FLNRORD Supplement to Canadian Highway Bridge Design Code

The Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNRORD) Supplement to CHBDC is to be read and utilized in conjunction with the CSA S6:19 Canadian Highway Bridge Design Code (CHBDC).

This supplement to CHBDC is considered an integral part of the FLNRORD Bridge Guidelines, Standards and Specifications (BGSS) website.  Included in this supplemental document are referenced bridge design code clauses where additional or modified guidance is provided for the CHBDC clause; changes are noted that delete or modify CHBDC text, or additional commentary is provided for reference by designers.  All commentary within this document is denoted by italicized text.  The text under each specific clause is considered additional and supplemental to the information provided in the CHBDC. Where variations are not specified herein, CHBDC Section 3 shall be followed for Forest Service Road bridges. 

 This is a working draft and currently, specifically pertains to Section 3, Loads of CHBDC. The Ministry will continue to expand this Supplement, as a work in progress, to capture additional sections of CHBDC.

 Long span, continuous multi-span or otherwise complex or unique bridges may require special engineering investigations, and some deviations from common Ministry standards.  These deviations must be approved by the Ministry Engineer. 

 Bridges that are exclusively designed for light recreational use, such as for pedestrians, cyclists, snowmobiles, horses and/or all-terrain vehicles, are not currently addressed in the Supplement.  A Ministry Engineer shall be consulted for information relating to Ministry design requirements for these types of recreational use structures. 

This document supersedes any previous related Ministry standards that conflict with the guidance provided herein.

The Canadian Highway Bridge Design Code CSA S6:19 (CHBDC) applies subject to each of the CHBDC sections specified herein by section number and title, being amended, substituted or modified, as the case may be, in accordance with the amendments, substitutions and modifications described herein as corresponding to each such CHBDC section.

The CHBDC shall apply for the design and construction of Ministry of Forests, Lands, Natural Resource Operations and Rural Development (Ministry) Forest Service Road (FSR) bridges and other structure types that are referenced within the scope of CHBDC.

The “Ministry of Forests, Lands, Natural Resource Operations and Rural Development Supplement to the Canadian Highway Bridge Design Code CSA S6:19” (Supplement) shall also apply for the design and construction of Ministry bridges and other Ministry structure types that are referenced within the scope of CHBDC.

In the event of inconsistency between the Supplement and CHBDC, the Supplement shall take precedence over CHBDC.

The following definitions/interpretations shall apply and, where referenced in CHBDC, shall replace the CHBDC definitions for these terms:

“Highway” shall be interpreted to mean “Forest Service Road”.

“Ministry” refers to the Ministry of Forests, Lands, Natural Resource Operations and Rural Development

“Normal Traffic” shall be interpreted to refer to truck and/or lane loading associated with the following Ministry standard traffic loads: BCL-625, LOH, HOH, L-100, L-150 and L-165.

“Regulatory Authority / Owner” shall be interpreted to refer to a Ministry Engineer.

“Supplement” refers to the Supplement to the Canadian Highway Bridge Design Code CSA S6:19.

“Temporary Structure” shall be interpreted in Section 3 to refer exclusively to structures built as bypasses or other short-term structures (e.g.; retaining walls) that are used during construction of a longer-term structure at a site and are then removed.  Other structures (e.g.; all steel portable bridges, log bridges, etc.) shall not be considered temporary in the context of Section 3, even if they may only be intended for use on an FSR for a short period of time.

Commentary:

The above definition of “Temporary Structure” replaces the definition of “Temporary Structure” in CHBDC clause 1.3.3 which is “a structure with a design life of less than five years.”

3.3.1 Abbreviations

BCL – British Columbia Loading

BGSS - FLNRORD Bridge Guidelines, Standards and Specifications website

CHBDC – CSA S6:19 Canadian Highway Bridge Design Code

“CL-W” – shall be interpreted for FSR bridges to mean “design traffic loading”, thereby referring to the specific normal design traffic loading scenario being used for a site.

CSA – Canadian Standards Association

FLNRORD – Ministry of Forests, Lands, Natural Resource Operations and Rural Development

FSR – Forest Service Road

L – ### - Logging Truck Loading (GVW in tons)

LOH – Light Off-Highway

HOH – Heavy Off-Highway

3.4.4 Serviceability Limit States

Delete and replace with:

Structural components shall satisfy the requirements for the serviceability limit states specified in the applicable sections of this Supplement for the appropriate loading combinations.

Superstructures shall be proportioned so that the maximum deflection due to factored traffic load, including dynamic load allowance, does not exceed:

  • Span/350 for concrete slab girder bridges, all steel portable bridges and inverted channel compo-girder bridges;
  • Span/450 for all other bridges (including: steel girder/concrete deck bridges, steel girder/timber deck bridges, compo-I-girder/concrete deck bridges).

Commentary:

Span deflection limits like those described above have been successfully used for decades in FSR bridge design. 

Add the following:

CHBDC provides detailed directions regarding the use of various limit states for bridge design.  These directions apply unless otherwise specified by the Ministry.

Commentary:

Tables S3.1 and S3.2 provide example limit states scenarios that need to be considered when engineers are designing superstructures for FSR bridges.  The intention of providing these tables is to provide engineers a typical indication of the Ministry’s expectations for the thoroughness of the limit states design process.  The tables provide common examples only and do not portray all situations. 

It is important to note that these tables do not provide detailed information relating to the use of limit states for the design of substructures.

Table S3.1: Superstructure Design Fatigue Limit States and Serviceability Limit States Examples
Limit State Combination Type of Superstructure Example CHBDC Clause References Comments
FLS1 General 1.4.2.1, 3.4.3, 3.5 Applies only to steel components (including rebar)
Reinforced concrete superstructures 8.5.3.1 Applies to reinforcing bars
Steel superstructures 10.5.4, 10.17 Applies to girders, bracing, steel decks, girder splices, welds, bolted connections, stud shear connectors
SLS1 General 1.4.2.1, 3.4.4, 3.5 According to Section 3.5, Table 3.1, SLS1 loading includes dead loads, however not always (e.g. not for deflection of wood components according to 9.4.2.)
Reinforced concrete superstructures 8.5.2, 8.12 Addresses cracking and deformation limitations
Steel superstructures 10.5.3, 10.18.2.3
  • Addresses yielding of sections: 10.5.3.3 applies: “Members of all classes of sections shall be proportioned so that general yielding shall not occur. Localized limited yielding may be used.”
  • Addresses slipping of bolted joints: 10.18.2.3 applies for continuous girders: “Joints of primary members subjected to stress reversal shall be designed as slip-critical connections.”
  • FSR bridge steel decks do not meet the definition of “orthotropic steel decks” therefore the deflection provisions of 10.16.4 do not strictly apply.
Wood superstructures 9.4.2 Deflection of wood components caused by live load is limited to Span/400
SLS2 General 1.4.2.1, 3.4.4, 3.5 According to CHBDC Clause 3.5, Table 3.1, SLS2 applies for superstructure vibration only.  For FSR bridges SLS2 shall apply for bridge deflection limits.
Reinforced concrete superstructures 8.5.25
Steel superstructures 10.5.3.5
Wood superstructures 9.4.2

 

Table S3.2: Superstructure Design Ultimate Limit States Examples
Ultimate Limit States Combination Types of Loads Considered (in addition to dead loads and earth pressures)
1 Live loads
2 Live loads plus other strains (including thermal forces, etc.)
3 Live loads plus other strains (including thermal forces, etc.) plus wind load on bridge plus wind load on traffic
4 Other strains (including thermal forces, etc.) plus wind load on bridge
5 Earthquake loads
6 Loads from streams, ice and debris torrents
7 Loads from ice accretion plus wind on bridge
8 Loads from vehicle or vessel (boat) collision with bridge
9 No additional loads, however uses an increased load factor for dead loads

 

Add the following:

Dead loads for bridges with a concrete running surface shall include an allowance for a 50 mm future concrete overlay over the full area of the bridge deck. 

For timber deck bridges, design dead loads shall include an allowance for a 75 mm thick layer of running planks, if running planks are not already included in the bridge design.

These loads shall be noted on the detailed design drawings.

Replace the unit weight of softwood in Table 3.4 with

  • Untreated Softwood: 6.0 kN/m3; and
  • Treated Softwood: 7.0 kN/m3

Add the following:

The Ministry is developing guidance to be incorporated for future versions of this document.

3.8.1 General

Add the following:

Ministry standard traffic loads are the BCL-625, LOH, HOH, L-100, L-150, and L-165 as described in this section.

Commentary:

Place holder for brief introduction to LOH and HOH traffic loads.

3.8.2 Design Lanes

Delete and replace with:

In addition to the standard bridge widths outlined in this section, vehicle horizontal tracking requirements shall be considered when determining appropriate bridge widths for sites that are not on straight horizontal alignments.

The WB-19 design tracking vehicle is to be used together with a minimum clear distance to guardrails of 500 mm unless otherwise specified by the Ministry Engineer.

Figure S3.1: WB-19 Design Tracking Vehicle

Figure S3.1 WB-19 Design Tracking Vehicle

Commentary:

The following reference documents published by FPInnovations are available on the Ministry website for designers to review:

Standardizing the Design of Approach Alignment to Bridges on Forestry Roads in British Columbia: Review and Analysis

and

Field Testing to Validate Standardized Bridge Approach Curve Design Recommendations.

3.8.2.1 Single Lane Bridge Widths  

Standard single lane bridge widths:

  • 4.268 m for BCL-625, LOH and L-100; and
  • 4.876 m for HOH, L-150 and L-165.

Bridges with deck widths less than or equal to 6.0 m shall be considered single lane bridges and shall not be designed to carry two lanes of traffic. 

Commentary:

FSR bridges are typically single lane, although in special situations, such as high traffic volumes, challenging horizontally curved alignments or poor sight lines for vehicles approaching from opposing directions, bridges wider than 6.0 m are occasionally utilized.

3.8.2.2 Two-Lane Bridges

3.8.2.2.1 Two-Lane Bridge Width

Standard two-lane bridge widths:

  • 9.144 m for BCL-625, LOH, and L-100; and
  • 10.973 m for HOH, L-150, & L-165.

Bridges wider than 10.973 m are extremely rare on FSRs.  For such bridges, the Ministry Engineer shall provide direction in relation to design lanes and analysis methodologies.

Commentary:

The 9.144 m two-lane bridge width is based on two 4.268 m single lane bridge widths for the BCL-625, LOH, and L-100 traffic loads, with additional deck width to accommodate passing of vehicles.  The 10.973 m two-lane bridge width is based on two Pacific Truck P16 bunk widths of 15 ft with additional deck width to accommodate passing of vehicles.    

3.8.2.2.2 Two-Lane Bridge Safety

Two-lane bridges shall either be 9.144 m wide or 10.973 m wide, based on the standard two-lane bridge widths in Section 3.8.2.2.1.  Single lane bridges shall not be designed with a width greater than 6.0 m and less than the standard two-lane bridge width, other than where required by tracking. 

Signage at the bridge site shall indicate to drivers whether a bridge is suitable for two-way traffic or one-way traffic only. 

Pullouts at approaches shall be provided as required to accommodate vehicles waiting to cross a bridge.

Commentary:

When a bridge width is greater than 6.0 m and less than 9.144 m it may be difficult for drivers to determine whether the bridge is wide enough to safely accommodate two-way traffic when vehicles are simultaneously approaching from opposite directions.  This uncertainty can create safety concerns.

Since FSR bridges are normally assumed to be single lane, and signed to be narrowed roadways, any bridges that are designed to carry two-way traffic should have alternate signage that clearly indicates that the bridge is suitable for two-way traffic.

Most FSR bridges have narrow ( 6.0 m) single lane widths that clearly do not allow two-way traffic on the bridge.  When vehicles travelling in opposite directions meet near such a bridge, one vehicle needs to wait at the end of the bridge to allow the other vehicle to cross.  Pullouts at such bridge sites provide the needed space for the waiting vehicle.

A standard two-lane bridge with a width of 9.144 m on a reasonably straight horizontal alignment will clearly provide two-way traffic on the bridge for most vehicles.

3.8.3. Traffic Loads

3.8.3.1 Normal Traffic

3.8.3.1.1 CL-W Loading

Delete and replace with:

Bridges shall not be designed for loads less than the BCL-625 traffic load.  Alternatively, bridges should be designed for the LOH, HOH, L-100, L-150 or L-165 traffic loads as specified in the contract documents.

3.8.3.1.2 CL-W Truck & 3.8.3.1.3 CL-W Lane Load

Delete and replace with:

The BCL-625 traffic load consists of the BCL-625 truck and the BCL-625 lane load defined in Figure S3.2.1 and Table S3.3.

The LOH traffic load consists of the LOH truck and the LOH lane load defined in Figure S3.2.2 and Table S3.3.

The HOH traffic load consists of the HOH truck and the HOH lane load defined in Figure S3.2.3 and Table S3.3.

The L-100 traffic load consists of the L-100 truck and the L-100 lane load defined in Figure S3.2.4 and Table S3.3.

The L-150 traffic load consists of the L-150 truck and the L-150 lane load defined in Figure S3.2.5 and Table S3.3.

The L-165 traffic load consists of the L-165 truck and the L-165 lane load defined in Figure S3.2.6 and Table S3.3.

 

Figure S3.2.1: BCL-625 Traffic Load

Figure S3.2.1: BCL-625 Traffic Load

 

Figure S3.2.2: LOH Traffic Load

Figure 3.2.2 LOH Traffic Load

 

Figure S3.2.3: HOH Traffic Load

Figure 3.2.3 HOH Traffic Load

 

Figure S3.2.4: L-100 Traffic Load

Figure S3.2.4 L-100 Traffic Load

 

Figure S3.2.5: L-150 Traffic Load

Figure S3.2.5 L-150 Traffic Load

 

Figure S3.2.6: L-165 Traffic Load

Figure S3.2.6 L-165 Traffic Load

Table S3.3: Traffic Load Dual Wheel Footprint Dimensions
  Units Traffic Load
BCL-625 LOH L-100 HOH L-150 L-165
Transverse mm 600 800 800 800 800 800
Longitudinal mm 250 250 275 350 400 450

Commentary:

The clearance envelope for L-series vehicles has been determined from the previous Ministry standard methodology which described placement of these vehicles laterally on a bridge deck based on an eccentricity from the centreline of the bridge, which varied depending on the design vehicle and the bridge deck width.

3.8.4 Application

3.8.4.1 General

Delete and replace with:

The following requirements shall apply:

  1. Truck axles that reduce load effect shall be neglected.
  2. The uniformly distributed portion of the lane load shall not be applied to those parts of a design lane where its application decreases load effect.
  3. For FLS and SLS Combination 2, the traffic load shall be one truck only placed at the centre of the design lane.  The lane load shall not be considered.  The lateral wheel load distribution shall be 50%-50%.  For L-165 bridges, the L-150 truck shall be used for FLS and SLS Combination 2 analysis.
  4. For SLS Combination 1 and for ultimate limit states, the traffic load shall be the truck load increased by the dynamic load allowance or the lane load, whichever produces the maximum load effect.  This load shall be placed longitudinally and transversely within the design lane at a location and in a direction that produces the maximum load effect.  The lateral wheel load distribution and minimum lateral offsets for the truck and lane load shall be as specified in Figure S3.2.1 to Figure S3.2.6 and shall be in accordance with Section 3.8.4.2 for two-lane bridges.

3.8.4.2 Multi-Lane Loading

3.8.4.2.1 Normal Traffic Only

Delete and replace with:

Two-lane bridges shall either be 9.144 m wide or 10.973 m wide, based on the standard two-lane bridge widths in Section 3.8.2.2.1.  Single lane bridges shall not be designed with a width greater than 6.0 m and less than the standard two-lane bridge width, other than where required by tracking. 

The number of design lanes for traffic shall be determined from Table S3.4. 

For single-lane traffic loading, the lateral wheel load distribution shall be as specified in Figure S3.2.1 to Figure S3.2.6.  For the two-lane loading scenario, the lateral wheel load distribution for both vehicles shall be 50%-50% and the traffic load shall be multiplied by a modification factor of 0.9.  Design lanes that are loaded shall be selected to maximize the load effect.

Table S3.4: Traffic Loading Based on Bridge Width
Traffic Load Wc
(Deck Width, m)
n
(number of design lanes)
We
(Lane Width, m)
BCL-625 ≤ 6.0 1 We = Wc
> 6.0 1 & 2 We = Wc & Wc/2
LOH ≤ 6.2 1 We = Wc
> 6.2 1 & 2 We = Wc & Wc/2
HOH ≤7.4 1 We = Wc
> 7.4 1 & 2 We = Wc & Wc/2
L-100 ≤ 6.936 1 We = Wc
> 6.936 1 & 2 We = Wc & Wc/2
L-150 ≤ 8.152 1 We = Wc
> 8.152 1 & 2 We = Wc & Wc/2
L-165 ≤ 7.952 1 We = Wc
> 7.952 1 & 2 We = Wc & Wc/2

Bridges designed for off-highway traffic loads with deck widths greater than 6.0 m and less than 8.152 m shall be designed for two-lanes of BCL-625 loading in addition to the design traffic load.

Commentary:

Bridges wide enough for two lanes of traffic shall be designed for a single lane traffic loading scenario and a two-lane traffic loading scenario.

At some sites with horizontally curved alignments, a bridge designed as single lane for a design tracking vehicle may be wide enough to allow two short trucks to pass on the bridge.  For this reason, the Ministry requires all bridges wide enough for two-way traffic to be designed as two-lane bridges as described herein. The deck width limits in Table S3.4 are based on two clearance envelopes for the respective traffic load in Figure S3.2.1 to Figure S3.2.6.

Bridges designed for off-highway traffic loads, that are wider than 6.0 m for tracking purposes, are wide enough for two-lanes of BCL-625 traffic.  Therefore, these bridges shall also be designed for two-lanes of BCL-625 traffic.

3.8.4.3 Local Components

Delete bullet points c) and d) and replace with:

  1. For the design of decks and other components whose design is governed by axle loads, the tandem axle increased by the applicable dynamic load allowance shall be considered.  For BCL-625 traffic loading axle no.4 shall also be considered. 
  2. For deck overhangs or adjacent to a curb, railing, or barrier, the minimum distance from the centres of the wheels to the guardrail shall be as specified in Table S3.5.
Table S3.5: Minimum Lateral Offset - Design of Components Governed by Axle Loads
Units Traffic Load
BCL-625 LOH HOH L-100 L-150 L-165
mm 300 400 400 400 400 400

 

Commentary:

The minimum lateral offsets in Table S3.5 are based on the wheel being positioned against the curb, railing or barrier.

3.8.8 Barrier Loads

3.8.8.1 Traffic Barriers

Delete and replace with:

Bridge curb, railings and barriers shall be in accordance with Ministry standard designs and guidelines.

Commentary:

Ministry barrier design loading is provided in Table S3.6 and is outlined in Associated Engineering Ltd. report “Development and Testing of CL-2 and CL-3 Barriers, Rev.1”.

Table S3.6: Barrier Design Criteria
Factored Design Forces Containment Level
CL-13 CL-2 CL-3
Transverse Load, FT. kN - 45 120
Longitudinal Load, FL, kN - 20 40
Vertical Load, Fv, kN - 20 20
Load Application Height, mm2 - 450 510
Minimum Barrier Height, mm2 - 525 585

Note:

  1. When completing an analytical evaluation of a barrier, these forces represent factored forces; resistances should be calculated assuming nominal material strengths.
  2. Height measured from travel surface.
  3. Only MFLNRO standard drawings shall be used for CL-1 containment level design. 

 

3.10.1 General

3.10.1.2 Reference Wind Pressure

Delete and replace with:

The following simplified reference wind pressure shall be used for the design of FSR bridges.

The hourly mean reference wind pressure, q, shall be as follows:

  • For in-service conditions, qis, shall be 700 Pa. 
  • For construction, qc, shall be 300 Pa.

Alternate values of hourly mean reference wind pressure for construction may be used for supply install contracts or as approved by the Ministry Engineer.

The maximum allowable wind speed for construction activities shall be 20 km/h unless further detailed analysis is undertaken.  The maximum allowable wind speed for construction activities shall be stated on the design drawings.

Design reference wind pressures shall not be increased to account for wind funneling.

Commentary:

The simplified reference wind pressure methodology has been adopted to address the nature of the materials procurement process for FSR bridges, where the designer may not know the intended location of a structure.  An in-service hourly mean reference wind pressure of 700 Pa is equivalent to some of the highest 50-year return period values in CHBDC Table A3.1.1 for B.C. locations.  A construction hourly mean reference wind pressure of 300 Pa has historically been used for construction wind loading for forestry bridges in B.C.  This approximately reflects the 95th percentile hourly mean reference wind pressure for locations in B.C. with a return period of 1 year.  This construction wind loading is deemed appropriate given the accelerated construction methods used for typical forestry bridge construction, which results in very short periods between the non-composite and composite conditions of steel girder composite concrete deck bridges.  The maximum allowable wind speed for construction activities is intended to provide a conservative limit on the wind speed in which crew or equipment may be on a composite concrete deck steel girder bridge while it is in non-composite condition. 

The CHBDC 20% increase for sites that may experience wind funnelling has been assumed to not apply for the development of the supplement’s wind loading methodology.  Instead 0% increase is assumed.  The 20% increase for sites with funnelling has not been used in order to keep wind loading reasonable in relation to the lower risks associated with wind loading on FSR bridges in comparison to highway bridges.

3.10.1.3 Gust Effect Coefficient

Delete and replace with:

A gust effect coefficient (Cg) of 2.0 shall be used for FSR bridges.

3.10.1.4 Wind Exposure Coefficient

Add the following:

A wind exposure coefficient (Ce) of 1.0 shall be used for FSR bridges except for sites where the height of the top of the superstructure (H) above low water level is known to be >25 m in which case CHBDC Table 3.9 shall apply.

3.10.1.5 Non-uniform Loading

Delete and replace with:

Consideration of non-uniform loading is not required.

3.10.2 Design of the Superstructure

3.10.2.4 Wind Load on Live Load

Delete and replace with:

The horizontal wind load per unit exposed frontal area of live load shall be calculated in accordance with Clause 3.10.2.2., except that Ch shall be taken as 1.2.  For “in-service” wind loading, the exposed frontal area of the live load shall be assumed to be 5 m high x 20 m long, located on the bridge to produce the maximum load effect. 

3.11.7 Debris Torrents

Replace the last sentence with:

All bridges subject to potential debris torrents or debris flows shall be designed to accommodate the debris torrents and debris flows without damage to the structure or approaches unless otherwise directed by the Ministry Engineer.

If a site has debris torrent/flow potential, the design professional responsible for the general arrangement design of the structure shall ensure that reasonable investigations into the probability and size of debris torrents/flows are undertaken.  The design professional shall identify the opening size and design requirements that would be needed to accommodate debris torrents/flows for estimated return periods of 1, 10, 50, and 100 years. This professional shall discuss their conclusions with the Ministry Engineer at the initial design stage in order to obtain Ministry direction relating to acceptable risk for structure design at the site.

3.12.1 General

Delete and replace with:

All bridges subject to potentially damaging ice loads shall be designed to accommodate the ice loads without damage to the structure unless otherwise directed by the Ministry.

If a site has significant ice loading potential, the engineer responsible for the general arrangement design of the structure shall investigate the probability and severity of reasonably anticipated ice loads.  This professional shall discuss their conclusions with the Ministry Engineer at the initial design stage in order to obtain Ministry direction relating to acceptable risk for structure design at the site.

Delete and replace with:

Not required unless specified by the Ministry.

Delete and replace with:

Not required unless specified by the Ministry.

Delete and replace with:

Not required unless specified by the Ministry.

3.16.1 General

Add the following:

As a minimum, FSR bridges shall be designed for the following construction loads:

  • Self weight of the structure, supported at the bearings, including all deck panels in position but un-grouted.
  • A vertical live load of 445 kN (40-ton equipment + 10-ton panel) distributed over a length of 4 m, positioned on the bridge to produce maximum load effect, eccentricity = 100 mm.
  • Load factors in accordance with CHBDC.
  • Minimum dynamic load allowance of 10% (assumed design speed = 10 km/h).

The maximum permissible construction equipment loads shall be stated on the design drawings.

Wind loads on construction equipment need not be considered provided the maximum allowable wind speed for construction activities in Section 3.10.1.2 is followed.