Sandwich Joints

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

In the course of the development of new types of sandwich towers, questions regarding the connection technology also had to be answered. For this purpose, Schaumann and Keindorf proposed a new type of connection, which is to be used as a steel sandwich tower in the favoured combination. At the DEWEK 2008, the principle of the connection was presented (see list of publications), whereby the term sandwich joint was chosen for the connection type.

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).

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 Bolts bei der vorgeschlagenen Verbindungsvariante, dem Sandwich Joint, eingespart werden.

Compared to the grouted joint, the sandwich joint has the advantage of being a double-shear pipe-in-pipe connection. This is because, in analogy to two-shear screw connections, it has two shear joints, as the comparison in the following figure illustrates. As a result, the Sandwich Joint is able to transmit twice the shear force with the same overlap length. In this case, higher tower head loads could be transmitted with the same material input. Conversely, it is possible to shorten the overlap length LÜ for the same force size F, thus saving material in the joint area (length of the additional pipe section).

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

Criteria	Advantages	Disadvantages
Material	no annular ring flanges	add. pipe section in overlap area
	no bolts	weld on add. thrust ribs
Load transfer	no eccentricity in normal force direction
	higher tolerance for imperfections
	compensation of misalignment possible
	damping through composite material possible
	less fatigue critical components
Transport		sections without dimensionally stable ring flanges
Montage		higher installation effort
Maintenance	lower than for preloaded bolts

The force transmission within the Sandwich Joints

In the context of Christian Keindorf's dissertation, stress analyses were carried out for the sandwich joint, taking into account the adhesive contact, in order to show the stresses in the filling material. For this purpose, a bending moment was applied at which the upper steel tower section (ST 355 with tST = 28 mm) reaches the yield strength (S355) on the bending compression and bending tension side. Inner and outer tube of the lower sandwich section each have a plate thickness of t-1 = t+1 = 12 mm consisting of the higher strength steel S460. Therefore, the sandwich tower section is designated SES 460 in this case.

The shear stress distribution shows that stress peaks occur at the edges of the overlap area. The curve corresponds qualitatively to the expected shape (see above, explanation of the principle).

Furthermore, the normal stresses in the steel pipes and in the filler material must be known in order to be able to dimension the sandwich joint. The second figure shows the normal stress curves over the height of the supporting structure. The stress plot shows that there is a continuous transmission of force from the upper steel tower section to the inner and outer tubes of the sandwich tower section.

At the lower transition of the sandwich joint, the assumption is confirmed that the filler material resists slippage of the upper steel tube, depending on its stiffness and compressive strength, because it has direct compressive contact with it. At this position, the compressive stresses in the filler material reach a peak value, but remain below the characteristic compressive strength. In addition, the stress curves are used to confirm further findings. These include, for example, the theoretically determined stress differences between the inner and outer pipe and the participation of the filler material in the load transfer within the sandwich tower section. Due to this involvement of the filling material in the load transfer, the inner and outer tubes of the sandwich tube are significantly relieved on the bending pressure side so that they are not stressed up to the yield point at all.

Last but not least, the sandwich joint is expected to provide greater structural damping because the steel tubes do not directly collide. The filling material between the tubes has a higher material damping than steel, which reduces the transmission of vibrations to the lower supporting structures (sandwich tower section, foundation section, anchor cage or monopile).

Further design variations of the sandwich joint

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 basically also possible offshore, whereby the sandwich joint can be used as a connection to the Monopile or central pipe of a Tripods. However, for this, the sandwich joint must meet the special design requirements of offshore towers. However, this new joint variant could save additional ring flanges and bolts. Many years of transferable experience with the required assembly technology already exist in the case of realized grouted joints for WTGs.