Engineering Structures 40 (2012) 479–486

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Engineering Structures

journal homepage: www.elsevier .com/ locate /engstruct

The Maria Pia Bridge: A major work of structural art q

A.P. Thrall a,⇑, D.P. Billington b, K.L. Bréa c

a Department of Civil and Environmental Engineering, Princeton University, Engineering Quadrangle E321, Princeton, NJ 08544, USAb Department of Civil and Environmental Engineering, Princeton University, Engineering Quadrangle E323, Princeton, NJ 08544, USAc Skanska USA Building, Empire State Building, 350 5th Avenue, 32nd Floor, New York, NY 10118, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 August 2011Revised 6 February 2012Accepted 7 February 2012Available online 10 April 2012

Keywords:ArchesBridgesCompetitionHistoric sitesIron

0141-0296/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.engstruct.2012.02.032

q This document is a collaborative effort.⇑ Corresponding author. Present address: Departm

Geological Sciences, University of Notre Dame, 159 Fit46556, USA. Tel.: +1 5746312533.

E-mail addresses: athrall@nd.edu (A.P. Thral(D.P. Billington), kyrsten.brea@skanska.com (K.L. Bréa

This paper presents a technical, historical, and aesthetic study of the Maria Pia Bridge over the DouroRiver in Porto, Portugal which was designed and built by G. Eiffel and Cie. between 1875 and 1877.Through these analyses, this paper demonstrates the significance of this bridge due to its (1) economyas shown through the design competition, (2) efficiency and safety under self-weight, live, and windloads as revealed by finite element analyses, and (3) elegance of form which is evaluated through the aes-thetic motivation of the designer, international acclaim, and an on-site visual analysis. The primary pur-pose of the paper is not to suggest that designers today emulate the Maria Pia form, but to shed light onthe thought process of the engineers at G. Eiffel and Cie. The conceptual design was imagined throughconsidering the site constraints, the forces acting on the structure, and the erection procedure, ultimatelyleading to an economical project that won the design competition. This design-build approach can beused more often in 21st century design.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Gustave Eiffel (1832–1923) and his company, G. Eiffel and Cie.,designed and built the Maria Pia Bridge over the Douro River inPorto, Portugal (completed in 1877) (Fig. 1). It is a two-hinge, ironarch bridge with a span of 160 m and a rise of 42.935 m [1]. It wasthe first of Eiffel’s great arch bridges. The second, the GarabitViaduct (completed in 1884), is better known but follows closelyits design and construction.

This paper will demonstrate the significance of the bridgethrough its economy, efficiency, and elegance. A detailed descrip-tion of the design competition will highlight the economy of thedesign. Technical analyses reveal the structure to be efficient andsafe under self-weight, live, and lateral wind loadings. The ele-gance of the design is demonstrated through the aesthetic motiva-tion of the designer, international acclaim, and an on-site visualstudy. Finally, a discussion of the relevance of this research to21st century bridge design will be presented.

The main purpose of all of these studies will be to demonstratethe design and construction process of the primary engineers: Gus-tave Eiffel and Théophile Seyrig (1843–1923). They formed G. Eiffeland Cie. in 1868 – a partnership which lasted for 8 years and

ll rights reserved.

ent of Civil Engineering andzpatrick Hall, Notre Dame, IN

l), billington@princeton.edu).

resulted in the design and construction of many works, includingthe Maria Pia Bridge [2,3]. Seyrig wrote a report on the analysisof the bridge titled ‘‘Le Pont sur le Douro à Porto’’ which this paperrelied on for details related to the structure [1]. This paper willpresent the analysis techniques and the design-build mentality ofboth engineers which were fundamental to winning the designcompetition and to the success of the final built structure.

2. The design competition

The economy of the design is shown through the design compe-tition held by the Royal Portuguese Railroad Company that openedon May 1, 1875. Four companies (Mead, Wrightson and Co., Sociétédes Batignolles, Cie. Fives-Lille, and G. Eiffel and Cie.) collectivelyprovided six designs. Seyrig lists each design anonymously, butLoyrette (1985) and Cruz and Cordeiro (1981) provided detailsrelated to the companies and costs of the projects [2,4]. Eiffel’s pro-posal (costing 965,000 francs) was 31% less in cost than the nextlowest design [1]. The final cost exceeded the original bid, butwas still 9% less than the next lowest design [4]. See Fig. 2 for ele-vation drawings of each proposal and the designer and cost (whenknown).

The site for the bridge crossing – a 400 m wide, 61 m deepvalley between Via Nova de Gaia and Porto – would complete a raillink between Lisbon and Porto, but posed an engineering challengedue to soil instability at the riverbed. Any alternative site wouldadd an additional 12 km of travel [1]. The 150 m wide Douro River,that runs through this valley, ranges in depth from 15 to 20 m with

Fig. 1. Maria Pia Bridge (foreground). Photograph by Bréa.

(A)

(B)

(C)

(D)

(E)

(F)

(G)

Mead, Wrightson and Co.2,750,000 F

Société des Batignolles1,895,000 F

Société des Batignolles

Cie. Fives-Lille1,410,000 F

Cie. Fives-Lille

Fig. 2. Proposals for design competition including designer and cost (in Francs). G.Eiffel and Cie.’s proposal cost 965,000F [2,4]. Image reprinted from Seyrig [1].

480 A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486

high river currents and is subject to flooding that can elevate thewater level by 10 m [5]. Nearly 15 m beneath the water surface,surveys had revealed the existence of a thick layer of sand. Atthe time, it was less expensive to divide a span longer than150 m into two smaller spans. However, these soil conditionswould make in-water piers inadvisable. Though bridge spans hadbeen increasing steadily, none had reached 160 m [1]. This compe-tition provided the impetus for engineers to consider thischallenge.

Mead, Wrightson and Co. proposed the most expensive design(Fig. 2A). This design featured a 160 m central arch resting on ma-sonry piers. On either side of the arch, two 82.5 m half-arches

spring from these piles to the masonry abutments on either sideof the valley. The arches were to be erected simultaneously startingat each side of the river such that the half-arches would balancethe central arch during erection [1].

Société des Batignolles proposed two designs featuring bow-string trusses. The first, less expensive design spanned the entireriver with a 170 m bowstring truss with a 22.5 m depth at midspan(Fig. 2B). The main truss members of this design would be in steelrather than iron. Seyrig raised concern over the design with respectto potential dangerous oscillations from wind excitation. Further-more, the center of the gravity of the bowstring truss is signifi-cantly below deck level. With wind loads around 270 t, theresulting moment from the wind could result in the uplift of theentire structure. The bowstring truss is supported by masonrypiers which might not be able to sustain the high lateral wind loads[1]. Société des Batignolles’ second design (Fig. 2C) featured a120 m bowstring truss with a depth at midspan of 15 m. This de-sign resulted in an in-river pier requiring a foundation in the sandyriverbed [1]. This second design was highly problematic both withrespect to its higher cost in comparison to their first proposal anddue to the difficulty with the in-river pier foundation.

Cie. Fives-Lille’s proposed the second least expensive proposal(Fig. 2D) consisting of two metal piers on either side of the riverwith two 86 m struts supporting the central span. The deck is thendivided into four equal spans of 78 m. The large surface area of thedeck compared to the other designs would result in high windforces in the structure. The same company submitted a second,more expensive project (Fig. 2E) which places a pier in the river.This design is subject to the same problems with foundation designas previously discussed [1].

There were two other designs originally proposed prior to theopening of the competition, but were withdrawn (Fig. 2F and G).See Seyrig (1878) for a review of these designs [1].

G. Eiffel and Cie.’s design was chosen from among these propos-als on the basis of cost.

3. Erection of the bridge

One of the advantages of G. Eiffel and Cie.’s design was its erec-tion method. This design crossed the river by means of a centralarch. The 354.38 m long deck is supported at ten points, includingthe crown of the arch and the abutments. The bridge was con-structed from 1450 t of iron, 750 t of which lies in the arch alone.To avoid the unstable soil conditions, no supporting piers or false-work were placed in the river-bed [5].

The erection sequence of the arch began with the constructionof the piers on either side of the river which were built with theuse of cranes (Fig. 3A). Arch panels were pre-assembled and thenlifted into place, where they were suspended by cables and pivotedfrom the main supports (Fig. 3B). Eight cables on either side of theriver supported the cantilevered arch during erection. The archpanels were mounted in succession until they met at midspanwhen the final keystone was added (Fig. 3C). When the two halvescame together there was a negligible horizontal deviation at thecenter [5]. Eiffel stated that, ‘‘At the junction of the two intradoshalf-arches at the keystone, we could observe that the work inthe shop and the assembly had been done so carefully that, be-tween the two arch portions, the horizontal deviation was limitedto approximately one centimeter, and it disappeared as a result ofthe assembly pins’’ [5]. This attests to the precision of G. Eiffel andCie.’s workmanship and design.

The deck was assembled on embankment platforms and waslaunched as the arch construction progressed towards the center[5]. The deck was launched with the use of rollers and poweredonly by human labor at a rate of 9 m/h [4].

Fig. 3. Erection sequence. Image reprinted from Eiffel [5].

Table 1Geometric properties of arch. The first column indicates the node (see Fig. 5), thesecond and third columns indicate the horizontal and vertical coordinates, respec-tively, the fourth column provides the cross-sectional area, the fifth column providesthe moment of inertia in the plane of the arch, and the sixth column provides themoment of inertia perpendicular to the plane of the arch. Data reprinted from Seyrig(1878) Tables 1 and 14 [1].

A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486 481

4. Detailed design of the arch and original calculations

The span and the depth of the arch were dictated by the siteconstraints. The shape of the arch itself is derived from preliminarycalculations on the bridge. Initially, a parabolic arch was assumed,but high bending moments resulted. Instead, the shape of the archwas eventually selected through hand sketches which balanced areasonable distribution of bending moments with the designers’interest in developing an elegant shape. With the shape of the archdefined, a crescent form (meaning an arch that is narrowest at thehinge and deepest at the crown) was selected since the highestbending moments would be experienced to the left and right ofthe crown. The greatest depth of the arch was therefore assignedto the crown and the crescent form followed from there. The cres-cent form had previously been employed for roof beams in trainstations, but this is the first time that it was used for an archbridge. Out of the plane of the bridge, the arch is widest at the baseand narrows toward the crown to counteract the effects of windand horizontal oscillations from train traffic [1].

Even with these characteristics of the arch pre-determined, asSeyrig points out in his article, there is no direct method to deter-mine the cross-section properties on the first attempt. The self-weight of the arch is of course initially unknown as is the distribu-tion of the material in the arch. A trial and error approach, as notedby Seyrig, would be long and tiring. Instead, a combination ofmethods was employed to arrive at a detailed design of the archtruss [1]. First, Bresse’s theory of circular arches was employed todetermine a preliminary value of the arch thrust [1,6]. To employthis theory, it is assumed that the neutral fiber of the arch is closeto that of a circular arch, that the cross-section is symmetric aboutthe neutral axis, and that there is not much variation of themoment of inertia along the arch (although the arch deepens atthe crown, the cross-sectional area increases toward the hingesmaking the the moment of inertia sufficiently close to constant)[1]. With these assumptions, Seyrig asserts that it is then possibleto employ Bresse’s tables for the design of circular, iron arches withconstant cross-section [1,6]. Using these tables, Seyrig finds a pre-liminary value for the thrust and therefore all external loads actingon the bridge (including self-weight from the deck) are known.Then the arch is divided into two systems: (1) one arch with verti-cal and diagonal members and (2) the same form with vertical anddiagonal members inverted. Graphic statics is then employed to

Fig. 4. Design drawings. Image reprinted from Seyrig [1].

calculate the stress in each member for each of the two systems.The two systems are then superimposed to find the total stressin each member [1]. Fig. 4 shows drawings highlighting the useof graphic statics in the design of the Maria Pia Bridge.

Due to the prominence of the structure and the its long span,however, a special commission was formed to investigate its safetyand stability. This investigation employed the force method ofanalysis with numerical integration to make the final detailedcalculations on the arch. More specifically, this analysis methodwas employed to calculate the horizontal reactions and stressesin the arch under various loadings, including self-weight, three sta-tic live loads, and wind loads. For example, to calculate the hori-zontal reaction, the horizontal degree of the freedom at the hingewas released and a given load was applied to the arch. The horizon-tal translation can be calculated by considering the effects of bend-ing and axial compression on each section of the arch. Then theload is removed and a horizontal thrust is applied. The horizontaltranslation due to only the thrust can be found in the same man-ner. By requiring that the total horizontal displacement be zero,the horizontal thrust can be determined. For such calculations,each half of the arch was divided into 11 sections and the sectionproperties of each segment were projected onto the neutral axis.See Table 1 for the geometry and section properties of the arch.Numerical integration was then employed for the calculations[1]. See Winter et al. (1964) for a review of this method of analysis[7]. See Thrall (2008) for a more detailed discussion of how thismethod applies to Seyrig’s calculations [8]. Note that both axialand bending deformations were considered in this method. If theaxial effects had been neglected, the horizontal thrust under self-weight would have been just 1.5% different. This difference in hor-izontal thrust is effectively negligible and shows that axial effectscould have been ignored altogether. Alternatively, as Parker dis-cusses in his 1881 thesis on the bridge, the horizontal reactions

Node x (m) y (m) A (m2) I (m4) IW (m4)

1 2.80 3.00 0.293296 0.246 14.47432 8.40 9.00 0.273948 0.588 9.40303 14.10 14.55 0.264448 1.153 7.44014 20.40 20.42 0.252848 1.848 5.69255 25.25 24.20 0.241848 2.463 4.93466 31.00 28.30 0.236048 2.863 3.73717 39.75 32.75 0.225298 3.486 2.63318 49.15 36.85 0.222048 3.758 2.04419 59.20 40.35 0.222848 4.220 1.6443

10 69.60 42.25 0.228348 4.609 1.424611 80.00 42.65 0.228348 4.696 1.3515

Table 2Material properties. This table provides the

482 A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486

could have been solved for directly [9]. This direct approach is usedin Eiffel’s discussion of his calculations for the Garabit Viaduct [10].

assumed material properties include the Mod-ulus of Elasticity (E), the allowable stress (r),and the density (q).

Property

E 16 Mt/m2

r 6000 t/m2

q 7.8 t/m3

Table 3Geometric properties of piers. The first columnindicates the height (measured from the height ofthe arch hinges) corresponding to the assumedsection properties. The second column providesthe assumed moment of inertia in the plane of thearch (I).

Range (m) I (m4)

0–5 0.1495–10 0.12510–15 0.10315–20 0.08420–25 0.06625–30 0.05030–35 0.03735–40 0.02640–42 0.018

5. Analysis under self-weight, live, and wind load

This paper evaluates the behavior of the Maria Pia Bridge underself-weight, three static live loads, and lateral wind load. Fig. 5 pro-vides an elevation drawing of the bridge including numberingschemes for the piers and the nodes of the arch. Note that theintersection of the deck with the arch has been modeled as twopiers closest to the crown. Table 2 provides the material propertiesemployed for these analyses. The reader is referred to Table 1 forthe properties of the arch given by Seyrig [1]. Seyrig did not pro-vide geometric properties for the piers. We assumed a reasonablevalue for the cross-sectional area (0.0280 m2) that is concentratedon the edges of the pier (assumed to be constant for its entirelength). Based on plans for the bridge, Piers 3 and 4 range in width(in the plane of the bridge) linearly from 4.8 m at the base to1.532 m at the top [11]. To simplify the piers, we considered onlynine sections, each assumed to have a constant width. See Table 3for the moments of inertia calculated for these constant cross-sec-tion sections. Piers 3 and 4 range from 0 m at the base to 42 m atthe top. Piers 1, 2, 5, and the piers connecting the deck to the archeach start at a higher elevation, but are assumed to have the samesection properties according to their relative locations. Piers 1–5are assumed to be fixed to the ground. Piers 1, 2, and 5 have rollerconnections to the deck. Piers 3 and 4 are pin connected to the deck[11]. The piers connecting the arch to the deck are assumed to havea fixed connection at the arch and a roller connection at the deck.Seyrig also did not provide any details related to the cross-sec-tional area and moment of inertia of the deck. To find reasonablevalues for these properties, we applied an asymmetrical live load(chosen because it presents an extreme in deck-arch interaction)to a finite element model of the bridge with estimates of deckproperties. These estimates were refined until our results matchedclosely those presented by Seyrig. The cross-sectional area and mo-ment of inertia were chosen to be 0.0326 m2 and 0.1000 m4,respectively.

All analyses presented in this research were performed usingStructural Analysis Program (SAP). This study assumed an allow-able stress of 6000 t/m2 based on the value that Eiffel used in hispaper on the Garabit Viaduct [10]. Note that throughout the text,

Fig. 5. Elevation of bridge with numbering scheme for piers (

the authors employ the units used by G. Eiffel and Cie. at the timeof the design (e.g. tons as the unit of force).

5.1. Self-weight analysis

An analysis of the arch under self-weight was performed bybuilding a two-dimensional model of the neutral fiber of the archalone (neglecting the piers and the deck). Hinges on either sideof the arch were modeled as pin constraints in the plane of thearch. Table 1 provides the geometry and section properties of thearch. Data related to the lateral bracing was not available. There-fore, to capture the full self-weight of the arch, we applied pointloads based on Seyrig’s calculations of the self-weight at each noderather than relying on a self-weight load case in SAP. These pointloads were back-calculated from the values for moment that Seyrig

top) and numbering scheme for nodes of arch (bottom).

A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486 483

provided in his numerical integration tables for calculating thehorizontal thrust under self-weight only [8].

When analyzed, this model found a horizontal reaction negligi-bly different from Seyrig’s value. The stresses of the extreme fiberof the upper chord of the arch are on average 4.6% different fromSeyrig’s values. At the extreme fiber of the lower chord, the averagedifference is just 1.3%. These comparisons provided confidence inour modeling strategy.

An efficient two hinge arch design has minimal bending underself-weight. A consideration of the magnitude of bending underthis loading will therefore begin our evaluation of the efficiencyof this design. To visualize the amount of bending, we superim-posed the shape of the arch with the magnitude of bending inFig. 6. The solid line of Fig. 6 shows the shape of the arch. The dot-ted line shows the moment (divided by 100 for scaling) added tothe vertical coordinate of the arch. When the moment is zero,the plots exactly superimpose. A positive moment (above the archline) implies compression in the top of the element; a negative mo-ment (below the arch line) implies compression in the bottom ofthe element. Overall the bending moments are small. For example,the highest bending moment occurs at node five, where the stressfrom bending is 600 t/m2 at the upper chord and 570 t/m2 at thelower chord. The stress due to bending even at the largest pointof moment is just 10% of the assumed allowable stress of 6000 t/m2, thereby indicating low bending stresses under self-weight.

Horizontal Position (m)

dedda )001/mt( tne

moM

fo thgieH ot

)m( hcr

A

100500-50-1000

10

20

30

40 MomentArch

Magnitude of Moment at Node 5

Fig. 6. Moment under self-weight (dotted line, quantity divided by 100) added tothe geometry of the arch (solid line). The distance between the two lines indicatesthe magnitude of the bending moment at that node. The lines coincide when thereis zero moment.

(A)

(B)

(C)

Fig. 7. Live loadings. Diagram (A) shows the live load applied to the entire arch, (B) show80m of the arch.

This study shows that G. Eiffel and Cie. chose a form that mini-mized bending under self-weight [12].

5.2. Live load analysis

The original live load calculations considered three static load-ings: (A) uniformly distributed over the entire arch, (B) distributedover one half of the arch, and (C) distributed over the middle 80 mof the arch with a loading of 4 t/m for each case (see Fig. 7) [1]. Weapplied these same loadings (assuming the same magnitude) inour full SAP model.

Fig. 8 shows the bending moment envelope along the arch forall three loadings. From 0 to 70 m along this diagram, the maxi-mum and minimum moments are given by Load B (consideringboth halves of the arch). Load B is equivalent to loading the archat its quarterpoint and therefore, one would expect the highestbending to occur at the quarterpoint as it does in Fig. 8. The spikearound 25 m can be attributed to the location where the pier inter-sects the arch. The minimum moment along the rest of the arch isalso from Load B. The maximum moment from 70 m to 80 m is gi-ven by Load C. This distribution of the live load bending momentenvelope demonstrates the reasoning behind selecting a crescentform for the arch.

5.3. Wind analysis

We applied a static, lateral wind load on the arch of the bridgealone. The out-of-plane moment of inertia values for each sectionof the arch were provided by Seyrig and are given here in Table 1[1]. Seyrig assumed a maximum wind load of 275 t/m2 which cor-responds to a wind speed between 37.3 m/s and 47.4 m/s. How-ever, these high wind speeds would not likely occur over theentire structure at all times while a train was also crossing. Instead,Seyrig assumed a wind load of 150 t/m2 with the presence of atrain [1]. In the efficiency results discussed in the following section,we considered two, unfactored load combinations: (1) self-weightand 275 t/m2 wind load and (2) self-weight, live load, and 150 t/m2

wind load. These lateral loads are applied as point loads at eachnode of the arch using magnitudes provided by Seyrig [1].

To understand the behavior of the arch under lateral wind load,Fig. 9 shows the bending moment of the arch under the 275 t/m2

load in the lateral direction. The arch is shown in plan below the

s the live load over one half of the arch, and (C) shows the live load over the middle

Fig. 8. Bending moment envelope under three static live loadings with elevation ofarch for reference.

Table 4Efficiency (e) in the upper and lower chord of the arch (denoted by subscript U and L,respectively) under Load Combinations 1 and 2.

Node Combination 1 Combination 2

eU eL eU eL

1 0.729 0.493 0.566 0.7412 0.664 0.513 0.703 0.8263 0.628 0.461 0.801 0.779

484 A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486

plot. This shows the highest bending moment occurring at thehinge of the arch and the lowest moment occurring toward thecrown. The shape of the arch in plan – widest at the hinge andnarrowest at the crown – therefore follows the bending momentdiagram of the arch under wind load.

5.4. Evaluation of efficiency of the form

Through the analyses described in the previous sections, wehave shown that there is minimal bending in the arch under self-weight, that the shape of the arch in elevation follows the live load

Fig. 9. Bending moment under lateral wind load with plan of arch for reference.

bending moment envelope, and that in plan the arch follows thebending moment diagram under lateral wind load. These studiestherefore suggest that the designers’ at G. Eiffel and Cie. selectedan appropriate form based in structural logic.

Another method for evaluating the form of the bridge is toconsider its efficiency (defined as the stress in the member dividedby the allowable stress). Though buckling typically controls thecapacity for compression members, the slenderness ratio of themost highly stressed member (near the hinge of the upper chord)is 29.5 [13]. As a result, the Euler buckling stress is so high(5,360,000 t/m2) compared to the assumed allowable stress thatbuckling could not control the design of these members. Therefore,the stress is calculated simply by dividing the axial force by thecross-sectional area.

We calculated the efficiency of each member of the upper andlower chord of the arch for two unfactored load combinations (1)self-weight and 275 t/m2 wind load and (2) self-weight, live load(worst scenario of three live loadings), and 150 t/m2 wind load.Table 4 provides these calculated efficiencies for both load combi-nations. Fig. 10 shows a visual representation of the efficiencies ofthe upper and lower chords under Load Combination 2. Based onthese studies, it is clear that the arch was designed to be very effi-cient while remaining within the safety limits of the assumedallowable stress.

6. Elegance of design

The elegance of a structure can be considered through threeperspectives: (1) the aesthetic motivation of the designer, (2) inter-national acclaim, and (3) an on-site visual analysis.

4 0.567 0.421 0.711 0.7525 0.566 0.357 0.743 0.6666 0.457 0.347 0.556 0.6437 0.432 0.281 0.506 0.5238 0.407 0.302 0.551 0.4769 0.349 0.433 0.259 0.777

10 0.338 0.429 0.302 0.78511 0.304 0.466 0.492 0.482

CL

0.80.70.60.50.40.3

Fig. 10. Efficiency of the arch under Load Combination 2.

A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486 485

The aesthetic motivation of the designer was clearly describedin Seyrig’s report. Seyrig argued that engineers too quickly tendto neglect the study of form. He went on to comment that thedesire to provide economic savings often overrides the desire toproduce an elegant form. Seyrig’s ‘rules of taste,’ requires thatone not lose sight of the fact that the aesthetic considerations areas important as the economic considerations when designing astructure. The Maria Pia Bridge managed to combine economy withpure elegance. Seyrig even wrote that the choice of form for thisbridge gives the impression of rigidity and power while maintain-ing the lightness possible from metal construction. Furthermore,Seyrig makes note that no extra material was wasted for construc-tion or ornamentation, suggesting a focus on economy of materials[1]. Indeed, the form is visually superior to the alternativespresented in the design competition. The next lowest cost design(Fig. 2D) is visually awkward – the slender struts supporting thecentral span give the appearance of a broken arch and the massiveabutments create an unpleasant contrast with the delicate metal-work. This further indicates the balance of economy and elegancethat the G. Eiffel and Cie. designers found.

The bridge has received much international acclaim, includingbeing awarded the distinction of becoming the 11th InternationalHistoric Civil Engineering Landmark by the American Society of Ci-vil Engineers (ASCE) in 1990. This is Eiffel’s only bridge to receivethis award [14]. In the nomination for this distinction, MariaManuela dos Reis Martins of the Universidade do Minho Unidadede Arqueologia wrote ‘‘With such a remarkable building Eiffelbrought about new technical teachings to the world [of] civil engi-neering and encouraged several engineerings [sic] to set up similarbuilding processes in many other engineering works, mainly inwhat concerns the crossing of wide deep valleys’’ [15]. A furthertestament to the significance of the bridge is Eiffel’s later master-piece the Garabit Viaduct (completed in 1884). G. Eiffel and Cie.was given the contract for this bridge without the traditional de-sign competition solely based on the success of the Maria Pia[2,16].

Finally, an on-site visual analysis of the structure can indicateits elegance. The Maria Pia appears to embrace its surroundingsas one approaches the bridge. It does not create a disturbancethrough the ostentation of the Bon Marché, but rather it is throughthe simplicity of its design that this iron form appears welcomed inthe natural setting of the Douro Valley. The appearance of lightnessfound in the iron lattice-work is a counterpoint to the rough, denseand rocky terrain of the river and valley that the bridge spans [1].At the same time, the iron span appears to spring from the earthbelow. The hinged supports are articulated above the massive ma-sonry structure to create an instant understanding of the purposeof these supports: to concentrate at a single point the weight ofthe whole.

No matter how one approaches the bridge, be it by boat, car oron foot, the Maria Pia does not disturb the landscape. It serves onlyto add to the charm of this old European city. The surrounding areaon the Porto side of the bridge is now filled with shanties, while afunicular runs adjacent to this settlement, up the side of the hill.The winding walk through this make-shift village affords the on-looker occasional views of the bridge, seemingly telling a story ofthis once vibrant commercial area through its material, span, andstructure. The absence of an in-river pier provides a clear pathfor navigation down the river and allows onlookers the chance towatch as luxury ships travel down the river. The old boats of thevarious Port-houses line both the Porto and Vila Nova de Gaia riv-erbanks, re-emphasizing the historical trade in wine on which thecity was once dependent. Approaching the bridge from the Southvia boat, the viewer sees the São João Bridge – the Maria Pia’s con-crete rigid frame replacement – span the river in a greater leap, itsstrength evident in its slim deck design. Traveling under the bridge,

the viewer is able to fully see its arc, its seeming lightness of struc-ture and the train tracks which had at one point supported the railtraffic between the two sides of the river down to Lisbon and be-yond. The Maria Pia with its 1870s era iron structure, latticed piersand deck were to the viewer reminiscent of what would eventuallybe one of Eiffel’s most memorable works, his namesake Tower.

The Maria Pia gives expression in its design to the difficulties ithad to overcome in order for it to be constructed. The stone pierson either side of the river enclose the arch as it comes to its hingedsupport, rising 15 m above the shore-line. This can be attributed tothe repetitive flooding of the Douro River, often times rising abovethe first floor of the buildings along the river bank. The finishedstructure shows, in some part, the circumstances and conditionsof its construction.

The Maria Pia invites contemplation of the evolution of struc-ture and a historical point in time for this beautiful valley rich inport wine and Portuguese literary culture.

7. Relevance to today

This paper has examined the Maria Pia Bridge as a significantstructure through its economy, efficiency, and elegance. However,the purpose of this paper is not just a case study of a 19th centurybridge. Rather, as scholars it is our duty to the profession to presenttechnical, historical, and aesthetic studies of great works worthy ofawards like the ASCE International Historic Civil Engineering Land-mark distinction which have never been fully documented in civilengineering literature. From such studies, 21st century engineerscan understand how a design-build mentality, which characterizedthe engineers of G. Eiffel and Cie., can lead to significant workstoday.

Beginning with the industrial revolution, there has been a tradi-tion of great engineers performing planar analyses and using a de-sign-build mentality to develop economic designs. Thomas Telfordexperimented with arches and suspension bridges in iron. JohnRoebling pushed the limit of suspension bridge design with hisNiagara River Bridge (1855), Cincinnati Bridge (1866), and Brook-lyn Bridge (1883) [17]. Robert Maillart designed concrete arches,evolving from his Stauffacher Bridge design (1899) which copiedstone arch forms using concrete as the material to the SalginatobelBridge (1930) which featured a box section design [18]. OthmarAmmann simultaneously built the world’s longest spanning arch(the Bayonne Bridge) and the world’s longest bridge (the GeorgeWashington Bridge) in 1931 [17]. Each has used different materialsand different forms to generate major works. Each designed a ma-jor work which has received the distinction of being an ASCE His-toric Civil Engineering Landmark [14]. The common thread amongthese designers is their conceptual design process which focusedon planar analysis and an emphasis on erection process. Engineerstoday can keep these lessons in mind when developing conceptualdesigns so that owners and designers can recognize the potentialfor visually significant and constructionally economic bridges inthe future.

8. Conclusion

This paper has discussed the economy of the bridge based onthe design competition that earned G. Eiffel and Cie. the contract.Analyses of the structure under self-weight, live, and wind loadshave demonstrated its safety and efficiency. Within this disciplineof efficiency and economy, the bridge also exhibits elegance asshown through the aesthetic motivation of the designer, interna-tional acclaim, and by an on-site visual analysis. Based on thesequalifications, the Maria Pia Bridge can be deemed a major workof structural art as it adheres to the following tenets: efficiency

486 A.P. Thrall et al. / Engineering Structures 40 (2012) 479–486

(minimum material consistent with satisfactory performance andassured safety), economy (competitive construction cost consistentwith minimal maintenance requirements), and elegance (aestheti-cally striking consistent with efficiency and economy). These qual-ifications act as scholarly guidelines through which a discussioncan be framed. Structural art is a continuum on which designscan be judged, with major works following all three guidelines.All bridges designed with aesthetic motivation and with an effi-cient form can be structural art. It is the degree to which theyare economic and elegant that makes certain bridges major works.Christian Menn wrote that ‘‘The optimization of economy and ele-gance requires more than the craftsmanship component of engi-neering. It requires creativity, fantasy, and sensitivity to visualform. These talents collectively constitute the art of engineering’’[19]. Here Menn identifies the key to great structural art in theengineer’s play between economy and elegance within a disciplineof proper form. This paper has shown that the Maria Pia Bridgemeets this definition of a major work of structural art. Further-more, this paper has demonstrated how developing the conceptualdesign with the construction process, site constraints, and forcesacting on the structure in mind was critical to the success of theengineers at G. Eiffel and Cie. This approach could be used moreoften by designers and owners today.

The Maria Pia Bridge was continually in service from the date ofits completion until 1991 when its use was replaced by the SãoJoão Bridge. After this, it was closed to service, but still standstoday [4]. The bridge remains a beautiful reminder of the region’stradition of large span efficiently designed bridges.

Acknowledgements

This material is based upon work supported by the NationalScience Foundation Graduate Research Fellowship under GrantNo. DGE-0646086. The authors are also grateful for financial sup-port from the National Science Foundation Grant No. 0308549,the Princeton University Gordon Y.S. Wu Chair, the Princeton Uni-versity School of Engineering and Applied Sciences, the PrincetonUniversity Department of Civil and Environmental Engineering,and the Norman J. Sollenberger Fellowship. The authors would also

like to thank the staff at the Archives Nationales du Monde du Tra-vail and the École Nationales des Ponts et Chaussées for their assis-tance. Finally, the authors are grateful for the advice and guidanceof Ted Zoli of HNTB Corporation.

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