Steel sections are manufactured and shipped to some standard lengths, as governed by rolling, transportation and handling restrictions. However, most of the steel structural members used in structures have to span great lengths and enclose large three dimensional spaces. Hence connections are necessary to synthesize such spatial structures from one- and two-dimensional elements and also to bring about stability of structures under different loads. Thus, connections are essential to create an integral steel structure using discrete linear and two-dimensional (plate) elements.
A structure is only as strong as its weakest link. Unless properly designed, the connections joining the members may be weaker than the members being joined. However, it is desirable to avoid connection failure before member failure for the following reasons:
To achieve an economical design, usually it is important that the connections develop the full strength of the members.
Usually connection failure is not as ductile as that of steel member failure. Hence it is desirable to avoid connection failure before the member failure.
Therefore, design of connections is an integral and important part of design of steel structures. They are also critical components of steel structures, since
They have the potential for greater variability in behaviour and strength,
They are more complex to design than members, and
They are usually the most vulnerable components, failure of which may lead to the failure of the whole structure.
Thus designing for adequacy in strength, stiffness and ductility of connections will ensure deflection control during service load and larger deflection and ductile failure under overload. Hence, a good understanding of the behaviour and design of joints and connections in steel structures is an important pre-requisite for any good design engineer.
Welded connections are direct and efficient means of transferring forces from one member to the adjacent member. Welded connections are generally made by melting base metal from parts to be joined with weld metal, which upon cooling form the connection. The welded connections in a majority of the cases may be categorised as fillet weld or butt (or groove) welds as shown in Fig. 7.1.
Fillet welds, as shown in Fig. 7.1(a), are made against two surfaces of adjacent plates to join them together. The merits of the fillet welds are:
no prior edge preparation is necessary,
simple, fast and economical to make, and
does not require very skilled labour.
not appropriate to transfer forces large in magnitude,
poorer performance under fatigue loading, and
less attractive in appearance.
Butt welds, as shown in Fig. 7.1(b), are made by butting plate surfaces against one another and filling the gap between contact surfaces with weld metal, in the process fusing the base metal also together. In order to ensure full penetration of the weld metal, normally the contact surfaces are cambered to obtain gap for the weld metal to flow easily.
The merits of butt welds are:
easily designed and fabricated to be as strong as the member,
better fatigue characteristics, compared to fillet welds,
better appearance, compared to fillet welds, and
easy to detail and the length of the connection is considerably reduced. The demerits of the butt welds are:
more expensive than fillet welds because of the edge preparation required, and
require more skilled manpower, than that required for filled welds.
It has been pointed out that steel sections are linear elements produced in certain convenient lengths due to constraints on manufacturing and transportation. Therefore connections are necessary to provide continuity, where required, as well as to create three-dimensional steel structures. One of the most efficient and possibly direct ways of providing connections in steel structures is by way of welding. Welding is the process of joining two pieces of metal by creating a strong metallurgical bond between them by heating or pressure or both. It is distinguished from other forms of mechanical connections, such as riveting or bolting, which are formed by friction or mechanical interlocking. It is one of the oldest and reliable methods of jointing. Welding was quite an art prevalent in ancient Greece to make bracelets. It was probably a forging process, where metals were heated and hammered together. Modern welding has been in existence since World War I. It was mainly used for repairing damaged ships. After 1919, the use of welding as a construction and fabrication method began to develop. Since then many improvements and developments have taken place. Today there are over 50 different welding processes, which can be used to join various metals and their alloys.
Advantages of welding
Welding offers many advantages over bolting and riveting. Some of the advantages are listed in the following.
Welding enables direct transfer of stress between members. Hence, the weight of the joint is minimum. Besides efficiency, design details are very simple. Less fabrication cost compared to other methods due to handling of fewer parts and elimination of operations like drilling, punching etc. The most striking advantage of welded structures is in the area of economy. Welded structures allow the elimination of a large percentage of the gusset and splice plates necessary for riveted or bolted structures. Time is saved in detailing, fabrication and field erection. In some bridge trusses it may be possible to save up to 15% of the steel weight by resorting to welding. Welding also requires considerably less labour for executing the work.
Welding offers air tight and water tight joining of plates and hence ideal for oil storage tanks, ships etc.
Welded structures usually have a neat appearance as against the cluttered surface of bolted or riveted connections. Fig. 7.2 shows a comparison of riveted plate girder and a welded plate girder. Further, welded connections offer the designer more freedom for innovation in his design concept. It enables him to use any cross section and the best configuration to transmit forces from one member to another.
The range of application of welding is very wide. For example, connection of a steel pipe column to other members can be made very easily by welding whereas it is virtually impossible by bolting or riveting. Welding is practicable even for complicated shapes of joints.
There is no need for holes in members connected by welding except possibly for erection purposes. This has direct influence in the case of tension members as the problem of determining the minimum net section is eliminated. This also results in a member with a smaller cross section.
Welded structures are more rigid compared to structures with riveted and bolted connections. The rigidity of welded structures is due to the direct connection of members by welding. In bolted or riveted structures, the connection is established through angles or plates, which deflect under loads, making the structure flexible.
It is easier to make design changes and to correct mistakes during erection, if welding is used. It is also a simple procedure to strengthen the existing structures with welding.
*. A truly continuous structure is formed by the process of fusing the members together. This gives the appearance of a one-piece construction. Generally welded joints are as strong or stronger than the base metal, thereby placing no restriction on the joints.
Due to this continuity advantage, a very large number of steel frames have been constructed all over the world.
Stress concentration effect is considerably less in a welded connection. Some of the lesser important advantages of the welding processes are: relative silence of the process of welding and fewer safety precautions.
Some of the disadvantages of welding are:
Welding process requires highly skilled manpower
Experienced manpower is needed for inspection of welded connections. Also, non-destructive evaluation may have to be carried out to detect defects in welds
Welded joints are highly prone to cracking under fatigue loading
Costly equipment is essential to make welded connections
Proper welding can not be done in the field environment
Large residual stresses and distortion are developed in welded connections
In the earlier days, combination of bolting, riveting and welding was not practiced. Structures were completely welded, bolted or riveted. Presently both are used in a structure except that both connection techniques are not used in one and the same joint. The present trend is to use welding for workshop connections or splices, and high strength bolts for field joints.