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Federal Highway Administration > Publications > Research > Structures > Covered Bridge Manual

Publication Number: FHWA-HRT-04-098
Date: APRIL 2005

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Chapter 9. Design and Analysis Specifications

This chapter covers the various codes, specifications, and guidelines that are most applicable in working with covered bridge design, and discusses the organizations that promulgate them. These specifications include one specific to bridge design, others specific to the three major structural materials, one for design loads on structures, and the primary building codes used in the United States. All viable codes are subject to regular revisions, some of them sweeping. One such major shift is the one that currently takes structural design from allowable stress methods to limit State design methods. Covered bridge designers and analysts must be familiar with all of these codes, but also may need to use methods and theories not found in any of them.

For purposes of this discussion, the various cited documents (codes, specifications, guidelines, etc.) are presented without distinction as to their technical classification or title. Each provides important reference and/or regulatory information and restrictions. To the extent relevant to work on covered bridges, they are cited in the given context; the intent is not to support one over another, except as noted otherwise. Designers must abide by the design codes mandated by each State, but most codes are sufficiently open-ended so that differing approaches can be used at the engineer's discretion.

American Association of State Highway and Transportation Officials

Covered bridge work is governed by several guidelines, most notably by those documents published by AASHTO. The organization published its first edition of the Standard Specifications for Highway Bridges in 1931, and has been updating this publication periodically since then.[2] The specifications relevant to covered bridges offer guidance on loads and combinations thereof, requirements for foundations, and sections on various materials, including timber, steel, and reinforced concrete. The materials portions of the AASHTO specifications were developed to complement and to modify specifications promulgated by the organizations representing the various industries, including:

Each of the material-specific sets of design guidelines, identified above, is generally aimed at the building industry. Accordingly, the specifications focus mainly on stationary structures, generally subjected to uniform loading only or so assumed, albeit sometimes with concentrated loads, and only very rarely moving loads (most frequently, crane loads in steel and concrete industrial buildings).

The AASHTO specifications, on the other hand, deal almost exclusively with highway bridges- structures that are subject to large moving loads in an open environment, exposed to harsh environmental attack. Further, bridges often represent a significant capital investment, intended and expected to serve an extended life, often targeted as 50-75 years. In practice, off-system bridges (those not on the State highway system or not under the State's jurisdiction) often receive little maintenance and may require major rehabilitation or even replacement before the end of their design lives. Major bridges and those on primary routes are often maintained with more vigilance for more years, due to the increasingly immense costs to replace them. For these and other reasons, AASHTO has intentionally adopted generally conservative provisions, usually based on the broader specialty specifications, but with its own selective modifications.

Therefore, the AASHTO specifications offer guidelines that are useful for general concepts related to covered bridges, while referring to other specifications for special needs, most notably the timber provisions of NFPA.

Initially and for many years, the AASHTO bridge specifications were intended to determine and limit stresses. This approach is called working stress, allowable stress, or service load design, depending on the consulting agency or organization. This approach involves calculating stresses from various loads and comparing them with allowable stresses, including provisions for group load combination factors.

During the latter part of the 20th century, the bridge design industry began to address the inconsistent factors of safety inherent in the working stress method. The steel and concrete industries had begun to adopt a different approach based on strength, rather than stresses. Similarly, this alternate methodology is known by various identifiers-strength, ultimate strength, or limit states approach. This approach involves establishing factors of loading predicated on their probabilities of occurrence or confidence in predictions of various loads. The term load factor design now identifies that approach in the bridge industry.

For many years, AASHTO permitted use of either the working stress approach or the load factor approach to design and maintain both portions of the specifications. Some State agencies adopted the load factor approach, while many continued the older working stress approach.

Continued evolution of the load factor approach led to the adoption of a refinement termed load and resistance factor design (LRFD). This involves assigning capacity reduction factors based on material and member behavior, in addition to load factors based on the probability of an individual loading or loading combination. The intent of this newer design method is to allow designers to provide structures with more uniform reliability for all components. The older working stress design process often leads to nonuniform factors of safety.

AASHTO has stated that the 17th edition (2002) will be the last of the standard specifications and will receive only editorial corrections hereafter. After 2007, the LRFD specification will be the only bridge design specification that AASHTO routinely supports.

At this time, several State and most local government agencies continue to use the working stress approach for bridge design because of its familiarity and relative ease of use. More importantly, the switch to LRFD does not significantly alter the design results for the shorter, simple-span structures representing the vast majority of bridges, not only covered timber bridges.

The timber industry also has been pushing its design methods toward ultimate strength, but, as in the building industry, design work on timber-framed covered bridges continues almost invariably to use the working stress approach. In large part because analytical work on historic covered bridges involves such assumptions, using a more sophisticated design specification may seem unwarranted.

The National Design Specifications for Wood Construction

The NDS was first printed in 1944, originally published by the National Lumber Manufacturers Association.[3] The NFPA assumed subsequent publishing responsibilities, which now have been transferred to the AF&PA. In 1992, the American National Standards Institute (ANSI) accredited AF&PA, and the NDS gained approval as an ANSI standard. It currently is identified as the ANSI/AF&PA National Design Specifications for Wood Construction.[3] As noted above, the NDS is prepared for use on buildings; however, bridge structure applications are mentioned. The NDS remains the most recognized publication on timber specifications, and it is available in both allowable stress and LRFD formats.

The AASHTO specifications cite specific reference to the NDS, but some of the more common provisions found in the AASHTO specifications were lifted directly from the NDS. AASHTO includes a few special modifications, such as load duration factors for vehicular live loading.

Specifications for Minimum Loads

The reasonable design loads on covered bridges include snow loading, which is not addressed in the AASHTO specifications. In addition, the wind loading provisions contained in the AASHTO specifications are often overly simplistic and conservative for covered bridges, particularly when compared to the provisions in those specifications that are aimed more at the buildings industry. Accordingly, covered bridge engineers should seek additional guidance beyond strict adherence to the AASHTO specifications.

A popularly cited reference in establishing design loads is the Minimum Design Loads for Buildings and Other Structures.[8] It is published by the American Society of Civil Engineers (ASCE), and is often referred to as ASCE 7. Like the NDS, this publication was published previously as a jointly sponsored standard between ASCE and ANSI. The previous edition is identified as the ANSI/ASCE 7[8]. Hence, this standard is highly respected as a thorough compendium and is quite applicable to covered bridge work.

Other Building Specifications

The buildings industry is governed by a host of codes, some national (in name only, as certain regions of the country tend to adopt a single code), and others local. Those national codes more commonly cited include:

An International Building Code (IBC) was published with the intention of eventually superseding the three codes noted above, because of the difficulties for designers working in various regions of the country. Also, because of differences of opinion related to fire safety and other issues in the new IBC, the National Fire Protection Association recently announced that it would develop its own complete building code as an alternative to the IBC. Thus, the likelihood of a single national code has been reduced significantly.

Many States also publish and maintain their own version of code requirements. Hence, it is very important that covered bridge engineers know the local code requirements, particularly as the requirements relate to topics with special interest to covered bridges (e.g., minimum ground snow loads).

Glued-Laminated Timber Specifications

Because rehabilitation projects of historic covered bridges occasionally involve the use of glued-laminated (glulam) components, it is appropriate to include a brief mention of this engineered wood material in this chapter. The American Institute of Timber Construction (AITC) was the standard of the industry with respect to this topic for many years; it periodically published the Timber Construction Manual, which contains various sections related to glulam members.[9] The Institute also published a large, three-ring volume of material that contains useful information about this topic. The AASHTO specifications refer to AITC specifications, as do the NDS for glulam products. More recently, the American Plywood Association, in promulgating the most current specifications for the glulam industry, has replaced the AITC.

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