Highway Bridge Maintenance Planning and Scheduling

338 Pages·2016·37.818 MB·English

by  Hurt, Mark A., Schrock, Steven D

HIGHWAY BRIDGE
MAINTENANCE
PLANNING AND
SCHEDULING
MARK HURT
Bridge Design, Bureau of Structures and Geotechnical
Services, Kansas Department of Transportation
STEVEN D. SCHROCK
Department of Civil, Environmental, and Architectural
Engineering, University of Kansas
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ix
CHAPTER 1
Introduction
“A bridge is to a road as a diamond is to a ring.”
– Anonymous
Overview
The inventory of bridges on public roads in the United States is discussed. The bridge pres-
ervation process is introduced. Bridge preservation practices before the establishment of
the National Bridge Inspection Standards (NBIS) are examined through the case of such
practice in the state of Kansas. The development of the NBIS is presented. The nature of
ongoing development in bridge inspection and evaluation practices is illustrated by the
case of the I-35W Bridge collapse. The implementation of bridge management systems
(BMS) to manage bridge inspection and condition data, and the growth of BMS are dis-
cussed. An overview of the layout of the book is provided.
1.1 BRIDGES IN THE UNITED STATES
The previous statement may appeal to the vanity of those who work with
bridges, but it also reflects a truth: bridges are critical assets that provide
important value, but at a cost. Value, in that each highway bridge is a solu-
tion to a problem of how to carry traffic across a river or gorge or other
obstacle such as conflicting lanes of traffic. Cost, in that the solution comes
at a price, each section of a bridge deck costs several times more than an
equivalent area of roadway both to construct and to maintain over the life
of the bridge.
According to the Federal Highway Administration (FHWA), in 2010
the road network of the United States included over 4,083,768 miles of
public roads and more than 604,493 bridges [1]. A bridge is defined by
the FHWA as, “a structure including supports erected over a depression
or an obstruction, such as water, highway, or railway, and having a track
or passageway for carrying traffic or other moving loads, and having an
opening measured along the center of the roadway of more than 20 feet.”
(Figure 1.1)
Highway Bridge Maintenance Planning and Scheduling Copyright © 2016 Elsevier Inc.
http://dx.doi.org/10.1016/B978-0-12-802069-2.00001-5 All rights reserved. 1
2 Highway Bridge Maintenance Planning and Scheduling
Figure 1.1 Bridge Opening.
The total length of those bridges is 16,349.5 miles [2], less than 0.5%
of the total miles of public road. The amount expended by all levels of
government in the United States in 2010 on public roads and bridges was
$205.3 billion. Of this, $60 billion was spent on system rehabilitation, which
is defined as, “capital improvements on existing roads and bridges that are
intended to preserve the existing pavement and bridge infrastructure.”
Twenty-eight and half percent, $17.1 billion, of the system rehabilitation
expenditures were for bridge-sized structures. This does not include the
system rehabilitation funds spent on highway structures with an opening of
20 ft. or less. These small spans and culvert structures are used most often to
convey drainage or sometimes to provide a single lane underpass through a
roadway berm. These structures are more numerous than bridge-sized struc-
tures and are subject to most of the same maintenance issues as the larger
structures. The cost of work on structures with an opening of less than 20 ft.
conducted under system rehabilitation projects is captured in the $42.9 bil-
lion in highway expenditures. Rehabilitating structures for preservation is
considerably more expensive than rehabilitating an equal length of roadway.
Part of the cost of bridges is also the acceptance of risk. A study of bridge
failures in the United States over the period of 1989–2000, by Wardhana
and Hadipriono of Ohio State University, found cases of 503 failures [3].
Failure was defined as the incapacity of a bridge or its components to per-
form as specified in the design and construction requirements. Conditions
of either collapse (total or partial) or distress constitute failure of a bridge
and result in its removal from service until either repair or replacement. A
distressed bridge is one with one or more components in such condition
that the facility is rendered unserviceable. An example would be excessive
deflections in the superstructure resulting in a dip in the bridge deck that
would render the bridge unusable for traffic. Almost all of the failures with
identified conditions were either partial or total collapse. The consequences
of the collapse of a bridge can be quite severe and, in the worst case, result in
fatalities. In the cases studied, there were 76 fatalities and 161 people injured.
Introduction 3
To characterize a bridge as having failed in the study, it was not only
implied that it became unserviceable, but that it became unserviceable sud-
denly and unexpectedly. Of the 503 bridge failures studied by Wardhana
and Hadipriono, 266 failed due to high-water events, 103 failed due to
either overloading or vehicular impacts, and 45 failed due to other events
such as fire or earthquakes. The failures of only 48 bridges were attributed
to either deterioration or fatigue. The most common way for a bridge to fail
was to be subjected to an extreme event.
A far more common end to the life of a bridge is deterioration that
accumulates and results in a progressively less serviceable structure. Under
the wear of traffic loads and exposure to the weather and to agents such
as salts used to melt snow and ice on roadways, steel corrodes, and con-
crete cracks and spalls. The wearing surface of the bridge deck may become
rough enough to require slowing traffic. The supporting members of the
structure may lose enough material that their ability to bear load is reduced,
requiring the restriction of heavy trucks from the bridge. Thankfully, slow
deterioration rarely results in a sudden failure with the attending risk of
injury to bridge users; however, it may still result in significant economic
impact by disrupting traffic. This is particularly true for the movement of
commercial freight by heavy trucks.
The cost of bridges makes them a significant investment for owners and
operators of highways. The risks and consequences of bridge failure require
owners and operators to act to maintain their bridges in good repair. In the
United States, these actions have come to be classified as bridge preservation. The
FHWA defines bridge preservation “as actions or strategies that prevent, delay
or reduce deterioration of bridges or bridge elements, restore the function
of existing bridges, keep bridges in good condition and extend their life” [4].
Bridge preservation has become increasingly important to the owners and
operators of highway bridges in the United States due the age and numbers
of bridges in their inventories. According to data from the National Bridge
Inventory (NBI) maintained by the FHWA, as of 2013 the average age of
bridges carrying traffic on public roads in the United States was 43 years [5].
This is due to the rapid expansion of the highway system and public roads in
general after World War II. Figure 1.2 shows the decade of construction for
bridges on public roads in the United States constructed between 1910 and
2010. For bridges constructed in the post-World War II period and prior to
adaption of the American Association of State Highway and Transportation
Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge
Design Specification, the anticipated service life was 50 years.
4 Highway Bridge Maintenance Planning and Scheduling
Figure 1.2 Bridges by Decade of Construction.
Over 11% of the bridges on the NBI in 2013 are categorized as structur-
ally deficient [6]. A bridge is categorized as structurally deficient when one of
its major components – the deck, superstructure, or substructure – is rated
as poor during a bridge inspection, or when it is evaluated to be inadequate
either for load-carrying capacity or for its waterway opening. Structural
deficiency does not automatically imply an imminent danger to the travel-
ing public using the bridge. It does imply impairment to the operation of
the bridge in that some heavy truck traffic will not be allowed to use the
bridge. And it also implies that work is required to restore the condition of
the bridge. The poor rating of one or more bridge components is almost
always due to deterioration. Deterioration comes about as a function of en-
vironmental exposure and use over time. The average age of a bridge rated
structurally deficient was 65 years old. By 2023, one in four of the existing
bridges in the inventory will be 65 years or older if left in service [7]. For
these bridges, preservation actions will be required to maintain them in full
service.
A bridge on the NBI may be also considered deficient if it is function-
ally obsolete [6]. A bridge is categorized as functionally obsolete if either the
geometry of its deck, the clearance for roadways under the bridge, or the
width of the roadway at the approaches to the bridge deck are inadequate. It
may also be considered functionally obsolete if either its load-carrying ca-
pacity or waterway opening is inadequate, but not to the degree to be con-
sidered structurally deficient. If a bridge qualifies as structurally deficient, it
is not also considered functionally obsolete. Almost 13% of bridges on the
Introduction 5
2010 NBI were functionally obsolete. The total number of deficient bridges
in the 2010 NBI was 146,636, over 24% of the total inventory.
1.2 BRIDGE PRESERVATION PROCESS
The term bridge preservation should not be taken to focus solely on the
particular maintenance actions to keep a bridge in good condition. These
are actions such as sealing open cracks on a bridge deck. Implementing
these actions and developing effective strategies for their deployment re-
quires owners and operators to assess the condition of the components of
the bridge and to know the relevance of any defects found. Bridge pres-
ervation may be defined as a process consisting of three general activities:
inspection, evaluation, and maintenance (Figure 1.3).
The cornerstone of the bridge preservation process is inspection. In-
spection provides information as to the physical condition of bridge com-
ponents. An initial inspection provides a baseline for review throughout the
life of the structure. Subsequent inspections alert the owner to changes in
condition and to any current needs. Maintaining records of bridge inspec-
tions allows an owner to track deterioration. Combining the information
available from the records of an inventory of bridges over time allows the
owner to intelligently predict rates of deterioration and anticipate future
needs. Inspection procedures and the intervals at which inspections are con-
ducted are determined by policies adopted by the bridge owner. For bridges
on the NBI, those policies are set forth in the NBIS.
Evaluation is an assessment of a bridge’s ability to safely carry traffic.
Bridge inspectors evaluate the condition of bridge elements during the
inspection process. The evaluation step in the bridge preservation process
Figure 1.3 Bridge Preservation Process.
6 Highway Bridge Maintenance Planning and Scheduling
is an assessment of the bridge as a whole. Bridge owners must determine
whether a bridge is safe to remain open after experiencing an extreme
event, such as a large flood or a fire. Although if a significant amount of
damage is apparent it may be obvious that a bridge needs to be removed
from service, often an engineering analysis is required to determine the de-
gree of impairment suffered by the structure. An engineering analysis may
also be required to assess the effect of a change in site conditions, such as
experienced from stream degradation.
A structural analysis conducted to determine the load-carrying capacity of
the existing bridge components in their current condition, noting any loss in
capacity due to deterioration or damage, is a load rating. Older bridges may
have lower load-carrying capacity than desired for the highway route they ser-
vice not only due to the effects of deterioration, but the loading used for their
initial design may have been significantly less than current standards require.
The current AASHTO LRFD design truck is the HL-93, a 72,000 pound
truck with a maximum axle load of 32,000 pounds. Its load effects are com-
bined concurrently with those of a uniform load of 640 pounds per ft. per
lane. It was not until 1944 that the design specifications of the predecessor to
AASHTO, the American Association of State Highway Officials (AASHO),
recommended a minimum design truck load for highways with heavy truck
traffic, the H15-S12. The H15-S12 loading consisted of checking for the
effects of either a 54,000 pound truck or a 480 pound per ft. lane load with
a 13,500 pound concentrated load. This was still considerably heavier than
the first weight limits for trucks on public roads in the United States. These
were enacted by four states in 1913: 18,000 pounds gross vehicular weight
(GVW) in Maine; 24,000 pounds GVW in Pennsylvania and Washington;
and 28,000 pounds GVW in Massachusetts [8] (Figures 1.4 and 1.5).
All bridges on the NBI are required by the NBIS to be load rated for
the HL-93 truck configuration, note that the concurrent lane loading is not
used [9]. The HL-93 truck configuration is known as HS20-44 truck con-
figuration in previous design specifications. Two load ratings are reported
to the FHWA: operating and inventory ratings. The operating rating is the
maximum permissible weight of truck in the chosen load configuration
to which the bridge may be subjected. The inventory rating is the maxi-
mum permissible weight of truck in the chosen load configuration, which
may safely utilize the bridge for an indefinite period of time. For example,
an inventory rating of 39 tons for the HL-93 truck configuration would
imply that a truck weighing 39 tons with axles spaced and apportioned
similar to the HL-93 should be able to use the bridge indefinitely without
Introduction 7
Figure 1.4 HL-93 Design Truck. (Adapted from AASHTO LRFD [10]).
causing undue distress on the structure. A HL-93 design truck has a front
axle weight of 8,000 pounds and two rear axles each with 32,000 pounds. A
39 ton (78,000 pound) truck in the same configuration would have a front
axle of 8,666 pounds (78/72 × 8) and two rear axles of 34,667 pounds.
Maintenance consists of those actions to sustain a bridge in operation
despite onslaughts by both deterioration and damage. A broad spectrum
of actions will fall into this activity, from actions as simple as cleaning the
bridge wearing surface, to as involved as the removal and reconstruction of
bridge decks. Bridge maintenance actions can be generally categorized as
preventative or substantial.
Figure 1.5 H15-S12 Design Truck. (Adapted from 1941 AASHO Design Manual [11]).