Ultimate limit state of steel structure

Like other building structures, the limit states of steel structures are divided into two categories: bearing capacity limit state and serviceability limit state. The former corresponds to the structure or component reaching its maximum bearing capacity or exhibiting deformation unsuitable for continued bearing, including overturning, strength failure, fatigue failure, loss of stability, the structure becoming a kinematic system, or excessive plastic deformation. The latter corresponds to the structure or component reaching a certain specified limit value of normal use or durability performance, including deformation, vibration, and local damage that affect normal use (or appearance).

Strength failure refers to the failure of a certain section of a component or a connection due to stress exceeding the material strength. A steel component with holes breaking at the weakened section is a general strength failure. In the truss bridge shown in Figure 1-9, if the lower chord member with the largest force breaks, the entire bridge can no longer continue to bear the load. Steel structures also have a special case, that is, brittle fracture in a low-stress state under specific conditions. Inferior materials, unreasonable construction, and low temperatures can all contribute to this fracture.

Steel used in civil steel structures has good plasticity, and it will strengthen after yielding, showing that the tensile strength fu is higher than the yield strength fy. The plasticity of the material can be considered appropriately when designing steel structures. However, the use of the plastic working stage should not cause excessive deformation. If the tensile chord member of the truss uses fu instead of fy as the bearing limit, it will be unsuitable for continued bearing due to excessive deformation.

Hyperstatic beams or frames can allow full plasticity to occur at the section with the largest force, forming a so-called plastic hinge. When the load continues to increase, this section works like a real hinge. Multiple hyperstatic structures can have several plastic hinges without losing bearing capacity until the number of plastic hinges increases to form a kinematic system. Of course, there are conditions for reaching this limit state, that is, the possibility of losing stability is prevented.

Steel components have small sections due to high material strength, and the plates that make up the components are also thin, making instability a very important aspect of the bearing capacity limit state. Compressive stress is the reason for the instability of the components. Except for axially tensile members, compression members, beams, and compression bending members all have compressive stress to varying degrees. Therefore, instability is also universal in steel structures. If the main compression chord member of the truss bridge loses stability, the entire structure will lose its bearing capacity. However, some local instability phenomena do not constitute the limit of bearing capacity. Readers will learn about this aspect from the following chapters.

Many steel components are used to withstand repeated live loads, such as bridges and crane beams. Under repeated cyclic loads, fatigue failure may occur.

The vast majority of bearing capacity limit states are irreversible. Once they occur, they lead to structural failure, so they must be treated cautiously. The deformation and vibration limits in the serviceability limit state are usually within the elastic range and are reversible. For reversible limits, the requirements for reliability can be relaxed somewhat.