Dowels rivets and bolts
Rivets and bolts transmit load by a combination of bearing pressure, shear and friction on the interface between the components being joined.
The structural principle of the rivet and the bolt is just the same as that of the traditional dowel or peg. In essence, whenever it is desired to transfer load from one plate to another, the method is to drill a hole in each plate and then insert a circular bar through these holes. The system relies on the pressure at the interface of the plate and the bar to transfer load and also on the shear stresses in the cross section of the bar. Iron and steel have adequate shear strength so that simply increasing the cross sectional area increases the capacity of the bar. To get adequate bearing between the bar and the plate, the ductile character of wrought iron and steel helps the action of the joint. The holes drilled in the plates have in practice to be slightly larger than the bar. If the circle of the bar and the circle of the hole remained perfect, they would only touch at a point. With wrought iron and steel connections, both bar and hole will distort significantly to give a larger area of contact. This principle is used for the dowel, the bolt and the rivet, but it was the rivet which was the predominant means of connecting wrought iron and steel from the mid-19th to the mid-20th century.
This curious diagram is an attempt to illustrate the flow of forces between steel plates connected by a bar. The principle is the same whether the bar is a dowel, a rivet or a bolt. The lower diagram shows the forces flowing through the plate going around the hole cut in the plate and concentrating behind the bar. It should be apparent that at the cross section of the plate that goes through the hole, the stresses are most heavily concentrated. Also that for a reasonable contact area between the plate and the bar, a degree of ductile deformation is required. Were the hole in the plate and the bar to remain perfectly circular they would touch only at a point. The positive virtue of a ductile material like steel or wrought iron is that by deforming by quite small amounts it spreads the loading over a reasonable area. The upper diagram is a section through the bar showing the transfer of load between the plates and the sections of the bar that have to transfer the load by shear stresses. In order to make the diagram clearer and to give room to indicate the shear planes the plates are shown slightly apart. In practice of course, rivets and bolts clamp the plates together. Indeed the friction between the plates gave traditional joints added strength and it is utilised by modern high strength friction grip bolts.
Rivets make very effective connections between components in both wrought iron and steel.
In essence, the rivet is simply a bar with its ends hammered over to prevent it slipping out of the hole. This is easy to do with soft metals like gold, silver, or copper and has been a traditional method of jointing since prehistory. It was impossible to achieve with cast iron because cast iron cannot be squashed, it simply fractures. It is a very effective way of joining ductile metals such as wrought iron and steel. Rivets were heated in portable braziers and held in position with tongs while the protruding end was bashed over with a big hammer to form strong reliable and positive fixings. In fact, the connection had more strength than simply the shear across the cross section of the rivet since the hammering of the head gripped the plates together and load could be transferred through friction between the plates. Also, the heat and pressure would often induce some pressure welding if the conditions were favourable.
The Eiffel Tower used 2.5 million rivets. On site they were fitted by this type of four person team. The rivet was heated to red heat in a portable brazier and transferred from brazier to pre-drilled hole by the person holding it in position with the tongs. Finally while restrained at the far side it was beaten with a hammer to form the fixing head. Look at any large scale nineteenth century structure and consider the man power and organisation required.
The change from cast iron, with elements that fitted together, to wrought iron, joined by rivets, had a significant effect on the appearance of joints.
In terms of the visual expression of the joint, the change from cast to wrought iron was a step backwards. The relation between the parts was limited by the overlapping of flat surfaces. While this was ideal for the fabrications of steam boilers, it made the connection of columns and beams, struts and ties more difficult. Initially, sections were small in size and restricted in profile to flats, tees and angles. Larger sections would be made up from combinations of these basic profiles, and here there was scope for richer and more varied joint design. As the 19th century progressed, the introduction of steel and the development of the industry meant that the rolled sections got larger and the jointing tended to be cruder.
This column head is a particularly interesting juxtaposition of a timber roof purlin with localised increase in depth, a wrought iron edge beam made up from riveted angles and flats, cast iron decorative brackets with both opposing curves fixed by bolts and a slender cast iron column.
Rivet patterns at joints contributed to the visual interest of large structures.
The most visually interesting structures of this period tended to be the very large showpiece structures built for the great exhibitions. Eiffel's great tower for the Paris exhibition of 1888 was a tour de force of joint design. As the scale of the structures got larger, it was not sensible to increase the size of the rivets, there just had to be more of them. At close quarters, the pattern of the uncountable numbers of rivets arranged in consciously designed patterns gives human scale to the vast structure. The care with which the geometry of these patterns was organised showed that engineers like Eiffel were as alive to the potential of rewarding detailed design as any architect. The Gallery of Machines for the same exhibition was not only a gigantic enclosure of space, but its structure was built up from riveted sections in a way that demonstrated a controlled richness of detail that can still be admired in the famous photograph a hundred years later.
