EXPERIMENTAL STUDY ON PREFABRICATED HIGHWAY STEEL BRIDGE Yongzuo Zhu, male , was born in 1948, a Member of Planning Division, Ministry of Communications of China, has undertaken research management for a total of fifteen years. He has applied himself to carry out bridge test for a long time He is in charge of technology developing and quality management of steel bridges.
ABSTRACT The purpose of this project is to study factors of Prefabricated Highway Steel Bridge (PHSB) during design theoretically and experimentally. The factors include dynamic or impact factor caused by moving vehicle, panel lateral maldistribution factor caused by vehicular eccentricty within the trusses, shear maldistribution factor caused by connecting clearance in end posts, shear impact factor caused by the slope of approach ramps. The conclusions are of guiding significance for both improving the bridge and bridging practice.
Guanyao Xu is male, was born in 1965, Ph.D., a Member of China Steel Construction Society, a Fellow of China Operations Research Society, has undertaken research, development and consulting work on steel bridge and bridge test for a total of ten years. Now he took the task of steel bridge health monitoring system and mainly studied the fatigue response of steel bridge members under variable-amplitude long-life loading.
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EXPERIMENTAL STUDY ON PREFABRICATED SECTIONAL HIGHWAY STEEL BRIDGE YONGZUO ZHU Planning Division, Ministry of Communications, Beijing 100736, China; GUANYAO XU Beijing System Engineer Institute of Engineer Equipment, Beijing 100093, China
[email protected]
the value of µ is 0.16 and 0.35 correspondingly. The
1. INTRODUCTION Under the action of moving vehicle loads, the stress or deflection of the bridge structure is larger than that
simplest PHSB is one wherein a single truss of panels along each side of the deck forms the main girders. This type of construction is referred to as Single Single (SS) .
caused in static situation. The increment of stress or deflection is called as dynamic influence, the ratio of which to stress or deflection in static situation is called as dynamic coefficient. In the design criterion of bridge, the dynamic influence is usually called as vehicle loads impacting coefficient or dynamic coefficient to bridge structure. In practice the dynamic coefficient is oftentimes defined as the ratio of the maximum dynamic deflection to the static deflection at mid-span of a bridge, use the letter µ to respresent the dynamic coefficient.
Figure 1 A SS PHSB
With regard to steel bridge, µ is prescribed in the design criterion for road and bridge of China as follows,
The main factors affecting vehicles’ impacting
15 37.5 + L
coefficient to PHSB namely the first natural frequencies
where L is the span or the length of influence line, whose
of vehicles are studied in this article. And a correctional
unit is m.
calculating formula of dynamic or impact coefficient is
µ=
As to Prefabricated Highway Steel Bridge as shown
of PHSB, structure of deck system, the mass and speed
put forward.
Moreover, panel lateral distribution
in Figure 1, we find out through a great deal of field tests
coefficient caused by vehicular eccentricity within the
that its impacting coefficient is not only relative to the
trusses,
span, but also to the mass of vehicles, the running speed
connecting clearance in end posts and shear impact
and the structure status of deck system etc. When a
coefficient caused by the slope of approach ramps are
170kN heavy camion passes through a 21m long SS steel
studied
bridge respectively at the speed of 15km/h and 30km/h,
significance
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shear
as
distribution
well. for
The both
coefficient
caused
by
conclusions
have
guiding
improvement
and
practical
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application of steel bridges.
f1
2. STUDY ON DYNAMIC COEFFICIENT OF PHSB 2.1
Factors Affecting Dynamic Coefficient of PHSB
and Their Analysis
2P 1+ G
From table 1, we can know that L=15m P/G=3.23 f1 =0.366 f 1 L=21m P/G=1.65
f1 =0.482 f 1 . So, as to
a certain PHSB, P’s effect on f1 is great, the large P is, the less f1 is. In fact, the first natural frequencies f 1
Factors affecting dynamic coefficient of PHSB mainly include dynamic characteristics of structure of span mainly the first natural frequencies f1 , proportional relationship between live load P and dead load G, structure of deck system and vehicle speed V.
of a PHSDB is usually that caused by the action of live loads, which is f1 , not f 1 . Only when the wind load is considered, the first natural frequencies is f 1 , not f1 . So, for a PHSB, the item of P/G should be included in µ . From the above, we know that to express P/G’ s effect by
Influence toµcaused by f1, P and G
1)
f1
=
For permanent bridges, because the mass of dead load is much larger than that of live load, the influence can be left out. For PHSB, as the length of span is usually shorter than 39m as shown in Table 1, and the mass of dead load is smaller than that of live load, the shorter the span is, the greater the ratio of dead load to live load is.
1/ 1+2P/G
is feasible. Field experiments
show that the mass of loads P’s effect on µ is very great, the larger P is, the smaller µ is. 2)
Influence to µ caused by structure of floor system
For permanent bridges, the thickness of faceplates of deck is more than 10mm, above which there is cement concrete pavement or asphalt concrete pavement so tires won’t directly act on the steel faceplate. Obvious clash will be heard only when vehicles pass the expansion and
Table 1 Realation Between Live Loads & Dead Loads Span m
Construction
G
P
kN
kN
P/G
contraction installationof bridge floor. For PHSB, there are two kinds of deck structure called wooden deck structure and steel deck structure. To be
15
SS
108.5
350
3.23
convenient for installing, the wooden deck faceplates are
18
SS
130.2
250
1.92
put on the longitudinal stringers, and the latter are put on
21
SS
151.9
250
1.65
the transverse beams. The wooden deck is only 190mm
24
DS
229.6
500
2.18
wide and there are gap of 5~15mm wide, so a so-called
30
DSR
357.0
500
1.40
washboard road is formed actually.
36
DSR
428.4
630
1.47
The faceplates of steel deck are directly put on the
39
DSR
464.1
500
1.08
transverse beams. A single steel faceplate is only 3 metres long and 3~6mm thick, and there are 3~10mm
Mass of loads affects µ because of its effect on f1. Formula of mass of loads effect on f1 is provided (Fryba,1972) as follows:
slots among the faceplates, so tires will directly act on the faceplates of steel deck. Figure 2 is a steel deck, and figure 3 is its working status. Therefore, for PHSB, the influence caused by gap should be considered.
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should be adopted in order to enhance the system reliability of PHSB. Influence to µ caused by the velocity V of moving
3)
vehicle tructure of floor system Field tests show under the velocity of from 0 to 30 km/h, the greater velocity is, the greater µ is. Figure 2 A Steel Deck
2.2
The Improved Dynamic Coefficient
The first natural frequencies f 1 , the ratio of live load P to dead load G, and the velocity V of moving vehicle have more influence on the impact of trusses.The impact factor of truss can be explained as follows: µ=1+
Figure 3 Working Status of Steel Deck
0.0157+0.4068×log
f1 2P 1+ G
×
V 60
where the maximum value of µ is less than 0.5 and V is less than 50 km/h..
The method used to deal with floor slots’ effect on
Experiments show that when a JIEFANG CA10B
3. STUDY ON THE PANEL LATERAL MALDISTRIBUTION FACTOR CAUSED BY VEHICHE ECCENTRICTY WITHIN THE TRUSSES OF PHSB
camion passes through a PHSB with wooden deck system at the speed of 20km/h, the value of µ of
Field test adopted a PHSB of DD construction as
impacting coefficient in AASHTO design criterion for bridges of the USA can also be applied to Prefabricated Sectional Highway Steel Bridge.
transverse beams is from 1.90 to 2.0. For PHSB with steel deck system, the value of µ of transverse beams
shown in Figure 4. The span of bridge is 27 metres.
is from 0.2 to 0.3. Through analysis of systematic reliability, we know that under the action of track load 50, a 21m long DSR steel bridge, the reliability index of invalidation mode of bending stress of a transverse β =2.357, that of invalidation mode of the compressed stress of upper diagonal bracing β=2.834 and that of thedeflectionin the middle of span β=4.532, The system reliability index of the bridge β=2.274. It can be seen that transverse beams have the greatest effect on the system reliability. So, if transverse beams are not to be changed, steel decks
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Figure 4 A PHSB of DD Construction
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Test results are as following:
A PHSB is a through bridge. The advantage of the
Case 1, e=0. While the trucks were at the central line of
through bridge is the simplicity of the panels of the
the bridge, the distribution factor of panels lateral
bridge, and the deficiency of the through bridge is to set
maldistribution was shown as in Figure 5.
end posts to bear large shears. The posts don’t bear the same shear because of the connecting clearance between the end panels and end posts.
e=0
Field experiment shows the maldistribution factors of shears of end posts varies from 6.0% to 120%. 0.27
0.23 0.27
0.23
Figure 5 Distribution Factor of Panels Case 2, e=0.20 metre eccentricty apart from the central line of the bridge. While the trucks were at the eccentricty line of the bridge, the distribution factor of panels lateral maldistribution was shown as in Figure 6.
5. SHEAR IMPACT FACTOR CAUSED BY THE SLOPE OF APPROACH RAMPS OF PHSB The shear of the end panels is sensitive to the slope of approach ramps. The greater the slope is, the greater impact factor the shear of the end panels is. Field experiment shows the impact factors of shears of
e=0.20
end posts varies from 1.2 to 2.0 according to the changes of the slope from 8% to 17%.
0.21 0.24
0.29
0.26
Figure 6 Distribution Factor of Panels
6. CONCLUSION Viewed overall above, many factors such as
Case 3, e=0.50 metre eccentricty apart from the central
dynamic or impact factor caused by moving vehicle,
line of the bridge. While the trucks were at the
panel lateral maldistribution factor caused by vehicular
eccentricty line of the bridge, the distribution factor of
eccentricty within the trusses, shear maldistribution
panels lateral maldistribution was shown as in Figure 7.
factor caused by connecting clearance in end posts, shear impact factor caused by the slope of approach ramps,
should be considered in the design and practical
e=0.50
application. 0.18 0.20
0.32
0.30
References
Figure 7 Distribution Factor of Panels
Fryba L,1972: Vibration of solids and structures under
4. EXPERIMENTAL STUDY ON SHEAR DISTRIBUTION COEFFICIENT AT THE END POSTS OF PHSB
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moving loads. Noordhoff International Publishing, Groningen, The Netherlands.
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