Impact on Supporting Substructures

(1) Notwithstanding the requirements of this Section for the protection of supporting substructures, barrier fences shall be provided in accordance with the Transport Planning and Design Manual (TPDM) to keep damages and injuries to errant vehicles and their occupants to a minimum.

(2) All supports shall be capable of resisting the main and residual load components acting simultaneously. Forces in the direction of normal travel Fdx shall be considered

separately from forces perpendicular to the direction of normal travel Fdy.

3.6.2.2.1 Risk ranking procedure for supports

(1) The risk ranking procedure specified in Clause NA.2.11.2.3 of the UK NA to BS EN 1991-1-7 for road bridge supports and Clause 2.5.1(c) of PD 6688-1-7 for foot/cycle track bridge support shall be followed to determine the risk ranking factor Rde.

(2) When determining the factors F2 and F8, the design values of annual average daily

traffic (AADT) and percentage of heavy goods vehicles (HGVs) with three or more axles of the considered roads should be determined on a project-specific basis. The traffic flow data of the considered roads based on the annual traffic census of the Transport Department may also be referred. Where such information is not available, the AADT values given in Table 3.23 with the assumed proportion of HGVs with three or more axles taken as 10% may be used.

Table 3.23 – AADT Values for Factor F2 and F8

Carriageway Type AADT

1 lane 15,000 2 lane 33,333 3 lane 58,333 4 lane 75,000 Dual 1 lane 30,000 Dual 2 lane 66,666 Dual 3 lane 116,667 Dual 4 lane 150,000

(3) The risk ranking procedure for gantry supports should be the same as that for road bridge supports, except that factor F7 for deck stability should be taken as 2.0, while

3.6.2.2.2 Impact on road bridge supports

(1) The equivalent static design forces due to vehicular impact on road bridge supports and the associated adjustment factor Fa shall be determined in accordance with Table 3.24.

The design forces and adjustment factor Fa given in Table NA.1 and Clause

NA.2.11.2.4 of the UK NA to BS EN 1991-1-7 respectively shall not be used.

Table 3.24 – Equivalent Static Design Forces due to Vehicular Impact on Members Supporting Road Bridges over or adjacent to Roads

Force Fdx in the direction of normal travel (kN) Force Fdy perpendicular to the direction of normal travel (kN)

Point of application on bridge support

For all road bridges

Main load

component 1650 825

At the most severe point between 0.75m and 1.5m above carriageway level Residual load

component 825 415

At the most severe point between 1m and 3m above carriageway level

Minimum forces for robustness

Main load

component 250 250

At the most severe point between 0.75m and 1.5m above carriageway level Residual load

component 165 165

At the most severe point between 1m and 3m above carriageway level

Notes: (1) The adjustment factor Fa shall be applied to all main and residual load components

except for the minimum forces for robustness. (2) The adjustment factor Fa shall be taken as:

Fa = 1.0 for bridges over roads where speed limit < 72kph

Fa = 1.0 for bridges over roads where speed limit ≥ 72kph and Rde ≤ Ta = 2.4

Fa = 2.0 for bridges over roads where speed limit ≥ 72kph and Rde > Ta = 2.4

Fa = 1.0 for bridges over roads where speed limit ≥ 72kph and Rde > Ta = 2.4 with

supports protected by L3 containment level barriers with full working width

(2) When L4 containment level barriers with full working width are provided to protect the bridge supports, the main and residual load components of the minimum forces for robustness should be adopted.

(3) When L4 containment level concrete rigid barriers with a minimum lateral clearance of 400 mm between the barrier and the bridge support are provided, the design static forces should be taken as either only a residual load component specified for road bridges or the main and residual load components of minimum forces for robustness, whichever gives the most unfavourable effect on the structure. Provisions given in Clause 2.7 of PD 6688-1-7 shall not be used.

3.6.2.2.3 Impact on foot/cycle track bridge supports

(1) The equivalent static design forces due to vehicular impact on foot/cycle track bridge supports and the associated adjustment factor Fa shall be determined in accordance with

Table 3.25. The design forces given in Table 1 of the PD 6688-1-7, and the provisions and adjustment factor given in Clause 2.5.1(d) of PD 6688-1-7 shall not be used.

Table 3.25 – Equivalent Static Design Forces due to Vehicular Impact on Members Supporting Foot/Cycle Track Bridges over or adjacent to Roads

Force Fdx in the direction of normal travel (kN) Force Fdy perpendicular to the direction of normal travel (kN)

Point of application on bridge support

For foot/cycle track bridges over roads where speed limit ≥ 72kph

Main load

component 1650 825

At the most severe point between 0.75m and 1.5m above carriageway level

Residual load

component 825 415

At the most severe point between 1m and 3m above carriageway level

For foot/cycle track bridges over roads where speed limit < 72kph

Main load

component 825 415

At the most severe point between 0.75m and 1.5m above carriageway level

Residual load

component 415 205

At the most severe point between 1m and 3m above carriageway level

Minimum forces for robustness

Main load

component 165 165

At the most severe point between 0.75m and 1.5m above carriageway level

Residual load

component 85 85

At the most severe point between 1m and 3m above carriageway level

Notes: (1) The adjustment factor Fa shall be applied to all main and residual load components

except for the minimum forces for robustness. (2) The adjustment factor Fa shall be taken as:

Fa = 0.5 for bridges with Rde ≤ Tc = 2.4

Fa = 1.0 for bridges with Rde > Tc = 2.4

Fa = 0.5 for bridges over roads where speed limit ≥ 72kph and Rde > Tc = 2.4 with

supports protected by L3 containment level barriers with full working width

(2) If the risk ranking factor Rde is larger than Tc = 2.4 and when L4 containment level

barriers with full working width are provided to protect the bridge supports, the main and residual load components of the minimum forces for robustness should be adopted. When L4 containment level concrete rigid barriers with a minimum lateral clearance of 400 mm between the barrier and the bridge support are provided, the static design forces should be taken as either only a residual load component of Fdx = Fdy = 165 kN or

gives the most unfavourable effect on the structure. Provisions given in Clause 2.5.1(f) of PD 6688-1-7 shall not be used.

(3) If the risk ranking factor Rde is less than or equal to Tc = 2.4 and when L3 containment

level barriers with full working width are provided to protect the bridge supports, the main and residual load components of the minimum forces for robustness should be adopted. When L3 containment level concrete rigid barriers with a minimum lateral clearance of 400 mm between the barrier and the support of foot/cycle track bridges are provided, the static design forces should be taken as either only a residual load

component of Fdx = Fdy = 85 kN or the main and residual load components of minimum

forces for robustness, whichever gives the most unfavourable effect on the structure. Provisions given in Clause 2.5.1(f) of PD 6688-1-7 shall not be used.

3.6.2.2.4 Impact on gantry supports

The equivalent static design forces due to vehicular impact on gantry supports and the associated adjustment factor Fa shall be determined in accordance with Table 3.26.

Table 3.26 – Equivalent Static Design Forces due to Vehicular Impact on Members Supporting Sign Gantries over or adjacent to Roads

Force Fdx in the direction of normal travel (kN) Force Fdy perpendicular to the direction of normal travel (kN)

Point of application on bridge support

For gantries over roads where speed limit ≥ 72kph

Main load

component 1650 825

At the most severe point between 0.75m and 1.5m above carriageway level

Residual load

component 825 415

At the most severe point between 1m and 3m above carriageway level

Minimum forces for robustness

Main load

component 165 165

At the most severe point between 0.75m and 1.5m above carriageway level

Residual load

component 85 85

At the most severe point between 1m and 3m above carriageway level

Notes: (1) The adjustment factor Fa shall be applied to all main and residual load components except for

the minimum forces for robustness.

(2) The adjustment factor Fa shall be taken as 0.2 for gantries with Rde > 0.5.

(3) The minimum forces for robustness shall be adopted for gantries over roads where speed limit < 72kph or Rde ≤ 0.5.