Structural engineering is a major branch of civil engineering, and realistically the first thing that comes to mind of many, when the words civil engineering are mentioned. It deals with designing the body of all types of structures and their supplementary components, connections, attachments per the criteria given in codes, to ensure public comfort and safety. As we mentioned in the previous post in Construct Magazine, failures in civil engineering, structures can fail in so many ways but they can all be grouped under several categories. Let’s list them below…

Structural Failure Types in Materials and Members:

Structural members can fail as a result of many reasons such as:

• Yielding
• Buckling
• Rupture
• Fatigue
• Creep
• Corrosion
• Fire
• Thermal effects

Yielding of the material should be the first one we should mention. For yielding, everything ultimately boils down to the types of forces, which are compression, tension and shear. A structural member therefore yields as a result of not being able to have adequate resistance against one or more of these forces. Note that there is also the “moment”, which we often call flexure, the turning effect of force (Moment = force x distance to point of rotation), but that also can be reduced to these basic forces as far as failure reason. In fact, even compression failures are often ultimately a result of shear failure, as described in Mohr’s Circle, so in structural (and geotechnical) engineering, the name of the game is often shear. If you are not an engineer and want to familiarize yourself more with all these terms, see our post summary of structural engineering fundamentals in ConstructMagazine.Com

Buckling happens when a slender member under compression suddenly buckles even if it didn’t yield, when the compressive force reaches a critical value with respect to the members geometry and support conditions, even if that force caused a stress less than the yield strength of the material. This phenomenon was put in formula by Euler three centuries ago. Buckling failures are often sudden which means they often have sudden, catastrophic consequences. Buckling failures are also called geometrical failures because it is a result of the geometry of the members and how they are connected with other members and their support conditions. There are several types of buckling failures. Reinforced concrete or steel columns both can buckle but for steel structures this is a little more often.

Rupture is when a structural steel member gets torn similar to paper.

Fatigue is a failure type which mainly metals are prone to, which means mostly structural steel and to a point reinforcing bars of RC. Wood structures can also experience fatigue failure in their own way. Fatigue happens as a result of repeated cycling loading even below yield point, where the number of loading repetition is usually in the order of at least tens of thousands or sometimes even millions, depending on the load intensity, frequency, the structure and the material itself. Fatigue is one of the main design criteria for steel structures.

Another failure mechanism is creep, where similar to fatigue the load is below yield point, but this time it is a constant, sustained, very long term loading, in terms of several years. As a result of creep the material keeps deforming at a slow rate, more at first, then slowly decreasing and approaching to zero in years, but the accumulated deformation may be hard to ignore in the long term. Concrete structures are especially prone to creep, and it is a major design criteria because all structures are built for long term. Wood structures can also experience creep, although this is less critical for them. Creep in steel structures are low.

Corrosion of steel is a major concern anywhere we have steel, such as reinforcement in concrete or structural steel. Corrosion happens as a result of moisture interaction so the steel must be properly enclosed. In RC this is achieved by a proper minimum concrete cover amount over rebar, usually at least 2 inches.

Another failure initiating mechanism is fire, which steel and wood are prone to, but not concrete itself. Again, the concrete cover over rebar protects the reinforcing in concrete and for structural steel this is achieved by proper architectural details or protective paint.

Thermal expansion and contraction, if not properly accounted for, can cause large unanticipated stresses and deformations in the structure, leading to collapse. Unlike what many of us would expect, concrete and steel have very close thermal expansion coefficients, which is a very fortunate thing for us because if this wasn’t the case, this would be a big concern and make all design and construction much more difficult for reinforced concrete structures.

Also see our post earthquake related structural failures in Construct Magazine, where we discuss earthquake effects on structures and the resulting failures.

Reinforced Concrete and Structural Steel:

To this point we listed the main failure mechanisms that can occur in structural members. Now let’s take a closer look at the two most common structural materials, Reinforced Concrete (RC) and Structural Steel.

Reinforced Concrete (RC):

Concrete, the most common construction material, is mainly used for its compressive strength. It can fail simply by being crushed from compression, as in a compression test or when it is loaded too much in a structure, such as an overloaded column. This indicates a serious design failure if the loads were not anticipated correctly, or a serious construction failure which can stem from a number of things of which we will mention a few below.

Concrete is a brittle material. It fails suddenly without warning and with little deformation. This is not a desirable behavior and one of the main reasons that we reinforce it with steel, which is ductile, meaning, steel can undergo a considerable amount of deformation before failing so it does not fail suddenly, which we highly prefer. Concrete is strong in compression but weak in shear, and even weaker in tension, to the point that we assume it as zero in design. Steel is strong for all three, which compensates for concrete’s shear and tensile weakness. Steel’s compressive and tensile stress are practically the same with each other and far higher than concrete’s, even high strength concrete. Steel’s shear strength is lower than its tensile and compressive strengths, but still much higher than concrete’s shear strength. Plus, reinforcing in concrete is arranged such that even when steel is supposed to take shear forces at a particular location in a structural member, it is aligned such that it will be in tension itself. Concrete and steel union can be resembled to a happy marriage. Concrete protects steel from corrosion and fire, and steel takes on the tensile and shear stresses, while also helping in compression and also taking the excess amount of deformation by its ability to elongate large amount before losing strength. This especially becomes critical during strong earthquakes where the loads can be excessive and cyclic and the earthquake energy entering to the structure must be absorbed by large amount of (plastic, in other words, yielding) deformations (beyond elastic, in other words, below yielding deformations), which only steel can do.

Some common failure modes can be:

When we place too much rebar in concrete, you may first think this is even better even though it may be more expensive. But this is not the case. Too much rebar means, the steel will not fail first, but concrete, which is brittle and fails suddenly without warning so this is very undesirable, which is called over-reinforced concrete. Therefore the goal of a reinforced concrete member, be it a column, beam, slab, wall, footing… is to meet load and deformation demands from it by optimally making use of concrete and steel. The codes heavily regulate all these, by establishing criteria based on engineering theory, tests and past experience. For example in the US, there is the ACI 318, Building Code Requirements for Structural Concrete, which is such a useful and detailed source that it is also referenced and adopted by many other countries in the world. It deals with the rebar details and the concrete itself, to which an RC building design must comply to avoid failures.

Corrosion of rebar is a very common concern in RC structures. Water leaks must be prevented, cracks must be repaired and there must be adequate concrete cover over rebar while pouring the concrete in the first place. Concrete cover also protects rebar from fire, which can render steel considerably weaker or even useless.

Concrete can also experience chemical failures, such as carbonation, alkali silica reaction. All of these can contribute to much greater problems within the structure.

Structural Steel:

The next most common construction material is structural steel. We said above that steel is strong for all types of forces, which are compression, tension and shear and it can also tolerate a lot of deformation before yielding. Structural steel systems are designed to effectively make use of these characteristics, but failures do occur. Note that we did not list bending moment here separately, because it produces these forces in members. Another type of moment is torsional moment, which can also cause failures, although this failure mode is less common in structural engineering.

Steel structures tend to be more flexible than concrete ones. Therefore they are sensitive to failures resulting from vibration, which can cause fatigue, resonance – which happens if the vibration period matches the natural period of the structure which amplifies the vibration forces, loosening of connections, user discomfort, damage to nonstructural elements and noise.

Steel is prone to fatigue as discussed above.

Thermal stresses are also critical consideration for steel structures. Expansion and contraction from temperature increases and decreases respectively, must be accounted for.

Steel members are connected by welds and or bolts, which can fail in many ways.

The connection points are the most vulnerable in steel structures generally. They can fail as a result of improper welding, bolting, member usage or arrangement.

Buckling failures, especially for columns, are sudden and catastrophic. Buckling occurs when a slender member is under compression. Buckling can occur in different ways. Slender members must be braced adequately. Even beams may undergo buckling, often in the form of lateral torsional buckling.

Rupture must be prevented. Under certain conditions, a steel member may be torn like paper.

As we mentioned above for the reinforcement of concrete, steel is sensitive to corrosion and fire. To protect from these, structural steel members must be adequately wrapped by fire and water resistant architectural details.

Above we listed some failure types for two of the most common construction materials, concrete and steel. The material failures also have structural failure consequences. Also see our post earthquake related structural failures in Construct Magazine.

Construction Failures:

Construction phase failures either affect the construction phase itself or their results can surface long after the structure starts its service life. Construction is a very complex phenomenon with lots of real life variables that cannot easily be controlled, as opposed to manufacturing something in a controlled factory environment or designing things on computer. To build what is shown in the designs and specifications in real life opens the door to failures at every small step of the way. Therefore, highly organized, standardized processes are needed to prevent or minimize failures. Because of this reason, construction and project management always go together.

Below we give several very common, well known examples out of thousands of different ways things can fail because of or during construction activities as far as structural work:

Concrete Mix:

A concrete mix can be properly designed but in the end the batch plant is going to prepare the mix, and the field crew will make the pour. So many things can go wrong: using improper / defective materials or incorrect proportions including water, concrete truck waiting too much before it dumps the concrete to the pump, inadequate mixing, not taking precautions for too hot or too cold weather concreting, segregation of aggregates and sand during pour, inadequate vibration while concrete is wet which leaves air bubbles inside, or too much vibration which causes segregation, sagging of formwork, inadequate curing of concrete after the pour, concrete not filling the forms properly, ordering inadequate amount of concrete to the site which hardens before the next batch arrives, which causes cold joints between the two pours and so on… Because of all these numerous and important steps, inspection and quality control play very critical role in construction. Pay attention to the number of ways we have just listed that can cause failure, just for a concrete mix and placement, which was not even a full list.

Reinforced Concrete:

Proper placement of rebar is of utmost importance for RC structures. Placing bars in a configuration other than what is shown on approved structural plans will often lead to failures later. This is why, before each pour, rebar placement is carefully checked and approved. During a very strong earthquake that cost more than 50,000 lives in Turkiye in 2023, it was seen that many buildings collapsed due to improper, inadequate placement of rebar. A very common occurrence was inadequate usage of column ties. A column tie is a horizontal bar that encircles the vertical column bars, to hold them together. It also encircles the core of the concrete column, laterally bracing it, which considerably increases the strength and ductility of the column. This is especially important during cyclic earthquake action. Proper placement of ties is especially critical at beam column connections, to make the connection stronger and more ductile, increasing its moment and energy absorption capacity. Near many buildings that collapsed, there stood buildings with little damage, in which nobody got hurt or very few people suffered minor injuries. This was because for those buildings, simply the code was followed (and not even the latest one but the previous one) and proper, honest construction practices were applied. Even only if the column ties were properly placed everywhere given everything else remained the same, hundreds and may be thousands of lives would have been saved (because each non collapsing building also means diverting the rescue efforts to other locations). Inadequate column ties led to beam column connections failing under cyclic lateral forces of earthquake which caused collapse of entire buildings.

Another bad practice is not to provide enough splice lengths to locations where two bars must join each other. Another one is not making the end of ties like a hook, which considerably increases its strength. Not tying reinforcing bars properly to each other can cause them to be misplaced during concrete pours, because of the weight of the crew and equipment and impact of wet concrete. These are just a few examples among so many.

Structural Steel:

During steel construction, many things may also go wrong or be performed poorly. Here are some examples:

Proper welding, bolting practices are of highest importance, which must be thoroughly inspected.

The erection sequence of members are critical for not only intended work schedule but often also due to structural reasons for frame to brace itself properly during erection, while it can be more vulnerable than its finished state. Temporary supports, braces must be kept in place until that part of the frame can hold itself together.

The members may be fabricated to wrong dimensions. This can happen especially in field fabrication, where the environment is less controlled than the shop.

Timely delivery of steel is a critical item in all projects that involves structural steel. So many things can go wrong here such as submittal approvals, supplier workload, dimensional coordination with the work of many other trades that will either be wrapped around or attached to steel members.

Members may get damaged such as warps, bends, dents during transport, loading or unloading.

Long term storage may lead to corrosion.

Proper alignment of members are critical, not only for dimensional accuracy, but also for avoiding unaccounted undue stresses and stress concentrations in members.

Temporary works:

Construction branch also involves design to a certain degree, such as the design of temporary structures, such as formwork, scaffolds, braces that will make the construction work possible. The forms must be strong enough to carry wet concrete, which is heavier than the hardened concrete. In addition, there will be construction loads, vibrations and impacts from the equipment, wet concrete and crew during pour. The forms not only must support wet concrete but also prevent sagging and any unwanted deformations. Forms must be clean and free of debris before the pour otherwise the quality of concrete will be affected.

By: Ahmet Tuter

Reference:“How to Construct: Introduction to Civil Engineering, Structures and Construction” – A. Tuter, August 2024

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