«HIGHWAY INFRASTRUCTURE INTRODUCTION Highway infrastructure protection historically has been the primary consideration in determining TS&W limits as ...»
Highway infrastructure protection historically has been the primary consideration in determining
TS&W limits as the weights and dimensions of trucks in particular determine the costs that
highway agencies must bear to construct and maintain a highway system to serve present traffic and
that anticipated in the near future. This Chapter is intended to acquaint the reader with the technical and practical side of TS&W interaction with the infrastructure elements. Pavement deterioration increases with axle weight, the number of axle loadings, and the spacing within axle groups. The axle loads and spacing on trucks also affects the design and fatigue life of bridges.
Truck dimensions influence roadway design -- truck width affects lane widths, trailer or load height affects bridge and other overhead clearances, and length affects intersection and curve design. And conversely, truck designs are determined by existing pavement and bridge strength and roadway geometry.
Pavement types analyzed in this Study include flexible, asphaltic concrete; and rigid, portland cement concrete. Bridge features included in the analysis are span length and type of member support -- simple or continuous. The list of roadway geometry features analyzed includes interchange ramps, intersections, and mainline curves. Alternative truck configurations analyzed, in terms of their interaction with highway infrastructure features, include single-unit or straight trucks and single- and multitrailer truck combinations.
OVERVIEW OF INFRASTRUCTURE IMPACTSThe TS&W characteristics -- axle weights, GVW, truck length, width, and height -- affect pavements, bridges, and roadway geometry in different ways, as shown in Table VI-1.
VI-1 Table VI-1 Highway Infrastructure Elements Affected by TS&W Limits Highway Infrastructure Element Axle GVW Axle Truck Truck Truck Weight Spacing Length Width Height Pavement Flexible E E Rigid E e Bridge Short-Span E E E Features Long-Span E e E Clearance e E Roadway Interchange e E e Geometric Ramps Features Intersections E e
IMPACT OF WEIGHTThere are two aspects of truck weight that are interdependent and that interact with the highway infrastructure -- axle weight (loading) and GVW. As shown in Table VI-1, the effect of axle weight is more significant to pavements and short-span bridges, whereas GVW is of more significance to long-span bridges.
Generally, highway pavements are stressed by axle and axle group loads directly in contact with the pavement rather than by GVW. The GVW, taking into account the number and types of axles and the spacing between axles, is distributed among the axles and determines axle loads. Over time, the accumulated strains (the pavement deformation from all the axle loads) deteriorate pavement condition, eventually resulting in cracking of both rigid and flexible pavements and permanent deformation or rutting in flexible pavements. If the pavement is not routinely maintained, the axle loads, in combination with environmental effects, will accelerate the cracking and deformation. Proper pavement design relative to loading is a significant factor in pavement life, and varies by highway system and the number of trucks in the traffic stream.
VI-2 Axle groups, such as tandems or tridems, distribute the load along the pavement, allowing greater weights to be carried and resulting in the same or less pavement distress than that occasioned by a single axle at a lower weight. The spread between two consecutive axles also affects pavement life or performance; the greater the spread, the more each axle in a group acts as a single axle. For example, a spread of 9 to 10 feet results in no apparent interaction of 1-axle with another, and each axle is considered a separate loading for pavement impact analysis or design purposes.
Conversely, the closer the axles in a group are, the greater the weight they may carry without increasing pavement deterioration beyond that occasioned by a single axle, dependent on the number of axles in the group. This benefit to pavements of adding axles to a group decreases rapidly beyond 4-axles.
Axle loads also have a beneficial effect on short-span bridges -- that is, bridge spans that are shorter than the truck, thereby resulting in only 1-axle group, front or rear, being on the span at any time. While spreading the axles in an axle group is beneficial to short-span bridges, it is detrimental to pavement. It is not GVW but the distribution of the GVW over axles that impacts pavements.
However, GVW is a factor for the life of long-span bridges -- that is, bridge spans longer than the wheelbase of the truck. Bridge bending stress is more sensitive to the spread of axles than to the number of axles. The FBF takes into account both the number of axles and axle spreads in determining allowable GVW.
In the context of roadway geometrics, increasing GVW affects a truck's ability to accelerate from a stop, to enter a freeway, or to maintain speed on a long grade. Acceleration from a stop influences the time required to clear an intersection. Acceleration into a freeway affects the determination of acceleration lane length requirements. Inability to maintain speed on a long grade requires the construction of truck climbing lanes. Some of these effects can be ameliorated by changes in truck design, primarily to engine and drive train components. The GVW also has a second order effect on offtracking -- that is, on how the rear axle of a trailer tracks relative to the steering axle of the truck. Other truck characteristics affected by roadway geometrics are discussed in more detail later in this Chapter.
IMPACT OF DIMENSIONS
The dimensions of trucks and truck combinations have various effects on the three elements of highway infrastructure. The most significant effects relate to length, particularly when combined with GVW. Width has a limited effect on swept path -- the combination of offtracking and vehicle width. Swept path affects highway geometrics in terms of interchange ramp or roadway intersection design which is based on mapping a maximum swept path that the truck encroaches on the shoulder, over the curb, or into another lane of traffic. Height regulations are intended to ensure that trucks will clear overhead bridges, bridge members, overhead wires, traffic signals, and other obstructions.
In general, truck length -- or more specifically wheelbase -- has a strong effect on bridge stress for long-span bridges. The longer the wheelbase the shorter the distance from the support member to where the load is being applied (the moment arm) when the truck is in the middle of VI-3 the span. The shorter the truck the greater the concentration of load at the middle of the span, and the longer the distance (moment arm) to the support member for the bridge span member. A truck at mid-span is the loading condition for the maximum stress in a simple supported span. This is not the case for some continuous supported spans: when a truck is straddling the center pier of a continuous span, increasing the truck length can increase the stress in the span at the pier.
The effect of truck wheelbase on offtracking is reduced considerably if the combination is articulated, especially in a multitrailer combination. Low-speed offtracking affects interchange and intersection design, and high-speed offtracking affects lane width.
Bridges are critical to the safe and efficient movement of people and freight on the Nation’s highways. This section discusses the important considerations that have influenced the decision making and investments of Federal and State transportation officials for bridges.
Most highway bridges in the United States were designed according to the design guidelines of the AASHTO. These guidelines provide traffic-related loadings to be used in the development and testing of bridge designs, as well as other detailed requirements for bridge design and construction.
Dynamic effects (vibration resulting in bridge loads that vary above and below that load resulting trucks operating at higher speeds. In bridge design, design loadings (in the static condition) are adjusted upward to account for dynamic effects. To minimize the dynamic effects of extra-heavy nondivisible loads on some bridges, permits often require the truck to cross at a very slow speed, depending on its GVW.
A key task in bridge design is to select bridge members that are sufficiently sized to support the various loading combinations the structure may carry during its service life. These include dead load (the weight of the bridge itself); live load (the weights of vehicles using the bridge); and wind, seismic, and thermal forces. The relative importance of these loads is directly related to the type of materials used in construction, anticipated traffic, climate, and environmental conditions.
For a short-span bridge (for example, span length of 40 feet), about 70 percent of the load-bearing capacity of the main structural members may be required to support the traffic-related live load, with the remaining 30 percent of capacity supporting the weight of the bridge itself. For a long bridge (for example, span length of 1,000 feet), as much as 75 percent A substantial amount of the background material is drawn from the TRB Special Report 225, Truck Weight Limits: Issues and Options, 1990 and from the 1981 U.S. DOT Report to Congress under Section 161, An Investigation of Truck Size and Weight Limits.
In most instances, the loading event that governs bridge capacity is a design vehicle placed at the critical location on the bridge. In certain cases, a lane loading simulating the presence of multiple trucks on a bridge is the governing factor. Bridges are also affected by the dynamic impact and lateral distribution of weight of trucks; dynamic impact is determined by speed and roadway roughness, and the lateral distribution of loads varies with the position of the truck(s) on the bridge.
The methods used to calculate stresses in bridges caused by a given loading are necessarily conservative; therefore, the actual measured stresses are generally much less than calculated
stresses. Providing for a margin of safety is necessary to bridge design because:
C The materials used in construction are not always completely consistent in size, shape, and quality;
C The effects of weather and the environment are not always predictable;
C Highway users on occasion violate vehicle weight laws;
C Legally allowed loads may increase during the design life of a structure; and C Overweight loading is occasionally allowed by permit.
The adjustment of the nominal legal loading is reflected in the safety factors, which are selected so that there is only a very small probability that a loading condition that exceeds load capacity will be reached within the bridge’s design life.
The margins of safety used by bridge designers in the past have been reduced in recent bridge design procedures. Use of new design procedures and computer-aided engineering and design has enabled more precise analysis of load effects and the selection of smaller bridge members. Also, the competition between the steel and concrete industries has led each group to foster lower costs for their own material. For example, many designs now proposed for steel bridges reduce the safety factor by reducing the number of girders, which increases their spacing.
Design and construction of highway bridges in the United States has been governed by the AASHTO's Standard Specifications for Highway Bridges since 1931, with subsequent revisions.
In the early 1990s AASHTO decided to develop an entirely new bridge code to incorporate state-of-the-art bridge engineering that is based on the load and resistance factor design (LRFD) approach. 2 In 1993, AASHTO adopted LRFD bridge design specifications on a trial basis, as an FHWA http://www.ota.fhwa.dot.gov/tech/struct/dp99lr.html, February 19, 1998.
The LRFD method applies statistically determined factors to bridge design parameters, using a series of load and resistance factors to account for variabilities in loads and material resistance.
The specifications use statistical methods and probability theory to define the variations in loading and material properties and the likelihood that various load combinations will occur simultaneously. 4
Past studies of the impact of truck weight limit changes on bridges were based on various percentages of the yield stress for steel girder bridges, such as 55 percent or 75 percent. The yield stress, a property of the particular type of steel, is the stress at the upper limit of the elastic range for bridge strain. The elastic range of a structural member is the set of stresses over which the deformation -- the strain of the member -- is not permanent. In the elastic range, the member returns to its former size and shape when the stress is removed. There is no permanent set in the structural member. For this discussion, strain is the elongation of a steel girder when (1) a portion of the strain becomes permanent at a stress level above the yield stress; and (2) the girder continues to elongate, or stretch, under increasing load until it ruptures or fails. Beyond the elastic range, there is permanent elongation of the bridge girder, that is, for those stresses that are greater than the yield stress. However, in structural steel there is considerable strain before failure occurs.
BRIDGE INVENTORY AND OPERATING RATINGS