Substringer system with 5 main girders = 11,100,000 lb (97 lb/sf) Multi-girder system with 8 main girders = 11,000,000 lb (96 lb/sf)

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1 Memo To: John Clute Iowa Department of Transportation, Bridge Division From: Gary Krupicka \ Todd Horton Project: NHSX-34-1(68)--3H-65 CC: Brad Hofer (IaDOT), Matt Tondl (HDR) Date: March 6, 2006 Job No: RE: US 34 Missouri River Crossing Bellevue Bridge Project No. NHSX-34-1(68) 3H-65 River Crossing Bridge Options Introduction This memo summarizes the two options considered for the river crossing bridge for the new US 34 alignment. s and steel weights are based on a 3-span river bridge unit with a total length of 1,285 feet ( spans), and an out-to-out deck width of The overall bridge layout is shown in Figure 1. The design vehicle considered is the AASHTO HS-25 truck and lane vehicle with a limiting deflection of L/800. Steel Plate Girder Option A variable depth sub-stringer system with 5 main girders and a variable depth multi-girder system with 8 main girders were studied for the steel plate girder option (Figures 2 & 3). The girder weights were determined based on initial line girder runs and the miscellaneous steel was estimated based on past jobs of similar span lengths. The total structural steel weights for each system are: Substringer system with 5 main girders = 11,100,000 lb (97 lb/sf) Multi-girder system with 8 main girders = 11,000,000 lb (96 lb/sf) PDM Bridge of Euclaire Wisconsin was contacted to discuss the relative cost difference between the Multigirder system and the Substringer system. Their opinion was the Substringer system would be preferable and somewhat less expensive due to the reduced number of plate girders that would require handling and shipping. Review of existing structures with similar span lengths also shows a predominate preference towards the Substringer system. One additional consideration is the multi-girder system would require a ½ inch thicker deck based on the current configuration. Truss Option A 3-span constant depth Warren truss without verticals (Figures 1 & 4) was studied for the truss option. The Warren truss was selected for this site due to it s efficiency at these spans, and due to the clean and modern look of the truss. The truss main member weights were determined based on in-house truss design software while the miscellaneous steel was estimated based on past jobs of similar span lengths. For geometric considerations, the total truss length for analysis was 1,269 feet with a maximum span of 517 feet. Final cost and weights were prorated to the base length of 1,285 feet for direct comparison to the Plate Girder option. The prorated total structural steel weight is: 3-span constant depth truss = 10,500,000 lb (92 lb/sf) IaDOT raised the question whether a single span truss would be more cost efficient than a 3 span continuous truss. Based on our experience of similar structures across the country, the relative cost difference between a multi-span continuous truss and a single span truss would be a wash. The single span truss would require a greater main span and consideration of additional approach structure. Therefore, for the purpose of comparing a truss option to a plate girder option, it would be reasonable to compare the three span truss to HDR Engineering, Inc Indian Hills Drive Omaha, NE Phone (402) Fax (402) Page 1 of 3

2 the plate girder to establish relative costs. The relative advantages and disadvantages of the two possible truss span configurations are not discussed herein. Comparative A weight take-off of the plate girder and truss options was performed to establish a relative cost comparison between the two superstructure alternatives. The weight take-offs were sent to PDM Bridge for a fabrication and shipping cost estimate. The following steel fabrication and shipping cost estimates were provided by PDM Bridge: Plate Girder option = 1.95 $/lb. delivered (assuming weathering steel) Truss option = 2.03 $/lb. delivered (painted system) Jensen Construction was contacted to estimate the erection costs of the two options. It was their opinion that the truss option would be $300,000 to $500,000 more expensive to erect then the plate girder option. A base erection cost was estimated at $0.40 per lb. based on past experience. Assuming $400,000 additional erection cost for the truss option, the following relative per pound erection cost was calculated as 0.40 $/lb for the plate girder option and 0.44 $/lb for the truss. In addition to the structural steel cost difference, roadway fill cost savings due to the reduced structure depth of the truss was calculated as $500,000. River pier costs were assumed to be comparable and not enough to affect the relative cost. Life Cycle s for painting were determined as shown in Appendix A. The present value of the future painting for the truss was calculated as $1,450,000. The present value of the initial and future painting for the steel plate fascia girders was calculated as $560,000. The initial painting cost for the steel plate fascia girders was not included in the delivered unit prices for the plate girder steel. Life Cycle s for other future events were considered similar in cost for the two options are not included in the relative cost comparison. These future events include: annual maintenance, biennial inspection, deck overlay, and deck replacement. The following table summarizes the relative cost comparison for the two options. Unit Rate for Struct. Steel - Erected _ Structural Steel - Erected _ Relative Grade Reduction _ Future Painting _ Comparative _ Steel Plate Girder $2.35/lb $ 26,100,000 $ 0 $ 560,000 $ 26,660,000 Steel Truss $2.47/lb $ 25,900,000 -$ 500,000 $1,450,000 $ 26,850,000 Other consideration In addition to the cost difference there are advantages and disadvantages to the two structure options: Advantages of the Plate Girder option: Steel elements are below the deck and shielded from salt spray Fewer erection pieces No fracture critical members Possibly reduced long term maintenance and inspection cost Advantages of the Truss option: Cantilever construction will eliminate temporary piers in the river (multi-span truss) Smaller cranes required for lighter pieces Lower profile resulting in less fill at the Nebraska levee Shipping would not require barging HDR Engineering, Inc Indian Hills Drive Omaha, NE Phone (402) Fax (402) Page 2 of 3

3 Appendix A LIFE CYCLE COST CALCULATIONS Future Painting for the Truss Option Assumptions: Complete field painting at 35 years. Touchup painting of 10% of the surface area at 17.5 years. Painting surface area of 190,000 SF. Field Painting costs at $12/SF to $15/SF. Use $13.5/SF. Inflation adjusted discount rate of 3%. Present Value s: Year (n) Base Present value Factor 1/(1.03)^n Present Value Touch-up Painting 17.5 $250, $150,000 Complete Painting 35.0 $2,500, $900,000 Touch-up Painting 52.5 $250, $50,000 Complete Painting 70.0 $2,500, $330,000 Touch-up Painting 87.5 $250, $20,000 Painting and Future Painting for the Steel Plate Fascia Girders Assumptions: Total $1,450,000 Include initial shop painting, since not included in delivered steel price for weathering steel. Complete field painting at 35 years. Touchup painting of 10% of the surface area at 17.5 years. Painting surface area of 64,000 SF. Field Painting costs at $10.0/SF. Shop painting at $3/SF. Inflation adjusted discount rate of 3%. Present Value s: Year (n) Base Present value Factor 1/(1.03)^n Present Value Initial Shop Painting 0 $190, $190,000 Touch-up Painting 17.5 $64, $38,000 Complete Painting 35.0 $640, $230,000 Touch-up Painting 52.5 $64, $14,000 Complete Painting 70.0 $640, $83,000 Touch-up Painting 87.5 $64, $5, Total $560,000 HDR Engineering, Inc Indian Hills Drive Omaha, NE Phone (402) Fax (402) Page 3 of 3

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