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The Design of Concrete TanksRobin Atkinson of CCP Consulting Engineers compares traditional methods of designing reinforced

concrete rectangular tanks with modern computer based methods and discusses how existing software

can be developed to make the whole design process more efficient.

TANKS have traditionally beendesigned by reference to

published tables derived fromelastic thin plate theory.

These tables cover isolatedrectangular panels with various

proportions, and edge conditionsand loading as shown below.Interpolation is required and thevalues cannot represent the realinteraction between adjacent wallsand the base. The base rarely

provides the assumed fixity and itis not easy to calculate theseaffects accurately.

Engineers now have access topowerful computers and suitablesoftware which should enablethem to design tanks moreaccurately as complete structuresand show worthwhile savings.This has not become a routine

method however because the timespent generating the structure andloadings and dealing with theoutput can be such that manytanks can still be designedeconomically and quicker byhand.

It is pointless to aim for theextreme accuracy of a PerfectAnalysis for a concrete structurewhen the variability of factorssuch as ground conditions and

panel thickness can affect theresults by as much as 20% insome cases. The aim, therefore, is

to develop a practical and quicktheoretical analysis procedure thatcan give results to between 5%and 10% of a Perfect Analysis.

The following methods canmeet these criteria and show that a

balance can be struck betweenaccuracy and practicality.

Grillage MethodThis method subdivides each

wall and slab panel of the tankinto a grillage of rectangular

beams which are connected at thepanel junctions.

The beams are given propertiesrelating to their orientation,spacing and the panel thickness.The support conditions are applied

at the base panel nodes as springswhich can be specified to suit

piles or the stiffness of theground. The loads are appliedonto the beams.

The above model producesacceptably smooth momentdiagrams and the results have anerror of less than 4%. They can

be plotted as shown in the cut-away view below or againstsections through the structure.The values must be divided by the

beam width to produce values permetre width.

The grillage pattern can bemade coarser to reduce the inputtime and model size but at theexpense of some loss of accuracyand smoothness of the results plot.This can be more practical for

large multiple tank structures.

Finite Element MethodThis method subdivides each

panel into a mesh of smallelements. The element thicknessis specified and the supports orsprings can be added at nodes inthe same way as for the grillage.The pressure loads on the panelscan be applied by specifying theintensity on the panel as a whole

Finite Element Analysis (FEA)programs can produce a colouredresults value contour plot as

shown in the following diagram.

The results from FEA canimprove on the accuracy andsmoothness of the grillage methodand can be presented with values

per metre width.In some FEA programs the

results plots are based on the

average or centre of element valueso it is important in these cases touse small elements at panel joints.The coarse mesh option thereforeis not always appropriate withFEA but the speed of meshgeneration and loading inputoutweighs this.

The FEA method can showvery high local force values at

point loads such as at pilesupports so these need to bemodelled carefully to reflect theirreal width and load spread.

ComparisonsThe grillage method uses

general purpose 3-D Space Frameanalysis software which is usedregularly by Engineers in manydesign offices, whereas the FEA

programs are more specialised andare not used as generally. So thiscan limit its use to the largerorganizations.

The preparation of the grillagemodel is not as fast as FEA meshgeneration because the beamsmust be defined for each beamspacing and panel thickness

condition.The application of uniform

loading is equally fast by bothmethods but hydrostatic loading ismuch faster by FEA by virtue ofits full panel loading facility.

A Pentium computer can nowanalyse the above FEA model inless than 5 minutes so run timesare no longer a real issue.

The output from the grillagemodel is more familiar toStructural Engineers but it doesneed to be converted manually toshow the results per metre width.

This requires a degree of vigilanceby the Engineer.

DevelopmentsThe grillage method needs to be

automated further so that thedesigner can specify the panelwidth and beam spacing and letthe computer calculate the beam

properties and output the results ina per metre width format. Theloading input method also needsto be enhanced to allow globallyvarying loads to be applied to the

panel as a whole.FEA programs which are to be

used for tank analysis should beable to access and plot the resultsat the element boundaries and plotmoment diagrams.

Both analysis methods wouldbenefit from a library of tank

models which could be modifiedor multiplied by simple input fromthe Engineer.

The most valuable benefithowever, would be the automationof the reinforcement detailing

process. The results could easilybe fed through a post processor tocalculate the requirements to BS8007.

The Engineer could specify themain parameters and preferredspacing and the computer couldthen produce reinforcement

proposal diagrams for review.

The information could then bepassed to the detailing program toproduce drawings to a standardformat similar to the wallelevation below.

The details would then bechecked and edited wherenecessary for holes and items notcovered by the main tank analysis.

It is quite possible therefore, toautomate the design process formany tanks and bring cost

benefits to the water industry.One final note: Engineers;

dont throw out your tablesbecause with all this computersophistication, someone has stillgot to check the result!Grillage used QSE SPACE

FEA used STAAD III