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FIGURE 3 Maximum load effects of AASHTO HS20 live load for
different bridge span lengths.
particular permit vehicle and the load rating process consid- gage width. The AASHTO evaluation specifications (MCEB
ers general truck traffic loads. These two groups of loads have 2000) refer to the AASHTO design specifications (Standard
very different probabilities of occurrence (Fu and Moses Specifications . . . 2002) to guide how load distribution should
1991; Fu and Hag-Elsafi 1996). The latest AASHTO bridge be done in bridge evaluation for the standard gage width of
evaluation specification [Guide Manual for Condition Evalu- 6 ft. Although these guidelines may be adequate for bridge load
ation and Load Resistance Factor Evaluation (LRFR) of rating of general truck traffic, they are unable to cover all bridge
Highway Bridges 2003] has adopted a probabilistic concept evaluation scenarios for permit review. There are many situa-
of prescribing different load factors for the standard bridge tions where these guidelines are not applicable. For example,
load rating and the bridge evaluation for permit review. permit loads may not simultaneously appear in two or more
lanes on the bridge, as likely as nonpermit vehicles. In addition,
the gage widths of permit vehicles may not be the standard 6 ft.
LIVE LOAD DISTRIBUTION FACTOR These factors leave ample of room for interpretation and alter-
natives, which could lead to different results of permit review.
In Eq. 1, LL is defined as the live load effect distributed to the
particular component being evaluated. The load distribution de-
pends on how many vehicles are used to load the bridge, the IMPACT FACTOR
material types of the structural components involved, and their
structural arrangement. It also depends on how the wheel lines In Eq. 1, LL also includes the so-called impact factor meant to
are arranged in the transverse direction, namely the vehicle's cover the dynamic amplification of the vehicle load. According
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FIGURE 4 Maximum load effects of AASHTO H20 live load for
different bridge span lengths.
to the AASHTO bridge design specifications (Standard Speci- NCHRP Synthesis of Highway Practice 143:
fications . . . 2002) this factor can be as high as 1.3 or 30% above Uniformity Efforts on Oversize/Overweight
Permits 1988.
the static load effect. Many transportation agencies adjust that
factor in bridge evaluation for permit review, particularly when
the bridge capacity is otherwise below the required level. When
This synthesis study focused on the uniformity efforts in
a lower impact factor is used in LL, the rating factor in Eq. 1 can
OS/OW permit issuance. The report summarized the reasons
become higher and therefore more likely to reach the 1.0 level
for nonuniformity in permit procedures as follows. From the
to allow issuance of the permit. This also imposes a requirement
states' perspectives, the difficulty in common permit proce-
for the permit vehicle's operation to control the driving speed,
dures includes concerns about physical, safety, economic,
braking, and/or acceleration to limit impact when crossing the
legal, and political factors. More specifically, the following
bridge. Apparently, different jurisdictions used different prac-
factors were identified as contributors to the observed
tices with respect to this factor.
nonuniformity: inadequate funding and staffing, continuing
changes in state policies, inadequate data for analysis, pres-
In this study, a literature search was undertaken with re- sure from the trucking industry, concern about federal pre-
gard to bridge evaluation for permit review and other possi- emption, a lack of constituency, concern about reducing
bly related subjects. The identified previous research efforts standards, and national effort having little chance for suc-
reported in the literature are reviewed next. Some of the cess. From the federal perspective, only a limited degree of
issues possibly causing nonuniformity are addressed in these intervention was considered possible; therefore, the federal
research reports and papers, with respect to bridge evaluation government preferred to have the states take the lead toward
for permit review. a higher level of uniformity.
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It was also concluded in this study that among all the ef- In practice, bridge weight-limit posting is typically de-
forts aimed at achieving better uniformity only the NETC has termined based on bridge inspection and bridge load rating.
been able to succeed in developing relatively uniform per- Bridge inspection, in the context of its relation to weight-
mitting procedures. (An update of the NETC activity is given limit determination, is necessary to obtain the information
in chapter five.) The reasons were summarized as follows: for properly evaluating the strength of the bridge and its be-
havior and performance under the load. On the other hand,
· Recognition of the importance of the issue by the chief this synthesis emphasized that bridge inspectors are often
administrative officers of the involved state DOTs. not involved in the following structural load rating, possibly
· A set of issues was selected for resolution that all par- using the inspection results. This "discontinuity" may con-
ticipating DOTs believed were critical and for which tribute to nonuniformity or inconsistency in bridge weight-
the probability of achieving success was very high. limit determination, which in turn affects the uniformity in
· Full cooperation and participation was achieved by the permit issuance.
technical individuals of the DOTs who were responsi-
ble for issuing permits. In the issue of bridge load rating, the synthesis noted that
· Within that framework of mutual cooperation, each of the relevant AASHTO specifications for the practice allow
the states was willing to drop its "jurisdictional barriers." for considerable leeway for the use of engineering judgment
· The participating states presented a uniform position to in evaluating or posting bridges. This leeway has resulted in
the trucking industry. considerable variation in the ways different states evaluate
· The NETC did not attempt to include all permit re- and post bridges. This issue will be discussed further in chap-
quirements within the regional agreement; therefore, ters three and four.
each state can deal with the exceptions in the usual way
and no situation is excluded.
"Overload Permit Checking Based on Structural
· A concerted, centralized staff effort was funded to de- Reliability" (Fu and Moses 1991) and "New Safety-
velop and implement this program. Based Checking Procedure for Overloads on
· Every state gained and none lost anything from this Highway Bridges" (Fu and Hag-Elsafi 1996)
agreement.
· The participating states believed that it was inevitable Currently, a common practice in issuing overweight permits
that uniform procedures will be required by the federal is to use the bridge load rating formula Eq. 1 provided ear-
government, and that it is much more efficient for the lier, prescribed in the AASHTO specifications (MCEB 2000
states to take the lead before they are preempted. and its previous editions). Note that these bridge rating for-
mulas are intended for use in evaluating bridges against typ-
It was also concluded that the NETC experience illustrates ical truck traffic loads, and not necessarily for very heavy and
that it is possible to enhance better uniformity. However, it occasional permit loads. These two papers represent the first
appears that it cannot be accomplished initially on a national research efforts focusing on the issue of different probabili-
scale. Rather, it should begin on a regional or even a subre- ties of occurrence of legal vehicle traffic and permit vehicles
gional basis as the NETC was able to do. Then, it would re- above the legal load level done for the Ohio and New York
quire that the appropriate policy and political as well as tech- DOTs, respectively.
nical interaction take place within and between regions.
The research projects reported in these publications ana-
Note that in view of contemporary concerns about a fast re- lyzed data of the respective states for normal truck traffic and
sponse to natural and terrorist driven disasters, harmonization permit truck traffic and developed different live load factors
must be accomplished readily to facilitate permit reciprocity (A2 in Eq. 1) specifically for bridge evaluation in permit re-
across multistate areas in time of disaster and to eliminate the view, to maintain the same bridge safety as for bridge load
conflicts in OS/OW permitting. rating intended to cover general truck traffic. These live load
factors are different for single trip permits and multitrip (e.g.,
annual) permits. In general, the live load factor can be
NCHRP Synthesis of Highway Practice 108: smaller for less frequent permit loads. This concept has been
Bridge Weight-Limit Posting Practice 1984 adopted in the latest AASHTO bridge evaluation specifica-
tions (Guide Manual . . . 2003).
For overweight permit issuance, weight limit owing to bridge
capacity (load rating) is often a critical factor. Understanding
how the bridge weight limits are determined is therefore rel- Overload Permit Procedures (Noel et al. 1992)
evant to this study. NCHRP Synthesis 108 summarizes the
practice of bridge weight-limit posting in the United States As reviewed earlier, overweight vehicle review uses the same
as of 1984. Besides the administrative aspects of weight-limit load rating concept of the AASHTO specifications (MCEB
posting, engineering practices, which are more relevant to 2000); namely, the vehicle requesting a permit is placed
the current study, were also addressed in that study. on a bridge structure in a mathematical model, replacing the
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standard vehicle, such as the HS20. If the induced load effects 4000000
(moment, shear, etc.) do not exceed the capacity allowed,
3500000
the vehicle will then be allowed to cross the bridge and, oth-
erwise, not. Although there may be many bridges needing 3000000
evaluation for a permit type (e.g., annual permits without 2500000
specified routes), analyzing every bridge in the jurisdiction is
costly and can become particularly difficult. Therefore, there 2000000
have been some models suggested and developed to cover a 1500000
group or population of bridges within a jurisdiction. Noel et
1000000
al. (1992) represents a typical effort in this direction for
bridges designed to the AASHTO H15 load. Hereafter, these 500000
bridges are referred to as H15 bridges. Figure 2 shows the
0
H20 load whose 75% proportional reduction is the H15 load 1997 1998 1999 2000 2001 2002 2003
(15/20 = 75%).
FIGURE 5 Number of overweight/overwidth permits issued by
states.
The reason for focusing on H15 bridges in this study was
that they were considered to be the bottleneck in approving
The proposed formulas can be useful for a first screening
overweight permits in Texas. These bridges typically have
of permit vehicles if the formulas' validity is confirmed. This
the lowest capacity, because the H15 design load has been
type of screening can reduce the work load required, because
obsolete for some years and it induces lower load effect than
otherwise every permit vehicle needs to be analyzed. It also
the HS20 design load.
should be noted that those permit vehicles that fail the
screening may still be permissible; however, they will need
As a result, the study developed a formula of maximum
to be analyzed on a case-by-case basis. Sometimes additional
gross weight as a function of the permit vehicle's wheel-
requirements (such as reduced speed for reduced dynamic
base (the distance between any two axles) and the bridge's
impact) may be needed on the permit vehicle's operation.
span length. A similar formula was also proposed as a func-
tion of the wheelbase only. It is more restrictive than the
first one (with span length as another variable), because it Bridge Analysis Simplified (Bakht and Jaeger 1984)
uses the most restrictive gross weight for all span lengths.
Furthermore, an empirical modification factor was also In this paper, Bakht and Jaeger proposed an overweight per-
suggested to cover the gage width of the permit vehicle mit review method. The method's concept is that the safe
other than the standard 6 ft. It should be noted that the permit load can be the worst combination(s) of the maximum
analysis used in this study included only one permit vehi- vehicle loads the bridge is likely to have sustained during its
cle in one lane (not multiple vehicles in all lanes available) lifetime. The procedure was derived using theoretical as-
on the bridge. sumptions regarding the probability distribution of the truck
3000000
Non-Divisible Single Trip
2500000 Permits
Non-Divisible Annual
Permits
2000000
Divisible Single Trip
Permits
1500000 Divisible Annual Permits
Overwidth Permits
1000000
500000
0
2001 2002 2003
FIGURE 6 Overweight/overwidth permits issued by states by type.
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loads that have been experienced. Although the probabilistic Figure 6 shows these permits broken down by type for
concept appears to be reasonable, it is not clear whether the 3 years (20012003), indicating nondivisible trip permits
theoretical model is applicable to all real bridges, because the dominating the population. Nondivisible loads here refer to
truck load spectrum for each bridge can be very much dif- those that cannot be cost-effectively divided, such as a large
ferent. It has not been reported since then that this procedure transformer, a house, a piece of construction equipment, etc.
has been applied to real cases of permit review. It should be pointed out that the figure shows "head counts"
for the permits, not indications of how frequently each per-
"National Commercial Motor Vehicle Size mit type of vehicles appears on the road. Statistically, the an-
& Weight Enforcement Trends" (2004) nual permits represent significantly more trips than the sin-
gle trip permits. Therefore, the annual permit vehicles may
FHWA routinely monitors the practice of OS/OW vehicle op- appear much more often on the roads. FHWA (LRFD Bridge
eration. Apart from the earlier FHWA statistics published in Design Specifications 2004) also indicated that "Overweight
1991, these statistics derived in 2003 may currently be the permit issuance continues to increase, with annual or multi-
latest presented. Figure 5 shows the number of overweight/ ple trip permits becoming more commonplace." In addition,
overwidth permits issued in the states from 1997 through "Freight tonnage moved by truck is forecast to continue to in-
2003. An annual increase in permit issuance can be seen. crease." This has been a concern of FHWA for some time.
