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Sandwich-Towers for WindEnergy Converters

P. Schaumann, C. Keindorf, Institute for Steel Construction, Leibniz Universität Hannover, ForWind - Center for Wind Energy Research

1 Introduction

The capacity in wind energy has increased significantly with-in the last few years in Europe. Due to ongoing research anddevelopments wind energy converters (WEC) get more andmore efficient and economic. However, the requirements onthe supporting structures will be also increased with thedevelopment of bigger turbines. The tower constructions areusually manufactured as tubular steels or pre-cast concretesegments or lattice structures. The structural design of tubu-lar steel towers is dominated by ultimate and fatigue limitstate. Especially the shell buckling leads to high dimensionsfor the steel sections during the design process. Therefore,lower tower sections must be assembled with shell seg-ments, which have often a thickness of 30 mm and more.This trend is in contrast to the fabrication costs, transporta-tion and steel tonnages. With regard to the next generationof WEC with bigger turbines as well as larger towers new orimproved solutions must be developed for tower sectionswith respect to stability and fatigue. This contains the choiceand combination of materials for new tower variants, theincrease of shell stability, innovative joint techniques, fabri-cation processes as well as questions to the fatigue life ofeach component.

2 A New Tower Concept

The intention of every engineer, planning steel construc-tions, is to increase the bearing capacity and if possible sav-ing weights simultaneously. With regard to axially com-pressed steel shells the use of high-strength steels could beone opportunity for this challenge. But the comparison inFig. 1 for buckling loads of cylindrical steel tower sections(ST) with various steel grades shows that only the use ofhigh-strength steels could not satisfy the intention. The sec-tion ST S235 has a length (or height) of H = 30 m, a diame-ter of D = 5.5 m and a constant shell thickness of t = 50 mm. Because the thickness is greater than 40 mmthe yield strength fy,k has to be reduced from 235 to 215 MPawith regard to EN 10025-2.

For the other tower section the shell thickness can bedecreased concerning the yield stress ratio. Thus, for exam-ple the comparable thickness for a steel shell with S460 canbe calculated to:

(1)

Therefore, the ideal buckling stresses are calculated accord-

y,k S235

ST S460 ST S235

y,k S460

f 215t t 50 24mm

f 460

DEWI MAGAZIN NO. 33, AUGUST 2008 65

EXTERNAL ARTICLE ENGLISH

P. Schaumann

66 DEWI MAGAZIN NO. 33, AUGUST 2008

ing to DIN 18800-4 [1] and for the same boundary conditions(BC 2 at the bottom and top of the sections). The bucklingreduction factor κ2 decreases with increasing the relativeslenderness λs. As result the κ2 of the ST S460 section with0.61 is significant lower as for the ST S235 section with 0.93.The ratio of utilization between the real buckling stress andthe yield stress falls from 85 % for ST S235 to 61 % for ST S460and 31 % for ST S690. Thus, only the use of high strengthsteels is not recommended for tower sections of WEC with-out any stiffeners. But nevertheless to use the increase ofstrength and reduction of weight due to high strength steelsa sandwich cylinder offers a new alternative solution fortower sections of WEC as shown in Fig. 2. This new kind ofhybrid tower consists of two steel shells which are bondedtogether with a core material. Compared to a steel towersection the shell thickness is splitted to an inner and outersteel face.

The core between the inner an outer steel face increases thestability of the shells. It works together like a sandwich orcomposite shell. Different composite shell theories are usedto estimate the stability of such double skin shell construc-tions. Within a numerical pre-design the use of high strengthsteels for the inner and outer steel face is also considered tocompare various types of tower configurations. The goal is tofind the best combination of steel faces with a core materialin the ultimate limit state. The following variants are pre-ferred:

• sandwich section with elastomer core (SES)• sandwich section with grout core (SGS)• sandwich section with concrete core (SCS)

The diameter and the shell thicknesses will be assumed con-stant over the section length such as done for the mono-coque steel section in Fig. 1. With the idea of a double skinshell construction for WEC the following facts have to beinvestigated:

• increase in shell stability• additional load capacity due to the core• using high strength steels• assembly two smaller shell thicknesses• reducing steel and overall masses• decreasing weld deposit and pre-heating• additional injection process• new hybrid tower variants• new type of connections

The goal is, to find the best combination of materials andshell thicknesses to satisfy important criteria’s in the designand fabrication phases. However, the economy would beplaying the decisive role finally. But with this new tower con-cept a new kind of hybrid tower is feasible like shown in Fig.3. This type consists of different tubular sections, where theupper section is a monocoque steel shell and the lower oneis a sandwich shell construction. Compared to a monocoqueconstruction of the same face materials, this new tower-con-cept produces structures with higher overall buckling loads

Fig. 1: Comparison of tower sections with varioussteel grades with regard to shell buckling

Fig. 2: Sandwich shells with a core material as fullspace stiffener for tower sections of WEC

and offers new types of connections between the sections,which will be presented later.

The following chapters deal with design criteria’s in the ulti-mate limit state. Especially for the shell buckling of suchsandwich cylinders a model scale test series is carried out toanalyze the influence of suitable core materials. Furthermorethe experimental results are compared to numerical simula-tions including measured geometrical imperfections. Withina parameter study the use of high strength steels for theinner and outer face is also considered to compare the vari-ous types of tower configurations and the overall masses.

3 Ultimate Limit State

Over the years a significant literature has evolved of methodsof analysis and design for sandwich constructions subjectedto various mechanical and environmental loads. An overviewof the methods and theories is included in [7]. To analyze thestability of sandwich cylinders the laminate composite shelltheory is used presented by Vinson in [7]. Therefore, effectsof anisotropy and asymmetry to the mid-plane of sandwichshell cross section can be considered. The details and theapplication of the sandwich shell theory for tower sectionare already presented in [6].

The geometry of such sandwich cylinders is shown in Fig. 3with the length L, the radius R0 of the mid-plane and the shellthickness h. The definition for deformations (u, v, w) is basedon the cylindrical coordinate system (x, θ, z). The mid-plane

of the sandwich shell is used as reference surface, which is incase of symmetry the mid-plane of the core material. Thus,the core is defined as layer 0 with the thickness t0. Thenomenclatures for the other layers with thicknesses t-1 forthe inner steel face and t+1 for the outer steel face are shownin Fig. 4.

A parameter study is carried out to check if the classical shelltheory for laminate composites in [7] is also applicable forsandwich cylinders of such tower section shown in Fig. 2. Forthe calculations the dimensions in Fig. 1 are used for themonocoque steel sections ST S235 and ST S460, which willbe compared with two configurations for sandwich towersections. The first sandwich construction is a combination ofsteel-grout-steel (SGS), where a grout is used as core materi-al. The second one is a combination of steel-elastomer-steel(SES). In this case a polyurethane with excellent bondingcharacteristics is taken into account for the core. The variousthicknesses and the material properties are summarized inTab. 1.

The criterion of shell buckling plays a decisive role for thestructural design of tower sections. Typical steel sections ofWEC with a ratio between r/t = 60 - 100 have allowable char-acteristic or real buckling stress σx,Rk which are usually lowerthan the yield stress fy,k. Herein, the real buckling stress de-pends on the relative slenderness of the shell, the type ofloading and the class of imperfection. For axial compressionthe buckling reduction curve κ2 has to be used in Germany[1]:


Fig. 3: A new kind of hybrid tower for WEC

Fig. 4: Definition of the geometry for a sandwich shell(x-axis in axial direction of the shell)


(2)

A cylindrical steel shell can be optimized so that no reductionof the yield stress is necessary as shown in Fig. 5. This level isreached if the relative slenderness is lower or equal 0.25 [1]:

(3)

Thus, for each cylindrical steel shell loaded by axial compres-sion the optimized elastic critical buckling stress σx,cr,opt can beestimated for every kind of steel grade as follows:

(4)

In this way the best configuration concerning the shell stabil-ity and utilization in the elastic range would be reached if thefollowing elastic critical buckling stresses are:

for S235: σx,cr,opt = 3760 MPafor S355: σx,cr,opt = 5680 MPafor S460: σx,cr,opt = 7360 MPa

For the sandwich shells a linear buckling analyses is carriedout to estimate the core thickness, which is necessary to getthese optimized elastic critical buckling stresses. All results ofthe parameter study are summarized in Fig. 6 to compare thecritical buckling stresses derived from numerical simulationswith values of shell theories for monocoque shells according

to [1] and composite shells presented by Vinson in [7].Therefore, the results for SGS and also for SES agree very wellbased on the chosen configuration of face and core thick-nesses. To check a wide range the core thicknesses is variedbetween t0 = 0 – 80 mm. Thus, the numerical and theo-retical results are plotted in Fig. 6.

Therefore, a sandwich shell with steel faces of S460 has anoptimized elastic critical buckling stress estimated to σx,cr,opt = 7360 MPa. This value is also plotted as limit line in Fig.6, which is crossed due to the curve for SGS S460 nearly t0 =68 mm. This is the core thickness that belongs to the opti-mized configuration for the sandwich tower section SGSusing high strength steel faces with t-1 = t+1 = 12 mm (Tab. 1).

Because the SES with an elastomer core is weaker the opti-mized core thickness would be nearly 100 mm. Since thetower sections for WEC are normally designed in the elasticrange it is not recommended or necessary to increase thebuckling stresses and core thicknesses over the optimizedvalues. For the sandwich tower section with S235 the opti-mized critical buckling stress (3760 MPa) is reached when t0 = 30 mm for SGS S235 and t0 = 35 mm for the SES S235 (notplotted in Fig. 6).

With these optimized core thicknesses the steel faces can beutilize up to the yield stress and no reduction due to shellbuckling is necessary in the elastic range (s. Eqs. 3 and 4). Acomparison for the real buckling loads in Fig. 7 shows theincrease in shell stability which is possible with sandwich

y,k y,k

x,cr,opt y,k2 2

Sx

f f16 f

0.25

y,k

Sx 2

x,cr

f0.25 for 1.0

x,Rk 2 y,kf

Type Layer thicknesst-1 / t0 / t+1

in mm

E-Moduleof core

E0 in MPa

Poissonratio

0

ST S235 50

ST S460 24- -

SGS S235 25 / t0 / 25

SGS S460 12 / t0 / 1233800 0.20

SES S235 25 / t0 / 25

SES S460 12 / t0 / 12870 0.36

Tab. 1: Parameters for the sections ST, SGS and SES

Fig. 5: Buckling reduction curve κ2 for cylindricalshells loaded by axial compression [1]


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tower sections in contrast to monocoque steel tower sec-tions.

Herein, the real buckling loads are calculated with regard toDIN 18800-4 [1] where the steel cylinder ST S235 is definedas reference type with 100 % buckling load. In comparison tothe reference cylinder ST S235 in Fig. 7 the ST S460 has a sig-nificant lower buckling load (-30 %). Thus, the ST S460would be not economic and is cancelled as an alternativesolution for tower sections.

But with sandwich tower sections a significant increase inoverall buckling loads is possible. For example the bucklingload of SES S235 is with 195 MN (+21 %) much higher as forthe reference cylinder ST S235. The value for SGS S235 iseven 204 MN which is an increase of 27 % compared to thereference type. It has to be mentioned that the bucklingloads correspond to the bearing capacity in the ultimate limitstate, since no reductions due to the overall shell bucklingare necessary. Therefore, the load capacities of the corematerials are additionally considered in Fig. 7. Herein, thecompressive strength of the grout material is much higher asfor the elastomer core. Thus, the ratio of load capacity ishigher for the SGS S235 as for the SES S235. The values areestimated in the elastic range according to [2] for compositestructures. But this national standard is not applicable for allstructural design calculations of the sandwich constructionbecause of the higher slenderness the cylinders can not be

declared as composite column structures. However, thestress-strain relations can be used and they are valid up tothe limit of elasticity which is also the design limit for sup-porting structures of WEC. Furthermore, with regard to theeconomy the additional load capacity of the grout core isbeneficial or even necessary to justify the extra costs for thiscore material.

With these assumptions a further increase would be possiblewith the configuration as SGS S460. In this case the steelfaces can be also loaded up to the yield strength and the loadcapacity of the core material increases according to thestress strain relation and due to the higher core thickness.This type of tower section is very interesting because simul-taneously to the increase in bearing capacity a reduction inoverall mass is possible as shown in Fig. 8.

The intention of every planning engineer to increase loadcapacities coupled with saving tonnage, as formulated in theintroduction, seems to be possible with sandwich shells fortower sections of WEC. The comparison of tonnage in Fig. 8is based on the following mass densities:

Steel: ρS = 7850 kg/m³Grout: ρG = 2280 kg/m³Elastomer: ρE = 1150 kg/m³

The ST S235 with 50 mm shell thickness has been defined asreference tower section with 100 % tonnage again. The use

Fig. 6: Comparison of critical buckling stresses for S460depending on the core thickness

Fig. 7: Increase of buckling loads with sandwich towersections

of high strength steel leads to a reduction, the ST S460weights only 48 %. But the buckling load of this type is to lowin contrast to the reference cylinder (s. Fig. 7) and is can-celled for an alternative design study.

The higher buckling loads for SGS S235 and SES S235 are onlypossible with additional tonnages due the core material. Butthe use of the high strength steel S460 in combination witha grout material as core the tonnage can be decreased (-13%). Together with the increase in buckling loads (+45 %) theSGS S460 is a more lightweight structure with great shell sta-bility compared to the ST S235 and offers a very interestingnew alternative solution for tower sections.

However, in the comparison it has also to be taken intoaccount that two cylindrical steel shells for one sandwichtower section have to be manufactured which produceshigher costs. Additionally the costs for the injection processof the core material have to be considered. On the other sidethere are saving in costs for welding possible because thevolume for the seam welds decreases in square with the shellthickness. In comparison to a reinforced concrete tower sec-tion a sandwich tower section has the advantage that thesteel faces function as formwork shells during the productionprocess.

The design study above is carried out only for axially com-pressed tower sections, but also the other loads of WEC such

as bending and torque has to be taken into account. The nod-ding moment, the torsion and the thrust of the turbine dominate the stresses in the tower sections. Therefore thelaminate shell theory presented in [7] is also applicable. Forexample the buckling due to torsion and a comparisonbetween the buckling loads for various monocoque andsandwich tower sections is already presented in [6].

In contrast to conventional composite structures whereshear connectors are used the forces between the layers of asandwich cylinder should be transferred over adhesion. Thiscriteria that the adhesive bonding between the steel facesand the core is ensured at the whole contact area in the elas-tic range up to yield stress of the steel faces, has beenassumed for the design study above. Whether this assump-tion is justified was the interest of a buckling test series at theInstitute for Steel Construction of the Leibniz UniversityHannover. The experimental results of this test series arepresented in the next chapter.

4 Shell Buckling Tests

A model scale test series with sandwich cylinders was carriedout to analyze the shell buckling and the influence of differ-ent core materials. The test specimens are loaded by uniformaxial compression. The deformations and strains are meas-ured online by optical 3D sensors to localize critical zones.The buckling tests were carried out on a 600 kN servo


Fig. 8: Comparison of tonnage between the sections


hydraulic testing machine. The test setup is shown in Fig. 9.

The test specimen has a top and a bottom plate at the endsand is vertical positioned between the supporting elements.As core three different injectable materials were tested. Thefirst grout was the SikaGrout 311 from Sika DeutschlandGmbH, the second one was V1/10 from Pagel SpezialbetonGmbH, both consist of mineral components. Additionally anelastomer (two-component polyurethane) from ElastogranGmbH was used as core material. The most importantmechanical properties of all core materials are listed in Tab.2.

The values for the module of elasticity E0 are taken from thedatasheets of the companies. In contrast to both grout mate-rials the elastomer has a significant lower value, thus theelastomer core is much weaker. The values for the compres-sive strength after 1 and 28 days were measured in the labo-ratory. The high early-strength is important for injectionprocesses in situ. Since two different grout materials basedon mineral components were tested, the type SGS (steel-grout-steel) gets the extensions SGS_s for Sika and SGS_p forPagel. As steel material the X2CrTi12 was used for inner andouter steel shells. The yield stress of this steel grade corre-sponds to a S235.

The geometric data of the tested cylinders are summarized inTab. 3. All test specimens have a length (height) of 700 mm.

In addition to the buckling tests with sandwich cylinders theinner and outer steel shells were also used for stand alonetests as steel cylinders. Therefore, the inner steel shell with0.7 mm was ST_1 and the outer steel shell with 0.8 mm wasST_2. These thin steel faces were chosen to get a high slen-derness for elastic shell buckling.

The layer configuration for the sandwich cylinders is fixeddue the geometry of inner and outer steel shell. Thus, thecore has a thickness of 10.9 mm. It has to be mentioned thatthe core thickness can not be too thin for the model scaletests because the core materials must be injectable.Therefore, the maximum grain size for both grouts was 1mm and for the elastomer core significant lower as 1 mm,which is an advantage for this composite material. The injec-tion processes are shown in Fig. 10.

The injection process of the elastomer was carried out withbottom and top plate to ensure a closed cavity. The elas-tomer was filled in with an inlet valve. At the outlet valve avacuum pump was used to support the injection. After 10minutes coupling processes of the two components of theelastomer began. During this exothermic reaction the elas-tomer expands (6 % of core volume). Due to this expansionit is ensured that the elastomer is bonded at the whole con-tact area to the steel faces.

The injection of both grout materials was carried out without

E0 fck after1day

fck after28 day

Company Corematerial

[MPa] [MPa] [MPa]

Sika Grout 311 37000 28 78

Pagel V1/10 35300 39 91

Elastogran Elastomer 870 18 18Tab. 2: Parameters of core materials [4]

Fig. 9: Test setup for sandwich shell buckling tests

the top plate (s. Fig. 10). A closed cavity was in these casesnot necessary. The shrinkage of the grout materials can becompensated with additive. All injection processes could becarried out without any problems. Some cuts at the sand-wich cylinder after the buckling tests approved the correctbonding without any holes or leaks.

The buckling tests were carried out after the optical meas-urements of geometrical imperfections for all test specimensand the injection processes. Both sandwich cylinders withgrout as core material (SGS_s and SGS_p) were tested oneday after injection. All buckling tests were carried out dis-placement controlled. During the tests the strains weremeasured using strain gauges attached to the outer surfaceof the cylinders. The displacements in axial direction wererecorded online by inductive sensors. The applied axial forcewas measured with a load cell.

All test results of shell buckling are summarized in Fig. 11.The axial force could be increased for the sandwich cylinders.The buckling loads of all sandwich test specimens were overthe limit of elasticity of the steel faces which is estimated to178 kN (Npl,ST_1 = 75 kN from inner steel face and Npl,ST_2 =103 kN from outer steel face).

The buckling loads of SGS_p and SGS_s are very high (356and 306 kN). The higher value for SGS_p can be explainedwith the higher early-strength of the Pagel Grout V1/10 com-

pared to the Sika Grout 311 in Tab. 2.

These test results attest that the grout materials participateat the bearing capacity as known from composite columns.For both sandwich variants with grout the same post buck-ling behavior can be observed as the steel variants. The sud-den drop (collapse) in bearing capacity is typical for bucklingmodes of shells under axial compression. But in contrast tothis the SES with an elastomer core has a very good postbuckling behavior. This kind of stability based mainly on theexcellent bonding characteristics of the elastomer whichcould also transfer the forces between the layers in the plas-tic range of the steel shells. Furthermore, it can be recog-nized that the nonlinearity of the SES-curve started near 180kN which corresponded approximately to the limit of elastic-ity for the steel faces. This can be explained with the lowerstiffness and the lower compressive strength of the elas-tomer core. It is weaker compared to the core with groutmaterials.

The stability of sandwich shells can be mainly optimized withthe thickness, the module of elasticity and the compressivestrength of the core material. Since elastic shell bucklingcould be avoided other failure modes occurred in the plasticrange of the sandwich cylinders for example face wrinkling asshown in Fig. 12.

Face wrinkling can occur in a sandwich construction either


LengthL

RadiusR0

Layert-1 / t0 / t+1

Type ofcylinder

[mm] [mm] [mm]

ST_1 72.55 0.7

ST_2 84.20 0.8

SGS_s

SGS_p

SES

700

78.40 0.7 / 10.9 / 0.8Tab. 3: Geometry of test specimens

Fig. 10: Injection of core materials for SES and SGS

Fig. 11: Buckling tests with steel and sandwich cylinders[4]

Fig. 12: Face wrinkling of a sandwich cylinder (SGS)

Fig. 13: Failure modes for sandwich constructions [8]

Fig. 14: Comparison of ultimate axial force for all testedcylinders


when subjected to a compressive buckling or in the compres-sive face during bending (s. Fig. 13). A wrinkle that becomesunstable causes an indentation in the core if the compressivestrength of the core is lower than the tensile strength.

The second mode of face wrinkling is possible if the wrinklecauses a gap between the core and the faces if the tensilestrength of the core is lower than the compressive strength.Whichever case applies, a poor adhesive core will undoubt-edly reduce the allowable wrinkling stress of the sandwich.In Fig. 12 it can be recognized that the second mode of facewrinkling occurred where the wrinkle cause a gap betweenthe core and the steel faces. Outside of the area of face wrin-kling the bonding was intact for all tested sandwich cylinders.

As result the shell stability of sandwich constructions couldbe ensured up to the limit of elasticity with sufficient bond-ing behaviors for all tested core materials. The increase inbuckling loads was very high and the steel faces could bestressed over the yield strength. Furthermore, the additionalbearing capacity due to the core materials in the elasticrange offered a further increase in buckling loads comparedto monocoque steel constructions.

In addition to the buckling tests numerical simulations werecarried out to analyze the sandwich shell buckling modes andto compare it with test results. In contrast to typical geomet-rically and materially nonlinear buckling analysis with includ-ed imperfections (GMNIA) these numerical simulations wereexecuted with measured real geometrical imperfections. Theresults have a good agreement with the test results (Fig. 14).As theoretical values for the sandwich shells the axial force at

the strain of failure was chosen. Therefore, the proportion-ate axial forces of each layer were determined with the fol-lowing equation based on the measured stress-strain-rela-tions.

(5)

The calculation of the theoretical results for SGS and SES arealso summarized in Fig. 14. The values for Ntheory agree verywell to the measured axial loads (Nexp) compared for the pointat failure (displacement, strain). The comparison shows thatthe consideration of additional bearing capacities due to thecore materials is valid, which offers a good performance con-cerning the sandwich shell stability of tower sections.

Conclusions

Alternatively to a steel tower section for WEC a sandwichtower section was analyzed with regard to the stability. Atfirst a comparison between cylindrical shells with varioussteel grades showed that a shell with high strength steels hasa lower buckling load if the shell thickness is reduced accord-ing to the ratio of yield stresses. Thus, a monocoque steelshell with S460 or S690 would be not economic without anystiffeners and is not recommended as an alternative solutionfor tower sections of WEC. But with sandwich shells in com-bination with high-strength steels a significant increase inoverall buckling loads is possible.

The sandwich shell consists of an inner and outer steel face,which were bonded to a suitable core material betweenthem. Two grout and one elastomer core were investigated.

theory 1 1 0 0 1 1N A ( ) A ( ) A ( )



A parameter study with linear buckling analyses showed asignificant increase in shell stability for sandwich cylinders.Therefore, the inner and outer steel faces could be loaded upto the yield stress considering an optimized core thickness. Inthis case the core materials operated as full space stiffenerand produced an increase of critical buckling stresses. Thegoal was to find the best combination of steel faces with acore material in the ultimate limit state for sandwich towersections of WEC. Due to the reached plastic buckling loadsthe combination of high-strength steels is in principle possi-ble to get tower sections, which will be optimized withregard to stability and weight. The design study was carriedout only for axially compressed tower sections, but also theother loads of WEC such as bending and torque has to betaken into account.

Several buckling tests were carried out to check the bondingcharacteristics and ultimate bearing capacities of sandwichcylinders. The test series showed a significant increase inbuckling load capacity, which also depends on the compres-sive strength of the core materials. The failure criteria for allvariants of tested sandwich shells is more a local failure dueto face wrinkling in the plastic range and not due to an over-all shell buckling. As result the shell stability of sandwich con-structions could be ensured up to the limit of elasticity withsufficient bonding behaviours for all tested core materials.

Furthermore, a comparison between theory, experiment andsimulation was carried out. As result the consideration ofadditional load capacities due to the core materials was valid.Thus, the sandwich shells in combination with high-strengthsteels could be offered a new alternative solution for towersections of WEC.

However, the fatigue limit state must be also considered.Therefore, a method for post weld treatment is preferred toincrease the fatigue strength of the steel faces. Furthermore,new joint techniques are also needed for hybrid tower con-structions with sandwich sections. This will be an objectiveof a presentation at the DEWEK 2008.

Acknowledgements

The authors would like to thank the German companiesBRUGG Rohrsysteme GmbH, Elastogran GmbH, Pagel Spe-zialbeton GmbH as well as Sika Deutschland GmbH for thetest specimen and core materials. Further thanks should alsobe given to SIAG AG which manufactured the test setup. Inaddition the authors would like to acknowledge for thefinancial support of the project GROW (grout structures forOWEC) provided by the German Federal Ministry for theEnvironment, Nature Conversation and Nuclear Safety. Theauthors are also grateful to the Institute of Building Materialsand Institute of Materials Science of the Leibniz UniversityHannover for material tests on the steel and core materials.

References

[1] DIN 18800-4 (1990): Stahlbauten - Stabilitätsfälle, Schalenbeulen,NABau im DIN e.V., Beuth-Verlag.

[2] DIN 18800-5 (1990): Verbundtragwerke aus Stahl und Beton -Bemessung und Konstruktion, NABau im DIN e.V., Beuth-Verlag.

[3] DIN EN 1993-1-6 (2007): Bemessung und Konstruktion von Stahlbauten– Teil 1-6: Festigkeit und Stabilität von Schalen, Eurocode 3, BeuthVerlag.

[4] Schaumann, P.; Keindorf, C. (2007): Structural Design of Hybrid-Towersfor Wind Energy Converters, Paper No. 05 of the ConferenceProceedings of the EWEC, Brussels

[5] Lam, K. Y. and Loy, C. T. (1998): Influence of boundary conditions for athin laminated rotating cylindrical shell, Elsevier Composite Structures,Vol. 41, pp 215-228.

[6] Schaumann, P.; Keindorf, C.; Matuschek, J. and Stihl, T. (2006): Schalen-beulen von Sandwichzylindern mit einem neuen Elastomer als Ver-bundwerkstoff, Stahlbau, Vol. 75, No 9, pp 748-753.

[7] Vinson, J. R. (1993): The behaviour of shells composed of isotropic andcomposite materials, University of Delaware, USA, Kluwer AcademicPublishers.

[8] Zenkert, D. (1997): An introduction to sandwich construction, Warley,West Midlands, Engineering Materials Advisory Services, Ltd.

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