The animal kingdom has many different criteria for classification. The presence and the type of the skeleton is a particularly important one as the animals are divided into two large groups; vertebrates and invertebrates. The existence and the type of the skeletal structure is a major defining characteristic. While there are no vertebrate and invertebrate buildings, they too can be defined in the terms of their “skeleton.” When we talk about the building skeleton, as a metaphor, of course, what we actually have in mind is the building’s structural frame. The properties of a structural frame are what is going largely to define the properties of the building. The main purpose of the structural frame is to hold the building’s weight, the weight of the frame itself, to stand the test of time, and react to major structural challenges such as winds, earthquakes or fires. This is why, in this article, we will explore the properties of structural steel and how they affect the building as a whole.
Strength of Steel
Depending on the sources, there are many different types of properties of steel. In this article, the focus will mainly fall on the following: strength, toughness, ductility, weldability, and durability. The first property on the list is the strength, or, more specifically, yield strength – and their relation to metal constructions. Yield strength is defined as the amount of stress a material can undergo before exhibiting plastic (or irreversible deformation). The amount of stress that would cause plastic deformation is called the yield point. Those who are more familiar with properties of steel might think at this point that steel frequently reaches its yield point since material fatigue is the number one reason for the structural failures of steel framed buildings. However, this is not the case. In order to say that a material has reached its yield point, it needs to suffer 0.2% plastic deformation, something rarely caused by forces such as weight, wind and corrosion, the main culprits in the case of material fatigue of steel. This is why steel is rightfully synonymous with strength and widely used in high-rise buildings as it provides a very strong structural basis.
Ductility and Toughness of Steel
The next two important properties of steel, which are very closely related, are toughness and ductility. Ductility can be understood as the flexibility of steel for the sake of simplicity. Ductility of a steel component denotes the amount of tensile strain the component can endure before suffering plastic deformation. Toughness is often confused with strength, however, toughness is a separate property in its own right. Toughness is different from the strength in the sense that the damage stemming from the lack of toughness will mostly result in visible fracture, and always result in brittle fracture. What can be a tiny break in the material resulting from the lack of strength to handle the given amount of stress, turns out to be a rapidly expanding crack when the material lacks toughness. Factors that increase the risk of brittle fracture are very cold temperatures, stress that causes the material to elongate, and its thickness. Even if we eliminate the weight factor, the increased thickness can steel result in steel being less able to withstand the stress exerted. This is the reason why it is important to carefully plan and aim for optimal structural steel beams for buildings situated in areas that have very big temperature amplitudes throughout the year. The great variations in temperature on the annual level might increasingly become a factor in building planning in the future as the climate change might lead to colder winters and warmer summers, especially in areas with moderate continental climate. This is especially important for steel as iron is its major component. Iron is known to greatly suffer from increased brittleness at lower temperatures.
Weldability of Steel
Weldability of steel is another very important property, but the term can be a bit misleading. The implication of the name and of the context in which it is used can suggest that there are weldable and non-weldable types of steel, which is most definitely not true. Every type of steel can be welded together, including two different types of steel if the proper filler material and welding methods are applied. There are differences, however, in the welding methods that are used depending on the chemical composition and carbon level in the steel piece. For example, the most widely used type of steel for structural purposes is the low carbon steel containing less than 0.15% of carbon per unit of mass. Low carbon steel, also known as mild steel, is, for example, a material of very high weldability. It requires no pre-heating and nor it demands a high level of protection from outside contamination in order to achieve the good weld. Apart from a very favorable strength-to-weight ratio, the property of good weldability is another reason why mild steel is so widespread in the construction industry as the material allows for great design flexibility for not high a cost or compromised structural integrity.
Durability of Steel
The last, but definitely not the least important property of structural steel is durability. Steel durability can be understood in two, equally legitimate ways. First, structural steel is very durable in the sense that it can support buildings for decades without a significant reduction in capacity if properly maintained. This is the reason why New York skyline is still filled with buildings from the early 20th century such as those that comprise the Rockefeller Centre. Another aspect of looking at steel durability is actually in the recyclability of the material. Steel is relatively easy to recycle and at a recycling rate of around 90% worldwide, it really pays to invest in the effort. As project planners aim for more sustainable buildings in the future, this property is becoming increasingly important and it is one of the main reasons why it is expected that the new-born trend of building wood-framed structures in the US is to inevitably falter. The carbon footprint of steel is shrinking while the sustainability scores for steel framed buildings are rising.
Other than the five properties of steel mentioned, there are many more that can determine the designing and erection of structural steel frames for metal constructions. However, depending on the case-by-case basis, these are either too irrelevant for properly assessing the bigger picture, or they are too complex and with too many relevant variables to fit within the scope of the article. Keep checking our blog in the future for more content on the topic.