Structural Concepts – Static and dynamic load basics

  • Structural concepts revolve around what is known as the static equilibrium of a body
  • When acting and reacting forces on a structural body are equal this is what is known as the structure’s state of equilibrium
  • Stabilisation systems help to combat swaying bridges/buildings making use of tuned/spring mass dampers
  • Prestressed elements/materials help to create structures which have a higher shear resistance, less diagonal tension and resist displacement from external forces
  • People housed in buildings act like natural dampers and their impact is taken into consideration

Defining and understanding the basic structural concepts of engineering and how they are expressed in physics and mathematics is a key aspect for students. This enables them to enter the realms of structure design and development on the right foot. The purpose of this quick guide is to provide a generic yet helpful description of the various engineering structural concepts. Hopefully helping students out there get a grasp of their practical value and their meaning.

Structural Concepts

Static Load/Energy Structural Concepts

Static Equilibrium

When a body isn’t moving in any direction, while being at its original intended position, we say that it is in a state of equilibrium. Engineers calculate the sum of the acting and reacting forces on a structural body in order to figure out their total sums. If the result is equal this implies that the structure will remain in a state of equilibrium.

Centre of Mass/Gravity

For estimation and static analysis purposes, it is important for engineers to be able to determine the centre of mass/gravity (or centroid). This may be for an object or structural element, or connected sets of these. The centre of mass is the only point on a rigid body that can be used to suspend this body. The point it will stay in a state of equilibrium. If the vector from the centre of gravity to the centre of the Earth isn’t passing through a structure’s base, the structure cannot be in equilibrium as tipping forces are constantly acting on it.

Cross Sections and Second Moment of Area

The second moment of area is a geometrical property of a plane/area which indicates how its points are distributed in regards to an arbitrary axis. This is a very important element in all structurak concepts. For construction engineers, this geometrical property is important. It is used to help calculate the deflection of a beam, and thus the stress that is applied to it.

Beam Bending

As structural beams are subjected to loads, they tend to bend in a particular direction. This is an important property for engineers that helps to calculate the stresses and beam deflection, which are monumentally important structural concepts to understand. Allowing engineers to figure out if the bending and deflections are within limits, and balance the loads uniformally or equally to make the bending acceptable.

Shear and Torsion Forces

Shear forces are applied on two different points of a material and their vectors have opposite directions. This results in shearing stress and breakage, or cutting, or cracking of the material. For example, if a huge load is applied on the bottom of a floor which is held in place by a column, that is away from this load, then the floor will be subject to shear stress. Torsion forces on the other side are those that tend to twist a structural element. These are usually the result of improper static load analysis and bad predictions.

Stress Distribution

The self-explained stress distribution is a key part of analysing the stress-strain of a given system. Calculating the internal stresses throughout the connected elements of the particular system based on the applied external forces. The goal is to make adjustments that help increase the symmetric level and homogeneity of this load distribution. Whether this has to do with individual sets of elements, or even the distribution of the loads of a multi-storey building on the ground soil.

Span and Deflection Loads

Imposing load can result in the displacement of a structural element this is called a deflection load. The displacement may be measured in absolute distance or even angle, and it is usually used for beams. The beams are supported by points. The deflection is proportional to the distance of these points, which is called the span. This determines the distance which is vital when looking to modifying the design.

Stiffness and Internal Load Transmission

The stiffness of a structure is a physical measure of how well it can resist deformation forces. This should not be confused with elasticity. Stiffness is a property of how well or how effective internal forces are transmitted between the various structural elements of a building. From tension and shearing forces, to compression and torsional loads, all are considered internal forces. We want these to be distributed between the various elements and end up being carried by as many elements as possible. This means that no single beams have to withstand the forces. Engineers usually utilize bracing members to create force transmission paths and increase the stiffness of buildings.

Column Buckling

Column buckling is the structural failure of a column – an extremely important structural concept. This happens when subjected to an overwhelming amount of axial compressive stress. Resulting in its deformation and lateral deflection. This makes columns unable to carry loads, so they are usually completely failing. The problem with buckling is that it can occur with loads that were calculated to be within the compression force limit. Unfortunately, unforeseen bending of the column changes this limit altogether.

Prestressed Elements or Materials

Pre-stressed elements or even materials such as pre-stressed concrete are often used by engineers. They create structures that can better resist displacement from external forces. As a consequence they can resist cracking due to impact or shocks and demonstrate better longevity. Creating much higher compressive and tensile strength, as well as a much better resistance to vibrations, leading to a safer structure. Pre-stressed elements are generally:

  • Lighter
  • Have higher shear resistance
  • Generate less diagonal tension in concrete sections
  • Can constitute more compact structural members

It is not difficult to see how these characteristics can have an extremely positive impact of the stability and longevity of any structure.

Vertical Loads and Horizontal Movement

Vertical loads leading to horizontal movements are very important and must be considered for their symmetry and magnitude. Many engineers in the past have underestimated the importance of vertical loads in traditional structures. In some cases this can lead to the collapse of outer edges and total failure of supports. Load distribution and structural geometry are key elements to consider in relation to the vertical loads analysis, and are extremely important structural concepts.

Structural Concepts

Dynamic Load/Energy Concepts

Conservation of Energy and Momentum

For the structural engineer, it is important to keep in mind the fundamentals of the energy and momentum conservation. This often helps to predict the behaviour of a building under wind loads and earthquakes.

Pendulum Systems

In the same context, evaluating the pendulum characteristics of a structure is a very important structural concept. This helps to devise stabilization systems that will bring the structure back to equilibrium. While allowing it enough elasticity to accept and withstand external loads. For example, skyscrapers or bridges can start swinging back and forth after a strong wind hits them.

Free Vibration

Free vibration occurs when a building is taken out of its stable equilibrium. This may be due to an occurrence such an earthquake or similar natural event. As a consequence, the building vibrates or enters a pendulum situation without any additional external interference. Expressed in cycles of vibration, with the number of cycles indicating the natural frequency of the building, free vibration is important . It is possible to mitigate and bring free vibration to a complete stop using dampening elements.

structural concepts = skyscraper harmonic damper

Harmonic damper of Taipei 101 skyscraper, as seen from 89th observatory floor.

Resonance

When the vibration of a building meets its natural resonance this leads to the very dangerous situation of vibration at the maximum magnitude. This is important for engineers to calculate, determine, and consider. As a result they can then make design interventions, avoiding resonance in a particular frequency range that is close to the natural frequency of the structure. Or they can increase the damping which allows the building to withstand such a scenario.

Damping in Structures and Vibration Absorbers

Damping elements in structures help reduce the displacement caused during the free vibration. Normally this will only last for short periods of time (minutes). After determining the free vibration, resonance, etc, an engineer will decide on the dampening ratio required for the creation of a safe structure. Placed on the top of high structures, tuned/spring mass dampers (also known as harmonic absorbents) are very important. This is where the magnitude of the free vibrations is the greatest and the dampers are most useful.

Considering Human Bodies

People in buildings are act as natural spring mass dampers, which is an interesting structural concept to be aware of. Those moving dynamically inside a structure are inducing dynamic loads and structural vibration. This may be hard to estimate or calculate with precision but it is wrong to treat people as static loads. Therefore engineers are likely to take the presence of people into consideration as additional dampers.

About: Bill Toulas

Passionate engineer and new technologies advocate, writing about the ways they shape our world and amplify our very existence. Believes that engineering is the art of changing this world forever, every day, little by little, and sometimes all at once.

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