r - The Hong Kong Polytechnic University

450
35
400
30
350
300
25
( 1.0E- 6)
m
250
200
150
100
50
1 day
10 days
30 days
one year
σ
20
15
10
5
1 day
10 days
30 days
one year
0
0
0 50 100 150 200 250 300
0 50 100 150 200 250 300
Tim e Segm ent (s)
Tim e Segm ent (s)
Figure 2 Extreme strain distributions parameter, μ, σ, derived from different initial distributions
Probability Density Function
0.012
0.01
0.008
0.006
0.004
0.002
Mean recurrence interval of 1 day
Mean recurrence interval of 10 days
Mean recurrence interval of 30 days
Mean recurrence interval of 180 days
Mean recurrence interval of one year
0
100 200 300 400 500 600 700 800 900 1000
Extreme Strain (1.0E-6)
Figure 3 PDF of extreme strain distribution for mean recurrence intervals of 1 day, 10 days, 30 days and one year
CALIBRATION OF IMPACT FACTOR FOR EXISTING BRIDGES
Impact factors of existing bridges may be significantly different from code specified values. Therefore, in the
present study the impact factor of existing bridges is calibrated through a reliability approach. The following
design equation, recommended by the current LRFD code (AASHTO 2004) for considering situations with only
dead loads and vehicle live loads, was used in the calibration process.
φ R ≥ 1.25DC
+ 1.50DW
+ 1.75LLl + 1.75(1 + IM ) LL t
(23)
where R = nominal value for the resistance, DC = effects due to design dead loads excluding the weight of
wearing surface, DW = effects due to wearing surface weight, LL
l = effects due to design lane load, LL
t =
effects due to the most demanding between truck and tandem load, IM = LL t , dyn
LLt
-1= 0.33 denotes the
dynamic load allowance(impact factor), in which LL
t , dyn = effects due to the most demanding between truck
and tandem load including the dynamic effects, and φ = resistance factor. For prestressed-concrete bridges,
the current AASHTO LFRD code suggests a resistance factor of 1.0 and 0.85 for moment and shear, respectively,
based on the study by Nowak (1995).
In order to calibrate the impact factors for existing bridges, a group of seven prestressed concrete girder bridges
were selected as benchmark bridges in this study. The detailed configurations and properties of the seven
prestressed concrete girder bridges are described in Deng and Cai (2010). By assuming certain statistical
properties of the impact factors under different road surface conditions (RSCs) and explicitly modeling the IM
as an individual random variable, calibrations can be performed with respect to different RSCs (Deng et al.
2011).
The proposed IMs have been chosen with the aim of balancing three different needs, i.e., (1) obtaining reliability
indices consistent with the target reliability index of 3.5 for all bridges under any RSC, (2) ensuring as much as
possible consistency with the AASHTO LRFD code (2004), and (3) minimizing the modifications to the IM
values suggested by the AASHTO LRFR manual (2003).
Table 2 provides, for all the RSCs considered, (1) the minimum values of IM required to reach the target
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