As an engineering student or engineer, you know how important it is for structures to be able to handle different kinds of loads.
But have you ever thought about the unique problems that axial load presents? In contrast to shear force, torsional load, and bending load, axial load applies force directly along the axis of a structure.
This means that when designing, building, and maintaining structures, a whole new set of things need to be taken into account.
In this article, I will go into detail about axial load and cover everything you need to know to build structures that can handle this important force.
So buckle up, and let us get started!.
Introduction to Axial Load
Formal definition:
A force with its resultant passing through the centroid of a particular section and being perpendicular to the plane of the section.
Axial load is a type of load that puts pressure on a structural member along its axis.
Unlike shear force, torsional load, and bending load, axial load creates more compressive stress than tension or shear force.
Shear Force, Torsional Load, and Bending Load: Differences
Shear force causes stress to be spread out along the plane of an object, while torsional load causes stress to be spread out around the longitudinal axis of the object.
When a load is bent, it creates normal stress and transverse shear stress.
Normal stress includes both axial and transverse stress, while transverse shear stress includes both torsional and transverse shear stress.
Importance of Axial Load
Axial loads are important because they can change both the structure of the implant and the bone around it.
In engineering, axial load is a very important part of how columns, beams, and trusses are made.
In biomechanics, axial loads can change how bones move, which can cause fractures or joint replacements, among other injuries.
Because of this, it is important in both engineering and medicine to understand how axial load works.
Difference between Axial Loading and Transverse Loading
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Structural Members and Axial Loads
Trusses and columns are two common types of structural members that are mostly made to carry axial loads.
Trusses: Characteristics and Applications
Trusses are structural members that are made to carry axial forces in their members.
These forces can be tension, compression, or reversible tension/compression, depending on the worst case loads and load combinations.
Truss members use less material to support the same amount of weight.
This makes them great for bridges or roofs that need to be strong but not too heavy.
But truss members are free to move and can only carry loads in one direction.
This means that they are not strong enough to hold up against loads that move side to side or bend.
Columns: Characteristics and Applications
Columns are the vertical parts of steel building frames that hold up floor girders or floors that are subjected to heavy axial compressive loads.
They are mostly made to resist compressive axial loads, but depending on how they are set up and how they are made, they can also resist bending and shear forces.
Columns can be round, square, or rectangular, and they can be made of concrete, steel, or wood, among other things.
Frame Members: Characteristics and Applications
Depending on how they are set up and made, beams and columns can carry both transverse and axial loads along their length.
They are often used to hold up floors, roofs, and walls in building and construction projects.
But, unlike truss members, frame members do not have to support only axial loads; they can also support transverse loads.
Determining Maximum Axial Load
When building a structure, it is important to know how much axial load a certain member or structure can handle.
Calculating Maximum Axial Load for Columns
To find out how much axial load a column can handle, you can figure out its KL/r and then look up the value of cFcr in a table.
The stress in the cross-section of the column can be found by using the equation AP=f, where f is assumed to be the same all over the cross-section.
Buckling is known to be a failure limit state for columns, and Equation gives the critical buckling load Pcr for columns (3.1).
But to fully figure out the critical buckling load for a given column, you need more equations and methods, and the design must take into account how the column ends and the properties of the material.
Approximating Maximum Load Carrying Capacity
Doing a steel design and looking at the interaction ratio is a good way to get a rough idea of the most weight a member can carry.
The interaction ratio is the ratio between the most weight a member can carry and how much weight it is actually carrying.
The reciprocal of that ratio tells you how much more weight each member can carry before it breaks.
It is important to remember that this method only gives an estimate.
The actual maximum load that a member can carry may be lower or higher than the calculated value.
Designing for Maximum Axial Load
When designing structures, members are sized roughly based on architectural drawings and other relevant documents, and their weights are figured out using information from most codes and other civil engineering literature.
But structures must be built to handle the critical load, which is the largest load that could act on them.
This is done by adding up all the loads that a structure could carry over its lifetime.
This includes both live loads and dead loads, as well as loads caused by wind, earthquakes, and other possible loads.
Critical Load in Long Slender Columns
The critical load is the most axial weight that a column can hold before it starts to bend.
Euler's Formula: Calculating Critical Load
Euler's formula can be used to find the critical load: Pcr = (2EI)/(KL)2, where Pcr is Euler's critical load, E is Young's modulus of elasticity, I is the minimum second moment of area of the cross-section of the column (area moment of inertia), K is the column effective length factor, and L is the unsupported length of the column.
Significance of Critical Load
The critical load is important for figuring out how long, thin columns react to axial compressive force because it does not depend on how strong the material is.
This means that when building thin structures that might bend, engineers need to pay extra attention to the slenderness ratio, which is the length of the column divided by its smallest radius of gyration.
A high slenderness ratio means that small compression loads are more likely to cause the structure to break.
Buckling happens when a straight column that is being compressed along its length suddenly bends. This is a failure limit state for columns.
Axial Load Cells and their Applications
Load cells that measure force along a single axis are called axial load cells.
Working Principle of Axial Load Cells
Axial load cells work by turning the force that is applied to them into an electrical signal that can be read and written down.
They use strain gauges to measure how much axial loading changes the shape of something.
When a force is put on the load cell along its axis, the strain gauges bend, which changes their resistance.
The change in resistance is then turned into an electrical signal that can be measured.
Applications of Axial Load Cells
Axially mounted load cells are used in many fields, like aerospace, automotive, and manufacturing.
Some common ways that axial load cells are used are:
- Measuring the force on structural parts of buildings and bridges, like beams and columns, while they are being built or used.
- Testing uses, like figuring out how much force is needed to squeeze or stretch something, or how much force is needed to break or deform something.
- Keeping an eye on things like hydraulic presses, cranes, and lifts to make sure they are working safely.
- I'll list more in the bottom of this article.
Other Aspects of Axial Load
Axial Wind Load
Axial wind load is the force that wind flow has on a building.
In the past, wind forces, especially in coastal areas, have caused many buildings to fall down.
Civil engineers use ASCE 7-16 modified equation 2.2, which takes into account the structure's height above ground level and how important it is to people's lives and property, to figure out the wind speed and pressure at different heights above ground level.
Civil engineers use a formula that takes into account things like the projected area, wind pressure, drag coefficient, exposure coefficient, gust response factor, and importance factor to figure out axial wind load.
One formula is F = A x P x Cd, where F is the force or wind load, A is the projected area of the object, P is the wind pressure, and Cd is the coefficient of drag.
Fatigue Strength
Calculating a structure's fatigue strength under axial and bending loads can be done using analytical methods based on the ratio of the fatigue strengths for axial and bending loading.
In these methods, the fatigue strength under rotary bending load is changed into the fatigue strength under axial load.
To find out how an analytical model works, high-cycle fatigue tests can also be done under both loading conditions.
Also, plane stress models can be used to figure out how long a material will last when used on its surface, where one of the main stresses is usually zero.
Lastly, S-N curves can be used to find the maximum allowable stress at N cycles and a fatigue strength reduction factor kf.
Ball Bearings and Maximum Axial Load
Radial ball bearings with a retainer (or cage) are mostly made to handle radial loads, but they can also handle axial loads.
The amount of axial load that can be put on a bearing depends on its size and is usually given as a percentage of the bearing's radial load rating.
When the difference between the diameter of the bore and the diameter of the outer ring is big, the bearing can take axial loads that are up to 50% of the radial static load.
The raceways in thin-section bearings are shallower, which makes them less able to handle axial loads.
A angular contact bearing should be used if the bearing needs to handle a heavy axial load.
These are made differently on the inside than deep groove ball bearings, so they can handle higher axial loads.
The maximum axial load for ball bearings with a certain inside diameter depends on several things, such as the size of the bearing, the depth of the bearing raceway, and whether or not it is subjected to heavy radial or moment loads.
The amount of axial load that can be put on a bearing is often given as an approximation of the bearing's radial load rating.
SKF provides minimum axial and radial loads for single bearings and bearing pairs set up in tandem or back-to-back/face-to-face configurations.
The most stress that can be put on ball bearings depends on how they are made on the inside.
Axial Loading in Structures
When a force is put on a structure directly along an axis of the structure, this is called axial loading.
When there is a point load, the stress near the point of loading is much higher than the average stress.
This causes very complicated deformations because the stress states are very complicated.
Normal stress and shear stress are both ways to measure the average stress over a cross section.
No matter where along the cross section you look, the amount of stress is the same.
A point load is a force from the outside that is concentrated in a small area.
Use cases
Here are some ways that axial load could be used:
| Used in: | Description: |
|---|---|
| Design of a Column | Columns are a great example of a structural member that is made to support axial loads. For example, in buildings, columns support the weight of the floors and roof above, which creates a compressive axial load that the column must resist. Axial load is an important thing to think about when designing columns to make sure they will not bend or break under the force. |
| Bridges | When designing bridges, axial load is also a very important thing to think about. Compressive axial loads are caused by the weight of the bridge and the vehicles it carries. The bridge must be able to withstand these loads. Bridges can also be affected by things like wind, earthquakes, and traffic, all of which can cause bending moments and shear forces. To make safe and useful bridges, it is important to know how these loads interact with each other. |
| Tower Design | High compressive axial loads are put on towers like transmission towers, cell towers, and wind turbines. When making these structures, you have to think about the way they will be loaded, the materials, and the height and width of the towers, among other things. To make sure these structures are safe and last a long time, you need to know how axial loads affect them. |
| Manufacturing and testing | Axial load is also an important concept in manufacturing and testing, where materials and products must be able to withstand certain loads without breaking or deforming. With testing tools like axial load cells, you can find out how much axial load a material or product can take before it breaks. |
| Aerospace Applications | Axial loads can be very important when designing rockets, missiles, and other vehicles for use in space. Axial loads can be caused by the vehicle's weight, its speed, or vibrations. To make safe and effective systems, it is important to know how axial loads affect the structural parts of a vehicle. |
Conclusion
As we have seen in this article, axial load is a key part of designing and building structures that can stand up to the forces that are put on them.
When working with this important force, there are a lot of things to think about, from understanding the unique challenges of long, thin columns to adding axial load cells to your engineering toolkit.
But axial load is also a reminder that engineering is a complicated field that is always changing.
As we continue to push the limits of what is possible, we will inevitably face new problems that will require us to think creatively and work together to solve.
So, the next time you work with axial load, keep an open mind and be ready to learn.
Who knows what new things are around the next corner?
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