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正常使用状态与极限状态
件常常由较薄的板件组成（薄壁腹板和薄壁翼缘等）。上述两种情况可能引起极限破坏状态下的不同形式的屈曲。ASD法和LRFD法都把以前应力法的某些要素和极限状态分析的某些新要素结合起来。基本设计目的是使结应力法强调结构在使用荷载状态（即预期的使用情况）下的特性，而承载力法主要与抗力极限有关。有些工作状态——例如变形、抗火性及锈蚀——两种方法都适用。此外，假设使用荷载与承载力法尽可能的同样精确，因为使用荷载是极限设计荷载来源的基础。尽管如此，承载力设计集中于确定结构的极限状态。以下是钢结构的两个极限值：
SERVICE CONDITIONS VERSUS LIMIT STATES
Stress methods emphasize behavior at service (i.e.,actual anticipated usage) load conditions, while strength methods relate primarily to limits of resistance. Some service behavior—such as that pertaining to deformations, fire resstance, and rusting—applies to both methods. Moreover, the service load is visualized as accurately as possible for the strength methods because it service as the basis for derivation of the factored load.
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