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Purpose

This notice amends the requirements of the NZTA Bridge manual for vehicle live loads used in bridge design and bridge deck evaluation. This notice is an interim step in the project to replace HN-HO-72 discussed in TAN #22-04.

NZTA Bridge manual(external link)

General information

Amendment 4 to the Bridge manual 3rd edition and amendment 1 to the Bridge manual commentary, both published in May 2022, introduced improved evaluation loadings covering the effects of HPMVs including the effects of platoons of vehicle travelling at minimum spacings. Figures C7.8 and C7.9 of the Bridge manual commentary show that the resulting negative moment effects for both single vehicles and pairs of vehicles travelling in the same lane can be substantially greater than those resulting from the previous evaluation loading. This shortfall in the previous evaluation loadings also implies that the existing HN (normal) design loading with one pair of HN axle loads has lower margins over the effects of conforming HPMV on continuous spans than for simply supported spans.

Amendments to deck evaluation loadings are also presented to correct small errors in the current provisions, and introduce a requirement to consider additional axle groups in the same lane or in adjacent lanes where that results in increased demands.

Key changes

Outline of design normal live load changes

This TAN amends the HN (normal) loading to include a second pair of HN axles positioned in the adjacent span for calculation of negative moment effects. This is relevant for continuous span lengths in the 25m to 60m range. For shorter continuous spans the effects of long semi-trailers, long draw-bar truck-and-trailers etc, are represented by a new loading arrangement comprising two 160kN tandem axle groups with the standard uniform load. For these double HN and double tandem loadings and the standard uniform load, a 0.90 reduction factor is applied.

This approach using two design vehicles or tandem axle groups for negative moment effects follows the AASHTO LRFD bridge design specifications and supporting research reports.

This TAN also amends the load factor for normal live loads used for frequently occurring SLS load effects for reinforcement stress range checks.  This amendment applies for both positive and negative moment effects caused by the amended loading.

Amendment to Bridge manual clause 3.2.2 Loads

At the end of the second sentence of the third paragraph of a. HN (normal) loading add: 

"except where required for calculation of negative moment effects as described below"

Insert the following after the third paragraph of a. HN (normal) loading:

"For calculation of negative moment effects in longitudinal members the HN (normal) loading shall consist of the worst of the following loading variations for the member under consideration:

• HN (normal) loading as specified above with one pair of axle loads.
• 90% of the effect of two HN axle pairs (120kN each axle, spaced at 5m) placed at a minimum gap of 25m from trailing axle of the first pair to lead axle of the second pair combined with 90% of the effect of the 3.5kN/m2, 3m wide uniform load placed as specified above. The two axle pairs shall be placed in adjacent spans to produce worst load effects.
• 90% of the effect of two tandem axle groups (80kN per axle spaced at 1.25m) spaced from 8m to 12m apart (from trailing axle of the first pair to lead axle of the second pair) combined with 90% of the effect of the 3.5kN/m2, 3m wide uniform load placed as specified above. The two axle groups shall be placed in adjacent spans to produce worst load effects."

Axle track transverse dimensions and wheel contact areas for each axle shall be as for the HN load element axles shown in figure 3.1. An amended Bridge manual figure 3.1(external link) is available for use.

Amendment to Bridge manual clause 4.2.1 Reinforced concrete and prestressed concrete – General

Replace the text of  j. Design for fatigue with:

"In the application of NZS 3101 clause 2.5.2.2, the stress range due to repetitive loading to be considered in flexural reinforcing bars shall be that due to normal vehicle live loading at the serviceability limit state, with a reduced load factor of 1.2 (ie 1.2LLxI).

In the application of NZS 3101(1) clause 19.3.3.6.2, the stress range due to frequently repetitive live loading shall be that due to normal vehicle live loading at the serviceability limit state, with a reduced load factor of 1.2 (ie 1.2LLxI). The stress range due to infrequent live loading shall be taken to be that due to the transient components of any of the serviceability limit state load combinations in table 3.2 that include traffic loading."

Amendment to Bridge manual clause 7.5.1 Deck capacity and Evaluation – General

In the first sentence of the fourth paragraph, replace “vehicle wheels” with “vehicle axles and their associated wheels”.

Add a fifth paragraph as follows:

"The concurrent presence of vehicle axles in adjacent lanes shall be considered where this results in higher deck demands. Axle positioning shall be based on marked lanes and the restriction imposed by the 3m wide load lane of HN loading. The accompanying lane factors of 7.4.4 (f) shall apply."

Amendment to Bridge manual clause 7.5.4 Reinforced concrete decks: plate bending analysis

In the table within 7.5.4 (b) Live Loading and analysis, replace “tables 3.1 to 3.6” with “tables 3.1 to 3.5".

In the paragraph under the table, replace “The slab shall be analysed for the loads given in tables 7.8 and/or 7.9” with “The slab shall be analysed for the loads given above”.

Add a second paragraph as follows:

"For posting, HPMV and 50MAX evaluations, consideration shall be given to the concurrent presence of additional axles or axle sets forming part of the same vehicle. If the increase in demands as compared with those resulting from the most adverse axle or axle set alone is significant, the combined demand shall be utilised in the evaluation. The reference vehicles described in 7.4.4 may be used to determine the effects of concurrent presence. If the most adverse axle or axle set is not exactly replicated in a reference vehicle, small adjustments may be made to the reference vehicle to suit."

Commentary

The above is similar to previous draft versions of this TAN adopted for some bridge designs and also clause 2.4.1.1 Vehicle loading of Standardised designs – NZTA standardised design solutions for use on state highway Roads of National Significance. Variations from previous guidance should be noted however, namely:

• The minimum gap between HN vehicles has been increased from 18m or possibly 20m to 25m.
• For calculation of negative moment effects in longitudinal members the HN (normal) loading elements are all multiplied by 90%. Multiple lane reduction factors are not applied separately to the second pair of HN axle loads.
• Accompanying lane factors in accordance with clause 3.2.4 of the Bridge manual are not being amended at this stage. For clarity, all HN load elements applied in accordance with the modified clause 3.2.2(a) are to be multiplied by the appropriate multi lane factor from clause 3.2.4. The dynamic load factor shall be applied to all load elements in accordance with clause 3.2.5.

Implementation

This TAN #26-01 shall be implemented immediately for projects instigated after publication of this TAN. For existing highway projects, confirmation should be sought from NZTA, or the relevant road controlling authority for non-state highway schemes, whether this amendment shall be used, or designs may still be undertaken in accordance with the project requirements applying prior to the issue of this TAN.

Further opportunities and clarifications

For clarification and discussion of further opportunities, please direct queries to the Lead Advisor Structures at structures@nzta.govt.nz.

Document review

This technical advice note will be withdrawn when a future amendment of the Bridge manual replacing these requirements is implemented. Cancellation will be confirmed at that time.

Publication details

Publication title: Interim amendments to Bridge manual HN loading

Published: 30 January 2026

Reference: #26-01

Version: 1

Location: nzta.govt.nz/bridge-manual

Further information

If you have any questions or feedback, please contact the Lead Advisor Structures at structures@nzta.govt.nz.

For more information visit the Highways Information Portal