Bridge Design to Eurocodes

22 - 23 November, 2010Institution of Civil Engineers

Bridge Design to Eurocodes - UK Implementation

22 - 23 November, 2010Institution of Civil Engineers

Design Illustration – Bridge Abutment DesignTim Christie, Mark Glendinning, John Bennetts, Steve Denton – Parsons Brinckerhoff

Introduction

• Paper illustrates preliminary design of a bridge abutment to determine base slab proportions

• Purpose of this presentation is to highlight some specific issues and differences from past practice

• Based on UK National Annexes and PD 6694-1 recommendations (including use of Design Approach 1)

Structure dimensions and notation

Dimensions to bedetermined

• Integral bridge abutment now more typical, but simple structure illustrates Eurocode issues more clearly

Presentation of calculations

• Calculations are presented in parallel columns for SLS (characteristic), and STR/GEO combinations 1 and 2:– reduces calculation effort– facilitates direct comparisons

Illustration of calculations

Outline of preliminary design method (see paper for detail)

Determineactions and effects

DetermineBheel to prevent sliding (drained)

Determine minimum total B to satisfy:i. sliding (undrained)ii. middle 1/3 rule – SLSiii.middle 2/3 rule – ULSiv.drained bearing resistance (ULS)v. undrained bearing resistance (ULS)vi.settlement (SLS) (approximate

method)

Actions and effects

• Most significant change is requirement to do two calculations for ULS (in Design Approach 1), using:

– STR/GEO Combination 1 partial factors– STR/GEO Combination 2 partial factors

Permanent actions

Variable actions

Materials

STR/GEOCombination 1 G > 1 Q >> 1 M = 1

STR/GEOCombination 2 G = 1 Q > 1 M > 1

Partial factors for bridge abutment design

Horizontal actions - Partial and model factor values

ActionCombination 1 Combination 2

F M Sd;K F M Sd;K

Active pressure

1.350.95 1.0 1.2

1.01.01.0 1.25 1.2

1.0Traffic surcharge 1.35 1.0 - 1.15 1.25 -

Braking / accleration 1.35 - - 1.15 - -

Partial and model factors used for bridge abutment design(ULS, Persistent Design Situation, STR/GEO)

Traffic load groups

• Traffic loads are grouped into multi-component actions• No special vehicles (LM3) considered in this example

Traffic group

Vehicle(load model) in

Group

Representative value in Group

Vehicle Surcharge Braking / acceleration

gr 1a Tandem system and udl (LM1) Characteristic Characteristic -

gr 1b Single axle(LM2) Characteristic Characteristic -

gr 2 Tandem system and udl (LM1) Frequent Frequent Characteristic

Characteristic values of multi-component traffic groups (see EN1991-2 Table NA.3)





method)

Bheel required to prevent sliding

• Combination 2 governs heel length, Bheel , in this case





method)




Dependent upon the resultant line of thrust of horizontal and vertical action


method)

Minimum total base length, B

V

H

eheel = M/V

i. Undrained sliding:

B > H / cu;d

ii. Middle 1/3rd at SLS:

B > 1.5 eheel;SLS

iii. Middle 2/3rd at ULS (GEO):

B > 1.2 eheel;ULS

M

• M is moment about P

Minimum total base length, B iv., v. bearing resistance calculation method

• Iterative calculation using EN1997-1 Annex D method varying B, starting with minimum from i., ii. and iii.

• Final iteration uses B = 8.6m, Bheel = 6.25m

Minimum total base length, B iv., v., vi bearing resistance and settlement verification

Notes:1 R/A = cd Nc bc sc ic + qd Nq bq sq iq + 0.5 d B N

b

s

i2 Settlement verified using simple method based on mobilising a sufficiently

small fraction of ground resistance

2

1

Conclusions

• Detailed design would be required to verify preliminary sizing of foundations

• Simple method for preliminary design presented, illustrating application of EN1997-1 and PD 6694-1

• Combination 2 governs sizing of foundation in this case– might not always do so, but typically does

Bridge Design to Eurocodes

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