General arrangement design


General arrangement (GA) and relationship to structural design drawings

This section describes Ministry requirements related to design drawings for bridges and major culverts.

In this section, and in other Ministry documents, the terms “conceptual drawings” and “general arrangement drawings” are used interchangeably. Both terms refer to a drawing set showing some, but not all, of the details required to fabricate and construct a bridge.

Typically, the general arrangement drawings consist of a set of drawings illustrating the components and configuration of the proposed bridge or major culvert crossing, overlain on topographical site plans and profiles, in relation to the road and stream. The general arrangement drawings will also show approach the approach road and barriers (if applicable) and provide construction details and specifications. The general arrangement drawings may reference specific Ministry standard drawings and documents as provided within the BGSS website.

Structural bridge design drawings are prepared subsequent to, and elaborate upon, the general arrangement drawings. The structural design drawings, are detailed design drawings, which show the individual bridge components and connections to be fabricated. Structural design drawings will be completed by a Structural Design Engineer who will draw on and reference Ministry standards and specifications within the BGSS website.  The structural design drawings will show all aspects of the bridge components, including all structural details required to fabricate and assemble the bridge structure materials. Detailed material and fabrication specifications in the form of notes on the drawing are also required.

In combination, the general arrangement drawings and the detailed drawings shall provide all information necessary to fabricate and install a bridge meeting all requirements and specification in a safe manner.

Sample Standard Bridge General Arrangement Drawings – Simple (PDF, 6.1MB)

Sample Standard Bridge General Arrangement Drawings – Complex (PDF, 2.2 MB)

See a list of all standard drawings

Ensure that general arrangement (GA) drawings clearly depict the proposed components and configuration of the bridge or major culvert in relation to the forest road, stream, and stream banks.

Bridge and major culvert general arrangement drawings should include, but definitely not be limited to, the following:

  • Site location key map;
  • GA Designer’s name (and seal as applicable);
  • Name of the stream, road, and station (km) and adequate information to detail the location of the structure;
  • Traffic loading scenario (e.g.; BCL-625 etc.)
  • Design vehicle configuration used for horizontal alignment (e.g.; WB-19, etc.);
  • Design code references, specifically those from the most recent version of the CAN/CSA S6 – Canadian Highway Bridge Design Code and the Canadian Foundation Engineering Manual;
  • Expected life of the structure in place (temporary or permanent);
  • Design high-water elevation for bridges and design discharge;
  • Clearances between the design high-water level and soffit (low point of underside of superstructure) of bridges;
  • Details of debris passage or management strategies, if required;
  • Road approaches and grades, including width requirements (e.g., allowance for vehicle side tracking) and side slopes, to a sufficient distance back from the bridge to show potential problems, or to the end of the first cut or fill;
  • Dimensioning and labelling of component parts;
  • Connection requirements for component elements;
  • Drawing scales;
  • Relevant site plan and profile data;
  • Location (vertical and horizontal) of proposed structure relative to field reference points;
  • Deck elevations at bridge ends;
  • Possible ford or temporary bridge crossing locations;
  • Road and bridge or culvert signs;
  • Approach barriers, if required;
  • Critical elevations of substructure components;
  • Scour protection: dimensions, composition, extent of placement, design slope, design high water, and other considerations;
  • Rip rap extents;
  • Limits of construction for contract purposes;
  • Special provisions related to the unique nature of the site and crossing, including specific instructions to bidders related to process or results, as appropriate; and
  • Special instructions relating to material erection, installation standards, requirements, or methods as deemed necessary.

A Professional Engineer registered to practice in the province of British Columbia shall design all bridge components.

Detailed structural bridge drawings should include, but definitely not be limited to, the following:

  • Design code references, specifically those from the latest version of the CSA S6 – Canadian Highway Bridge Design Code and the Canadian Foundation Engineering Manual;
  • Design traffic load
  • Individual member shapes, dimensions and connection details;
  • Material specifications and CSA references;
  • Steel grades, impact category, finish;
  • Timber species, grades, preservative treatment;
  • Concrete strength, slump, and air entrainment;
  • Bearing materials and connections;
  • Superstructure elements, configuration, and connections;
  • Dimensions and sizes of components;
  • Fracture critical and primary tension components;
  • Weights of:
    • Single girder;
    • Assembled steel girders plus bracing and diaphragms;
    • Deck panels, ballast walls, caps and footings.
  • Girder or stringer arrangements and connections;
  • Span lengths;
  • Bridge width;
  • Curb and rail configuration, connections, and component elements;
  • Bridge identification requirements;
  • Field fabrication details;
  • Abutment elements, configuration, and connections;
  • Piers;
  • Location and sizes of piles or posts;
  • Pile-driving specifications, minimum expected pile penetrations, set criteria, and required service level capacities;
  • Field welding requirements;
  • Bracing and sheathing configurations;
  • Foundation requirements, material types and depth, and compaction level;
  • Location of girder support and lift points;
  • The installation procedure assumed for the design;
  • Notes stating that:
    • only low impact lifts are allowed;
    • four equal length slings/chains shall be used for lifting;
    • angle of lift shall be less than 30 degrees from vertical;
  • Allowable wind velocity during construction procedures shall be stated on the drawings, when necessary;
  • Maximum construction equipment loads;
  • Where expansion joints are used, sufficient information shall be provided to facilitate installation at various temperatures.

Note that the detailed drawings are frequently supplemented by shop drawings prepared by a fabricator.

 

Log bridge superstructure on log crib general arrangement drawing requirements

Since log stringer and crib materials are variable in nature and finished dimensions are not uniform, log bridge drawings are somewhat schematic.

In addition to the requirements provided in the general arrangement drawing requirements, log bridge superstructure and log crib drawings should include, but not be limited to, the following:

  • Schematic layout indicating width and span;
  • Reference source for stringer and needle beam sizing;
  • Minimum stringer, curb, and needle beam dimensions;
  • Stringer, curb, needle beam, and crib logs specifications, including species, quality characteristics of acceptable logs, and seasoning;
  • Stringer-to-cap bearing details, including shim types and stringer and cap- bearing width and surface preparation;
  • Dap details at log connections;
  • Needle beam locations and connection details, if applicable;
  • Space to add stringer, curb, and needle beam sizes as part of the as-built record;
  • Deck layout, indicating tie sizes and spacing, plank thickness, and connections;
  • Other material specifications, including sawn timber, hardware, and shims;
  • Excavated depth relative to scour line for mudsill or bottom bearing log;
  • General layout and arrangement of front, wing wall, deadman, and tieback logs, and their connections to each other and to the bearing log or cap;
  • Description of crib fill material;
  • Layout and description of in-stream protection, if applicable; and
  • Rip rap protection layout and specifications (as required).
 

Major culvert general arrangement design drawing requirements

In addition to the requirements provided in the general arrangement drawing requirements, major culvert general arrangement design drawings should include, but not be limited to, the following:

  • Location of the culvert, such as a key map;
  • Design vehicle load;
  • Fill height, depth of cover, maximum and minimum cover requirements;
  • Design slopes of fill and riprap;
  • Culvert invert elevations at the inlet and outlet;
  • Culvert specifications and dimensions: opening dimensions, length, corrugation profile, gauge, material type, and inlet bevel specifications;
  • Site preparation requirements;
  • Embedment requirements, including a description of the substrate and any rock used to anchor the bed material in the pipe;
  • Foundation details;
  • Backfill and installation specifications;
  • Installation camber;
  • Culvert gradient;
  • Seepage barrier details if required;
  • Special attachments or modifications;
  • Inlet requirements (rip rap layout, stilling basin, etc.);
  • Outlet requirements (rip rap layout, stilling basin, backwater weir for fish passage, etc.);
  • Rip rap specifications, including dimensions and configuration;
  • Design high-water elevation and design discharge, inlet or outlet control;
  • Connection details for pipe sections; and
  • Any existing improvements and resource values in the vicinity of the culvert that would influence or be influenced by the structure.

 


Site plan requirements

Site plans shall conform to the following requirements, and shall be prepared by capable survey crew working under the technical direction of a forest professional or a engineering professional who is a member in good standing of the Association of BC Forest Professionals (ABCFP) or Engineers and Geoscientists BC (EGBC). 

Site conditions should be such that the ground surfaces and any existing structures within the limits of survey are clearly visible (i.e., snow free).

The site survey(s) will be conducted with a theodolite/transit or total station with a relative precision not less than 1:1000 horizontally and 1:300 vertically.

Field referencing and bench marks will be established with a relative precision not less than 1:1000 horizontally and 1:300 vertically.

Extend the site plan in the following directions:

  • Upstream to show any bend which may influence the current pattern at the site, either at normal stage or in flood (generally, assume a distance up to seven (7) stream widths or to the second bend, minimum of 20 m);
  • Downstream three (3) stream widths, or to the limit of possible location changes; or to show all creek cross-sections, minimum 20 m;
  • Back from each bank to cover potential overflow channels, or to well above the high water mark; and
  • Along the existing or proposed road location a minimum of 50 m back from each existing or proposed abutment, or far enough to show approach problems.  The surveyed area shall be large enough to cover possible bridge and road location changes.

Establish a control traverse to gather site information and tie to the road P-line.

Reference points. Establish two reference points at each site to allow for relocation and control surveys. The references will consist of a ground hub and a reference back-site in a tree or a reference ground hub back sight, on tangent with a transit hub, on each side of the stream crossing. Both reference hub and back-site will be placed out of the right-of-way to avoid disturbance.

Benchmarks. Are elevational reference points. Establish at least two benchmarks, away from any potential disturbance, to be placed on firm fixed objects which will not be disturbed, one on each stream bank. Preferably place the benchmarks where they will be visible from both proposed bridge abutments and within 1500 mm above high water level.  Where elevations have not been established, use reference elevation datum of 100.000 m and note as "assumed datum of 100 m." Generally install 200 mm spikes in the side of a blazed, live tree (greater than 50 mm diameter), using the head as a bench mark.

Construction reference stakes. Establish a minimum of two pairs of offset construction reference stakes to the existing centreline or P-line at each site; to allow for establishment of the location of the proposed bridge centreline with horizontal measurement. Establish one pair on each side of the crossing, right and left of the centreline, away from potential disturbance and preferably outside the right-of-way. Where possible, stakes should be placed at approximately the same elevation as the road surface to allow for level, tape measurement.

Present Water Level (PWL). Collect spot elevations every 10 m along both sides of the stream to an accuracy sufficient to determine stream surface slopes.

High Water Level (HWL). Collect spot elevations where evidence is clear; note evidence.
Stream flow pattern. Use floats dropped at several points across the stream to collect relative flow velocities. Note measured velocities; and signs of bank erosion (overhanging trees and roots, vertical banks, areas where present channel differs from previous channels, etc.).

Site features. Note sloughs, abandoned channels, overflow channels, sand or gravel bars, bedrock, boulder areas, log jams, debris accumulations, fords, vegetation boundaries, trails, ice-jam areas or other significant features.

Soil and rock types. Note descriptions and boundaries of soil and rock types as seen on the surface, including stream bed substrate, particularly where exposed rock may affect bridge structure or abutment location.  Note any test-hole locations and their logs.

Spot elevations. For existing bridge structures within the limits of survey obtain:

  • Spot elevations for each corner of the existing deck and/or edges of the existing travelled road surface;
  • Spot elevations for the upper corners of the existing abutments;
  • Spot elevations showing the perimeter of the abutments where they contact the ground surface and the outline of the edges of the existing bridge; and
  • The outline of the outside perimeter of the existing structure.

Other items. Provide descriptions and locations for other existing items:

  • Structures such as: buildings, fences, roads, driveways and gates;
  • Utilities such as: power poles with numbers, height, and direction of wires; gas lines with any surface markings and structures; and
  • Right-of-way markings such as: Forest Service Road, pipe line, gas line, easement, railway and power line R/W pins.

Photos. Descriptive photographs (including descriptive captions) shall be taken to show the bridge crossing site and surrounding area. At minimum, take photographs of: 

  • The proposed and existing bridge sites from upstream and downstream;
  • The existing and proposed bridge sites from a reasonable distance up-chain and down-chain on the road approaches;
  • Each stream bank from the opposite stream bank, in sufficient numbers to show existing and proposed abutment locations, and the full extent of the surveyed stream banks with captions noting signs of bank erosion;
  • Along the road centreline away from the bridge, in each direction, from the existing and proposed bridges;
  • Upstream and downstream from the proposed and existing bridge sites;
  • HWL evidence;
  • Site features;
  • Difficult or convoluted ground conditions, and bedrock outcrops; and
  • Other existing items

Other data. In addition to other data points required, collect sufficient data points for production of the following profiles and cross-sections:

  • For proposed crossings, take a road centreline profile, including soundings where appropriate, of the expected crossing line to include: approach alignment (a minimum of 50 m each side or adequate distance to resolve any approach or alignment problems), high water line, present water line, wetted perimeter, top of banks and other topographic features;
  • If the reach is uniform, take one other section about three to five stream widths (minimum 15 m) downstream from centreline and upstream from centreline.  If the reach is non-uniform, take two or more sections below centreline at points of change, and do the same upstream; and
  • Where a bridge structure exists, cross-sections shall be taken, parallel to the road centreline, along the upstream and downstream edge of the bridge to show stream bed and ground lines adjacent to the abutments.
     

Using a preferred scale of 1:200, plot the following information on the plan drawing:

  • The designation, location, and description of reference points and benchmarks, including elevation and datum;
  • Contours, as follows:  
    • The contour interval shall be 0.5 m; this may need to change on rock cliffs; 
    • Make a conspicuous warning note if different intervals are used on the same plan; 
    • Show all cross-sections and points read; 
    • Extend contours across the stream bed where possible particularly at the proposed crossing location and at possible sites for a work bridge or ford; and
    • Accuracy should permit interpolation to a 0.3 m error on rock, around existing bridge structure components and along the stream banks;
  • PWL and date of data collection; show spot elevations every 10 m along both sides of stream to an accuracy sufficient to determine stream surface slopes;
  • HWL show spot elevations where evidence is clear and if possible join with a dashed line; describe evidence;
  • Stream flow patterns; 
  • For existing bridge structures show:
    • Spot elevations for each corner of the existing deck (indicate "deck");
    • Spot elevations for the upper corners of the existing abutments (indicate as "top abut" or "top crib");
    • Spot elevations showing the perimeter of the abutments where they contact the ground surface; and
    • Outline of the perimeter of the existing structure;
  • Show and label significant features from data collection such as sloughs, abandoned channels, overflow channels, sand or gravel bars, boulder areas, log jams, debris accumulations, fords, vegetation boundaries, cabins, trails, ice-jam areas or other significant features.  Provide descriptive notes to describe such features;
  • Show and describe boundaries of soil and rock types as seen on the surface, including the streambed, particularly where exposed rock may affect bridge structure or abutment location.  Show test-hole locations and their logs;
  • Other existing structures, utilities and rights-of way;
  • Vegetation boundaries and types;
  • Location of stream cross-sections taken;
  • Land status and right-of-way boundaries, where known;
  • Control traverse;
  • North arrow and magnetic declination;
  • Key map showing location of bridge site, direction and distance to town;
  • Legend of symbols and lines;
  • Scales; and
  • Title Block, showing:  bridge structure number, FSR or forest road name, km location, stream name, who completed the survey, who completed the drawing, date of survey, and drawing number.

Summary of requirements:

 

Drawing

Scales

Creek Cross Sections

1:200 Horizontal and Vertical

Road Centreline Profiles (on existing/proposed bridge centrelines)

1:200 Horizontal and Vertical

Detailed Bridge Centreline Profiles (for existing and proposed bridges-extends 10 m from each bridge end)

1:100 Horizontal and Vertical

Creek Centreline Profile

1:200 Horizontal and Vertical

Details for road and bridge centreline profiles: Requirements include, but are not limited to:

  • Use a stationing of 0+000 at the beginning of the town side;
  • For a detailed bridge centreline profile of an existing bridge: on the same drawing, using distinct line types, show the streambed and banks along the upstream and downstream edge of the bridge (where possible), and show ground lines adjacent to the abutments;
  • Show the present water level and right bank/left bank; and
  • Show high water level.

Details for creek centreline profile: Show the profile of the stream bed and water surface for the length of the site plan including stream bed gradient.

Title blocks: Show the bridge structure number, FSR or forest road name, km location, stream name, who completed the survey, who completed the drawing, date of survey, and drawing number.

 


Bridge alignment

Bridge horizontal alignment, vertical alignment and plan geometry shall be designed to suit site conditions, to accommodate present and future anticipated traffic, and to meet road user safety and stopping sight distance criteria.

The approach road alignment (vertical and horizontal) shall provide a smooth transition to the bridge. Approach road vertical curves shall be utilized where required but shall not extend onto the bridge deck. Where economically possible, the road approach shall have a minimum 15 m length, immediately at the end of the bridge that is tangent to the bridge. The Ministry, on a site-specific basis, shall specify the appropriate design vehicle to be used for tracking analysis in relation to determining horizontal alignment design. Bridge approaches shall have adequate drainage controls to minimize sediment deposition onto bridge decks and into streams.

Additional information related to bridge approach roadway design is available in the Engineering Manual, and in a guidance document entitled Standardizing the Design of Approach Alignment to Bridges on Forestry Roads in British Columbia: Review and Analysis (PDF, 3.1MB) which was created by FP Innovations for the Ministry.

Unless otherwise specified by the Ministry, a vehicle turnout shall be provided at one end of a single lane bridge (within both approach sight lines) to accommodate passing vehicles. Turnouts may be needed at both ends of the bridge for longer span bridges or for other safety reasons.

Bridge deck widths shall comply with Ministry standard bridge deck geometry, as described in the BGSS, the Supplement to CHBDC (CSA S6) and on Ministry Standard Drawings.

The standard deck width on bridges is:

 
Supplement to CHBDC (CSA S6)Traffic loading Standard deck width mm (ft)
BCL-625, LOH, L-100 4267 (14’)
HOH, L-150, L-165 4876 (16’)

Decks shall be widened to accommodate vehicle side-tracking on horizontal curves and to accommodate site specific safety concerns such as an allowance for snowbank accumulation. The design vehicle used for side-tracking will be specified by the Ministry where required for specific projects.

Additional information related to bridge width determination in locations with horizontal roadway curves is available in Standardizing the Design of Approach Alignment to Bridges on Forestry Roads in British Columbia: Review and Analysis (PDF, 3.1MB).

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Skew in plan view is permitted up to an angle of 30 degrees for:

  • Steel I-girder bridges with composite or non-composite concrete decks
  • Precast concrete reinforced slab bridges

Standard square bridge details shall be modified as required.

The longitudinal gradient on bridge decks shall not exceed 4% in the interior of B.C. or where bridges may be used in snow and ice conditions.

No bridges are permitted to be designed in such a manner that water will “pond” on the deck.  Deck drainage on steel girder bridges with concrete decks shall be achieved by providing a 2% transverse cross-fall to either side from a crowned centreline. 

 


Hydrologic design 

Forest Service Road bridges must meet Ministry requirements for hydrologic design, including consideration of climate change to accommodate the design flood, including any floating debris or ice where applicable, without resulting in damage to the structure, approaches, abutments, downstream resources or environmental values.

Bridges and culverts need to be designed for peak flow return periods according to the following from the Forest Planning and Practices Regulation (FPPR):

74  (1) A person who builds a bridge across a stream or installs a culvert in a stream for the purpose of constructing or maintaining a road shall ensure that the bridge or culvert is designed to pass the highest peak flow of the stream that can reasonably be expected within the return periods specified below for the length of time it is anticipated the bridge or culvert will remain on the site:

 
Anticipated period the bridge or culvert will remain on the site Peak flow return period

For a bridge or culvert that will remain on site for up to 3 years

10 years

For a bridge that will remain on site from 3 to 15 years

50 years

For a bridge that will remain on site for over 15 years

100 years

For a culvert that will remain on site for over 3 years

100 years

For a bridge or culvert within a community watershed that will remain on site for over 3 years

100 years

(2) A person may build a bridge that will not conform to the requirements of subsection (1) if

  1. the bridge will pass the flow that will occur during the period the bridge remains on the site,
  2. the construction of the bridge occurs during a period of low flow, and
  3. the bridge, or a component of the bridge that is vulnerable to damage by high flow, is removed before any period of high flow begins

(3) A person may install a culvert that will not conform to the requirements of subsection (1) if

  1.  the installation is temporary and the person does not expect to subsequently install a replacement culvert at that location,
  2.  the stream in which the culvert is being installed is not a fish stream,
  3.  the culvert will pass the flow that will occur during the period the culvert remains on the site,
  4.  the installation of the culvert occurs during a period of low flow, and
  5.  the culvert is removed before any period of high flow begins.

The runoff effect of a stream depends on many factors, most of which are not readily available or easy to calculate, such as:

  • Rainfall (e.g., occurrence of cloudbursts; hourly and daily maxima);
  • Snowpack depth and distribution, and snow melt;
  • Contributory watershed area, shape, and slope;
  • Topography and aspect;
  • Forest and ground cover;
  • Soil and subsoil composition;
  • Weather conditions;
  • Harvesting and road or other upslope development or disturbance;
  • Drainage pattern (stream order, branches; lakes and swamps); and
  • Stream channel shape, length, cross-section, slope, and “roughness.”

Because topography, soil, and climate combine in infinite variety, design the drainage for specific sites individually from available data for each site. In addition, consult those who have long experience in maintaining drainage structures in the area, as well as observing evidence of local activity/events.

There are too many analytical and empirical methods for estimating stream discharge to be discussed at any length in this section. Engineering professionals, in the course of carrying out their function as designer of a bridge or a major culvert, are ultimately responsible for establishing the design discharge for a structure.

Methodologies for determining design flood discharge include:

  • Working from available evidence of flood flows of the stream in question;
  • Gathering evidence of flood flows in other streams, relating these to their drainage basin characteristics, and then, by comparison to the characteristics of the basin under consideration, estimating a flood flow; and
  • Relating meteorological data to stream basin characteristics and estimating flood flow through empirical methods.

Obtain the necessary data for these methodologies from various sources, such as the following four possibilities:

Use site-specific data at, and adjacent to, the proposed crossing to estimate the maximum flow. Records of culverts and bridges within the vicinity that have successfully withstood known flood events can provide useful information in the estimation of flood flows.

Use stream basin characteristics such as length, slope, order, roughness, vegetative characteristics, and elevation band, combined with meteorological data, in empirical approaches to determine design flood flows.

Use data from studies done on other streams in the vicinity, with similar characteristics, to provide information on relationships and comparative values.

Use hydrometric records. The Water Survey of Canada publishes Surface Water Data (annual reports of readings on hydrometric stations throughout the province), as well as Historical Stream Flow Summaries in which mean values and annual peaks are tabulated.  Use these stream flow records to project design flood flows from theoretical analysis.

Determining design flood discharge usually involves applying several different methods and then using judgement to select an appropriate design value. In all stream flood discharge determinations, compare the proposed opening size with historically problem-free existing stream crossings serving similar drainages in the same area.

Compare the flood discharge estimates derived from the site information with other data and theoretical derivations. Base the final selection of design discharge and resulting bridge opening or major culvert size, taking into account these comparisons together with consideration of debris potential, ice jams, and any other local factors that might influence the structure opening.

One reference that has been used in BC for design discharge determination recommends use of Table C-1. This table should be considered by professionals responsible for determining design flows for a specific site, however other methods should also be considered as appropriate, and as described in this section.

Table C-1: Guidelines for Selection of Methods for Estimating Design Peak Flows

(Source: Manual of Operational Hydrology in British Columbia, B.C. Ministry of Environment, 1988)
Drainage area (km²) Availability of hydrometric data
    Less than 5 years More than 5 years
None On site Nearby watershed On site Nearby watershed

<10

rational formula   

unit hydrograph or model

unit hydrograph transfer or model

frequency analysis

frequency analysis

10 - 100

Regional   

unit hydrograph or regional

regional   

frequency analysis

frequency analysis and regional

>100

Regional   

regional   

regional   

frequency analysis and regional

frequency analysis and regional

For bridges not subjected to debris flows, the bridge soffit elevation should be the higher of:

  • The water level corresponding to the design flood discharge  under ice-free conditions, plus an allowance (typically 1.5 m unless otherwise justified) for floating debris; or
  • The water/ice level caused by ice jams having a return period comparable to that of the design flood (where applicable).

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.

See Supplement to CHBDC (CSA S6), Section 3.11.7

 


Scour protection

The design of scour protection for bridges shall be based on requirements for hydrologic design and rip rap respectively for FSR bridges.