Bridges

Waka Kotahi NZ Transport Agency issued in early 2022 a substantial amendment (amendment 4) to the Bridge manual (3rd edition). A feature of the amendment is a significant upgrade of the design load combinations and load factors.

The Bridge manual design load combinations and load factors prior to amendment 4 first appeared in the Highway bridge design brief of 1972. Gradual refinement and additions had taken place over the years, but there had been little change to key combinations and factors. There were obvious inconsistencies within the combinations and with international practice, and Waka Kotahi commenced a project to review and update the combinations and factors.

A detailed comparison of the Bridge manual load combinations and factors with those contained in a range of international bridge design standards was carried out, to confirm where changes to the Bridge manual were necessary or desirable. Documents reviewed included AS 5100.2:2017, the AASHTO LRFD bridge design specifications, the Canadian highway bridge design code and the relevant Eurocodes and associated UK National Annexes. Based on the comparison, revised combinations for the Bridge manual were developed, noting the desire of Waka Kotahi to harmonise with AS 5100 where appropriate.

The revised combinations have been implemented in amendment 4. Revised load factors have been partially implemented. Full implementation awaits completion of new design traffic load models. Amendment 4 also incorporates an upgraded set of load combinations for bridges during construction.

The paper describes the outcomes of the reviews of international practice that were carried out and presents the revised load combinations as implemented. The similarities and the differences to current AS 5100 combinations and factors are highlighted. It is hoped the paper will provoke discussion which will lead to even closer harmonisation between the Bridge manual and AS 5100.