For a structural engineer, designing a structure entails, ensuring that the structure resists all possible forces. Some of the forces a structure needs to resist include:
- The mass of the contents of the structure including the structure itself and other nonstructural materials; (dead load.)
- Transient forces from the occupancy of the structure (i.e. people moving inside the structure, equipment operating inside the structure, and more), (live load)
- Environmental forces such as wind, earthquake, snow, and rain. (lateral load)
The use of steel as a construction material has become rampant over the years, hardly can you find a sophisticated structure (for example; skyscraper) that does not have parts of steel. Additionally, steel is a very popular construction material because of its properties. Some of which are;
- The availability,
- High strength to weight ratio,
- Ductility
- Elasticity
- Good tensile strength
- Good yield strength
- Good thermal conductivity
However, there is common saying that everything that has an advantage, has a disadvantage as well, and steel is no different. There are some noteworthy cons one must consider when using steel as a construction material. Some of these points are:
- Susceptibility to corrosion
- Reduction in strength due to high temperatures.
- Poor fire resistance
- Cyclic loading makes steel susceptible to fatigue
- Steel undergo buckling, as a result of high compression and flexural stresses.
Additionally, the structural engineering firm responsible for a steel construction design must ensure that the work done by the structure is large enough to resist the possible steel failure:
- Shear failure
- Flexural failure
- Compression failure
- Tensile failure
Shear Failure
Shear failures occur due to a lack of adequate shear resistance between the materials. Shear failures are known to occur in connections between members (like beams and columns). Examples of connections where shear failure can occur include:
- Member to column connection.
- Member to girder connection.
The design of steel connections is a not so easy task. The shearing forces of connections are usually very high; therefore, a structural engineer must take that into consideration when designing connections. Additionally, to avoid shear failure, a structural engineer must never underestimate the force a connection must withstand. Bolts, welds or a combination of both, constitutes most connections in steel structures.
Flexural Failure
Flexural failures refer to the bending or yielding of the steel. This type of failures occurs in flexural members and in some cases compression members, such as:
- Members
- Girders
- Columns that are subjected to bending stresses.
When an element of flexural member buckles as a result of flexural loadings, flexural failure occurs at that point. Steel members are usually at risk of buckling, this is because of the strength of steel, an efficient steel member design equals slim members. However, buckling seldom occurs in heavier, stocky members.
Additionally, tension and compression forces in steel members occur as a result of flexural loadings. And compression forces can result in lateral-torsional buckling if it is high enough to laterally displace or buckle an unrestrained section of the steel member.
However, an engineer can prevent buckling from flexural loadings by providing lateral restraints to a member. notwithstanding, a member’s failure is very much a function of its material strength. Therefore, if the flexural loading is greater than the material strength of the member, the member would fail.
Compression Failures
Compression failures occur as a result of compression in the axial axis of the member. When this compression results in buckling of an element of a steel member. This type of failure occurs in compression members like columns and braces.
In designing a column, it is important to consider a parameter known as the slenderness ratio. The slenderness ratio refers to the cross-sectional geometry of a member to the length of the member. This parameter is important because of its relationship with buckling. The higher the slenderness ratio of a member, the higher the member’s susceptibility to buckling and vice versa.
However, if the compressive stress is greater than the material strength can handle, the would fail.
Tensile Failures
Tensile failures are as a result of tension in a member. This type of failure is most common in brace members or hangers. When tension stretches a member beyond its material strength, tensile failure occurs.
Notably, this failure occurs in stages, starting from yielding to necking and finally, material failure, which occurs where the cross-section is least.
Conclusion
The strength and reliability of steel as construction materials cannot be overemphasized, however, its efficiency can only be guaranteed when the structural engineer designs the structure, taking all possible failures into consideration.
