truss and girder comparison

8/11/2019 Truss and Girder Comparison

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HDR Engineering, Inc. 8404 Indian Hills DriveOmaha, NE 68114-4098

Phone (402) 399-1000Fax (402) 399-4979

www.hdrinc.com

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MemoTo: 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: 14925

RE: US 34 Missouri River Crossing Bellevue BridgeProject No. NHSX-34-1(68)3H-65River Crossing Bridge Options

Introduction

This memo summarizes the two options considered for the river crossing bridge for the new US 34 alignment.Costs and steel weights are based on a 3-span river bridge unit with a total length of 1,285 feet (385-515-385 spans), and an out-to-out deck width of 89-2. 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 maingirders were studied for the steel plate girder option (Figures 2 & 3). The girder weights were determinedbased on initial line girder runs and the miscellaneous steel was estimated based on past jobs of similar spanlengths. 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 Multi-

girder system and the Substringer system. Their opinion was the Substringer system would be preferableand somewhat less expensive due to the reduced number of plate girders that would require handling andshipping. Review of existing structures with similar span lengths also shows a predominate preferencetowards the Substringer system. One additional consideration is the multi-girder system would require a 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. TheWarren truss was selected for this site due to its efficiency at these spans, and due to the clean and modernlook of the truss. The truss main member weights were determined based on in-house truss design softwarewhile 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 costand 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 continuoustruss. Based on our experience of similar structures across the country, the relative cost difference betweena multi-span continuous truss and a single span truss would be a wash. The single span truss would requirea greater main span and consideration of additional approach structure. Therefore, for the purpose ofcomparing a truss option to a plate girder option, it would be reasonable to compare the three span truss to


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the plate girder to establish relative costs. The relative advantages and disadvantages of the two possibletruss span configurations are not discussed herein.

Comparative Cost

A weight take-off of the plate girder and truss options was performed to establish a relative cost comparisonbetween the two superstructure alternatives. The weight take-offs were sent to PDM Bridge for a fabricationand 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 thatthe truss option would be $300,000 to $500,000 more expensive to erect then the plate girder option. A baseerection cost was estimated at $0.40 per lb. based on past experience. Assuming $400,000 additionalerection cost for the truss option, the following relative per pound erection cost was calculated as 0.40 $/lb forthe plate girder option and 0.44 $/lb for the truss. In addition to the structural steel cost difference, roadwayfill cost savings due to the reduced structure depth of the truss was calculated as $500,000. River pier costswere assumed to be comparable and not enough to affect the relative cost.

Life Cycle Costs for painting were determined as shown in Appendix A. The present value of the futurepainting for the truss was calculated as $1,450,000. The present value of the initial and future painting for thesteel plate fascia girders was calculated as $560,000. The initial painting cost for the steel plate fascia girderswas not included in the delivered unit prices for the plate girder steel. Life Cycle Costs for other future eventswere considered similar in cost for the two options are not included in the relative cost comparison. Thesefuture 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 forStruct. Steel

- Erected _

StructuralSteel - Erected

Cost _

RelativeGrade

ReductionCost _

FuturePainting

Cost _Comparative

Cost _

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


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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 Costs:

Year(n)

BaseCost

Present valueFactor

1/(1.03)^n

Present ValueCost

Touch-up Painting 17.5 $250,000 0.60 $150,000

Complete Painting 35.0 $2,500,000 0.36 $900,000


Complete Painting 70.0 $2,500,000 0.13 $330,000


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Total $1,450,000

Painting and Future Painting for the Steel Plate Fascia Girders

Assumptions:

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 Costs:

Year(n)

BaseCost

Present valueFactor

1/(1.03)^n

Present ValueCost

Initial Shop Painting 0 $190,000 1.00 $190,000


Complete Painting 35.0 $640,000 0.36 $230,000


Complete Painting 70.0 $640,000 0.13 $83,000


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Total $560,000


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truss and girder comparison

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