Seismic Assessment and Retrofit of Pre-Northridge Earthquake Panel Zones in Steel Moment Frames

Summary

Steel moment frame design prior to the 1994 Northridge Earthquake (1985-1994 era) favored 1) panel zones that would yield before the adjacent beams and 2) column splices made with partial joint penetration (PJP) welds. The welding requirements used during this era were not as stringent as modern requirements developed after the Northridge Earthquake. Consequently, pre-1994 welds are susceptible to brittle fracture, as was demonstrated in failures of beam-to-column connections during the Northridge earthquake. Weld fracture in structural components can result in undesirable behavior of the moment frame during a seismic event. First, a weak panel zone mechanism may result in excessive shear deformations within the panel zone and significant column flange bending at the corners of the panel zone resulting in high strains at the welds connecting the beam flange to the column flange.  Second, similar issues with weld materials and construction practices that put beam-to-column connections at risk in the Northridge earthquake also put welded column splices at risk for weld fracture. Neither of these issues have been comprehensively investigated nor associated modeling guidance and performance metrics developed.  

The first stage of this project is to issue a task order to the new IDIQ contractor supporting 2EG to convene a group of steel and welding experts to develop a comprehensive research plan. This research plan will outline the requirements for conducting research into the performance of these pre-1980 frames. The plan will likely include a combination of experiments and high-end analyses to develop an understanding of the behavior of pre-Northridge earthquake weak panel zone assemblies and column splices with PJP welds. High-end nonlinear finite element analysis software will be used develop 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. Results from this project will develop new, state-of-the-art criteria for modeling and assessment of existing pre-Northridge earthquake weak panel zone assemblies and welded column splices. Moreover, repair options to strengthen these systems also will be investigated. Developed criteria will focus on incorporation into ASCE 41 and also provide guidance on retrofit solutions of these components.

Description

Objective - This project will assess the seismic performance of pre-Northridge earthquake weak panel zone assemblies and column splices made with partial joint penetration (PJP) welds in steel moment frames. These two structural components are combined in this project because they employed the same welding requirements and materials. 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. Similarly, strain demands on welded column splices with PJP welds may trigger weld fracture. These fracture mechanisms can inhibit the intended ductility capacity of the panel zone assembly and/or the adjacent beam-to-column connection required to dissipate energy. Results from this project will develop criteria for modeling and assessment of pre-Northridge earthquake weak panel zone assemblies and column splices made with PJP welds for incorporation into ASCE 41 and provide guidance on retrofit schemes of these assembles to improve 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. It was shown during the SAC project (see FEMA 355D (FEMA 2000)) that weak panel zones trend towards low levels of plastic rotation, which implies reduced overall ductility. A primary cause of this reduced capacity is that a weak panel zone mechanism may result in excessive shear deformations within the panel zone 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, these weld fractures can preclude the formation of plastic hinges in the beams which is contrary to the “balanced yield” approach adopted for assessment of existing steel moment frames in ASCE 41.

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 strength of the panel zone in reference to beam strength, 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.

Additionally, pre-1994 era moment frames also employed column splices between floors that utilized partial joint penetration (PJP) welds. The depth of penetration within the flanges depends on the era and regional construction practices and standards. There is limited information available to the practicing community regarding modeling approaches and assessment criteria for evaluating these PJP welded column splices. Bruneau and Mahin (1990, 1991) investigated the failure modes of welded column splices and reported unsatisfactory response of splices using PJP welds, which failed in a startling brittle manner when tested under pure bending; splices with CJP welds were noted to have performed satisfactorily. The tests highlighted the poor weld requirements utilized in this era. 

Through the combined use of experiments and high-level analyses, this project will develop a methodology to model and assess a weak panel zone and column splice that uses PJP welds in existing steel moment frames. Means to repair and strengthen these elements also will be examined. The methodology will be aligned with criteria in ASCE 41-17. Additionally, guidance will be provided as to methods to retrofit a weak panel zone subassembly and column welded splice that do not satisfy the newly developed criteria.

What is the research plan?  The first stage of this project is to issue a task order to the new IDIQ contractor supporting 2EG to convene a group of experts to outline the needed test setups and welding processes to support the development of a comprehensive research plan. The group will consist of highly-qualified practitioners and academic researchers who specialize in the evaluation of steel moment frames and welding processes. This research plan will outline the requirements for the first large research task order in the upcoming new IDIQ contract. The report will detail a combination of experiments and high-end analyses used to develop an understanding of the behavior of pre-Northridge earthquake panel zone assemblies and PJP welded column splices. Testing will not be lab-dependent so that work can commence at any laboratory capable of providing the force and deformation demands. 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 U.S. 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 U.S., 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. The details of this work will be contained in the task order SoW provided to the IDIQ contractor. 

The existing IDIQ contract has expired and a new contract will be awarded through a competitive RFP. This process will be completed in FY2020 with the work described here as the subject of the first task order under the new IDIQ.