19 research outputs found
Fracture Investigation of Welded Cruciform Connections
As one of the main failure modes of steel structures, fracture in welded connections has widely been discussed based on experimental investigations and numerical simulations. However, the mechanical properties of the weld and Heat Affected Zone (HAZ), such as stress-strain relationships and fracture strains under various stress states, have rarely been considered in these analyses. Therefore, in this paper, the fracture process of welded connections is discussed to investigate the effects of the inhomogeneity of mechanical properties in the weld zone. Tensile tests are conducted on welded cruciform specimens fabricated using 8 mm or 12 mm fillet welds and finite element models are developed by considering or ignoring the material inhomogeneity in the weld zone. The simulation results are compared with the experimental and it is concluded that the assumption of homogenous properties within the weld zone using the properties of the base metal will underestimate the strength of the welded cruciform specimens and using the mechanical properties of the three material areas in the weld zone will increase the accuracy of the simulation results. Using the free parameters calibrated by the fracture strains of the three material areas, the fracture process of the welded cruciform specimens is simulated using the fracture model LMVGM, and the comparison shows that the mechanical properties of the weld and HAZ should be included in the investigation of fracture in welded connections to obtain reliable simulation results
An Assessment to Benchmark the Seismic Performance of a Code-Conforming Reinforced-Concrete Moment-Frame Building
This report describes a state-of-the-art performance-based earthquake engineering methodology
that is used to assess the seismic performance of a four-story reinforced concrete (RC) office
building that is generally representative of low-rise office buildings constructed in highly seismic
regions of California. This âbenchmarkâ building is considered to be located at a site in the Los
Angeles basin, and it was designed with a ductile RC special moment-resisting frame as its
seismic lateral system that was designed according to modern building codes and standards. The
buildingâs performance is quantified in terms of structural behavior up to collapse, structural and
nonstructural damage and associated repair costs, and the risk of fatalities and their associated
economic costs. To account for different building configurations that may be designed in
practice to meet requirements of building size and use, eight structural design alternatives are
used in the performance assessments.
Our performance assessments account for important sources of uncertainty in the ground
motion hazard, the structural response, structural and nonstructural damage, repair costs, and
life-safety risk. The ground motion hazard characterization employs a site-specific probabilistic
seismic hazard analysis and the evaluation of controlling seismic sources (through
disaggregation) at seven ground motion levels (encompassing return periods ranging from 7 to
2475 years). Innovative procedures for ground motion selection and scaling are used to develop
acceleration time history suites corresponding to each of the seven ground motion levels.
Structural modeling utilizes both âfiberâ models and âplastic hingeâ models. Structural
modeling uncertainties are investigated through comparison of these two modeling approaches,
and through variations in structural component modeling parameters (stiffness, deformation
capacity, degradation, etc.). Structural and nonstructural damage (fragility) models are based on
a combination of test data, observations from post-earthquake reconnaissance, and expert
opinion. Structural damage and repair costs are modeled for the RC beams, columns, and slabcolumn connections. Damage and associated repair costs are considered for some nonstructural
building components, including wallboard partitions, interior paint, exterior glazing, ceilings,
sprinkler systems, and elevators. The risk of casualties and the associated economic costs are
evaluated based on the risk of structural collapse, combined with recent models on earthquake
fatalities in collapsed buildings and accepted economic modeling guidelines for the value of
human life in loss and cost-benefit studies.
The principal results of this work pertain to the building collapse risk, damage and repair
cost, and life-safety risk. These are discussed successively as follows.
When accounting for uncertainties in structural modeling and record-to-record variability
(i.e., conditional on a specified ground shaking intensity), the structural collapse probabilities of
the various designs range from 2% to 7% for earthquake ground motions that have a 2%
probability of exceedance in 50 years (2475 years return period). When integrated with the
ground motion hazard for the southern California site, the collapse probabilities result in mean
annual frequencies of collapse in the range of [0.4 to 1.4]x10
-4
for the various benchmark
building designs. In the development of these results, we made the following observations that
are expected to be broadly applicable:
(1) The ground motions selected for performance simulations must consider spectral
shape (e.g., through use of the epsilon parameter) and should appropriately account for
correlations between motions in both horizontal directions;
(2) Lower-bound component models, which are commonly used in performance-based
assessment procedures such as FEMA 356, can significantly bias collapse analysis results; it is
more appropriate to use median component behavior, including all aspects of the component
model (strength, stiffness, deformation capacity, cyclic deterioration, etc.);
(3) Structural modeling uncertainties related to component deformation capacity and
post-peak degrading stiffness can impact the variability of calculated collapse probabilities and
mean annual rates to a similar degree as record-to-record variability of ground motions.
Therefore, including the effects of such structural modeling uncertainties significantly increases
the mean annual collapse rates. We found this increase to be roughly four to eight times relative
to rates evaluated for the median structural model;
(4) Nonlinear response analyses revealed at least six distinct collapse mechanisms, the
most common of which was a story mechanism in the third story (differing from the multi-story
mechanism predicted by nonlinear static pushover analysis);
(5) Soil-foundation-structure interaction effects did not significantly affect the structural
response, which was expected given the relatively flexible superstructure and stiff soils.
The potential for financial loss is considerable. Overall, the calculated expected annual
losses (EAL) are in the range of 97,000 for the various code-conforming benchmark
building designs, or roughly 1% of the replacement cost of the building (3.5M, the fatality rate translates to an EAL due to
fatalities of 5,600 for the code-conforming designs, and 66,000, the monetary value associated with life loss is small,
suggesting that the governing factor in this respect will be the maximum permissible life-safety
risk deemed by the public (or its representative government) to be appropriate for buildings.
Although the focus of this report is on one specific building, it can be used as a reference
for other types of structures. This report is organized in such a way that the individual core
chapters (4, 5, and 6) can be read independently. Chapter 1 provides background on the
performance-based earthquake engineering (PBEE) approach. Chapter 2 presents the
implementation of the PBEE methodology of the PEER framework, as applied to the benchmark
building. Chapter 3 sets the stage for the choices of location and basic structural design. The subsequent core chapters focus on the hazard analysis (Chapter 4), the structural analysis
(Chapter 5), and the damage and loss analyses (Chapter 6). Although the report is self-contained,
readers interested in additional details can find them in the appendices
Strengths and Fracture Strains of Weld and HAZ in Welded Connections
This paper investigates the strengths and fracture strains of weld and heat affected zone (HAZ) in welded connections for both the longitudinal and transverse directions and compares them to those of the base metal. A series of miniature coupons, including miniature flat plates, notched round bars and grooved plates, were extracted from the three zones of a butt weld and tested using a custom-built jig. The true stress-strain relationships and fracture strains of the base metal, weld and HAZ materials were obtained for both directions from the miniature coupon tests and corresponding numerical simulations. The fracture strain data were used to calibrate the Lode angle modified void growth model (LMVGM) for predicting the fracture strain of the three material zones at any given stress state. The following major conclusions were drawn: (1) The weld was generally isotropic in terms of both strength and fracture strain. The weld also had the highest values of yield and tensile strengths among the three materials in both directions, but the lowest fracture strain in both directions except for the longitudinal direction with stress triaxiality above 0.21 to 0.30, for which the base metal had the lowest fracture strain. (2) The HAZ had higher yield and tensile strengths but smaller fracture strain in the longitudinal direction than in the transverse direction. The same anisotropic characteristic applied to the base metal. Meanwhile, the HAZ had higher yield and tensile strengths than the base metal as well as similar but slightly larger fracture strains in both directions. (3) The yield and tensile strengths of the weld and HAZ can be approximated using the empirical hardness-strength correlation functions, except that the functions tend to overestimate the strengths of the weld by about 10%. (4) For the weld, HAZ and base metal, the fracture surfaces tilted towards stress states with high stress triaxiality and low Lode angle parameter, indicating that fracture can initiate more easily at these stress states. Note that the above conclusions are limited to the tested AS350 grade steel and the selected welding parameters
Stress-resultant plasticity for frame structures
Two versions of a bounding surface plasticity model implemented in stress-resultant space and applicable to the analysis of steel, reinforced concrete, or composite beam-columns are discussed. One is a two-surface model appropriate for steel members with a finite elastic region. The second, which is developed for reinforced concrete and composite steel-concrete members, employs a single outer bounding surface with an infinitely small loading surface that is degenerated to a point. Plasticity-based assumptions employed in the formulation of these models are reviewed and predicted plastic flow directions are evaluated against data from more fundamental fiber-type analyses of the beam-column cross sections. Results of these comparisons support the use of Mroz\u27s kinematic rule in stress-resultant space and lead to recommended improvements in the bounding surface formulation
Strength And Ductility Of Concrete Encased Composite Columns
Concrete encased composite column design provisions of the American Concrete Institute Code (ACI 318), AISC-LRFD Specification, and the AISC Seismic Provisions are reviewed and evaluated based on fiber section analyses that account for the inelastic behavior of steel and concrete, including the effects of strength and confinement on the concrete\u27s stress-strain properties. Trial column designs are analyzed to evaluate their strength and ductility as a function of the ratio of structural steel to gross column area, the nominal compression strength of concrete, and confinement of concrete by seismic hoop reinforcing. The analyses highlight known differences in the calculated nominal strength requirements between the ACI 318 and AISC-LRFD provisions and suggest a review of criteria used to establish the limits of the provisions. Compared to columns with low-to medium-strength concrete, columns with high-strength concrete (fâČc = 110 MPa) are shown to rely to a greater degree on hoop reinforcement to provide the necessary ductility for seismic design
Inelastic models for composite moment connections in RCS frames
An analysis model for joints between reinforced concrete columns and steel beams is presented. The inelastic relations account for stiffness degradation and pinching behavior associated with different force transfer mechanisms in the joint. The model is validated by comparison to test results, and is implemented into a computer program for performing inelastic static and dynamic analyses of three-dimensional mixed steel-concrete frames
Inelastic analyses of a 17-story steel framed building damaged during Northridge
A series of two- and three-dimensional static and dynamic inelastic frame analyses are performed for a 17-story steel moment frame building damaged by the 1994 Northridge earthquake. The primary objectives of the study are to: (1) exercise state-of-the-art inelastic static and dynamic analyses for the evaluation and design of steel buildings; (2) establish to what degree frame analyses can be used to predict the types of brittle connection damage that occurred during the Northridge earthquake; and (3) investigate the reliability of the analyses and the influence of modeling parameters on computed performance indices. In general, this study shows that calculated interstory drift ratios and curvature demands obtained from inelastic time history analyses correlate reasonably well with the pattern of connection damage observed in the building. However, there is significant scatter in the computed deformation demands that are strongly dependent on the degree to which three-dimensional torsion, secondary structural elements and strength/stiffness degradation (associated with connection fractures) are modeled in the analyses. Further, comparisons of static and dynamic analyses indicate that for this building static pushover analyses do not capture higher vibration modes that are significant. © 1997 Elsevier Science Ltd
Illustrating a Bayesian approach to seismic collapse risk assessment
In this study, we present a Bayesian method for efficient collapse response assessment
of structures. The method facilitates integration of prior information on collapse response with data
from nonlinear structural analyses in a Bayesian setting to provide a more informed estimate of the
collapse risk. The prior information on collapse can be obtained from a variety of sources, including
information on the building design criteria and simplified linear dynamic analysis or nonlinear static
(pushover) analysis. The proposed method is illustrated on a four-story reinforced concrete moment
frame building to assess its seismic collapse risk. The method is observed to significantly improve the
statistical and computational efficiency of collapse risk predictions compared to alternative methods.Non UBCUnreviewedThis collection contains the proceedings of ICASP12, the 12th International Conference on Applications of Statistics and Probability in Civil Engineering held in Vancouver, Canada on July 12-15, 2015. Abstracts were peer-reviewed and authors of accepted abstracts were invited to submit full papers. Also full papers were peer reviewed. The editor for this collection is Professor Terje Haukaas, Department of Civil Engineering, UBC Vancouver.Facult
Comparison of Seismic Performance & Recovery Metrics for a 1970s vs Modern Tall Steel Moment Frame Building
This study benchmarks the performance of older existing tall steel moment resisting frame (MRF) buildings designed following historic code-prescriptive requirements (1973 Uniform Building Code) against modern design standards (2012 International Building Code). The comparison is based on risk-based assessments of alternative designs of a 50-story archetype office building, located at a site in San Francisco, CA. The following metrics are compared: (i) mean annual rate of collapse, λc (ii) average annual loss (AAL), and (iii) average annual downtime (AAD). The mean annual frequency of collapse of the the 1973 archetype building is 28 times greater than the equivalent 2012 building (28·10â⎠versus 1·10ââŽ), or approximately 13% versus 0.5% probability of collapse in 50 years. The expected AAL is 65% higher for the 1973 than the 2012 building (0.66% versus 0.40% of building replacement cost); and the AAD to re-occupancy is 72% greater for the 1973 than the 2012 building (8.1 vs 4.7 days). The AAD to functional recovery for the 1973 building is twice that of the 2012 building (10.4 vs 5.0 days). An evaluation of the results at various earthquake ground motion shaking intensities suggests that existing 1970s tall steel moment frames are far from complying with modern design requirements in terms of both collapse safety under extreme ground motions and damage control in small to moderate magnitude earthquakes. Furthermore, while modern building code requirements provide acceptable seismic collapse safety, they do not ensure a level of damage control to assure a swift recovery after a damaging earthquake.Applied Science, Faculty ofNon UBCCivil Engineering, Department ofReviewedFacult