Objective - This project will assess the seismic performance of pre-Northridge earthquake panel zone assemblies in steel moment frames. During a seismic event, a weak panel zone mechanism may result in excessive shear deformations resulting in fracture of the welds around continuity plates, fracture of the column, or fracture of the complete joint penetration (CJP) weld at the bottom flange connection of the adjacent beam. These mechanisms can inhibit the intended ductility capacity of the panel zone assembly or the beam-to-column connection required to dissipate energy. Results from this project will develop criteria for assessment of pre-Northridge earthquake panel zone assemblies for incorporation into ASCE 41 and provide guidance on possible retrofit schemes of these assembles to improve panel zone performance in existing steel buildings.
What is the technical idea? Steel moment frame design prior to the 1994 Northridge Earthquake (1985-1994 era) favored panel zones that would yield before the adjacent beams framing into the joint. This weak panel zone design philosophy is contrary to the “balanced yield” approach adopted for assessment of existing steel moment frames in ASCE 41. A weak panel zone mechanism may result in excessive shear deformations within the panel zone during a seismic event resulting in fracture of the welds around continuity plates or fracture of the column initiating from the corner of the panel zone adjacent to the column flanges experiencing high bending strains. Column flange bending has also been reported in specific cases to result in fracture of the CJP weld at the bottom flange connection of the adjacent beam (Kim et al. 2015). Consequently, other failure modes within the beam-to-column connection can preclude the formation of plastic hinges in the beams.
It was shown during the SAC project (see FEMA 355D (FEMA 2000)) that relatively weak panel zones are seen to trend towards low levels of total plastic rotation, which implies reduced overall ductility. Test results showed that panel zone subassemblies with a ratio of shear demand to the shear yield capacity that is greater than about 1.1, do not achieve an appropriate level of well-behaved ductility.
Acceptance criteria is provided in ASCE 41-13 (ASCE 2014) for checking the acceptability of a panel zone against a desired performance level. However, these permissible forces and deformations are independent of the above noted ratio, the axial load acting through the panel zone, and the effects of adjacent connections and column parameters. These effects can be more problematic when weak panel zones are the primary source of inelastic actions and energy dissipation (i.e., the beam hinge within the beam-to-column connection assembly does not yield). Consequently, the permissible values for a panel zone may be too high for weak panel zones. Note that a relatively strong panel zone can be made weak when the adjacent beam-to-column connection has been retrofitted by means other than haunches or brackets that can increase the robustness of the bottom flange connection. Moreover, ASCE 41-17 (ASCE 2018) recently adopted (1) a simplified method to check the possibility of CJP weld fracture associated with column flange bending in pre-Northridge earthquake steel construction (see Kim et al. 2015), and (2) criteria to include the effects of an axial load acting through the panel zone. However, these new provisions are either not fully supported technically or are based on results for a limited number of tests. Therefore, a significant effort is needed to provide substantive guidance regarding seismic assessment and retrofit of weak panel zones to practicing engineers.
Through the combined use of experiments and high level analyses, this project will develop a methodology to assess a weak panel zone in existing steel moment frames. The methodology will be aligned with criteria in ASCE 41-17. Additionally, guidance will be provided as to how to retrofit a weak panel zone subassembly that does not satisfy the newly developed criteria.
What is the research plan? A combination of experiments and high-end analyses will be used to develop an understanding of the behavior of pre-Northridge earthquake panel zone assemblies. Three full-scale tests of panel zone assemblies replicating construction practices employed in the 1980s will be conducted in the Structural Testing Lab at NIST, depending on laboratory availability. Should the NIST lab be unavailable, then testing at an outside facility will be considered. A high-end nonlinear finite element analysis software, such as ABAQUS (Dassault Systemes, 2016), will be used develop models to study behavior and inform the test specimen development as well as models that replicate the tests and provide a testbed for conducting a parametric study to evaluate additional assemblies within the bounds of the tested assemblies.
Collaboration with American Institute of Steel Construction and the Lincoln Electric Company, the foremost US manufacturer of welding equipment and associated welding rods, will be necessary to construct test specimens representative of the pre-Northridge era. Pre-Northridge connections utilized a less ductile weld rod metallurgy than is currently used in the US, therefore, use of weld filler metal corresponding to this earlier construction era is essential in specimen construction, to be performed by an outside fabricator familiar with construction practices prior to the Northridge Earthquake.
Results from this project will develop criteria for assessment of pre-Northridge earthquake panel zone assemblies for incorporation into ASCE 41. Further, guidance on retrofit schemes of these assembles will be developed.