006 Steve Kite - Long Span CSB

Pushing the Limits of Cable Stayed Bridges – The Partially Earth Anchored Solution

Bergen, Norway

Steve Kite, Associate Director, Arup

6th Symposium on Strait Crossings Extreme Crossings and New Technologies

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Outline

Current Cable Stayed Bridges Challenges for longer spans Partially Earth Anchored System Aerodynamics Construction Stay Cable Vibrations Conclusions

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Research Project

Research Stages Stage 1 – Feasibility Study Stage 2 – Specimen Design

Research Partners Arup GS Engineering & Construction Professor Niels Gimsing

Current Cable Stayed Bridges

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Stonecutters Bridge, Hong Kong

1018m main span

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Sutong Bridge, China

1088m main span

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Russky Bridge in Vladivostok, Russia

1104m main span

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Increase in Span Lengths

Emergence of long span cable stay bridges

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Spans greater than 1200m

Suspension Bridge form More costly to construct and maintain Less redundancy in load paths Almost impossible to replace the cable system Dehumidification of the main cable system

Challenges for longer cable stayed spans

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Challenges for longer cable stayed spans

Compressive stresses in the deck require substantial strengthening of the deck

During the cantilever erection, the cable-stayed main span depends entirely on the strength and stiffness of the deck for lateral stability

Longest stay cables will be several times that of the longest hanger cables in a suspension bridge which means that problems related to individual cable vibration will be more severe

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Bridge Type Studied

Dual 2-lane road Main spans of 1400m, 1600m and 1800m studied Design criteria taken as those from Incheon Bridge,

Korea

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Deck Plate Thickness (fy = 460 MPa)

Plate thicknesses for 1400m, 1600m, 1800m spans

Impractical beyond 1400m span

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Governing aspects

Axial force from Permanent Loads Wind buffeting forces Aerodynamic stability Galloping and flutter

Aerodynamic serviceability Vortex shedding and low wind speed buffeting

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Critical Design Stages

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Buffeting Deflections

Buffeting deflections are significant However, stresses due to wind buffeting are

generally less than the in-service wind stresses

In-service [m] Max-cantilever [m]

Span Mean wind Buffeting Total Mean

wind Buffeting Total

1400 4.7 4.1 8.8 6.4 6.3 12.8

1600 7.4 6.3 13.7 10.2 10.0 20.2

1800 10.0 8.3 18.3 13.9 13.7 27.6

Design 10-minute mean wind speed at deck level during construction = 49.5 m/s Iu = 13.9%, Iv = 11.1%, Iw = 7.0%.

Partially Earth Anchored System

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Introduce a global tension into the deck by anchoring some of the stay cables to the ground

Axial force diagram for PEA cable stayed bridge (Gimsing)

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Longest back stay cables fixed to anchor blocks

1400m 400m 400m

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Key Benefits

Lower compression forces in the deck than a self anchored cable stay bridge

Smaller anchorages and more economical cable system than a suspension bridge

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Wind Effects

Transverse Displacements

Transverse Bending Moment

Tra

nsve

rse

Dis

plac

emen

t (m

)

Local Chainage

NON-LINEAR

STATIC

Ver

tical

Ben

ding

Mom

ent (

kNm

)

Local Chainage

NON-LINEAR Max.

STATIC Max.

Construction Challenges

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Construction Challenges

If construction case governs then no benefit to partially earth anchored system

Need a technique to build the bridge whilst limiting in the axial load in the deck


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Challenge is How to Construct It?


Option A

Option B

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Tie-cable System

Temporary longitudinal restraint connection between deck and tower

Deck erected by the balanced cantilever method. Erection proceeds until the length of the cantilever is such that the axial deck forces due to wind and gravity would exceed the in-service forces.

Horizontal tie cables are introduced to connect the deck cantilevers together with a beneficial tension force

Deck erection continues by the cantilever method

After mid-span closure the temporary deck / tower connection is released and the temporary tie cables are de-tensioned and removed


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Tie-cable System


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Erection Sequence

Cantilever until temporary forces are governing


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Erection Sequence

Install tie cable and stress against tower


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Erection Sequence

Continue cantilevering whilst incrementally increasing the tie force


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Erection Sequence

Install second cable


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Erection Sequence

Continue cantilevering and controlling tie forces


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Erection Sequence

Deck closure


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Erection Sequence

Remove ties and transfer tension to the deck


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Tie Cable Construction

Advantages: Reduce axial compression in the deck Reduce bending moment in the deck due to wind load Reduce deflection of the bridge due to wind load

Lateral Displacement at Cantilever Tip (m) under

Without Tie Cables With Tie Cables

Mean Wind 6.4 3.0 Buffeting Wind 6.3 3.6

Total 12.8 6.6


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Estimated Total Steel Quantity

In-service Condition Traditional Cantilever Construction Tie-cable Construction

29,700 ton 41,700 ton 30,600 ton


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Basis of Economic Comparison

Typical cable stay bridge - Foundations 20 – 25% - Towers & piers 5 – 10% - Cable system 10 – 15% - Deck 30 – 35% - Miscellaneous 15 – 20%

Comparison of steel superstructure quantities Cable steel taken as 1.5 times cost of deck steel Construction cost of suspension bridge anchorage

15% to 35% of superstructure


Relative cost of deck, cables and anchorages

Span

Self Anchored Cable Stay Bridge

Suspension Bridge

Partially Earth Anchored Cable Stay Bridge

Stay Cable Vibrations

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Cable damping

Longer cables more susceptible

Even with a textured surface, a damping ratio of approximately 0.5% to critical is still required (3.14% log-dec)

For ultra-long cables it is difficult to achieve this damping in practice


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Concept of optimal damping


Cable damping calculations

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xc

Damper geometry

xc tends to be governed by bridge geometry As L increases, xc/L decreases


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Damping achievable

0.000

0.010

0.020

0.030

0.040

0.050

1000 1200 1400 1600 1800

x c/L

op

timal

dam

ping

Main Span Length (m)

Single damper at fixed absolute distance from cable end

xc/l Optimal damping (log-dec)

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Position of external damper for 1400m span

xc = 20m ~5m


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Stay Cable Damping


0.000

0.010

0.020

0.030

0.040

0.050

1000 1200 1400 1600 1800

Min

imum

xc/L

Main Span Length (m)

One or two dampers to achieve 3.15% log-dec damping

Single Damper (xc/L)

Damper at Each End (xc/L)

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Tower Side Damper


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Tower Side Damper


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Challenges and Solutions

Beyond ~1200m, control of cable vibrations will start to become difficult with traditional systems

Up to ~1500m span, the addition of a tower side damper will be an effective means of controlling vibrations

Beyond ~1500m span, as well as a tower side damper cross cables could be needed


Conclusions

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Conclusions

Stay Cable Bridges with spans > 1200m pose significant technical challenges

Suspension Bridges likely to be more economical compared with traditional cable stayed bridges

Partially earth anchored system offers benefits by reducing the deck quantities required

Very long stay cables require additional damping to control vibrations

Technology available to increase the maximum cable stayed span beyond today’s 1104m record

Documents

006 Steve Kite - Long Span CSB