How to Design a Pedestrian Bridge

A pratt-style truss with raised deck on a screw pile foundation

Pedestrian bridges can be some of the most aesthetic structures out there, and are often the centerpiece of a development, park or golf course.  But their design has some special considerations and quirks that are unique to them, hence some specific knowledge and experience is important.

In this article I will walk through the six steps in the design of pedestrian bridges.

1. Governing design codes
2. Length, width and height
3. Design configuration
4. Loading
5. Analysis
6. Abutments

Governing Design Codes

The first step is to choose which design codes will govern the pedestrian bridge.  This is not as straight forward as it might sound, because the only comprehensive design code intended specifically for pedestrian bridges is by AASHTO and there are usually other, location specific building codes that govern above it.  That is, they may or may not specify it for use in that jurisdiction.  The design codes are:

• The local building code is the highest governing specification.  In Canada, the National Building Code takes precedence in the provinces of Manitoba, Saskatchewan, New Brunswick, PEI, Nova Scotia, and Newfoundland and Labrador.  It also takes precedence in all federal jurisdictions such as military bases and national parks.  In the other provinces, provincial building codes have been developed that take the place of the national code.
• The AASHTO LRFD Bridge Design Specifications, and in Canada, the Canadian Highway Bridge Design Code (CHBDC), contain provisions for pedestrian bridges but are intended for highway bridges.  In Canada, the CHBDC applies to “highway bridges,” a definition that contains some room for interpretation but if the bridge is adjacent to or part of a highway, design to this code will govern.
• The AASHTO LRFD Guide Specifications for the Design of Pedestrian Bridges is the only code that contains comprehensive coverage of pedestrian bridges.  Others contain bits and pieces, such as design loading values but few other requirements.  Local building codes are not usually very comprehensive in their coverage of pedestrian bridge design features.  Therefore, if your local building code specifies this AASHTO code for pedestrian bridges, it governs, but if not, it is a good idea to design to it anyway..

Since we are located in Canada, our pedestrian bridges have always been designed to all three of these:  Firstly, the provincial building code (or National Building Code of Canada in the applicable provinces) has a few minimal specifications like design live loading for footbridges, the design maintenance vehicle loading from the CHBDC is practical for maintenance vehicles, and a full pedestrian bridge design to AASHTO LRFD specifications covers many smaller details.

Once the design code is determined, the pedestrian bridge can proceed to the technical design phases.

Length, Width and Height

A bowstring truss with an inside width of 10 feet accommodates maintenance vehicles

In most cases the owner has a location, and therefore a relative bridge length, in mind.  If not, the required length might be determined from hydraulic analysis or geometric features.

The width is often determined from maintenance vehicle requirements.  Ten feet is usually sufficient for passenger vehicles which are built 8 feet wide plus mirrors.  If there is no maintenance vehicle required, 6 – 7 feet is a normal width.  The height of the truss is about 4½ – 5 feet.

The height of the bridge is determined by the requirements of what will pass underneath it (traffic, watercourse, etc.).  Pedestrian bridges are typically cambered (upward bow) by 1 – 2.5% of their span length, for aesthetic reasons and to allow for deflections without producing a visible downward bow that is distressing to the public.  Hence, if there is no camber it might be advised to increase the size of the truss members to minimize deflections.

Preliminary Engineering Report

However, if the abutment locations are not clear, or there are multiple viable options, a preliminary engineering report will determine the types, options and costs.  Preliminary Engineering Reports contains the following items:

• Hydrological and hydraulic analysis, to determine the elevation of the structure, if applicable.
• Traffic analysis
• Comparison of bridge types, steel vs. concrete, truss types, etc.
• Analysis of abutment types, and their pros and cons
• Analysis of any other factor affecting the structure location
• Preliminary structure sizing
• Compilation of a list of alternatives
• Estimates for each alternative
• Life cycle costing (net present value analysis)

If the AASHTO code is being used, a bridge width of 10 feet or greater requires a larger design vehicle, the H10, which is twice as heavy as the H5.  Thus, there is a significant cost premium at this threshold width.

The preliminary engineering report compiles all of the known information and presents a list of alternatives, with costs, pros and cons, and any other issues for consideration by the owner.  A recommendation is made, and the owner is given the chance to approve what will be designed before it is designed.

Design Configuration

In order to perform any kind of analysis on a structure, the configuration of the structure must be known.  This may require several iterations before the structure is finalized, but you must choose the type, size, and dimensions of each member of the bridge.

Truss Types

There are many truss types out there, that have been given different names for various applications, but for pedestrian bridges there are essentially four that are worth considering:

1. Pratt
   In a Pratt truss, the diagonals point outwards and upwards.  Hence, they are in tension and this is a very efficient configuration that will result in smaller members (i.e. cheaper).
2. Howe
   The Howe truss is the opposite of the Pratt, where the diagonal members point upwards and inwards.  Hence, they are in compression and this is a less efficient configuration that will result in large members (i.e. more expensive).
3. Warren
   

   A warren truss

   In a Warren truss, the diagonal members alternate back and forth, creating a triangle.  It is not significant how many vertical members there are (either in between every diagonal, or every two diagonals, or none at all).

4. Bowstring
   In a slight twist on these basic configurations, the upper chord of the truss is curved.  It can be fully or partially curved, that is, there could still be a partial vertical member at each bridge end.

There are some non-truss options as well, for example I-beam girders or a single bathtub girder.

Other details that affect the dead load of the structure, such as deck and railings, must be chosen prior to the analysis as well.

Loading

Like other structural engineering design projects, pedestrian bridge loading is taken from the design codes which the structure is being designed to.  The main loading specific to pedestrian bridges are:

• AASHTO LRFD Guide Specifications for the Design of Pedestrian Bridges
  • Pedestrian loading is specified as 90 psf, applied vertically to the deck.
  • For bridges with a width between 7 and 10 feet, the H5 design vehicle must be applied.
  • For bridge with a width greater than 10 feet the H10 design vehicle must be applied.
  • Load combinations are as per the main AASHTO LRFD Bridge Design Specifications with a few exceptions noted in the guide.  Equestrian, wind, and fatigue loading modifications are specified by the guide.
• Canadian Highway Bridge Design Code
  • Pedestrian live loading is specified by the code in section 3.8.9, a simple formula depending only on the bridge length.  The minimum and maximum is specified as 1.6 kPa and 4.0 kPa, respectively.
• Alberta Building Code
  • In our home province, pedestrian live loading is specified for the category of “footbridges” in table 4.1.5.3 as 4.8 kPa.  There are no other provisions specific to pedestrian bridges.

Analysis

Similar to other structural engineering problems, the bridge is modeled within a structural engineering software product and the design loading is applied to it.  Load combinations are modeled according to the design code being used, and the worst loading on each member is used for design, regardless of the load combination.

That being said, for aesthetic reasons each member type is usually made the same size.  For example, all truss diagonal members are made the same size as the largest one required.

In our case, we use S-Frame.  Like most other structural design software packages, S-Frame uses a finite element analysis engine to analyze the structure and report the stresses in each member.  It also performs the design (as opposed to analysis) by allowing you to choose member sizes and reporting whether or not resistance is greater than its loads.

The back-of-the-envelope check for each truss member involves the simple formula for tension or compression resistance:

C_r = Φ·A·F_y

Where:

• M_r = Resistance (kN)
• Φ = Resistance factor (0.9 for steel)
• A = Cross-sectional area (mm2)
• F_y = Yield strength (MPa)

However, compression members must also be checked for buckling and moment resistance.

There are a few more details prior to finalizing the design and proceeding to the design drawings:

• Connections are either welded or bolted.  Welded connections are designed according to the AWS D1.1 welding code.  Bolted connections require a project specific design for bolt shear, member shear, and block shear.

If any of these details, or truss member sizes, differ from the original assumption, the process proceeds back to the configuration step and the analysis is run until the dead loads and members sizes correlate.

Abutments

The substructure is often the most ambiguous part of the design because of the many options and unknown variables.  Normally, you would perform a geotechnical evaluation which includes drilling at the site to determine soil types and strength parameters.  There are usually two drill holes required, one at each abutment location.  A geotechnical engineer determines the soil resistance parameters in a geotechnical report, and the structural engineer designs the foundation.  The foundations are either:

• Footings are the cheaper alternative, and therefore first option to consider.  They are designed according to Terzaghi’s equation.  There is a short term design (ultimate strength) and a long term design (settlement) consideration required.
• Piles are used if the footings are not sufficient to handle the loading.  Pile resistance takes the form of end bearing and shaft resistance.  End bearing uses Terzaghi’s equation similar to a footing, and shaft resistance is calculated for each soil layer.  The pile strength is the summation of the two.

This is the typical work flow for any civil engineering project, and for large pedestrian bridges this would normally be the course that the project will follow.  However, my experience is that pedestrian bridges often have several complicating factors:

• Screw piles are an option
  For smaller sites, the relatively new alternative of screw piles are similar in cost to a footing.  Screw piles are helical augers that remain in the ground – they are like giant screws that get twisted into the ground and remain in place and can be sufficient to carry the loading of a small pedestrian bridge and even small buildings.  They need to be designed and estimated alongside the footing option.  Like piles, they have a side friction component and and end bearing component, which includes the area of each helix.
• Rock is present
  

  Rock anchors at a bridge abutment

  For larger projects, the geotechnical drill switches to a core drill once the rock is encountered, and geotechnical design involves “anchoring” piles into the rock with cast-in-place concrete.  If the rock is at the surface, “micropiles” aka rock anchors are used which are 1″ diameter (or thicker) reinforcing steel inserted about 5-10 m into the rock via drill holes and secured with epoxy.  Cast-in-place abutments are poured around the micropiles.

• The project is too small for a geotechnical evaluation
  If it doesn’t seem worth doing a geotechnical evaluation that includes drilling, several assumptions can be made.
  • A makeshift footing can be designed using precast concrete blocks.  If there is no vehicle loading to design for, and the material below the precast blocks is good, well-graded gravel (or can be replaced with such), this is an extremely cost-effective option.
  • Screw piles can be designed using worst case assumptions that result in larger screw piles, but the additional cost is less than the cost of a geotechnical evaluation.

Good luck with your pedestrian bridges, and let us know if there’s anything we can do to help!