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The Physics of Bridges

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The Physics of Bridges

The design of the bridge superstructure is based on a set of loading conditions
which the component or element must withstand. A bridge engineer must take into
account a wide variety of loads which may vary based on the duration of the load
(permanent or temporary), the direction of the load (vertical, longitudinal, etc.), and the
effect of the load (shear, bending, torsion, etc.).
In order to form a consistent basis for design, organizations like AASHTO, American
Association of State Highway and Transportation Officials, have developed a set of
standard loading conditions which are applied to the engineer's design model of the
structure. Other nations maintain their own set of design loads like the BS 5400 loads
utilized in the United Kingdom or the Ontario Highway Bridge Design Code, OHBDC,
loads utilized in the Canadian province of Ontario and elsewhere in that nation. (1)
Permanent loads, as the name would imply, are those loads, which always remain on a
bridge throughout its life. Although the term dead load is often used synonymously with
permanent loads, there are distinctions, which need to be made. Permanent loads are
divided into the following three major categories: dead load, superimposed dead load, and
The dead load on a superstructure is the aggregate weight of all superstructure elements
(i.e. those elements above the bearings). This would include, but not be limited to, the
deck, wearing surface, stay-in-place forms, sidewalks and railings, parapets, primary
members, secondary members (including all bracing, connection plates, etc.), stiffeners,
signing, and utilities. One of the first steps in any design of a superstructure is to compile
a list of ...

... middle of paper ...

...HTO load distribution specification, when appropriate.
The AASHTO code places the distribution of loads for concrete slabs within the same
section as that which describes the general design criteria for this element. For load
distribution criteria pertaining to other less common flooring, such as timber flooring
(wood), you can be referred to the AASHTO specifications directly for information.


1. American Association of State Highway and Transportation Officials (AASHTO): Standard Specifications for Highway Bridges, Washington, D.C., 1977
2. Bakht, B. and Jaegar, L.G.: Bridge Analysis Simplified: 1985
3. Serway: Physics for Scientists and Engineers: Vol. 1, 4th ed. 1996
4. Hoelsher, R.P. and Springer, C.H. and Dobrovolny, J.S.: Graphics for Engineers: Visualization, Communication, and Design, New York, New York, 1968

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