Improving the collapse risk of steel structures with high-performance steel

With the advent of material science, high-performance steel materials have been developed for seismic applications that potentially reduce the earthquake-induced collapse risk of steel frame buildings. The High Yield Point (HYP400) steel is one of those materials that is developed based on thermomechanical control process. Both its yield strength and notch toughness is enhanced compared to conventional seismic-resistant steels. This is achieved through refinement of the microstructural grain size. The application of such material in steel columns facilitates economic design and enhances the collapse capacity of steel moment resisting frames during severe seismic events. This paper first discusses the key aspects of the HYP400 steel and illustrates its potential through detailed nonlinear analyses and earthquake-induced collapse simulations of a steel frame building. The material cyclic hardening prior to the onset of local buckling is characterized on the basis of uniaxial cyclic coupon tests. Finite element studies are then conducted to investigate the pre- and post-buckling behavior of hollow structural section steel columns made of HYP400 steel under various loading histories. A versatile fiber-based deterioration model is then utilized that captures the steel material potential and facilitates computationally efficient collapse simulations. The effectiveness of the HYP400 steel for enhancing the collapse capacity of a steel moment resisting frame is demonstrated through incremental dynamic analysis of a case study steel moment-resisting frame.

Welded moment connections with highly dissipative panel zones for enhanced seismic performance of steel moment frames

Andronikos Skiadopoulos

Research conducted after the 1994 Northridge earthquake in the U.S. and the 1995 Kobe earthquake in Japan led to the development of today's pre-qualified beam-to-column connections for capacity-designed steel moment resisting frames (MRFs). Welded moment connections feature weld backing bars that should be removed after the execution of complete joint penetration groove welds at the bottom beam flange-to-column flange joint. On the other hand, advancements in steel and weld materials, as well as fabrication techniques, allow for the exploitation of simplified weld details at this location. In welded moment connections for seismic applications, the participation of the beam-to-column web panel zones in the energy dissipation is generally limited during an earthquake event. In such a design context, beam local buckling is likely even at modest lateral drift demands, thereby engendering structural repair costs in the aftermath of earthquakes. The primary reasons for such a seismic design practice are twofold. The first one relates to known limitations of panel zone design models that compromise our ability to effectively balance the seismic design of fully restrained beam-to-column connections. The second one relates to the panel zone kinking that could increase the fracture potential of beam-to-column connections. However, recent experiments in the literature provide controversial results regarding the same matter.This doctoral thesis aims at advancing the state-of-knowledge regarding the seismic design and behavior of steel MRFs with highly dissipative panel zones. A new panel zone design model was first developed. This model addresses the limitations of all available panel zone models in the literature. The model was validated thoroughly with about 100 available experiments that encompass a broad range of geometric parameters. The panel zone model can effectively enable a balanced seismic design of welded moment connections with inelastic panel zones. One step further, this thesis revisited the current detailing of welded connections. Simplifications in their fabrication process were proposed by intentionally keeping a customized beveled backing bar in place, without impairing the connection's ductility under cyclic loading. The proposed connection weld detail is substantiated by continuum finite element analyses and full-scale experiments on welded moment connections with highly dissipative panel zones. It is demonstrated that, contrary to the current design paradigm, a stable hysteretic response is achieved up until lateral drift demands of at least 7% rad, thereby diminishing the cyclic deterioration in story shear resistance. As such, the seismic stability of steel MRFs is only governed by global P-Delta effects. Quantitative seismic response characteristics of steel MRFs with highly dissipative panel zones through large-scale systel-level parametric studies were also provided. It is shown that steel MRFs with highly inelastic panel zones, have up to two times lower mean annual frequency of collapse than corresponding results with steel MRFs designed with the current status quo. It is also demonstrated that steel MRFs with inelastic panel zones, enjoy up to 50% reduction in residual story drift ratios at a design-basis earthquake; their beam-to-column connections do not experience fractures due to panel zone kinking; and local buckling in steel beams is very limited even at low probability of occurrence seismic events.

EPFL2022

Earthquake-induced loss assessment of steel frame buildings with special moment frames designed in highly seismic regions

Seong-Hoon Hwang, Dimitrios Lignos

This paper discusses an analytical study that quantifies the expected earthquake-induced losses in typical office steel frame buildings designed with perimeter special moment frames in highly seismic regions. It is shown that for seismic events associated with low probabilities of occurrence, losses due to demolition and collapse may be significantly overestimated when the expected loss computations are based on analytical models that ignore the composite beam effects and the interior gravity framing system of a steel frame building. For frequently occurring seismic events building losses are dominated by non-structural content repairs. In this case, the choice of the analytical model representation of the steel frame building becomes less important. Losses due to demolition and collapse in steel frame buildings with special moment frames designed with strong-column/weak-beam ratio larger than 2.0 are reduced by a factor of two compared with those in the same frames designed with a strong-column/weak-beam ratio larger than 1.0 as recommended in ANSI/AISC-341-10. The expected annual losses (EALs) of steel frame buildings with SMFs vary from 0.38% to 0.74% over the building life expectancy. The EALs are dominated by repairs of acceleration-sensitive non-structural content followed by repairs of drift-sensitive non-structural components. It is found that the effect of strong-column/weak-beam ratio on EALs is negligible. This is not the case when the present value of life-cycle costs is selected as a loss-metric. It is advisable to employ a combination of loss-metrics to assess the earthquake-induced losses in steel frame buildings with special moment frames depending on the seismic performance level of interest.

Wiley-Blackwell2017

Influence of embedded steel column base strength on earthquake-induced residual deformations

Hiroyuki Inamasu, Dimitrios Lignos

Recent experiments on steel columns under multi-axis cyclic loading suggest that a steel column may experience appreciable axial shortening due to local buckling. Reconnaissance reports from recent earthquakes indicate that a number of embedded column base (ECB) connections in steel frame buildings were found to be lightly damaged although they are expected to remain elastic. As a result, the amount of inelastic damage concentrated in the respective steel columns was much less than it was expected to be. This paper investigates the influence of the ECB strength on the residual deformations of first story interior steel columns in steel moment resisting frames (MRFs) under multi-axial cyclic loading. The evaluation is conducted with rigorous finite element (FE) simulations. The developed FE model is able to simulate the pinching hysteretic behavior of typical ECBs. The parametric study involves two main cases. In the first one the ECB remains in elastic. In the second case the inelastic behavior is spread to the ECB and the steel column itself. The simulation results suggest that the resultant plastic deformation within the steel column is much smaller in the latter. The onset of column local buckling is delayed; therefore, the residual column axial shortening becomes fairly minimal. In particular, it is reduced by 50% at lateral drift demands associated with low-probability of occurrence earthquakes.

2018

Welded moment connections with highly dissipative panel zones for enhanced seismic performance of steel moment frames

Andronikos Skiadopoulos

EPFL2022