Types of Standard Bridges in Alberta

BF 1753 in the County of Warner

Standard bridges are an excellent option for small sites where it is desirable to save money.

A standard bridge is a bridge of pre-designed components, for example, abutments, piers, and girders.  Alberta Transportation currently maintains three main types of standard bridges (defined by their girder type) with variations for abutments and piers.

The three types of standard bridges in Alberta are:

  • Type SLC
  • Type SLW
  • Type SL

Type SLC

This type was intended for paved highways and higher volume gravel roads.  It contains a concrete deck which can be utilized as a wearing surface or it can be paved over with asphalt.

They are available in single or three span configurations.  The two-span configuration was removed due to lack of use.  The available girder lengths are 8 m to 20 m (8 m to 16 m for the 510 mm deep variation, often called SLC-510, and 14 m to 20 m for the 700 mm deep variation, often called SLC-700).

The maximum pier height is 6 m.  Hence, the largest bridge configuration possible with a standard type SLC bridge is 60 m length (three 20 m spans) by 6 m pier height.  The maximum backwall height is 600 mm, hence the opening must account for the headslope.  Openings larger than this do not qualify for a standard bridge and will require a site specific major bridge design.

The only abutment type is cast-in-place concrete, and the abutment is an aesthetic vertical wall with a 45 degree wingwall.

Type SLW

BF 1174 in Mountain View CountyThe type SLW is a variation of the type SL which is intended for paved local roads.  The original type SL had no deck options – it must contain traffic directly on the girders – therefore the girders were redesigned with a bit more reinforcing steel to accept an asphalt deck and renamed SLW.

The type SLW standard bridge is available in configurations 6 m to 14 m, in one, two, or three spans.  The maximum pier height is 5.5 m.  Hence, the biggest opening for a type SLW bridge is 42 m long (three 14 m spans) by 5.5 m pier height.  However, there is an option for a “high backwall” in which a 2.5 m backwall can be used on a single span only.  If this is feasible for the required streamflow hydraulics, a 2.5 m x 14 m “square” opening on the single span high backwall abutment is a feasible alternative.

There are three options for substructure types:

  1. Steel is generally the preferred option because it is usually cheaper and easier to install, and can be installed in any time of year.  However, it is less aesthetic.  Steel substructures utilize HP310x94 structural members as piles and pile caps.
  2. Cast-in-place concrete requires pouring of concrete which is not possible in the Alberta winter (except for during some southern Alberta chinook events) but has stronger aesthetic properties in situations where this is desirable.  Cast-in-place concrete substructures utilize H-piles in the abutment and more aesthetic round piles in the piers, where they are more visible.
  3. Precast concrete abutments are rarely used.  They can be aesthetic like cast-in-place concrete but fabricated in a precast plant and shipped to the site, eliminating the winter construction problem.

Additionally, the wingwalls can be 45 degrees or straight (in line with the abutment), to match the site topography.

Type SL

BF 79604 in Lethbridge CountyOn most County roads and local roads, the type SL is the cheapest option where there is no asphalt deck and traffic drives directly on the girders.

The span configurations are the same as type SLW since the SLW came later as an adaptation of the type SL for paved roadways.  The maximum opening for the three span structure is 42 m (three spans of 14 m) by a pier height of 5.5 m.  Again, the high backwall option exists for single spans, whereby the maximum opening is a 2.5 m by 14 m “square.”

Other Criteria for Standard Bridges

Choosing a standard bridge is a relatively straight forward process, but there are indeed many other criteria not shown here.  In essence, the Canadian Highway Bridge Design Code (CHBDC) must be followed and choosing a standard bridge for a site does not negate the requirements to abide by the code.  The engineer responsible for the standard bridge design does not know the intricacies of each site, therefore the engineer in charge of each site must make sure that each criteria in the CHBDC is met.

  • Pile tip elevations must be at least 5 m below streambed.  The standard bridge design requires a geotechnical investigation which confirms the site specific conditions and identifies the types of soils present at the site.  For example, if there is high bedrock, the minimum pile depth will not be achievable which will require non-standard substructure design, that is, either drilled, cast-in-place, or rock socketed piles.
  • Hydrotechnical considerations such as size of riprap and dimensions of erosion control must receive a site specific design.
  • All standard bridge types were designed for CL-800 vehicle loading according the CHBDC.  If this is not the design criteria for the site, the standard bridge is not applicable.
  • The design loading assumptions on the first page of the standard bridge drawing set must be checked for applicability.  This includes water and ice loads, earth pressure, and wind loads.

When all of the criteria are met, standard bridges present a strong alternative to a custom major bridge design.  They are significantly cheaper to design as well as to build, and there are many engineers, contractors and suppliers who are familiar with the standard parts.

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