6.1 Types of Connections
A structure is an assembly of various elements or components which are fastened together through some type of connection. If connections are not designed properly and fabricated with care, there may be a source of weakness in the finished structure, not only in their structural action but also because they may be the focus of corrosion and aesthetically unpleasing. Where as the design of main members has reached an advanced stage, based upon theories which have been developed and refined, the behaviour of connections is often so complex that theoretical considerations are of little use in practical design. By their very nature, connections are a jumble of local effects. Most connections are highly inderminate, with the distribution of stress depending upon the deformation of fasteners and the detail material. Local restraints may prevent the deformation necessary for desirable stress redistribution.
Following are the requirements of a good connection in steelwork:
It should be rigid, to avoid fluctuating stresses which may cause fatigue failure
It should be such that there is the least possible weakening of the parts to be joined
It should be such that it can be easily installed, inspected and maintained.
The following are the common types of connections used for structural steel work;
Rivets, bolts and welds are used extensively, and frequently the economic advantage of one over the other two is so small to be uncertain. However, at one time, riveting prevailed but it has been superseded in importance by welding and high-strength bolting.
6.2 Historical Notes on Rivets
Rivets were the accepted method for connecting the members of steel structures for many years. Today however they no longer provide the most economical connections and are obsolete. It is doubtful that you could find a steel fabricator who can do riveting. It is however desirable for the designer to be familiar with rivets even though he or she will probably never design riveted structures. He or she may have to analyze an existing riveted structure for new loads or for an expansion of the structure. The purpose of these sections is to present only a very brief introduction to the analysis and design of rivets. One advantage of studying these obsolete connectors is that while doing so you automatically learn how to analyze common bolts. These bolts are handled exactly as are rivets except that the design stresses are slightly different.
6.3 Rivet and Riveting
Riveting is a method of joining together structural steel components by inserting ductile metal pins, called rivets, into holes of the components to be connected from coming apart. A rivet consists of (i) a shank of given length and diameter and (ii) a head known as manufactured head. The size of the rivet is defined by the diameter of the shank. Riveting is essentially a forging process during which a hot rivet is driven in its plastic state and a head is formed at the other end. The head so formed at the other end of the rivet with the help of a riveting hammer and a buckling bar is known as driven head.
Rivets driven in the field during the erection of a structure are known as field rivets. Rivets driven in the fabricating shop are known as shop rivets. Both these types are known as hot driven rivets since the rivets are heated to a temperature ranging between 1000o F to 1950oF before driving. Field rivets are driven by a hand operated pneumatic riveting hammer, while the shop rivets are driven by “bull” riveter. Some rivets are driven at atmospheric temperature. They are known as cold driven rivets which are squeezed or driven to fill the holes and to form the heads by application of large pressure. However, they are smaller in diameter, ranging from 12 mm to 22 mm. Strength of cold driven rivet is more than hot driven rivets. Rivets driven by hand operated riveting hammer are known as hand driven rivets while those driven by power operated equipment are known as power driven rivets. Some times, even the field rivets may also be power driven.
6.4 Types of Rivetd Joints
There are two types of riveted or bolted joints.
Lap joint: The first is the lap joint in which the plates to be connected overlap each other. (Figure 6.1)
Butt joint: The second is the butt joint in which the plates are to be connected butt against each other and the connection is made by providing a cover plate on one or both sides of the joint. (Figure 6.2)
Figure 6.1 Single Riveted lap joint
Figure 6.2 Double riveted butt joint
The following definitions are used for riveted or bolted joints
Nominal diameter: The diameter of the shank of a rivet before riveting is called the nominal diameter.
Effective diameter or gross diameter: The effective or gross diameter of a rivet is equal to the diameter of the hole it fills after riveting
Gross area: The gross area of a rivet is given by its gross diameter.
Pitch: The distance between centres of any two adjacent rivets is called the pitch.
Gauge: A row of rivets parallel to the direction of force is called a gauge line. The normal distance between two adjacent gauge lines is called the gauge distance.
Edge distance; It is the distance between the edge of a member or cover plate and the centre of the nearest rivet hole.
6.6 Failure of a riveted joint (Figure 6.3)
Figure 6.3: Failure of Riveted joints
Consider a riveted joint shown in Figure 6.3. The joint may fail in any of the following manners.
Tearing of the plate between rivet holes: The strength of the plate is reduced by rivet holes and the plate may tear off along the line of the rivet holes as shown in Figure b.
Shearing of rivet: The rivets fail by shearing if the shearing stress exceeds their shearing strength. In lap joints and single cover butt joints, the rivets are sheared at one plane only. In a double cover butt joint, the rivets are sheared at two planes as shown in Figure c.
Bearing of plate or rivet: The plate or rivet is crushed if the compressive stress exceeds the bearing strength of the plate or the rivet as shown in Figure d.
iv) Edge cracking; The plate will crack at the back of a rivet if it is placed very near to the edge of the plate as shown in figure e. This failure is prevented if the minimum edge distances are provided.
The first three types of failures determine the strength of a joint. The rivet value or strength of rivet is determined by the types of failure described in shearing and bearing of rivets.
6.7 Assumptions in the theory of riveted joints
Certain assumptions are made while deriving expressions for the strength of riveted joints as follows:
The tensile stress is uniformly distributed on the portions of the plate between the rivets.
The friction between the plates is neglected.
The shearing stress is uniformly distributed on the cross-sections of the rivets.
The rivets fills the holes completely.
The rivets in a group share the direct load equally
Bending stress in rivets is neglected
The original strength of a section is reduced by rivet holes. The efficiency of a joint is the ratio of the joint and the original strength of the member without rivet holes. At the weakest critical section, the number of rivet holes should be minimum for maximum efficiency.
6.9 Design of riveted joints for axially loaded members
The diameter of a rivet is generally calculated by the following formula;
= 6t where d is the rivet diameter in mm and t is the thickness of plate in mm Number of rivets required for the joint = Load/Rivet value
The rivets are arranged bearing in mind the following points:
The arrangement should satisfy the gauge, pitch and edge distance requirements
The strength of joint should be increased gradually towards the joint for uniform distribution of stress in the rivets.
The cg of each rivet group should coincide ith the centreline of the connected members. It is not possible practically to follow this condition in some cases e.g the
angle connection with gusset plate. The small eccentricities are usually neglected.
The centreline of all members meeting at a joint should coincide at one point only otherwise the joint will twist out of position,
The strength of member reduces due to rivet holes. The reduction in area due to rivet holes is minimum if rivets are arranged in a zig-zag form.
Figure 6.4 shows a joint in the lower chord of a roof truss. Design the riveted connections if the permissible stresses are:
at = 150MPa
pf = 250MPa
vf = 80MPa
Figure 6.4 A truss joint