Load Factor Design (LFD), adopted by AASHTO in the mid-1970, is currently used for
bridge superstructure design. However, the AASHTO specifications do not have any
LFD provisions for foundations. In this study, a LFD format for the design of shallow
foundations for bridges is developed.
Design equations for reliability analysis are formulated. Uncertainties in design parameters
for ultimate and serviceability limit states are evaluated. A random field
model is employed to investigate the combined inherent spatial variability and systematic
error for serviceability limit state. Advanced first order second moment
method is then used to compute reliability indices inherent in the current AASHTO
specifications. Reliability indices for ultimate and serviceability limit states with different
safety factors and dead to live load ratios are investigated. Reliability indices
for ultimate limit state are found to be in the range of 2.3 to 3.4, for safety factors between
2 and 3. This is shown to be in good agreement with Meyerhof’s conclusion
(1970). Reliability indices for serviceability limit state are found to be in the range of
0.43 to 1.40, for ratios of allowable to actual settlement between 1.0 to 2.0. This appears to be in good agreement with what may be expected. Performance factors are then determined using target reliability indices selected on the basis of existing risk
levels.
next to an author's name indicates that all files or directories associated with their ETD are accessible from the Virginia Tech campus network only.
![[VT]](http://scholar.lib.vt.edu/images/ETD-db/restricted.gif)
