Types of joints

• The main function of a joint is to transmit load from one part of a structure to another
• This usually results in the joint being subjected to a lot of stress and weakening the structure
• Certain types of joints allow stress to pass uniformly accords the joint
• Examples are the butt-weld and scarf joints
• The former is used when joining metal and the later is used when joining wood/timber
• There are various other types of joints including:
• Lapped joints
• Mortised joints
• welded joints
• bolted, screwed or nailed joints
• brazed and soldered joints and
• riveted joints
• Joints are named in accordance with the material they are holding together e.g rivets,
• These are also known as pinning
• the material/method used to hold the materials together e.g. nailed joints
• Or the way in which the members of the joint are brought together e.g mortised joints

Lapped joints

Joint types and pinnings

• In a lapped joint which is also known as a lap joint
• In a lapped/lap joint the two pieces being joined overlap without over one another
• They overlap over each other hence the name lapped joint
• The two adjacent pieces may be glued, nailed, screwed, welded, bolted or riveted together
• the load is transferred at the two ends of the joint and
• stress will be at greatest at these points
• The strength of these joints depend on their width rather than upon the length of overlap
• Joints can be secured in different ways
• This is known as pinning
• There are various types of pinning:

a) bolted and nailed joints

• lapped joints in wood are often bolted or nailed together
• bolts, nails and screws resist sheer

b) Mortised joints

Dowels and mortised joints

• In mortised joints
• A hole(or more) is made in  one of the timber beams
• the adjoining beam is carved out to fit into this hole
• the end of this beam is glued into the hole of the first beam

b) Dowels

• Are similar to mortised joints
• Dowels are often used in making furniture
• Holes are made into both beams and
• Small wooden cylinders are inserted and glued to hold the beams together

c) Welded joints

• Are commonly used in steel-work
• It has several advantages:
• This is because they are cheaper than rivets
• They are also lighter
• They also offer less streamlining resistance to air or water
• Welding also creates joints that spread stress evenly
• Welding is achieved in various ways
• In electric arc-welding electricity is used to heat an electric rod
• The rod melts and the molten metal is spread along the joint and solidifies joining the metal in the process
• Welding has several disadvantages:
• It is difficult to obtain a perfect weld thus there is always the possibility of human error during the welding process creating inherent weaknesses in the joint
• Welded joints also do not stop cracks from developing and spreading through steel structures

d) Brazed and soldered joints

Soldering joints are often used in electronics. Image credit youtube.com

• involve the joining of metal plates with molten metal that solidifies to make a continuous joint
• Solder is an alloy of tin and lead
• It has a lower melting point
• A small hot iron element is used to melt it
• Brazing uses an alloy of zinc and brass and a very hot flame
• Brazing creates a more flexible joint
• Brazed joints pass stress more evenly

e) Gluing

• Is often used in making furniture
• Often in scarf joints the two pieces of timber are glued together
• A glued joint passes stress evenly

f) Riveted joints

• involves the use of rivets
• White hot rivets are placed in a hole drilled through a lapped joint
• Their ends are then hammered flat
• Upon cooling the rivet contracts and pulls the joint together
• The more rivets used in making a joint the stronger it is
• Because they share the stress
• Rivets have several advantages:
• Riveted joints are reliable
• the joint stops cracks because of its discontinuity
• The joints can slip a little thus redistributing the load and avoiding stress concentrations
• The rivet holes can fill up with rust thus locking the joint and allowing it to transmit shearing forces equally between the joined plates
• It acts like glue and as a result rivet joints improve with age

To access more topics go to the Combined Science Notes page.