What are connections?

In steel structures, connections are the members that join two or more steel members. The connections are the most important members of any steel structure, and hence, they are designed more carefully. The majority of the construction cost of steel structures is incurred due to the connections. The design of connections is done as per the recommendations and specifications of the American Institute of Steel Construction (AISC).

Types of structural members

A typical steel structure mainly consists of beams, girders, columns, tension members, compression members, and roof trusses. The steel members are manufactured using standard structural steel, specified by the American Society for Testing and Materials (ASTM). Generally, carbon steels, high strength low alloy steels, corrosion-resistant high strength low alloy steel, quenched and tempered alloy steels, and forged steels are used in the manufacturing of steel members. The steel members in steel constructions are used in the form of structural shapes. The most commonly used structural shapes are I-shape, Z-shape, Hollow Structural Section (HSS), Angles (L-shape), C-section, T-section, sheets, bars, rebars, and joists. The majority of the shapes are hot rolled, while some of the shapes are cold rolled.

The various types of connections of steel members are as follows-

• Bolted connections
• Welded connections
• Riveted connections

Bolted connections

Bolted connections are the most commonly used connections for assembling steel members. A bolt is a fastener consisting of a head at one end and a shank on the other. Some portion of the shank is threaded to allow the nut to be bolted, whereas the remaining portion is kept as it is. The portion in which threads are present is known as threaded length and the remaining length of the unthreaded shank is known as grip length. The bolts are mostly made of stainless steel. For water-proofing purposes, bronze bolts are preferred. To connect the members using the bolted connection, holes of diameter larger than the bolts, are drilled in the members at specified intervals as prescribed by the standards, and the bolts are inserted through the holes and the nuts are tightened on the threaded portion on the other end. The bolted connections can be used for members carrying tension, compression, moments, shear, and all other types of forces. The grade and diameter of the bolt and the spacing between bolts should be considered as recommended by AISC.

Advantages of bolted connections

• Less labor is required, and skilled professionals are not necessary
• The method is easier and faster.
• The bolted connections are faster made relative to other methods.
• The structures can be used immediately after connections are made.
• The connection can be easily disassembled and assembled. Hence, altering the members is possible.

Disadvantages of bolted connections

• The area of the bolt is reduced due to the threaded portion, and hence, the load-carrying capacity is reduced.
• It is uneconomical.
• The connections become loose due to the vibrations.

Welded connections

Welding is the process of joining the parts of metals by the process of heating and fusing using molten filler metal. After cooling, the metal of weld and the metal of members act as a single component. The metal used as filler is chosen as per the recommendations of the American Welding Society (AWS). Fillet welding, groove welding, skip welding, slot welding, and plug welding are the types of welds used for welding metals. In the case of welding the members of steel structures, the fillet weld is mostly preferred. The weld size and length need to be determined before welding. It is decided as per the recommendations of AISC. The members are placed as per the design and welded together.

Advantages of welded connections

• Design is simple, and the material required is less.
• The method is economical.
• Drilling holes is not required. It saves labor and time.
• Welding provides proper water resistance.

Disadvantages of welded connections

• The metal of the member becomes brittle at the spot of welding due to the heat effect.
• If any welding crack occurs, it expands rapidly.
• The welded member can cause damage due to lamellar tears, weld shrinkage, and residual stresses.

Riveted connections

Riveted Connection is an outdated technique used for the connection of steel members and is succeeded by the bolted connections. A rivet is a fastener consisting of a head and a cylindrical shank below it. A rivet is made up of mild steel or high tensile steel. Depending upon the usage, the rivets are classified as hot driven rivets, shop rivets, field rivets, and cold driven rivets. For riveting, holes are first drilled into the members and then rivets are driven into the holes. Hot-driven rivets are first heated and then they are inserted into the holes and beaten at the other end to form a head at another end. The hot rivet shrinks after cooling and the members are joined together. Other types of rivets are driven directly by applying high pressure using machinery.

Advantages of riveted connections

• The riveted joints can sustain vibratory forces without loosening.
• It is durable.
• The maintenance cost is less.

Disadvantages of riveted connections

• More labor is required.
• It is uneconomical.
• The process of installation causes much noise, thus disturbing the surrounding.
• Alteration is not possible without damaging the structure.

Lap joints and butt joints

The steel members are connected in two ways- Lap joints and Butt joints. A lap joint is a joint in which the members overlap each other. A butt joint is a joint in which the members are not overlapped, but simply placed and joined. The lap and butt joints are formed using the bolted and welded connections. But mostly, the bolted connection is preferred.

To form a lap joint, two members are overlapped and bolted directly. The lap joint can be classified as a single bolted lap joint or double bolted lap joint based upon the number of rows of bolts provided. In the case of a single row of bolts, it is known as a single bolted lap joint, and in the case of two rows of bolts, it is known as a double bolted lap joint.

To form a butt joint, two members are joint, but not overlapped and a cover plate is placed on either one side or both sides of the members. The assembly of cover plates and members is then bolted. If the cover plate is provided only on one side, then the joint is known as a single cover butt joint and if the cover plate is provided on both sides, then it is known as a double cover butt joint.

Context and Applications

This topic is studied in the following courses:

• Bachelors of Technology (Civil/Mechanical Engineering)
• Masters of Technology (Structural Engineering)

Practice problems

1. Which of the following connections is not a type of connection of steel members?

1. Bolted connections
2. Roller connections
3. Welded connections
4. Riveted connections

Answer- b

Explanation– Roller connections are not a type of connection of steel members.

2. What is the unthreaded shank portion of a bolt known as?

1. Grip length
2. Thread length
3. Welded length
4. Braced length

Answer- a

Explanation- The unthreaded shank portion of a bolt is known as grip length.

3. Which of the following is not a type of welding?

1. Fillet welding
2. Plastic welding
3. Groove welding
4. Plug welding

Answer- b

Explanation- Plastic welding is not a type of welding.

4. Which of the following is not a type of rivet used in steel structures?

1. Hot driven rivet
2. Shop rivet
3. Cold driven rivet
4. Porous rivet

Answer- d

Explanation- Porous rivet is not a type of rivet used in steel structures.

5. Which of the following statement is false regarding the bolted connections?

1. Bolted connections are economical.
2. Less labor is required for bolted connections.
3. The method of bolted connections is easier and faster.
4. Altering of the members is possible in the bolted connections.

Answer- a

Explanation- The statement "Bolted connections are economical" is false.