A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
4. Are washers necessary for proper operation of bolts? (BF4)
“Fastener” is a general term used to describe something which is used as a
restraint for holding things together or attaching them to other things.
The main physical distinction between screws and bolts is that screws are
entirely full of threads while bolts contain shanks without threads. However,
a better interpretation of the differences between the two is that bolts are
always fitted with nuts. On the contrary, screws are normally used with
tapped holes.
High friction grip bolts are commonly used in structural steelwork. They
normally consist of high tensile strength bolts and nuts with washers. The
bolts are tightened to a shank tension so that the transverse load across
the joint is resisted by the friction between the plates rather than the bolt
shank’s shear strength.
The purpose of installing washers in a typical bolting system is to distribute
the loads under bolt heads and nuts by providing a larger area under stress.
Otherwise, the bearing stress of bolts may exceed the bearing strength of
the connecting materials and this leads to the loss of preload of bolts and
the creeping of materials. Alternatively, flanged fasteners instead of using
washers could be adopted to achieve the same purpose.
Fig. Washer
5. What is the behaviour of internal force in preloaded fasteners?
The force in a bolt in a bolted joint depends on the preloading force applied
to it during the tightening operation. For instance, when the preloaded bolt
is tightened with a certain force, the bolts’ internal force will not increase
significantly if the external applied force on the bolted joint does not exceed
the preloading force. It looks like the bolt does not feel the external applied
387