In the 19th and 20th centuries, many important steel structures were riveted, such as the Eiffel Tower and automobile skeletons. In the late 20th century, a considerable number of structures were welded together, including automobile skeletons.
However, some structures needed to be lighter and more resistant, such as aircraft fuselages and steel bridges, and these use the screw connection technique.
What is the difference between riveted and bolted connections, and why do bridges use bolted connections instead of welded connections?
Let's delve deeper into this.
From the perspective of supporting force, riveting has better shear bearing capacity but lower tensile bearing capacity. Bolted connections can withstand tension and shear resistance. Welding can also withstand tension and shear strength, but is susceptible to tearing.
From a detachability perspective, bolted connections are detachable joints, while riveting and welding are not.
From a quality assurance perspective, bolted connections are better than riveting, which is better than welding. Welding is the least easy to check quality, so civil aviation aircraft use less welding.
In terms of changing the material properties of parts, welding has the greatest impact, and the residual stress and deformation are serious. This is tolerable for bridges and cars, but is not ideal for aerodynamic shapes such as aircraft surfaces.
From a cost perspective, bolted connections are more expensive than riveted and welded connections.
From the perspective of the extra weight added, bolted connections are more expensive than riveted and welded connections.
For connections of different materials (such as aluminum and titanium, composite and titanium, different series of aluminum alloys), welding is not a good choice (different materials are commonly joined in planes, so the range of applications is seriously affected by welding) .
From the comparison above, you can see why cars and bridges are more likely to use welded and bolted connections, while airplanes are more likely to use riveted and bolted connections.
PS1: Weld defects are relatively difficult to control, resulting in inconsistent fatigue properties.
PS2: Hot riveting, especially with larger titanium rivets, is also commonly used in airplanes.
Further Explanation: Each connection technique is advancing and has given rise to different types.
For example, riveting can be divided into one-sided riveting and self-piercing riveting.
Self-piercing riveting is currently most commonly used in the automotive industry and the equipment is expensive.
One-sided riveting is mainly used in situations where the structure is not open and is a low-cost application in the packaging industry.
There are more advanced riveting techniques on planes that are difficult to locate.
New technologies such as welding, laser welding and friction stir welding are also being used in the aerospace industry.
Laser welding has a small heat-affected zone and small deformation.
The stirring mechanism of friction welding has not been studied clearly and differs from other welding methods.
Therefore, the previous view of welding used in automobiles and riveting in airplanes is no longer accurate.
Regarding bridges, I don't know much, but intuitively speaking, most of these large structures consider cost factors and are not sensitive to weight, and often do not use the latest technological methods.
If we consider only the mechanical properties of the connection method itself, without taking into account the complexity of fabrication and on-site construction, bolted connections and welding are more likely to be used in civil engineering due to their reliability and economy.
Screws have the best reliability, especially high strength screw friction type connections. In engineering, it is always preferable to use more controllable technology with less dispersion, which leads to lower costs and greater reliability. Therefore, friction-type high-strength screw connections are currently an ideal method for such engineering requirements.
Rebi tes
In civil engineering, steel structures are generally thicker and larger, and the rivets used are not the same as cold rivets. They need to be heated first, and then the straight end is secured with a riveter.
Heating > Installation > Forging and forming
In civil engineering, steel structures are generally thicker and larger, and the rivets used are not the same as cold rivets.
They need to be heated first, and then the straight end is secured with a riveter.
In this process, the rivet actually went through two processes: heat treatment and forging.
In modern industrial conditions, most heat treatment and forging is done in metallurgical and mechanical processing plants.
The initial temperature of heat treatment and the speed of heating and cooling need to be controlled. The forging is also finished with high-precision dies.
In the field processing environment, it is clear that none of them can be controlled well at low cost.
For most on-site civil engineering installations, after rivet formation, the temperature is reduced in the external natural environment. This process is actually “annealing”.
Annealing reduces the strength of steel and increases ductility.
However, it is different from factory heat treatment, in which the steel is heated in an electric furnace with a thermometer and the temperature is controlled by an automatic control device.
Engineers dislike the uncontrolled annealing that occurs naturally in the field.
The rivets are naturally annealed into the component and it is impossible to know exactly how much the resistance will be reduced and how much residual stress will remain.
Even if it suffers some accidents, such as rain or strong wind, it may even cause “tempering”, which is the opposite of “annealing”.
The final performance of this type of hot rivet is highly variable.
To ensure the overall reliability of the design, the rivet strength will not be fully utilized, which will increase the number of rivets required, resulting in waste and complicated design.
Furthermore, the construction is complicated and each rivet needs to be heated before use. Now these hot rivets are not widely used in the field of civil engineering.
Welding
Welding is a more versatile way.
In the past, it was thought that welding was a very simple thing, but after taking metallic structure courses and reading a little information, the truth was not. The welding process involves many things, which makes it a very extensive and complicated subject. The entire process is a large collection of diverse physical and chemical reactions.
Welding can be a specialized profession and is not just welding at a higher vocational or technical school, but also a specialization at a research university. Many software such as MARC and NASTRAN have developed welding modules to simulate the welding process. Even in the field of civil engineering, there are still a large number of scholars studying the influence of welding on the structure, which shows the complexity of welding. Welding will melt the materials in the connection area, and materials near the connection area will also withstand high temperatures.
Therefore, in civil construction conditions, as the temperature is higher than that of the rivets, the residual stress caused by cooling and recrystallization can even reach the yield limit of the material. The effect of heat treatment on the surrounding area is uncontrollable, the strength and toughness of the surrounding materials will change. At the same time, the welding process is not only a physical change, but also reacts with the surrounding gas and welding flux to produce some waste.
On-site welding is generally manual welding and it is inevitable that errors will occur, resulting in defects such as undercuts, false welds and welds. These types of failures can have a significant impact on structural stiffness and fatigue performance.
Not all materials can be welded easily, especially in the on-site construction environment. For example, aluminum has high requirements in the factory and the scenario is even more challenging. Although steel is the most commonly used material in civil engineering, its weldability varies widely. High-strength steels and alloys generally have poor weldability due to different materials, the high melting point of oxides in aluminum, very rapid cooling and reactions during welding.
In the factory environment, materials such as steel can be welded using argon arc welding and other welding techniques that require more equipment. However, the construction site is limited by the simple environment, and arc welding is generally used, making welding such materials unrealistic.
Although the raw materials are high-strength steel or common steel, they are obtained through various cold and hot processing in the factory environment. As a result, it is almost impossible to spot weld welds with the same properties.
Rivet
Compared to riveting and welding, bolting is much more controlled in the field construction environment, without a heating process that could cause uncontrolled heat treatment.
Both components and screws are produced in a factory environment, so the product consistency is quite good and can be screwed on site. Pressure-bearing joints are similar to rivets, but the strength and consistency of the screw are better than that of a rivet.
Friction joints are not created equal and there is one problem involved in field construction – friction control. Friction is influenced by contact surface pressure and surface roughness, but shear and torsion screws, torque wrenches and surface preparation techniques can now solve this problem.
The shear and twist screw has a rounded, rivet-like head on one end without angles, and the screw is screwed through a spline (or plum head) on the other end. There is a thin neck between the spline and the screw, and the spline is twisted when the torque generated by the friction between the component and the screw reaches the torsion limit of the neck.
The torque wrench can be used to tighten large hexagonal screws to achieve the same effect as shear and twist screws. It won't be too hard to cheer because someone has a cold today, nor will it be too exciting because someone is going next door tonight (if the bolts are too tight, they'll break).
Surface treatment can be done at the factory by sandblasting or applying anti-rust paint after sandblasting.
Using friction-type connection, the transfer of force between members is through friction, so the performance of the connection is basically the same as that of the member itself. It is more in line with the design concept, and the strength, stiffness and fatigue performance are guaranteed.
Diagram of how to use the wrench
Do you think I want to twist the screw connections? NO!
Anyone who has installed steel structures on site knows how frustrating bolted connection installations can be.
Screw holes may not match the screw for various reasons (manufacturing error, welding deformation and force deformation…), and it is not uncommon for screws to deviate by one millimeter.
The Reinhardt sledgehammer is used to drive the shear pin into the hole and make the hole match.
However, the two components did not fit together and the screws could not be tightened.
Drilling and patching in place can weaken the component too much.
Sometimes repair welding or steel sleeve repair is required, which is very troublesome.
In many cases, connection plates cannot be connected directly between components and do not forget to cut the slot that was unscrewed from the screws.
All of this results in a significant increase in material usage.
The screws are more expensive than common Q345\Q235 by weight, so the price of screw connections is very high.
There is no such problem in welding and it can be welded directly without much difference in position.
Another advantage of welding is that it is quick.
At the same time, the welding torch can be used not only for connecting but also for cutting, allowing quick correction of construction errors.
Most of the time, welding can be done directly between members, without the need for additional plates and with less material.
The strength of steel that can be welded by arc welding is generally not very high, and the strength of the weld may be higher than that of the base material.
Furthermore, the welding area completely covers the entire component, leaving sufficient safety and reliability margins.
Therefore, welding is a common method during on-site installation because it is convenient.
However, several methods cannot be applied when it is not possible to obtain the required stable quality in a particular environment at a sufficiently low cost.
The reason for the unstable quality of on-site welding is that the external environment is not controllable and human operation cannot be trusted.
Therefore, if welding is transferred to a factory environment, the results will be very different.
Automatic welding machines, closed workshops and processes such as straightening, grinding and heat treatment after welding help adjust welding deformation and reduce residual stress.
Furthermore, large-scale flaw detection equipment can help detect the quality of welds and repair them.
Therefore, the best way to produce some non-standard components in a factory environment is still welding.
Riveting can also be improved by using better technologies such as the heating process.
In the past, coal furnaces were used, but now electric furnaces have emerged that use the principle of eddy current heating to quickly heat the rivet.
As the rivet is heated and then cooled, it will exert considerable pressure on the plate and may compress it.
Furthermore, as rivets themselves are relatively good in terms of toughness, they are sometimes used in structures subject to dynamic loads. It is still necessary to continue to use rivets to repair some old steel bridges.
For example, the image below was selected from the Guangzhou Daily news about the repair of the Haizhu Bridge.
Due to the bulky nature of civil engineering, the use of hot rivets is decreasing in the field. However, cold rivets (such as pull rivets) are still useful when connecting light layers and thin plates, as they are lighter and use less material than screws.
In addition, thin plates are not easy to weld, and cold rivets are suitable for connecting different types of materials, especially for connections that require less strength. In civil engineering, they are typically used to connect thin-walled steel and profiled steel plates, which are very thin components. A common example is temporary fencing on construction sites.
So, which connection form should be considered:
- the requirements of mechanical properties;
- construction conditions;
- the money allowed.
Each connection type has its own scope of application.
There are still many uses for rivets in fields like aerospace, but that's not my field, and it's time for others in the aerospace industry to come forward more.
I will summarize from a shipbuilding point of view.
Riveting was commonly used in ship construction before World War II, but is now obsolete. Despite their obsolescence in the shipbuilding industry, riveted joints are still used in modern aircraft construction, but they are very different from the riveted joints used on pre-World War II ships.
Due to the need to reduce empty weight, the majority of materials used in aircraft are aluminum and composites. Aluminum is not easy to weld, while composites cannot be welded. Considering that the aircraft must also be waterproof, riveting is the best option for aircraft construction.
Bolted connections are mainly used in interior construction (buildings, bridges, cranes and deck-mounted installations of ships/ocean platforms). Bolted structures are easy to remove, but they are not watertight, and the bolts themselves are prone to rust (water can accumulate in the bolt grooves).
Welding is essentially the only method of joining components currently used in the marine industry (completely replacing riveting) and is also used in land-based construction. Compared with bolted connections, welding has the advantage of impermeability. Compared with riveting, welding has the advantage of fast speed, and the quality of contemporary welding technology is more reliable. The disadvantage is that it is not easy to disassemble and the components must be exploded or cut during disassembly, which hinders the reuse of raw materials.
Why was riveting eliminated by the shipping industry?
In addition to the slow speed of construction, the riveted structure of the hull before World War II can be compared to a soda cookie, and the new Titanic built with modern welded technology can be compared to a piece of plasticine.
The key to any great metal structure is actually the connection point of the components!
This is true whether it is a ship, plane, vehicle or rocket.
Two connected boards cannot be stronger than a single composite board.
Regarding welding: You should not rely completely on the results of welding in the factory.
Even if the welding material is stronger than the original material, the original material at the boundary line will still be weakened after welding. To remember!
Example
Let me take a real project as an example (I like to learn through practical examples), using a steel structure factory building with a portal steel structure as an example.
Due to the high cost and difficulty of using rivets in common steel-framed buildings, we will discuss welding and bolting.
Our requirements for a crane plant are as follows:
- The plant must meet reliability requirements, including safety, comfort and durability.
- The crane is a motorized equipment and needs to meet the dynamic load demands of the crane.
- We need to consider whether the workshop needs to be demolished after the factory moves and how much space can be repurposed for materials.
Let's start with the connection between the steel door pillar and the foundation.
The pillar base is divided into rigid connections and hinge joints.
For electrical equipment, we tend to make a rigid connection because the dynamic load of the crane, especially the horizontal braking load, can easily cause overall instability.
We can weld or screw the base of the column, but welding is not very easy to do.
Because the column base is connected to the foundation under the column, if it is fully welded, it is easy to cause problems such as insufficient welding seam, and it is difficult to ensure the stability of the column during the welding process.
Are you afraid of looking at the welding stick in your hand while pouting and looking at the hanging pillar?
Second, let's talk about beam-to-column connections. Does not matter.
As long as they are rigidly connected, both bolts and welding are acceptable, but welds are prone to rust, so steel frame manufacturers need to be reliable.
If the screw holes are even slightly off, you could be crying in a minute.
Again, the wind-resistant pillar must be hinged.
So don't hesitate, grab the bolt and take a stab.
Lastly, if one day the factory fails (boss don't hit me), how easy will the screw connection be? Two people can ruin three factories in one day and sell them at night.
In short, if the connection needs to be hinged, use screws.
If it is a rigid connection, the choice between welded and bolted connections depends on the situation, the difficulty and the skill level of the welders.
Bolted connections are easier and require less skill, but present higher requirements for the manufacturer in terms of precision and quality control.
Welded connections can save money, but present higher requirements for material strength and welder skill.
After the work is completed, the inspection unit will be asked to perform an inspection, but the manufacturer will likely be unhappy if there are errors and may even offer a discount.
However, if you are confident in your bolted connections and think that the government might demolish your factory land, you can use bolted connection to claim compensation for the factory and then find new land to build on.
Opinion A
I've just finished evaluating the basic research on process equipment and will share my thoughts.
The biggest problem with both riveting and bolting is that they conflict with the larger goal of weight reduction.
The screws also present problems with slipping and loosening of the threads.
Both riveting and screwing have the advantage of simple principles and proven technology.
There are many types of welding, including friction welding, stir welding and laser arc welding.
The advantage of welding is that weight reduction can be achieved.
The disadvantage is that some metal materials have poor weldability and are prone to cracking and deformation.
Welding the same material and different materials requires different welding techniques.
In addition to the process, manufacturing automated welding equipment is also difficult.
Opinion B
Welding
Advantages: Suitable for different formats, saves steel, can be automated and has high production efficiency.
Disadvantage: Quality is strongly influenced by welding consumables and handling.
Riveting
Advantages: Reliable force transmission connection, toughness, good plasticity, easy to check quality, commonly used in structures subject to dynamic loading.
Disadvantages: Scrap steel and extra work involved.
Screws are divided into common screws and high-strength screws.
Common screws are easy to handle and should not shear.
High-strength screws contain the advantages of ordinary screws and riveted joints and can now be used instead of riveted joints.
Opinion C
Defective riveted parts can be easily observed, making them useful in aerospace, shipbuilding, bridges and other areas.
Welding has high efficiency and resistance, making it widely used in the manufacture of machines, equipment and automobiles.
Bolted connections are convenient for disassembly, and are mainly used in the assembly process. For example, welding on airplane windows is problematic for maintenance, and welding during flight is risky.
Riveting can make defects found during maintenance easy to repair. For example, riveting metallic structural parts of a car chassis can compromise production efficiency and strength.
In the National Stadium (Bird's Nest) project, welding of all parts creates concentrated stresses in certain areas and makes construction inconvenient due to the large amount of welding in high-altitude locations. The most reliable solution is to make “branches” at the factory and assemble them on site.
A riveting professor repaired a riveted bridge built by the Japanese by heating the rivets with gas welding and then breaking them with a sledgehammer when they were cold.
Repairing a cracked weld inside a bridge is difficult as the entire bridge cannot be demolished and re-welded.
Opinion D
The answer provided addresses only two points, although the advantages of riveting are not clear.
Bolting provides good toughness and ductility in both the connection plate and screws.
It maintains its load capacity even under large deformations and has strong seismic resistance.
It is easy to observe when the node load is exceeded.
The toughness of the solder is not that good, especially at low temperatures, which makes it prone to brittle damage.
During construction, bolting the construction may not be as convenient as welding, but the construction quality is easy to control.
Welding, especially field welding, can produce defects such as cracks, blisters, slag, welding leakage, unmelted and penetration welding.
It has high requirements for welding personnel and it is difficult to observe the construction quality.
The technical content of common ultrasonic non-destructive testing is superior to high-strength screw torque testing and has higher equipment and labor costs.
Opinion E
Based on my own intuition, let me discuss a few concepts.
The riveting process is the simplest. The rivet itself is a casting and riveting can be achieved just by drilling holes in the object. The disadvantage is that the rivet can be deformed and collapsed if it receives a sufficiently large longitudinal impact.
The screwing process is more complicated because not all screws are self-working. The screw thread itself needs to be made on a lathe, and the hole in the joint also needs a machine tool to engrave the negative pattern. Compared to riveting, screws are stronger when facing direct impact. However, screws may loosen over time if the joint is subjected to prolonged reciprocating vibration and the screw's threaded surface is particularly prone to rust.
The welding process is the most complex. In terms of advantages, welding not only splices but also fuses materials. Therefore, welding must be better than the previous two types in terms of strength, water tightness, air tightness and electrical conductivity.
Opinion F
Disadvantages of welding:
Deformation can be severe, as the high welding temperature leads to stress concentration in the weld. Therefore, many high-precision and high-requirement surface connections use threads and adhesive bonding.
It's difficult to inspect. After welding, special testing equipment is required to detect any defects, which can increase costs.
Location restrictions can also be an issue.
It's difficult to dismantle. If a welding error is made, the joint must be cut and welded again.
Advantages of welding:
It has good connection performance and is practical for welding large equipment with various shapes and sizes of materials.
It has good rigidity, overall performance and sealing.
Opinion G
Screws can be easily removed, but they take up space and add weight, so they are used sparingly if they do not need to be removed. Welding requires equipment and man hours, and not all materials can be welded. Riveting is becoming less common.
Opinion H
Stationary objects such as bridges, towers, and construction bolts are commonly riveted or welded, while moving objects or parts are welded or riveted due to the fact that movement can loosen the bolts. It is not feasible to check all screws before each flight.
Welding is the most stable option because both riveted and bolted joints connect two parts by friction, which is inadequate if there is a force in the translational direction between the two parts. Welding fuses two components into a single unit.
If there is a force in the direction of translation between two pieces, it is possible to consider fixing them with screws if they are supported by each other. For example, if one part has grooves and the other has convex rails, they can be fixed with screws if they match. The principle is to convert the translational friction force into pressure against the assembly, greatly improving the rolling force and stability.
Opinion I
Riveted joints are a broad category, and screws are one of the most common types. High-strength bolting or riveting can provide greater strength than welding, but the disadvantage is higher cost. Airplanes use riveted links which are stronger and lighter than normal screw links such as good screw and huck screw, etc. Because planes require high safety standards, they cannot be welded and must use rivets. High-speed trains don't require such high standards, so they typically use welding. Cars have even lower standards and may use welding, although some car manufacturers like Land Rover have made cars with full aluminum bodies and riveted like airplanes, but they are extremely expensive.
Bridges are not my specialty, but for some special requirements, such as the need to keep the bridge light when using high strength steel, welding may not be possible due to the poor welding performance of high strength steel. In addition, there may be corrosion resistance requirements near the shore, and because corrosion-resistant steel is also difficult to weld, riveting is often used to ensure safety.
