Ring flange connections as an L-or T-variant

Segment approach for the design of annular flanges [Seidel, 2001]

Ring flange connections are used in all cases where application tubular sections or components are to be positively connected with each other. They represent an alternative if welds are not suitable due to technical or economic criteria.

For Ring flange connections prestressed bolts are usually used, which have the usual dimensions of M12 to M30 in steel construction. 

In the field of wind energy even Bolts are used up to a size of M64 (see size comparison for bolted connections). For example, schedule preloaded Ring flange connections are carried out between the steel tower sections for wind turbines. This Ring flange connections are exposed both in static and in dynamic ways extreme loads. Ring flanges are usually unilateral than with L-shaped flanges, internal cross-section angeordent (see figure at right). In the foot and flanges of T-shaped cross-section are used.

Many other applications for Ring flange connections are in mechanical and plant engineering.

Assessment in the ultimate limit state

FE-model of the ring flange segment
Stress plot for tensile stress

Evidence of ultimate bearing capacity of Ring flange connections is usually the most highly loaded segment which is cut out of the overall system. This segment approach is illustrated in the figure above. The tensile force in the shell plate of the segment is obtained by integrating the stresses. Advantages are the relatively low computational cost and availability of analytical models. A disadvantage is that the spatial structural behavior can not be mapped. 

The viability of the excised segment depends depending on the dimensions of the load capacity of the flange and / or the carrying capacity of the bolt. For the failure of the bolt above all their tension resistance shall prevail.

The calculation of total models is very complicated, but in principle leads to a more economical Assessment, since rearrangement effects can be captured.

Assessment in the limit state of fatigue

General screw force Traction History [SCHAUMANN et al. 2007]

In the border state of fatigue there is usually an elastic system behavior. The transfer function of the bolt force FS as a function of the tower bending moment MR or the proportionate strength casing plate Z has on the flexural tension side of a nonlinear course on (see figure at right). It is possible to identify four areas:

Area 1:   Linear Gradient, reduction of the prestressing                    force at fully depressed contact zone

Area 2:   Successive gaping of the Fugue

Area 3:   Gaping connection ("edge support")

Area 4:   Progressive plasticization up to fracture

 

In addition to FE methods exists a variety of different analytical method for describing the Bolts force curve, which reference is made at this point to the relevant technical literature.

Ring flanges and their structural behavior under consideration of imperfections

The SKI Ingenieurges. mbH employs inter alia to with the structural behavior of Ring flange connections, taking into account imperfections. The following failure modes are first to be distinguished: 

  • Failure mechanism A: Bolt failure
  • Failure mechanism B: Bolts failure and plastic hinge in the shell plate
  • Failure mechanism C: Plastic hinges in the casing plate and flange

Ring flanges can be analyzed with the help of segment models or even half models, with prestressing forces can be controlled individually each bolt.

 

 

 

 

 

 

 

 

 

 

 

 

The structural behavior differs significantly between the bending compressive and flexural tensile side because a gaping may occur under tensile stresses. For the FE model contact elements have been implemented. As a result, the following voltages on the scope set.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In order to investigate the influence of imperfections on the structural behavior of RFL is preferred by SKI, the half model. As imperfection mainly come into question:

  • Gaping of the flange
  • Gaping tube side
  • Parallel gaping

The FE model was validated with experimental results in the framework of the dissertation of Dr.-Ing. M. Seidel were obtained at the Institute for Steel Construction of the Leibniz Universität Hannover. The load curves show a very good agreement, see comparison in the chart between segment model (blue curve) and experimental results (red curve). The third Curve (green) is used as a reference model for a different ring flange, which was then probed with a rubber seal.