comparison of industry-standard nonlinear dynamic …
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Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE
COMPARISON OF INDUSTRY-STANDARD
NONLINEAR DYNAMIC ANALYSIS
METHODS WITH OBSERVED DAMAGE
ON A RC BUILDING
A. Kozmidis1, M. Melek
2, L. Massone
3 and K. Orakcal
4
ABSTRACT
Observations from recent earthquakes such as Chile (2010) and Christchurch (2011) showed
vulnerabilities of reinforced concrete buildings with bearing wall lateral force resisting systems.
The observed failure patterns included not only web crushing due to shear demand but also
flexural failures followed by the buckling and/or fracture of longitudinal wall reinforcement and
concrete spalling and crushing. The observed vulnerabilities in modern buildings have raised an
intriguing question: “Are the analysis methodologies currently used by the engineering
community able to predict the response of shear wall buildings with acceptable level of
accuracy?” In order to provide an answer for this question, a building in Chile, which has
experienced damage during the 2010 M8.8 Maule Earthquake, was assessed using industry
standard nonlinear analysis methodologies. Comparisons of the analysis results with the
observed damage patterns were conducted and acceptability of these techniques was investigated
to provide a better understanding of the shortcomings of currently used industry standard
analysis and design methodologies. Results have shown inconsistencies in the observed damage
versus nonlinear time history analysis results specifically in regard to the location of damage and
shear failure observations. Better representation of shear-flexure interaction effects, attenuation
relationships and soil structure interaction is required for further studies on the topic.
1Senior Engineer, Arup, Los Angeles USA
2 Consultant, Oklahoma City USA
3 Associate Professor, University of Chile, Department of Civil Engineering , Santiago Chile
4 Associate Professor, Bogazici University, Department of Civil Engineering, Istanbul Turkey
Kozmidis A, Melek M, Massone L, Orakcal K. Comparison of industry-standard nonlinear dynamic analysis
methods with observed damage on a RC building. Proceedings of the 10th
National Conference in Earthquake
Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
Comparison of Industry-Standard Nonlinear Dynamic Analysis Methods
With Observed Damage on a RC Building
A. Kozmidis1, M. Melek
2, L. Massone
3 and K. Orakcal
4
ABSTRACT Observations from recent earthquakes such as Chile (2010) and Christchurch (2011) showed
vulnerabilities of reinforced concrete buildings with bearing wall lateral force resisting systems.
The observed failure patterns included not only web crushing due to shear demand but also
flexural failures followed by the buckling and/or fracture of longitudinal wall reinforcement and
concrete spalling and crushing. The observed vulnerabilities in modern buildings have raised an
intriguing question: “Are the analysis methodologies currently used by the engineering community
able to predict the response of shear wall buildings with acceptable level of accuracy?” In order to
provide an answer for this question, a building in Chile, which has experienced damage during the
2010 M8.8 Maule Earthquake, was assessed using industry standard nonlinear analysis
methodologies. Comparisons of the analysis results with the observed damage patterns were
conducted and acceptability of these techniques was investigated to provide a better understanding
of the shortcomings of currently used industry standard analysis and design methodologies.
Results have shown inconsistencies in the observed damage versus nonlinear time history analysis
results specifically in regard to the location of damage and shear failure observations. Better
representation of shear-flexure interaction effects, attenuation relationships and soil structure
interaction is required for further studies on the topic.
Introduction
For more than half a century, typical mid to high-rise residential and commercial buildings in
Chile has been constructed using reinforced concrete bearing wall systems. Experiences from
four major earthquakes during this time frame (Valdivia M9.5 1960, Valparaiso M7.5 1971,
Valparaiso M7.8 and Maule M8.8 2010) have shaped the construction and design practice of the
Chilean engineering community. Following the 1985 Valparaiso Earthquake, the 1983 version of
the ACI requirements for reinforced concrete (ACI 318-83, 1983) was adopted, with some
exceptions that notably reduced concrete cover and relaxed boundary element confinement
requirements [1]. The impact of these exceptions on the structural damages observed in some of
the bearing wall systems after the 2010 M8.8 Maule Earthquake is currently under study by the
engineering research community.
Of particular importance is that the 2010 earthquake occurred in a county with a modern seismic
design code as well as good engineering and construction practice. During the earthquake, one
1Senior Engineer, Arup Los Angeles
2 Consultant, Oklahoma City
3 Associate Professor, University of Chile, Chile
4 Associate Professor, Bogazici University, Turkey
Kozmidis A, Melek M, Massone L, Orakcal K. Comparison of industry-standard nonlinear dynamic analysis
methods with obersved damage on a RC building. Proceedings of the 10th
National Conference in Earthquake
Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
building suffered complete collapse (Alto Rio Building), whereas numerous buildings have
experienced partial collapse or severe damage. Consequently, performance of the existing
reinforced concrete building stock has served as a platform to provide valuable information on
the vulnerabilities existing in the modern building codes. Large axial stress ratios, relaxations in
the code after 1985 earthquake, poor detailing and wall configuration were some of the primary
causes of damage [2]
This paper attempts to predict the seismic response, using a commonly used nonlinear analysis
software: Perform3D (CSI, 2011) [3], by investigating a tall building, located in Santiago, Chile.
The building structural system, reinforced concrete bearing walls, has suffered substantial
damage during the 2010 Earthquake. The reports and site investigations show that the building
has experienced concrete crushing followed by reinforcement buckling at lower levels during the
earthquake. Herein, we try to answer the question “Can the damage be predicted for buildings
with the industry-standard analysis methods?”. Industry standard nonlinear analysis tools and
software were used to model the building in order to predict the observed damage. Comparisons
and recommendations are provided within this paper.
Chile Earthquake
Chile is familiar with large intensity earthquakes in its history due to its location relevant to
tectonic plates. Nazca plate which situates by the west of Chile has been one of the most active
tectonic plates, moving at an absolute rate of 8cm/year to east [4]. The eastward movement is
causing the plate to be subducted beneath the South-American plate by creating a convergent
plate boundary in Chile. Subduction zone running through shores of Chile is depicted in Figure 1
with a pink line. This particular formation of subduction zone is known to produce largest
earthquakes due to the fact that it allows stress to be built up before the energy is released. These
energy releases were usually occurred in large magnitude in history (1960 Valdivia Earthquake –
Mw 9.5, 1985 Valparaiso Earthquake – Mw 7.8) 2010 Chile earthquake was one of these largest
magnitude earthquakes occurring with a moment magnitude of 8.8 on Richter Scale and a
rupture zone of 500km by 140km. This is the sixth largest earthquake ever recorded. Cities of
Concepcion, Arauco and Coronel, have experienced the strongest shaking with either VIII or IX
on the Mercalli Scale. The epicenter and seismic hazard of the earthquake is shown on figure 1.
The earthquake was followed by tsunamis in major cities of Chile, regions of Japan and has also
caused small size damages in shores of San Diego. According the official resources, the total
estimated loss of lives is 525 in addition to 25 people missing. Low to severe damage have
occurred in more than 200 hundred buildings and one of them collapsed completely.
Structural damages have been observed mostly in tall buildings with bearing wall systems.
General damage mechanisms observed in buildings in Chile is due to spalling and crushing of
concrete and buckling of vertical reinforcement at levels close to the ground level. [2] Some of
the later assessments on the damaged structures have shown that typical 20cm horizontal
reinforcement spacing and 90 degree hooks were used at wall ends. This has resulted in a less
dense confinement zones in the wall ends. Use of walls with smaller thickness (15cm-20cm) has
also resulted in comparably small concrete core areas. Further damages of reinforcement
buckling were observed in the web boundaries of T and L shaped walls due to confinement
problems. The causes of the damages being located at areas close to ground level can be
explained by concrete quality.
Figure 1. Seismic hazard map for Maule earthquake [5]
Building Description and Observed Damage
Benchmark building, is located in the capital city of Santiago which is the highest populated
urban area of the country. The building is 18-story tall with reinforced concrete bearing wall
system as its primary seismic force resisting system. There are also two additional basement
floors below podium level. Concrete with a cubic strength of 30 MPa and conventional steel,
A630-420H, conforming to national code requirements was used in the construction of the
building. Construction of the building was finished in 2007 and the initial design adopted the
Chilean codes after 1985 Earthquake. Percentage of wall to floor area ranges between 2.5-3.5%
which is in line with Chilean construction industry standard of the buildings constructed at the
same era. Typical wall layout for the building includes flanged and non-flanged walls with
uniform reinforcing of 8 to 12 mm diameter rebar and additional reinforcing at ends with varying
rebar diameter from 16 to 32mm. Figure 2 depicts rendering of the building and shows the
typical plan layouts for various floors.
Based on the investigations conducted after the earthquake, there were several areas in the
building which observed nonstructural failure of partition walls. In addition to several
nonstructural damages, the building has also experienced reinforcing steel bar buckling due to
crushing of concrete. Damages have occurred mostly in the regions of webs of T shaped walls at
subterranean levels close to the podium level. This is due to the backstay effect with recessed
walls at basement levels. Per on-site investigations after the earthquake, observed damages for
the building have occurred on gridlines N, Q and U of the first subterranean level. The areas of
damage are depicted in figure 3.
Model Description
The analyses for the investigated buildings were carried out in Perform 3D (CSI, 2011) which
enabled the use of nonlinear analysis methods while explicitly modeling fiber elements with
constitutive material models. Per as-built drawings, use of 90-degree hooks at wall ends was
observed. In the light of aforementioned reason and considering the omission of confinement
requirements in the existing code, the analysis model assumes unconfined concrete stress strain
curve using Hognestad model. 2.5% of Rayleigh damping was considered in the analysis per
Section 3.2.1 of LATBSDC 2011 Guidelines.
Figure 2. Benchmark building, perspective view (left), typical plan layout of walls of upper
floors (top right), typical plan layout of walls of basement levels (bottom right).
Figure 3. Observed damage at gridline Q (left) [6] and damage key on the wall layout (right)
Wall Modeling
Walls are modeled with industry standard fiber modeling techniques to capture stress
strain relationship throughout each fiber element. Fixed fiber sections are considered at the edges
to capture the behavior precisely. Typical macro modeling of structural walls has evolved
throughout the years to include custom response characteristics. Recent macro modeling
techniques such as multiple-vertical-line element model, MVLEM (initially proposed by
Vulcano et al. later improved by Orakcal, Wallace and Conte), uses multiple fibers connected to
rigid base to predict flexural behavior. Each fiber has its own response characteristic due to its
force-displacement relationships. Perform 3D software uses a similar approach to model
nonlinear flexural behavior of concrete walls. Although flexural modeling approach of the
industrialized software is similar to that of the MVLEM, modeling of wall shear response
incorporates more flexibility. Perform3D allows the user to model inelastic or elastic shear
material separately from the flexural fiber model, whereas the MVLEM elements incorporate
horizontal springs at which shear deformations are concentrated. Based on the observed damages
which are governed by flexure and due to the height of the building, use of elastic shearwall
material was considered in the Perform3D analysis model. [7]
Coupling Beam Modeling
Post-1985 buildings in Chile mainly include non-structural components in between walls.
This is due to the poor performance of slender coupling beams in 1985 earthquake [8]. In the
benchmark building, walls are connected through the slab for most of the cases. However,
building also includes few coupling beams per as-built drawings. Although the failure
mechanism did not include failure of coupling beams, for the sake of industry-standard approach
of modeling, coupling beams are explicitly modeled in this study by lumped plasticity approach.
Modeling parameters of plastic hinge rotation and residual strength ratios for these coupling
beams are considered based on Table 6-18 of ASCE41-06.
Seismic Hazard
Considering there is no record close to the building, couple of various records varying in
peak ground acceleration was used for the analysis. Figure 4 shows three acceleration records
that are applied onto the building. Histories are obtained from records provided from different
ground motion stations in central Santiago and Maipu which is considerably closer to the site of
the benchmark building. Figure 5 shows spectral accelerations of each record. Given the soil
conditions, it is less likely that the building behavior is well represented by the Santiago record.
Figure 4. Ground motion records, TH18 Santiago-Conjunto Villa Andalucia (top), TH19
Santiago-Conjunto Villa Andalucia-corrected (middle) and TH2021 Maipu-Centro de
Referencia de Salud [3],[9]
Figure 5. Spectral accelerations of hazards for each direction
Analysis Results
Nonlinear dynamic analyses on the building show that some walls on the building undergo
failure of concrete during earthquake. However the building is able to dissipate energy. Coupling
beams have performed well with varying plastic hinge rotations of 0.004 to 0.01 radians. This is
in line with the observed damage that no major energy dissipation has occurred and no damage
observed for coupling beams.
Story Drifts
Figure 6 shows building drift time-history for each record. It is observed that maximum residual
drift in the building is approximately 0.1%. This value would fall into Immediate Occupancy
performance criterion per structural performance levels defined in Table C1-3 of ASCE41-06.
Figure 7 depicts the extrema of inter-story drifts on the building with transient drift limit states
shown per ASCE41-06. Drift values stay within 1% which can conclude to building satisfying
drift requirements for Life Safety performance level. Although buildings in Chile are not
designed for any probabilistic or deterministic earthquake definition, these drift results are
satisfying an industry-standard performance level under DBE level earthquake.
Figure 6. Building drifts vs time history plots
Figure 7. Interstory drifts with limit states
Energy dissipation
Analyses have shown the inelastic energy dissipation is ranging between 20-50% of total energy
dissipation for different records. It is also noted that vast amount of the ductility in the system is
provided with shear walls where inelastic energy dissipation of these components is larger than
coupling beam energy dissipation of the building. Figure 8 shows the representation of energy
dissipation for each record.
Figure 8. Energy dissipation chart for the building
Strain Gauges
Axial strain outputs on the structural wall elements have shown areas where compressive strains
have exceeded crushing strain in level 1 and below. These numerous wall locations are depicted
in figure 9.
Figure 9. Zones of concrete crushing at Level 1 per analysis Shear Wall Behavior along Gridline Q
The structural wall located along gridline Q, which has observed concrete crushing and rebar
yielding at wall base, has been further investigated to provide an answer to identifying damage.
Shear demands on the wall were compared with capacity per information provided on as-built
drawings. The results shown in Figure 10 states that shear demands on the lower levels of the
wall close to the podium exceed capacity.
Figure 10. Shear demand versus capacity on wall at Gridline Q
Although the analysis showed deficiency in the shear behavior, axial strains measured from the
analysis model state minor yielding of steel reinforcement with strains lower than concrete
crushing strain at reverse cycles. To further the investigation on the capacity of the gross
structural wall section, PM capacity of the wall at first subterranean level is calculated. Figure 11
shows some critical data points on the PM curve of the wall. The capacity is exceed by nearly
5% on the first subterranean level whereas the demands on the lower level do not exceed
capacity. Considering the use of concrete crushing strain at 0.003 in conventional designs and to
form this PM curve, 5% overcapacity would not generate local crush of this wall.
Figure 11. PM curve of wall at crushing strain 0.003 on Gridline Q subterranean level 1
Comparison of Model Results with Observed Damages
As mentioned earlier in the paper, damages occurring at basement levels were compared with
strain gauge information provided by the building model. Although actual damages were mostly
concentrated along the transverse axis of the building at first subterranean level, analysis results
showed that concrete crushing occurs at several locations, at which concrete damage was not
observed during the earthquake. Another contradictory outcome of the analysis was that there
was no shear failure observed on the building, whereas one of the damaged walls that is assessed
more in detail have shown that the shear capacity of the section is not adequate. This outcome
raises the question of the impact of shear on initiating the flexural failure considering they are
not independent behaviors.
Conclusions
The analysis results presented in this paper have shown that the building dissipates hysteretic
energy to the range of 50%, with crushing of concrete in several areas. Although there were
various damages predicted by the analysis, locations of these damages did not coincide with the
observed damage locations. It is also seen that damages have been highly varying in between
different seismic hazards measured in discrete stations.
Considering the damages occurring in subterranean levels, further investigations need to be
conducted on the effect of soil on basement levels. More accurate results may be obtained with
better representation of soil structure interaction after conducting series of sensitivity studies on
the bathtub analogy of the subterranean soil. Further refinement of seismic hazard based on
consideration of attenuation relationships and frequency content of the ground motion can help
to better predictions on the damage. This paper did not include any attenuation relationships
during the analysis which might have led to conservatism in the approach. The lack of a
connection between shear and flexure failure mechanism is also providing a setback for more
realistic predictions. Shear-flexure interaction of the failure mechanism will further need to be
investigated.
This study will be succeeded by comparing damages in several other benchmark buildings to
provide a stock of data while taking into account attenuation relationships, shear flexure
interaction effect and soil structure interaction.
References 1. NEHRP Consultants Joint Venture. Comparison of U.S. AND Chilean Building Code Requirements and Seismic
Design Practice 1985-2010. NIST GCR 12-917-18, 2012.
2. Wallace JW, Massone LM, Bonelli P, Dragovich J, Lagos R, Lüders C, Moehle J. Damage and Implications for
Seismic Design of RC Structural Wall Buildings. Earthquake Spectra Volume 28 No. S1, 2012.
3. Boroschek LR, Contreras V, Kwak DY, Stewart JP. Strong Ground Motion Attributes of the 2010 Mw 8.8
Maule, Chile, Earthquake. Earthquake Spectra Volume 28 No. S1, 2012.
4. CSI Perform-3D V5-0. Computer and Structures, Inc. Berkeley, CA,2011.
5. U.S. Geological Survey (USGS),2010. USGS Earthquake hazards program: Magnitude 8.8 Offshore Maule,
Chile, available at http://neic.usgs.gov/neis/eq_depot/2010/eq_100227_tfan/neic_tfan_w.html.
6. Idiem, Inspeccion Post Sismo Del 27 de Febrero de 2010 Edificios Sol Oriente I y II. Chile, 2010
7. Orakcal K, Wallace JW and Conte JP, Flexural Modeling of Reinforced Concrete Walls – Model Attributes.
ACI Structural Journal Title No. 101-S68, 2004
8. Massone LM, Bonelli P, Lagos R, Lüders C, Moehle J, Wallace JW. Seismic Design and Construction
Practices for RC Structural Wall Buildings. Earthquake Spectra Volume 28 No. S1, 2012.
9. Civil Engineering Department of University of Chile, Chile, 2013. Earthhquakes of Chile, available at
mhttp://terremotos.ing.uchile.cl