Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/222250032

“NewNaturalDraftCoolingTowerof200mofHeight,”

ARTICLEinENGINEERINGSTRUCTURES·DECEMBER2002

ImpactFactor:1.84·DOI:10.1016/S0141-0296(02)00082-2

CITATIONS

24

READS

618

4AUTHORS,INCLUDING:

W.B.Krätzig

Ruhr-UniversitätBochum

108PUBLICATIONS632CITATIONS

SEEPROFILE

Availablefrom:W.B.Krätzig

Retrievedon:30October2015

Engineering Structures 24 (2002) 1509–1521www.elsevier.com/locate/engstruct

New natural draft cooling tower of 200 m of height

Dieter Buscha, Reinhard Harteb, Wilfried B. Kratzig c,∗, Ulrich Montagd

a RWE Solution AG, Kruppstrasse 5, 45128 Essen, Germanyb Department of Civil Engineering, University of Wuppertal, Pauluskichstrasse 7, D-42285 Wuppertal, Germany

c Department of Civil Engineering, Ruhr-University Bochum, D-44780 Bochum, Germanyd Kratzig & Partner Engineering Consultants, Buscheyplatz 11-15, D-44801 Bochum, Germany

Received 22 January 2002; received in revised form 17 May 2002; accepted 29 May 2002

Abstract

In the years 1999 to 2001 a new natural draft cooling tower has been built at the RWE power station at Niederaussem, with200 m elevation the highest cooling tower world-wide. For many reasons, such structures can not be designed merely as enlargementof smaller ones, on the contrary, it is full of innovative new design elements. The present paper starts with an overview over thetower and a description of its geometry, followed by an elucidation of the conceptual shape optimization. The structural consequencesof the flue gas inlets through the shell at a height of 49 m are explained as well as the needs for an advanced high performanceconcrete for the wall and the fill construction. Further, the design and structural analysis of the tower is described with respect tothe German codified safety concept for these structures. Finally, the necessity of extended durability of this tower is commented,the durability design concept is explained in detail and illustrated by virtue of a series of figures. 2002 Elsevier Science Ltd.All rights reserved.

Keywords: Natural draft cooling towers; Reinforced concrete shells; Design for durability

1. Introduction: the new 965 MW lignite powerblock at Niederaussem

The RWE Energie AG, the largest German electricityproducer, has operated since 1961 a lignite power plantat the small village of Niederaussem, 20 km west of Col-ogne. This power plant is composed of eight singlepower blocks with a total capacity of 2700 MW. Thesingle power stations possess net degrees of efficiencyfrom 31.0% (1961) to 35.5% (1974), depending on theirindividual ages.

Starting in 1998, a new power block is under construc-tion with an intended net capacity of 965 MW of elec-tricity (gross capacity: 1027 MW). Because of increasingenergy prices in Europe and of growing consciousnessof limited natural resources, this new station is equippedwith highly innovative novel technologies in order toachieve an utmost degree of efficiency. After com-

∗ Corresponding author. Tel.:+49-234-322-9051; fax+49-234-321-4149.

E-mail address: wilfried.b.kraetzig@sd.ruhr-uni-bochum.de(W.B. Kratzig).

0141-0296/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0141-0296 (02)00082-2

pletion, it will possess with clearly over 43% the highestelectrical net degree of efficiency (gross degree: 48.5%)of all fossil fueled (coal and lignite) power plants world-wide, and it likewise will become the largest lignitepower block in the world.

One of the required innovative technology steps ofthis new power station is an increased steam temperature(580°C) and pressure (270 bar) at turbine entrance, thenext a reduced pressure in the condenser, both stepsrequiring a considerably larger amount of cooling water.Solely these two single measures raise the net degree ofefficiency of the plant by 2.7%, compared to latest stan-dard technologies. This increase requires a remarkablyenlarged cooling component, namely a natural draft coo-ling tower, 200 m high, by expectation the tallest coolingtower and the largest shell structure in the world. It isanticipated that such a tower increase in height and coo-ling capacity may finally enhance the total net efficiencyof the electricity generation towards 45%. Clearly, natu-ral lignite resources will be preserved thereby, and theamount of carbon dioxide released into the environmentwill be reduced considerably. But design and construc-tion of such giant tower required a series of innovative

1510 D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

structural technology steps which probably will start anew cooling tower generation. Fig. 1 gives a survey overthe existing power station with the new block added bycomputer visualization.

2. The world’s highest cooling tower

2.1. Geometry of the tower structure

Due to Fig. 2, the total height of the cooling tower is200 m. Its base diameter measures 152.54 m, that oneof the tower shell 136.00 m, and the top opening is 88.41m wide. Both the outer and inner shell surfaces possessareas of more than 60 000 m2 equivalent to over 10 soc-cer fields each.

The shell structure is composed of two hyperbolicshells of revolution both meeting at the throat, and exhi-bits in its main parts a wall thickness between 0.22 and0.24 m, increasing towards the lower shell rim. The toprim is stiffened by an edge member of U cross-section,extending into the interior of the tower shell by 1.51 mwith a shank-height of 1.20 m. In order to reduce crack-ing-sensibility due to wind vibration, this edge memberis pre-stressed by four SUSPA tendons with eight mono-wires of 150 mm2 cross-section each of steel quality1 570/1 770 N/mm2. The lower edge member is formedby a thickening of the shell up to 1.16 m. Fig. 3 offersan impression of both edge members. The complete shellis constructed of a special acid-resistant high-perform-

Fig. 1. Computer vision of the future lignite power plant Niederaussem.

Fig. 2. Overview over the geometry of the cooling tower.

ance concrete of 85 N/mm2 of compression strength,called ARHPC 85/35 as explained later.

The cooling tower shell is supported by 48 meridionalcolumns 14.68 m high, built of reinforced concrete C45/55 to Eurocode EC 2. Their thickness ranges from1.16 m on top to 3.10 m above foundation, their width is1.40 m. All columns have been founded on a reinforced

1511D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

Fig. 3. Details of lower and upper edge member of the tower shell.

concrete base ring of dimensions 6.60 m×1.80 m, restinggenerally on rather well consolidated gravel soil. Softersoil had to be exchanged, and along some areas—at thewater inlets and the water outlet—the ring-width had tobe enlarged, leading to a rotationally non-symmetricfoundation.

The further tower components are big but rather con-ventional. The interior of the tower is captured by thelarge water basin for collection of the re-cooled water.Its basin plate and walls consist of water-proof concreteC 30/37 0.20 m thick, founded on 0.15 m of concretebase layer C 12/15 over an anti-freeze layer of 0.30 m.The fill construction and the water distribution aredesigned as a prefabricated reinforced concrete beam-column structure, also made of high-performance con-crete ARHPC 85/35. A brief outline of the completestructure is delivered in [4].

2.2. Conceptual design and shape optimization oftower meridian

Cooling towers of such size cannot be consideredmerely as extrapolations of smaller ones. At such largedimensions, the shape of the meridian in interaction withthe loading conditions is of much higher importance forthe states of stress (structural safety), for the initiationof concrete cracking (durability), for the elastic stability(overall stiffness) and for the vibration properties(dynamic load amplification) of the structural response,compared to smaller towers. In such latter cases, shapesmay by all means be selected unfavorably without disad-vantages, since their design is governed more pronounc-edly by minimum code requirements, like minimum wallthickness and minimum reinforcement. For large highcooling tower shell, the shape-finding process in the con-

ceptual design phase thus is of highest importance andhas to balance a series of different aspects to an opti-mum solution.

Generally as illustrated in Fig. 4, the meridional shapeof a hyperboloidal cooling tower shell consists of a lowerand an upper hyperbola branch, which both meet at thethroat. The hyperbola axis need not correspond with thetower axis. Thus the curvature of the meridian variesover the tower height, in general with a maximum at thethroat. As has been pointed out by Kratzig and Zerna[15], maximum size as well as uniformity of the distri-bution of membrane stresses, consequently the load levelof crack initiation, the safety against instability, and thenatural frequencies—as intensity measures of thedynamic response—are severely effected by the shapeof the meridian: Greatest possible uniformity of the cur-vature influences favorably all mentioned aspects, seeBusch et al. [5].

Total tower height h, column height hC and lower shellradius rL are generally fixed by the thermal design, like-wise the throat radius rT with small admissible varia-bility. The upper shell radius rU must be not smaller thanrT for reasons of an unperturbed steam flow into theenvironment. All other parameters in Fig. 4 are freewithin certain design limits, and can be selected in orderto optimize the above mentioned aspects including con-struction and architectural points of view. Since theupper shell parts above the throat are of minor impor-tance in this context, we will concentrate solely on thelower hyperbola branch.

As one observes from Fig. 4, the angle bL of the shellinclination at the lower rim is restricted by

Fig. 4. Basic parameters of a natural draft cooling tower.

1512 D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

Fig. 5. Shape-finding process: dependence of min f on hT and bT.

tanbL�(rL�rT) / (hT�hC).

The sign of equality herein designates the smallest poss-ible value of bL, at which limit condition two conicalfrusta (with straight generatrices) meet at the throat in abreak point of infinite curvature. The maximum anglebL is limited by the maximum possible inclination of theform-work system for the shell construction, by experi-ence noticeable below �20°. It is an interesting fact thatmost of the above mentioned technical aspects improvefor bigger bL, except for the aesthetics of the structure:A cooling tower generally is perceived as more pleasantfor medium values of bL.

Such exemplary variations of basic shell parameterscan be seen from Figs. 5 and 6. They show—as partresults of the shape optimizations [3]—the lowest naturalfrequencies min f (all for circumferential wave numbern=5) and the lowest elastic buckling safeties min n (aGerman codified design condition requires n�5) for thegeneral tower geometry from Fig. 2. Both design para-meters generally improve for higher bL. In these pre-design studies the shell openings and the thickened wallparts around both holes have not been considered, inorder to maintain rotational symmetry of the structure.While examining Fig. 5 one should be aware that anincrease of the lowest natural frequency from a certain

Fig. 6. Shape-finding process: Dependence of min n on hT and bT.

level by 10% will reduce the dynamic wind action byeven more than 10%.

The total amount of shell reinforcement in Fig. 7,designed on basis of the guideline VGB [18], is determ-ined by the usual minimum requirements of 0.3% of Ac

in meridional and circumferential direction, and—fordurability reasons—by 0.6% in circumferential directionin the upper half of the shell, distributed on both sidesof the shell cross-section. Clearly, this parameter influ-ences the economy of the tower. Because of identicalwall thickness of all variants, the minimum constructionrequirements for the reinforcement have been selectedidentically for all designed variants in Fig. 7. Theobserved increase of reinforcement there is a clear indi-cation of the growing unevenness and rising peak valuesof the tension stresses in the shell, as the angle bL

reduces and consequently the throat height hT increases.Consequently from Figs. 5 to 7, unfavorably shaped

cooling towers may exhibit up to 40% more steelreinforcement. In strong wind velocities, they thus willsuffer greater tension stresses at lower gale velocities,leading to earlier and wider crack-damages in the shell.This will probably result in a shorter life duration, ashas been demonstrated in Harte and Kratzig [10] for arecently constructed cooling tower.

After a series of systematic pre-design analyses [3],the following optimized parameters of the shell middlesurface had been fixed for the tower to be executed. Withregard to the general hyperbola equation

r(z) � r0 � �{1 � (hT�z)2 /b2},

we finally have chosen for the lower (upper) branch:

r0 � �1.0730 (42.3828) m

a � 43.7030 (0.2472) m

b � 105.5967 (7.9419) m

hT � 142.0000 (142.0000) m.

Fig. 7. Shape-finding process: dependence of total shell reinforce-ment on hT and βT.

1513D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

Both hyperbola branches meet at the throat with continu-ous (zero) tangent. At the lower rim, angle bL as wellas inclination of the column axes measure 17.8°. Thiscooling tower variant has been pictured in reduced scaleon Fig. 2.

2.3. Inlets for cleaned flue gas

An interesting detail of this new tower is the inlet ofthe cleaned flue gas stream into the shell by two tubesmade of glass-fiber reinforced resin, both 6.50 m diam-eter at an axial height of 49.00 m above ground. Asshown in Fig. 2, this requires two openings of 9.00 mwidth in the shell at an axial distance of 19.0°�14.00 m.

In all German operating power plants, the flue gas hasto be cleaned for sulfur- and nitrogen-oxides; in Nied-eraussem this is executed by chemical washing pro-cesses. This cools down the flue gas temperature from240°C to �80°C. In order to avoid re-heating of the cle-aned flue gas for release over the classical smoke-stack,the latter is mixed to the cooling tower vapor and therebydistributed into the environment. Thus, a smoke-stackis saved, but necessary for the gas inlet are those twomentioned neighbored openings. They will generallycause stress concentrations, and weaken the shell wallto an important extent by reducing the lowest bucklingsafety as well as lowest natural frequency of the coo-ling tower.

In order to keep these perturbations of the shellresponse small, the washed flue gas has, up to now, beenguided from the release of the purification plant, here49.00 m above ground, down to the lower shell rim.There over the water distribution, they were led into theinterior of the tower, which causes considerable velocitylosses of the flue gas stream. For efficiency reasons, thepipes in Niederaussem were conducted at same heightstraight and with free spans—Fig. 1—into the shell,requiring both holes in Fig. 2.

Consequently, both tubes carry heavy loads into theshell, each 2000 kN in vertical and ±400 kN in horizon-tal direction. To counteract all degrading shell effects,the surroundings of both openings were re-strengthenedby thickening of the shell wall up to 45 cm and by con-siderable additional reinforcement. Both measures aimedat a recovery not only of the critical natural frequenciesand buckling safeties of the unperturbed shell, but alsoof the original mode shapes. Fig. 8 demonstrates theresult by comparison of the lowest elastic vibrationmode: Frequency as well as vibration mode shape of thefinal solution matches nearly perfectly with those onesof the hole-free shell.

But in spite of this re-strengthening, noticeableresponse effects of the shell openings remain in thetower construction. Strong non-axisymmetric effectswere added to the original non-symmetric soil con-ditions, leading to additional shell bending. To reduce

Fig. 8. Comparison of lowest natural vibration modes with/withoutflue gas inlets.

these deficiencies, intermediate ring-stiffeners—like ontop of the shell—were considered during the design pro-cess, as proposed for example by Form et al. [7] orGould and Guedelhoefer [9]. But although such ringstiffeners tend to equalize all states of stress over thecircumference, they finally had been rejected for econ-omical reasons.

2.4. High-performance concrete

The most spectacular new construction element of thiscooling tower is the earlier mentioned high-performanceconcrete, especially developed for the shell and the fillconstruction.

In spite of the chemical gas-washing process, the cle-aned flue gas still contains low concentrations of SOx

and NOx. As a consequence of the flue gas injection intothe vapor, the inner face of the upper shell will beattacked chemically by low concentrated acids with pH-values from 3.5 to 6.0 or just by condensed steam, aslong as in winter-service conditions the condensationpoint lies within the shell wall. Both corrosive fluidswill, in winter months, permanently attack the concreteof the inner shell face over the planned service lifetimeof 55 years.

The classical counteraction against such corrosiveattacks in Germany is by coating the inner tower surfacewith co-polymeres on acrylic-venyl-resin basis, asdescribed in Engelfried [6]. This however does not seemto be manageable in the present case, mainly because ofthe limited life-duration of the curing of �12 years. Inview of the few and short service breaks of modernpower stations, a necessary multiple re-curing of theextremely large inner shell surface of more than 60 000m2 seems impossible. In order to completely excluderehabilitation measures of the concrete, a new high-per-

1514 D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

formance concrete with high acid resistance has beendeveloped and successfully placed in-situ, see Hillemeierand Huttl [12] or Lohaus [16].

The innovative element of this concrete is anextremely dense package of the aggregate and a ratherlow amount of cement. As far as possible, water is sub-stituted by additional portions of liquifier, and the micro-scale package is improved by additives of micro-silicaslurry; for details see Hillemeier and Huttl [12]. Thisspecially designed and carefully tested mixture does notviolate the classical properties of cooling tower concrete,namely high early-strength, high structural density andhigh resistance against frost (Gould and Kratzig[8]).

The extremely dense package automatically leads toa concrete with high compression strength of fcm�85N/mm2. On the other hand, Young’s modulus Ec and thetension strength fctm had to be controlled at values of aC 35/45 in order to limit thermal stresses and reducecrack-widths on the shell exterior. Basic properties ofthis concrete measured in-situ are given in Table 1. Sincethey vary between those of a C 85 (compressionstrength) and a C 35/45 (stiffness and tensile strength),the new concrete mixture has been named SRB-ARHPC85/35 (SaureResistenter Beton—Acid Resistant HighPerformance Concrete).

3. Structural analysis and design

3.1. Design loads

Structural analysis and design of the cooling tower isbased on the German design regulations VGB-BTR [18].Main loading conditions for these structures are deadweight G, wind load W and thermal actions T. Becauseof the location of the plant-site, also seismic excitationsE had to be considered in the design. The highlyadvanced surveying and controlling in German coolingtower technology admitted a complete suppression ofinitial imperfections during the design.

In the guideline VGB-BTR [18], wind loads W aredescribed in detail for isolated towers. The basic valuesof the dynamic design pressure q0(z) for W for the decis-ive German wind zone II—maximum 3 s peak velocitieswith 50 years of return period—follow the exponentiallaw

q0(z) � 0.90(z /10)0.22,

Table 1In-situ mean values of material properties of SRB-ARHPC 85/35

Compression Tensile strength Bending tensile Modulus ofstrength (N/mm2) strength elasticity(N/mm2) (N/mm2) (N/mm2)

82.03 2.88 6.31 40 400

in which z describes the height above ground. To derivethe pressure distribution on the outer surface of the shellfrom q0(z), this function has to be multiplied by one ofthe normalized, dimensionless circumferential pressuredistribution functions cp(q) from [18]. Depending on thesurface roughness of the wind ribs, the distribution func-tion named K 1.4 has been selected:

qa(z,q) � jq0(z)cp(q).

The dynamic amplification factor f has been evaluatedto 1.07. As usual in Germany, internal suction is con-sidered only in the limit state of instability.

As clearly demonstrated in Fig. 1, the present toweris not an isolated building, it hence may be influencedseverely by wind interference actions of other coolingtowers or boiler houses of neighboring blocks. As afurther design basis for the wind loading of the new Nie-deraussem cooling tower, a series of wind tunnel investi-gations in the boundary layer wind tunnel of the Ruhr-University in Bochum have been carried out, which aredetailed in Busch et al. [5]. Special attention therein wasgiven to all negative (load-increasing) effects of the sur-rounding situation for the new tower. Positive effects,such as sheltering caused by existing buildings, wereneglected because of the long service life of the newplant and the possibly lower ages of existing ones.

All this finally led to 24 sets of different, direction-dependent wind loading conditions. Each loading con-dition consists of the axi-symmetric pressure distributionof the isolated tower, and alternatively of a non axi-sym-metric wind pressure distribution considering the powerstation environment. Decisive for the design was themore unfavorable alternative of both, respectively.

The temperature loading T in VGB-BTR [18] isdefined with air temperature differences of 25 K for stan-dard service conditions (warmer inner surface) and 45K for winter service conditions, the latter for examplefor air temperatures of �15°C outside and +30°C insidethe tower. In addition, sun radiation of 25 K (warmerouter surface) has to be considered. The seismic loadingconditions follow the German Standard DIN 4149.

3.2. Stress analysis and safety concept

The final design analysis of stresses and deformationsfor all single load cases has been computed within stan-dard linear finite element techniques, using the softwaresystem Femas (see Beem et al. [2]). Due to the direction-dependence of the wind loads caused by interference ofthe surrounding building environment, further due to theflue gas injection and due to uneven foundation and soilconditions, there existed no axi-symmetry in the FE-model. The computer model with a total of 50 919degrees of freedom, which has been selected for the finaldesign, comprises the shell, the supports, the foundationand the soil stiffness. No formal adaptive analysis has

1515D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

been performed, since more than 4 years of design workhave required detailed local model refinements until thedesired quality had been gained. Fig. 9, a 20% resolutionof the final FE-model, gives an impression of theapplied discretization.

Readers who are interested in the evaluated states ofstress in the tower shell for single load cases, are referredto the response maps in Gould and Kratzig [8]. Therethe main membrane forces and bending moments of apre-design version of this cooling tower (without fluegas inlets) have been portrayed.

As codified in VGB-BTR [18], the cooling tower hasbeen analyzed and designed for the following limitstates:

� the limit state of serviceability applying total safetyfactors g � 1.75 ÷ 2.10 for concrete in compressionand γ=1.71 for steel, applied to the following loadcombinations

G � W,

G � W � T,

G � W/3 � T � E,

G � 0.70W � T for proof of crack widths

limitation with T for standard service conditions;

� the failure limit state, applying partial safety factorsgG=1.00, gW=1.75 for the actions, and gms=1.00 forsteel, gmc=1.50 for concrete on the resistance side, for

G � 1.75W,

G � 1.75W � T;

� the instability limit state for the load combination

l(G � W),

Fig. 9. FE-mesh for final tower design including shell, supportsand foundation.

requiring minimum stability safeties of l�5.0 due toDIN 1045. According to VGB-BTR [18], the windloading W in this limit state contains also the influ-ence of internal suction.

The evaluation of the safety checks and the determi-nation of the required reinforcement is executed com-pletely computer-internally. For the failure limit state,the 2nd load combination can be omitted, since the loadcarrying influence of temperature gradients T is rathersmall, see Fig. 19, while their deformation influence maybe rather pronounced. The crack widths in the shell andtheir supports are generally limited to 0.2 mm.

Fig. 10 sketches the amount of reinforcement steelBSt 500 S, evaluated on basis of the mentioned limitstates, for the cooling tower shell. Because of the direc-tion-dependent wind loads, there have been defined twosectors with different meridional reinforcement. Theamount of circumferential steel is equal in both sectors,and additional reinforcement is required for the thick-ened shell wall around the flue gas inlets.

As usual, both faces of the shell are reinforced by anorthogonal mesh, arranged in symmetric manner. Toavoid initiation of vertical cracks, the circumferentialbars are placed outside, the meridional ones inside. Thespacing of the latter is arrange in 4-bar groups with dis-tances from 9.2 cm to 13.0 cm in the lower shell quarter,followed by �19.0 cm in the remainder. Their splicingis staggered at regular intervals, while the splicing of the

Fig. 10. Sector-dependent reinforcement distribution in the shell.

1516 D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

circumferential bars is random. The distances of circum-ferential bars vary from 8.2 to 20.0 cm. The minimumconcrete cover is 3.0 cm. Fig. 11 reports on constructionworks at the shell.

4. Design for durability

4.1. Crack-damage protection by multi-level-simulation technique

As explained in Section 2.4, the novel acid-resistanthigh-performance concrete ARHPC 85/35 forms aninnovative material platform for durability extension.But in order to really achieve a cooling tower withextended durability properties, this new material has tobe combined with advanced design and constructiontechniques.

One of the observed weaknesses of earlier coolingtower shells world-wide is cracking of the outer surfacein meridional direction. This phenomenon is due towind, thermal and hydro-mechanical effects in combi-nation with insufficient circumferential reinforcement,appearing then at rather low wind load levels. If in avirgin (uncracked) cooling tower shell wind loads in

Fig. 11. Scaffolding, reinforcement and concreting of the shell.

combination with temperature initiate crack-damage, theset of natural frequencies generally will decrease. Thisespecially holds true for the lowest frequencies, whichgovern the tower’s dynamic response behavior. Thus thehighly-tuned dynamic system of a cooling tower will beshifted towards the center of the peak of the wind spec-trum, for further increase of dynamic wind excitations.Consequently, such shift will probably lead to enlarge-ments of the existing crack-damage, both in terms ofcrack-lengths and crack-widths in the sense of a pro-gressive damage process (see Harte et al. [11]).

If we are aware that each face of the cooling towershell exceeds the area of 60 000 m2, it is evident thatintensive crack-repairs during the service lifetime of thetower, probably after a couple of gales, are generallyout of any question: The design has to guarantee crack-freedom up to rather high wind speeds, with sufficientlow probability of appearance, for long periods of ser-vice life. Advanced cooling tower design concepts thusrequire simulation of crack-damage for typical load com-binations.

Such evaluation of crack-damage exceeds by far theactual standard design technologies, requiring computersimulations of at least the materially nonlinear responsebehavior of the tower. In order to master such time-con-suming analyses, the original FE-model has beenreduced to 4222 degrees of freedom, neglecting the fluegas inlets as well as the foundation, and considering thetower due to its wind loading as an isolated structure.We have repeatedly experienced that numerical resultsfor towers with properly strengthened openings are in aglobal sense closely comparable to those without holes,as has been recently confirmed by Waszczyszyn et al.[19]. Consequently, we present here the results for a pre-design prototype of the Niederaussem cooling tower,alternatively intended to be built in concrete C 35/45.Since crack-initiation as well as final failure of the toweris mainly due to local tension failure, the gained resultswill differ only marginally from those expected for thefinal design in ARHPC 85/35. The reinforcement of thisprototype tower for crack simulation is determined alsodue to chapter 3.2 and mapped in Fig. 12, and thematerial data is given in Table 2. In all other respects,the tower geometry corresponds to the executed designof Fig. 2.

For such crack-damage simulations, reinforced con-crete has to be modeled as a two-component composite[13]. In this project we applied 6-parameter “assumedstrain” shell elements with a layered structure. Hereinthe material is modeled in 3D for concrete and by 1Dsteel layers (see Soric et al. [17]). Nonlinear phenomenaof the response have been considered in the kinematicsand of course in the material models, namely:

� cyclic elastic-plastic stress–strain behavior for con-crete in compression with a micro-damage component

1517D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

Fig. 12. Amount of reinforcement of shell and supports for proto-type tower.

Table 2Material properties for prototype tower (normal concrete)

Concrete C 35/45 (EC 2)Young’s modulus: Ec=34 000 N/mm2

Poisson’s ratio: n=0.2Compression strength: fcm=35.00 N/mm2

Tension strength: fctm=2.67 N/mm2

Reinforcement steel BSt 500 S (DIN 488)Young’s modulus: Es=2.1×105 MN/m2

Yield strength: fym=5.0×102 MN/m2

Tension strength: ftm=5.5×102 MN/m2 at strain limit:esu=0.01

after transgression of concrete strength, as describedin Bazant and Kim [1];

� linear-elastic behavior in the concrete tension rangewith brittle cracking due to the Rankine-stress-criter-ium, applied smeared-crack regularization;

� cyclic elastic-plastic stress-strain behavior of thereinforcement bars in tension and compression includ-ing the Bauschinger-effect;

� elastic-damaging bond behavior between reinforce-ment and concrete including bond-slip, cross-checkedby an experimentally gained tension stiffening curve.

This material model, which has to be evaluated at eachintegration point of each element-layer, is described indetail in Kratzig et al. [14]. During the incremental-iter-ative solution process of the structural analysis, an evalu-ated increment of the nodal degrees of freedom has tobe transformed from the global structural level down tothe single finite elements. There it is converted intostrain increments of single layers, and then solved—byapplication of the above mentioned respective constitut-ive laws—for the stress increments, which are then trans-formed back to the tangential stiffness and the internalnodal forces on global level. Theoretical background andfurther details of this rather complicated “multi-level-simulation-technique” can be found in Zahlten [20] oragain in Kratzig et al. [14]. Within this project, we reporton monotonic crack simulations for selected load combi-nations G � lW and G � lW � T, applying the designwind W on G respectively G � T by increase of l upto structural failure.

4.2. Discussion of results

The achieved results are documented first as load-dis-placement plots of the points A, B and C along the stag-nation meridian as marked in Fig. 13. Displacementsnormal to the middle surface, due to dead weight G andincreasing wind load W, are plotted as function of the

Fig. 13. Points of investigation for prototype tower.

1518 D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

Fig. 14. Load-displacement plots for G � lW.

wind load factor l in Fig. 14. Because of absence oftemperature, this gale response depicts the tower in “out-of-service” conditions. Obviously, there appears a linear,essentially crack-free response phase of the tower until

Fig. 15. Computed crack patterns for G � 1.0W and G � 1.5W(W=design value).

Fig. 16. Computed crack patterns for G � 2.0W and G � 2.3W(W=design value).

l�1.2, e.g. for wind intensities slightly higher than thedesign value l=1.0 (50 years return period). It was oneof the goals of this durability design, to extend the crack-free phase of the tower shell to a highest possible level,without few additional reinforcement and no thicknessincrease. After l�1.2, a gale with approximately 100years of return period, obviously wide-spread crackingstarts with intensive stress re-distributions as observedin Fig. 14, finally leading to failure at G � 2.37W.

Simultaneously to certain wind loading levels, corre-sponding states of crack-damage of the shell are evalu-ated and visualized on Figs. 15 and 16. Of course, thesingle line-elements there are no real cracks, they ratherstand for intensities and directions of cracks to beexpected from the assumed smeared crack model. Cracksof smaller crack-width than 0.05 mm are suppressedtherein by numerical filtering. This nonlinear crack-dam-age analysis uses comparable basic assumptions as thenow-a-days applied standard design techniques forcrack-width limitation, but its recognition is much moredesign-oriented.

1519D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

Fig. 17. Exaggerated deformations of the tower (factor: 40).

Fig. 15 proves the expectation that up to l=1.0 nocracks will appear in the shell. First cracking evolvessoon after l�1.2 and is evident at l=1.5. Fig. 16 givesan impression of the states of (computed) crack-patternsin the shell for storms of pressure intensity of 2.0,respectively 2.3, the design one, the latter corresponds toa 50% increase of the design wind velocity, immediatelybefore failure. As one observes from both Figures, forload combinations G � lW the cracking in the shell israther low, even for such extreme over-load factor likel=2.30. This is clearly a result of the careful structuralshape-finding optimization during the conceptual designphase, see Section 2.2 and Bischoff [3], and of fewreinforcement re-designs after first simulation results.Fig. 17 illustrates the deformation pattern of this pathto failure.

Fig. 18 then maps the displacements of the samepoints A, B and C from Fig. 13, if ultimate service tem-perature �T=45 K is added. Such tower state corre-sponds to a storm attack during “cold-winter-night-ser-vice” conditions. As expected, the thermal gradients

Fig. 18. Load-displacement plots for G � �T45 � lW.

Fig. 19. Load-displacement comparison for G � lW and G ��T45 � lW for point B.

introduce a considerable “pre-damage” into the structure.The comparison with the response of the “cold” toweron Fig. 19 for point B shows, that at l�1.2 the displace-ment of the “warm” tower is at least twice as high as of

Fig. 20. Computed crack patterns for G � �T45 � 2.0W and G ��T45 � 2.3W.

1520 D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

the “cold” one. Consequently, around point B the secant-stiffness has degraded to around 50% of the original one,caused by greater crack-damage. Although crackingstarts earlier in winter conditions and develops moreintensively than in out-of-service conditions, the failureload is influenced only marginally because of similarglobal crack-patterns in both cases closely before failure.This recognition can be confirmed by comparison ofboth crack-damages in Figs. 16 and 20.

As shell-experts will realize, the wind load factorsl=2.37, respectively 2.31, at failure are rather high, com-pared with the required partial safety factor for windl=1.75 in [18]. This fact is due to design details of thetower, namely slight circumferential over-reinforcementsin the upper shell allowing for nonlinear stress re-distri-butions, and the excellently optimized shape of the shell.We feel obliged to remark, that the wind failure loadfactor l�3.3 of a Polish tower, evaluated in [19], israther unrealistic for unknown reasons (erroneous refer-ence loads, reinforcements, material models).

5. Concluding remarks

The cooling tower of the new power block at the RWEelectricity station Niederaussem presently is the highestcooling tower in the world at 200 m (see Fig. 21). Ithas just been completed and will start service—with thecomplete power block—in mid 2002. This high-tech-

Fig. 21. View of the completed cooling tower shell in January 2000.

nology power station component exhibits a series of newinnovative design elements, namely the careful shape-finding pre-design of the shell, the high positioned fluegas injection, and the novel acid-resistant high-perform-ance concrete SRB-ARHPC 85/35. Intensive nonlinearcomputer simulations of the anticipated damage evol-ution and repeated re-designs have been executed inorder to maintain the structure’s durability over the fullservice life of 55 years. This paper has reported on thesefeatures, which all have contributed to a landmark engin-eering structure, as part of an advanced highly efficientelectric energy plant with savings of �30% of fossil fuelcompared to power stations of the eighties.

References

[1] Bazant PZ, Kim S-S. Plastic fracturing theory for concrete. ASCEJ Engng Mech Div 1979;105:407–28.

[2] Beem H, Konke C, Montag U, Zahlten W. Femas —FiniteElement Moduln allgemeiner Strukturen. In: User’s ManualRelease 3.0. Bochum: Institute for Statics and Dynamics, Ruhr-University; 1996.

[3] Bischoff M. Pre-design and optimization of a natural draft coo-ling tower. Diploma thesis (in German), Institute for Statics andDynamics, Ruhr-University Bochum, 1995.

[4] Busch D, Harte R, Kratzig WB, Montag U. World’s tallest naturaldraft cooling tower, near Cologne, Germany. IABSE—StructEngng Int 2001;11:107–9.

[5] Busch D, Harte R, Niemann H-J. Study of a proposed 200 mhigh natural draught cooling tower at power plantNiederaussem/Germany. J Engng Struct 1998;19:920–7.

[6] Engelfried R. Surface protection measures for cooling towershells for REA operations. In: Wittek U, Kratzig WB, editors.Natural draught cooling towers. Rotterdam: Balkema; 1996. p.199–206.

[7] Form J, Krings W, Mazur H, Peters HL. Analysis and construc-tion of ringstiffened RC cooling tower (in German). Beton- undStahlbetonbau 1980;75:205–12.

[8] Gould PL, Kratzig WB. Cooling tower structures. Chap. 14 In:Chen WF, editor. Handbook of structural engineering. BocaRaton: CRC Press; 1997. p. 14.1–14.32.

[9] Gould PL, Guedelhoefer OC. Repair and completion of a dam-aged cooling tower. J Struct Engng ASCE 115(3):576–93.

[10] Harte R, Kratzig WB. Large-scale cooling towers as parts of anefficient and ecologic energy generating technology. Submittedto Thin-Walled Structures.

[11] Harte R, Kratzig WB, Noh S-Y, Petryna YS. On progressivedamage phenomena of structures. Computational Mechanics2000;25:404–12.

[12] Hillemeier B, Huttl R. High performance concrete—an exampleof acid resistance. BFT—Beton- und Fertigteiltechnik2000;28:52–60.

[13] Hofstetter G, Mang HA. Computational mechanics of reinforcedconcrete structures. Braunschweig: Vieweg, 1995.

[14] Kratzig WB, Konke C, Mancevski D, Gruber K. Design for dura-bility of natural draught cooling towers by life-cycle simulations.J Engng Struct 1998;20:899–908.

[15] Kratzig WB, Zerna W. Resistance of hyperbolic cooling towersto wind and earthquake loading. In: Pister KS, editor. Structuralengineering and structural mechanics. Englewood Cliffs, NJ:Prentice-Hall; 1980. p. 419–45.

[16] Lohaus L. High-performance concrete—an alternative to coating.

1521D. Busch et al. / Engineering Structures 24 (2002) 1509–1521

In: Wittek U, Kratzig WB, editors. Natural draught cooling tow-ers. Rotterdam: Balkema; 1996. p. 207–13.

[17] Soric J, Montag U, Kratzig WB. On increase of efficiency ofcomputational algorithms for elasto-plastic shell analysis. Engin-eering Computations 1997;20:75–97.

[18] VGB. Guideline. Structural design of cooling towers. Technicalguideline for the structural design, computation, and execution ofcooling towers. Essen: VGB Technische Vereinigung der Groß-kraftwerksbetreiber, 1990.

[19] Waszcyszyn Z, Pabisek E, Pamin J, Radwanska M. Nonlinearanalysis of a RC cooling tower with geometrical imperfectionsand a technological cut-out. J Engng Struct 2000;22:480–9.

[20] Zahlten W. A contribution to the physically and geometricallynonlinear computer analysis of general reinforced concrete shells.Technical Report No. 90-2, Ruhr-University Bochum, 1990.