Seismic Response of Atwood Building, Anchorage, Alaska

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Seismic Response of Atwood Building, Anchorage,
Alaska
Presented in State of Alaska Seismic Hazards
and Safety Commission 6th December, 2006
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Atwood Building Instrumentation project

• Funding Sources
• U.S. Geological Survey under ANSS program
• NSF from Alaska EPSCoR
• University of Alaska
• Research Team
• U. Dutta, ENRI/UAA and GI/UAF
• He ( Helen) Liu, SOE/ UAA
• Z. Yang, SOE/UAA
• N. Biswas, GI/UAF
• F. Xiong, SOE/ UAA
• W. Scott, SOE/UAA
• T.Kono, SOE/UAA

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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Seasonal Frost effect
• Conclusions

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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Seasonal Frost Effect
• Conclusions

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Anchorage Strong Motion Network

• .

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Site response Map of Anchorage

• The site response map at three different
  Frequencies( 0.35 Hz, 1 Hz and 5.0 Hz). 1 Hz and
  5Hz maps were obtained from the analysis of
  earthquake data with maximum PGA 75 cm/sec/sec,
  while 0.35 Hz map was obtained from Micro-tremor
  data.

Atwood Building
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Atwood Building/Downhole Array Instrumentation

• Instrumentation of the building is to monitor its
  behaviour under earthquake shaking
• Instrumentation of the foundation soil downhole
  array to study Soil-Structure Interaction (SSI)
  in building with shallow foundation
• Downhole construction was sponsored by NSF EPSCoR
• The instrumentation of both building downhole
  by ANSS of USGS

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The Atwood Building and Downhole Preparation
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Atwood Building -elevation and plan views
Parking Garage
Building Details The building is 38.5m x 38.5m
in plan MRSF building with 14.63m x 14.63 m in
plan center steel shear walled core. The roof is
80.5m above the ground level. The building
foundation consists of 1.52m thick mat below the
core and 1.37m thick mat at the perimeter.
Exterior and core mats are connected through grid
beams.
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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Seasonal Frost Effect
• Conclusions

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Accelerometer deployment in Atwood
Building
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Delaney Park Downhole Array Depth Profile
DPDA Consists of 6 boreholes and one surface
accelerometers. The deepest sensor is located in
a glacial till formation with shear wave velocity
gt 900m /s, corresponding to engineering
bedrock. The sensors are arranged such a way so
that characteristics of major formations can be
studied.
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Schematic Plan of the Delaney Park Downhole Array
30m
Enclosure
5
1
1m
61m
10m
0m
7
3
Data Recorder
4.5m
2 m
2.9 m
1.7 m
1.7 m
4.5m
45m
18m
2
6
4
1.8 m
2.1 m
7.6 m
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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Seasonal Frost Effect
• Conclusions

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Recorded Earthquakes Since Dec. 2003
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Impulse Testing
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Plot of Transfer function of Atwood Building from
12/15/03 event
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Plot of Singular values of spectral density
Matrices
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Plot of Mode Shapes from Atwood Building
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Modal Identification Results - fundamental
periods of AB
by ARTeMIS extractor
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Identification Modelling
Recorded Seismic Data
Instrumented Structure
Identified Structural Parameters
FE Model
Comparing
Calibrate FE Model
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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Seasonal Frost Effect
• Conclusions

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Final FE model for AB -3d view plan view
Plan View
3D- View
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Assumptions for the Initial Model

• Floor diaphragms and the roof were assumed to be
  rigid in their own planes since 77 mm-deep
  composite metal decking is topped with 65 mm of
  concrete.
• The mass was assumed to be uniformly distributed
  in all floors without considering the large
  variation in locations of partitioning walls, and
  other non-symmetric masses.
• Center-to-center dimensions of the steel frame
  elements were used to develop the initial model.
• The composite action between concrete slabs and
  steel beams was ignored in the initial model.
• The structure is almost double symmetrical
  two-direction 5 eccentricity was incorporated in
  the initial model to account for accidental
  torsional effects.
• The contribution of non-structural elements to
  the overall building stiffness was not
  considered.

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Refined Modeling Approach

• To capture the fundamental dynamic properties,
  several approaches were used
• more accurate mass mass eccentricity
• correction in mass moment inertia
• modification in panel zone rigidities of
  beam/column connections
• evaluation of composite functions between the
  concrete slabs and steel beams
• adjustment in damping to consider the effects of
  nonstructural components. The sensitivity of
  natural periods to certain factors was also
  studied.

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Example Panel zone modeling
Typical Interior Subassemblage
H
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A Sketch of End Offsets in a Frame
Elastic Response of Panel Zone
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(No Transcript)
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Comparison of natural periods among the initial
and refined FE models, and the identified results
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The first 4 mode shapes in EW and NS directions
of the final mode
Mode 1. E-W T 2.19s
Mode 1. N-S T 1.81
Mode 2. E-W T 0.71s
Mode 2. N-S T 0.57s
Mode 3. E-W T 0.39s
Mode 3. N-S T 0.30s
Mode 4. E-W T 0.22s
Mode 4. N-S T 0.21s
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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Conclusions

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Comparison of recorded and FE mode simulated
results in EW direction


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Discussion on FE analysis

• The most effective ways to improve the accuracy
  of FE modeling of buildings of this type are to
  refine the mass calculation (quantity,
  eccentricity and moment inertia) and evaluate the
  panel zone rigidity of the beam/column
  connections.
• Comparison of the recorded motions with dynamic
  analysis results obtained using the best fit FE
  model shows that a FE model could be refined to
  give very good fit of the observed responses.
• Recorded seismic data can be very useful in
  improving the accuracy of FE modeling used in
  typical engineering design.

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Outline

• Introduction
• Building and Instrumentation Description
• Structural System Identification
• Finite Element Modeling
• Time History Analyses
• Seasonal Frost effect
• Conclusions

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Results from more earthquakes
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Results of the frost effect study
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Schematic model of the soil-foundation and the
soil properties
78 m
44 m
78 m
Reinforced Concrete Grid Beam
Frozen Silt and gravel
E 10 GPa, v0.35

h1.5m
Sand and Gravel, E 10 GPa, v0.35, h8.5m
Silt and lean clay, E 20 GPa, v0.45, h7m
Clay and silty clay , E 20 GPa, v0.45, h31m
Silt, sand and clay, E 40 GPa, v0.45, h7m
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Lumped model for the building superstructure
Krot
Khor
Kver
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Results of the Frost effect study
Soil Springs Coefficient Computed from FE model
and Elastic Solution
Elastic Solution is the solution obtained from a
shallow foundation embedded in a homogeneous half
space for unfrozen condition with elastic
properties of the first layer.
The foundation consist of spreading footings
connected by grid beams was treated as mat
Foundation for both FE and elastic solution
Seasonal Frozen Soil effect on Fundamental
Frequency From recorded data and FE model
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Results of Frost effect
We extended the study of the frost effect on a RC
type building, as it is common in the cold
region. Thus if a RC building of same geometry
like Atwood Building (AB) exist under the similar
site condition with mass 1.5 times that of AB and
having stiffness 4 ( model 1) and 8 (model 2)
times AB.
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Summary of Seasonal Frost effects

• There is 13 variation in the building
  fundamental frequency identified from earthquake
  vibrations and the ambient noise recorded in
  winter and summer.
• The seasonal frost has a clear influence on the
  building fundamental frequency. At relatively
  small shaking the change in fundamental frequency
  due to seasonal frost is around 4. This is also
  confirmed by FE modeling.
• FE model shows that the seasonal frost could
  increase the foundation stiffness in horizontal
  direction by one order of magnitude.
• If other effect remain the same, the effects of
  seasonal frost on the building dynamic property
  is much more prominent for RC type building than
  steel-frame building.

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Methodology

• Standard Spectral Ratio

• Horizontal to Vertical Spectral Ratio

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Horizontal to Vertical Spectral Ratio

• We have studied the H/V spectral ratio at
  different depths in order
• To study the stability of the methods and to
  predict the resonant peaks.
• To compare the results of site response by H/V
  methods with the direct spectral ratio methods.
• Two prominent peaks are present in the H/V
  spectral data 1) 1.4 Hz (2) 4.2 Hz.

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Site Response
We have computed the standard spectral ratio of
sites from surface to 30 m depth using the
station BH- 60 as the reference site. It is
noticed that the spectral peaks of the SSR
matches with the spectral peaks of the H/V
methods. But the spectral amplitude of all the
peaks are not always the same. Possible
reasons 1. Vertical component may have own site
response produced by S-P conversion.
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Standard Spectral Ratio of Vertical Component
If the incoming waves are not exactly vertically
incident, the S-wave window on the vertical
component may contain the significant S-P
converted waves. We like to test whether in the
DPDA such condition exists or not. We have
noticed a relative high peaks 1) at 2.8 Hz with
site amplification 2 and 2) at 7.8 Hz with a site
amplification nearly 3. Thus the assumption of
vertical component free of site response is not
valid.
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• THANK YOU !!