The sandwich joint than two sleek tube-in-tube connection

With the development of novel Sandwich Towers also raise questions about the connection technology had to be answered. Therefore a new connection type was proposed by Schaumann and Keindorf , which should come as steel sandwich tower for use in the favored combination. On the DEWEK 2008, the principle of the compound was presented (see list of publications), which sandwich joint was chosen for the connection type of the term.

It is a double-shear tube-in-tube connection, in which the upper steel tower section will be collected between inner and outer tubes of the already established sandwich tower section. Therefore, the upper portion of the core layer may not be filled at the factory to the upper steel pipe can be pushed up to a certain overlap length. Only after the upper steel tower section is vertically aligned and fixed, the filling of the gap made with a composite in-situ (see figure below).

New type of connection between steel and sandwich tower sections

The principle and structural behavior of a sandwich joints

Within the overlap region, the friction forces and moments are transmitted from the upper steel tube over the composite material in the inner and outer tubes of the sandwich tower section. The power transmission takes place via shear forces in the composite material, as is the case in a similar manner with grouted joints. So far as the composite material not only serves as a stiffening but also as a force-transmitting element between the tower sections. As a result, two annular flanges and a plurality of screws, in the proposed connection variant, the sandwich joint, can be saved.

Compared to the grouted joint of the sandwich joint has the advantage that it is one of the two sleek tube-in-tube connections. For in analogy to two-shear bolted connections he has two shear bands, as it illustrates the comparison in the following figure. As a result, the sandwich joint able to transmit the thrust for the same double overlap length. In this case, higher tower head loads could be forwarded with the same material used. Conversely, it is possible to shorten the overlap length LÜ with the same amount of force F, which material is saved in the connection area (length of the additional length of pipe).

Force transmission in the analogy to one and two-shear bolted connections

Advantages and disadvantages of the sandwich joints against ring flange

Unlike bolted ring flange connections can use the sandwich joint misalignments and imperfections, such as Runouts are equalized. Furthermore, doing away with some fatigue-critical areas (hot spots), because no screws and ring flanges are available. Also, no maintenance is required to control the bolts to possibly biasing force losses. On the other hand, the assembly work at sandwich joint is rated higher. Therefore, the advantages and disadvantages for a better overview are tabulated.

Advantages and disadvantages of the sandwich joints against ring flange

The force transmission within the Sandwich Joints

As part of the Doctoral thesis of Christian Keindorf stress analysis for the sandwich joints were done in consideration of the adhesion contact to show the stress in the filler material. To a bending moment was applied, in which the upper steel tower section (ST 355 = 28 mm with TST) reaches the yield strength (S355) on the bending pressure and Biegezugseite. Inner and outer tubes of the lower sandwich section each have a sheet thickness of t-1 = t +1 = 12 mm consisting of the higher-strength steel S460. As such, the sandwich tower section is referred to in this case with SES 460.

By means of the shear stress distribution can be seen to occur at the edges of the overlap area spikes. The curve corresponds qualitatively to the expected form (see above, Notes to the principle). 

Furthermore, the normal stresses must be known in the steel pipes and the filling material in order to make a sizing for the sandwich joint can. For this purpose, the normal stress gradients are shown in the second image over the height of the support structure. Based on the stress plots can be seen that a continuous power transmission from the upper steel tower section takes place to the inner and outer tubes of the sandwich tower section.

At the bottom of the sandwich transition joints, the assumption is confirmed that the filling material against the sliding of the upper steel tube has a resistance as a function of its stiffness and compressive strength, providing as it has direct pressure contact with the latter. At this position, the compressive stress reached a peak value of the characteristic, however, remains below the compressive strength in the filling material. Also, be confirmed by the stress gradients further insights. For example, these include the theoretically calculated stress differences between the inner and outer tube and the participation of the filling material on the load transfer within the sandwich tower section. Due to this involvement of the filler load transfer at the inner and outer tube of the sandwich pipe be relieved significantly to the bending pressure side so that they are not stressed to its yield point.

Not least, increased structural damping is expected by the sandwich joint, because the steel tubes are not pushed each other directly. The filler material between the tubes has a higher material damping than steel, whereby the transfer of oscillations to the lower load-bearing structures (sandwich tower section, foundation section, anchor basket or monopole) can be reduced.

Shear stress distribution in the filling material [dissertation Keindorf, 2010]
Normal stress gradients in the steel pipes and the filling material [dissertation Keindorf, 2010]

Further design variations of the sandwich joint

Different material combinations for sandwich joint
Further design variations (onhsore and offshore)

On the basis of experimental results in this  thesis by Christian Keindorf  come for the sandwich tower sections that are to be connected with steel tower sections, several material combinations considered. In the variant with an elastomer filling no shear ribs and adhesion promoters are required because the tested composite material has an extremely high tensile bond strength on grit blasted steel surfaces.

Alternatively, a grout with adhesion-promoting additives in the connection area are used, which also should have a good pump and fluidity and high compressive strength. For this variant, shear web, recommended as the grouted joints to the strut bearing capacity to significantly increase both in the ultimate limit state and serviceability limit state of fatigue. This effect was demonstrated by measurement / 2009 explained in detail in Bautechnik Heft 11/2009 , with reference to the experimental results, a design proposal was derived. 

In an onshore location, for example, can a foundation section a frictional transmission between sandwich tube and foundation are obtained. In this case ("reverse power flow") the forces and moments are analogous to the connection to the top steel tower section is initiated.

The use of a sandwich tower section is in principle also possible offshore, with offers of sandwich joint for connection to the central tube of a monopile or tripod. For the sandwich joint, however, the specific design requirements of the offshore towers must meet. However, further annular flanges and screws could be saved by this new connection variant. For the required mounting technique already exist many years of transferable experience in unrealized Grouted Joints for offshore wind turbines.