bridge design to eurocodes
TRANSCRIPT
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)
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)
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)
Bheel required to prevent sliding
• Combination 2 governs heel length, Bheel , in this case
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)
Outline of preliminary design method (see paper for detail)
Determineactions and effects
DetermineBheel to prevent sliding (drained)
Dependent upon the resultant line of thrust of horizontal and vertical action
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)
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