AESTHETICSOFBRIDGE DESIGN

Fritz LeonhardtProfessor, Technische Hochschule StuttgartStuttgart, Germany

There is an old Latin saying: "Degustibus non disputandum est—onecannot argue about taste." In fact,people's tastes can be very different,especially in aesthetics. Thus it ap-pears as if there could not be anyclear judgment, as far as aestheticsare concerned, whether a buildingor a structure can be consideredbeautiful or not. Views on beautydiffer widely, especially in architec-ture. I think, however, that these dif-ferences in judgment are mainly be-cause most people never spend muchthought on why they consider onebuilding beautiful and why they dis-like another one. Few of us havebeen taught the laws of beauty andwe have never trained ourselves tofind out why we like to see one ob-ject while we dislike another.

In fact, it takes training to learn tosee and to conceive and then ex-plain impressions, which we perceivethrough our eyes. It requires a longtraining, which must also reach intohistory in order to find out whysome of the old buildings, monu-ments or sculptures have been con-sidered beautiful and great artthrough centuries, why they have

been called classical art or classicalbeauty and have kept this designa-tion though views and tastes werechanging as the centuries passed. Asto the masterpieces of modern artand architecture, we also speak ofclassical beauty, but this judgmentwill only be confirmed in the future,if and when these masterpieces doin fact satisfy the criteria of classicalart.

Pieces of classical art move oursoul when we see them, and we aretouched if we are sensitive to beau-ty. give us something that wefeel enriches us. For this reason, somany people travel like pilgrims farand wide in order to see classicalart and to gain those deep impres-sions. These facts are sufficient proofthat beauty or, for the engineer, theaesthetic quality of design, of formand of shape can be defined and thatthere must be laws and rules forachieving beauty. If this is the case,then there must be both good andbad taste. It takes much learning toachieve the skill of really seeing andto develop good taste, but it is also aquestion of talent, which God mayor may not have laid into our cradle.

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Emphasized is the importance of artistic considerations,in addition to engineering design, to achieve beauty inbridges. Based on extensive experience, the author out-lines his rules to achieve aesthetical structures, andshows by example how to blend aesthetics into bridge design.

RULES FOR AESTHETICS

Being the son of an architect, Ihave been trained almost from child-hood to see and to analyze beauty.When I started to design bridges 35years ago, I tried hard to design notonly safe, but also aestheticallypleasing bridges. I was fortunateenough to belong to a group of engi-neers and architects which had thetask of improving the aestheticalpart of bridge design for the hun-dreds of Autobahn bridges that hadto be built at that time. We traveledmuch in order to see and contem-plate bridges, we had long discus-sions about the subject, and wegained more and more insight andknowledge that enabled us to de-velop tentative rules for good forms.Put into words, these rules mightbe valid for the design of any tech-nical product which is fabricated tosatisfy a certain purpose or need andwhich is not made only for art's sake,like sculptures.

I. There is the rule of good order—order of the systems and orderof direction of lines and edges. Forbridges, order of systems means tochoose a beam, an arch, a suspen-

sion or a frame, but never to mixsystems. Disorder cannot lead tobeauty.2. We have to observe the oldlaws of good proportions, of har-monic proportions between lengthand height, between span clear-ance and depth, between the sup-porting and the supported struc-tures. Sometimes contrast leads tobeauty, for instance the impres-sion of slenderness of a beambridge can be increased by thethickness of the piers.3. We should aspire to simplicityand pureness of structure and re-strict ourselves to few and simpleelements. We should avoid all use-less additives or ornaments. A de-sign of a bridge is only perfect ifnothing can be omitted.4. Beauty of technical productswill reach lasting value only if theproduct fulfills its purpose andgives good service. Technical pro-ducts must be both serviceableand suitable.5. The product, or the structure,must be shaped in a way to alloweasy fabrication or construction.This means, that the material be-

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ing used has an influence on thedesign. The final shape should alsoexpress special qualities of the ma-terial, for instance high strengthshould lead to slenderness orgracefulness.6. The design must also considereconomy as one of the main aimsfor technical products. Uneconom-ical shaping will not lead to lastingdesign value or classical appear-ance.7. The presently available rangeof technical sciences and of meth-ods of fabrication will always beof influence on the design. Thismeans that with progress in sci-ence and technology, we will alsohave a steady development in theform and appearance of our struc-tures. A well-designed bridge ofthe 1930's looks differently froma good, modern bridge.8. One of the most valuable in-fluences on design is creativityand intuition, which allows us tofind and to develop new forms,which are more suitable, or im-proved fabrication and construc-tion.Excellent examples of perfect

technical forms are modern airplaneslike the Caravelle which comply per-fectly with these rules and whichare considered to be beautiful byeverybody. They have reached clas-sical beauty.

ENGINEER-ARTIST COOPERATION

It is often said by engineers thatsound engineering by itself mustlead to beauty. This is not true atall. Sound engineering, of course, isa necessity for any good design oflasting value, but beyond the ration-al thinking of the engineer, there

must be the aim and willingness tocreate beauty. This depends not sim-ply on the application of rules andlaws, but mainly upon the artistictalents of the designer, upon hissense of beauty, his intuition andhis ability to judge and foresee theimpression which his design willmake.

Only a few men have the specialgifts and talents for creating beau-ty of technical products, which pri-marily fulfill the purpose of service-ability. This kind of art demandssensitivity and sobriety simultane-ously. Unfortunately, most engineers.have no talent in this field, becausethe profession of engineering at-tracts persons who have a talent forrationalism and logical thinking, butnot sensitive feeling. Therefore,there should be collaboration be-tween engineers and artists. Bothmust cooperate in trying to find asolution which will satisfy both.

The artist should not dominatebut serve, just as the engineer servesthe purpose of his structures. Theartist must refrain from forcing theproducts of his fantasy upon theengineer and he must refrain frominsisting on fashionable extras andso-called styling. The collaborationof the two should be governed by asense of serving with a willingnessto understand each other; they mustbe master servants and not kings. Itis seldom that we find such people.

In rare cases, both gifts—being agood engineer and a good artist—are combined in one person. Thismay lead to the best results. In myexperience, however, the cooperationof several talented persons is morereadily possible and, hence, is to bepreferred.

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RULES FOR AESTHETICS SHOWN BY EXAMPLE

An example of disorder of a structureis this old lift bridge, where the

members of the different trusses haveall possible directions and sizes.

Trusses can be designed to give agood appearance if the chords areparallel and if the diagonal bars haveonly two directions, as this steel trussbridge may show. In addition toorder of directions, there must begood proportions between the chordmembers and diagonal bars.

A second example of lack oforder and of good proportions isthis highway bridge with a spanof about 1000 ft. The latticework of this large arch, andespecially the lateral bracing,do not show good order.

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A steel arch-bridge with the same span, but which looks good, hasbeen built by engineers who collaborated with a good architect. Therules of good order, simplicity and good proportions were observedin a perfect way.

An excellent example ofobtaining good order, byreducing the number ofstructural elements to aminimum, is the first four-Ievelcrossing in the world near LosAngeles. This difficult designwas solved with two elementsonly—solid slabs and circularcolumns. This structure hashigh aesthetical value.

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February 1968

Beam bridges depend mainlyupon their slenderness for goodappearance. An overpass bridgeof the early days of thruwayshas heavy reinforced concretebeams, heavy piers andabutments and Iarge wing walls.The driver feels handicappedby the narrowness of theopenings -and the masses of thestructure; his feeling ofopenness is disturbed. Suchbridges no longer fulfill theirpurpose, looked at with ourpresent perspective.

Progress in engineering, throughthe introduction of prestressedconcrete, allowed us to buildmore slender beam bridges andenabled us to develop overpassbridges with a satisfactoryappearance. The slendercontinuous beam rests on fewcolumns; large wing walls arereduced to small abutments ontop of the embankment. The firstrule for good appearance, therule of order, requires that alllines of the superstructure are tobe parallel. All edges must followthe axis of the highway, no matterwhether it is curved vertically orhorizontally. The second rulerequires good proportions betweenspan lengths. In particular, endspans must be sufficiently long, sothat a small slope for the end ofthe embankment can be used,preferably paved with dark stone.

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The impression of slenderness can be emphasized by increasingthe cantilevered width of the deck slab and by good proportionbetween the depth of the fascia beam and the depth of themain girder. Fascia beam depth should also be in good pro-portion with the length of the main span—at least 1/100 ofthe span length for spans up to 150 ft. or 1/4 to 1/5 of the

total depth for longer spans.

The type of railing, too, influences the look of the bridge. Light-Iookingsteel railings, preferably with equal vertical bars, allow you, whiledriving over the bridge, to look through the railing just as if it did notexist. With a steel wire rope inside the hand rail, it gives excellent safety.

Many overpasses cross thruways at a skew angle. One of the difficulttasks is to develop good-looking bridges for such skew crossings.Some overpass bridges in California were the firstto achieve this. Looking along the thruway the span lengthappears to be decreased, thus distracting from the impression ofslenderness. Therefore, such bridges should be designed extremelyslender. This can be achieved with hollow boxes, widely cantileveringdeck slabs and rather deep fascia beams. The torsional stiffness of thehollow box beam permits such bridges to be supported with onecolumn at the center line. Even hinged columns can be used for threeor four spans, because torsional moments can be taken up at theabutments only or at some intermediate locations with double columns.This type of design has the great advantage of a bridge with normalrectangular superstructure.

For long, multi-span bridges forelevated highways, theintermediate piers present themain aesthetic problem. Thefirst example shows a continuous,hollow slab on transversal frames,with only a small additionaldepth for the frame beam. Alledges of the superstructurefollow the curve of the highway.Notice the good proportionsbetween the spacing of theframes and the distance betweenframe legs.February 1968 21

If prefabricated girders are used, quite often capping beams on the piers orcolumns are required. They tend to be heavy, because they have to carry heavyloads. This type of support does not look attractive but it can easily be improvedby moving the capping beam upwards, so that its depth disappears fully orpartially within the depth of the longitudinal girders. The Russians showedsuch a solution many years ago; girders rest on the bottom flanges of thetransversal beams. Good solutions of this type have been proposed for the

Crosstown Expressway in Chicago.

For an elevated railroad bridgein Rotterdam, the column carriesa thick table plate on which theprefabricated girders are placed,with recesses for the bearings athalf depth. With girders andtable plate having the samedepth, the bridge looks likehaving a continuous main girder.

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For a very wide bridge with six lanes, a good-looking solution consistedof two hollow box girders and two rows of columns, with cross girdersbetween the two main girders of equal depth and equal spacing. Thismeans good order. Good proportions for such a bridge are found in thelongitudinal and transversal spacing of the columns. In such cases thetransverse distance of the columns should be less than about half thelongitudinal distance and the number of columns for each span shouldbe small. If such rules are not observed, one gets a disorderly lookingforest of columns.

A good example of a long bridge on pairs of slender columns is thisbridge along a steep hill in the Rhine valley (design by Dr. Wittfoth).

For bridges within cities, crossingsquares, plazas or shopping streets withits many pedestrians, it is most im-portant to dematerialize the supportingelements of the bridge, so that thebridge deck appears to be floating inair, thus disturbing the street life aslittle as possible. In Dusseldorf, such abridge was built after long designstudies. The columns for this bridgeare very slender and widely spaced, thespans being between 100 and 130 ft.

For the four-lane portion of thebridge, the columns are shaped like aY. In order to attain maximum slender-ness of the columns, steel was used tosupport the prestressed concrete bridge.

However, it is not only the columnsbut also the cross section of the super-structure which gives this light andfloating impression. Cross sections show

no distinction between main girder anddeck slab. It was developed from a nar-row, central hollow box and an ex-tended, cantilevered deck slab, but alledges have been avoided by curvingthe bottom surface. This has the effect,more or less, that the depth of thestructure can not be visualized. Thedeep portion of the main girder almostdisappears behind the rather deepfascia beam and gives the impressionof extreme slenderness. The relativelyheavy prestressed concrete superstruc-ture virtually eliminates traffic noiseunder the bridge, so that in fact thestreet life is not disturbed by this fly-over bridge.

At the end of such a bridge, it is im-portant to extend it until it almosttouches street level, and to make theabutment as small as possible.

-^20 --------1--

11.50 --- ------ -t,ta- re

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The above pedestrian bridge is a very slender,hollow-plate structure resting on two columns,strengthened at the top by means of a flat cone.Again, a rather deep fascia beam and different colorsincrease the appearance of slenderness. The structureends at rather low points in order to reduce the sizeof the abutments.

This pedestrian bridge is about300 ft. long and forks into twodifferent directions on one end. It issupported at several points withstay cables extending from a singletower at the beginning of the fork.Having no visible extra elementsfor anchoring the stay cables to theplate or to the top of the tower wasimportant for the bridge's goodappearance. The hollow plate is ofconstant depth and has a simplerectangular cross section. Foraesthetics, it was most important tojoin the different curves—vertical andhorizontal—with graceful transistions,avoiding any sudden change ofcurvature. This means that at nopoint should a circle meet a straightline without a parabola in between.

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One of the bestlooking river crossingsis this single-spanbeam bridge, en-castered at both ends,over the River Traunin Austria. It is theslenderness andgraceful curvature ofall lines which makesit so attractive.

For multi-span riverbridges with straightgrade, it is preferableto have beams withparallel edges onheavy piers and abut-ments with parallelwing walls.

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ii. is ammcuxr to nna gooa-iooxmg solutions for paunched beamson high piers. Aas-Jacobson of Norway solved this problem withdouble columns in a satisfactory way.

For Finsterwalder'sbridge across theRhine River inBendorf, which hasthe longest span forprestressed concretewith almost 700 ft.,an effort was made tosuppress the veryIarge depth of thesuperstructure at themain piers by extend-ing the face profileof the pier intothe beam.

For multi-span bridges withparallel edges, single columnswith a cap also give goodappearance. The cap, however,must be limited to good pro-portions in relation to the widthof the structure. Caps look badif they are as wide or evenwider than the hollow-box beam.

If circular or elliptical columns aresufficiently wide for placing twoof the modern, rubber potbearings, then we can eliminatethe cap and support the wholesuperstructure, even for ratherwide bridges, directly on thecolumn. This can be seen on thisbridge crossing the Mosel.

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In the design of valley bridges, thedevelopment was from heavy andwide masonry piers to slendercolumns. An old bridge on theAutobahn shows masonry piersfitting well into the free landscape,but almost closing the viewinto the valley.

Today, slender doublecolumns, or even slendersingle columns, are moreoften considered to bethe best solution. Singlecolumns can supportstraight and wide bridgesonly if the superstructureis of great torsionalrigidity.

The best looking bridge on such slender single columns was designed byFinsterwalder for the Elztal viaduct, with spans of about 120 ft. The shallowslab is thickened by a flat, mushroom cap to get the necessary fixitywith the columns, which are as high as 320 ft.! Many examples alreadyprove that continuous beams on rather high, slender piers least disturb

the beauty of the landscape.

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The same can be said of the approachbridges over Lake Maracaibo. Here,too, the silhouette shows a very goodappearance, but the interesting piershave too many legs and too heavyhorizontal bracing beams, when viewedat a skew angle.

The longest span achieved thus far withprestressed concrete is in the MaracaiboBridge, designed by Professor Morandi.Looking at this bridge from an architecturalpoint of view, there is no doubt thatthere are good proportions and goodorder of lines, providing the bridge isconsidered at right angles to its longitudinalaxis. However, if the towers are viewedat a skew angle, less favorable impressionresults. There are too many elements indifferent directions, with dimensionsnot in proportion to the tasks. This exampleshows that designs should be studied notonly from drawings, but also fromsmall-scale models which can be regardedwith a critical eye from all possibledirections.

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During the last 15 years, bridges have been developed in Germanywith different types of stay cables, suspending continuous beams,So far mostly steel bridges have been built, like this bridge crossingthe Rhine in Cologne.

To show what can be done with stay cables, and how long-span bridgeslook when the stay cables are parallel, a bridge crossing the Rhinein Diisseldorf is shown below. The impression of slenderness of thebeam has been intensified by using near white for the fasciabeam and a very dark gray for the web of the main beam. In the nearfuture, there will be more prestressed concrete bridges withmultiple stay cables for longer and longer spans.

From an architectural point of view, the access bridges for pedestriansto the previous Rhine River bridge are interesting. The usual stairswere avoided by designing curved and slender ramp bridges,which have been very well received by the citizens of Dusseldorf.

CONCLUSION

If we consider the development ofbridge design from an aestheticalpoint of view, or the development oftaste concerning beauty of bridges,we find a preference for very slenderstructures supported by slender piersor columns. We prefer simplicityand restriction to a minimum of ele-ments. We prefer continuous andsteady long lines, either straight orin smooth curves, which correspondto fast-moving traffic. We avoidheavy, clumsy structures with largeabutments.

The reduction of structures tominimum sizes, or dematerialization,

corresponds to the general aim ofengineering to fulfill the purpose ofstructures with a minimum of mate-rial, effort and cost. This is also asound principle of nature for what-ever nature develops.

Bridge designs must be governedby valid rules for simple order andgood proportions. Aesthetics forbridge design is a necessity for thefuture as it is for architecture, or foranything that we build. Unattractivebuildings or structures not only de-valuate real estate, but cause sick-ness of man's soul. We must recog-nize these facts in order to beconvinced of the high responsibilitywhich engineers also have towards.beauty in their structures.

Discussion of this paper is invited. Please forward your discussion to PCI Headquartersbefore May 1 to permit publication in the August 1968 issue of the PCI JOURNAL.

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