Steel tubular structures with its unique advantages likeconvenience in construction, less land space occupation, aesthetic merit andunambiguous force transfer are increasingly widely used for transmission anddistribution poles. For splicing steel tubular structures, bolted flange plateconnection is considered as one of the important and practiced solutions. Besidesexperiments, many investigations including theoretical study and numerical simulationhave been done in the past. To verify related design codes for flange-plateconnections, an experimental investigation on 63 specimens of unstiffened connectionsfor CHS (Kato et al., 2005). In the theoretical study, typicallyT-stub analysis and yield line theory are used to develop suitable designmodels (Wang et al., 2013). During an experimental investigation, theoreticalstudy and numerical simulation, the behavior of specific topic like bendingbehavior of flange plate, or study of prying action, or behavior offlange-plate under axial load are studied.
These studies have given a strongfoundation for further research and study. However, for practical application,simplified design steps need to put forward.1.1.1 Methodof Analysis:There are two methods of analysis,elastic design and plastic design. For thedesign of steel structures in the ultimate load criteria,plastic analysis is used. When the structure resists the applied loadcontinuously till it yields, it is known as plastic condition.
Thestress-strain curve for mild steel and idealized stress-strain response forplastic analysis is shown in Figure 1.In plastic analysis, while determining the ultimate load capacity, strength ofthe joints is of major concern. Plastic hinge is the section where thestructure has reached the yield stress and it will not deform on any furtheradditional load. The formation of plastic hinges either in the joint or in themember is based on the strength of the joints and consequently, it decides thecollapse mechanism. If hinge is to be developed at the joint, then the jointshould be detailed with adequate ductility to resist rotation. The basicassumptions of plastic theory of analysis are as follows:a) The material shows a lower yield point and canundergo considerable strain without any increase in the stress as shown inFigure 1.b) Plane section before bending remains plane evenafter bending.c) The relationship between compressive stress andcompressive strain is same as that of tensile stress and tensile strain.
d) When fully plastic moment is attained at asection, a plastic hinge is formed, which can undergo rotation of anymagnitude.e) Effect of axial and shear force on the plasticmoment capacity is neglected.f) The deflections are considered so small that theequations of static equilibrium hold good as if its undeformed structures.
1.1.2 FailureCases:The cross-section of T-shape connection is identical tothe circular tubular flange connection. There are three typical failure casesof T-shape connections as shown in Figure 2. (F. Huang et al., 2017):a) Whenthe flange plate is relatively thick, the bending deformation of the flangeplate is small and the bolts are pulled off as shown in Figure 2.
(a). In thiscase, the bolts have achieved the ultimate stress while the flange plate is inelastic condition. Prying action does not exist in this mode of failure.
b) Whenthe deformation of the flange plate is equal to that of the bolts, a plastichinge is formed at the weld line and the bolts are pulled to failure as shownin the Figure 2. (b). c) Whenthe flange plate is relatively thin, the deformation of flange plate is largerthan that of the bolts as shown in the Figure 2. (c). The plastic hinge isformed at the flange plate both at the weld line and bolt line and in thismode, flange plate bends in parabola while no bolts yield.
1.1.3 YieldLine Method of Analysis:Yield line method of analysis is a simple and efficientmethod of calculating the collapse load of relatively thin plates ofrigid-perfectly plastic material. The plate deforms plastically at collapseload and separated by segments connected by plastic hinge lines, known as yieldlines.1.1.4 PryingAction:When the bolts are subjected to tension, the center lineof bolts acts as a hinge and the tensile force pushes the plate between bolts inthe upward direction and pushes the edge part of the plate outside bolt in the downwarddirection as shown in Figure 3.
The phenomena of plates to push downwards is knownas 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.
Theprying action can be seen more in the unstiffened connections than in the stiffenedconnections. As stiffeners share part of the tensile load, prying action is lessin the stiffened connections. In other words, in stiffened sections, two transverseand two radial yield lines shares part of the tensile load, the prying action isless.