The Design and Installation of the Hopland Casino Bridge

Transcription

1 The Design and Installation of the Hopland Casino Bridge Paul C. Gilham, P.E. Chief Engineer Western Wood Structures, Inc. Tualatin, Oregon USA 1. Summary The Hopland Band of Pomo Indians constructed a Glued-Laminated Timber (GLULAM) Bridge to provide access to their new casino facility from a parking lot located across a 54.9m wide ravine. The ravine bisected the site and was environmentally sensitive. The sensitivity of the site dictated that no intermediate piers would be allowed and limited the access during construction. A threehinged arch was chosen as the main structural element to span the 54.9m. The construction of the bridge was accomplished entirely outside the environmentally sensitive areas. Figure 1. Hopland Casino Bridge 2. Project Overview The Hopland Band of Pomo Indians determined to build a casino with adjacent parking on land near Hopland, California. A deep ravine bisected the site selected. In order to meet the parking requirements of the casino, a secondary parking lot would need to be constructed on the far side of the ravine. Access to this secondary parking lot would be by means of a new bridge across the ravine. The creek bed in the bottom of the draw was considered environmentally sensitive. Furthermore the downstream neighbors were very sensitive to turbidity in the stream and were vigilantly watching the construction of the casino and parking lots to make certain that construction did not affect the stream s clarity. For this reason, the project manager desired a clear span bridge to connect the secondary parking lot to the main lot and casino.

2 3. Design Criteria The distance between the tops of the banks at the bridge crossing was 54.9m. There is an elevation difference of 2.74m from end of the bridge to the other. This equates to a 5% slope for the length of the bridge. The project manager determined that a single lane bridge with room for pedestrians would be adequate. One traffic lane 3.66m wide was placed adjacent to the 1.83m wide walkway. Therefore the total width of the roadway was 5.49m. The local building official required that the bridge be designed to conform with the 1997 UBC requirements for pedestrian live load, rail-opening requirements, and for wind and seismic loading. UBC specifies a 4.79 MPa live load for pedestrian bridges, which is greater than the 4.07 MPa specified by AASHTO. The UBC allows a live load reduction based on the influence area for the member under consideration with a maximum reduction of 50%. The UBC specifies that rail elements shall not allow a 102mm sphere to pass through the rail. This is more restrictive than AASHTO which specifies a 152mm sphere for 381mm above the deck and an 204mm sphere above this elevation. The UBC specifies a 36 m/s wind speed with exposure B and the bridge is in seismic zone 4. The requirements for vehicle loading were based on AASHTO Standard Specifications for Highway Bridges. A H10-44 (9.09 metric tons) vehicle was used as the design vehicle. This was to allow for emergency vehicle access to the parking lot. The H10 vehicle was considered an infrequent load case where a load duration factor, C D, of was considered appropriate. AASHTO specifications do not require the use of an impact factor in the design of timber bridges. Therefore the loads from the vehicle were not increased for impact. 4. Bridge Layout Western Wood Structures, Inc. engineers determined that a three-hinged glued-laminated (GLULAM) timber arch with a longitudinal GLULAM deck panel floor system would be the most economical solution for this site. Additionally, the owners preferred the graceful appearance of the curved arches and the open sightlines provided by the suspended floor system. The bridge measures 54.9m horizontally, center to center of arch pins. The arches have a 42.67m radius of curvature resulting in a 40 degree spring angle. This angle provides for an economical yet graceful structure. A lower spring angle results in higher moments in the arch members and horizontal thrust at the bridge ends. As the spring angle is increased, the member forces and horizontal thrust are reduced but the arch appears boxy. The arches are spaced 6.71m apart. This allows sufficient room for the curb and rail system and the floor beam hangers. To further enhance the open sight lines, the owners chose a rail system made with a 76mm x 76mm x 6mm square steel tube for the top rails and 6mm diameter stainless steel cables for the intermediate rails. 5. Bridge Component Design 5.1 Main Arches The primary structural elements of the bridge are the GLULAM arches. These members are 273mm x 1143mm Douglas Fir, 24F-V8 members. All GLULAM on this project conformed to ANSI/AITC A190.1 specifications. The pedestrian live load governed the design of the deck. Due to the influence area of 301m 2, the live load was reduced to 2.56MPa. The total live load applied to the arches was 35,217 kg. Both full live load and unbalanced live load at one end of the bridge was considered. The dead load plus unbalanced live load case governed the design of the bridge with the highest bending plus compression interaction equation result of The live load deflection due to full live load was 28mm = L/1955.

3 The arches are laterally supported by a series of chevron braces and cross struts. This system also provides the required strength and stiffness for the lateral loads due to wind and seismic. The vehicle clearance was set at 3.5m at the lowest cross strut. This low clearance was used to prevent vehicles heavier than the design vehicle from crossing the bridge. The maximum unbraced length of the arch is 7.02m from the pin to the first cross strut. The arches were analyzed using the general purpose finite element analysis program SAP2000 by Computers and Structures, Inc. The members were checked for conformance to the requirements of the 1997 National Design Specification for Wood Construction using Western Wood Structures, Inc. in-house program SAPSTRESS. With this program, the arches are divided into 14 sections. The appropriate unsupported length for both axes are input for each section. The program extracts the analysis results for each element in a section, chooses the highest moment, axial force and shear for the section and then using the member size, unsupported length and allowable stresses, checks the section for adequacy. The arches are treated with Pentachlorophenal in heavy oil. The maximum retort size for this product has an 2.44m diameter. The offset for the full arch halves is over 3.66m. Therefore a moment splice was designed to allow the arch halves to be cut in half and then bolted together after treating at the jobsite. The moment splice reduced the shipping length from 30m to 15m which reduce shipping costs dramatically. These splices consisted of side plates at the top and bottom of the arch and a series of (4) 100mm diameter shear plates with 19mm diameter by 305mm dowels in the end grain of the arch halves. The design of the splice accounted for the actual compressive stress, which partially offset the tension from the actual bending moment. Three rows of nine 25mm diameter bolts were required to resist the applied tension force due to combined moment plus axial loading at the top of the arch. Two rows of five bolts were used at the bottom of the arch for the unbalanced load case where the tension occurred on the bottom face of the arch. Figure 2 shows a detail of the moment splice. The arches are pinned at the supports and at the crown. A welded steel plate arch base assembly was attached to the concrete abutment with 25mm diameter cast-in-place anchor bolts. Another welded steel assembly was attached to the arch by means of 25mm diameter machine bolts. The bolts were designed to transfer the axial forces and shear forces to the steel assembly and a 89mm diameter steel pin was used to connect both assemblies together. At the arch crown, similar welded steel assemblies and a 76mm diameter pin are used. All steel assemblies and connecting hardware on this project were hot dipped galvanized conforming to ASTM A123 to provide corrosion protection. 5.2 Floor Beams 171mm x 609mm DF 24F-V4 GLULAM floor beams are suspended from the arches using steel rod hangers spaced at 13.66m o.c. Since there is no waterproof wearing surface on the deck, the floor beams were designed using wet stresses. The influence area for each floor beam is 20.06m 2. This influence area does not meet the threshold area for live load reduction requiring that the floor beams be designed for the total 4.79 MPa live load. Two vehicle load cases considered the rear axle of an H10-44 vehicle on the floor beam. In the first vehicle load case, the wheels are positioned to produce the maximum moment in the floor beam. In the second case, the wheels were positioned to produce the maximum shear. The rear axle of the H10-44 vehicle has a design load of 7,272 kg. The pedestrian live load controlled the design for both bending stresses and shear stresses. The live load deflection ratio for the floor beams is L/650.

4 The floor beams are suspended from the arch using 25mm diameter steel rods with #3½ clevises at each end. These rods attach to the arches by means of welded steel hanger assemblies and (4) 100mm diameter shear plates on each side of the arch member. At the bottom of the rod, welded steel floor beam hangers support the floor beams. These welded steel assemblies are non-redundant fracture critical members. Therefore continuous inspection and Non Destructive testing of the welds was required. Figure 2. Arch Moment Splice Detail 5.3 Longitudinal Deck This floor beam spacing and vehicle load allowed for the use of 131mm longitudinal decking. This deck system is designed in accordance with AASHTO and also used wet use factors. To meet the requirements of a longitudinal deck design, 131mm x 152mm deck stiffeners were installed centered between the floor beams. The deck panels were laid out in two and three span continuous panels. Four interior rows of 914mm panels plus two outside rows of 1062mm panels were used to make up the m distance from the outside face of curbs. A 10mm gap was detailed between panels to allow for swelling of the panels during the wet season. The floor beam elevations are positioned to provide a 1524m vertical curve at the roadway. The deck panels are allowed to bend in their weak axis under their self-weight to match this curvature. The end joints of the deck panels are staggered. The continuity of the deck panels over the floor beams allows the deck system to act as a horizontal beam to withstand the wind and seismic loads. The deck is attached to the floor beams by means of two L127x127x8 angles that are bolted to the floor beams. Dome head bolts are used to attach the deck to the angles. 5.4 Non-structural Elements Non-structural elements included the 63.5mm x 286mm rough sawn running plank, 171mm x 305mm GLULAM curbs and a rail system made with galvanized steel tubes and ¼ diameter stainless steel cable rails. The rough sawn finish on the running plank provides for a slip resistant surface for pedestrian traffic. 5.5 Preservative Treatments The arches, floor beams, deck panels and curbs on this project were preservatively treated with Pentachlorophenol in a P9 or heavy oil carrier. This treatment results in the chocolate brown color of the arches. The sawn running plank was preservatively treated with Pentachlorophenol in a light solvent carrier. All treating was performed in conformance with AWPA specification C2 and C28 and the Best Management Practices for the use of Treated Wood in Aquatic Environments as developed by Western Wood Preservers Institute. These practices ensure that the treated members

5 have been adequately cleaned before they arrive on the jobsite and that the fixation of the treating chemicals is sufficient to minimize leaching into the surrounding environment. All of the fabrication of the timber components, including drilling of holes, routs for timber connectors and end cuts was completed before the members were preservatively treated. This provides an intact treating envelope completely around each member. In this way, the wood is protected against insect and fungus attack. This sequence of fabrication and preservative treating provides for a minimum 50-year service life for the bridge. 6. Bridge Erection 6.1 Site Conditions As mentioned earlier, access to the site during construction was limited by the sensitivity of the bottom of the ravine. This required that all assembly and erection be performed from outside the ravine slope. An additional difficulty was that the East side of the ravine was only accessible by a road with very tight corners. This severely limited the size of crane available to lift members from this side and prevented the trucks with materials from accessing this side of the site. The decision was made therefore to set a complete arch section from the West side using a 140 ton crane and then set single arch halves from the East side using a smaller 70 ton crane. 6.2 Pre-assembly On the East side of the ravine, material for two arch halves was walked in approximately 200m by forklift from the main road. The arch members were assembled into halves by making up the moment splices. The floor beam hanger assemblies, the hanger rods and the cross brace connectors were attached to the arch members. The weight of each arch half was approximately 7500 kg. Figure 3. Assembly of the West arch section On the West side of the ravine, two arch halves were assembled and stood up. The GLULAM deck panels were stacked at the center to provide temporary shoring under the arches. The cross braces and chevron bracing were attached to the arches to form an arch section. This assembly was completed using two forklifts. The entire section only weighed about 20,000 kg. At this point, the pre-assembly was complete and the cranes were called in for the bridge installation. The crane companies were requested to have their cranes on site and set up with the counterweights the evening before the bridge was erected. Construction on the following day would start at first light and continue until the arches were installed and enough bracing attached for the arches to be fully stabilized.

6 6.3 Arch Erection The arch erection was accomplished using the 140 ton crane in the West parking lot and the 70 ton crane in the East parking lot. Both cranes were able to set up adjacent to the abutments. This made for a 20m lifting radius. Using GLULAM arches minimized the weight of the arch system. This kept the crane sizes down and minimized installation costs. An additional 40 ton crane with a man basket was brought in to allow access to the pin at the arch crown. Figure 4. Lifting the West arch section with a 140 ton crane. Figure 5. Lifting the first arch half from the East side. The arch section on the West side was first lifted by the 140 ton crane and set on the West abutment. A hydraulic pin press was used to press the pins into to the steel base hinge assemblies. The crane continued to hold the arch section with the crown at the appropriate elevation. The temporary offices of the California Gaming Commission needed to be evacuated as the arch section was lifted directly above them. This was completed in less than ten minutes. Once the West section was installed, the 70 ton crane lifted an arch half. This arch half was attached to the abutment with the steel pin and then a workman in the manbasket installed the pin at the arch crown. The second arch half was then installed in a like manner. Two cross braces and a pair of chevron braces were then installed. At this point, the 140 ton crane was released and both the 140 ton crane and the 70 ton crane were sent back to their yard. Setting of the arches took approximately 10 hours. The 40 ton crane was then used to set the remainder of the arch bracing. This crane would also be used to set the floor beams.

7 6.4 Installation of deck system To continue the work, it was decided that access would be required from the ravine. This access was necessary to access the floor beam to hanger connection and the decking attachments. However, no roads were available and none could be built to allow access for the manlift. At the deepest point, the bottom of the ravine is 12m below the bridge deck. The solution to this was to lift a JLG manlift and place it in the bottom of the ravine on pads. By using the remaining crane to lift the JLG manlift, disturbance to the ravine bottom was eliminated. The steel angles for the deck to floor beam connection and the floor beam hangers were attached to the floor beams. The crane next lifted the floor beams so they could be attached to the hanger rods. The height of the clevises on the hanger rods was adjusted to build in the camber in the deck. With the floor beams installed, the longitudinal GLULAM deck was walked out onto the bridge with one of the forklifts. The axle load on the front of the forklift, lifting a deck panel with the boom extended, created a load on the hanger just under the design live load. A workman in the JLG manlift then made up the connections to the floor beams. Figure 6. Setting the longitudinal GLULAM deck with a hydraulic boom forklift. Finally, The curb and rail system were installed onto the GLULAM deck. The curb is attached to the deck with dome head machine bolts. The posts for the rail system were then bolted to the curbs. The final step in the construction process was installing the running plank. The running plank is attached with lag screws with the heads countersunk to provide a smooth wear surface. A rough sawn finish was specified for these plank to provide slip resistance. A crew of three carpenters erected this entire bridge in less than three weeks.

8 7. Bridge Costs and Schedule 7.1 Material Costs The materials expenses on this bridge included the preservatively treated GLULAM arches, floor beams, deck and curbs, the sawn lumber running plank, and the galvanized steel connecting assemblies. The cost for these materials including truck freight from Portland, Oregon to the site was US$254, This does not include the cost of the concrete abutments. 7.2 Erection Costs The cost of labor (including per diem for three workers) and equipment to install this bridge was US$68, The equipment requirements we kept to a minimum by maximizing the amount of pre-assembly. The 140 ton crane and the 70 ton crane were used for one day only. The 40 ton crane was used for one week. 7.3 Design and Construction Schedule The design of the bridge and abutments took 3 ½ months to complete. This included the work required to prepare the soils reports and the final site surveys. The sitework was then interrupted for approximately one month due to heavy rains at the site. Fortunately, the bridge manufacturing processes continued during this time. The manufacturing and fabrication of the GLULAM members and steel assemblies required 3 months. Delivery to the jobsite and jobsite installation required one additional month. So the total time from execution of the contract to completion of the job was 7 ½ months. 8. Conclusion Figure 7. The Hopland Casino Bridge Completed May The Hopland Casino Bridge uses three hinged GLULAM arches to span 54.9m from a new Casino to the overflow parking area. The clear span structure met all of the owner s criteria for load capacity and minimal site impact. The sensitivity of the site added required that the erection be completed from the ends of the bridge. The light weight of the structure aided in the ability to do this.