Connections in Steel Structures
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Connections are structural elements used for joining different members of a structural steel frame work. Any steel structure is an assemblage of different members such as beam, columns, and tension members, which are fastened or connected to one another, usually at the member ends. Many members in steel structures may themselves be made of different components such as plates, angles, I beams, or channels. These different components have to be connected properly by means of fasteners, so that they will act together as a single composite unit. Connections between different members of a steel frame work not only facilitate the flow of forces and moments from one member to another, but also allow the transfer of forces up to the foundation level.
A structure is only as strong as its weakest link. Unless properly designed and detailed, the connections may become weaker than the members being joined due to following reasons:
• A connection failure may lead to a catastrophic failure of the whole structure.
• Normally, a connection failure is not as ductile as that of a steel member failure.
• For achieving an economical design, it is important that connectors develop full or little extra strength of the members it is joining.
To properly design a connection, a designer must have a thorough understanding of the behavior of the joint under loads. Different modes of failure can occur depending on the geometry of the connection and the relative strengths and stiffness of the various components of the connection. To ensure that the connection can arry the applied loads, a designer must check for all perceivable modes of failure pertinent to each component of the connection and the connection as a whole.
3. COMPONENTS OF A CONNECTION
Connections mainly include any or in combination with some of the components given below:
• Bolts (Shop or Site)
• Welds (Shop or Site)
• Connecting Plates
• Connecting Angles
4. CLASSIFICATION OF CONNECTIONS
Connections are basically classified as:
1. According to the type of connecting medium used:
i) Riveted connections
ii) Bolted connections
iii) Welded connections
iv) Bolted–welded connections
v) Pinned connections
2. According to the type of internal forces the connections are expected to transmit:
i) Shear (semi rigid, simple) connections
ii) Moment (rigid) connections
3. According to the type of structural elements that made up the connections:
i) Single plate angle connections
ii) Double web angle connections
iii) Top and seated angle connections,
iv) Seated beam connections, etc.
4. According to the type of members the connections are joining:
i) Beam to beam connections
ii) Column to column connections (column splices)
iii) Beam to column connections
iv) Column base plate, etc.
5. DISCUSSION AND REVIEW
The above classification of connections is elaborately discussed in this heading. All these connections shall be designed in accordance of IS 800:2007 (Standard Code of practice for General Construction in Steel).
5.1. According to the type of Connecting Medium Used:
These are the connection which are classified according to the connecting medium is used. They are discussed below.
5.1.1. Riveted Connections
For many years rivets were the sole practical means of producing safe and serviceable steel connections. A rivet is made up of a round ductile steel bar called shank, with a head at one end. The length of the rivet to be selected should be longer than the grip of the rivet, sufficient to form the second head. The behaviour and design of riveted connections are very similar to bearing type of bolted constructions. The installation of the rivet requires heating of the rivet and inserting it to an oversize hole applying pressure to the performed head while at the same time squeezing the plain end of the rivet using a pneumatic driver form a round head. During this process the shank of the rivet completely or nearly fills to the hole. Further, in the process of cooling, the rivet shank length reduces; thereby causing some clamping force, owing to this a riveted joint is intermediate between a friction and a bearing type connection.
Riveting has been traditionally limited to railway bridges in India. However, with the introduction of HSFG bolts, which are better suited under cyclic loading than rivets, their use is discontinued even in railway bridges in most countries. Riveting became obsolete as the cost of installed high strength structural bolts became competitive with the cost associated with the four or five skilled tradesmen needed for a riveting crew.
Riveting is no longer used in engineering structures due to:
1. The introduction of high strength structural bolts.
2. The necessity of pre heating the rivets prior to driving.
3. The labour costs associated with large riveting crews
4. The cost involved in careful inspection and removal of poorly installed rivets.
5. The high level of noise associated with driving rivets.
5.1.2. Bolted Connections
Bolted connections are connections whose components are fastened together primarily by bolts (fasteners). Depending on the direction and line of action of the loads relative to the orientation and location of the bolts, the bolts may be loaded in tension, shear, or a combination of tension and shear. For bolts subjected to shear forces, the design shear strength of the bolts also depends on whether or not the threads of the bolts are excluded from the shear planes. Because of the reduced shear areas for bolts whose threads are not excluded from the shear planes; these bolts have lower design shear strengths than their counterparts whose threads are excluded from the shear planes.
Two types of bolts are used in bolted connection:
1. Bearing type bolts
2. High strength friction grip bolts (HSFG)
The most common type is bearing bolts in clearance holes, often referred to as:
1. Black bolts
Black bolts are also referred to as ordinary, unfinished, rough, or common bolts.
They are least expensive bolts. They are primarily used in light structures under static load such as small trusses, purlins etc.
2. Turned bolts
These are similar to unfinished bolts, with the difference that the shanks of these are formed from hexagonal rods. Since these are expensive, it’s usage limited to structures with no slippage connections.
3. Ribbed bolts
These bolts have a round head similar to a rivet and raised ribs parallel to the shank. The actual diameter of the ribbed bolt is slightly larger than the hole into which it is driven experiences a relatively tight fit.
They are popular since they are economical, both in terms of material and installation costs.
The main disadvantage of bearing type of bolted connections is that the elements undergo some slip even under a small shear, before being able to transfer force by bearing. This is due to clearance between the bolts and the holes. Such a slip causes increased flexibility in the lower ranges of load and unexpected joint behaviour in some situations. In such cases high strength friction grip (HSFG) bolts are used.
In HSFG bolted joints, high strength bolts (8G or 10K grade) are pre-tensioned against the plates to be bolted together, so that contact pressure is developed between the plates being joined. When external shear force is applied, the frictional resistance to slip between the plates prevents their relative slip. These bolted joints achieve higher stiffness in shear because of frictional resistance between the contact surfaces. Only when the externally applied force exceeds the frictional resistance between the plates, the plates slip and the bolts bear against the bolt holes. Thus even after slip, there is a reserve strength due to bearing.
The HSFG bolts are expensive both from material and installation points of view. They require skilled labour and effective supervision. Due to their efficient force transfer mechanism they have become very popular recently.
Bolted Moment Connection Bolted Splice Connection
Figure 1. Bolted Connection
1. The bolting operation is very silent, in contrast to the hammering noise in riveting.
2. Bolting is a cold process; hence there is no risk of fire.
3. Bolting operation is more quicker than riveting.
4. Less man power is required in making the connections.
1. The bolted connections, if subjected to vibratory loads, results in reduction in strength if they get loosened.
2. Bolted connections for a given diameter of bolt, have lesser strength in axial tension since the net area at the root of the threads is less.
3. Unfinished bolts have lesser strength because of non uniform diameter
5.1.3. Welded Connections
Welded connections are connections whose components are joined together primarily by welds. Welds can be classified according to:
• The types of welds: groove, fillet, plug, and slot
• The positions of the welds: horizontal, vertical, overhead, and flat
• The types of joints: butt, lap, corner, edge, and tee
Although fillet welds are generally weaker than groove welds, they are used more often because they allow for larger tolerances during erection than groove welds. Plug and slot welds are expensive to make and do not provide much reliability in transmitting tensile forces perpendicular to the faying surfaces. Further more, quality control of such welds is difficult because inspection of the welds is rather arduous. As a result, plug and slot welds are normally used just for stitching different parts of the members together.
Welding will eliminate the need for punching or drilling the plies of material that will make up the connection, however the labor associated with welding requires a greater level of skill than installing the bolts. Welding requires a highly skilled tradesman who is trained and qualified to make the particular welds called for ina given connection configuration. He or she needs to be trained to make the varying degrees of surface preparation required depending on the type of weld specified, the position that is needed to properly make the weld, the material thickness of the parts to be joined, the preheat temperature of the parts (if necessary), and many other variables.