Design Loads for Box Culverts

Design Loads for Box Culverts for the SM1600 Design Loading

ABSTRACT

The new SM1600 design loading of the draft Australian Bridge Design Code AS 5100 is
complex. The interpretation of some aspects of the loading and its application in the practical
design of large precast reinforced concrete box culverts is not clear. In an attempt to clarify these
aspects, published material relating to the development of the SM1600 design loading is
reviewed, and the SM1600 loading is briefly compared with previous Australian codes, with
overseas codes, and with current knowledge.
It appears that the SM1600 loading was not developed with box culverts and other short span or
buried structures in mind. Modifications are suggested to the AS 5100 loading provisions for the
design of box culverts, particularly in regard to live load surcharge and compaction pressure. The
fatigue design provisions of AS 5100 are such that check for fatigue from moment and shear
effects will usually be required for precast box culverts under shallow fill. In the past, this was
only required for railway traffic loadings.
The use of the accompanying lane factors, together with the different uniform lane loads
associated with the M1600 and S1600 design vehicles, appears to add complexity to the design of
large precast box culverts and other short span or buried structures that is not required for these
structures.
Accordingly, it is suggested that the SM1600 loading be modified with a view to simplifying it
for the purpose of designing culverts and other short span buried structures, to a tandem or a
tri axle loading applied without uniform lane load or accompanying lane factors. The limit for the
length of short spans and associated axle loads as well as the fatigue loading should be decided as
part of this modification.
The adequacy of the current M1600 triaxle load is reviewed using available data from Culway
sites, and the use of this data for verification and adjustments of design vehicle loads is briefly
discussed.