| Abstract | Load effects under traffic conditions and vehicle characteristics in Thailand are
calculated to examine the appropriateness of AASHTO (1996) design loads for the design of
highway bridges in Thailand. Bridge considered is the two-lane simple span slab-on-girder
bridge. Span length varies from 10 m to 50 m. Load effects are the mid span moment and the
support shear in the critical girders, which are the extreme left and right girders. Three cases of
torsional rigidity of bridge structure are considered. They are low, medium, and high torsional
rigidity cases. The data for traffic conditions and vehicle characteristics are obtained from
many sources and real observations on highway No. 3214. Only heavy vehicles are considered
in the calculations of maximum load effects. Heavy vehicles comprise buses, small trucks, and
large trucks. The characteristics of these heavy vehicles are modeled to use in the calculations
of maximum load effects. The calculations of maximum load effects are performed for small
percentages of large trucks and large percentages of large trucks. The Monte Carlo simulation
technique is introduced in the calculations of maximum load effects. The presence of one, two,
three, and four heavy vehicles on the bridge are simulated separately in four categories. The
time periods of simulations are one day, one month, one year, and 75 years for the presence of
one, two, three, and four heavy vehicles on the bridge, respectively. The results of each
category are extrapolated to obtain the maximum load effects in 75-year time period, which is
the service life of the bridge. The maximum load effects in 75-year time period of each
category are combined to obtain the total maximum load effects in 75-year time period. The
AASHTO load effects are divided into two parts. The first part is the AASHTO load effects
calculated by considering three cases of torsional rigidity of bridge structure. The second part
is the AASHTO load effects calculated from the simplified procedures in the AASHTO
specifications (1996). Three cases of girder spacing are considered in the second part. From
the results of simulations, the presence of one heavy vehicle on the bridge governs the
maximum load effects for all span lengths under consideration. The maximum load effects in
the critical girders are greater in low torsional rigidity bridges than in medium and high
torsional rigidity bridges. The percentages of large trucks have no effect on the load effects in
the critical girders. From the results of comparison of simulated load effects with the
AASHTO load effects, a nonconservative situation occurs in low torsional rigidity bridges.
More uniform variations of load effect ratios over the span lengths are observed in high
torsional rigidity bridges than in medium and low torsional rigidity bridges. Therefore, the
AASHTO design loads are appropriate for the design of high torsional rigidity bridges. The
AASHTO load effects calculated from the simplified procedures in the AASHTO
specifications (1996) are conservative for larger girder spacings and nonconservative for
smaller girder spacings. The consideration of torsional rigidity of bridge structure is required
in the calculation of the AASHTO load effects for small girder spacing bridges. |