Ishibashi, Tomohiko

Kitada, Toshiyuki

Yamaguchi, Takashi

Osaka City University Repository

Mem. Fac. Eng., Osaka City Univ., Vol. 47, pp. 25-30 (2006)
Experimental Study on Vibration Control of Steel Poles Using Chloroprene Rubber
Tomohiko ISHIBASH(, Toshiyuki KITADA" and Takashi YAMAGUCH("
(Received September 29, 2006)
Synopsis
Recently, it is pointed out that the bridge vibration caused by the traffic causes damage to a pole type steel
structures such as lighting poles or marker poles on elevated bridges. Some damages induce collapse in the
severe case and its collapse may cause secondary accidents. Therefore, vibration problem of the pole type steel
structures should be solved as soon as possible. As the number of such steel poles which should be maintain
sufficiently is very large, it is important to avoid resonance and to improve damping performance of them
economically. The presented herein are discussed a proposed vibration controlling technique for steel poles by
using chloroprene rubber base and the effectiveness of this method based on the experimental results..
KEYWORDS: Pole Type Steel Structure, Vibration Control, Resonance, Damping Performance,
Chloroprene Rubber
1. Introduction
Vibration of pole type steel structures becomes serious problem in these days!) because vehicles weight is
getting heavier, the number of them is increasing significantly, and rubber supports are adapted to many bridge
structures due to improving seismic performance.
To avoid resonance and to improve damping performance of the steel poles, which are set onto the
superstructures with various setting conditions and natural frequencies, becomes very important. For example,
IMD (Impact Mass Damper)2) or TMD (Tuned Mass Damper)3) may be attached to the steel pole in order to
improve damping performance. IMD does not have enough damping performance to control such traffic
vibration. On the other hand, TMD is effective except for economical aspect. Accordingly, some other
economical and practical methods for vibration control of steel pole structures will be strongly needed.
It is known that the rigidity of the base of a steel pole affects the vibration characteristic significantly. It is
possible to change the base rigidity by installing the rubber sheet under the base plate of the steel pole
structures. Proposed are seismic isolation rubbers and damping rubbers made from various materials. In this
study, the chloroprene rubber, which is a general material and not expensive, is applied to the basement rubber.
Effectiveness of the rubber sheets for vibration control is investigated experimentally by using scaled
specimens of an actual steel pole structure.
2. Vibration Test
2.1 Specimens and Experimental Cases
The specimens used in this study are shown in Fig. 1. The size of the specimens is almost 1/2 of actual
lighting poles. The height of the specimens is 5000mm. 200N weight is set on the edge of the arm section as
shown in Fig. 1. This weight is corresponding to that of an illuminator. Prepared are the chloroprene rubber
sheets varying the thickness (3mm, 6mm, 15mm) and hardness (45Hs, 65Hs, 80Hs) of the sheet. The
geometrical configurations of the rubbers are shown in Fig. 2. The size of the chloroprene rubber sheets is
almost same as that of the base plate. The rubber is separated into two parts so that to replace. Summary of
experimental cases are shown in Table 1. The case without the rubber sheet is named as in-nOn-plane) and
out n(out-plane).
* Nasu Denki-Tekko Co., Ltd., Student, Doctor Course of Dept. of Civil Engineering
** Professor, Dept. of Civil Engineering
*** Associate Professor, Dept. of Civil Engineering
- 25-
1, 050
We i g h t
(200 N)
HO 5X3. 2
Steel Band
""x
'" '
Fig. 1 Geometrical configurations
of the specimen (unit: mm)
Fig. 2 Dimensions of
chloroprene rubber sheet (unit: mm)
Table 1 Summary of experimental cases
Case name vibration Hardness(Hs) Thickness(mm)direction
in_n --- ---
in r 45 3 3
in_f_45 9 45 9
-
in r 45 15 15
in r 65 3 3
in_r_65_9
in-plane
65 9
in r 65 15 15
in r 80 3 3
In r 80 9 80 9
in f 80 15 15
out_n --- ---
out f 45 3 3
out r 45 9 45 9
out r 45 15
out-plane
15
out r 65 9 65 9
out f 80 9 80 9
2.2 Loading and measurements
The plane view of the loading frame for the vibration test is shown in Fig. 3. Main beams made of the H-shape
steel as shown in Photo 1 correspond to the main girder of the bridge. Detail of the support points is shown in
Photo 2. The pins covered with rubber are used to realize simple supported condition at each support points.
The hydrofluoric actuator with the capacity of 1000kN is jointed to the center of the loading beam and applies
the idealized bridge vibration as shown in Fig. 3. In case of applying in-plane vibration, the specimen is set at the
center part of the main beam. In case of applying out-plane vibration, the specimen is set at vicinity of the support
point.
In this study, free vibration and forced vibration test are executed. All experimental cases are tabulated in Table
1. In free vibration test, 16 cases as shown in Table 1 are executed. Case in n(normal case of in-plane) and
out_n(normal case of out-plane) are prepared for comparison focusing on the hardness and thickness of the
chloroprene rubber and the direction of the pole's vibration.
The initial displacement for free vibration is given at the top of the pole by human power. As for the forced
vibration test, the same 16 cases as tabulated in Table 1 are also carried out. The harmonic force is given as the
displacement, whose amplitude is ±5mm. Its frequency is set to be in a range of ± 10% for the natural
frequency of the pole.
In this experiment, the displacement of the beams, the accelerations at the top of the pole along in-plane and
out-plane direction and the strains at the vicinity of the top of the welded rib plate are measured by digital strain
- 26-
meter with 100Hz sampling rate.
Actuator
\
Support points)f'"
P---
. ! I
-~, , ~
Setting position of
pole specimen 1
1
r
.~. . •...~ ".1')1)'1",[1" 1(1
.-.~
Support points --'-j--'
Fig. 3 Plane view of loading frame (unit: mm)
Photo 2 Details of support point
3. Test Results and Discussions
The results of the free vibration tests are summarized in Table 2.
It is found that first natural frequencies have been changed slightly by using rubber for both directions. The
change of them is limited to almost 5%.
Logarithmical damping coefficients have been improved up to almost twice of them for each original case. But,
there is little influence on change of logarithmical damping coefficient by varying thickness or hardness of the
rubber.
The power spectrums obtained from the free vibration tests, in_n and in_r_45_9 are plotted in Fig. 5. It is
considered from these results that the shapes of the spectrum have not be changed significantly by installing the
rubber. In fact, there is little difference of the shape of the spectrum between the original case and other cases with
rubber.
Table 2 Results of the free vibration test
(a) in-plane (b) out-plane
Case name First natural Logari thmicalfrequency (Hz) damping coefficient
in n 1. 477 0.016
in r 45 3 1. 477 0.024
in r 45 9 1.440 0.025
in r 45 15 1.440 0.022
in r 65 3 1.465 0.025
in r 65 9 1. 453 0.030
in r 65 15 1. 428 0.027
in r 80 3 1. 465 0.020
in r 80 9 1. 465 0.030
in r 80 15 1.440 0.033
Case name First natural Logari thmicalfrequency (Hz) damping coefficient
out n 1. 428 0.018
out r 45 3 1. 392 O. 041
out r 45 9 1. 361 0.050
out r 45 15 1.367 0.032
out r 65 9 1. 398 0.032
out r 80 9 1. 416 O. 032
- 27-
-t
----+----------1
.- --- --····-----1
i
I
...----- -~~1
I
- ---------1
I
1.61.51.41.3
1---- - - --
I-~--' ----- .-
I---~- ------------
r-----~ --.--
I
O.OEtOa
1.2
3.0Et06
'""'~ 2.5Et06
~ 2.0Et06
~
~
~ 1.5Et06
tl
<l)
~ I.OEt06
~~ 5.0Et05
1.61.51.41.3
3.0Et06
'""' 2.5Et06u<l)
en
N 2.0Et06<~
~
E 1.5Et062
tl
<l)
0. 1.0Et06en
...
<l)
~ 5.0Et0500...
O.OEtOO
1.2
Freque.ncy(Hz) Frequency(Hz)
(a) in_n (b) in_r_45_9
Fig. 5 Power spectrums in free vibration test
1. 1001. 0501.0000.950
I-------~.- .--- -- i n_n
... _------ -
- -----~--- ---- - in_f_65_3
1---------- _ __ i n_f _65_9
·-.-inf65 151
Frequency ratio
(b)Hardness 65Hs
;:; 800
;: 700
r:.:<..
:; 600
cd 500
u
400
0. 300
e
< 200
u
~ 100
s:::& 0
0.900
. - - ~-- ~ -- - .------_. t
--in n
-II-- i n_r _45_3
It< i n_r_45_9
.- i n f 45 15 :-
1. 000 1. 050 1. 100
Frequency ratio
(a)Hardness 45Hs
500
.....
0
u 400cd
"""'
c:
0 300.-
cd
u
200
0.
E
<>:: 100
u
'-E
cd ~-
s::: 0»
Cl
0.900 0.950
Frequency ratio
(c) Hardness 80Hs
Fig. 6 Resonance curve in forced vibration test
(in-plane)
--in n
10.0
--in.n
---=-=-_ i n_r~6U
Fig. 7 Time history of acceleration
at the top of the pole
..----.c-a-- ,---- -.-- -- --
-400 L-_...L.- ~ _
0.0 2.0 4.0 6.0 8.0
Time (sec)
500 l.--~400300
'" 200~
""0 100
'" 0....
.,
~ -100
u
<:
-200
-300
1. 1001.050
- in_f_80_3
in_f_80_9
---tr--- in f 80 15
1.000
..... 5000
u
cd
"""' 400
s:::
0
.-
cd 300
u
0. 200
E
<>::
jfL
~IOO.~
& 0
0.900 0.950
- 28-
The resonance curves of in-plane direction obtained through the forced vibration tests are shown in Fig. 6.
It is found that the response of the cases with the 9 mm rubber and I5mm rubber decrease greatly in the cases
with any hardness of the rubber.
As shown in Fig. 6(b), the response of in_r_65_3 is larger than that of the original case(in_n) . It might be
caused by the phase difference between the pole and the beam. Because it is observed that the phase of the
specimen is changed greatly at vicinity of resonance point, and that the displacement of the beam is mitigated in
high frequency ratio range. In addition, the acceleration at the top of the pole in the case in_65_3 isn't so
different from that of the case in_n (normal case) as shown in Fig. 7.
Frequency ratio
(a)Hardness 45Hs
1. 1001.0501.000
,--- -_..._------.-
I
r----·-·
I
o
0.900 0.950
Frequency ratio
(b)Thickness 9mm
;) 150
+'
u
til
c.r...
C
o.~ 100
til
U
.....
4-<
.....
...-<
~ 50
<C
u
.....
E
til
C
>,
Cl
1. 1001. 0501.000O. 950
--out_n
---- ou t_r_45_3
out_r_45_9
··-+--out r 45 15
o w:
0.900
~ 50
<C
u
.,...,
E
CIl
c
>,
Cl
~ 150
+'
u
CIl
c.r...
c
S 100
+'
CIl
U
.,...,
4-<
.,...,
...-<
Fig. 8 Resonance curve in forced vibration test (out-plane)
O~~~~~~
OBO OB2 OB4 OB6 OB8 lDO lD2 lD4 lD6 lD8 1.10
Frequency ratio
Fig. 9 Resonance curve of stress degree
in forced vibration test (in-plane)
180
160
140
N
~ 120
"-
z
ll00
bJl
Q)
'0
'" 80
'"Q)
.:0
(/)
60
40
20
--in.n
. ..--.. in.r.45.9
in.r.65.9
~in.r.80.9
The resonance curves of out-plane direction of
forced vibration tests are shown in Fig. 8. The
effect of installing the rubber can be also confirmed,
but this effect is less than that of in-plane direction.
This is caused by the reason the response of case
ouen is smaller than that of case in_no It is
considered that the method of installing the rubber
sheet is effective in case of large response.
For the same reason, there is little effect in the
case of out_r_80_9(Fig. 8(b)). It relates that the
fixed condition doesn't change. On the other hand
the result of the case, out_r_45_9 is quite different
from other results. It is considered to be caused by
the appearance of shearing mode.
The resonance curves of the stress at the
welding of the rib plates obtained from the forced
vibration test are shown in Fig. 9. The effect on the
stress is larger than amplification of the
displacement. This is because that the impact
power might be softened by the rubber sheet.
- 29-
4. Concluding remarks
As an economical vibration control method, the method installing the chloroprene rubber sheet to the basement
()f the pole type steel structures is proposed. The effectiveness of this method is investigated experimentally
through the free/forced vibration test assuming the bridge vibration. Concluding remarks of this study are
"ummarized as follows.
(1) At the resonance frequency, dynamic amplification factor of the pole installed the rubber sheet can be
reduced to about half that of the pole without the rubber sheet. But it is necessary to use the rubber that
thickness is more than 9mm in order to obtain the effect.
(2) This method is more effective in stress reduction than dynamic amplification factor reduction, because of
softening of the impact power.
(3) The larger response brings the larger change of dynamic rigidity and can getthe larger effect.
(4) Decrease of frequency isn't so much (about 5% in large).
5. References
j) Foundation of Road Management Technology Center, Commission of Inquiry about Safety of Road
Attached Structure: Damage and Measures Example ofRoad Attached Structure, 200l.
2) K.Sumi, THonjo, KMorikita, KArima, M.Nanjo: A Study of the New Type Damper for a Light Pole,
Journal of Structural Engineering, Vo1.39A, 1993.
3) Tlshibashi, TKitada: Vibration Characteristic of a Lighting Pole on an Elevated Highway Bridge and
Effect of TMD for Mitigating Vibration, International Workshop on Structural Health Monitoring of
Bridges/ Colloquim on Bridge Vibration 2003.
- 30 -


Experimental Study on Vibration Control of Steel Poles Using Chloroprene Rubber

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Experimental Study on Vibration Control of Steel Poles Using Chloroprene Rubber

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