire for some systems that already have low periods and are in the acceleration sensitive region of the response spectra. A complimentary or alternative option is to improve the repairability of structures so they can be returned to service after severe earthquakes. Various replaceable fuse concepts have been explored. The most practical concepts are already being used in practice. The most repairable concept for braced frames is BRBFs. The most repairable concept for moment frames is DuraFuse Frames (DFF). FEMA P-58 analysis confirms that DFF moment frames are a cost-effective path to functional recovery.
This technical bulletin summarizes the content from a presentation given by Dr. Richards at the SEAU Continuing Education Conference on March 3, 2021.
Repair Requirements for Welded and Bolted SMF Connections
For small earthquakes, special moment frames (SMFs) will respond elastically. For moderate and severe earthquakes some amount of inelastic behavior is expected. Welded moment frames are susceptible to damage that is the most difficult to repair. For moderate earthquakes that require repairs, bolted SMFs, including DuraFuse Frames (DFF), might be repaired without replacing any steel. For severe earthquakes, DuraFuse Frames offers better repairability than other SMFs because beam yielding is prevented and the fuse is accessible. Experimental testing has demonstrated that DFF fuses have sufficient energy dissipation capacity to handle multiple, maximum considered events (MCEs). DFF fuses will only require replacement after severe events that result in large residual drifts.
Impact of Importance Factor and Post-Yield Stiffness on SMF Weight and Residual Drift
Steel special moment frames (SMF) are susceptible to unrepairable residual drifts. Two strategies for controlling residual drifts are designing with a higher importance factor and/or using SMF with higher post-yield stiffness. To investigate these two strategies, four-story SMF were designed considering different importance factors (1.0 or 1.5) and different post-yield stiffness [typical reduced beam section (RBS) or typical DuraFuse Frames (DFF)]. The SMF were analyzed with response history analysis (RHA) using eleven ground motions, scaled to design earthquake (DE) and maximum considered earthquake (MCE) levels. Designing with the higher importance factor increased weights 79-96%, but did not reduced residual drifts enough to ensure repairability for the I=1.5 RBS after DE loading. In contrast, designing with higher post-yield stiffness (DFF) had significant impact on residual drifts, with DFF having up to 34% lower residual drifts than RBS designed with the same importance factor. In addition to reduced residual drifts, DFF were 10-18% lighter and had up to 12% lower floor accelerations than comparable RBS frames.
M-Factors for the DuraFuse Frames Arrow Connection
Most pre-Northridge buildings with steel special moment frames (SMF) do not meet current seismic performance standards. When engineers evaluate such buildings and consider retrofit options, they often perform linear analysis and use m-factors to account for nonlinear effects. This bulletin describes the development of m-factors for the DuraFuse Frames Arrow connection. Data from full-scale experimental testing were used to develop a general backbone curve for the connection. The m-factors obtained from the curve were 4.7 for life safety (LS) and 5.9 for collapse prevention (CP). These m-factors are higher than those for welded haunch retrofits and reflect the excellent performance of the DuraFuse Frames Arrow connection.
DFF Arrow vs. Welded Haunch for SMF Retrofit Projects
Most pre-Northridge buildings with steel special moment frames (SMF) do not meet current seismic performance standards. One method to retrofit such buildings is to modify the beam-to-column connections. This bulletin compares the welded bottom haunch retrofit with the DuraFuse Frames (DFF) Arrow retrofit. The DFF Arrow retrofit has greater rotation capacity and prevents beam yielding, making functional recovery much more likely. The installation costs for the two retrofits are comparable.
Shake Table Testing of Durafuse Frames
The large high performance outdoor shake table (LHPOST) at the University of California San Diego was upgraded between 2018 and 2022. As part of the upgrade, an accessory frame was added that can be used for non-structural component testing. The accessory frame, called the Modular Testbed Building (MTB2), uses DuraFuse Frames® for lateral resistance since it must be able to withstand repeated severe earthquake shaking. In April 2022, the first system characterization tests were performed on the MTB2. The building was subjected to twenty earthquake ground motions of varying intensity. At least ten of the earthquakes would be characterized as strong. Under the design-level shaking, the building experienced story drifts around 2% and the fuse plates experienced minor yielding. The same fuse plates were used for multiple design-level records without requiring replacement. No damage was observed in the beams or columns. At the conclusion of testing, the fuse plates were removed without difficulty, and the MTB2 was switched to its buckling-restrained brace configuration.
Criteria for Replacing Fuse Plates after a Severe Earthquake
DuraFuse Frames (DFF) have replaceable fuse plates that prevent beam and column damage
during severe earthquakes. This technical bulletin reviews criteria for determining if fuse plates need to be replaced after an earthquake. Fuse plates should be replaced if residual drifts are causing serviceability problems or if there is evidence that maximum story drifts have exceeded 4%. Otherwise, experimental testing indicates that fuse plates will work for another design event.
Technical Bulletin 27