LECTURE # 38
BOLT STRENGTH

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The strength is specified by stating the
minimum proof strength, or minimum proof
load, and the minimum tensile strength
The proof load is the maximum load (force)
that a bolt can withstand without acquiring a
permanent set

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
The proof strength is the quotient of the
proof load and the tensile-stress area
The proof strength is about 90 percent of
the 0.2 percent offset yield strength
ASTM threads are shorter (deals mostly
with structures) and generally loaded in
shear
TENSION CONNECTIONS-THE
EXTERNAL LOAD
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A clamping force,(preload) Fi is applied by
tightening the nut before external force, P is
applied
Fi = preload
P = external tensile load
Pb = portion of P taken by bolt
Pm = portion of P taken by members

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Fb = Pb + Fi = resultant bolt load
Fm = Pm – Fi = resultant load on the
members
The load P is tension, and it causes the
connection to stretch, or elongate, through
some distance d
Pb
d 
kb
kb
Pb  Pm
km
d 
and
Pm
km

Since, P = Pb + Pm
kb P
Pb 
kb  k m

The resultant bolt load is
kb P
Fb  Pb  Fi 
 Fi
kb  k m
Fm < 0
Fig. 15.9
Fig. 15.9
Fig. 15.10
Fig. 15.9
Fig. 15.10
Fig. 15.11
Fm  Pb  Fi 

km P
kb  km
 Fi
Fm < 0
These results are valid only as long as some
clamping load remains in the members
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In all cases, the members take over 80
percent of the external load
Making the grip longer causes the members
to take an even greater percentage of the
external load
TORQUE REQUIREMENTS
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A high preload is very desirable in
important bolted connections
If the overall length of the bolt can actually
be measured with a micrometer when it is
assembled, the bolt elongation due to
preload Fi can be computed using formula
d  Fi l  A E 
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The nut is simply tightened until the bolt
elongates through the distance d
This ensures that the desired preload has
been attained
The elongation of a screw cannot be
measured, because the threaded end is
often a blind hole
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The torque wrench has a built-in dial
which indicates the proper torque
With impact wrenching, the air pressure is
adjusted so that the wrench stalls when
the proper torque is obtained
Or in some wrenches, the air automatically
shuts off at the desired torque
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The snug-tight condition is the tightness
attained by a few impacts of an impact
wrench
Or the full effort of a person using an
ordinary wrench
When the snug-tight condition is attained,
all additional turning develops useful
tension in the bolt
Turn-of-the-Nut Method
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The turn-of-the-nut method is the easiest
and least expensive method for installing
fasteners with the proper bolt tension.
The procedure generally works as follows.
An iron worker tightens the bolt and nut
as tight as possible using a spud wrench
or a pneumatic impact wrench
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A chalk mark or paint is then made on the bolt
and nut
The bolt is tightened further by either
hammering on the spud wrench or using a
pneumatic impact wrench until the rotating part
has rotated the required amount
The paint or chalk mark shows how far the part
has rotated and the rotation is always measured
relative to the rotation of the bolt.
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The turn-of-the-nut method requires that
you compute the fractional number of turns
necessary to develop the required preload
from the snug-tight condition
For heavy hexagon structural bolts, the
turn-of-th-nut specification states that the
o
nut should be turned a minimum of 180
from the snug-tight condition under
optimum conditions
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A good estimation of the torque required
to produce a given preload is
Fi d m
T 
2
 1    d m sec   Fi  c d c

 
2
  d m   l sec  
tan   l  d m
Fi d m  tan    sec   Fi  c d c

 
T
2  1   tan  sec  
2
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Th diameter of the washer face of a
hexagonal nut is the same as the width
across flats and equal to 1.5 times the
nominal size
dc = (1+1.5d)/2 = 1.25d

T 


 d m   tan    sec  
  0.625  c  Fi d

 
 2d   1   tan  sec  


Define a torque co-efficient K
 d m   tan    sec  
  0.625  c
K    
 2d   1   tan  sec  
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T = K Fi d
The coefficient of friction depends upon
the surface smoothness, accuracy, and
degree of lubrication
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On the average, both f and fc are about
0.15
K = 0.26 for f = fc =0.15 no matter what
size bolts are employed and no matter
whether the threads are coarse or fine
BOLT PRELOAD-STATIC LOADING
kb P
Fb 
 Fi  CP  Fi
kb  k m
kb
C 
kb  k m
Fm  1  C  P  Fi

The tensile stress in the bolt can be found
by dividing both terms of first equation by
the tensile-stress area, At
Fi
CP
sb 

At
At
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The limiting value of sb is the proof
strength, Sb
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With the introduction of a load factor n
C n P Fi

 SP
At
At
S P At  Fi
n
CP
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Any value of n > 1 ensures that the bolt
stress is less than the proof strength
JOINT SEPARATION
For safe joint, external load be smaller than
that needed to cause the joint to separate
If separation does not occur, then the entire
external load will be imposed on the bolt
Let Po be the value of the external load that
would cause joint separation
Fig. 15.15
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At separation, Fm = 0,

(1 – C)Po – Fi =0
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Let the factor of saftey against joint
separation be
Po
no 
P
Fi
no 
P (1  C )
Fig. 8.18
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Fig. shows stress-strain diagram of a good
quality bolt material
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No clearly defined yield point
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Fracture at the tensile strength
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No matter how much preload is given to
bolt, it will retain its load-carrying capacity
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The pr-tension is the muscle of the joint
and its magnitude is determined by the
strength
If the full bolt strength is not used, then
money is wasted
Good quality bolts can be preloaded into
the plastic range to develop more strength
A bolt will either fracture during tightening
or not at all
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It is recommended for both static and fatigue
loading that the following be used for
preload :
0.75 F P
Fi  
0.90 F P
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for reused connections
for permanent connections
Where FP is the proof load, obtained from
the equation FP = At SP
SP is the proof strength obtained from tables
8-4 to 8-6
For other materials, an approximate value is
SP = 0.85 SY
Fig.
8.17
Fig.
8.18
Fig. 8.19