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The Engineering & Capacities of Timber Bridges

Timber bridges are designed according to the principles of engineering mechanics and strength of materials, assuming the same basic linear elastic theory applied to other materials. The method used for design is the allowable stress design method, which is similar to service load design for structural steel. In this method, stresses produced by applied loads must be less than or equal to the allowable stresses for the material. A design method called load and resistance factor design (LRFD) is used for timber design in other countries, but not in the United States. Progress is being made toward development of such a method in the United States; however, adoption is several years away.

Wood strength and stiffness vary with species, growth characteristics, loading, and conditions of use. As a result, one set of allowable design values for all species and design situations would result in very uneconomical design in most cases. Conversely, tabulated values for all potential conditions would result in so many tables that they would be unusable. Rather than using either of these approaches, timber design is based on published tabulated values that are intended for one set of standard conditions. When these conditions differ from those of the design application, the tabulated values are adjusted by modification factors to arrive at the allowable values used for each design. This approach produces more realistic design values for a specific situation. In general terms, the basic timber design sequence is as follows:

  • Compute load effects and select an initial member size and species.
  • Compute the applied stress from applied loads.
  • Obtain the tabulated stress published for the specific material.
  • Determine appropriate modification factors and other adjustments required for actual use conditions.
  • Adjust the tabulated stress to arrive at the allowable stress used for design.
  • Compare applied stress to allowable stress. The design is satisfactory when applied stress is less than or equal to allowable stress.

A bridge must be designed to safely resist all loads and forces that may reasonably occur during its life. These loads include not only the weight of the structure and passing vehicles, but also loads from natural causes, such as wind and earthquakes. The loads may act individually but more commonly occur as a combination of two or more loads applied simultaneously. Design requirements for bridge loads and loading combinations are given in AASHTO Standard Specifications for Highway Bridges (AASHTO). AASHTO loads are based on many years of experience and are the minimum loads required for design; however, the designer must determine which loads are likely to occur and the magnitudes and combinations of loads that produce maximum stress. Methods and requirements for determining the magnitude and application of individual loads are presented first, followed by discussions on loading combinations and group loads. Additional information on load application and distribution related to specific bridge types is given in succeeding chapters on design.

Dead Load

Dead load is the permanent weight of all structural and nonstructural components of a bridge, including the roadway, sidewalks, railing, utility lines, and other attached equipment. It also includes the weight of components that will be added in the future, such as wearing surface overlays. Dead loads are of constant magnitude and are based on material unit weights given by AASHTO (Table 6-1). Note that the minimum design dead load for timber is 50 lb/ft3 for treated or untreated material. Dead loads are commonly assumed to be uniformly distributed along the length of a structural element (beam, deck panel, and so forth). The load sustained by any member includes its own weight and the weight of the components it supports. In the initial stages of bridge design, dead load is unknown and must be estimated by the designer. Reasonable estimates may be obtained by referring to similar types of structures or by using empirical formulas. As design progresses, members are proportioned and dead loads are revised. When these revised loads differ significantly from estimated values, the analysis must be repeated. Several revision cycles may be required before arriving at a final design. It is often best to compute the final dead load of one portion of the structure before designing its supporting members.

Vehicle Live Load

Vehicle live load is the weight of the vehicles that cross the bridge. Each of these vehicles consists of a series of moving concentrated loads that vary in magnitude and spacing. As the loads move, they generate changing moments, shears, and reactions in the structural members. The extent of these forces depends on the number, weight, spacing, and position of the loads on the span. The designer must position vehicle live loads to produce the maximum effect for each stress. Once the locations for maximum stress are found, other positions result in lower stress and are no longer considered.

Terminology

Vehicle live loads are generally depicted in diagrams that resemble trucks or other specialized vehicles. The terms used to describe these loads are defined below and shown in Figure 6-1.

Gross vehicle weight (GVW) is the maximum total weight of a vehicle.

Axle load is the total weight transferred through one axle.

Axle spacing is the center-to-center distance between vehicle axles. Axle spacing may be fixed or variable.

Wheel load is one-half of the axle load. Wheel loads for dual wheels are given as the combined weight of booth wheels.

Wheel line is the series of wheel loads measured along the vehicle length. The total weight of one wheel line is equal to one-half the GVW.

Track width is the center-to-center distance between wheel lines.

Vehicle Diagram

Standard Vehicle Loads

AASHTO specifications provide two systems of standard vehicle loads, H loads and HS loads. Each system consists of individual truck loads and lane loads. Lane loads are intended to be equivalent in weight to a series of vehicles (discussed in the following paragraphs). The type of loading used for design, whether truck load or lane load, is that producing the highest stress. It should be noted that bridges are designed for the stresses and deflection produced by a standard highway loading, not necessarily the individual vehicles. The design loads are hypothetical and are intended to resemble a type of loading rather than a specific vehicle. Actual stresses produced by vehicles crossing the structure should not exceed those produced by the hypothetical design vehicles.

Truck Loads

There are currently two classes of truck loads for each standard loading system (Figure 6-2). The H system consists of loading H 15-44 and loading H 20-44. These loads represent a two-axle truck and are designated by the letter H followed by a number indicating the GVW in tons.

Truck Loading Diagram 1
Truck Loading Diagram 2

Lane Loads

Lane loads were adopted by AASHTO in 1944 to provide a simpler method of calculating moments and shears. These loads are intended to represent a line of medium-weight traffic with a heavy truck positioned somewhere in the line. Lane loads consist of a uniform load per linear foot of lane combined with a single moving concentrated load, positioned to produce the maximum stress (for continuous spans, two concentrated loads — one placed in each of two adjoining spans — are used to determine maximum negative moment). Both the uniform load and the concentrated loads are assumed to be transversely distributed over a 10-foot width. AASHTO specifications currently include two classes of lane loads: one for H 20-44 and HS 20-44 loadings and one for H 15-44 and HS 15-44 loadings. The uniform load per linear foot of lane is equal to 0.016 times the GWV for H trucks or 0.016 times the weight of the tractor truck for HS trucks. The magnitude of the concentrated loads for shear and moment are 0.65 and 0.45 times those loads, respectively.

Application of Vehicle Live Load

Vehicle live loads are applied to bridges to produce the maximum stress in structural components. The designer must determine the type of design loading and overload (when required), compute the absolute maximum vehicle forces (moment, shear, reactions, and so forth), and distribute those forces to the individual structural components. The first two topics are discussed in the remainder of this section. Load distribution to specific components depends on the configuration and type of structure; it is addressed in subsequent chapters on design.

Design Loading

Vehicle live loads used for design vary for different locations and are established by the agency having jurisdiction for traffic regulation and control. Bridges that support highway traffic are designed for heavy truck loads (HS 20-44 or HS 25-44). On secondary and local roads, a lesser loading may be appropriate. To provide a minimum level of safety, AASHTO specifications give the following minimum requirements for bridge loading:

Bridges that support interstate highways or other highways that carry or may carry heavy truck traffic are designed for HS 20-44 loading or the alternate military loading, whichever produces the maximum stress (AASHTO 3.7.4).

Bridges designed for less than H 20-44 loading also must be designed to support an infrequent heavy overload equal to twice the weight of the design vehicle. This increased load is applied in one lane, without concurrent loading in any other lane. The overload applies to the design of all affected components of the structure, except the deck (AASHTO 3.5.1). When an increased loading of this type is used, it is applied in AASHTO Load Group IA, and a 50-percent increase in design stress permitted by AASHTO (see discussions on load groups in Section 6.19).

The Engineering & Capacities of Pedestrian Bridges

Sidewalks are provided on vehicle bridges to allow concurrent use of the structure by pedestrians, bicycles, and other nonhighway traffic. Sidewalks are subjected to moving live loads that vary in magnitude and position, just as do vehicle live loads. For design purposes, AASHTO gives sidewalk live loads as uniformly distributed static loads that are applied vertically to the sidewalk area (Figure 6-26). The magnitude of the load depends on the component of the structure and the length of sidewalk it supports. When a member supports a long section of sidewalk, the probability of maximum loading along the entire length is reduced. As a result, loads vary and are based on the type of member and sidewalk span (AASHTO 3.14.1).

Sidewalk floors, floorbeams (longitudinal or transverse), and their immediate supports are designed for a live load of 85 lb/ft2 of sidewalk area. Loads on longitudinal beams, arches, and other main members supporting the sidewalk are based on the sidewalk span:

Span Length

  • Up to 25 ft.
  • 25 ft. to 100 ft.

Sidewalk Load

  • 85 lb/ft2
  • 60 lb/ft
Pedestrian Bridge Capacities

It should be noted that sidewalk loads given in AASHTO are intended for conditions where loading is primarily pedestrian and bicycle traffic. If sidewalks will be used by maintenance vehicles, horses, or other heavier loads, the designer should increase the design loading accordingly.

Sidewalk loads are distributed to structural components in a manner similar to dead load. The load supported by any member is computed from the tributary area of sidewalk it supports. If bridges have cantilevered sidewalks on both sides, one or both sides should be fully loaded, whichever produces the maximum stress. In cases where the maximum design load in an outside longitudinal beam results from a combination of dead load, sidewalk live load, and vehicle live load, AASHTO allows a 25-percent increase in allowable design stresses, provided the beam is of no less carrying capacity than would be required if there were no sidewalks.

All information provided can be found in the Timber Bridges: Design, Construction, Inspection and Maintenance. Written by: Michael A. Ritter, Structural Engineer, USDA Forest Service.