TC BOLT
ASSEMBLY
introduction TO FRICTION
For every two surfaces that come
in contact there is friction. Friction opposes the direction
of motion, such as when you try to push a box across the
floor the frictional force is what is opposing the motion.
To calculate the friction between these surfaces you must
determine their friction coefficient, which describes the
resistance of the the two surfaces to slide across each
other. For instances, the rougher the surfaces are the
greater the coefficient of friction will be. Since this is
based on a microscopic level of the two surfaces the amount
of surface area touching between the two objects has no
effect on the friction coefficient. However, the force at
which the two surfaces are being pressed together does make
a difference, this is call the normal force. The normal
force is another name for the action reaction forces between
two objects. So the force of friction is equal to the
friction coefficient multiplied by the normal force. 
Friction between the
threads
Friction
plays an important role in the functionality of bolts.
As was discussed on the PHYSICS OF BOLTS page,
the action reaction (normal) forces increase as
the nut is tightened onto the bolt (see picture
below). And as we just went over, the frictional force
increases as the normal force increases. This
is why the nut becomes more difficult to turn as it is
tightened down and why it stays in place once it is
tighten down. If there was no frictional force between
the threads to oppose motion the nut could easily come
loose. This is why people often add a lubricant to the
threads when they are trying to loosen a bolt. 
Diagram by Taylor TharpFriction and torque
This
plays an especially important role when it comes to
pretension bolts. Pretension bolts use a special tool
for installation; as you may have noticed these bolts do
not have a hex shaped head to hold the bolt while the
nut is tightened down. Instead pretension bolts have a
spline at the end of the shaft. The tool that is used
grabs on the spline and the nut at the same time and
turn them in opposite directions which tightens them up
faster. As the Normal force between the threads
increase, the frictional force also increases which in
turn also increases the amount of torque (rotational
force) needed to turn the nut onto the bolt. When the
torque becomes so great that the nut and bolt can barely
turn anymore the rotational force breaks off the spline
along a groove between the spline and the shaft of the
bolt called the shear plane. The shear plane is
specifically designed to be the weakest point along the
shaft so that it intentionally breaks off at this point
and not somewhere else along the bolt.
Everything about these bolts is
specifically designed so that the spline breaks off
when it reaches a specific torque, which varies based
on the diameter of the bolt. However, it is easier and
more accurate to measure the normal force than the
torque using a load cell which measures the
compression caused by the action reaction forces.
Below is a table from the AISC (American Institute of
Steel Construction) Thirteenth Edition published in
2009 that specify the minimum normal force that the
bolts must reach based on the bolt diameter.
From AISC Manual 2009 13
Edition