许用应力（ASD)概念普及

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Allowable Stress Design (ASD) is also referred to as the service load design or working stress design (WSD). The basic conception (or design philosophy) of this method is that the maximum stress in a structural member is always smaller than a certain allowable stress in bridge working or service conditions. The allowable stress of a material determined according to its nominal strength over the safety factor. Therefore, the design equation of the ASD method can be expressed as:∑σi≤σall=σn/Fs
where σi is a working stress due to the design load, which is determined by an elastic structural analysis under the design loading conditions. σall is the allowable stress of the constructional material. The σn is the nominal stress of the material, and FS denotes the safety factor specified in the design specification. Selection of allowable stress depends on several factors, such as the design code, construction materials, stress conditions, etc. Taking the allowable of SS400 (a structural steel in Japanese design code) in tension as an example, the allowable stress shall be taken as 140 MPa when its thickness is larger smaller than 40 mm but 125 MPa for thickness larger than 40 mm. When it is in compression, the buckling may also be considered in selecting the allowable stress.

The ASD method is very simple in use, but it cannot give a true safety factor against failure. All uncertainties in loads and material resistance are considered by using the safety factor in ASD. Although there are some drawbacks to ASD, bridges designed based on ASD have served very well with safety inherent in the system. Currently, ASD design method is still used in the bridge design specifications in Japan.

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