Forensic Engineering 7th Congress
Performance of the Built Environment
November 15 – 18, 2015
Hilton Miami Downtown Hotel
Miami, FL
Some Aspects of the measurement of vibration intensity for the assessment of Building Structures 1.
A.P. Jeary1, T. Winant1 and J. Bunyan1
STRAAM Group. 40 Wall Street, New York City, NY 10005; PH (212) 367-5706;
email: [email protected]
ABSTRACT
Building commissioning has, for a long time, been concerned with an assessment of
many parts of a building, but has remained silent on the state of the basic fabric of the
structure itself. Recently, improvements in technology and assessment techniques have
made it practicable to assess the structure itself, and add a new capability to the
commissioning process.
Using accelerometers that are extremely sensitive, it is possible to use excitation from
natural sources (such as wind or traffic) to obtain information about the in service
performance of the structure, from an analysis of tiny movements in a process that is
analogous to the use of an electrocardiogram to judge the state of health of a human.
The measured tiny movements reflect the movements of the structure even up to very
large motion, and give a direct measure of the capacity of the structure, and the presence
of any anomalies and their location.
The techniques and analysis have been used in buildings in the USA and in New
Zealand, to support decisions about the viability use, and even whether construction
work is safe to continue. Examples will be given in the presentation.
THE BACKGROUND
In the 1930’s German railways organizations became concerned that vibrations
generated by trains might cause damage to domestic housing close to the railway lines
(Reiher and Meister, 1931).
Accordingly they set about making a series of measurements designed to show the
vibration intensity that could be tolerated by unreinforced domestic housing. They
produced a sophisticated set of equations to define the vibration intensity. Subsequently
the Building Research Establishment in the United Kingdom accumulated a large body
of information during the 1960’s through to the 1970’s (Steffens, 1973). More recently
information obtained from tests conducted by the US Bureau of Mines (Siskind et al,
1981) mainly on housing subjected to nearby explosions provided a large data base of
the effects of vibration on unreinforced structures. This latter database refers to the two
earlier ones and produces a statistical analysis of the likelihood of damage based on the
vibration intensity. Unfortunately, the complex analysis of the vibration intensity was
replaced with the simpler measure of peak particle velocity.
While the German measures of vibration intensity were based on physical properties
the instrumentation available at the time did not lend itself easily to measures such as
the Vibrar scale. In this respect the available instrumentation played a large part in the
simplification of the measurements.
THE EQUIPMENT USED
The electronic revolution began with the invention
of the transistor in 1947, and its commercialization
in 1954. Nevertheless, the improvements in
measurement sciences only gathered pace slowly.
For much of the 1950’s and the 1960’s the use of
thermionic valves was essential, and as a result
precluded the use of high precision measurements
in the field. Devices that were used as transducers
(able to change physical quantities to proportional
voltages) were based on such devices as coils or
resistors moving past another coil or resistor.
Effectively then, until about 1973 measurement
devices were essentially mechanical. From 1973
onwards, electronic technology allowed the use of
force balance techniques. In this the shell of the
instrument and a seismic mass move in
synchronization because the mass is forced to Figure 1: Dallas Instrument Blast Monitor (1970's)
move in sympathy by the use of electronic
feedback through a small electric motor. This type of device is an accelerometer. An
alternative is to use a device that is tuned to measure velocity. Older mechanical devices
were arranged to operate at their frequency of resonance and to show the motion
between a seismic mass and the shell of the instrument. Early shells weighed as much
as two tons. The seismic mass was later arranged, with a pen that made a mark on a
slowly rotating drum. The advantage of using a velocity measuring device is that,
because it operates at resonance, the output is much larger than that of an accelerometer.
But because the operation of a velocity-measuring device occurs at its frequency of
resonance the frequency range is limited and it is quite variable in the useful
measurement range. As a result, the range is limited. Early measurements were largely
restricted not only to velocity measurement, but also to a frequency range that was
limited to 4 Hz and above, but later became 2 Hz. Such a low frequency limit has an
annoying limitation that very large modern structures have frequencies of resonance
that are much lower than 2 Hz. That was the situation in the 1970’s, but electronics
technology has moved very quickly since that time. Developments have allowed the
use of servo accelerometers down to zero frequency (indeed such devices are
commercially sold as tilt meters). The acceleration of the development of electronic
capabilities has meant that large quantities of data can be measured, processed and
stored quickly. This development is explored in more detail below.
THE DATABASES
The German and British information from their
databases have been quoted and taken into
account in the US database produced by the US
Bureau of Mines in 19813. There have been no
efforts to undertake such a large study since that
time, and as a result, this database therefore
represents the latest version of measurement, yet
it is nearly 35 years old, and was not conducted
with the benefit of the electronic capabilities
available now. The data in the US study was
based on correlating vibration intensity with
observed damaged in domestic housing. The
study was well conducted and the statistical
Figure 2: Damage thresholds from RI 8506
analysis was particularly good. The equipment
typically used is shown if Figure 1, and the results
of the most significant study are shown in Figure 2. The results in Figure 2 form the
base of the current approach to contract administration in the US. As can be seen from
Figure 2 the values of vibration intensity for the threshold of damage is relatively
indistinct. When this factor is coupled with the fact that the tests were conducted on
domestic housing, and that the frequency range considered did not extend below 2 Hz.,
then it is somewhat surprising that it is common to find that these recommendations
form the base for most US contracts when the issue of vibration intensity (for the
purpose of monitoring blasting, tunneling or demolition effects on adjacent structures)
are being considered.
MODERN EQUIPMENT
Modern electronic capabilities have revolutionized the approach to the measurement of
vibration. Firstly, the noise floor possible in electronic devices has reduced to such a
low level that it is possible to measure down to such small quantities that it is even
possible to make measurements comparable with the wavelength of visible light.
Secondly, the storage of vast quantities of data at a low cost (in the last five years) has
meant that collecting data several hundred times a second, allows a near real time
capability and digital storage that is effectively limitless. Finally, the instrumentation
capabilities have progressed in direct proportion. It is now possible to manufacture
accelerometers that are Micro-electro mechanical devices (MEMS devices are typically
used for gaming), while at the same time driving down costs. The electronic
conditioning circuits have also become more reliable and more stable. This last factor
has been used additionally for velocity measuring devices. It is possible using modern
electronic circuits to compensate for the resonance curve of a velocity measuring
device and extend the frequency range of operation downwards. This is a compromise
which involves sacrificing some dynamic working range (the noise floor at low
frequencies is also amplified). Nevertheless, it is now possible to extend the working
range down to 1 Hz, and the possibility of extending further is feasible.
THE CODIFICATION AND CONTRACT SCENARIOS
It can be seen from Figure 2 that a reasonably conservative approach is to determine
that all vibrations with a PPV value of less than 2 inches per second are unlikely to
cause damage in domestic housing. Codes of practice call up other documents,
including but not limited to the Minimum Design Loads for Buildings and Other
Structures – SEI/ASCE7-98 and updates (ASCE 2010). This document includes a
requirement about serviceability, and whilst serviceability limits are not specified the
US Bureau of Mines is generally considered to be a reasonable choice. The
extrapolation to incorporation into contract documents that specify a PPV value to be
measured within 30 inches of the ground has logical deficiencies for the following
reasons:
1) The value ascribed is often 1 inch per second (to provide a safety factor).
2) The original concept was that although buildings were able to amplify a
ground level vibration the maximum magnification was limited to a value
of 4 times.
3) The frequency limits of the original data have been removed from the
requirements
4) The criteria are applied to all structures, not just domestic housing
As a result of these factors a potentially serious situation has been allowed to develop
in that magnification factors of greater than 4 lead to some parts of structures being
exposed to a vibration intensity that is significantly above the thresholds that are
assumed to be associated with damage.
AN EXAMPLE THAT DEMONSTRATES THE PROBLEM
Measurements were made at a church building as
nearby ground clearance involved blasting that induced
vibrations in the structure. The purpose of the
measurements was to check the response of the
structure. This allowed the comparison of the
conventional methodology with a more modern
approach. An accelerometer was placed close to a
velocity measuring device within 30 inches of the
ground. Additionally, four accelerometers were placed
at roof level so as to capture the response of the two
transepts and the northern nave.
At the outset a baseline of the behavior of the structure
was established. This involves analyzing for the
presence of resonances. These were identified and the
major responses are listed in Table 1. Each resonance
has its own characteristic frequency and a characteristic
displacement throughout the structure. When a structure
is characterized in this way it is possible to invoke the
principle of simple harmonic motion and convert acceleration measurements to a
velocity equivalent.
Figure 3: Plan of the church building
Measurements of the response in these different
positions were subsequently captured on a continuous
basis, and the response at the time of the blasting was analyzed. On each day that
blasting was conducted the response at all of the measurement positions was analyzed
and compared not only with the PPV value but also with the vibration intensity
measured at ground level.
Table 1: Modal identification of the Church
Orientation
Mode
Freq (Hz)
Description
X
NS1
2.78
NS Translational
Y
EW1
2.98
EW Translational
Y
EW2
3.37
Nave EW Translation
1
4.60
1st Torsion Nave only
2
5.10
Probable torsion of Nave about Bell tower
3
6.80
Torsion of the East transept
4
7.30
Torsion of the West transept
The response of the church was obtained from the time history of the response at the
time of the blasting event. Figure 4 gives an example of a measured acceleration time
history in the East-West direction.
Figure 4: Example of the measured acceleration response at the time of a blast
Figure 5: Triaxial output from a velocity measuring device Ordinate Scale: 0.1 in/sec/ division. Abscissa Scale 0.1 secs/division What is particularly important is that the measured acceleration response is ‘clean’. The
frequency is easily measured and is consistent, so that the conversion to PPV is
straightforward. Direct measurement of velocity contains more high frequency
components and is therefore more problematic. Figure 5 gives an example of the direct
measurement of velocity. Because of the quantity of ‘noise’ in the signal the application
of the principle of simple harmonic motion is problematic with this waveform.
Subsequently, a more detailed analysis using a proprietary version of the random
decrement technique was used on the East-West behavior of the church structure. This
version of the random decrement (Jeary, 1986) allows the measurement of the nonlinear characteristics of the frequency of resonance and the damping, and is known as
a ‘Randec’ signature. Damping is particularly important because it is a measure of
energy dissipation. Any damage in a structure will cause energy dissipation by, for
instance, the working of cracks. In this case the characteristics are shown in Figure 6.
A monolithic structure tends to have a frequency characteristic that approximates a
straight line, while the damping characteristic is expected to gradually rise as the
amplitude of response increases. Figure 6 shows clearly that this is not the case for this
church, and the frequency jumps between two values, and the damping value
dramatically decreases at higher amplitudes. Coincidentally, the frequency settles to a
more stable value at these larger amplitudes too. This shows that the East-West
behavior of the church involves some independent behavior over a series of amplitudes.
The supposition is that the Eastern transept is able to have its own resonance in the
east-West direction. If this were to be the case then the maximum stresses would occur
at the interface of the transept with the nave, and cracking would be likely to occur. A
visual inspection confirmed this to be the case.
Figure 6: Randec analysis showing frequency and damping non-linearities in the East-West direction
Finally the relative responses at the various measurement positions were analyzed with
the following result:
Table 2: Averaged multipliers of response for all measurement positions
Position Average Individual Multiplier Multiplier NS EW Vert Ground 1.00 1.00 1.00 1.00 Tower east 7.16 2.96 10.05 8.46 Tower West 4.30 2.18 5.22 5.51 North roof 7.59 1.54 9.46 11.77 Bell tower 1.85 0.93 1.74 2.88 Table 2 gives rise to considerable concern since some individual responses include a
multiplication of the near ground vibration intensity by a factor of over 10. The vertical
amplification of the intensity above the nave reached a value of nearly 12, and since
the accelerometers were securely attached to stonework only the imposition of a PPV
value of 0.16 inches per second at ground level would result in the vibration intensity
being kept within the guidelines promulgated by the US Bureau of Mines.
THE SOLUTIONS AVAILABLE
The use of PPV measurements and the specification that measurements must be made
within 30 inches of the ground are the root of the problem. Modern accelerometers are
usable down to zero frequency and they are calibrate-able on-site in real time simply
by using Earth’s gravity to introduce a 1g signal. This avoids the problem of sending
instruments to be calibrated every year. Accelerometers do not have the low frequency
limit imposed by the use of velocity measuring devices, and so can be used for any
structure. Finally, there are three databases that show the vibration intensity that
probably causes damage under different conditions. If the energy in the vibration is
used instead of an arbitrary (but simple) measurement, then acceleration can be used
directly to determine vibration intensity limits that are likely to induce damage in
unreinforced parts of a structure. The use of acceleration rather than velocity introduces
a sliding scale for thresholds of damage that are a function of the frequency. But this
frequency is a structural parameter rather than a function of the ground-borne vibration.
As a result, the values are structure specific and the rules of simple harmonic motion
can be used to convert from velocity to acceleration.
AN ALTERNATIVE APPROACH
A standard for load testing has been introduced in Brasil (BNDS, 2006). This standard
refers to several other documents (Koch, 1953, RILEM, 1985, RILEM, 1984, ISO,
1986, DIN, 1939) including the early German work and the three databases including
the US Bureau of Mines data. It then goes on to recommend the use of the Vibrar scale
for the assessment of vibration intensity.
The Vibrar rating is an empirical parameter which is very useful to analyse the damage
levels of all structures. It was originally developed by Koch (1953) based on a large
database relating structural damage level with other parameters. The Vibrar rating V is
given by:
V = 10 log (160 π4 A2 f 3)
(1)
Where A is the amplitude in Centimetres and f is the frequency of oscillation, then V
is the vibration intensity in Vibrars. The Vibrar scale derives from the work of Koch
(1953) in which the following indicators of possible damage states are quoted (Steffens,
1973):
Table 3: Vibrar rating and prediction of damage (after Koch, 1953)
V
10 - 30
30 - 40
40 - 50
50 - 60
Damage level
None
Light damage
Severe damage
Collapse
The value of V=40 is normally taken as the threshold that roughly equates to a PPV of
2 inches per second for domestic structures. However, the larger values of V do not
appear to have a good correlation with damage states for larger engineered structures,
and some caution is recommended here. In the case of unreinforced masonry structures,
the predictions line up well with current criteria.
There have been several attempts to indicate a rational approach to the assessment of
vibration intensity that will cause damage to a structure. The problem is compounded
by the differing state of health of different structures, and so any system must
necessarily be limited to being a guide. The added problem of using velocity as the
basic measurement system merely compounds the difficulties, and makes the
extrapolation to structures
with
frequencies
of
resonance below 1 Hz.
pointless.
Steffens (1973) produced a
graph showing the
relationship between the
characteristics shown in
terms of acceleration,
velocity and displacement
(Figure 6). This graph was
produced in 1973, and while
it is appropriate to suggest
that such data are old and
may have been superseded
by more modern
information, it is germane to
appreciate that this graph
was produced at the same
time that the current basis
for the entire approach to
vibration intensity and its
effects in the US was also
being arranged.
A more modern approach is
produced below, but it will
be noted that there is little new information that supports these criteria that were
produced on the basis of large scale experimentation that was a feature of the 1960’s,
1970’s and 1980’s.
Figure 7: Steffens comparison of the effects of vibration intensity on structures The following table shows a comparison of various measures for potential indicators of the effects of vibration intensity on structural damage, as
contained in several worldwide codes. The values for peak particle velocity are included as a comparison.
Velocity Velocity
mm/sec
in/sec
At 4 Hz.
At 40 Hz
DIN4150/ISO
At 1 Hz. DIN/ISO
Acc'n
Acc'n
At 4 Hz
DIN Rating
g
g
Vibrar
Vibrar
Acc (g)
Vibrar
percentile
Threshold
1
0.2
0.0079
0.000512 0.005124
-2.2
0.00013
-8
Human
Discomfort
50
2
0.0787
0.005124 0.051239
18
0.00128
12
Perception
Panic
99
20
0.7874
0.051239 0.512391
37
0.01281
32
Structural
Cosmetic
15
0.5906
0.038429 0.384293
35
30-40
Light cracking
0.00961
29
Damage
Minor
30
1.1811
0.076859 0.768586
41
40-50
Severe
0.01921
35
BS7385
Major
60
2.3622
0.153717 1.537172
47
50-60
Collapse
0.03843
41
CONCLUSIONS
The use of velocity to measure vibration intensity through the application of limits set
on the value of peak particle velocity is a hangover from the technology of the last
century and introduces several difficulties that are becoming steadily more important
with the increasing size of some structures. Additionally, the use of data sets that were
produced from tests on domestic housing should be used only as a guide rather than the
currently used definition in contract documents.
Finally, structures amplify the vibration intensity by much more than was observed in
the US Bureau of Mines tests. A more rational approach would be to use an acceleration
based set of measurements that are usable at the low frequencies inherent in structures
taller than about 70 feet (eight storeys tall). The production of acceleration charts that
are consistent with established data sets should be a priority. An example of a church
that was subject to nearby blasting showed that the amplification by the structure
reached a value of nearly 12.
REFERENCES
1 Reiher H., and Meister F.J. (The Effect of Vibration on People) Forschung auf dem
Gebeite des Ingenieur-Wesens. V2n No. 11, 1931.
2.Steffens R.J. Structural Vibration and Damage. Building Research Establishment.
1973
3. Siskind D.E, Stagg M.S., Kopp J.W. and Dowding C.H. Structure Response and
Damage Produced by Ground Vibration From Surface Mine Blasting. RI8507 U.S.
Bureau of Mines Report of Investigations. 1980.
4. Jeary A.P. : Damping in tall buildings. Conference on the second century of the
skyscraper. ASCE/ Council on tall buildings. Chicago Jan 1986.
5. Associacao Brasileiro de Normas technicas. Non Destructive Testing.Dynamic
Loading Tests on Large Structures. ABNT NBR 15307, 2006
6. Koch H W. (1953). Determining the effects of vibration in buildings, V.D.I.Z. 95, 21,
744-747.
7. Rilem Dynamic behaviour of Concrete Structures - Final report of Rilem 65 MDB
Committee. March 1985. Elsevier, Amsterdam. ISBN 0-44442624-8 vol.13.
8. Rilem Dynamic behaviour of concrete structures. Recommendations of good practice
for methods of testing and design. (Rilem 65MDB committee). Rilem Conference on
the Long term
observation of structures. Budapest Sept. 1984.
9. ISO/DIS 4866 Mechanical vibration and shock - measurement and evaluation of
vibration effects on buildings - guidelines for the use of basic standard methods. UDC
69.058:534.834. International standards organisation 1986.
10.
DIN 4150 Protection against Vibration in Building Construction. German
Institute for Standards. Berlin 1939.