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22
HYSTERETIC DAMPERS FOR THE PROTECTION OF STRUCTURES
FROM EARTHQUAKES
R.I. Skinner, R.G. Tyler, A.J. Heine and
W.H.
Robinson*
SYNOPSIS
The development of hysteretic dampers for the protection of
structures against earthquake attack, carried out at the Physics and
Engineering Laboratory over the past six years, is described.
Details of both steel and lead devices and their application to
bridges and base isolated buildings are given. Steel devices are
designed to absorb energy by plastic deformation in torsion or
bending, while lead devices rely on plastic extrusion or shear.
The characteristics of PTFE sliding bearings are also described and the
possibility of using this type of bearing to permit sliding on base
isolated systems, and to allow dissipation of energy in joints in
conventional structures, referred to. The most promising development
is in the lead rubber bearing in which the properties of load-bearing
and damping are combined in one unit.
INTRODUCTION
The development and testing of hyster-
etic dampers at the Pjysics and Engineering
Laboratory was initiated in 1970 and has
proceeded rapidly since 1973 in order to
provide dampers for particular base isolated
structures,
which were at the same time
investigated by the Laboratory!, and were
the subject of theoretical studies^.
There have been a number of publications 2-13
giving details of various types of devices
and it is the intention in this paper to
give the latest details of research and to
write a commentary on the characteristics
of the devices, rather than repeat the details,
in order that a choice may be made between
the various types for a particular applic-
ation.
Up to the present the use of simple
types involving the loading of either
steel or lead into the plastic range has
been favoured, because of the need for
minimum maintenance. In addition bridge
bearings of rubber or PTFE have also been
the subject of tests, as a knowledge of
their characteristics is required for their
use in conventional ductile design of
bridge decks and also in base isolation
systems.
TYPES OF DEVICE
Work on steel energy dissipating
devices has been carried out in the
Engineering Seismology Section and on lead
devices in the Materials Science Section;
a range of devices is now available, the
principle of design being that energy
is dissipated by the plastic yielding of
either steel or lead.
1. Steel Hysteretic Dampers
Attention has been directed towards
the production of mild steel devices of
solid cross section, which do not become
unstable at high levels of plastic strain.
Black mild steel to BS 4360/43A or bright
steels to a similar composition have been
*Physics and Engineering Laboratory, DSIR,
Gracefield, Lower Hutt.
found to be the most suitable, preferably
heat treated for 5 hours at 620 C following
fabrication. In design, welding is kept
well away from highly strained zones other-
wise rapid failure results.
1.1 Torsion Beam Device
The first hysteretic damper developed
by the laboratory was of the torsional beam
type (Plate 1) for the Rangitikei Bridge
project
13
'
14
(Plate 2 and Fig. 1). In
the device the short sections of the beam
between the loading arms are overstrained
in torsion and bending. The initial
testing of models was followed by the testing
of the full scale device of 450 kN capacity
with a range of movement of up to 80 mm
(Plate 2).
The device offered a means of providing
a comparatively large dissipating force
from a welded fabrication using 600 mm
plate.
However, it is likely now that, for
large forces, the flexural beam device
(Section
2.1.5)
would be preferred in any
future application requiring a steel
device,
as this can be fabricated with a
minimum of welding using cast steel
arms.
1.2 Round Steel Cantilever
The tapered round steel cantilever
(Figure 2) was developed to provide damping
in a horizontal plane when used in conjunct-
ion with rubber bearings in base isolation
buildings.
The taper is designed to yield
over its whole length. Damping forces up
to about 100 kN for a movement of + 75 mm
can be provided (Figure 3) using steel
rounds commercially available, but above
this size fabrication of the base becomes
inconvenient as rounds above 150 mm
diameter may be difficult to obtain
commercially.
This type of device was originally
considered for the base isolated William
Clayton Building in Wellington , but was
later abandoned in favour of the lead-rubber
bearing (Section 2.2) which was simpler and
less expensive to fabricate and install.
BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING VOL 13. NO 1 MARCH 1980
23
Fig.
1: Details of Rangitikei Bridge
Plate 1: Torsion beam hysteretic damper in test
machine Plate 2: South Rangitikei Rail Bridge under
construction
25
1.3 Taper Plate Cantilever
The taper plate device (Plate 3) was
originally suggested as a simple alternat-
ive to the torsion beam type when space
permitted the use of a cantilever arm and,
accordingly a test programme was carried
out to establish the design parameters
(Figure 4). In practice however, it was
found that the fabrication of the welded
base,
necessary to keep welding well away
from the taper, proved more difficult
than at first thought, mainly because of
the tendency for the buttresses to shrink
away from the taper plate following welding.
This was however, overcome by a special
welding and machining procedure for the
fabrication of a 240 kN device for a motor-
way overbridge in Dunedin (Figure 5).
The taper plate design also found
application in a chimney at Christchurch
designed to rock on its base (Fig. 6 and
Plate 4) for which application the fixity
for the device was provided by extending
the plate, which was produced economically
mainly by profile cutting. The flexibility
of the bolted support system was not
established by testing, however, and it is
felt that, should further applications be
found for it, then more testing would be
required to establish this and check whether
the design charts (Figure 4) need to be
revised for this design.
1.4 Round Bars
The tenacity of ordinary reinforcing
bars in continuing to resist earthquake
loading, after concrete has spalled away
from them suggested that plain round mild
steel bars could be used to provide damping
in a base isolated system, provided a bend
or loop is introduced into the length of
the bar (Figure 7), to allow for extension,
without premature tensile failure, during
excursions in the horizontal plane. In
general for such excursions a mixture of
bending and torsion overstrain occurs in
a bar of this shape. For small excursions,
up to the elastic limit, the bars behave as
double cantilevers with maximum stress at
the fixities. Testing of bars of various
diameters (Plate 5) has revealed maximum
heat generation at the fixities for the
recommended design conditions
7
for varying
directions of horizontal attack on the
device,
thus indicating that bending at
these points is the predominant effect.
15
The method was first proposed as an
alternative to the use of lead-rubber
bearings for the William Clayton Building"
1
but was rejected because of the greater
simplicity of the lead-rubber device
(Section 2.2) and the possible embrittlement
of the steel following aging after overstrain.
However, the use of steel, in this particular
arrangement, may appeal to many engineers
since there is a progressive locking up as
horizontal deflection increases and the
bars straighten. In addition, in the
event of uplift on a bearing under disaster
conditions,
work will be done on the bars
vertically to straighten them, up to the
ultimate tensile capacity of the
bars.
This could have advantages at the corners
of buildings. Also the method may now be
regarded more favourably, as the collation
of results on specimens deformed cyclically,
stored for a few years, and again deformed
(Section 1.6) indicates that age embrittle-
ment is most unlikely to be a problem. Thus
the possible need for replacement during
the building
1
s lifetime is diminished.
Care would need to be taken in design, how-
ever,
to ensure that windstorms or thermal
effects do not carry the bars into yield"?.
Otherwise fatigue of the steel during normal
service would need to be designed for.
1.5 Flexural Beam Damper
In the flexural beam damper, loads
applied to the ends of the cranked arms
(Figure 8 and Plate 6) cause the circular
beam element to behave alternately as an
eccentrically loaded strut or tie depending
on the direction of loading. The cranked
loading arms produce a favourable geometry
in that the alternate bowing up and down
is compensated for in the changes in arm
leverage;
the geometrical effect produces
a near rectangular hysteresis loop (Figure
9).
The loading arms are of cast steel
and welding is confined to the ends of the
beam away from the loaded length of the
beam. A 300 kN device was designed for
the Cromwell Bridge
13
. The bridge is
connected to the fixed abutment through
a set of six dampers (Figure 10) to allow
damped relative movement during a severe
earthquake.
Neglecting direct stresses the load
Q, and the deflection +y are given by:
f d~
6r
(1) where d = diameter of beam
r = arm length
(Figure 8)
and y = 2Lr
e(2)
f = 350 MPa
and
= plastic stress
appropriate to design
strain levels = +0.03
for 100 cycles to
failure
7
.
= beam length.
For the prototype d = 114 mm, r = 280 mm
and L = 500 mm which gives Q
d
= 309 kN and
y = 74 mm, i.e. the stroke has the usual
value of about 150 mm.
By fitting smaller diameter beams to
the standard loading arms the capacity may
be reduced to about 150 kN while fitting
longer beams enables the stroke to be
increased by about 25%. The patterns
for the castings can be made available
by the Laboratory. If a device of load
capacity greater than 300 kN is required
then new patterns would need to be prepared.
1.6 Aging Tests on Steel Devices
A selection of the results for small
black mild steel specimens which were
loaded and allowed to age both naturally
and artificially during model tests for
the torsion beam dissipator (Section 2.11)
are given in Table 1. The results
indicate that, for the materials and strain
levels which have been adopted in the design
Fig.
2 Round Cantilever damper
Plate 3: Cantilever plate damper in test machine
Plate 4. Chimney at Christchurch de-
signed to step (From Beca,Carter, Hoi I-
ings & Ferner)
Plate 5. Failed round bars
Fig.
7 Base isolation method using round bars
11-000
ELASTOMERIC
BEARING
ENERGY
DISS
I
PAT
OR
SECTION A-A
CROSS-SECTION
THROUGH PIER AT
DISSIPATOR
Fig.
5 Cantilever plate dampers in Dunedin Motorway Overbridge (from
Park and Blakeley 13)
28
Fig.
8 Flexural beam damper Fig. 9 Force-displacement hysteresis loop for
flexural beam damper
TOTAL
SUPERSTRUCTURE MASS = 2500 TONNES
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1PERATURE
T
EMENT
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f
ELEVATION
TT
PLAN
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kN
'I'lTIT
DIRECTION
OF ACTION OF
ENERGY
DISSIPATORS
12
Om
3 5m DEEP
STANDARD
STEEL
TRUSS
SECTION AT PIER
ELASTOME
BEARINGS
Fig.
10 Flexural beam dampers in Cromwell
Bridge (from Park and Blakeley 13)
•HHHT
' -
Plate 6: Flexural beam damper at
extreme of travel in test frame
29
of steel dissipators, embrittlement is not
likely to be a problem following excursions
into the plastic range,
2.
Lead Hysteretic Dampers
2.1 Extrusion damper
The research on using, the plastic
deformation of lead for hysteretic dampers
began in 1971 with the invention of the
lead extrusion damper
9
'
1
. In the
extrusion damper the lead absorbs energy
by being extruded back and forth through
an orifice (Figure 11). On being extruded
the deformed lead recrystallizes immediately,
thereby recovering its original mechanical
properties before the next extrusion stroke.
Accordingly, the amount of energy absorbed
is not limited by work hardening or fatigue
of the lead. The damper behaves like a
"coulomb damper" or "plastic solid" in
that its force displacement curve is
nearly rectangular and it has very little
rate dependence. A particular advantage
of the extrusion damper is the long stroke
which is possible, as this depends only on
the buckling of the connecting rod which
can be designed accordingly.
Twelve extrusion dampers designed to
operate at a force of 150 kN and a stroke
of 500 mm
(
+ 250 mm) were manufactured
by Auckland Nuclear Accessories Company.
The MOWD installed these dampers in the
overpass bridges at Bolton Street and
Aurora Terrace in Wellington, with six
dampers in each bridge (Plates 7 and 8).
These two sloping bridges are supported
on glide bearings, with the deck motion
in the longitudinal direction due to
earthquake attack or braking vehicles
being resisted by the extrusion dampers
and the transverse motion by cantilevered
columns.
Since the extrusion dampers
have a high stiffness and low creep
rate,
any movement of the bridge due to
the braking of heavy traffic is kept to
a minimum
(<
1 or 2 mm/yr) .
The manufactured extrusion damper
consists of a steel tube (OD ^150 mm) with
an extrusion orifice at its midpoint; two
pistons are joined by a tie rod which is
extended from one piston to form a push
rod, with lead surrounding the tie rod
and filling the volume contained between
the pistons. The lead to steel interface
is lubticated and the lubricant retained
by chevron seals amounted in the pistons.
One end of the tube has a flange attached
to a spherical bearing while at the other
end the connecting rod is attached to a
similar spherical bearing. The bearings
were designed to take a vertical movement
of 100 mm of the bridge without any bending
moment being applied to the damper. The
damper plus end bearings weighs 150 kg,
has a length of 1500 mm and is contained
in a diameter of 260 mm.
A prototype of the manufactured extrus-
ion dampers was tested at the Physics and
Engineering Laboratory, DSIR
1
, intermit-
tently over a seventy day period, at a
frequency of 0.9 Hz for strokes of 150 mm
(+ 75 mm) and 250 mm ( + 125 mm). The
damper had a rectangular hysteresis
loop (Figure 12) and performed well over
the twenty tests each of 4.8 to 7.6 cycles
giving a total of 123 cycles and a distance
travelled of 53.2 m. The maximum power
consumption was ^68 kW while the maximum
force was 150 kN with a minimum of ^75 kN
after seven cycles soon after the previous
test.
The results showed that the damper
was well behaved in all aspects of its
performance showing no evidence of wear
or failure and that it should be capable
of operating through many major earthquakes.
2.2 Lead-rubber bearing
12
The lead-rubber bearing represents
the most promising development in the field
of base isolation. For this device a
laminated elastomeric bridge bearing is
modified by placing a lead plug down its
centre (Figure 13). The bearing carries
the weight of the structure and supplies a
horizontal restoring force while the plastic
deformation of the lead plug produces
damping. A 356 x 356 x 140 mm lead-rubber
bearing, containing seven 3 mm thick
steel plates, six 16 mm rubber plates and
a lead plug of ^100 mm diameter has been
tested at 0.9 Hz with vertical loads and
strokes of up to 450 kN and + 68 mm
respectively (Figure 14). This bearing
completed a total of
34 0
cycles and operated
satisfactorily at temperatures of -35 + 5°C
and 45 + 5°C. More recently a 650 mm
diameter x 297 mm lead-rubber bearing has
been tested with a range of lead inserts of
up to 3 MN and +90 mm. The results of
these tests together with the tests now in
progress on the lead-rubber bearings for
the Scamperdown (Figure 15), Toe Toe and
Waiotukupuna Bridges and the William Clayton
Building (Figure 16), (all MOWD) are being
used to prepare a design procedure for the
lead-rubber bearings.
The good performance of the lead-
rubber bearing is most likely due to the
fact that at ambient temperatures the
lead is being "hot worked" so that during
its deformation the lead recovers most of
its mechanical properties almost immediately.
Furthermore, all the lead confined by the
steel and rubber plates is forced to deform
uniformly in pure shear.
2.3 PTFE Sliding Bearings
Testing of PTFE sliding bearings was
initiated at the Laboratory because of the
possibility of using them in base isolation
systems in buildings, while at the same
time providing data to bridge designers,
as PTFE sliding bearings have been used in
long-span bridges for about two decades to
accommodate temperature movements. For
this application, the coefficient of friction
of pure dry PTFE sliding on stainless steel
is usually taken to be about 0.03.
Dynamic tests have been reported in
the literature but usually these have been
carried out at quite slow speeds, when
coefficients of friction of about 0.03,
or slightly greater, obtained, usually with
Plate 8: Wellington Motorway Overbridges
(b) ENERGY DISSIPATORS
AND
ABUTMENT FRICTION
SLABS
AS
PRIMARY SEISMIC FORCE RESISTING MEMBERS LEAD/RUBBER DEVICE
Fig.
15: Lead-rubber bearings in Scamperdown Bridge
(from Park and Blakeley 13)
t 0 C fc A
Cross section showing lead-rubber bearings at footings
Artist's impression of completed building
Fig.
16: Base isolated William Clayton Building
(from Megget 14)
33
a suggestion that if the speed is increased
then perhaps the coefficient of friction
will also go up. In fact, it does so
markedly, as tests at the Laboratory have
shown
1
^.
For conditions equivalent to
a moderate to severe earthquake, viz. a
travel of 150 mm and simple harmonic
motion at frequencies up to 0.83 Hz, giving
a maximum velocity of
3 8
cm/sec, frictional
coefficients up to 17% were obtained for
the pressures normally employed in bridge
bearings (Figure 17). Thus the damping
obtained from PTFE bearing in the dry
state is rate dependent; a near rectangular
hysteresis loop is obtained with friction
peaking up at the beginning of the stroke
(Figure 18).
A promising development for.base
isolated structures is in the field of
lubricated PTFE bearings. For the
conditions given above a frictional
coefficient of less than 2% was obtained
(Figure 17) for a type of greased lubricated
bearing which has been employed by one
manufacturer of bridge bearings for more
than a decade. This opens up the possib-
ility of using a combination of lubricated
PTFE and rubber or lead-rubber bearings in
a base isolation system to reduce the
transmitted horizontal shear to a minimum.
There may be a cost restriction however as
PTFE bridge bearings tend to be more
expensive than rubber bearings of the same
capacity.
Lubricated PTFE layers bonded to steel
plates,
sliding in stainless steel, have
been used at the Laboratory in reciprocating
motion in research applications, following
the original work establishing the low
coefficient of friction, and no failures
have occurred. One such example was the
use of a patchwork of bonded PTFE pieces
(Plate 9) as a slider operating as part of
the 'sandwich
1
in tests on lead-rubber
bearings for the William Clayton Building.
Many hundreds of cycles have been performed
with a maximum velocity of
4 0
cm/sec at
25 MPa pressure. In the normal way a
sandwich of two lead-rubber bearings would
have been tested in double shear but the
tractive force would have been too great
for the testing machine available; hence
the use of the PTFE layer. The lubricant
is replaced around the patchwork of PTFE
pieces after each test to keep the
coefficient of friction down to a minimum,
and as a precaution against failure,
because of the tendency of the lubricant to
be driven out from between the sliding
faces during operation. It is felt that
failures of lubricated PTFE bearings for
rotating shafts, reported in the literature,
are caused by the bearing changing from a
lubricated to an unlubricated state, with
the consequent rise in friction, and heating,
of the bearing. In another application
in the laboratory a slider lined with
lubricated PTFE pieces has been used at
slower speeds in the testing of steel
flexural beam absorbers. In a base
isolated building it has to be borne in
mind that there will be little movement at
the bearings borne in mind that there will
be little movement a the bearings prior to
the earthquake movement, Hence it should
be possible to seal off the bearing against
the ingress of dust and to insure that
lubricant is available at the bearing
surfaces for the short time the bearing
is required to slide. Sealing of the
bearing against dust ingress has been
found to be most necessary as very high
coefficients of friction have occurred
when cement dust has been introduced between
the bearing surfaces during testing . A
French group has developed a metal to
metal sliding bearing in combination with
rubber in a development for a base isolated
nuclear power station
18
and the character-
istics of this type of bearing needs to
be compared with those of PTFE.
The application of PTFE sliding
layers in joints within conventionally
designed buildings has also been examined
19
and it has been concluded that where such
joints can be included to secure panels
and internal partitions, the friction
generated during sliding, caused by
racking of the building would assist in
providing damping of either earthquake
or wind generated oscillations. Such
a joint has already been employed to
allow temperature movements in curtain
walling in high-rise buildings^O(Figure 19).
3. Marketing of Devices
The mechanical energy dissipating
devices developed by DSIR are patented
through Development Finance Corporation
of New Zealand, and are manufactured and
marketed by a firm or firms selected by
the Corporation. Their cost includes
any royalty payable to the Corporation at
the time of sale. The costs of devices
installed in bridge structures to date
are given elsewhere
13
.
4.
Discussion
The design parameters related to
the bilinear loop characteristics for
steel and lead devices are given in
the companion paper on the base isolation
of buildings
1
^. When laminated rubber
mounts are used, and when it is appropriate
to provide hysteretic damping at the
locations of the mounts then it is simple
and economic to use lead-rubber bearings.
Hence the lead-rubber bearing offers the
cheapest way of base isolating a building
since the functions of bearing and damping
are contained in one unit, reducing both
the unit cost and the installation cost.
For bridges, this use of the same bearing is
a natural development, as plain unmodified
laminated rubber bearings have been used
to allow temperature movement in bridge
decks for about two decades. Because of
the ability of lead to recrystallise at room
temperature following deformation, the lead
inserts are not likely to fatigue as a
result of the comparatively slow temperature
movements,
whereas with steel devices,
repetitive movement into the plastic range
needs to be taken into account and prefer-
ably eliminated by using them as a connection
to a fixed point as in the case of the Crom-
well Bridge, where expansion occurs at the
20n
18-
16-
Ld
U
Ld
O
U
8
y
Q: 6
Stroke t 72-5mm
Figures give
temperatures at start
of each test °C
^-At 0°C [J,^ 24-0-0-45p
A20°
30-
KEY
A
Stow 0-2 Hz approx
•
Fast 0-83 Hz .
$ Intermediate 0-5 Hz
+ Lubricated - slow
^
Lubricated - fast
T
40°
A
20°
+ 21°
+ 20°
5 10 20 30
BEARING PRESSURE p(MN/m
2
)
Plate
9:
PTFE sliding bearing used
in
test
on
lead-rubber bearing, which
is
below
the
plate
shown
Fig.
17:
Friction characteristics
of
PTFE
bearings
for
first cycle
of
loading
Fig.
18:
Force-displacement loops
for
tests
on pure PTFE sliding layers
for
pressure
of 23
MPa
Fig.
19:
PTFE joint
for
curtain walling
(from Dupont
20)
35
other end of the structure. For stepping
structures,
where damping only is required,
steel devices are to be preferred from
the point of view of simplicity and
expense,
although the lead extrusion
dissipator could also be employed. Again
if lubricated PTFE mounts are used it
would be appropriate to use steel beam
dampers.
Research to date has not demonstrated
any significant age embrittlement of
the steel beam dampers. While the present
policy is to provide for the possible
replacement of devices should this be
necessary, it is most likely that, for
a return period of about 100 years for the
most severe effects of a major earthquake
at any particular location in New Zealand,
the devices will survive the life of a
structure, without requiring replacement.
With replacement unnecessary it is
practical to use plain round bars embedded
in concrete at either end being used
as energy dissipating devices. Lubricated
PTFE sliding bearings offer possibilities
for favourable application in base
isolation systems where sliding is combined
with the use of rubber bearings for central-
ising the building; in addition their
use as a damping element in joints within
conventionally designed buildings should
be considered.
REFERENCES:
1. Skinner, R.I., Beck, J.L. and Bycroft,
G.N. "A practical System for Isolating
Structures from Earthquake Attack",
International Journal of Earthquake
Engineering and Structural Dynamics,
Vol.
3, 1975, pp. 297-309.
2.
Skinner, R.I. and McVerry G.H. "Base
Isolation for Increased Earthquake
Resistance of Buildings" Bulletin
of the New Zealand National Society
for Earthquake Engineering, Vol 8, No.
2,
June 1975, pp
93-101.
3. Skinner, R.I., Kelly, J.M. and Heine,
A.J. "Hysteretic Dampers for Earth-
quake-Resistant Structures",
International Journal of Earthquake
Engineering and Structural Dynamics,
Vol.
3, 1975, pp.287-296.
4*
Skinner, R.I., Heine A.J. and Tyler
R.G. "Hysteretic Dampers to Provide
Structures with Increased Earthquake
Resistance". Proceedings Sixth
World Conference on Earthquake Eng-
ineering, New Delhi, January 19 77.
5. Tyler, R.G. "Tapered Steel Cantilever
Energy Absorbers", Bulletin of the
New Zealand National Society for
Earthquake Engineering, Vol. 11,
No.
4, December 1978, pp. 282-294.
6. Tyler, R.G, and Skinner, R.I.,
"Testing of Dampers for the Base
Isolation of a Proposed
4-storey
Building against Earthquake Attack",
Proceedings Sixth Australasian
Conference on the Mechanics of
Structures and Materials, University
of Canterbury, New Zealand, August
1977,
pp.376-382.
7.
Tyler R.G., "A Tenacious Base
Isolation System Using Round Steel
Bars",
Bulletin of the New Zealand
National Society for Earthquake
Engineering, Vol. 11, No. 4, December
1978,
pp.273-281.
8. Tyler, R.G. "Dynamic Tests on Laminated
Rubber Bearings", Bulletin of the
New Zealand National Society for
Earthquake Engineering, Vol. 10,
No.
3, September 1977, pp.143-150.
9. Robinson, W.H. and Greenbank, L.R.
"An extrusion Energy Absorber
Suitable for the Protection of
Structures During an Earthquake",
International Journal of Earthquake
Engineering and Structural Dynamics,
Vol.
4, 1976, pp.251-259.
Robinson, W.H. and Greenbank, L.R.,
"Properties of an Extrusion Energy
Absorber", Bulletin of the New
Zealand National Society for Earth-
quake Engineering, Vol. 8, No. 3,
September 1975, pp.187-191.
11.
Robinson, W.H., Test of ANAC's
Extrusion Damper, PEL Report No.
5
91,
January 1977.
12.
Robinson, W.H. and Tucker, A.G.,
"A Lead-Rubber Shear Damper",
Bulletin of the New Zealand National
Society for Earthquake Engineering
Vol.
10, No. 3, September 1977,
pp.151-153.
13.
Park R. and Blakeley R.W.G. "Seismic
Design of Bridges". Structures
Committee Summary Vol. 3, Road
Research Unit, National Roads Board,
New Zealand, November 1978.
14.
Beck, J.L. and Skinner, R.I., "The
Seismic Response of a Reinforced
Concrete Bridge Pier Designed to
Step".
Int. Journ. of Earthq. Eng.
and Struct. Dyn. Vol. 2, 1974, pp.
343-358.
15.
Meggett, L.M. "Analysis and Design
of a Base-Isolated Reinforced
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NZ Nat. Soc. for Earthquake Engineer-
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Vol. 11, No. 4, December 1978,
pp.245-254.
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Blakeley R.W.G., et al. "Recommendat-
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May 1979.
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September
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18.
Jolivet, F. and Richli M.
,
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TABLE 1. STRAIN HISTORY OF TORSION BAR SPECIMENS
N.B.
All specimens rectangular bars in black mild
steel.
Carbon content
0.20°/o
except for
No.
779 which was
0.0296.
No prior heat
treatment. Some specimens showed cracks
in cycling prior to aging but these did
not get noticeably worse subsequently.
There were no fractures.
Strain history
prior to aging
Stra.i n history
folloxvurig natural aging
Strain history following
artificial aging
Specimen
No
Date
Strain*
(± %)
No.
of
cycles
Date
Strain*
(± *)
No,
of
cycles
Date
lrti ficial
Aging
Time (brs)
at
100°C
Strain*
(± *)
No of
cycles
Cross
Section
(mm )
.751
12/72
1.9
400
8/12/78
3-8
5.7
22
12
13/12/78
36
3-8
5.7
7.6
22
17
4
25
x 25
752
12/72
3.8
30
7/12/78
3-8
5-7
60
10
757
1
1/73
3.8
407
29/11/78
3.8
128
12.5-50
758
1
1/73
3.9
300
6/12/78
3.9
30
766
2/75
.4
•
7
1.4
2.
8
4.3
5
8
8
8
4
5/12/78
1.4
2.8
4.3
2
124
24
*i
778
4/75
2.
2
500
1/12/78
2.
2
1
20
11/12/78
12
2.
2
3.4
42
7
it
779
4/75
2.
1
500
4/12/78
2,
1 120
11/12/78
12
2.
1
3.4
42
7
^Calculated maximum shear strain
