Steel tubular structures with its unique advantages like
convenience in construction, less land space occupation, aesthetic merit and
unambiguous force transfer are increasingly widely used for transmission and
distribution poles. For splicing steel tubular structures, bolted flange plate
connection is considered as one of the important and practiced solutions. Besides
experiments, many investigations including theoretical study and numerical simulation
have been done in the past. To verify related design codes for flange-plate
connections, an experimental investigation on 63 specimens of unstiffened connections
for CHS (Kato et al., 2005). In the theoretical study, typically
T-stub analysis and yield line theory are used to develop suitable design
models (Wang et al., 2013). During an experimental investigation, theoretical
study and numerical simulation, the behavior of specific topic like bending
behavior of flange plate, or study of prying action, or behavior of
flange-plate under axial load are studied. These studies have given a strong
foundation for further research and study. However, for practical application,
simplified design steps need to put forward.

1.1.1 Method
of Analysis:

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There are two methods of analysis,
elastic design and plastic design. For the

design of steel structures in the ultimate load criteria,
plastic analysis is used. When the structure resists the applied load
continuously till it yields, it is known as plastic condition. The
stress-strain curve for mild steel and idealized stress-strain response for
plastic analysis is shown in Figure 1.
In plastic analysis, while determining the ultimate load capacity, strength of
the joints is of major concern. Plastic hinge is the section where the
structure has reached the yield stress and it will not deform on any further
additional load. The formation of plastic hinges either in the joint or in the
member is based on the strength of the joints and consequently, it decides the
collapse mechanism. If hinge is to be developed at the joint, then the joint
should be detailed with adequate ductility to resist rotation. The basic
assumptions of plastic theory of analysis are as follows:

a)    
The material shows a lower yield point and can
undergo considerable strain without any increase in the stress as shown in
Figure 1.

b)    
Plane section before bending remains plane even
after bending.

c)    
The relationship between compressive stress and
compressive strain is same as that of tensile stress and tensile strain.

d)    
When fully plastic moment is attained at a
section, a plastic hinge is formed, which can undergo rotation of any
magnitude.

e)    
Effect of axial and shear force on the plastic
moment capacity is neglected.

f)     
The deflections are considered so small that the
equations of static equilibrium hold good as if its undeformed structures.

1.1.2 Failure
Cases:

The cross-section of T-shape connection is identical to
the circular tubular flange connection. There are three typical failure cases
of T-shape connections as shown in Figure 2. (F. Huang et al., 2017):

a)     When
the flange plate is relatively thick, the bending deformation of the flange
plate is small and the bolts are pulled off as shown in Figure 2. (a). In this
case, the bolts have achieved the ultimate stress while the flange plate is in
elastic condition. Prying action does not exist in this mode of failure.

b)     When
the deformation of the flange plate is equal to that of the bolts, a plastic
hinge is formed at the weld line and the bolts are pulled to failure as shown
in the Figure 2. (b).

c)     When
the flange plate is relatively thin, the deformation of flange plate is larger
than that of the bolts as shown in the Figure 2. (c). The plastic hinge is
formed at the flange plate both at the weld line and bolt line and in this
mode, flange plate bends in parabola while no bolts yield.

1.1.3 Yield
Line Method of Analysis:

Yield line method of analysis is a simple and efficient
method of calculating the collapse load of relatively thin plates of
rigid-perfectly plastic material. The plate deforms plastically at collapse
load and separated by segments connected by plastic hinge lines, known as yield
lines.

1.1.4 Prying
Action:

When the bolts are subjected to tension, the center line
of bolts acts as a hinge and the tensile force pushes the plate between bolts in
the upward direction and pushes the edge part of the plate outside bolt in the downward
direction as shown in Figure 3. The phenomena of plates to push downwards is known
as prying action. The force required to resist this action is known as prying force.
The additional prying force is added to applied load to give total bolt force. The
prying action can be seen more in the unstiffened connections than in the stiffened
connections. As stiffeners share part of the tensile load, prying action is less
in the stiffened connections. In other words, in stiffened sections, two transverse
and two radial yield lines shares part of the tensile load, the prying action is
less.