Robert Tremblay

Inelastic seismic response of steel bracing members

Abstract A survey of past experimental studies on the inelastic response of diagonal steel bracing members subjected to cyclic inelastic loading was carried out to collect data for the seismic design of concentrically braced steel frames for which a ductile response is required under earthquakes. The parameters that were examined are the buckling strength of the bracing members, the brace post-buckling compressive resistance at various ductility levels, the brace maximum tensile strength including strain hardening effects, and the lateral deformations of the braces upon buckling. Equations are proposed for each of these parameters. In addition, the maximum ductility that can be achieved by rectangular hollow bracing members is examined.

Residual Drift Response of SMRFs and BRB Frames in Steel Buildings Designed according to ASCE 7-05

A recent study has shown that residual drifts after earthquakes that are greater than 0.5% in buildings may represent a complete loss of the structure from an economic perspective. To study the comparative residual drift response of special moment-resisting frames (SMRFs) and buckling-restrained braced (BRB) frames, buildings between 2 and 12 stories in height are designed according to ASCE 7-05 and investigated numerically. This investigation includes pushover analyses as well as two-dimensional nonlinear time-history analyses for two ground motion hazard levels. The two systems show similar peak drifts and drift concentration factors. The BRB frames experience larger residual drifts than the SMRFs; however, the scatter in the residual drift results is large. Expressions are proposed to estimate the residual drifts of these systems as a function of the expected peak drifts, the initial recoverable elastic drift, and the drift concentration factor of each system. When subjected to a second identical earth...

Comparative study of the inelastic seismic demand of eastern and Western Canadian sites

The damage potential of earthquake ground motions compatible with site-specific 2% in 50 year uniform hazard spectra is compared at two North American sites in areas of moderate seismic hazard: Montreal, in the east, and Vancouver, along the west coast. For Vancouver, crustal, subcrustal and Cascadia subduction ground motion earthquake ensembles are considered. Nonlinear dynamic analyses of bi-linear single-degree-of-freedom oscillators exhibiting various ductility levels and damage laws were performed to determine R factors required to prevent structural collapse for each site and each system. Then, inelastic response parameters were computed for the general design case, wherein a prescribed R factor is used for a given system irrespective of tectonic region or structural period. The results show that the R factors vary with the ductility level, the damage law, the structural period, and the tectonic region. Neglecting the latter two dependencies in design, as is current practice, may lead to significant discrepancies in the level of protection achieved for different structures in different regions.

Experimental Validation of Replaceable Shear Links for Eccentrically Braced Steel Frames

In the current design of steel eccentrically braced frames (EBFs), the yielding link is coupled with the floor beam. This often results in oversized link elements, which leads to overdesigned structures and foundations. In addition, the beams are expected to sustain significant damage through repeated inelastic deformations under design-level earthquakes, and thus the structure may require extensive repair or replacement. These drawbacks can be mitigated by designing EBFs with replaceable shear links. Two different replaceable link types with alternate section profiles, connection configurations, welding details, and intermediate stiffener spacings were tested. A total of 13 cyclic quasi-static full-scale cyclic tests were performed, including tests on EBFs with replaceable shear links, to study their inelastic seismic performance. The links exhibited a very good ductile behavior, developing stable and repeatable yielding. Additional inelastic rotation capacity can be achieved with bolted replaceable link...

Evolution of seismic design provisions in the National building code of Canada

The purpose of this paper is to provide a summary of the evolution of seismic design in Canada. This paper presents the significant changes to the approach taken in determining seismic hazards and seismic hazard maps, and describes the evolution of the seismic design provisions of the National building code of Canada. The introduction of important parameters in determining the seismic base shear such as the period of vibration of the structure, the influence of type of soil, and the concepts of ductility and energy dissipation capacity of elements and structures are presented. The levels of seismic design base shears, determined from different versions of the National Building Code of Canada, are compared for reinforced concrete frame and wall structures to illustrate the changes.

Performance evaluation of passive damping systems for the seismic retrofit of steel moment-resisting frames subjected to near-field ground motions

Passive friction and viscous damping systems for retrofitting steel moment-resisting frames located along the west coast of the United States are considered. First, preliminary design procedures are presented for friction as well as linear and nonlinear viscous damping systems. Thereafter, nonlinear dynamic analyses are performed on a six-story moment-resisting frame designed according to seismic provisions for California prior to the 1994 Northridge earthquake. A flexural strength degradation model is considered to account for the brittle behavior of pre-Northridge welded beam-to-column connections. The structure was subjected to three different earthquake ensembles including near-field records developed for major crustal earthquakes in California. The results of a parametric study indicate that, although both friction and viscous damping systems reduce significantly the response of the structure, they are unable by themselves to prevent fracture of welded beam-to-column joints. Connection retrofit measures of the types elaborated after the Northridge earthquake would still be required.

Shake Table Testing of Slender RC Shear Walls Subjected to Eastern North America Seismic Ground Motions

AbstractThis paper presents shake table test results on two identical 1:0.429 scaled, 8-story moderately ductile RC shear wall specimens under the expected high-frequency ground motion in eastern North America. The walls were designed and detailed according to the seismic provisions of the NBCC 2005 and CSA-A23.3-04 standards. The objectives were to validate and understand the inelastic responses and interaction of shear and flexure and axial loads in the plastic hinge zones of the walls taking into consideration the higher-mode effects. One specimen was tested under incremental ground motion intensities ranging from 40 to 120% of the design level. The intensity range was increased from 100 to 200% for the second specimen. The response of the walls was significantly affected by the second mode, causing an inelastic flexural response to develop at the base as well as at the sixth level. Dynamic amplification of the base shear forces was also observed in both walls. In the second wall, which was tested in t...

SEISMIC IMPACT LOADING IN INELASTIC TENSION‐ONLY CONCENTRICALLY BRACED STEEL FRAMES: MYTH OR REALITY?

Tension-Only Concentrically Braced Frames (TOCBF) exhibit deteriorating pinched hysteretic behaviour during strong earthquakes. Slender braces transit between an elastic buckling state, a restraightening state, in which they carry almost no load, an elastic tensile loading state as they are suddenly taut and, finally, a tensile yielding state. It has long been suspected that the sudden increase in tensile forces in the braces of TOCBSF creates detrimental impact loading on the connections and other structural elements. No experimental evidence, however, has been provided so far to confirm, or to quantify, this impact phenomenon. This paper addresses this issue through shake table tests of half scale, two-storey, TOCBF models. By normalizing the hysteresis loops of braces obtained from shake table tests to the yield strength of steel obtained from quasi-static tests, the increase in tensile forces in the braces was obtained. Results of dynamic tensile tests on steel coupons under similar strain rates as observed during the shake table tests showed that this increase in tensile forces is not the result of impact, but is rather caused by a yield strength increase of the steel under high strain rate. A procedure is proposed to estimate and account for this increase in tensile forces in the braces at the design stage.

Modeling and Testing Influence of Scaling Effects on Inelastic Response of Shear Walls

Monotonic and cyclic quasi-static testing was performed on ductile reinforced concrete shear-wall specimens designed and detailed according to the seismic provisions of the National Building Code of Canada and CSA-A23.3-04. The tests were carried out on full-scale and 1:2.37 reduced-scale wall specimens. The behavior under cyclic loading was characterized by ductile flexural response up to a displacement ductility of 4.0. At this deformation level, inelastic shear deformations in the plastic hinge contributed to approximately 20% of the total deformation. In the subsequent cycles, strength degradation took place due to shear sliding developing along the large flexural cracks at the wall base. Shear sliding was not observed under monotonic loading and the specimens exhibited significantly higher ductility capacity. Excellent agreement was found between prototype and reduced-scale walls. The inelastic response and failure mode observed under cyclic loading could be adequately reproduced using a finite element analysis program. Simpler models with frame elements and lumped plastic hinges could capture the wall flexural response well, but shear deformations could not be reproduced.

Seismic Design and Performance of Steel Moment-Resisting Frames with Nonlinear Replaceable Links

Although moment resisting frames (MRFs) designed according to the latest seismic codes can provide life safety during a design level earthquake, they are expected to sustain significant damage at flexural yielding locations in the beams. The design of the beams for strength and drift control considerations are also interlinked, often resulting in overdesign of other elements, such as diaphragms and foundations. These drawbacks can be mitigated by introducing replaceable links at the locations of expected inelastic action. A five-story prototype MRF building with replaceable links in a high-seismic zone was designed. Four full-scale beam-to-column subassemblages representing the first floor exterior connections in the prototype building were experimentally evaluated. The results demonstrated that MRFs with replaceable flexural links can provide strength and ductility equivalent to existing MRFs while minimizing the effect of the added links on the elastic stiffness of the system. Finite-element models were...