Short-Span Bridge Friendly Suspensions-Research Element 6 Of The OECD
DIVINE Project
Robert J. Heywood
Queensland University of Technology, Australia
Figure 1. The six-axle articulated test vehicle fitted with air suspensions crossing Cameron's Creek.
ABSTRACT
A study of the response of short-span bridges was
conducted to investigate the influence of vehicle
suspensions on the dynamic response of bridges. Three
bridges have been tested with plans to test two more bridges
as part of the series. This paper refers to the tests conducted
on the bridge over Cameron' s Creek. The objectives of this
study were i) to measure the dynamic responses of short
span bridges to vehicles fitted with air or steel suspensions,
ii) to rate the steel and air suspensions in tenns of the
European Community requirements for road friendly
suspensions, iii) to detennine how the dynamic bridge
response for air and steel suspended vehicles varied when
axle hop was excited, iv) to identify and understand the
important parameters involved in the dynamic response of
short span bridges, and v) to recommend measures for
reducing the dynamic response of bridges to the passage of
vehicles.
Road transport technology--4. University of Michigan Transportation Research Institute, Ann Arbor, 1995.
365
ROAD TRANSPORT TECHNOLOGY-4
The results of the study indicate i) air suspensions met
.the requirements for road friendly suspension unlike the
steel suspensions, ii) the significant differences between air
suspensions and steel suspensions lead to different dynamic
responses in short span bridges, iii) when axle hop
vibrations were excited, the dynamic response of the
bridges was sensitive to vehicle speed, bridge natural
frequency and road roughness.
To date three bridges have been tested with plans to test
two more bridges as part of the series[4,S). This research has
shown that the dynamic response of these short-span
bridges can, under certain conditions, lead to large increases
in both the peak deflections/stresses and the number of
stress cycles experienced during the passage of a vehicle
fitted with air suspensions. Under other conditions the
dynamic response was small for air suspended vehicles
compared with similar vehicles fitted with steel
suspensions.
The study demonstrated that maintaining smooth
approaches and profiles across bridges is imperative to
reducing damage to bridges. When the road roughness
excited axle hop, the bridge and the suspensions coupled
leading to large dynamic increments for vehicles fitted with
air suspensions travelling at critical speeds. It is believed
'road friendly' suspensions are likely to be short span
'bridge friendly' except for bridges that dynamically couple
with axle hop vibrations and provided suspension dampers
are operating efficiently. For vehicles to be friendly to
short-span bridges, wheel forces with axle hop frequencies
need to be controlled.·
This paper illustrates the dynamic response of shortspan bridges using bridge and vehicle response data
collected during the passage of air or steel suspended sixaxle articulated test vehicles crossing· the bridge over
Cameron's Creek. Two of these vehicles were almost
identical except for their suspensions. In this way the
relative effects of a steel and an air suspension were
investigated. The reasons that the air suspended vehicles
induced large dynamic responses in the bridge over
Cameron's Creek are discussed and recommendations for
improving the short span bridge friendliness of vehicle
suspensions are discussed.
INTRODUCTION
BRIDGE OVERCAMERON'S CREEK
The OECD DIVINE (dynamic interaction between
vehicles and infrastructure experiment) international
research project is investigating scientifically the influence
of vehicle suspension on road and bridge infrastructure[I,2).
One of the six research elements is investigating the effects
on bridge infrastructure. Dr Cantieni(3) is investigating the
response of longer span Swiss bridges.
This paper
discusses the results from Australian research focused on
short span (less than 15 m) bridges.
~
-
_
to Raymond Terrace
Cameron's Creek is a relatively modem (1976) four
span, simply supported, prestressed concrete deck unit
bridge, the most common short span bridge in Australia
(Figure 1 and Figure 2). It is located on Bucket's Way to
the north of Sydney, New South Wales, AustraIia. The
t. Body bounce bump to Stroud
l
9.0 m
f
,
{Steel suspensIOns)
7
£
,____ !r-;7/axI_e_d_etectors_r-;;::::j·(:::::::;:::::;-"r~;:+:;::::~I_(lriniv.;j1
N~-
--;r..--r,
-
detector
-o-G{4-8
-
-~/B-
a.Sm'
I
f--_ _ _ _..1.-_ _ _ _-l.-_ _ _ _~.--_-=o:...::D:.!(4-c:...:.14..:.j
, \
'-
-
I
914m
I
914m
914m
f
914m
Plan
West
0:r,~~m!...-----'8=.53=..:m"-'-----~f~O.~" m
East
- __ Ai
.0/
:iii __dh~""""""
-r-'------------r-~-
O(~}'
~--
Axle hop bump.(300X25 mm)
repaired approach
0(4-8)
'Q(4_14)
0(1-8}
Section
Figure 2. Cameron's Creek - Details and instrumentation layout.
366
axle
,
I
BRIDGE LOADING AND RESPONSE
Table I Details of test vehicles
Prime-mover
Trailer
Freightliner,
air suspension
Over the rear axle tri-axle tipper,
BPW air suspension, 1.23 m
spacing
6.05 t
0
t
Freightliner,
Hendrickson wallcing
beam, steel suspension
4.07
Gross
Laden Mass (t)
15.85 t
20.65 t
00
000
IP~
4.65
1
Vehicle Code
42.5
BA 42.5 1.23
42.5
BS 42.51.23
~2¥23t
12.48
Over the rear axle tri-axle tipper,
York 8 leaf steel susp'n, 1.23 m
spacing
6.35 t
16.15 t
o
00
19.75 t
000
3.71
prestressed concrete deck units act compositely with a castin-situ reinforced concrete slab which is the running
surface. The superstructure is supported on reinforced
concrete abutments and piers via elastomeric rubber bearing
strips.
suspensions of vehicle BS 42.5 1.23 did notf61• Thus the
research Was aJ:>le to compare the bridge response for
geometrically similar vehicles with either 'road-friendly' or
'non-road-friendly' .
ROAD ROUGHNESS
The 9.14 m (30 ft) spans have a fundamental natural
frequency of 11.3 Hz and damping coefficient (I;) of 1.5%
(logarithmic decrement cS = 0.51). One span has a mass of
110 t and a stiffness of 165 kN/mm determined from the
midspan deflection due to a 20 t tri-axle group placed at
midspan. The bridge was chosen because its natural
frequency is similar to the frequencies associated with axle
hop, providing the possibility of resonance.
TEST VEHICLES
A test vehicle with either steel or air suspensions fitted
was used to excite the dynamic response of the Cameron' s
Creek bridge. In this way, the influence of the suspensions
were compared. The trailer body was constant throughout
the tests. The tri-axle groups and the prime-movers were
interchanged to provide vehicles with almost identical
dimensions and properties except for the steel or air
suspensions. The details of the vehicles are provided in
Table I. The vehicles were loaded to their legal load of
42.5 t.
The suspensions have been characterised using the
European Communities drop test which defmes a 'road
friendly' suspensions as one with natural frequency less
than 2.0 Hz and damping greater than 20%. The air
suspended vehicle BA 42.5 1.23 satisfied the requirements
for 'road-friendly' suspensions.
The mechanical
TIlE ROAD PROFILE
This research has highlighted the importance of road
roughness on the dynamic response of short-span bridges.
The road profiles were measured by the Roads and Traffic
Authority of NSW using a laser profilemeter. A typical
profile is presented in Figure 4. In order to artificially
increase the roughness in a controlled manner, 300 by 25
high bumps designed to excite axle hop (AHB) and 6000 by
25 bumps to generate body bounce (BBB) were fitted to the
bridge surface (Figure 4). The southern approach to
Abutment A exhibited settlement and was repaired using
cold-mix asphaltic concrete. The result was a substantial
increase in short-wavelength roughness immediately before
the bridge.
PSD VERSUS SPATIAL FREQUENCY
The power spectral density versus spatial frequency7]
for the road profile has been determined (Figure 3). This
road is quite rough and clearly not of freeway standard.
However it is consistent with the quality of an ageing
infrastructure of secondary roads.
COMMENTS
The approaches to the bridge are much rougher than the
bridge itself. The reinforced concrete running surface is in
good condition. It is quite smooth except for the upward
camber in each span (induced by creep in the prestressed
367
ROAD TRANSPORT TECHNOLOGY-4
T- - -- -,- - ----,- ----
6.40
-1- -
-
-
-
I - - - - -
"'j - - - - -
t
\
I
I
,
I
J
J
I
I
approac,
,
7 - - - - - i - - - - - ,- - - - - -
1- -
i
I
I
I
I
l
l
-
-
-
-,
I
6.45 -I - - - -R~~cdh- - -;- - - - - -;'~!~ --~:)- --: -AHB-- - - ~ - - - - - ~ - - - - - ~ - - - - -:- - - - --:
~
E
~ 6.50
T-----:- --\ --:- -----:- -
~~
T-~~
6.55
.=
" '- I
-
-
-
: : - ' "-
-:
:
-
-
r
I
I
I
-
~
-
I
-
-
-
--.. ----- ~ -----.. -----.. -----,- -----,
~
-
-
-
-
-
I
-
-
-
-
,.
-
-
-
-
r - - - - - ,- - - - - -:
-
I
I
I
I
I __ - __ '_ __________
6.60
~
::
.!
is
I
:Abut A
,
I ,
"
- T - - - - - ;- - - - - - ,- .-~~J..
-, - - - I
I
6.65 ~ _____ :______ .Span 1 _ , S p a n 3 _
I
I
-20
o
-10
.! _ _ _
I
Span 2
-
20
30
I
~'".
I
.
--I
,~:
. :
:
7 - - - - - I - - - - - I' - - - - -,- - - - - -,
Span 4
10
"
'
40
50
60
70
80
Chainage (m)
Figure 4. Road profile - Cameron's Creek, northbound.
concrete deck units). Compared with the approaches, the
bridge surface does not exhibit the short wave length
roughness required to induce axle hop.
As the vehicles move onto the bridge they are excited
by the rough road profile. As they leave the bridge they
have been driving over the relatively smooth profile of the
bridge. The dynamic response of the first and the last spans
with and without the axle hop bwnp (AHB) fitted provides
important data for comparing the influence of road
roughness on the dynamic response of the vehicles and the
bridges.
10'
. ..
H'I
10'
. '"
.
J
I
I
1111\
I
I
I
I
I I11
Ill/
III
I
I
I
I
I
I
I
I1111
I I II1
I
r
I
I
I
I
I I 11
I I11
L L _
I I!. _ _ _
.!.. _ 1.
I
I t
I
I
I
I I1I
I
,
I
t
I
t
I
I
I I
.-. 10·'
1:
'-'
11>
r
Ii
5 10-'
.
!
I
_I_I_I..,! ,LI _ _ _ _ 1__ 1_
I
I
I
I111
I
I
I
I
1111\
I
J
l .1 1. Ltl
I
t I I I1
I I I11
I
I
I
I
II1
'
I
~1(rr--..!-.l--'"
Q
::
..
t
r
;:
r
I
I
I
I
I
11111
I
I
I
I
I I I r I
I
~
I
__ ~ _ ~ _ ~ ~ ~ ~ ~ ___ ~ _
tit
I
I I
l. _: _ I
r
lit
~ ~I
I
I
I
I
I
I
I
I
I
III
I
1
I
I
I I I
111111
I
I
t
I
I
IIII
I
I
111111
I
I
t
I
1
I
.- 10'" !
Cl
I
I
I
1 I I I I
t
I
I
I I I 11
1
lit
r---'--I-I-TI-J-ll---I-T-I-I-tlTlr---I--t-TI
-I.
I
t(r lO I
10"'
I
•
J
I
I
11 I
I
I
•
I
I I 1I
I
I
I
I
• I 1I
J
I
J
I
I
I1I
I
I
t
I
I1II
I
I
I
I
1111
I::
I
I J I :
I::
I
I : I I
:
t::
I I I I
10·'
10'
10'
Spatial frequency (cycles/m)
100
I
I
10
1
O.
Wavelength (m)
Figure 3. Power spectral density versus spatial frequency
for the road profile at Cameron's Creek.
DYNAMIC WHEEL FORCES
The dynamic load applied by each wheel of the tri-axle
group was measured simultaneously with the bridge
response. Figure 5 presents the dynamic force on each axle
and the sum for the three axles in the tri-axle group. From a
368
The dynamic force waveforms (Figure 5) are
characterised by body bounce (l to 3 Hz) and axle hop (8 to
15 Hz) frequencies. The sample power spectral densities
illustrate these differences (Figure 6). The dynamic forces
associated with the steel suspensions' body bounce
frequencies have a larger amplitude and a higher frequency
than the those of the air suspension. The peak total force
for the tri-axle group is approximately 270 kN for the air
and 300 kN or 10% more for the steel (Figure 5). An
analysis without considering dynamic effects would suggest
that the bridge would deflect 10% more under the influence
of the truck fitted with steel suspensions. This would ignore
the dynamic interaction between the bridge and vehicle,
which is very important for this bridge.
.
I
I
_
:
bridge response standpoint, this total dynamic force for the
tri-axle group is more important than the force on individual
axles. For example two axles vibrating 1800 degrees out of
phase are unlikely to excite a bridge but two axles vibrating
in phase will excite a short-span bridge.
The natural frequency of the bridge over Cameron' s
Creek is approximately 11.3 Hz. This corresponds to the
axle hop frequencies of. the dynamic axle group forces.
These high frequency vibrations are clearly evident in the
time and frequency domains (Figure 5 and Figure 6). They
are most evident when the tri-axle group is traversing the
repair prior to abutment A, the axle hop bwnp at pier 3 and
the approaches. When the tri-axle group is on the bridge
and the axle hop vibration has been damped, the high
frequency axle hop component is quite small. This is
consistent with the observation that the bridge surface is
much smoother and induces far less axle hop vibrations
than the approaches or the axle hop bwnp (Figure 4 and
Figure 3). A comparison of the axle hop vibrations for the
air and the steel suspensions shows that they are more
prQminent for the air suspension. This conclusion is even
stronger when the comparisons are made for the total
dynamic force. This increase in activity in the frequency
range of the bridge leads to substantial amplification of the
bridge response for some critical speeds.
BRIDGE LOADING AND RESPONSE
350
I----
Repair
r - - - - ~ .
300 t
f
., -
Span 1
--r--
TOiaT~CtOrCe~r1be-le
I
I
I
Span 2
I
-7--
I
-.--I
I
1---
1----1-----1
I
I
I
50
o
u
0.5
2.5
3.5
4.5
4
Time(s)
(a) Air suspensions, 59 km/hr over the axle hop bump (AHB), BA 42.5 1.54.
350 -
t
300
~o
~..
- - - - , - - - - ~ -
Repair
I
-'-----1-----1
., I
t : : :
t
I
I
I
I
I
I
----,----~----.,I
I
I
I
I
I
I
1
-
,
-
-
-
-
-1- -
-
-
-I
I
----r----~----.,-
I
200
~
.
oS
..
150
«
100
o
1.5
0.5
2.5
3.5
4.5
TIlDe (5)
(b) Steel suspensions, 62 kmlhr over the axle hop bump (AHB), BS 42.5 1.23.
Figure 5. Cameron's Creek - Dynamic axle forces for trailer tri axle group.
200.-.---------------~----~--_.
1.5 Hz
150
100
\---~-----4------p-----~-----
I
.. ~... j.... .L ..... l..... j.....
200
I -----~-----
100
:
I
50
I
5
:
I
:
I
I.
15
12.3 Hz
-~-----I------r-----i-----
I
:
I
I
--T-----~-----
10
3.3 Hz- - - ~ - - - - - - ~ - - - - - ~ - - - - -
300
20
2
Frequency of force function (Hz)
(a) Air suspensions, 47 kmlhr, Axle hop bump, BA 42.5
1.23.
OL---~----~----~----~==~
o
5
10
15
20
2
Frequency of force function (Hz)
(b) Steel suspensions, 67 km/hr, Axle hop bump, BS 42.5
1.23.
Figure 6. Cameron's Creek - Power spectral densities of dynamic wheel force- drivers side centre wheel for trailer triaxle group.
369
ROAD TRANSPORT TECHNOLOGY-4
I-Crawl
/_60kmlhr
_100kmlhr
-1.50
-2.00
-2.50
o
5
10
15
20
25
30
35
40
Distance to steer axle (m)
(a) D(4-8), Truck BA 42.5 1.23, Northbound, Axle hop bump.
1.00
0.50
_Crawl
_62kmlhr
-103 kmlhr
Deflection -0.50
(mm)
-1.00
I
-1.50 1_ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
I
::: t:::::::::::::::::::::::::::::::::::::::::::::::::.
o
5
10
15
20
25
30
35
40
Distance to steer axle (m)
(b) D(4-8), Truck BS 42.5 1.23, Northbound, Axle hop bump.
Figure 7. Cameron's Creek deflection waveforms.
BRIDGE RESPONSE
SAMPLE WAVEFORMS
The response of Cameron' s Creek bridge was found to
be sensitive to speed, roughness and vehicle suspension.
Figure 7 demonstrates the influence of speed and
suspension type. The deflection 0(4,8) is located in the
centre (8th bridge plank from the left) of the 4th span of the
bridge, the span immediately after the axle hop bump
(Figure 2).
With the axle hop bump (AHB) in place, vehicles
travelling at speeds approximately 60 kmJhr induced up to
10 substantial stress reversals during the passage of the
vehicle. At 100 kmIhr the number of cycles had reduced.
370
In the case of the air suspended vehicle, the peak deflection
was much larger at 60 kmIhr than at 100 kmJhr whereas for
the steel suspension the peaks were similar at these speeds.
Despite the softer air suspension giving smaller peak
dynamic wheel forces, the largest peak deflections induced
in Cameron' s Creek bridge corresponded to the passage of
air suspended vehicles, provided axle hop had been excited.
To assist in a more general comparison of the bridge
response it is helpful to consider the peak dynamic
deflection scaled against the peak deflection for a vehicle
traversing the bridge at a crawl speed.
DYNAMIC INCREMENT
The dynamic increment (DJ) provides a means of
comparing the peak dynamic deflection with the peak
BRIDGE LOADING AND RESPONSE
rt ---:- ---: ----: --- ---+- - -
150% T - - -~- - - -,- - -,- - -.., - - - -:- - - -
r - - -
~
I
I
I
I
I
I
:
I
:
I
;
I
1
I
I
I
I
I
I
I
I
I
I
I
I
I
J
I
I
I
I
:
I
I
I
I
~
125%
-
:
I
± ---:-: -- :
e 100% +
_1- -
-
_
E
Cl)
+
- - - ~ - - -:- - - -: - - - -; - - - ~ - - - ~
I
I
I
I
I
I
I
I
I
I
:
.J -
I
:
:
:
:
:
:
:
-
I
I
-;-
j
:
-'- _
I
Q)
~
- - -,- - - -,- - - -.- - - - - - -
I
- _.!. _ _ _ .l. _ _ _ I.... ___
1_ _ _ _ , _ _ _ _ 1 _ _ _
t
I
I
I
I
I
'
:
I
I
i
I
J ___ J
I
I
:
75% ~ ___ :____ :___ ~ ___ ~ ___ ~ ___ ~ ___ ~ ___ :____ :____: ___ ~ ___ ~
T
!
I
i
I
I
I
I
I
)1'
I
1
I
T
:
I
I
I
I
,
I
I
\
'
,
,
]
E"
,..
~
-
I
tt ---:-: ---:- --~- --~ ---~- --~ ---~ -- -:- -1-:\- --:-- -~- --~
ID
C
50%
C
I
I
I
I
I
I
I
I
I
r
I
[
I
I
I
I
I
I
I.
±- - - -:- - - -: - -
25%
~
I
I
I
I
I
I
I
I
I
I
I
I
I
J
- - - +- - - T- - - ~
_ _ _ 0(1.8)
- 0 - - 0(4.3)
_ _ _ 0(4.8)
--<>- 0(4.14)
:- - - -
1
I
I
o%~!~~~~~~~~~~~~~~~~~__~~
-120 -100 -80
-60
-40
-20
o 20 40 60 80 100 120
Velocity (km/hr)
(a) Air suspensions - BA 42.5 1.23,
1500/0 T - - -,- - - -,- - - -,- - -.., - - -
E
cs
~
50%
-
r- - - -,- - -
-1- -
-
-,- -
-., -
-
-,
I
I
t
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
1
I
I
I
I
I
+- --T- - -:-- --:- - --:- - -~:- - - ~-- - ~
I
:
I
I
I
I
I
1
I
I
I
I
I
:
I
I
I
I
I
I
I
1
I
I
i ---:- ---:- ---: --- ~ --- ~ --- ~ --- ~ - --:- ---:- --"\.
-- ~ --- ~
_ _ _ 0(1.8)
+
1
.;..
,
-0--0(4.3)
i
T
I
-c
l00"A. .1
~
1
Ell
75%
-
I
I
1
U
-,.. -
I
I
125% -± - - -:- - - -;- - --:- - - ~ ---
!
-
I
t
~
"T -
i
~
I
r
I
I
I
I
I
I
I
1
I
I
I
I
I
1
I
I
I
I
I
I
I'
,
I
I
J .•
I
I
1
J
"
t ---;- --i:- -- ~ --- ~ --- ~ --- ~ --- ~ ---:- ---:- - T~ - '
..
I
I
I
\1
t
I
I
I
I
I
1
t
I
I
I
/'
I
I
I.
J
1
1
1
I
~
---~
t
I
t ---:- --- -'- --~- --~ ---~ ---~ ---~ ---:- ---:- -~- --, ---~
'7
1
7
25% 1.
~
.
I
I
I
I
I';
I
-,
I
I
r
___ ,____ ' ___,_
_ ~ ___ .!. ___
I
I
± :.;
+,
:
~
,
I
,
I
~
I
I
;
I
I
I
I
--+-0(4.8)
--<>- 0(4.14)
,
_ - _ '___ ,I____ ,I
:
:
:
O%+t~~~~~~4-~~~~~==~~~~~~~
-120
-100
-80
-60
-40
-20
o
20
40
60
80
100
120
Velocity (km/hr)
(b) Steel suspensions - BS 42.51.23.
Figure 8. Cameron's Creek - Dynamic increment versus speed, No bumps
deflection reported at crawl speeds[8,9,IO). It is defined!S) as
follows:
DJ =
(8
dyn
-8
,)
SialIC
X
100%
(1)
OSlatic
where
8 dyn
8Sialic
DISCUSSION
The following general observations are made:
and
1. The dynamic increment (DJ) is small (less than ~5%) for
speeds less than 40 km/hr.
Dynamic increment (DJ) versus vehicle velocity graphs
are shoWn in Figure 8 and Figure 9. Positive velocities
refer to northbound traffic and negative velocities
correspond to the vehicles travelling in a southerly
direction. It should be noted that the vehicles travelled
within their lane and that Australian traffic drives on the left
2. Dls greater than 50% are relatively common. Dls of
100% or more were recorded. The largest DI recorded
was 137% for another air-suspended vehicle which was
excited by the pavement repair. These are much larger
than the 30% dynamic load allowance used in Australian
bridge design!II).
peak
=
dynamic
deflection
side of the road (Figure 2). Further, the dynamic increment
(DJ) has been calculated only for the deflections that are
directly under the zone of influence of the vehicle.
peak static deflection.
371
ROAD TRANSPORT TECHNOLOGY--4
3. The relationship between DJ and speed was different for
each suspension.
correspond to the deflections at the centre of the first and
last span of the bridge. Given a smooth road one would
expect their response to be similar to each other and for
both northbound and southbound traffic. This is not the
case.
4. The DJ due to air-suspended vehicles was less than the
DJ associated with steel suspensions, unless axle hop
was excited and it coupled dynamically with the bridge.
Figure 8a (air suspension and no bumps) illustrates that
the dynamic increment for D(I,8) is dramatically different
for northbound and southbound traffic. Generally the DI is
less than 25% except for speeds around 60 km/hr
5. The DJ due to steel-suspended vehicles tended to
increase with speed.
150%
T - - -,- - - -,- - -
t
I
125%
_
c
100%
E
~
u
.5
75%
U
E
c:
,..
C
500A>
-r - - - 'T - - -
-
T" -
~
t
-,- -
-.-"
:
:
:
:
:
:
I
I
I
I
I
I
I
I
I
I
I
!
I
:
I
I
:
I
-
-
I
:
I
-1- -
-
-
-
-
I
:
I
-I -
- - - ~ - - -:- - - -:- - - -:- - - ~ - - - ~
I
:
I
-I -
~
L__ ~ ___ ~ ___ !____~:___!___ J___
!____!___ J___
I
:
:
-
-
I
I
.. -
-
-
I
:
J
I
.... -
I
1- -
-
-
-
-
:
I
-I -
-
-
_____ 0(1.8)
I
:
I
-
J
I
:
1
I- -
--{j-
I
-I -
-
-
I
I
I
I
I
I
1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
- - - + - 0(4.8)
I
___ ,I ____ ,I ___ ..,I ___
I
I
I
I
I
I
I
1
I
I
I
I
I
~0(4.14)
___ ,
1
I
I
1
~---~---,.---,..-
I
0(4.3)
-I
!
I
T
-., -
:
-1- -
+
-
:
I
- -
-1- -
:
___ :____
t±
r - - -,- - -
-
:
~
t
-
:
t ---:- ---:- ---:- -- ---+- - -
-
CD
"-1- -
:
I
25%
O%L-~~~~~~~~~~~~~~~
-120
-so
-100
-60
o
-20
-40
20
40
60
so
100
120
Velocity (km/hr)
(a) Air suspensions - BA 42.5 1.23 AHB.
150%
- - -,- - - -,- - - -,- - -, - -T
T
T
125%
'T - - -
j- -
-,... -
100%
~
-,-- -
-.- -
-,---"'1
-
I
I
L
I
I
I
I
I
I
I
I
I
r
I
I
I
I
I
I
I
J
I
I
I
I
I
I
1
I
I
I
1
1
1
l l
1
1
I
1
I
!
1
I
'I
I---'----'----'-----
e
.5
-
1
J
T
E
-,- -
I
I
+- ___ :____ :____:_ - - ~ - - - +-- - ~ - - - ~ - - -:- - - -;- - --:- - - ~ -- - ~
Ii
-
-
I
l
l
~I
I
I
I
.I. - - - l. - I
I
I
75%
- -
±
,
U
.
E
~
-1- I
1
-
I
-1- _ I
I
1
-4 I
I
1
-
-
"" I
1
I
-
-
.... I
I
I
t
-
-
.. I
J
-
-
~
I
-
-
-1- -
I
r
I
I
I
-
-1- I
I
I
-
-1- -
:
-
.... I
I
I
I
-
-
1,1
- - - i J - 0(4.3)
..j
I
- - - + - 0(4.8)
I
~0(4.14)
SOO/of---:--Cl
',
,
t---:- ---:-T
25%
_____ 0(1.8)
- IL - - -'- - - '! - - - -'1 - - _.J1 - - - J1
1
::::::;:::::
'
,
i
O%+i~~~~~~~~~~~~~~~~~~~~~~~~~~~
-120
-100
-SO
-60
-40
-20
0
20
40
60
so
100
120
Velocity (km/hr)
(b) Steel suspensions - BS 42.5 1.23 AHB.
Figure 9. Cameron's Creek - Dynamic increment versus speed, Axle hop bump.
INFLUENCE OF ROAD ROUGHNESS
A more detailed consideration of the influence of road
roughness is beneficial. Deflections D(I,8) and D(4,8)
372
northbound where the dynamic increment jumps to almost
75%. This is related to the response of the bridge to the
axle hop induced by the repair immediately before span I
BRIDGE LOADING AND RESPONSE
for northbound traffic. By the time the vehicle has reached
the fourth span most of the axle hop has been damped out
and 0(4,8) registers a small DI. The reverse did not occur
for the southbound traffic as the northern approach to span
4 does not exhibit the short wavelength features required to
induce axle hop despite a relatively large depression.
limiting the opportunity for the axle hop vibrations to be in
phase for steel suspensions. Figure 5 confirms the notion
that the axle hop component of the total dynamic force for
the tri-axle group is more significant for the air suspended
vehicle at approximately 60 km/hr - the critical speed for air
suspensions at Cameron's Creek.
The above pattern is repeated when the axle hop bump
was installed over the pier immediately to the south of
span 4 for northbound traffic (Figure 9a). The response of
0(1,8) is similar with and without the axle hop bump as it is
too far away from the AHB to be substantially influenced
by it. However for 0(4,8) the maximum DJ increases from
25% to a maximum of 115% for northbound traffic whilst
exhibiting minimal changes for southbound vehicles. The
southbound vehicles have crossed the transducer prior to
crossing the AHB. This pattern is repeated for D(4,3).
Speed is important in determining the extent to which
axles in an air suspended group vibrate in phase. The
natural frequency of the bridge is most important. In the
case of Cameron's Creek this is 11.3 Hz which is consistent
with axle hop vibrations (Figure 6). For the purpose of
illustration, consider a series of axles with a static plus a
dynamic component with a 11.3 Hz natural frequency and
an amplitude of 10010 of static. Assuming the dynamic
component is induced by striking a defect then the dynamic
wheel forces will add as illustrated in Figure 10. For typical
axle group spacings, speeds of 50 to 60 kmJhr are likely to
result in the axle hop components summing whereas at
higher speeds they will tend to cancel. This is consistent
with observed behaviour. Thus speed is a critical issue for
air suspensions and the critical speed (vcrit.ah) is
approximately the axle hop frequency (fah) times the axle
SUMMARY
Thus it is concluded that the existence of short
wavelength defects in the running surface dramatically
increase the dynamic response of the bridge over
Cameron ' s Creek for the air suspended test vehicle
travelling at speeds of approximately 60 kmIhr.
For the steel suspended vehicle (Figure 8b and Figure
9b the response is different. Firstly the AHB tends to
induce confusion rather than the clear cut response of the air
suspension. Likewise the differences between northbound
and southbound are not as distinct. For example, 0(1,8)
experiences large responses in both directions, with and
without the AHB. It is concluded that the response of the
bridge over Cameron's Creek to vehicles fitted with steel
suspensions is more consistent with body bounce forces
rather than axle hop.
r
The evidence suggests that the dynamic response of
short span bridges will be small for high quality roads and
freeways for both steel and air suspensions.
spacing (s).
vcrit.ah
'"
(3)
s*fah
25
~
III
~
ca
Tri-de
20
.!:!
Q.
~
...
0
15
tJ)
CD
xc
10
u
E
ca
c:
5
50
>-
c
0
SUSPENSION BEHAVIOUR
0
0.2
0.4
0.6
0.8
1
Time (s)
The differences in design philosophy for steel and air
suspensions are substantial. Steel suspensions rely on
interleaf friction for damping whereas air suspension utilise
hydraulic dampers. Under static loads, load sharing is
achieved in air suspensions by air pressure equalisation.
Under axle hop conditions this does not occur as the air
does not have time to flow between the airbags. Thus air
suspensions behave independently for these high frequency
loads. If each axle in an air-suspended group strikes a
defect one axle hop cycle apart then each axle will vibrate
in phase and the total force for the group will include a
significant axle hop component (Figure 5).
In contrast, steel suspensions rely on pivoted beam and
rocker mechanisms to achieve load sharing within a group.
In this case, when an axle encounters a short wavelength
defect it induces a response in the other axles in the group
('cross-talk'). This tends to confuse the axle hop behaviour,
Figure 10. Influence of speed on dynamic axle
group loads
In the case of an axle hop natural frequency equal to the
bridge natural frequency (11.3 Hz) and a spacing of 1.30 m
(drive tandem), the critical speed would be 11.3*1.23 =
13.9 mls = 50 kmIhr. This is similar to the peak at 60 kmlhr
evident in the DJ versus velocity graphs.
BRIDGE VEHICLE INTERACTION
A constant axle load crossing a bridge will induce a
dynamic response in a bridge due to the pulse experienced
by the bridge. This effect increases with speed but is
relatively small at normal operating speeds. Of greater
significance is the dynamic response due to the dynamic
373
ROAD TRANSPORT TECHNOLOGY--4
variation in the axle and axle group loads. For the purpose
of illustration, assume that the bridge can be represented as
a single degree of freedom system with a natural frequency
(00) = 11.3 Hz and damping (0 = 1.5%. The corresponding
classical amplification factor versus the frequency of a
sinusoidal forcing function is presented in Figure 11. This
illustrates that body bounce frequencies are unlikely to be
amplified by the bridge but that the axle hop components
can be substantially amplified. The amplification factor is
33 for continuous forcing frequencies equal to the natural
frequency. Smaller amplification factors are anticipated as
a consequence of the limited number of cycles of load
applied as a vehicle crosses the bridge.
35
rLrr
30
,'<
,..,1
"'l: 11.3 Hz
,L 1,s'ib
1
I
25
fArTlP'n
0.00
0.20
0.40
0.60
o.SO
1.00
Time (5)
2O
Figure 12. Influence of frequency
Foetorf
15
I
10
I
I
5
o
l"...;'
o
2
4
6
8
~
10
SHORT-SPAN BRIDGE FRIENDLY SUSPENSIONS
I\.
",12
iooo..
14
16
18
20
ffequency of force funelion (Hz)
Figure 11. Magnification factor versus frequency
of force function for a single degree of freedom
approximation of Cameron's Creek bridge.
Figure 12 illustrates the simulated midspan deflection
due to a single 10 t axle with a 2 t (20%) dynamic
component at 1.5, 4.0 and 11.5 Hz crossing the single
degree of freedom model of Cameron's Creek at 50 kmIhr.
There are two lines for each frequency in Figure 12. The
central line is the deflection response if the bridge were to
respond in a linear relationship with the axle load. The
outer line includes the dynamic amplification. This figure
further illustrates that the body bounce modes (1.5 and
4 Hz) are transmitted without significant magnification
whereas at the axle hop frequency of 11.5 Hz significant
dynamic amplification occurs.
Thus it is concluded that the axle hop components of
the dynamic wheel force can induce large dynamic
fluctuations in bridges with natural frequencies in the axle
hop range (8 to 15 Hz). Axle hop components are
amplified by short-span bridges. In the case of the air
suspensions, the axle hop components within a group add at
some critical speeds before being amplified by the bridge,
inducing a large number of damaging stress cycles. This
behaviour was far less noticeable for the steel suspensions
tested.
In contrast, the dynamic body bounce components are
transmitted to these short-span bridges without significant
374
amplification. Road friendly suspensions are softer and
generate smaller peak loads than conventional suspensions.
As these body bounce forces are not amplified, the
corresponding bridge response is generally smaller for road
friendly suspensions.
The softer air-suspensions result in smaller peak
dynamic wheel forces. However there is a tendency for
axle hop to become more important. To minimise the shortspan bridge response, the component of the total axle group
force at axle hop frequencies must be controlled in addition
to the forces associated with body bounce.
Dampers provide the major control of axle hop in the
case of air suspensions. Their quality, performance,
geometric positioning and condition are vital in controlling
axle hop. Bridge responses induced by a petrol tanker fitted
with similar air-suspensions to that of the test vehicle
showed even larger excitation due to axle hop when the
tanker crossed the bridge at its critical speed. The tanker's
dampers were fitted in a less geometrically efficient
manner, confirming the importance of axle hop damping
(Heywood, 1995). The response of short-span bridges to an
air-suspended vehicle with non-functioning dampers
requires investigation.
It is suggested that the bridge response to axle hop may
be controlled on a axle group basis as well as for individual
axles. Should the axle hop vibration of adjacent axles be
out of phase to the extent that when added they cancel, then
the bridge will experience a smaller response. For single
axles this is not possible and it is anticipated that the bridge
response would be significant over a wide range of speeds.
For tandem and tri-axle groups, the existence of the other
axles in the group means that axles will be out of phase
except for a critical range of speeds. It is for these critical
speeds that other solutions are required. Possibilities
include adjacent axles having different natural frequencies
and in the case of tri-axle groups, unequal axle spacings.
BRIDGE LOADING AND RESPONSE
CONCLUSIONS
REFERENCES
The dynamic responses of three short span bridges to
vehicles fitted with air or steel suspensions have been
measured experimentally.
1. Road Transport Research.
Dynamic loading of
pavements. Report prepared by an OECD scientific
e{Cpert group, OECD Publications, Paris 1992.
The air suspensions met the European Community
requirements for road friendly suspensions whereas the steel
suspensions did not.
2. Sweatman, P.F. OECD Research in Dynamic Loading
of Pavements, OECD DIVINE Project Dynamic
Loading of Heavy Vehicles and Road Wear Mid-Term
Seminar, Paper 4. Sydney, Feb, 1995.
The peak bridge deflections were less for the air
suspended vehicles than for steel suspended vehicles unless
axle hop was excited and the vehicles travelled at critical
speeds.
When axle hop vibrations were excited, the dynamic
response of the bridges was sensitive to vehicle speed and
bridge natural frequency. At these critical speeds, mUltiple
fatigue cycles were induced.
The steel suspended vehicle applied the largest dynamic
wheel forces .. These are associated with truck body bounce
modes which are not amplified by short-span bridges.
The maintenance of smooth approaches and profiles
across bridges is a very important factor in reducing
. d3IIlage . to bridges. . This applies to short . and. long
wavelengths. It has been demonstrated that a cold mix
repair to a bridge approach induced axle hop which coupled
dynamically with the bridge.
'Road friendly' suspensions are likely to be short-span
'bridge friendly' except for those bridges that dynamically
couple with axle hop vibrations and provided suspension
dampers are operating efficiently.
Short-span bridge-friendly suspensions require axle hop
to be controlled. For groups of axles, different natural
frequencies between axles and/or unequal spacing if tri-axle
groups could reduce the dynamic response of sensitive
bridges.
ACKNOWLEDGMENTS
This research has been funded by the Roads and Traffic
Authority of New South Wales with in-kind contributions
from the National Road Transport Commission, Boral
Transport (trucks), Transpec Australia (air suspension),
BPW Germany (instrumented axles), York Australia (steel
suspensions), Hamalex Australia (suspension fabrication)
and Mercedes Benz (video). The contribution of Road User
Research, colleagues from OECD DNINE project and
QUT, and the Australian Road Research Board are
gratefully acknowledged. In particular
3. Cantieni, R.1. and Barella, S. Swiss Testing of Medium
OECD DIVINE Project Dynamic
Span Bridges.
Loading of Heavy Vehicles and Road Wear Mid-Term
Seminar, Paper 18. Sydney, Feb, 1995.
4. Heywood, R.J. DIVINE Element 6 Dynamic Bridge
Testing Short Span Bridges. OECD DNINE Project·
Dynamic Loading of Heavy Vehicles and Road Wear
Mid-Term Seminar, Paper 19. Sydney, Feb, 1995.
5. Heywood, R.J. 'Road-friendly' suspensions and short
span bridges, Australian Road Research Board Research
Report ARR 260, Dynamic Interaction of Vehicles and
Infrastructure, Proceedings of DNINE Special Session:
17th ARRB Conference, Ed K.G. Sharp, Melbourne,
.1994, pp. 39~65.
6. Sweatman, P.F., McFarlane S., Ackerman C., George,
R.M., Ranking of the road friendliness of heavy vehicle
suspensions: low frequency dynamics, National Road
Transport Commission, Melbourne, 1994.
7. ISO Draft 8608. International Organisation for
Standardisation. Mechanical vibration - Road Surface
profiles - Reporting ofmeasured data, Berlin, Germany,
1991.
8. Cantieni, R.I. Dynamic Behaviour of Highway Bridges
Under the Passage of Heavy Load EMPA Report No.
220, Dubendorf, 1992
9. Billing, J.R. and Green, G. Design provisions for
dynamic loading ofhighway bridges. TRR 95011,1984.
10. Ministry of Transportation and Communication.
Ontario Highway Bridge Design Code - Commentary.
Published by the Highway Engineering Division,
Toronto, Canada, 1993.
11. AUSTROADS. Bridge Design Code. AUSTROADS,
Sydney, Australia, 1992.
375