Meeting Tier 1 bridge spacings

Heavy vehicle combinations are often required to meet Tier 1 axle spacings for access over bridges. We have seen this recently with the National Class 2 Performance Based Standards (Tier 1) Authorisation Notice as well as the National Class 3 20m Long 3-axle Truck and 4-axle Dog Trailer Mass and Dimension Exemption Notice.  For operators who can meet Tier 1 this provides a fast, easy method of access.

Tier 1 bridge assessment works on a set of rules which are dictated by extreme axle spacings among groups of axles, which means that axle spacing is very important to meet Tier 1. If you don’t meet the required minimum axle spacings the mass of your combination will be reduced. In this article we will be going through the minimum axle spacings for a wide range of combinations to achieve full mass. Its worth noting that due to overall length limitations some of these minimum axle spacings will not typically be achievable and are included for completeness only (e.g. PBS Level 1 for a 26‑metre B-double).

If your combination doesn’t meet these minimum axle spacings, fear not, Advantia has also released a free online calculator which will enable all operators to calculate their eligible mass under Tier 1 for any combination type.

Furthermore, some specific networks may limit the total Gross Combination Mass (GCM) to below the sum of maximum axle loads. This is typical of the PBS Level 1 network (approximately 50 or 50.5t) and also networks like the PBS Level 2 B (Cubic) network in Victoria which only allows combinations up to 68.5 tonnes GCM.

With that being said, lets get started!

 

Photo credit – Hercules Engineering

Truck and dogs will almost always operate under Tier 1 mass limits, and this is the reason that larger dog trailers will typically have long drawbars.

The PBS Level 1 network will limit gross mass to 50.5t for single steer combinations and 50.0t for twin‑steer combinations. Where this is applicable this has been shown.

 

3-axle truck and 3-axle dog
PBS Level 1/2/3/4: 49.5t maximum

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-4	6.33	6.33	6.33
1-6	15.50	12.33	11.83
2-4	4.33	4.33	4.33
2-6	9.83	9.83	9.83
4-6	4.33	4.33	4.33

 

3-axle truck and 4-axle dog
PBS Level 1: 50.5t maximum
PBS Level 2/3/4: 57.5t maximum

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
1-7	17.50	17.33	14.33
2-5	6.83	6.83	6.83
2-7	11.50	13.33	12.33
4-7	5.00	6.83	6.83

 

3-axle truck and 5-axle dog
PBS Level 1: 50.5t maximum
PBS Level 2/3/4: 63.0t maximum

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
1-8	17.50	19.67	15.50
2-5	6.83	6.83	6.83
2-8	11.50	15.67	13.50
4-8	5.00	8.00	8.00

 

3-axle truck and 6-axle dog
PBS Level 1: 50.5t maximum
PBS Level 2/3/4: 68.5t maximum

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-6	10.00	10.00	10.00
1-9	17.50	22.00	16.67
2-6	8.00	8.00	8.00
2-9	11.50	18.00	14.67
4-9	5.00	9.17	9.17

 

4-axle truck (twin steer) and 3-axle dog
PBS Level 1: 50.0t maximum
PBS Level 2/3/4: 54.0t maximum

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-4	5.00	5.00	5.00
1-5	8.00	8.00	8.00
1-7	17.50	15.67	13.50
3-5	4.33	4.33	4.33
3-7	8.83	9.83	9.83
5-7	3.33	4.33	4.33

 

4-axle truck (twin steer) and 4-axle dog
PBS Level 1: 50.0t
PBS Level 2/3/4: 62.0t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-4	5.00	5.00	5.00
1-6	11.50	10.50	10.50
1-8	17.50	20.67	16.00
3-6	6.83	6.83	6.83
3-8	8.83	13.33	12.33
5-8	3.33	6.83	6.83

 

4-axle truck (twin steer) and 5-axle dog
PBS Level 1: 50.0t
PBS Level 2/3/4: 67.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-4	5.00	5.00	5.00
1-6	11.50	10.50	10.50
1-9	17.50	23.00	17.17
3-6	6.83	6.83	6.83
3-9	8.83	15.67	13.50
5-9	3.33	8.00	8.00

 

4-axle truck (twin steer) and 6-axle dog
PBS Level 1: 50.0t
PBS Level 2/3/4: 73.0t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-4	5.00	5.00	5.00
1-7	15.00	12.00	11.67
1-10	17.50	25.33	18.33
3-7	8.00	8.00	8.00
3-10	8.83	18.00	14.67
5-10	3.33	9.17	9.17

 

It should be noted that quad-axle groups are not entitled to full mass under Tier 1 bridge assessment, and as such any combination including quad-axle groups is limited to QML1 (equivalent to CML) e.g. 21 tonnes per quad-axle group.

For Super-Semi or other split-axle combinations, minimum internal space between axle groups of 2.5 metres should also be accounted for.

 

2-axle prime mover and tandem axle semi-trailer
PBS Level 1/2/3/4: 32.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-2	2.50	2.50	2.50
1-4	6.33	6.33	6.33
2-4	4.33	4.33	4.33

 

3-axle prime mover and 2-axle semi-trailer
PBS Level 1/2/3/4: 40.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
2-5	6.83	6.83	6.83

 

3-axle prime mover and 3-axle semi-trailer
PBS Level 1/2/3/4: 46.0t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-6	10.00	10.00	10.00
2-6	8.00	8.00	8.00

 

4-axle prime mover (twin steer) and 3-axle semi-trailer
PBS Level 1: 50.0t
PBS Level 2/3/4: 50.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-4	5.00	5.00	5.00
1-7	15.00	12.00	11.67
3-7	8.00	8.00	8.00

 

3-axle prime mover and 4-axle semi-trailer
PBS Level 1/2/3/4: 44.0t
QML2 is unavailable under Tier 1 so maximum mass is limited (except in NSW).

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-7	10.00	10.00	10.00
2-7	8.00	8.00	8.00

 

4-axle prime mover (twin steer) and 4-axle semi-trailer
PBS Level 1/2/3/4: 48.5t
QML2 is unavailable under Tier 1 so maximum mass is limited

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-4	5.00	5.00	5.00
1-8	15.00	12.00	11.67
3-8	8.00	8.00	8.00

 

3-axle prime mover and 3-axle super-semi-trailer (2+1 split axle)
PBS Level 1/2/3/4: 49.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
1-6	15.50	12.33	11.83
2-5	6.83	6.83	6.83
2-6	9.83	9.83	9.83
4-6	4.33	4.33	4.33

 

3-axle prime mover and 3-axle super-semi-trailer (1+2 split axle)
PBS Level 1/2/3/4: 49.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-4	6.33	6.33	6.33
1-6	15.50	12.33	11.83
2-4	4.33	4.33	4.33
2-6	9.83	9.83	9.83
4-6	4.33	4.33	4.33

 

3-axle prime mover and 4-axle super-semi-trailer (1+3 split axle)
PBS Level 1: 50.5t
PBS Level 2/3/4: 55.0t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-4	6.33	6.33	6.33
1-7	17.50	14.67	13.00
2-4	4.33	4.33	4.33
2-7	11.50	11.00	11.00
4-7	5.00	5.50	5.50

 

3-axle prime mover and 4-axle super-semi-trailer (3+1 split axle)
PBS Level 1: 50.5t
PBS Level 2/3/4: 55.0t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-6	10.00	10.00	10.00
1-7	17.50	14.67	13.00
2-6	8.00	8.00	8.00
2-7	11.50	11.00	11.00
4-7	5.00	5.50	5.50

 

3-axle prime mover and 4-axle super-semi-trailer (2+2 split axle)
PBS Level 1: 50.5t
PBS Level 2/3/4: 57.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
1-7	17.50	17.33	14.33
2-5	6.83	6.83	6.83
2-7	11.50	13.33	12.33
4-7	5.00	6.83	6.83

 

B-doubles

Photo credit – Vawdrey Trailers

 

7-axle tan-tan B-double (1-2-2-2)
PBS Level 1: 50.5t
PBS Level 2/3/4: 57.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
1-7	17.50	17.33	14.33
2-5	6.83	6.83	6.83
2-7	11.50	13.33	12.33
4-7	5.00	6.83	6.83

 

7-axle tan-tan B-double (1-2-2-2)
PBS Level 1: 50.5t
PBS Level 2/3/4: 57.5t

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-5	8.83	8.83	8.83
1-7	17.50	17.33	14.33
2-5	6.83	6.83	6.83
2-7	11.50	13.33	12.33
4-7	5.00	6.83	6.83

10-axle quad-tri B-double (1-2-4-3)
PBS Level 1: 50.5t
PBS Level 2/3/4: 65.0t
QML2 is unavailable under Tier 1 so maximum mass is limited

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-7	10.00	10.00	10.00
1-10	17.50	22.00	16.67
2-7	8.00	8.00	8.00
2-10	11.50	18.00	14.67
4-10	5.00	9.17	9.17

 

11-axle quad-quad B-double (1-2-4-4)
PBS Level 1: 50.5t
PBS Level 2/3/4: 65.0t
QML2 is unavailable under Tier 1 so maximum mass is limited

Minimum spacing (m)
Axles	PBS Level 1 roads	PBS Level 2 roads	PBS Level 3/4 roads
1-3	3.33	3.33	3.33
1-7	10.00	10.00	10.00
1-11	17.50	22.00	16.67
2-7	8.00	8.00	8.00
2-11	11.50	18.00	14.67
4-11	5.00	9.17	9.17

 

Again, if your combination doesn’t meet these minimum axle spacings, Advantia has also released a free online calculator which will enable all operators to calculate their eligible mass under Tier 1 for any combination type.