79 research outputs found
Seismic Loss and Downtime Assessment of Existing Tall Steel-Framed Buildings and Strategies for Increased Resilience
In areas of high seismicity in the United States, the design of many existing tall buildings followed guidelines that do not provide an explicit understanding of performance during major earthquakes. This paper presents an assessment of the seismic performance of existing tall buildings and strategies for increased resilience for a case study city, San Francisco, where an archetype tall building is designed based on an inventory of the existing tall building stock. A 40-story moment-resisting frame system is selected as a representative tall building. The archetype building is rectangular in plan and represents the state of design and construction practice from the mid-1970s to the mid-1980s. Nonlinear response history analysis (NLRHA) are conducted with ground motions representative of the design earthquake hazard level defined in current building codes, with explicit consideration of near-fault directivity effects. Mean transient interstory drifts and story accelerations under the 10% in 50-year ground motion hazard range from 0.19 to 1.14% and 0.15 to 0.81 g, respectively. In order to influence decision making, performance is reported as the expected consequences in terms of direct economic losses and downtime. Furthermore, to achieve increased levels of resilience, a number of strategies are proposed including seismic improvements to structural and nonstructural systems as well as mitigation measures to minimize impeding factors. Expected direct economic losses for the archetype building are in the order of 34% of building cost and downtime estimates for functional recovery are 87 weeks. The strategies presented in this paper enable up to a 92% reduction in losses and minimize downtime for functional recovery to 1 day or less
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 “benchmark” 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
building’s 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 “fiber” models and “plastic hinge” 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
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
Seismic Loss and Downtime Estimates of Existing Tall Buildings and Strategies for Increased Resilience
Tall buildings play an important role in the socio-economic activity of major metropolitan areas. The
resilience of these structures is critical to ensure a successful recovery after major disasters. Events
such as the Canterbury earthquake in 2011 have highlighted the impact of poor performing buildings
on the business continuity of downtown districts, where tall buildings are typically clustered together.
Following the 2011 earthquake, Christchurch’s Central Business District (CBD) red zone covered a
significant area of the city and more than 60% of the businesses were displaced (CERC 2012).
Until the introduction of Performance Based Seismic Design (PBSD) in the 1990s, buildings
were designed using conventional building codes, which follow a prescriptive force-based approach
based on the first mode translational response of the structure (FEMA 2006). Researchers and
engineers have raised concerns that the prescriptive approach of building codes is not suitable for tall
building design due to the significant contribution of higher mode effects (PEER 2010a). As a result of
these shortcomings, several jurisdictions in areas of high seismicity throughout the Unites States (e.g.
Los Angeles and San Francisco) have adopted a PBSD approach for the design of new tall buildings.
While new designs follow a more adequate approach, little is known about the seismic performance of
older existing tall buildings that were designed prior to the adoption of PBSD (Almufti et al. 2012).
This paper presents an assessment of the seismic performance of existing tall buildings in a
case study city, San Francisco, where an archetype tall building is designed based on an inventory of
the existing tall building stock. Non-Linear Response History Analysis (NLRHA) are conducted with
ground motions representative of the design earthquake hazard level defined in current building codes,
with explicit consideration of near-fault directivity effects. In order to influence decision making,
performance is reported as the expected consequences in terms of direct economic losses and
downtime. Once the performance of the archetype building is assessed, a range of structural and nonstructural
enhancements are explored for enhanced performance as well as mitigation measures for
increased resilience. Expected direct economic losses for the archetype building are in the order of
34% of building cost and downtime estimates for functional recovery are 87 weeks. The strategies
presented in this paper enable up to a 92% reduction in losses and minimize downtime for functional
recovery to 1 day
Risk-based seismic performance assessment of existing tall steel-framed buildings in San Francisco
This study presents the results of a risk-based seismic performance assessment of an archetype tall building representative of the existing tall building stock in San Francisco, CA. The archetype tall building, selected based on an inventory of existing tall buildings, is a 40- storey Moment Resisting Frame (MRF) representative of design and construction practice from the 1970-s to the mid-1980s. A Multiple Stripe Analysis (MSA) was conducted at 8 different intensity levels ranging from frequent to very rare seismic events, i.e. from 85% to 1% probability of exceedance in 50 years. Non-Linear Response History Analyses (NLRHA) were conducted with ground motions representative of each intensity level considered. The results of the NLRHA results were used to assess the probability of earthquake losses, considering collapse potential and the probability of the building deemed irreparable due to permanent residual drifts in the structure.
Based on the MSA results, the collapse fragility of the structure, assumed to follow a lognormal cumulative distribution expressed as a function of spectral acceleration at the fundamental period of the structure (T=5 seconds), has an estimated median of 0.15g and a dispersion of 0.30. A number of loss metrics were developed for the archetype building including: a loss function, which provides the annual frequency of exceeding a certain value of loss, e.g. the expected 500 year loss equals 0.6M or 0.46% of the building replacement cost; and loss exceedance rates, e.g. a loss of 10% building replacement cost or $13.5M has an exceedance rate of 95 years. The use of these results to benchmark the performance of the archetype tall building against the design intent in current building codes and to assess the impact of structural retrofit or other building enhancements is discussed
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
AN EVALUATION OF INFRASTRUCTURE FOR TSUNAMI EVACUATION IN PADANG, WEST SUMATRA, INDONESIA
Padang, West Sumatra, Indonesia is considered to have one of the highest tsunami
risks in the world. Currently, the strategy to prepare for a tsunami in Padang is
focused on developing early warning systems, planning evacuation routes,
conducting evacuation drills, and educating the public about its tsunami risk.
Although these are all necessary efforts, they are not sufficient. Padang is located
so close to the Sunda Trench and has such flat terrain that a large portion of its
populace will not be able to reach safe ground in the interval—less than 30
minutes—between the time the earthquake shaking stops and the tsunami arrives
at the shore. It is estimated that over 100,000 inhabitants of Padang will be unable
to evacuate in that time, even if they head for safe ground immediately following
the earthquake. Given these circumstances, other means to prepare for the
expected tsunami must be developed. With this motivation, GeoHazards
International and Stanford University partnered with Indonesian organizations—
Andalas University in Padang, the Laboratory for Earth Hazards (LIPI), and the
Ministry of Marine Affairs and Fisheries (KKP)—in an effort to evaluate the need
for and feasibility of developing Padang’s tsunami evacuation infrastructure. This
project team designed and conducted a course at Stanford University, undertook
several field investigations in Padang, and participated in a reconnaissance trip
following the September 30, 2009 earthquake. The team concluded that: 1) the
tsunami-generating earthquake is still a threat, despite the recent M7.6
earthquake; 2) Padang’s tsunami evacuation capacity is currently inadequate, and
evacuation structures need to be implemented as part of an effective evacuation
plan; 3) it is likely that previous estimates of the number of people unable to
evacuate in time are grossly low; and 4) a more engineering-based approach i
Weight gain and dietary intake during pregnancy in industrialized countries - a systematic review of observational studies
Background: Gestational weight gain (GWG) above the recently recommended ranges is likely to be related to adverse pregnancy outcomes and therefore a challenge in industrialized countries. Aims: We conducted a systematic review on observational studies in order to gain more evidence on whether diets with lower caloric/protein content or other diets might be associated with lower GWG. Methods: We searched in MEDLINE and EMBASE for observational studies written in English or German reporting associations between diet and GWG in singleton pregnancies of healthy women in industrialized countries. Results: We identified 12 studies which met the inclusion criteria. Five studies suggested significant positive associations between energy intake and GWG, whereas three found no significant association. Further significant positive associations of GWG were reported with respect to protein intake, animal lipids, energy density and a number of different food servings per day, whereas intake of carbohydrates and vegetarian diet were associated with less GWG. Conclusions: We suggest that GWG might be reduced by lower energy intake in pregnancy
Exposures to phthalates and bisphenols in pregnancy and postpartum weight gain in a population-based longitudinal birth cohort
Background: Experimental evidence suggests that exposures to phthalates and bisphenols may interfere with processes related to glucose and lipid metabolism, insulin sensitivity, and body weight. Few studies have considered the possible influence of chemical exposures during pregnancy on maternal weight gain or metabolic health outcomes postpartum. Objective: To examine the associations of early and mid-pregnancy bisphenol and phthalate urine concentrations with maternal weight gain 6 years postpartum. Methods: We analyzed urine samples for bisphenol, phthalate and creatinine concentrations from early and mid-pregnancy in 1192 women in a large, population-based birth cohort in Rotterdam, the Netherlands, and examined postpartum weight gain using maternal anthropometrics before pregnancy and 6 years postpartum. We have used covariate-adjusted linear regressions to evaluate associations of early and mid-pregnancy bisphenols and phthalate metabolites with weight change. Mediator and interaction models have been used to assess the role of gestational weight gain and breastfeeding, respectively. Sensitivity analysis is performed among women without subsequent pregnancies. Results: Among all 1192 mothers included in the analysis, each log unit increase in the average bisphenol A and all assessed phthalate groupings were associated with increased maternal weight gain. As a proxy for phthalate exposure, each log unit increase in averaged phthalic acid was associated with 734 g weight gain (95% CI 273–1196 g) between pre-pregnancy and 6 years postpartum. Mediation by gestational weight gain was not present. Breastfeeding and ethnicity did not modify the effects. Stratification revealed these associations to be strongest among overweight and obese women. Among women without subsequent pregnancies (n = 373) associations of bisphenols, HMW phthalate metabolites and di-2-ethylhexylphthalate metabolites attenuated. For phthalic acid, LMW phthalate metabolites and di-n-octylphthalate metabolites associations increased. Similarly to the whole group, stratification yielded significant results among overweight and obese women. Discussion: In a large population-based birth cohort, early and mid-pregnancy phthalate exposures are associated with weight gain 6 years postpartum, particularly among overweight and obese women. These data support ongoing action to replace phthalates with safer alternatives
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