Reliability based design (RBD) refers to any design methodology that are based on a rigorous reliability analysis. RBD is the only methodology available to date that can ensure self-consistency from both physical and probabilistic requirements and is compatible with the theoretical basis underlying structural design (Kulhawy & Phoon, 2002). It is considered as fundamental theoretical basis for all Load and Resistance Design (LRFD) and is capable of mitigating numerous logical inconsistencies inherent in current geotechnical design.
12. LLRRFFDD MMeetthhoodd
TThhee rreelliiaabbiilliittyy ooff aannyy ggiivveenn ppiillee iiss ddeessccrriibbeedd bbyy iittss
probability of failure ppf
f wwhhiicchh iiss ddeeffiinneedd aass tthhee
pprroobbaabbiilliittyy tthhaatt tthhee rreessiissttaannccee iiss ssmmaalllleerr tthhaann tthhee
llooaadd eeffffeecctt..
ppf
f == PP(( rreessiissttaannccee << llooaadd eeffffeecctt))
PPeerrffoorrmmaannccee ffuunnccttiioonn ffoorr ssuubbssttrruuccttuurreess::
³ å n i i fR h g Q EEqq.. ((33))
where Rn = calculated (nominal) resistance;
∅= resistance factor; Qi = nominal load effect; γi = load
factor; and η = factor to account for effects of ductility,
redundancy, and operational importance
13. LLRRFFDD MMeetthhoodd ((ccoonnttdd..))
Reliability index β associated with the linear
performance function can be calculated using the mean
value first-order, second moment method (MVFOSM)
EEqq.. ((44))
where QD and QL = nominal values of dead and live loads,
respectively; λR, λQD, and λQL = bias factors for resistance, dead load,
and live load, respectively; COVR, COVQD, and COVQL = coefficients
of variation for resistance, dead load, and live load, respectively; and
FS = factor of safety in the traditional allowable stress design (ASD).
14. LLRRFFDD MMeetthhoodd ((ccoonnttdd..))
TThhee bbiiaass ffaaccttoorrss::
m
R R
n
l = R
EEqq.. ((55))
where Rm = measured value of resistance. In this
study, Rm is taken to be the measured Davisson
capacity, whereas Rn is taken to be the predicted
capacity by any given prediction method
19. RReelliiaabbiilliittyy ooff DDrriivveenn PPiillee GGrroouupp
DDrriivveenn ppiilleess aarree most often used in groups and
connected to superstructures.
Group effects - the pile-soil cap interaction
induced by overlapping of stresses in soil and
changes in the density of the soil around the piles
due to pile penetration.
System effects - the interaction between groups of
piles and the superstructure they support.
Group effects and system effects - may alter
significantly the reliability of the foundation.
20. PPiillee GGrroouupp aanndd SSyysstteemm EEffffiicciieennccyy
PPiillee ggrroouupp eeffffiicciieennccyy ((ξ) ::
- the ratio of the ultimate capacity of a pile group
to the sum of the capacities of individual piles,
- a function of pile spacing and group size.
PPiillee ssyysstteemm eeffffiicciieennccyy ((χχ)) ::
- tthhee rraattiioo ooff tthhee llooaadd rreeqquuiirreedd ttoo bbrriinngg aallll tthhee ppiilleess iinn aa ppiillee ssyysstteemm ttoo
tthheeiirr uullttiimmaattee ccoommppuutteedd ccaappaacciittyy aanndd tthhee llooaadd rreeqquuiirreedd ttoo bbrriinngg mmoosstt
hheeaavviillyy llooaaddeedd ppiillee ttoo tthhee ssaammee ssttaattee ((BBeeaa,, 11998833))..
25. CCoonncclluussiioonn
Uncertainties in loading conditions and capacity
of the piles influence the reliability of pile
foundations.
Reliability-based LRFD is the only methodology
that can ensure self-consistency from both
physical and probabilistic requirements and is
compatible with the theoretical basis underlying
structural design
Based on the literature review, it seems
reasonable to establish the target reliability
between 2.0 and 3.0 for pile groups and as high
as 3.0 to 3.5 for single piles.
26. CCoonncclluussiioonn ccoonnttdd……
Values of the target reliability index βTS for single
piles required to achieve a specified target
reliability index βTG for pile groups can be
calculated for several pile capacity prediction
methods.
Both group effects and system effects can result
in added reliability of a pile system over that of
single piles.
There is a need of further research in the
reliability based design for the full range of
geotechnical design in order to achieve a rational
and practical design solution .