Timber Bridge Engineering & Capacities

We can build your timber bridge to AASHTO standards


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.

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.

In general terms, the basic timber design sequence is as follows:

  1. Compute load effects and select an initial member size and species.

  2. Compute the applied stress from applied loads.

  3. Obtain the tabulated stress published for the specific material.

  4. Determine appropriate modification factors and other adjustments required for actual use conditions.

  5. Adjust the tabulated stress to arrive at the allowable stress used for design.

  6. Compare applied stress to allowable stress. The design is satisfactory when applied stress is less than or equal to allowable stress.

Vehicular Bridges

Application of Vehicle Live Load and Design Loading

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.

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.

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).

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.