By Hal Iyengar, P.E., S.E.,Lawrence Novak, S.E.,

Robert Sinn, P.E., S.E. andJohn Zils, P.E., S.E.

CONSTRUCTED IN THE HEART OFTHE INDUSTRIALIZED CITY OFBILBAO in the Basque Country

of Spain, the fluid and curving formsof the recently opened GuggenheimMuseum have been immediately rec-ognized as one of the most complex,unique and important architecturaldesigns of this century. Variouslydescribed as a metallic flower, remi-niscent of ships’ hulls and prows, asystem of metal whorls, architectureas abstract art, and a tumble offreeform masses; the aestheticdesign has been universallyapplauded by both architecture crit-ics and the general public visitingthe museum. The innovative use ofstructural steel in concert withextensive reliance on modern com-puter capabilities by engineers,architects, detailers, and fabricatorsplayed a vital role in realizing theproject on time and within budget.

Modern Steel Construction / July 1998

Steel framing provided both thestrength and lightness needed for

the Guggenheim Museum’scomplex geometry

THE CHALLENGE

In 1991, architect Frank O.Gehry and Associates, SantaMonica, CA, was successful in win-ning a limited competition for themuseum design organized by BasqueCountry Regional government agen-cies and the Solomon R.Guggenheim Foundation. The designbrief for the museum called for cre-ating not only one of the great artmuseums, but one of the greatestbuildings in the world. Nearly twiceas long and tall as Paris’ GeorgesPompidou Center, the scale of the$100 million, 250,000 square footbuilding is such that the entire NewYork Guggenheim Museum designedby Frank Lloyd Wright can fit with-in only the central atrium space ofthe Bilbao museum.

The museum is entered belowgrade directly into the 50-meter(164’) tall central atrium space.From there, galleries of variousshapes and sizes radiate out in prac-tically all directions. Some so-called“classical” galleries are rectilinear ingeometry and are typically clad instone. Others are three-dimensional-ly curved and are generally clad intitanium metal. The largest “boat”gallery, some 140-meters (459’) long,25-meters (82’) wide without interiorcolumns, extends below the adjacentPuente de la Salve bridge. The gal-leries all include generous open vol-umes on the order of 15-meters (49’)to 20-meters (66’) tall without inter-nal floors or vertical supports.

Chicago-based Skidmore, Owings& Merrill, LLP was chosen as thestructural engineers for the projectbased upon a previous successful col-laboration with Gehry for a projectin Barcelona, Spain. The conceptualdesign process from a structuralengineering standpoint was uniquein that the building was withoutprecedent in terms of geometry,organization and scale. While mostbuilding structures are an extensionor derivative of earlier successfullyutilized systems, the Bilbao museumstructure would have to be devel-oped without the usual benefit of acomparable benchmark project.Furthermore, the architecturalthemes of fractured and irregularbuilding masses and surfaces wereexplicitly at odds with the normalstructural engineering precepts ofstability, organization, and regulari-ty in order to achieve a design whichis efficient and cost-effective. Thedesign challenge was therefore to

Modern Steel Construction / July 1998

The Guggenheim’s complex geometry was created by a structural steel “fabric”grid that provided support for the building’s titanian skin.

the structural system then evolvedover a period of a few monthsthrough an internal dialogue andthrough discussions with the archi-tectural team. One of the key con-cepts in the design philosophy wasto closely follow the architecturalform. By doing this, the curvature ofthe shapes proved to be advanta-geous in resisting lateral loads. Theconcept of a discrete bearing wallsystem in structural steel was devel-oped based on an organization of arelatively dense, diagonalized grid-work of members. Finally, the geom-etry of the exterior surfaces werestudied on the basis of horizontaland vertical slicing planes which ledto the germination of an idea oforganizing the frames in a similarlydisciplined, geometrically rigid fash-ion.

The structural system finallydeveloped for the complex titanium-clad forms may best be described asa three-dimensional diagonalizedfabric grid in structural steel. Thesystem has the ability to span longcolumn-free distances due to theoverall depth of the structure andlow self-weight of the frame struc-ture itself; while at the same timehaving significant stiffness againstlateral loads due to the ever-presentcurvature of the various geometries.This frame organization providessimilar material economies and thinstructural thicknesses historicallyassociated with reinforced concreteshell structures. The structural sys-tem concept was completed with theintroduction of the idea of shop-fab-ricated horizontal “bands” of truss-work which could be easily vertically“stacked” on-site, without a signifi-cant amount of temporary shoring orlateral bracing, in order to completethe full gallery wall structure. Thejoints would be made by providinghorizontal bearing plates at eachnode allowing both vertical and hori-zontal angle changes between mem-bers to occur while being consistentwith the truss stacking erection con-cept. The structural engineering con-cept for the complex three-dimen-sionally curved surfaces was finallyset; but the process of designing thestructure for the museum had, how-ever, only begun.

DESIGN PROCESS

Initial architectural designs werecreated with physical models ofcardboard, paper, wood, etc.Substantially complete physicalmodeling was then transferred elec-

create an organized, rational struc-tural system, within the fabric of thearchitectural design, which could bereasonably designed, detailed, andconstructed. In order to achieve this,a unique system in structural steelwas conceived.

STRUCTURAL SYSTEM

The initial stage of the projectwas focused on a search for anappropriate structural system forthe complex, doubly curved exteriorclad surfaces. These surfaces andtheir interior volumes are character-ized by their tall heights up to 20meters (66’) without internal struc-ture and their long, column-free dis-tances spanning between discrete,randomly located support points.Many of the three-dimensional sur-faces are themselves interconnectedwhile others are supported or later-ally braced by adjacent rectilinearblock shaped galleries.Traditionally, such free-form shapeshave been nearly exclusively framedin reinforced concrete—as in factother Gehry building designs ofsmaller scale have been. The scale ofthis project, however, demanded alighter structural system. Based onthese parameters, it was thoughtthat the structural system wouldrequire the following characteristics: 1. The structure should be equal-

ly applicable to a variety ofarchitectural forms so thateach element would notrequire a separate system.Member proportions anddetails should be simple and

conventional. 2. The structure should be disci-

plined and organized withoutimpacting or limiting thearchitectural design.

3. The structure should be fab-ric-like to follow as closely aspossible the architectural sur-faces in order to simplify theconnection of the exterior tita-nium cladding and interiordrywall systems.

4. The structural thicknessshould be as thin as possiblein order to minimize the depthof the void space between theexterior clad surface and theinterior drywall and to maxi-mize the usable floor space.

5. The structure should be ana-lytically verifiable, both struc-turally and architecturally, bycurrently available computerroutines.

6. The structure should be light-weight in order to span effi-ciently between discrete sup-port points.

7. The structure must be con-structed and controlled totight tolerances in the field inorder to fit the geometricallycomplex exterior and interiorcladding systems.For these objectives, the choice of

structural steel as the primaryframe material became a naturaldecision based on its low structuralself-weight and the ability to controland verify the structure in a shopenvironment. The overall concept of

tronically to computer mediumthrough a digitizing and probingprocess of the exterior surfaces. Thisdigitizing process resulted in a con-sistent three-dimensional wire net ofcontrol points that could be verifiedwith respect to the physical models.At this point, the geometric data wasmanipulated in the computer usingCATIA software which was original-ly developed for use in designingFrench “Mirage” fighter planes.CATIA was used to smooth andrationalize the various curved sur-faces as well as to create offset sur-faces locating the potential structur-al envelopes. With the offsetsurfaces defining potential locationsfor structural members available inCATIA, the architectural and struc-tural teams together developed adiscrete segmented “wireframe” ofnodal points and lines eventuallydefining the structural centerlines.The segmented structural wireframewas created by passing horizontaland vertical slicing planes at regularintervals through the offset surfacesin CATIA; the intersections becom-ing the locus of structural nodalpoints.

In creating the structural wire-frame models for the complex metal-clad surfaces in conjunction with thearchitectural team, a series of guide-lines were set in order to organizethe steel framework. These “rules”were imposed on the structuraldevelopment for the purposes of cre-ating a disciplined and regular pri-mary structure within the con-straints of the architectural design:1. All members would be

straight segments betweennodal points.

2. The grid spacing would beapproximately 3 meters by 3meters (10’x10’). This wasfound to be dense enough togenerally conform to thecurved surfaces while at thesame time allowing for rea-sonably dimensioned horizon-tal “band” trusses to be pre-fabricated and transported tothe site.

3. The structural nodal work-points would be a constant600mm (2’) dimension fromthe exterior clad surface.

4. Horizontal members would beat constant elevation except atsloping roof lines.

5. Inclined column memberswould be created by passing

vertical slicing planes normalto the ground surface throughthe offset surfaces in CATIA.The orientation of the columnweb would lie perpendicularto the exterior surface asdetermined by averaging thenormal vectors along the runof the column. The web orien-tation of the column wouldremain at a constant angle forthe full vertical run of the col-umn (no warping).

6. Diagonal members to be ori-ented in a tensile (Pratt)arrangement based upongravity load considerations.

7. Wherever possible, minimumsizes are to be used as follows(unless found analytically tobe insufficient structurally) tocreate economies in the struc-tural steel mill order and tosimplify the architectural/structural engineering coordi-nation process:— Vertically inclined columns:

standard, rolled, Europeanstandard HD 310mm x 310x 97 kg/m (approximatelythe cross-section of anHP12 x 63) and HD 260mmx 260 x 73 kg/m (slightlylarger than the cross-sec-tion of an HP 10 x 57) (50ksi yield).

— Corner vertical members:250mm (10”) diameter x10mm (.39”) wall thicknessseamless pipe section (42ksi yield).

— Horizontal members:160mm (6.3”) x 160 x 6mm(.24”) wall thickness squaretube (42 ksi yield).

— Diagonal members: 155mm(6.1”) diameter x 6mm wall(.24”) thickness seamlesspipe section (42ksi yield).

Standard connection geometriesand load-carrying capacities weredeveloped based on this limitednumber of sizes which could be easi-ly verified by the architectural teamin terms of interference with boththe exterior clad and interior dry-wall surfaces. The limited number ofcross sections for the frame mem-bers allowed for an efficient engi-neering verification process for all ofthe various imposed loads and loadcombinations. Upon analysis, it was

Modern Steel Construction / July 1998

In recognition of the museum’s prominence in the city, the owner’s wanted atower (above) to be included with the project.

Shown opposite is the building’s frame at the main entrance to the museum.

found that perhaps 95% of all of themembers in the complex three-dimensional frames were sufficientbased on the minimum number ofshapes with the remainder customsized based on their individualstructural requirements.

STRUCTURAL ENGINEERINGDOCUMENTS

Consistent with the uniquenature of the project, the structuralengineering design drawings wereorganized with a view toward theeventual use of the computer fordata manipulation by all members ofthe design and construction teams.In fact, the drawings were preparedprimarily as a check against thecomputer-based information withany discrepancies between the tworesolved by the various teams. Thedefinition of the geometry for themetal clad structures was based on aproject (x,y,z) coordinate system.Each nodal work point coordinatewas specified on the drawings as ref-erenced by inclined column markversus elevation. A schedule of pla-nar angles were prepared to definethe orientation of each column webin the project system. Further depic-tion of the frame geometry on therecord drawings was provided by aseries of three-dimensional isometricviews including continuations alongall interfaces between the variousforms. Each surface was also pre-sented in pure, unfolded elevationupon which the member sizes werekeyed to adjoining schedules for theverticals, horizontals, and diagonals.Standard framing plans were pro-vided for all gallery floors, roofs andparapets.

It was understood by all of theparties involved in the project thatthe computer files would be asimportant, if not more so, than thepaper drawings created. Very rigor-ous attention was paid to the dimen-sional accuracy of the computer-based information as this would bethe data actually used in the cre-ation of subcontractor shop draw-ings. To this end, the constructiondrawing computer files (in AutoCad.dxf format) for the project were sub-mitted to the contractors as part ofthe project documentation. For thecomplex metal clad structures, spe-cially created three-dimensionalcolor-coded “wireframe” files weresubmitted to the structural steel fab-ricator as well. These computer filesincluded the workline geometry ofthe frame in project coordinates as

Modern Steel Construction / July 1998

stone clad surfaces of the exterior, oronly partially expressed on the inte-rior with surrounding drywall enclo-sures; the structures for variousexternal complementary sculpturalforms featured the use of exposedpainted structural steel as a compo-nent in the overall architectural con-cept. These structures are unique insystem and geometry and also serveto demonstrate the powerful capabil-ities of the detailing routinesemployed by the steel fabricator forthe project. The exposed nature ofthese structures required tailoredsystems to be employed for each,quite different than those used forthe curved surfaces of the museumitself. These elements received amulti-coat paint system of shopapplied primer over sandblastedsubstrate, shop finish coat, and afinal coat applied in the field aftererection.

The 26m (85’) tall canopy or“Visor” overlooking the adjacentNervion River is approximately 25meters (82’) by 20 meters (66’) inplan and is supported on a singleslender, cantilevered vertical 2.0meter (6.5’) diameter steel pipepylon with a 40mm (1.6”) wall thick-ness. The Visor structure involvesfour cantilevered trusses taperingfrom four meters deep at the pylonto a minimum at the tip. The designfor the visor called for patterned andasymmetrical wind loads of up to200 kg/m2 (about 40 psf).

Without internal floors, the 50-meter (164’) tall figural “Tower” isnevertheless not without function.This element forms a visual markerextending the limits of the projectand embracing the existing Puentede la Salve bridge. The 200-meter(656’) long adjacent “boat” galleryextends below the bridge and even-tually is terminated at the towerbase. The exposed steel frame of thetower is partially clad in stone withthe internal structure visible tovarying degrees depending upon thepoint of observation. From a singlebase, the structure widens as itextends vertically and actually bifur-cates into two separate forms nearmid-height. All members are openwide-flange shapes with shop-weld-ed nodes at times requiring the join-ing of as many as 12 members at asingle point, all at non-orthogonalangles.

The museum was opened to thepublic in October, 1997. The univer-sal and repetitive nature of theunique system developed for the

Modern Steel Construction / July 1998

Pictured above is a horizontal truss prior to erection.

well as the member sizes indicatedby various colors keyed to a masterlist of section sizes. These computer-based wireframes were checked bythe steel detailer against the paperdrawings and formed the basis forthe creation of the steelwork shopdrawings.

STEEL DETAILING, FABRICATION & ERECTION

The steel fabricators for the pro-ject used a very powerful detailingprogram developed in Belgium,BOCAD, to create three-dimensionalgraphic files of the steel assembliesfrom the structural wireframesdeveloped by SOM. While each nodaljoint is unique in terms of the geom-etry of the interconnecting members,there were only a handful of differ-ent joint types that could be orga-nized in a subroutine and thenincluded in the BOCAD database.BOCAD was then able to completelydraw, three-dimensionally, theentire frame including connectionplates, bolts, member bevels, etc.From this fully three-dimensionaldata, individual piece drawings andsub-assemblies were culled in orderto create detailed and dimensionedshop drawings. This fully automatedprocess of shop drawing preparationwas responsible for creating pieceand assembly drawings virtuallyerror-free.

The horizontal truss “band” sub-assemblies were trial fitted in thefabrication shop to adjacent horizon-tal frames as well as bands immedi-

ately above and below. This resultedin very little site adjustment orreaming of bolt holes in the field andrelatively speedy erection of thesteel frames. The horizontal bearingplate at each node also forms theconnection point for the horizontaland diagonal members of the frame.The original design for the nodespecified shop welding of the hori-zontal and diagonal members to thenode plate but was later modified bythe steel fabricator, URSSA S. Coop,Ltd., to shop bolted connections toallow for more tolerance in the fit-upprocess. Partially complete frameswere found to have significant inher-ent stiffness, which allowed forstraight-forward erection with littletemporary bracing or shoringrequired to keep the membersaligned and within reasonable toler-ances. The steel fabricators as wellas the cladding system sub-contrac-tor utilized expected deflection plotsof the deformed structure plus a sig-nificant amount of spot surveying ofthe completed frame to verify thatthe erected structure was indeedwithin tolerance and behaving asanalyzed by SOM. All of the steel-work for the project was sandblastedto receive a shop-applied primer dueto the exposed and highly visiblepublic nature of the work on themuseum.

EXPOSED STEEL SCULPTURES

While the majority of the steel-work for the museum is concealedwithin the metallic titanium and

Project TeamOwner: Consorcio Del Proyecto Guggenheim Bilbao,

Bilbao, Spain

Struct. Eng.: Skidmore, Owings & Merrill LLP, Chicago

Architect: Frank O. Gehry and Associates, Santa Monica, CA

Architect/EOR: IDOM, Bilbao, Spain

Fabricator: URSSA, S. Coop. Ltd., Vitoria-Gasteiz, Spain (also includes detailing & erection)

Guggenheim Museum Bilbao result-ed in a unit steel price for the 3,900tons of structural steel comparableto a standard steel framed buildingproject and was a factor in the pro-ject opening on-time and within bud-get. Advances in the use of comput-ers for steel detailing and fabricationnow allow for the realization ofextremely complex projects whichperhaps ten years ago would havebeen deemed impossible. It can nowbe said that the versatility of steelhas been extended to new applica-tions in geometrically free-formarchitecture.

Hal Iyengar is a Retired Partner,Lawrence Novak is an Associate,Robert Sinn, Associate Partner, andJohn Zils, Associate Partner withSkidmore, Owings & Merrill LLP, inChicago.