COST EFFECTIVE STEEL BRIDGE DESIGN : A FABRICATOR’S PERSPECTIVE BO BOVARD - President
Augusta Iron & Steel Works, Inc. 1
TOPICS • • • • •
Weathering Steel HPS Fracture Critical Bearings Girder Spacing
• Crossframes • Welding/NGESW
• Flange Butt Splices • Stiffeners
Curved Girders
• Fabrication of: Plate Girders Box Girders Haunched Girders 2
WEATHERING STEEL
• High strength-low alloy • 4 times the corrosion resistance of carbon steel • ASTM A709 - Grades 50W, HPS 70W, & 100W • Forms tight protective oxide coating
WEATHERING STEEL WHY SHOULD I USE IT?
WEATHERING STEEL
SINGLE MOST IMPORTANT SELECTION FOR AN ECONOMICAL STEEL BRIDGE
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COST STUDIES
• Girder Optimization Studies • Optimize By Cost, Not Weight • Compare By Grades of Steel • Study Over 100 Bridges per Year 7
COST STUDIES
• FIRST COST SAVINGS : 7 TO 15% • Plus Future Maintenance Savings
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WEATHERING STEEL • Material costs 3 cents/pound more! • Painting costs significantly higher • Net costs favors W/S !
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WEATHERING STEEL
EXAMPLE • 1223 LF & 886 Tons • Paint : $ 270,674 • Material Extra : $ 53,160 • Savings : $ 217,514 • Life Cycle Cost Savings : $ 1.5 M
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JOINTLESS BRIDGES
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INTEGRAL ABUTMENTS
Bridge girders
Single row of straight piles
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JOINTLESS BRIDGES ADVANTAGES: • Use weathering steel; no paint on the ends
• No joints or bearings to purchase or maintain. • Smoother riding surface.
• Lower initial & life-cycle cost! 15
REFERENCE • Integral Abutments for Steel
Bridges; Ed Wasserman and J. Houston Walker, TN DOT Reasons
for jointless construction
Current
practice
Fiction and facts Design example and details 16
WEATHERING STEEL WHY SHOULD I USE IT?
WEATHERING STEEL MOST ECONOMICAL • THE WAY TO BUILD A STEEL BRIDGE
• SAVES 7 T0 15% • FUTURE MAINTENANCE SAVINGS
HIGH PERFORMANCE STEEL
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HPS GENERAL GOALS • High strength - 70 & 100 Ksi yield strength • Improved weldability - lower C and CE levels • High toughness - meet Zone 3 fracture critica requirements • Weathering capability
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HIGH PERFORMANCE STEEL • 14 CENTS/LB • CHEMISTRY CONTROL • ROLLING PROCEDURES • HEAT TREATING
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CHEMISTRY COMPARISON Pro. Old485W (70W)
Min.Q &T Max.
HPS-485W (1) (HPS-70W)
C
.19
-
.80
1.35 .035
Min.Q &T Max.
Mn
.11
1.15
P
S
Si Cu Ni Cr Mo V
-
- .25 .20
.04 .65 .40 .50 .70
-
- .40
- .02
- .10
- .35 .28 .28. 50 .04 .05
1.30 .020 .006 .45 .38 .38 .60 .08 .0
(1) Calcium treated for inclusion shape control, also requires .010 - .040 A1 and .015 N max.
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Processes for Producing Plate C onventional P rocessing 2400
H ot R olling
Therm o-M echanical C ontrolled P rocess ing (TM C cessi C ontrolled R olling
H eat Treatm ent
A ccelerated C ooling
2000 N orm alizing
F ° 1600 , e r u t a r 1200 e p m e T 800
400
0
Q uenching & Tem pering W a te r Q u e n c h
A ir C o o l
A ir C o o l
W a te r A ir o rW a te r C o o le d
p
A ir C o o l
A ir
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FRACTURE CRITICAL MEMBERS (FCM) • Member in tension or subject to tension. • Forces have no alternate path. • Member failure results in collapse of bridge, (non-redundant).
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FRACTURE CONTROL PLAN (FCP) • Sec. 12 AASHTO/AWS Welding Code. • Stricter fatigue requirements for every category. • Increased material toughness. • Increased mill testing. • Welding process, consumables, procedure. • Weld inspection. • Thermal cutting, curving, cambering. 33
FRACTURE CONTROL PLAN (FCP) • Should not be used as a crutch to be safe. • Major concern when specified but not justified. • May increase bridge cost without increasing the reliability! • Multi-girder bridges – not fracture critical! • Non-redundant structures: FCP requirements limited only to tension components.
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BEARINGS • Built-Up Rocker- approx. $1.50/lb • Spherical Bearing - approx. $1200 ea. • Pot Bearing - approx. $800 ea. • Elastomeric - approx. $225 ea!
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REFERENCE • Steel Bridge Bearing Selection and
Design Guide, Profs. Roeder and
Stanton, University of Washington Selection
guide
Design
guide
Details
and testing
Spreadsheet
for elastomeric bearing
design 36
GIRDER SPACING • Ideal Spacing = 11 to 12 ft. • Fewer Girders to Fabricate & Erect • Fewer Diaphragms, X-Frames, Bearings • Heavier Sections = Lower Cost/Lb.
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FLANGE BUTT SPLICES • WIDEN THE FLANGE? • THICKEN THE FLANGE?
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Bevel
STEP 1: Bevel and taper the 4 slab edges
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Burn
STEP 2: Burn 12 plates from slabs
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STEP 3: Fit up and tack weld 4 flange assemblies
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Run-off Tab
STEP 4: Attach 16 run-off tabs
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Weld
STEP 5: Weld and grind 8 splices
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STEP 6: Turn over 4 flange assemblies
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Weld
STEP 7: Back gouge, weld and grind 8 butt joints
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STEP 8: Remove and grind 16 run-off tabs
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Bevel
STEP 1: Bevel and taper the 4 slab edges
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STEP 2: Fit up and tack weld 3 slabs
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Run-off Tab
STEP 3: Attach 4 run-off tabs
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Weld
STEP 4: Weld and grind 2 splices
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STEP 5: Turn over 1 piece
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Weld
STEP 6: Back gouge, weld and grind 2 butt welds
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STEP 7: Remove and grind 4 run-off tabs
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Burn
STEP 8: Burn 4 flanges from 1 assembly
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FLANGE SPLICES • Width transitions increase labor for flange assemblies up to 35% • If you must increase width, do so at bolted field splice (do not clip corners of top flanges) • Allow fabricators to eliminate splices by providing extra material 59
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TRANSVERSE WEB STIFFENERS • How Many? • Least weight = least cost? • Cost Comparison: • Stiffeners Cost $150 ea. in place • +1/16” web t x 60” high x 100’ = $433 ( or 2.9 stiffeners ) 61
CROSSFRAMES
• Cross frames are very costly items. • Steel for girder flanges and webs cost about $0.90/lb. • Steel for X-frames = $3 to $5/lb. • Typical X-frame - up to $800 each!
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Approximate Cost = $600
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Approximate Cost = $200
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SKEWED BRIDGES
• PLACE PARALLEL TO SKEW? • PLACE NORMAL TO SKEW? • STAGGERED OR IN-LINE?
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STEEL COLLABORATION RECOMMENDATIONS
• 20 DEGREES OR LESS, USE EITHER • GREATER THAN 20 DEGREES, USE STAGGERED, NORMAL CROSSFRAMES
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STEEL COLLABORATION RECOMMENDATIONS CONT’D
• LEAVE OUT X-FRAMES BETWEEN UNITS IN PHASED CONSTRUCTION
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Dennis Mertz at the Univ. of • Prof. Delaware is developing a Bridge Designer’s Guide to Intermediate Cross-Frame Diaphragms.
• Discuss options with your local fabricator. 71
WELDING • Use fillet weld between web and flange. • Do not use full penetration welds. • Use single pass weld. Welds greater than 5/16” multiply welding time and cost. 5/16” (3/8”) 1 PASS
3/8”
1/2”
2 PASSES
5-6 PASSES 72
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NEW DEVELOPMENTS NARROW GAP ELECTROSLAG WELDING
• FHWA sponsored • Major increases in productivity & quality
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Burn
STEP 8: Burn 4 flanges from 1 assembly
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CJP Welds
Wide Plates
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IN SUMMARY • WEATHERING STEEL • HPS • Flange Transitions in Thickness Not Width • Jointless Bridges • Wider Girder Spacing • Elastomeric Bearings • Simple X-frames 125
FOR ADDITIONAL HELP: • Contact regional fabricators for guidance on selecting cost-effective details. • AISI: www.steel.org/infrastructure or (202) 452-7100.
• NSBA: www.aisc.org/nsba • NSBA: Bill McEleney (401) 943-5660 • Bethlehem Steel Construction Marketing 126
ANY QUESTIONS ?
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