Riveting stands as one of the most significant mechanical joining methods in the history of civil and structural engineering. During the late 19th and early 20th centuries, riveted connections were the standard for assembling steel frames in bridges, skyscrapers, towers, and industrial buildings. The technique provided a level of reliability and durability that allowed engineers to erect some of the most iconic structures still standing today. While modern welding and high-strength bolting have largely replaced riveting in new construction, understanding how riveting contributes to structural integrity is essential for engineers, preservationists, and historians working with historic steel frames.

The role of riveting extends far beyond simply holding pieces of steel together. When properly executed, a riveted joint distributes loads, accommodates thermal movements, and resists fatigue in ways that are distinct from welded or bolted connections. This article explores the metallurgical and mechanical principles behind riveted joints, examines their behavior under load, and highlights key historic structures that demonstrate their enduring strength.

The Riveting Process: Tools and Techniques

Riveting in structural steel construction was a labor-intensive, highly skilled operation. The process typically involved three distinct stages: heating, insertion, and forming the head. Each stage was critical to the final quality of the joint.

Heating and Handling

Steel rivets were heated in a portable forge or furnace to a temperature between 900°C and 1,000°C (1650°F–1830°F), at which the metal became bright orange-red and highly plastic. The heat served two purposes: it softened the rivet so it could be deformed without cracking, and it allowed the rivet to contract as it cooled, clamping the joined plates together with tremendous force. The hot rivet was quickly transferred to the hole using long-handled tongs—a task performed by a “heater boy” or “rivet passer” in a coordinated team.

Insertion and Bucking

Once the hot rivet was placed into the aligned holes of the steel members, a “holder-on” used a heavy sledgehammer or pneumatic tool (in later eras) to back up the rivet’s head, while a “riveter” on the opposite side formed the second head by hammering the protruding shank. The pressure created a tight, interference fit that, upon cooling and contraction, generated a powerful clamping force across the joint. This clamping action is what gave riveted connections their remarkable resistance to shear and slip.

Cooling and Shrinkage

As the rivet cooled from red-hot to ambient temperature, it shrank in length. Because the steel plates prevented the rivet from shrinking freely, the rivet came under tensile stress, effectively becoming a high-strength tensioned fastener. This preload clamped the plates together, creating friction between the faying surfaces. The resulting friction force was a key factor in the joint’s ability to resist slip and transfer load. In contrast, a loose-fitting bolt or a weld without preload relies solely on bearing or weld metal strength to transfer shear, making the rivet’s thermal contraction a unique advantage.

Metallurgy of the Rivet

The steel used for structural rivets was carefully specified. Typical materials included low-carbon steels (e.g., ASTM A502 Grade 1 or similar) that offered good ductility and formability when hot. The heated, hammered, and cooled cycle also altered the microstructure of the rivet. Rapid quenching was avoided; the rivet was allowed to cool in air, producing a relatively fine-grained ferrite-pearlite structure with adequate strength and toughness.

One important metallurgical consideration was that the rivet, being forged at high temperature, experienced recrystallization and grain refinement in the head and shank. This improved the mechanical properties of the shank compared to the original bar stock. The hammering (plastic deformation) also eliminated any casting segregation or porosity that might have been present, resulting in a more homogeneous material. Engineers today recognize that historic rivets often have higher strength and ductility than modern low-carbon steel fasteners because of this forging process.

However, the heating process also had to be controlled carefully. Overheating could cause burning—oxidation along grain boundaries—which embrittled the rivet. Underheating left the rivet too stiff to form a proper head without cracking. Teams of experienced riveters were essential to consistently produce sound connections. The skill involved is one reason why riveted structures from the early 20th century are often considered masterpieces of manual craftsmanship.

Types of Rivets and Joint Configurations

Structural rivets came in several shapes, though the most common for steel buildings and bridges were the button-head (round head) and the countersunk head. Button-head rivets were used on surfaces where protrusion was acceptable; countersunk heads were flush with the plate surface, often required in bridge decks or locations subject to aerodynamic drag.

The joints themselves were designed in two primary categories: lap joints and butt joints. In a lap joint, the plates overlap and are connected by a single row (or multiple rows) of rivets. In a butt joint, the plates are aligned end-to-end and covered with splice plates (one or two) riveted to both sides. The number of rows and the pitch (center-to-center spacing) were determined by the required strength and the stiffness of the connection. Rivet patterns in historic truss bridges often show rows spaced 3 to 6 diameters apart, with edge distances carefully calculated to prevent tearing.

Another critical design feature was the “grip length”—the total thickness of the plates being joined. Rivet diameters were typically chosen so that the grip length did not exceed 4 to 5 times the diameter, ensuring the rivet shank could be fully upset during installation. For thicker packs, built-up “field rivets” or special high-strength rivets were used, sometimes requiring multiple heaters and longer heating times.

How Riveting Contributes to Structural Integrity

Load Distribution and Stress Flow

Riveted joints transfer load through a combination of shear in the rivet shank and bearing between the shank and the plate. In a well-designed joint, the rivets are arranged so that the load path through the joint is as direct as possible, minimizing eccentricity and secondary bending. Because rivets are relatively closely spaced, the stress flow across a riveted splice is more uniform than in a bolted connection with fewer, larger fasteners. This reduces peak stresses at the holes and lowers the stress concentration factor, which is beneficial for fatigue life.

Friction and Slip Resistance

As noted earlier, the thermal contraction of the cooling rivet creates a clamping force that presses the plates together. For a typical ⅞-inch-diameter structural rivet, the clamping force can exceed 50 to 60 kN. This clamps the faying surfaces so tightly that friction alone can resist a significant portion of the applied shear load—often up to 30% to 50% of the ultimate shearing load of the rivet. This friction mechanism means that under service loads, a riveted joint experiences little to no slip, which helps maintain alignment and minimizes wear on the rivet holes. Over the life of a bridge, this slip resistance reduces the accumulation of damage that can lead to loosening or fatigue cracking.

Flexibility and Energy Dissipation

One of the most praised attributes of historic riveted frames is their ability to absorb energy during dynamic events such as earthquakes, wind storms, or temperature changes. Unlike a rigid welded connection, which can develop high stress concentrations and brittle failure if not properly detailed, a riveted joint allows a small amount of rotation or slip—typically a fraction of a degree—before full bearing is established. This “give” helps dissipate energy and reduces the peak forces transmitted through the frame. In early skyscrapers, the combination of riveted connections and the ductility of the steel allowed the structures to behave somewhat like a “steel fabric,” capable of redistributing loads after local yielding. This property is often cited in structural engineering literature as a key reason why many early 20th-century steel-framed buildings survived the 1906 San Francisco earthquake with minimal structural damage.

Fatigue Resistance

Fatigue failure in steel joints occurs when repeated loading causes microcracks to initiate and grow. Riveted joints exhibit excellent fatigue resistance compared to bolted or welded joints because the friction clamping prevents the rivets from working in the hole excessively. In bolted joints without preload, the fastener can rock in the hole under cyclic loading, fretting the plate and initiating cracks. Welded joints have sharp geometric transitions at the weld toe, a known stress raiser. Riveted joints have a more gradual transition and, because the rivet expands to fill the hole completely, there are no gaps or sharp corners to concentrate stress. Many historic riveted bridges have survived decades of heavy rail traffic with no detectable fatigue cracking in the rivets themselves—failures, when they occur, often happen in the plate at the net section, which is a sign of a well-designed connection where the rivets remain intact.

Historical Examples of Riveted Steel Structures

The Brooklyn Bridge (1883)

While primarily a suspension bridge, the Brooklyn Bridge makes extensive use of riveted steel in its stiffening trusses, towers, and anchorages. The bridge’s steel cables are anchored in massive stone abutments, but the superstructure includes thousands of hot-driven field rivets connecting the many built-up beams and lattice members. The bridge has carried ever-increasing traffic loads and has been subjected to corrosive salt air for over 140 years. Maintenance records show that the riveted connections have required relatively little repair compared to welded retrofits added later. The rivets have held their clamping force well, and many remain in excellent condition.

The Eiffel Tower (1889)

Gustave Eiffel’s masterpiece in Paris is a lattice tower constructed primarily of puddled iron (a material similar to low-carbon steel) joined by over 2.5 million rivets. The Eiffel Tower is a textbook example of how riveting allowed for efficient transfer of wind loads through a highly redundant, open framework. The connections are designed so that the rivets work primarily in shear, and the tower’s graceful curvature was achieved by varying the angle of the riveted joint lines. Engineers today note that the tower’s exceptional longevity—standing for over 130 years with minimal corrosion damage—is partly due to the tight fit of the riveted joints, which prevented moisture from seeping into crevices. Official Eiffel Tower history page.

The Woolworth Building (1913)

One of the earliest skyscrapers to use a fully triangulated steel frame, the Woolworth Building in Manhattan relied heavily on field riveting. Its 792-foot height required innovative connection designs to transfer loads from the slender columns at the top to the heavier columns at the base. The riveted gusset-plate connections in the building’s wind bracing system have performed superbly; even during hurricanes, the building’s lateral sway remains within acceptable limits. The Woolworth’s steel frame is still considered sound, and recent structural evaluations confirm that the original rivets retain nearly their full design capacity. National Park Service listing for the Woolworth Building.

The Forth Bridge (1890)

In Scotland, the Forth Bridge is a cantilever railway bridge composed entirely of steel tubes and lattice members connected by field-driven rivets. It remains a marvel of 19th-century engineering. The bridge uses millions of rivets to form its distinctive diamond-shaped trusses. Despite carrying heavy coal and freight trains for over a century, fatigue inspections have found very few cracked rivets. The design deliberately used riveted connections for their ductility and redundancy, which allowed the bridge to survive the dynamic loading from passing trains without requiring replacement of major members. The Forth Bridges website.

Preservation Challenges and Restoration Approaches

While riveted steel frames have proven remarkably durable, they are not immune to deterioration. Corrosion at the faying surfaces (the area where the two steel plates meet) is a common issue, especially in bridges exposed to deicing salts or coastal environments. Corrosion in these crevices can cause the rivet heads to “rust jack” or lose clamping force. In some historic structures, individual rivets have fractured at the shank due to overstress or thermal expansion cycles. The challenge for preservationists is to maintain historical authenticity while ensuring structural safety.

One restoration method is to replace damaged rivets with new hot-driven rivets of similar material and geometry. This requires training a new generation of riveters and maintaining forge equipment—a skill that was almost lost by the late 20th century. However, several historic bridge projects in the United States and Europe have successfully used hot riveting to restore original connections. For less critical members, high-strength bolts (often ASTM A325 or A490) may be substituted, but they alter the load path and stress distribution, which can affect the behavior of the frame. Bolt substitution also changes the historic appearance, which is often a concern for landmark status.

Another innovative approach is “cold driving” of rivets using hydraulic tools, but this method does not provide the same shrinkage clamping force as hot riveting. Consequently, cold-driven rivets are typically used only for reattachment of architectural details where load-bearing capacity is not critical. For major structural repairs, hot riveting remains the gold standard when historical accuracy and structural integrity are both required.

Understanding the original rivet patterns, pitch, and edge distances is crucial for any evaluation. Engineers must assess the remaining clamping force of the rivets, often through ultrasonic testing or by measuring the torque required to turn a rivet head—an indication of slip resistance. If multiple rivets in a joint have lost preload, the entire connection may need reinforcement with additional fasteners or steel plates, carefully designed to avoid overstressing adjacent members.

The Transition to Welding and Bolting

By the 1950s, structural welding had matured to the point where it could compete with riveting in terms of speed and economy. Welded connections required no pre-drilled holes, no heating forges, and no large crews. Furthermore, welded joints were more efficient in transferring load because they did not require splice plates or gussets, reducing the amount of steel used. However, early welded structures suffered from brittle fractures in some cases due to stress concentrations and poor notch toughness in the weld heat-affected zone. The lessons learned from those failures informed modern welding codes and filler metal specifications, but riveted connections never had that problem.

Bolting also gained popularity because high-strength bolts (torqued to a controlled tension) could provide clamping force similar to hot rivets without the need for heating. The A325 bolt, introduced in the 1940s, became a standard for steel construction. Bolted connections are easier to inspect, install, and maintain, and they can be loosened for disassembly if needed. For new construction, bolting and welding now dominate, but for historic preservation, the old riveted technology is irreplaceable.

Modern Studies and the Legacy of Riveted Connections

In recent decades, structural engineering researchers have revisited riveted connections to understand their long-term performance and to develop guidelines for evaluating existing structures. Load tests on full-scale riveted trusses have shown that the friction capacity of riveted joints is often higher than originally assumed, and that the slip factor (coefficient of friction) for rusted but still sound plates can be as high as 0.5. This has allowed engineers to rate historic bridges for heavier modern loads without extensive reinforcement.

Finite-element models of riveted joints have also shown that the residual stresses from the cooling process are not detrimental; in fact, the compressive stresses around the hole can actually increase the fatigue life of the plate by raising the threshold for crack initiation. This is contrary to earlier beliefs that the hammering process introduced harmful stress concentrations. Such findings underscore the ingenuity of early 20th-century engineering, which relied on empirical knowledge and careful workmanship rather than theoretical models.

The legacy of riveting extends into modern design: some architects and engineers are now using exposed rivet patterns in new structures as an aesthetic homage to industrial heritage. Moreover, the principles of preload, friction, and redundancy that made riveted joints so successful are being applied in modern high-strength bolted connections with tension-control bolts. The rivet remains an icon of the Machine Age, a symbol of the human effort that built the modern urban landscape.

Conclusion

Riveting was far more than a convenient joining method for early steel frames; it was a sophisticated technology that leveraged the physical properties of metal to create connections with exceptional strength, ductility, and durability. The clamping force generated by thermal contraction, the uniform stress distribution, the resistance to slip, and the energy absorption capacity all contributed to the structural integrity that allowed historic buildings and bridges to survive more than a century of use.

For professionals involved in the care of these historic structures, a thorough understanding of how riveting works—and how it continues to work—is essential. Preservation efforts that respect the original joint mechanics are more likely to succeed, both structurally and aesthetically. As we continue to study and admire masterpieces like the Eiffel Tower, the Brooklyn Bridge, and the Woolworth Building, we recognize that the humble rivet is a hidden hero of structural engineering, holding together the bones of our industrial heritage.