Humane Urban Aesthetic: US-183 Elevated Project in Austin, Texas

Dean Van Landuyt, Texas Department of Transportation

Designers at the Texas Department of Transportation were tunately these practices have dulled creativity and re• faced with building an elevated bridge 1.3 million ft^ sulted in ubiquitous, sterile architecture. By immediately through the city of Austin. Limited right-of-way and the applying the near-universal technique, many designers inherent size of an overhead highway meant that there ex• bypass (or aren't allowed to ask) the fundamental ques• isted the possibility of building an oppressive architecture tion that begins the creative aesthetic process: "What among the homes and businesses of the US-183 corridor. are we trying to express?". Instead of applying the accepted techniques of modern The designers at The Texas Department of Trans• bridge aesthetics, engineers used new fundamentals for de• portation (TxDOT) were faced with building 1.3 mil• signing appropriate regional bridge architecture. The aes• lion ft^ of elevated bridge through the city of Austin. thetic design philosophy was to build this important public Limited right-of-way and the inherent size of an over• structure so that it expressed the spirit of Austin as defined head freeway meant that there existed the possibility of by the city's innovative high-technology industry, tradi• building an oppressive architecture among the homes tional state architecture, and most importantly its humane, and businesses of the US-183 corridor. Engineers devel• livable quality. Engineers tapped into the iconoclastic cre• oped the philosophy that they should create an archi• ativity common to so many Austin artists. Aesthetics was tecture that reflected the values and culture of the given equal importance with the other constraints of bridge community. design: functional requirements and economic demands. Integrity of structure can only be achieved when all three of these elements are fully examined and integrated into THE CITY every level of decision making. The result is a sound, eco• nomical structure that reawakens the notion of creative Austin is a highly educated university city that is known detailing in large-scale bridge architecture. for its progressive technological and arts communities and the high quality of life that its residents enjoy. In• novation abounds in this city that is only the 27th larg• uch of the bridge aesthetic design today is a est in the country but 7th in the number of patents its result of engineers simply following tech• citizens are granted. A total of 480 software firms are M nique. Acceptable practices such as reducing headquartered here. The University of Texas is a major the number of structural elements, thinning members, cultural force not only in the curriculum it provides but and creating clean lines are readily embraced. Unfor• also in its progressive atmosphere for ideas that it fos-

60 VAN LANDUYT 61

ters. Students as well as artists are attracted to this oasis in a previous construction contract and currently carry of creative expression. Many of the arts are embraced all US-183 traffic. The elevated project was let in No• here, but Austin is perhaps best known for its thor• vember 1993, and expected completion is March 1997. oughly original music scene. The project is complicated by the need for four ele• Visitors and new residents are often surprised by the vated entrance and exit ramps that provide access to unexpected beauty of a Texas city. Set on the edge of the elevated mainlanes. Three of these ramps also need the hill country, Austin has an abundance of trees and to cross at-grade roads at a heavy skew. The other is a lakes. Austinites are protective of their environment, in• direct connector on the fourth level of the IH-35 inter• cluding the neighborhoods and architectural treasures. change that also crosses lower roadways at a large skew. With a population of 500,000, the city provides a del• The US-183 mainlanes will rise to the third level to pass icate balance of economic and cultural opportunities over the second level IH-35 mainlane bridges. US-183 without compromising its humane, small-town charac• frontage roads remain on the ground level. teristics that make it so livable. Structural Requirements

DESIGN PHILOSOPHY Generally speaking, the structural demands are not un• usual for a project of this type. The fourth-level direct The design philosophy for the US-183 elevated project connector requires a girder deep enough to span 54.9 is to balance the functional requirements, economic m (180 ft). Mainlanes are required to span 36.6 m (120 constraints, and aesthetic qualities of the bridge. The ft) over IH-35. Typical of most interchanges, the struc• functional requirements consist of project geometries tural system has to be able to accommodate flaring and structural demands. Economic considerations in• roadway widths where ramps merge with mainlanes. clude first cost and long-term maintenance of the struc• Perhaps the most unique feature is that the mainlanes ture. The visual elements should contribute to the over• have to cantilever 6.7 m (22 ft) over the frontage roads all aesthetic theme. Integrity of structure could be (Figure 1). achieved only if all three of these factors were fully ex• amined and incorporated into the bridge. Only a general aesthetic theme could be outlined be• Economic Requirements fore the structural design. Thereafter the myriad of aes• thetic decisions were made within the context of struc• The project is so large that it consumes a significant tural design where they were balanced by the functional portion of the budget of the local TxDOT district. En• and economic constraints. Consequently, designers had gineers were creative in their efforts to design a bridge to be sufficiently skilled to move quickly between and that would be as inexpensive as possible. However, de• capably within all three requirements. partment officials were willing to spend more money than for a typical bridge if all three frontage road lanes Functional Requirements could remain open during the daytime construction op• erations and aesthetics were improved. Because of the elegant shape of winged box girders, the ability to set Geometries segments at night while one lane of frontage road traffic was shut down, and the potential for a low square-foot US-183 is an urban corridor with two functions: to cost, a decision was made to build a precast segmental carry express traffic across the northern part of the city concrete superstructure. and to operate as an arterial with extensive commercial and residential development. To meet both of these needs, the corridor will consist of an elevated freeway and at-grade frontage roads that provide access to city Aesthetic Requirements streets and adjacent driveways. The elevated freeway will be composed of two separate, parallel structures Bridge architecture in the United States has become that each carry three lanes of mainlane traffic for a dis• standardized to the point that there is little regional tance of 2 mi. A gap 2.2 m (7 ft) wide between them identity. Fueled by the creative energy of Austin's artis• will allow for a possible future high-occupancy-vehicle tic environment, engineers took on the ambitious task bridge. To limit the purchase of expensive right-of-way, of designing a bridge that embodied the spirit of Austin. the elevated structures will be supported on columns in Three ideas were adopted to convey this feeling: ex• the wide median and will cantilever over the frontage pressing high technology, referencing architectural tra• roads. The frontage roads have already been completed dition, and creating a humane environment. 62 FOURTH INTERNATIONAL BRIDGE ENGINEERING CONFERENCE

I. Im 7.7 m (58 ft) •(3. 5' ) MA INLANE

-t PROJECT

6.7 m (22 ft)

1.6 m (38 ft) FRONTAGE ROAD

FIGURE 1 Mainline column and superstructure adjacent to frontage road.

High Technology Monotony can rarely, if ever, enrich the visual experi• ence. Unfortunately there is an explosion of sameness US-183 is also known as Research Boulevard—so in today's bridge designs. TxDOT designers sought first named because of the large number of high-technology to create a variety of elements but maintain harmony— firms located there. This area is home not only to in• a delicate proposition. Second, because of the large size novative software and microelectronic firms but also to of the project and the number of commuters and resi• the Phil M. Ferguson Structural Engineering Labora• dents on the frontage roads and city streets, it is im• tory. This University of Texas laboratory is known portant that a personal scale be used where possible. worldwide for its outstanding research in the design and Large elements could be divided into smaller, human- behavior of segmental concrete bridges, much of which size parts. Finally, the structure should manifest a hu• is funded by TxDOT. Engineers chose to expose the man presence. Too many of today's bridges exude a high-technology structure of the bridge as much as pos• coarse, simple, and austere appearance. US-183 design• sible. Elements typically hidden in other bridges were ers were free to utilize creative talents to build a bridge to be revealed where appropriate. with fine proportions and visual complexity and delight.

Architectural Tradition DESIGN PROCESS It is proper that Austin's largest highway project refer• ence other state architecture located in the city. Perhaps Superstructure the greatest architectural treasure is the century-old neoclassical state capitol building. Nearby buildings on The structural design began by optimizing span lengths the original 40-acre campus at the University of Texas originally set by roadway designers. Lower roadway are richly detailed in a Spanish mediterranean style. geometries force a direct connector span arrangement While referencing these wonderful structures, it was im• of 38.1 m-54.9 m-43.3 m-54.9 m-38.1 m (125 ft- perative that new designs not be hackneyed or slavishly 180 ft-142 ft-180 ft-125 ft) that is best buih by the old fashioned. balanced cantilever method. A minimum girder depth of 2.1 m (7 ft) is required for these spans. The remainder Humane Environment of the project (195 spans) can be most economically con• structed with the span-by-span construction method. Be• The most important aesthetic principle is to build a hu• cause the direct connector ties into the mainlanes, a 2.1- mane architecture for commuters, workers, and resi• m (7-ft) girder depth was chosen for the entire project. dents of the area. There are many ways to create a hu• Roadway designers had set columns for the span-by- mane environment, but three principles are followed. span bridges such that the average span length was 29.5 VAN LANDUYT 63

m (97 ft). Structural designers fully utilized the girder depth and rearranged the column layout. By extending the maximum span to 40.9 m (134 ft), the average length increased to 38.4 m (126 ft). More than 60 foun• dations, columns, and span construction cycles were thus eliminated. Geometric requirements dictate that the mainlane cross section be very large. An overall width of 17.7 m (58 ft) is necessary to support three lanes and shoulders. A 4.9-m (16-ft) nominal width box is required for sta• bility. A smaller scale is achieved by chamfering the ;n,- transverse joints between the segments. Thus the seg• mental persona of the bridge is suggested rather than hidden. Box girders typically are cast with a rigid corner at the intersection of the web and bottom slab of the box. A large chamfer creates a more organic feel that visually lightens the superstructure. Plinths, necessary to provide level bearing when the roadway surface is in a grade or cross slope, are accentuated to make the su• perstructure appear less earthbound. Moreover, they create a sense of order within the superstructure. These unique segments, which are designed to handle the heavy bearing and post-tensioning anchorage forces, are visually differentiated from typical segments. Harmony is achieved by incorporating these same elements into the smaller ramp girders [8.5 m (28 ft) wide] (Figure 2).

Substructure •4

Engineers started substructure design by examining the FIGURE 3 Small ramp piers in tandem to support flaring structural and geometric demands of the 260 columns box girder. on the project. Necessary bearing areas for the super• structure, available footprints, and type and magnitude I > of loadings were all categorized. It was determined that types. These include 115 mainlane piers, 132 small all demands could be met with as few as three column ramp piers, and 13 large ramp piers. The large ramp piers are needed to sustain severe balanced cantilever loadings from the direct connector. The small ramp piers are versatile enough to support ramp box girders, straddle bents, and the entire flaring superstructure re• gion (Figure 3). The small number of pier types meant that costs could be reduced because of reuse of forms and repeatability of tasks. Visual harmony is also more easily achieved when assimilating fewer elements. All of the columns on this project were originally designed as precast segmental piers post-tensioned with vertical tendons. Designers thought that this would be the most economical way to construct elements that were so repetitive. Close proximity to adjacent frontage roads, reduced field construction time, better quality control for casting complex column shapes, and the availability of the casting yard also made this an ap• pealing economic option. The resulting segmented ap• pearance of the columns would also fulfill the aesthetic FIGURE 2 Ramp girder and small ramp capital. vision. Similar to the superstructure segments, the con- 64 FOURTH INTERNATIONAL BRIDGE ENGINEERING CONFERENCE

needed to resist axial loads and moments. The larger bearing area combined with the large bases required for constructability hinted at the possibility of classical three-part columns.

Mainlane Columns

The bearings of the mainlane superstructure are so widely spaced that ill-proportioned, top-heavy piers commonly seen in bridges across the United States could have easily been designed. There was little to be done about the required top width of 5.2 m (17 ft), so en• gineers created the illusion of a smaller capital by elim• inating the concrete between the bearings (Figure 4). Load is directed downward by diagonal concrete com• FIGURE 4 Mainline capital. pression struts. Four steel pipes 0.20 m (8 in.) in di• ameter act as tension ties and reduce much of the flex- ural moment in the diagonals. The pipes are anchored struction history of the piers would be revealed and a on the exterior of the capital to create a well-confined more human scale presented at the all-important front• structural node. The concrete was also removed with age road level. After much deliberation, the winning the intention of providing a bearing ledge for erection bidder, Eby Contraction, asked the state to redesign the equipment (Figure 5). The contractor will dispense with mainlane and small ramp piers as cast in place in return tower scaffolding and instead fit a bracket into the for a small savings. TxDOT obliged. In keeping with opening to efficiently carry the tremendous weight of the already established aesthetic concept and creating longitudinal erection trusses and unstressed segments harmony with the still-precast large ramp piers, the ap• [about 635 Mg (700 tons)]. pearance of the cast-in-place piers remained almost The pier maintains an element of drama throughout identical to that detailed in the original plans. Trans• its height balanced by reassuringly correct structural verse joints at 1.2-m (4-ft) intervals were maintained as forms (Figure 6). Proportionally undersized tension were the oversized bases originally designed as cast-in- pipes at the top of the capital tenuously tether two large place connections between the first precast segment and diagonals. All of this is revealed by slicing away the the foundation. concrete between the bearings and abruptly interrupting The columns are the visual focal point of the project the horizontal rustications. The thrusts of opposing di• because of their close proximity to frontage roads and agonals are balanced by a single, inverted keystone but city streets. All three column types require superstruc• the visual duality is only briefly unified. A deep rusti• ture bearing areas larger than the column cross sections cation, defined by tracing the parabolic inside edges of

LONG ITUDINAL TRUSS

PIER BRACKET

FIGURE 5 Mainline capital notch used to support erection equipment. VAN LANDUYT 65

FIGURE 6 Mainline column.

the diagonals to their origin as vertical tangents, re• Ramp Columns sumes the duahty and slims the shaft. The division is finally and convincingly resolved by a large, banded The small ramp and large ramp piers harmonize with base. the mainlane piers. Because the ramp superstructure A large abacus (the flat slab at the top of the column) bearings are close together, a single undivided capital is crowns the diagonals for a variety of reasons. It in• used. The abacus, closely spaced horizontal rustications, creases the available bearing area not quite given by the and oversized lower capital terminus piece are similar flaring diagonals, references classical architecture, and to those on the mainlane piers. The shaft cross section should improve the long-term appearance of the pier. In is a square with its corners removed by the same heavy spite of sealed fingered joints in the superstructure, it is 0.20-m (8 in.) chamfers that are used on the mainlane almost a certainty that water will trickle down the bear• piers. The segmented shaft is supported on the base that ings and reach the top of the column. Smoothly detailed visually balances the capital. f columns that flare to follow the shape of the girder his• torically have provided a large canvas to display stains. I All US-183 piers are detailed to channel the water to Balancing Functionality, Economics, and confined, shadowed areas. A berm 32 mm (IV4 in.) tall Aesthetics ^ on the top of the capital is broken only above vertical rustications on the side of the abacus (Figure 7). A drip Good design requires the integration of functional re• bead formed with a 25-mm (1-in.) chamfer strip on th'; quirements, economic restraint, and aesthetic quality. underside of the abacus should prevent water fr'm Functional demands of the structure provide the origin reaching the compression diagonals. for all design. From this the engineer relies on creativity 66 FOURTH INTERNATIONAL BRIDGE ENGINEERING CONFERENCE

Berm-^ Berm Berm gaps and ver+Ical reveals Berm to channel water

Dr ip Bead —

FIGURE 7 Mainline column berm, rustications, and drip bead to reduce water staining.

to instill beauty and meaning while respecting the eco• the single most expensive architectural enhancement. nomic burden of the taxpayers. Most of the unique ar• While the extra cost of a complex form is essentially a chitectural features probably added little to the overall one-time cost amortized over many piers, the fabrica• cost of the bridge. The project bid at $39.00/ft^, about tion of the transverse pipes and the flaring reinforcing the same as other more traditional span-by-span seg• cage are incurred at every pier. Nonetheless details were mental bridges in Texas, but well below an average U.S. assimilated to control costs and meet structural de• bridge cost of approximately $70.00/ft^. The interplay mands. The void was shaped by smooth planes perpen• of the three requirements can perhaps best be under• dicular to the upstation and downstation faces of the stood by examining the design of the mainlane column. capital to allow a collapsing interior form to be easily Aesthetic design of bridge columns is frequently con• removed. strained by a need to build columns of varying height. Realities of structural demands dashed hopes of a Costs will increase slightly when a gracefully chang• more graceful mainlane pier. Early sketches were drawn ing cross section is introduced for a large number of to fine proportions using a smaller cross section for the constant-height columns; building designers often use entire column below the level on the capital void. Mo• economy of scale to afford their constant-height col• ments in the diagonal capital arms resulting from un• umns with gentle tapers or classically styled entasis (the balanced gravity loads and wind mandated the use of a subtle convex shape of the column shaft). However, wider column. costs become prohibitive for a project such as US-183 where mainlane columns range from 3 m (10 ft) to 13 m (43 ft). Therefore, unhke true classical columns the PUBLIC REACTION column shaft cross section was kept constant. It became an economic maxim then that visual com• All artists must first meet their own personal tests of plexity could be given to repetitive elements of constant truth and beauty. The individual painter or sculptor dimension. The capital could thus be flared on a num• need go no further, but the government designer has a ber of planes if it was maintained at a constant height duty to provide aesthetic quality to a diverse public. The and only the length of the shaft varied. Reveals and unusual visual styling of the US-183 elevated project chamfers were inexpensive additions because they were was determined by the engineers and technicians of the detailed with respect to necessary reinforcing steel cover TxDOT Design Division and supported at the admin• requirements, cage construction, and form assembly istrative level both at the division and the Austin dis• and removal. The void in the capital supports erection trict. The public did not provide any input into the ar• equipment, visually lightens the capital and reveals the chitectural theme. Public reaction, however, has been structural requirements of the capital but is probably overwhelmingly favorable as evidenced by news re- VAN LANDUYT 67

ports, letters to the editor of the local paper, letters to technology industry, traditional state architecture, and TxDOT, and enthusiastic citizen comments made to the most importantly its humane, livable quality. designers. Of course, all innovative public art will have To accomplish this, engineers tapped into the icono• its detractors (and perhaps it should lest it not be very clastic creativity common to so many Austin artists. In• innovative), but they have been very few. tegrity of structure can be achieved only when aesthetics is fully integrated with functional and economic de• mands at every level of decision making. The result is a bridge that reawakens the notion of creative expres• CONCLUSION sion in large-scale bridge architecture.

Engineers at the TxDOT were challenged by the task of designing a large elevated highway in the city of Aus• ACKNOWLEDGMENT tin that enhanced the human aesthetic appeal of the city. Rather than applying the accepted techniques of mod• A project of this magnitude requires the combined tal• ern bridge aesthetics, engineers used new fundamentals ents of a great number of people. There are many re• for designing appropriate regional bridge architecture. sponsible for the so-far successful completion of this The aesthetic design philosophy is to build this im• bridge, but the author would like to specifically thank portant public structure so that it expresses the spirit the many engineers and technicians who worked so dil• of Austin as defined by the city's innovative, high- igently on the design.