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Ultimate Strength Design Method

Civil Engineering and Design
Strength of Materials Basics and Equations | Mechanics of Materials
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Ultimate Strength Design Method

The allowable stress method has been replaced in most structural work by the ultimate strength design method, also known as the load factor design method, plastic design method, or just strength design method. This design method does not use allowable stresses at all. Rather, the member is designed so that its actual nominal strength exceeds the required ultimate strength.

The ultimate strength (i.e., the required strength) of a member is calculated from the actual service loads and multiplicative factors known as overload factors or load factors. Usually, a distinction is made between dead loads and live loads. For example, the required ultimate moment-carrying capacity in a concrete beam designed according to ACI 318 would be

Mu = 1.2Mdead load + 1.6 Mlive load

The nominal strength (i.e., the actual ultimate strength) of a member is calculated from the dimensions and materials. A capacity reduction factor, ), of 0.70 to 0.90 is included in the calculation to account for typical workmanship and increase required strength. The moment criteria for an acceptable design is

Mn ≥ Mu / Φ


Civil Engineering Reference Manual, Fifteenth Edition, Michael R. Lindeburg, PE