Specifications For Building Instrumentation
Derek A. Skolnik
Robert L. Nigbor and John W. Wallace
OBJECTIVE
OBJECTIVE
Determine minimum requirements for specifications of
sample rate, resolution, and time synchronization
STRATEGY
Quantify the sensitivities of ground motion intensity measures
and engineering response quantities to DAQ
APPROACH
1. Understand how engineers use strong-motion data
2. Simulate the noisy DAQ process
3. Perform sensitivity analyses
2
OUTLINE
 Introduction
 DAS Specifications
 DAQ Simulation
 Sensitivity Studies
 Conclusions
3
INTRODUCTION
Engineering use of Strong-Motion Response Data
 Traditional – validate modeling assumptions and develop
code provisions: fundamental period approximation
formulas
 Modern – tall building issues, structural health
monitoring (SHM)
(Goel & Chopra 1997)
4
INTRODUCTION
Tall Building Construction
 Alternative designs citing Chap 16 of ASCE 7
 NDA of 3D FEM w/ suite of motions & peer review
 Exposed fundamental issues: ground motion
selection, modeling guidelines, acceptance criteria
 LA-TBSDC publish document for LA-DBS (2008)
 Since 1965 LA requires accelerographs at base,
mid-level, and roof
 UCLA, LA-DBS & CSMIP
Stories
Channels
update requirements
10 – 20
15
 Deployment approval by
peer review panel
One Rincon Hill - MKA
20 – 30
21
30 – 50
24
> 50
30
5
INTRODUCTION
Structural Health Monitoring (SHM)
 Assess health of instrumented structures from measurements
 Detect damage before reaching critical state and allow for rapid
post-event assessment
– Potentially replacing expensive visual inspection which is
impractical for wide spread damage in urban areas
Arabdrill 19, UAE
I-35W Bridge 2007
Steel Joint Damage
1994 Northridge
6
INTRODUCTION
Strong Motion Instrumentation Programs (SMIP)
 CSMIP (CGS), ANSS & NSMP (USGS), K-net/KiK-net (Japan),
Taiwan Seismology Center (CWB)
 Provide real-time ShakeMaps and data for engineers and
scientists to improve hazard mitigation
 Since early 20th century with focus on ground monitoring
 Uniform structural instrumentation specifications are lacking
650 ground
170 buildings
762 ground
133 buildings
92 ground
51 buildings
7
INTRODUCTION
Intensity Measures (IM)
 PGA – Peak Ground Acceleration
 PGV – Peak Ground Velocity
 PSA – Peak Response Spectral Pseudo-Acceleration
 MMI – Modified Mercalli Intensity
Engineering Demand Parameters (EDP)
 PFA – Peak Floor Acceleration
 PID – Peak Interstory Drift
Advanced Engineering Analyses
 SID – System Identification, Model Updating
 SHM – Structural Health Monitoring
8
OUTLINE
 Introduction
 DAS Specifications
 DAQ Simulation
 Sensitivity Studies
 Conclusions
9
DAS SPECIFICATIONS
SMIP Specifications
 Recommended specifications for civil structures (buildings)
 Based on qualitative assessment and experience
Recommended
Specification
ANSS
(USGS 2005)
CSMIP
(CGS 2007)
Sensor Range
ADC Resolution
Sample Rate
±4g
16bits
200sps
± 4g
18bits
200sps
Sample Sync
Reference Time
Clock Stability
1% Dt
1.0ms
0.1ppm
0.2ms
0.5ms
1min/month
R (bits/g)
S (sps)
Tse (ms)
Guideline for ANSS Seismic Monitoring of Engineered Civil Systems, USGS Report 2005-1039
Integrated Tri-Axial Accelerograph, CGS/DGS SYSREQ 2007-TR
10
DAS SPECIFICATIONS
Data Acquisition Systems (DAS)
 Sampling – sample rate (sps = 1 / Dt)
 Quantization – resolution (LSB = Range / 2 Bits)
 Time stamp for synchronization of multiple channels
Sampling
Dt
LSB
Quantization
Sensor
.......110001001010
Reference
Time
11
DAS SPECIFICATIONS
Data Acquisition Errors
 Sampling – initial sampling instant and clock jitter
 Quantization – Differential Non-Linearity (DNL)
Sampling Instant
N  ti 2 ,  
Jitter
x j2
DNL
x j 1
analog
discrete
digital
xj
ti
ti 1
ti  2
12
OUTLINE
 Introduction
 DAS Specifications
 DAQ Simulation
 Sensitivity Studies
 Conclusions
13
DAQ SIMULATION
Baseline Earthquake Record Set
 30 EQ records downloaded from PEER, NCESMD, K-Net,
KiK-Net, COSMOS
 Selected to capture broad nature of earthquakes
 Digitally enhanced to increase resolution: resample to 2kHz,
zero-pads for filtering, band-pass filter 0.1-50Hz
10
Fourier Amplitude (g)
Number of EQs
8
6
4
2
0
0
0.5
1
PGA (g)
1.5
2
10
10
10
0
-2
-4
-6
0.1
0.4 1 2 5
20 50
Frequency (Hz)
14
DAQ SIMULATION
• Sinusoids
• Earthquake ground
motions
• Structural responses
Baseline
Signal
Jitter and initial
sampling instant
S
DAQ
Simulation
R
ADC
Digital
Signal
• SNR and Peak value
Analysis
Analysis
Sensitivity Analysis
• IM: PGA, PGV, PSA
• EDP: PFA, PID
15
DAQ SIMULATION
Clock Jitter
 Independent of sample rate
Sinusoidal Signal
 Can be neglected
X  t   A sin  2 f t 
160
0.1
SNR (dB)
140
Sampled Signal
100sps
xi  A sin  2 f ti 
Hz
1H
z
ti  i Dt
120
50
Hz
100
Jittery Sampled Signal
xi  A sin  2 f  i 
80
-9
10
-8
-7
-6
10
10
10
Standard Deviation of Jitter (s)
-5
10
i
N  ti ,  
16
DAQ SIMULATION
Initial Sampling Instant
Error in peak value (Ep)
 Depends on sample rate
Ep
 Biased error – always negative
0
10Hz
-10
250
Count (N=3000)
-5
PGAEError
p (%)(%)
analog
sampled
300
20Hz
-15
40Hz
-20
-25
200
150
100
50
100sps
-30
-5
-2.5
0
2.5
Sampling Instance (ms)
5
0
-5
-4
-3
-2
-1
0
Log PGA Percent Error
17
DAQ SIMULATION
X = EQ record
1. t0 ~ U (0,Dt)
 S = [50-500] sps  Dt=1/ S
2. ti = t0+ i·Dt
 R = [6-24] bits/g  res=g /2R
3. xi = interp(X @ ti )
4. xi =round(xi / res)·res
SNR
8
24
200sps
12b/g
6
4
2
20
Resolution (bits/g)
Number of EQs
10
30
40dB
50dB
20
16
12
8
0
30 35 40 45 50 55 60
SNR (dB)
100
200
300
400
Sample Rate (Hz)
500
18
OUTLINE
 Introduction
 DAS Specifications
 DAQ Simulation
 Sensitivity Analysis
 Conclusions
19
SENSITIVITY ANALYSIS
Intensity Measures
 PGA – 100sps, 6bits/g for error less than 5%
 PGV – 50sps, 8bits/g for error less than 5%
 PSA – 200sps, 8bits/g for error less than 5%
R=6
2
10
12
14
0.4
1
Period (s)
2
16 bits/g
2
10
10
50sps
10
100sps
0
10
200sps
-1
10
1
PSA Error (%)
1
PGA Error (%)
8
10
0
10
-1
10
500sps
-2
10
-2
6
9
12 15 18 21
Effective Resolution (bits/g)
24
10
0.03
0.1
4
8
20
SENSITIVITY ANALYSIS
Baseline Building Response Set
 Simulate responses to baseline EQ record set by
superimposed first few modal responses
 Assumptions: bounded by flexural & shear idealizations,
uniform mass and stiffness, zn = 5%
Flexural Bldg
 2n  2  x 
 2 H 
n  x   sin 
Shear Bldg
Tn   nT1
 RC SW
 Steel EBF
 Dual System 
Steel MRF .
RC MRF 
H
n 

H
0

H
0
mn  x  dx
m n  x   dx
2
N
ai  ag    nni an
n 1
21
SENSITIVITY ANALYSIS
Fundamental Period
 Depends on building structure and height... sort of
 Based on real data from instrumented buildings
 Empirical conventions in code (ASCE 7) are lower bounds
7
Flexural
Shear
Fundamental Period (s)
6
Ts  0.68  0.11N
T f  0.46  0.03N
5
4
3
ASCE 7-05 S12.8.2.1
2
Ta  Ct hnx
1
0
0
10
20
30
40
Number of Stories
50
(Goel and Chopra 1997-98, Naeim 1998)
60
22
SENSITIVITY ANALYSIS
Simulation
Digitizing
Structure Type:
Flexural/Shear
S
Processing
max AAn  PFAn
R
AA
Newmark
Integration
N
AA
ADC
RA = AA – Ag
Double
Integrate
Ag
RD
RD
LEGEND
N: No of Stories
S: Sample Rate
R: Resolution
Ag : Ground Acceleration
AA: Absolute Acceleration
RA: Relative Acceleration
RD: Relative Displacement
High-pass
Filter
Fc
SNR
max RDm  RDn  PIDmn
23
SENSITIVITY ANALYSIS
50
No correction
SNR = -31dB
0
-50
10
Detrend
SNR = -16.3dB
Displacement (in)
0
-10
5
Fc = 0.01Hz
SNR = -8.9dB
0
-5
5
Fc = 0.1Hz
SNR = 20.5dB
0
-5
5
Fc = 1.0Hz
SNR = 1.25dB
0
-5
0
10
20
30
40
Time(s)
50
60
70
24
SENSITIVITY ANALYSIS
Optimizing Frequency Cutoff
 High-pass 4th order acausal digital Butterworth filter
 A single floor of a 10-story bldg to one earthquake
 Resolution is important which corroborates Boore’s (2003)
findings of ADC quantization being a source of numerical drifts
Flexural: fn =
[0.4, 1.2, 1.9] Hz
Shear
Shear: fn =Flexural
[1.5, 9.3, 26] Hz
60
60
100sps
200sps
500sps
20
0
-20
8b/g
12b/g
14b/g
18b/g
-40
-60
0
0.2
0.4
0.6
Fc (Hz)
0.8
100sps
200sps
500sps
40
SNR (dB)
SNR (dB)
40
20
0
8b/g
12b/g
14b/g
18b/g
-20
1
-40
0
0.1
0.2
0.3
Fc (Hz)
0.4
0.5
25
SENSITIVITY ANALYSIS
Engineering Demand Parameters
 PFA – 100sps, 8b/g for error less than 5%
 PID – 100sps, 14b/g for error less than 5%
8
22
10
12
14
16
18
20 bits/g
22
10
10
10
10
Flexural
11
00
100sps
-1 -1
200sps
10
10
10
10
PID Error
PFA
Error (%)
(%)
PID Error
PFA
Error (%)
(%)
11
10
10
10
10
00
10
10
-1-1
10
10
Shear
500sps
-2 -2
10
10
33
-2-2
66
99
12
12 15
15
Stories
Stories(s)
(s)
18
18 20
20
10
10
33 10
10
20
20
30
40
30
40
Stories (s)
Stories
(s)
50
50
60
60
26
SENSITIVITY ANALYSIS
Time Synchronization Error
 Sync errors are additional to digitizing error
 PID – 200sps, 16b/g and sync to 1.0ms for total error < 5%
Flexural
2
Shear
2
10
10
10ms
10ms
1
10
1.0ms
0
10
0.1ms
-1
10
-2
10
PID Error (%)
PID Error (%)
1
10
1.0ms
0
10
0.1ms
-1
10
-2
3
6
9
12
15
Stories (s)
18 20
10
3
10
20
30
40
Stories (s)
50
60
27
OUTLINE
 Introduction
 DAS Specifications
 DAQ Simulation
 Sensitivity Analysis
 Conclusions
28
CONCLUSIONS
ANSS
(USGS 2005)
CSMIP
(CGS 2007)
Recommend
(Skolnik 2009)
Range
ADC Resolution
Sample Rate
±4g
16bits
200Hz
±4g
18bits
200Hz
±4g
20bits
200Hz
Sample Sync
Reference Time
0.05ms
1.0ms
0.2ms
0.5ms
1.0ms
Specification
Potential Improvements
 Other specifications – frequency response, dynamic range,
cross-axis sensitivity, sensor layout
 Improved simulations – non-uniform stiffness; vary damping
ratios, combo flex-shear shapes, non-linear responses
 Other engineering analyses – system identification
29
Support provided by NSF CENS and nees@UCLA
Future Publications
BSSA: A quantitative basis for strong-motion instrumentation (12/09)
EQS: A quantitative basis for building instrumentation
30