ARCH 342
STEEL STRUCTURES
Chapter 2
BOLTED CONNECTIONS
Main Reference:
William T. Segui 2007 and 2013.
«Steel Design», CENGAGE Learning,
4th and 5th Edition.
Prof. Dr. Nesrin YARDIMCI TİRYAKİOĞLU
2.1 INTRODUCTION
 A steel structure may be considered to be an assemblage of
various members that must be fastened together to make the
finished product (Connections and splices).
 If the necessary connections are inadequate, the result could
be collapse.
 At the beginning riveting was the accepted method used for
connecting the members of steel structures.
 For the last few decades bolting and welding used for making
structural steel connections.
 Selection of type of fasteners involves many factors,
including requirements of local building codes.
 Bolting of steel structures is very rapid field erection process
and requires less skilled labour than riveting and welding.
 For bolted connections it is possible to alter or disassemble
the structures and changes in connections are quite simple.
 Bolting is satisfactory for fieldwork.
2.2 TYPES OF BOLTS
Ordinary or common bolts (Machine bolts and unfinished bolts.):
 Easy to turn and need less turning space. Used in structures subjected
to static loads and for secondary members (purlins, girts, bracing,
etc.).
 Installed in the snug-tight condition.
High strength bolts (Pre-tension and high strength bolts.) :
 Tensile strength is two or more times greater than ordinary bolts.
Used for all types of structures.
 Installed in the snug-tight, pretensioned, or slip-critical
condition.
Bolt
Nut
Washer
Shank
Thread
Bolts:
M12, M16, M20, M24, M30, M36
Holes :
Standard
Oversized
Short-slotted
Long-slotted holes
• Common bolts : 4.6; 5.6; 6.8
• High strength bolts: 8.8; 10.9
• Mechanical
Bolt type
Fyb (N/mm2)
Fub (N/mm2)
properties of bolts
4.6
5.6
6.8
240
300
480
400
500
600
8.8
640
800
10.9
900
1000
Fub: Tensile strength Fyb : Yield stress ; 5= Fub /100 ; 6=10(Fyb / Fub)
d: Bolt diameter
M12~M14 : d=(12~14)mm
M16~M24 : d=(16~24)mm
M27 and › : d=27mm
h: Standard hole diameters
h=d+1mm
h=d+2mm
h=d+3mm
Snug-Tight
A snug-tight condition is defined as the tightness required
to bring the pieces into firm contact.
Pre-tension (Slip-critical)
Tensioning the high-strength bolts up to 70% of the
minimum tensile strength of the bolts. The installation of
tensioned bolts may consist of;
turn-of-the-nut method,
calibrated wrench tightening,
twist-off-type bolts,
direct-tension indicators.
2.3 SIMPLE BOLTED CONNECTIONS
Connections
of
structural
steel
members
are
of
critical
importance. An inadequate connection, which can be the «weak
link» in a structure, has been the cause of numerous failures.
Most structural failures are the result of poorly designed or
detailed connections.
In considering the behavior of different types of connections, it
is convenient to categorize them according to the type of
loading.
 Bolted connections in shear
 Bolted connections in tension
 Bolted connections in combined shear and tension
Bolts in tension
Bolts in shear
Bolts in combined tension and shear
Load transfer of bolts (Shear and bearing)
Bolt is required to prevent the movement of connected material in
a direction perpendicular to the axis of the bolt. Bolts may be
transmit the load in shear but if the material is not capable of
transmitting the loads into bolts the connection fails.
Bolt in single shear
The bolt has a tendency to shear off
along the single contact plane.
Plates slip in the direction of applied
force until they bear
against the bolt.
The lap joint : The simplest form but not commonly
used because of the tendency to deform
•Centre of gravity of the members are
not in line. This causes bending
in the connection.
•Used only for minor connections.
•Should be design with at least two fasteners in each line parallel to the
length of the member.
Bolt in double shear
There are two contact planes.
The butt joint: More common type, may be used for tension member splices.
•Total shearing force is split into two parts; bolts are in double shear; the
load-carrying ability twice as great as single shear connections.
•A more symmetrical loading condition is provided.
More than three members are being connected
•The bolts are in multiple shear. (Loads are tending to shear on
four separate planes.)
•Shear failure can occur on four planes.
2.4 FAILURE OF BOLTED
CONNECTIONS
Failure Modes
Failure of fasteners
Failure of parts being connected

Failure of fasteners (Shear connections)
Single shear
Double shear
: fv=P/Ab=P/(πd2/4)
and
: fv=P/2Ab=P/(2(πd2/4)) and
fv : Average shearing stress
Ab: Cross-sectional area of the fastener
d : Diameter of the fastener
P : Applied force on bolt
P=fvAb
P=2fvAb
Failure of the parts being connected
1.
2.
Failure resulting from excessive tension, shear or
bending in the parts being connected.
Tension on gross area and on effective net area must be
investigated and block shear must be considered.
Failure of connected part because of bearing exerted by
the fasteners.
For simplicity an average stress is used in computing the
bearing stress.
Bearing stress : fp =P/(dt)
Bearing load
: P =fp dt
t: Minimum thickness of the
part subjected to bearing
Failure
Modes
2.5 DESIGN OF BOLTED
CONNECTIONS
Bn
Ba  Bd 

Ba : Required strength of a bolt
Bd : Allowable strength of a bolt (allowable tensile strength,
allowable shear strength or allowable bearing strength of the
parts being connected)
Bn : Nominal strength of a bolt (nominal tensile strength,
nominal shear strength or nominal bearing strength of the
parts being connected)

: Safety factor ( = 2.00)
With the bolt numbers (n), nominal and allowable strength of a
bolted connection (Rn and Rd) may then be calculated as follows
R n  nB n
Rd  nB d
Rn
Ra  Rd 

Ra : Required strength of the connection
Rn : Nominal strength of the connection
Rd : Allowable strength of the connection
Connection is adequate if one of the following condition is satisfied :
For the connection
For one bolt
Ra  Rd or Ba  Bd
2.6 BOLTS IN TENSION
Area of threaded part is: An= 0.75Ab
Bnt=FubAn=Fub(0.75Ab)= Ab(0.75 Fub )
Fub: Tensile strength of bolt material
Ab : Gross area of one bolt
Fnt =0.75 Fub
Bdt=Bnt/Ω
:Safety factor(= 2.00)
Bat Bdt=Bnt/Ω
Bnt : Nominal tensile strength of a bolt
Bdt : Allowable tensile strength of a bolt
Bat : Required tensile strength of a bolt
Allowable Tensile Stress for Structural Bolts ( = 2.00)
Bolt
Grades
4.6
Nominal Tensile Stress
[N/mm2]
(Fnt = 0.75Fub)
300
Allowable Tensile
Stress [N/mm2]
(Fdt = Fnt /  )
150
5.6
375
190
8.8
600
300
10.9
750
375
With the bolt numbers (n), nominal and allowable strength of a
bolted connection (Rnt and Rdt) may then be calculated as
follows:
Rnt  nBnt
Rdt  nBdt
Rat  Rdt
2.7 BOLTS IN SHEAR
- 2.7.1 Bearing-Type Connections
- 2.7.2 Slip-Critical Connections
Clamping Force
Bearing
Bearing
Shear
Bearing
Shear
Bearing-Type Connection
Friction
Friction
Slip-Critical Connection
2.7.1 BEARING TYPE CONNECTIONS
Slip is acceptable; shear and bearing occur. (Common or high
strength bolts installed by snug-tight method)
SHEAR STRENGTH
Nominal shear strength of one bolt is:
Bnv=mFnv Ab
Fnv: Nominal shear stress of bolt material
Ab : Cross-sectional area of unthreaded part of the bolt
m : Number of shear planes
Fub: Tensile strength of bolt material
Based on the experimental studies, the nominal shear stress of bolt
material is given as,
Fnv  0.625Fub
Threads are in the shear plane
Threads are not in the shear plane
The test results revealed that there is no uniform force
distribution among the bolts in a connection. In bearing-type
connections, therefore, the initial reduction factor of 0.90 is
imposed on connections with lengths up to and including
965mm.
Thus: If the threads are not in the shear plane
the nominal shear stress of a bolt given in AISC 360-10 is,
Fnv  0.90(0.625Fub )  0.563Fub
If the threads are in the shear plane
the bolt area subjected to shear, denoted Av, is assumed
to be Av = 0.80Ab.
Thus, the nominal shear stress of a bolt given in AISC
360-10 is
Fnv  0.800.563Fub   0.450Fub
In the connections with common (unfinished) bolts (4.6,
5.6, etc.), the shear strength of the bolt shall be
obtained from above equation for all cases regardless of
the position of threads.
Allowable shear strength of a bolt is:
B dv
B nv mAb Fnv


,   2.00


Bdv  B av
Bnv
Bdv
Bav

:
:
:
:
Nominal shear strength of a bolt
Allowable shear strength of a bolt
Required shear strength of a bolt
Safety factor(= 2.00)
BEARING STRENGTH
Bearing strength is independent of the type of fasteners.
A possible failure mode from excessive bearing is:
Shear tear-out at the end of connected element
Rn/2=0.6FuLCt
0.6Fu
LC
t
Fu
:
:
:
:
Shear fracture stress of connected part
Clear distance from edge of hole to the connected part
Minimum thickness of part subjected to bearing
Ultimate tensile strength of the connected part
ll
Nominal bearing strength
of a bolt is:
Bnb=1.2FuLCt ≤ 2.4dtFu
1.2 Fu Lc t 
Bnb  min 

2
.
4
dtF
u 

Bnb
Bdb 

Bdb  Bad
Bnb
Bdb
Bab

Shear failure of plate
1.2FuLCt
:
:
:
:
Bearing failure of plate
Nominal bearing strength of a bolt
Allowable bearing strength of a bolt
Required bearing strength of a bolt
Safety factor(= 2.00)
s : Center to center spacing of holes
h : Hole diameter
Le: Edge distance to center of the hole
For the edge bolts
Lc=Le-h/2
For inner bolts Lc=s-h
2.4dtFu
Spacing and edge distance requirements
Minimum spacing and edge distances:
s ≥ 3d
Le ≥ 1.5d
Maximum spacing and edge distances
For corrosion protected elements
s ≤ 24t or 305mm
For non-protected elements
s ≤ 14t or 180mm
Le ≤ 12t or 150mm
t=tmin
d=bolt diameter
Nominal strength (load capacity) of a bearing-type connection is
lower of the followings;
(a) sum of the shear strengths of the bolts in the connection,
Rnv  nBnv
(b) sum of the each bearing strength of individual bolts both
end and inner bolts in the connection
Rnb  ne Bnbe  ni Bnbi
Rnv
: Nominal shear strength of the connection
Bnv
: Nominal shear strength of one bolt
n
: Total number of bolts
Rnb
: Nominal bearing strength of the connection
ne
: Number of end bolts
Bnbe : Nominal bearing strength of one end hole
ni
: Number of inner bolts
Bnbi : Nominal bearing strength of one inner hole
Thus, nominal strength of a bearing-type connection is:
Rn  min ( Rnv ; Rnb )
The connection is adequate if the following condition is satisfied:
Rn
Rd 
 Ra

Ra : Required shear strength of the connection
 : Safety factor