Experimental methods and techniques:
the Structural Engineer's viewpoint
Dario Coronelli and Roberto Felicetti
Dept. Civil and Environmental Engineering
PhD course: Experimental methods and techniques
in computer science and engineering
D. Coronelli, R. Felicetti
2
Objectives of this presentation
to convey the following aspects
- structural viewpoint on experimental mechanics
- current applications of experimental methods in Structural Engineering
- close relation between structural models and experiments
- added value from Computer Science
across different perspectives
from material characterization to structural behaviour
new structures vs. existing ones
real structures vs. virtual ones
the perfect recipe for a messy presentation...
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definitions of "structures"
the arrangement and interrelationship of parts in a construction
a complex system considered from the point of view of the
whole rather than of any single part
how is a "structural behaviour" involved in experimental tests ?
global and local response of complex structural arrangements
the primary object of structural experimental tests
non uniform loading in material characterization tests
parasitic effects in apparently simple tests on small samples
the loading equipment is itself a structure
loads and boundary conditions have to be imposed
the structural model becomes part of the experimental test
not all the structures can be tested as a whole
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Applications of experimental methods in Structural Engineering
Testing for Characterizing New Materials NM
Testing for the Design of New Structures NS
New types of structures and loads
Laboratory specimens and test methods
Numerical Analysis
Testing for the Assessment of Existing Structures ES
Measurement: Geometry, Materials Properties, Loads
Deterioration: Observation and Measurement
Models: Strength, Deterioration
Testing for Study of Historical Structures HS
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basalt fibre technology is similar to glass fibre
the molten rock (~1300°C)
flowing in a rhodium-platinum bushing
is drawn by blowing with air or steam
nice properties
• heat and chemical resistance
• high tensile strength and stiffness
• well matched density to cement
open issues
• brittleness in the long term
• sensitivity to Alkali Silica Reaction
• bond properties in cement paste
one example: testing the potential of mineral fibre (basalt)
for improving the fracture toughness of cementitious composites
a recurring issue in material characterization:
the difficult instatement of uniform load/strain conditions
filament properties
diameter = 12 μm
max load = 0.2 N
max stress =1500-2000 N/mm2
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6 bond tests on single basalt filaments specimen preparation
special
moulds
0.3 mm = 25∙Ø
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7 bond tests on single basalt filaments • displacement controlled tests
• a laboratory scale as a load measuring device
displacement measurement sample positioning on the scale plate
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elastic limit
load-displacement curves
0.0
5.0
10.0
Av. shear
str
ess (
N/m
m2)
0.0 0.1 0.2 0.3
Displacement (mm)
the change of slope indicates the onset of debonding
D. Coronelli, R. Felicetti
0 1 2 3 4 5
Displacement U ( m)
0.00
0.05
0.10 1__ = 1.7 Ø
P (N)
PE
f = 2.5 N/mm2 Lb= 0.32 mm
step A
y = 100 N/mm2 Ø = 11 m
PE Ø___
y
0 1 2 3 4 5
Displacement U ( m)
0.00
0.05
0.10 1__ = 1.7 Ø
P (N)
PE
f = 2.5 N/mm2 Lb= 0.32 mm
Pmax Ø Lb f + y - 2 f _______
Lb
step A
B
C = Pmax
y = 100 N/mm2 Ø = 11 m
0 1 2 3 4 5
Displacement U ( m)
0.00
0.05
0.10 1__ = 1.7 Ø
P (N)
PE
f = 2.5 N/mm2 Lb= 0.32 mm
Pmax Ø Lb f + y - 2 f _______
Lb
step A
B
C = Pmax
unstablestressdrop
fibre pull-out
y = 100 N/mm2 Ø = 11 m
y = 100 N/mm2
0.0 0.5 1.0x / Lb
0
50
100
step APE
step C
Pmax
step B
(N/mm2)
f = 2.5 N/mm2
Lb - acrit = 1__ acosh y__
f
0.00.51.0 a / Lb
0 1 2 3 4 5
Displacement U ( m)
0.00
0.05
0.10
P (N)
PE
f = 2.5 N/mm2 Lb= 0.32 mm
step A
B
y = 100 N/mm2 Ø = 11 m
Ø 10÷15 µm
x,u P, U
Lb
ainterfacialcrack
bonded(elastic diffusion)
debonded(slip-friction)
y
f
u
k1 k 500 000 N/mm³
K = ·Ø·k 20 000 N/mm²
FIBRE(Ef = 72 000 N/mm²)
= 1
Ø
4·K
Ef
0.6
Ø
CEMENT
9
y = 100 N/mm2
0.0 0.5 1.0x / Lb
0
50
100
step APE
step B
(N/mm2)
0.00.51.0 a / Lb
y = 100 N/mm2
0.0 0.5 1.0x / Lb
0
50
100
step APE
(N/mm2)
0.00.51.0 a / Lb
shear-lag model
pulled rod embedded in an elastic medium
elastic diffusion scale length 1/ ≈ 1.7∙Ø
total diffusion length ≈ 5∙Ø
elastic limit PE ≈ 5.3 Ø2 y
critical bonded length 1/ acosh(√ y/ f) ≈ 4.3∙Ø
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10 toughness indicators and comparison with the slip-friction model
Lf / 2
x
w
Lb(x, a)
w
f
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fixed rotation
direct tension
hinged platens
direct tencion splitting bending
increasing "structural" effects
material characterization often implies
tackling the not uniform distribution of stress and strain
• improving the test methods
• devising appropriate interpretation techniques (inverse analysis)
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Testing for design and verification of new structures
5.2 Design assisted by testing
Design may be based on a combination of tests and calculations.
Testing may be carried out, for example, in the following circumstances:
• if adequate calculation models are not available;
• if a large number of similar components are to be used;
• to confirm by control checks assumptions made in the design.
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an effect often neglected in the design of precast shell elements
2nd order effects due to loading + deformation
sagging curvature
the compressed chord
behaves like an arch
(downward thrust)
the pulled strands
behave like a rope
(uplift action)
some experience earned
by testing precast thin-shell elements
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0.000 0.002 0.004 0.006
Curvatura [1/m]
0
100
200
300
400
500
Mo
me
nto
[kN
·m]
M0 = 290 kNm
Multth
= 1030 kNm
a hyperbolic paraboloid shell element
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a long span hollow-core roof element
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16 the devil is in the detail...
bent rebar at a re-entrant angle
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Design of New Structures - NS
Testing and Numerical Modelling
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NS - Member Response, Testing and
Analysis
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one-way beam two-way slab
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Member Response, Testing and Analysis 19
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Measured
Displacement
on outer surface
Relevant phenomena on internal surface
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Laboratory Test 23
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Numerical Model
NLFE 3D
Abaqus v.13.0
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NS – New types of structures
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• Mexico City, 1985
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31 Testing for the Design of New Structures NS
TEST SPECIMEN: Structure and substructure
RESPONSE at study: Global and Local
• force, displacement, strain, crack opening
INSTRUMENTS
Measurements inside the Specimen
• Steel
• Concrete
FAILURE MODES and strain/crack patterns
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Punching shear
failure
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Attuatore
cerniera
biellebielle
funi controvento
nel piano orizzontale
cerniera cerniera
puntelli
(non a contatto)
Montante
contrasto
spinta
testa pilastro
mobile
travi
ancorate
a terra
biellebielle
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Numerical modelling
(Coronelli, 2010; Coronelli and Corti, 2014))
PRE: Test set-up Design
POST: Test interpretation
Pushover analysis
(Model implemented in SAP 2000 v.15)
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fire testing of structural assemblies
effects of fire on structural members
• rise of temperature (often with gradients)
• decay o mechanical properties (strength, stiffness)
• thermal strain (possibly restrained)
mutual interaction between
thermal and mechanical loads
• strain under load ≠ unloaded strain
• load-deflection interaction (buckling)
• stability of protective layers
D. Coronelli, R. Felicetti
the thermal curvature requires
a self-adapting loading system
(hydraulic jacks in parallel
hinged beams, etc)
Typical experimental setup: horizontal furnace
in most cases
just one isolated bay can be tested
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38 virtual extension of the tested structure
burning compartment
Fabienne Robert
CERIB, Epernon, France
cold
compartment
cold
compartment
stiffness
of the cold
sub-structure
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• vertical jacks: dead load
• horizontal jacks: tension-compression
• eccentric vertical jacks: rotations
• read displacements/rotations
• compute the target forces
• impose the target forces
closed loop interaction between numerical and real parts of the structure
[F] = [K]∙[u] + [c]
stiffness
matrix
initial
conditions
measured
displacement
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FULL SCALE 6m x 4m , 4.1m
MULTIDIRECTIONAL LOADING
•18 vertical jacks in compression 300 tons (1) - 50 tons (4) : stroke = 500 mm
30 tons (4) - 5 tons (9) : stroke = 400/500
•9 horizontal jacks C : 125 tons and T : 60 tons (3) : stroke 500
C : 50 tons (6) : stroke 400 mm
•2 specific vertical jacks 30 tons (2) for connections : stroke 1000
and rotation 10
TEMPERATURE RISE – POWER
16 MW (16 gas burners)
Temperatures of up to 1320
C
structural fire testing laboratory at CERIB (France)
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see Model-based Strategies for Real-time Hybrid Testing - J.E. Carrion and B.F. Spencer, Jr.
https://www.ideals.illinois.edu/bitstream/handle/2142/3629/NSEL.Report.006.pdf?sequence=2
Experimental testing methods in structural dynamics
quasi-static loading
predefined displacement or force time history
to investigate the hysteretic or cyclic behavior
of structural materials or components under earthquake loading
interaction with the structure and rate dependent behavior
of the structure are not considered
shaking-table test
the entire structure is subjected
to a ground acceleration history
dynamic effects and rate dependent
behavior can be completely modeled
reduced scale model due to limitations
on the size and payload capacity E-Defense - Japan UC San Diego
North Carolina State Univ.
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42 hybrid (or pseudodynamic) testing
the structure to be tested is divided into a physical component and a numerical model
the physical component is representative of the stiffness of the structure
the numerical model includes the mass of the structure (lumped at discrete locations)
and the structural damping
during the test:
the dynamic response
is calculated numerically
using time step-integration
the calculated displacements
are applied to the test specimen
(discrete DoF)
the forces required are measured
and fed-back to the model
to calculate the displacements
in the next time step
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43 implementation issues
numerical time step integration
Newmark method
Central Difference Method
(explicit)
management of time delays
1.compute target displacement
2.send target displacement
3. impose target displacement
4.measure restoring force and
displacement
5.send force and displacement
6.update response
ramp-hold loading procedure continuous pseudo-dynamic
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Distributed pseudodynamic
distributing the different substructures at geographically separated facilities
using the internet to link them with the main simulation computer
Fast-MOST Multi-Site Online Simulation Test
Computational Sites: Buffalo (deck)
UIUC
Lehigh
Experimental Sites: Berkeley
Boulder
Buffalo http://nees.buffalo.edu/
Real-time hybrid testing
the imposed displacements and response analysis are executed
at a signficantly higher speed, approaching the real time scale
rate dependent components can be tested
high performance physical testing system, computers and software for
numerical calculations and data acquisition are required
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Testing for the assessment of existing structures
two main tasks:
collecting the required information
historical survey, geometrical survey, structural details, material quality
possible distress, patologies, etc
data processing
hand computation, numerical analysis, categorization of structural condition
two corresponding lines of research:
developing new tools for onsite inspection
viability, reliability, low cost, readily available results
developing new procedures or strategies for data processing
scalable level of detail, safety assessment or maintenance planning
D. Coronelli, R. Felicetti
46 developing new low cost tools for onsite inspection
consumer electronics and portable computers allow interesting developments
in many cases the problem is to make objective and measurable
what is already perceived and recognized by a skilled inspector
discoloration of fire damaged concrete
300 - 600°C pink or red
600 - 900°C whitish grey
900 -1000°C buff
0
10
20
30
40
50
fatto
re d
i rifle
ssio
ne (
%)
400 500 600 700
lunghezza d'onda (nm)
rossoblue
800°C
400
200
20°C
verde
malta cementizia
Øobiettivo = 8 mm
media di 12 misuredeviazione std = 3-5%
600
spectrophotometer
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original picture
0.00
0.05
0.10
0.15
0.20
0.25
1
R-B
Red-Blue difference
-0.002
0.000
0.002
0.004
0.006
0.008
0 20 40 60 80
depth (mm)
color variation (x - y)
average
ordinary concrete(masked aggregate)
breakpoint
a digital image: millions of colour measurements
normalized Red-Blue difference
0.00
0.05
0.10
0.15
0.20
0.25
1
R-B / R+B
side view of a concrete core
0.31 0.32 0.33
0.33
0.34
0.35
x
y
ordinaryconcrete
flash illuminantauto white balance
D65
200
1
1 800°C
600
400
20
full image
masked aggregate
std dev ellipse (masked aggr.)
Mont Blanc tunnel [4]
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Delam Tool http://soundingtech.com/
Chain drag
identification of delaminated areas
in industrial concrete floorings
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- floor area: 8200 m2
- 7.5 km to be scanned
~15 mm
why the common inspection methods
are so sketchy?
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implementation of the impact acoustics technique
over a long linear path: a hand-driven hammering-trolley
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17 data processing 51
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data processing
two frequency peaks can be recognized:
one connected to the hammer mechanism (information about the blow intensity)
one ascribable to the delaminated pavement (related to crack extension)
besides the frequency drop
also the sound loudness
increases with the size
of delaminated area
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statistical frequency of loudness
indicator
the definition of a colour scale
allows to draft a damage map
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the damage map of the inspected pavement
about 8 hours data recording - 2 GBytes in digital audio files
85.000 tapped points - 2400 are more or less likely to experience a detachment
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+ +
re-thinking ordinary tools
-10
-5
0
5
10
am
plit
ud
e (
kN
)
drill bit
-4
-2
0
2
4
am
plit
ud
e (
V)
0 50 100 150
time ( s)
ultrasonic sensor
bit delay
(26 s)
time
of flight
AIC picker
AIC picker
t0 t1 t2
the time of flight of mechanical pulses
is monitored in real time (80 pulses/s)
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hot face
cold
face
depth
- sensitive and reliable method
- high definition of the results (10-20 pulses/mm)
- not influenced by the inherent heterogeneity of the material
- remarkable repeatability of the results
a scan of the pulse velocity is obtained
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Object of study:
System made of Parts and Elements
Assessment of SAFETY (ISO 2394):
Load effect < Strength
S < R
R requires measurements on the real structure
Deterioration effects
ISO 2394:1998 General principles on reliability for structures
ES – Assessment
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Geometry: Survey
A = cross section
R = R (Ac, As)
Material properties: Sampling, Testing
f = strength
R = R (fc, fs)
Model: R = R( A, f )
ES Testing for the Assessment of Existing Structures
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ES – DETERIORATION eg. Reinforced Concrete
ATTACK: Industrial Pollution, Marine Environment, Fumes, De-Icing Salt
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Corroded Steel Cross-section Measurement
Laboratory In situ
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Deterioration: Measurement and Observation
MEASUREMENT of quantities involved in Strength
(eg Material Properties and Geometry with deterioration)
Difficulties (No Standard Methods)
OBSERVATION of Damage and RATING
«Low, Medium, High, Very High»
Easily carried out visually – requires experience
ES - Strength and Deterioration Processes
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Condition Rating - Example 65
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Photographs/ Video
Image processing
Computer vision
Visual Methods and Digital Techniques
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Prestressed Concrete
Visible damage
Hidden damage
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Prestressed Concrete
Invisible damage
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Topics and Links
Structural Health Monitoring
http://shm.sagepub.com
Digital Image Correlation
Computing and Cultural Heritage
http://jocch.acm.org
ICT assistance in monitoring and restoration, Tools for reconstruction and
processing of digital representations
Archives of Computational Methods in Engineering
http://www.springer.com/engineering/computational+intelligence+and+c
omplexity/journal/11831
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D. Coronelli, R. Felicetti
Conclusions
The "structural" viewpoint involves the complex interrelationship
among the different parts composing a structural system
and the loading equipment
This perspective applies to any scale, from small material samples
to big structures exceeding the actual size of the lab facility
Design of New structure types and members:
combined use of testing and models (prediction and/or interpretation)
Existing and deteriorating structures:
standard measurement techniques missing
urgent need for some construction types
visual/digital developments of qualitative assessment
76
D. Coronelli, R. Felicetti
References
Coronelli D., (2007) “Condition rating of RC structures: a case study” Journal of Building Appraisal,
2007, Volume 3, Number 1, 2007 , pp. 29-51
Zandi Hanjari, K., Lundgren, K., Plos, M., Coronelli D. (2013). Three-dimensional modelling of
structural effects of corroding steel reinforcement in concrete. SIE, Structure and Infrastructure
Engineering (ISSN:1573-2479), vol. 9., pp. 702- 718.
Coronelli, D., Corti, G., (2014) Nonlinear static analysis of flat slab floors with a grid model ACI
Structural Journal Vol.111 No.2 March April, 343-351
Coronelli, D., Martinelli, L., Foti, F. “Analisi e progetto di piastre alleggerite in C.A. con azioni sismiche
/ Analysis and design of voided reinforced concrete slabs for seismic loading", 2015 (in Press)
Felicetti R. (2005), "Digital camera colorimetry for the assessment of fire damaged concrete", Proc. Fib
Task Group 4.3 Workshop Fire Design of Concrete Structures: What now? What next?, Milan,
Dec. 2-4, 2004, Gambarova P.G., Felicetti R., Meda A. and Riva P. (Eds.), Starrylink, Brescia,
2005, p.211-220
Felicetti R. (2006), "The Drilling Resistance Test for the Assessment of Fire Damaged Concrete",
Journal of Cement and Concrete Composites, V.28, p.321-329.
Felicetti R. (2010), "Assessment of an industrial pavement via the impact acoustic method", European
Journal of Environmental and Civil Engineering, V.14, p.427-439, DOI 10.3166/EJECE.14.427-
439.
Felicetti R. (2012), " Bond properties of mineral micro-fibre", Bond in Concrete 2012, Brescia, 18-20
June, 8 p.
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