THE M U S E U M BRIDGE for

SHINJI SHUMEIKAI

A POST-TENSIONED STEEL SPACE-FRAME

Katherine E. Hill, Associate Engineer

Saw-Teen See, Managing Partner

Leslie E. Robertson, Director of Design

Daniel A. Sesil, Partner

Leslie E. Robertson Associates, R.L.L.P. 211 East 46th Street

New York, New York 10017, U.S.A.

ABSTRACT

The Shinji Shumeikai Museum is now under construction in Shiga-raki, a rugged and mountainous area near Kyoto, Japan. The approach to the museum is via a processional bridge spanning 120 m from the mouth of a tunnel to the museum entrance, which is located on an adjacent ridge line. This steel bridge uses an innovative combination of elements from post-tensioned, cable-stayed, and cantilever bridges to produce an elegant form and a highly efficient structural system. This paper discusses some of the key elements of the bridge's structural design. Additionally, special attention is given to the self-draining steel and ceramic bridge deck, believed to be the first use of its kind.

KEY WORDS

bridge, steel, post-tensioned, space-frame, grating, ceramic in-fill, earthquake, typhoon, Japan, museum

PROGRAM AND SITE

The site for the Museum Bridge is a remote and wooded valley amidst the mountains of Shiga-raki in west, central Honshu, Japan. At one time, timbers from this very valley were used for temple construction in nearby Nara. Today, the area is a nature preserve resulting in strict regulations as to what can and cannot be constructed and strictly limits access to the site. Accordingly, the Museum Bridge minimizes its impact on the ecology by spanning the valley without intermediate supports.

The bridge is part of a larger complex that includes a reception pavilion with gardens, two tunnels, and an art museum. While the bridge is designed for vehicular traffic, access to the museum is intended to be on foot. Most visitors will park their cars or will arrive by busses at the reception pavilion located at the base of a steep incline, and will then proceed to walk through the hillside via a winding valley before entering a tunnel; they emerge from the tunnel onto the bridge from which, across the valley, they first catch their glimpse of the entrance to the museum.

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The ceremonial nature of the procession to the museum, combined with the high visibility of the bridge structure to pedestrians, calls for careful consideration of the aesthetic forms and of the structural detailing.While the basic design responsibility for the bridge rests with the structural engineers, Leslie E. Robertson Associates, R.L.L.P., the authors are deeply grateful to architect I.M. Pei (design consultant for the Shinji Shumeikai Museum) for his critique on many aspects of the bridge.

While intended primarily as a pedestrian bridge, both the bridge structure and the grating system were designed to support two lanes of stretched-limousines, with wheel loads up to 12 kN. The bridge need not carry heavy truck traffic as a separate tunnel provides both for service and for fire truck access to the museum. The addition of planking on top of the bridge deck permits the occasional accommodation of heavier wheel loads, for example, from the armored vehicles sometimes used to transport dignitaries.

Major earthquakes, typhoon winds and the dynamic behavior of the bridge under both foot and vehicular traffic provide the guiding design criteria for the most of the structural systems.

STRUCTURAL SYSTEM

The design of the bridge makes use of an innovative combination of elements from the technology of post-tensioned, cable-stayed, and cantilever bridges to produce a visually elegant and a highly efficient structural system. Figures 1 and 2 show the four primary structural components: the tunnel mouth, the space-frame structure, the arch, and the post-tensioned cable system.

Tunnel

With the stratigraphy of the underlying rock being of high quality, the tunnel was selected as the root for cantilevering of the bridge out over the valley. The cantilever is achieved by means of a moment couple formed between the axial compression in the space-frame structure at the bottom of the tunnel mouth and the axial tension in the post-tensioned cables anchored around the top of the tunnel mouth. The space-frame structure is structurally continuous with the floor of the tunnel and the cables are coupled to post-tension tendons buried in the wall of the tunnel. The tunnel itself is thus subjected to bending (but not tension) on account of gravity and lateral forces acting on the bridge. Except for the post-tension tendons, Shimizu Corporation, the general contractor for the entire project, is responsible for the design of the structure of the tunnel.

Space-Frame Structure

Structural steel was selected as the material of construction on account of the limited accessibility and ruggedness of the site and because it was felt that a light and airy bridge was aesthetically preferable to that which was possible for concrete construction. To the extent practical, in order to provide for speed and economy of construction, structural steel pieces are prefabricated into elements of a size that can be transported through the tunnels.

The main spanning element for the bridge is a three-dimensional steel space-frame structure only 2 m in depth, despite the 120 m span. The most of the members of the space frame are pipes. The space frame is arranged in a triangular configuration, with three top chords along the width of the 7500 mm roadway and a single bottom chord (Figure 3). Diagonal bracing in the horizontal, vertical, and sloped planes completes the truss geometry. The largest truss member is a mere 267 mm in diameter, with the remaining members 216 mm and 140 mm in diameter. The hierarchical arrangement of members is reflected in the choice of diameters; the diameter of a particular type of piece remains generally constant while the wall thicknesses and steel grade vary in accordance with loading and with performance criteria. Individual members are

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connected typically by pipe-to-pipe complete penetration welds; in non-visible areas, bolted end flanges are used for the field splices.

The space frame is fabricated and is erected on shores in an irregular profile such that, as the cables are post- tensioned, the bridge rises to a slightly cambered profile of about 400mm (about span/300). This methodology introduces bending moments into the space-frame...moments which are later relieved as imposed live loads are added onto the bridge. In this way, design bending moments are essentially halved and the axial loads from the post-tensioning are nicely balanced between the four longitudinal chords.

An additional two top chords are added at the cable anchorages, increasing the width of the space-frame structure to 11250mm in these locations (Figure 4). The upward thrust from the cables is resisted by tie down rods connected to the bottom chord. Following fabrication and during assembly, the tie-down rods are pre-shortened; this procedure bends the cross-section of the space-flame downward to allow for the later lengthening of the rods under post-tensioning. In this way, secondary stresses in the space-flame are reduced significantly.

At the top chord, the cables are anchored by threaded rods which pass through steel castings at the nodes of the space-frame structure. The castings accommodate the detailing of these highly visible connections and accept readily the variation in cable angles and the like at each connection.

At the museum-end of the bridge, the primary gravity support is located 6 m inward from the end of the bridge. To counteract the upward motion at the end of the bridge that would otherwise be created by imposed live loads on the main span, a system of tie-downs is employed, located at the very end of the bridge. The primary gravity support, when coupled to the tie-downs, effectively provides for full rotational restraint at this abutment while allowing for longitudinal expansion and contraction and for both lateral and torsional restraint under earthquake loading.

Cables

The cables, anchored at the tunnel mouth to the post-tension tendons buried in the tunnel, span to an inclined arch which functions in a manner similar to the vertical pier of a cable-stayed bridge. From the arch, the cables fan out to support the space-frame structure, being connected every 2 m along about half of the length of the bridge span. These cables are post-tensioned to provide a net upward force on the bridge, which force is counteracted with the application of imposed live loads. Post-tensioning jacks are required only at the deck-level of the bridge, not at the arch or at the mouth of the tunnel.

A second system of cables connects to a kingpost beneath the flee-spanning portion of the space-flame structure (Figure 1). These cables are post-tensioned to provide uplift forces and deflection control in a manner analogous to the use of deflected tendons in post-tensioned concrete beams. The post-tensioning of these cables is accomplished by vertical jacking at the kingpost, which allows the length of the kingpost to be increased with screw-type fittings.

Post-tensioning of the cables (kingpost cables and bridge-to-arch-to-tunnel cables) reduces the bending moments in the pipe structure, permitting the use of a shallower structure depth and smaller pipe diameters than would be possible for a non-post-tensioned bridge of this span.

The cables, utilizing a galvanized spiral-rope configuration, consists of six different cable diameters ranging from 22.4 mm to 60 mm. Following two larger-diameter cables, the cable diameters increase gradually in size as one goes from the base of the arch to the top of the arch (from the abutment of the bridge deck to the center of the span), resulting in high structural efficiency as well as a visually pleasing pattern. Aside from these aesthetic considerations, cable sizes were governed more by stiffness and performance criteria than by strength.

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Arch

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The use of an arch at the tunnel-end of the bridge allows for a significant reduction in cable sizes and for reduced axial compression in the bridge.

The arch slopes away from the tunnel mouth at an angle of 45 degrees from the horizontal. When viewed along the axis of the bridge, the arch consists of a built-up steel box section that follows a smooth curve similar to that of a parabola. The cross section of the arch varies along the length of the curve, evolving from a combination of structural and aesthetic considerations which results in high structural efficiency while giving visual definition to the surface of the arch.

The arch is pinned at the base with a bearing-type ball that is free to rotate in all directions. The arch is held in place onto the ball by the high compressive forces resulting from the post-tensioning of the cables.

ANALYSIS

The structural analysis was performed using the STAAD-III analysis program. While cable stiffnesses were reduced appropriately to allow for the sag, cables were treated as truss members within the model; cable sags are sufficiently small so that the truss analogy provides a reasonable approximation for purposes of the computer model.

The post-tensioning loads were modeled by using applied strains (in the guise of fictitious thermal loads). As noted earlier, these post-tensioning loads were adjusted carefully to provide uplift forces which counteract the imposed live loads...without over stressing the space-frame structure when only dead load is acting.

WIND TUNNEL TESTING

The aerodynamic behavior of a light-weight bridge span is always of concern to designers. The Museum Bridge, sited over a steeply sloping valley in an area subject to typhoon winds, is of particular interest. Accordingly, both a section model and a full aeroelastic model were constructed and were tested at the Boundary Layer Wind Tunnel of the University of Western Ontario. As well, a study was accomplished to demonstrate that lightweight electric vehicles, such as golf carts, would not be made unstable under sensible wind conditions.

The initial investigation made use of a section model with a geometric scaling of 1 to 60, with the model mounted in a rigid dynamic force balance. The tubular elements of the bridge were scaled directly on the appropriate diameter. No evidence of dynamic instability was found.

Encouraged by the results from the section model, a full aeroelastic model was constructed...this time to a scale of 1 to 100. An extensive site model of the rugged terrain was constructed with the goal of simulating the wind turbulence associated with the actual site conditions; the site model encompassed a scale area of 500 m by 2000 m. The test results indicated that the dynamic response of the bridge is characterized by buffeting action, without pronounced peaks at particular wind speeds. As well there was no rapid growth of response.., all for mean wind speeds at the deck level of up to 90 m/s. In short, the structural loads on the bridge structure are dominated by earthquake and not by wind.

GRATING SYSTEM

The largest structural element in the bridge is a pipe of only 267 mm diameter. It was realized at the outset of design that rainwater should not be allowed to cascade off of the bridge and that suitable drainage piping

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would need be provided. The required drain pipes proved to be as large as or larger than the structural elements, seriously impact the beauty of the design.

Borrowing an idea from tennis court construction, LERA developed a solution that makes use of a porous ceramic infill in a stainless steel grating. This ceramic material, properly bonded to the grating, provides a host of benefits:

�9 A free-draining roadway is achieved, not all that different from open steel grating, so that drainage piping is not required. In essence, the water passes through the grating, falling in drops to the land below, without creating erosion channels in the soils.

�9 Unlike the usual grating deck used for bridges, the ceramic in-fill provides a visually-opaque surface so that persons not accustomed to heights will be able to pass over the bridge without experiencing vertigo.

�9 A high-friction surface, in both the wet and dry states, allows a sure-footed experience for pedestrians and vehicles alike. As well, women in high-heels will be able to walk comfortably on the surface.

�9 A handsome, dignified and interesting walking and driving surface is achieved, compatible with the processional nature of the bridge itself.

Light weight and small depth is achieved so as to not detract from the elegant nature of the structure of the bridge. The low weight of these panels is particularly important when considering the very high levels of seismic excitation anticipated from the maximum credible earthquake.

The grating configuration consists of stainless steel bars, 5 mm by 50 mm, spaced at 33 mm on center. To reduce weigh, the ceramic material is 35 mm in total thickness, only partially filling the depth of the grating. Stainless steel was chosen for aesthetic reasons and to minimize the maintenance cost of the bridge. The remote location in the mountains, the difficulty in keeping the museum in operation while maintenance operations are underway and so forth, all add to the desirability of a low-maintenance surface.

The selected ceramic material is of a soft gray color which contrasts nicely with the color of the stainless steel bars of the grating, as well as with the gray paint to be used for the structure of the bridge.

The entire assembly has been tested for freeze-thaw resistance in accordance with Japanese codes and standards. Subsequent punch-through tests of the ceramic material shows the assembly to be conservatively designed.

ACKNOWLEDGMENTS

It is not practical to give proper credit to the many persons who have contributed to the design and to the construction of the Museum Bridge. Even so, it is essential to recognize the following:

Ms. Kaishu and Ms. Hiroko Koyama of Shinji Shumeikai, who provided essential inspiration and encouragement;

Dr. I.M. Pei, design consultant for the Museum, who first proposed the concept of the bridge and the tunnel as the ceremonial approach to the museum. While the structural design was entirely within the purview of Leslie E. Robertson Associates, the aesthetic design was a collaboration between Dr. Pei and Mr. Robertson. Additionally, Dr. Pei gave freely of his time and provided invaluable guidance throughout the course of the design;

Mr. Osamu Sato, Chief Architect of Kibowkan International, who assisted the authors in wending their way through some of the intricacies of the rules and regulations of Japan;

Mr. Yasumitsu Watanabe and Mr. Masayuki Kawasaki of Shimizu Corporation, Main Contractors for all aspects of the Museum, who provided essential coordination with the construction arm of Shimizu Corporation; and

Of most importance, the men and women of Leslie E. Robertson Associates, who provided their energies and their unsurpassed talents in creating this design.

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FIGURE 1: ELEVATION

FIGURE 2: PLAN AT SPACE FRAME

FIGURE 3: TYPICAL SECTION

FIGURE 4: SECTION AT CABLE ANCHORAGES