Small Pedestrian Bridge Concept Design

[tmalik3156]

Good day all.
We are going to design a small pedestrian bridge over a creek. It's 16.0 m long and about 4.0 m wide.
We are at the conceptual design stage now. The governing criteria is functionality and cost effectiveness - rather than aesthetics.

We could do a Pratt Truss or Warren Truss, or even a girder bridge made of rolled section.
If you have experience with design of similar bridge, please comment on which option would be best.

Some variable to consider are

1. Least material required
2. Least fabrication cost and time
3. Better vibration characteristics (very important !)
4. Use of handrails
5. Constructability
6. Stability and structural redundancy
7. Atmospheric corrosion resistance

Anything else you could think of. Your input will help us pick the most favorable option. Thank you.

 

[BridgeSmith]

What is the vertical profile of the bridge?

Truss superstructures are popular for pedestrian bridges, because the truss doubles as safety fence.

 

[tmalik3156]

@BridgeSmith: Vertical grade is theoretically flat. But probably a small camber at mid-span will be provided even though the bridge is only 16.0 m. Yes, a truss would serve as a pedestrian barrier. But truss members (typically hollow sections) require lots of welding.

 

[Lomarandil]

If cost-effectiveness is your criteria, I suspect you'll have a hard time beating a prefabricated option from Contech (or heck, even an Acrow/Bailey bridge)

 

[LittleInch]

Four metres wide??

you have some large individuals crossing this bridge then if you want to make it cheap.

Clear span of 16m?

Kind of limits you to steel of some sort.

Why not just buy a prefabricated one? People like this abound http://crescentmarineaccess.com/commercial.html

The potential liability of designing and building something like this is huge.

 

[BridgeSmith]

Vertical grade is theoretically flat. But probably a small camber at mid-span will be provided even though the bridge is only 16.0 m.

Then rolled steel beams (WF shapes) are a viable option. If you have the clearance available for a W24 section, that would be a fairly economical option.

If a concrete precaster who does prestressed bridge girders is nearby, and you're not particular about the exact initial or long-term camber, prestressed twin tee or trideck sections may be a good option, also.

Prefabricated modular bridges (AKA bailey bridges) aren't considered to be great looking, but may be an economical option, also. Acrow is one of the big manufacturers. They are easy to assemble (and disassemble, if that's a consideration), and the truss sections are above the deck, so the superstructure functions as a safety railing. I believe they have bolt-on handrails for them. They definitely have the advantage over traditional site-built bridges where speed of construction is a priority.

Four metres wide??

you have some large individuals crossing this bridge then if you want to make it cheap.

4m is wide for a pedestrian bridge, and wide enough for vehicles, so if there's not some way to prevent vehicles accessing the bridge, it should be designed for at least an H-10 truck. Overall, the pedestrian load will likely control the girder design, but the concentrated wheel load may govern the deck design.

 

[tmalik3156]

@Lomarandil, agreed. Contractor will likely end up asking a prefabricator. However, we need to come up with a conceptual design that meets various criteria mentioned.
@LittleInch, well 3.0 m is required for two pedestrians passing with some personal space around. Although COVID-19 is not tormenting us anymore, it's good to have some space! Then the cyclist will take 1.0 m. The bridge is on a trail, so 4.0 m would allow occasional maintenance vehicle as well. Agreed on prefab comment, but we need to provide a conceptual design at least - if not a detailed one.
@BridgeSmith, are rolled shapes are easily available in Atmospheric Corrosion Resistant steel? We will check with Canadian suppliers. Someone south of the border is creating some uncertainties in steel supply with solid (or empty) threats of tariffs! Bailey bridge - agreed. However, in this particular case the owner doesn't want a Bailey. Though aesthetics is not a paramount concern, the trail is popular, especially in summer.

 

[BridgeSmith]

are rolled shapes are easily available in Atmospheric Corrosion Resistant steel?

Yep. W shapes meeting ASTM A709 (bridge steel) are readily available in Grade 50W, AKA weathering steel, as are plates, tubes, HS bolts, and other structural shapes. The only thing we've had trouble sourcing in weathering steel are high strength anchor bolts.

 

[LittleInch]

@Lomarandil, agreed. Contractor will likely end up asking a prefabricator. However, we need to come up with a conceptual design that meets various criteria mentioned.
@LittleInch, well 3.0 m is required for two pedestrians passing with some personal space around. Although COVID-19 is not tormenting us anymore, it's good to have some space! Then the cyclist will take 1.0 m. The bridge is on a trail, so 4.0 m would allow occasional maintenance vehicle as well. Agreed on prefab comment, but we need to provide a conceptual design at least - if not a detailed one.
@BridgeSmith, are rolled shapes are easily available in Atmospheric Corrosion Resistant steel? We will check with Canadian suppliers. Someone south of the border is creating some uncertainties in steel supply with solid (or empty) threats of tariffs! Bailey bridge - agreed. However, in this particular case the owner doesn't want a Bailey. Though aesthetics is not a paramount concern, the trail is popular, especially in summer.

The thing here though is that is is not really what your title said - "small pedestrian bridge".

It's a one carriageway bridge, but this isn't "small". I'm typing this in a room I know is 4m wide and its enormous for a pedestrian bridge. And 16m long presumably bank to bank is a long way. You haven't mentioned buttresses yet or if you will need piling, scour control or other issues associated with foundations.

 

[AS_PEng]

I would suggest against Acrow/Bailey-type bridges. Considering a minimum 50-year service life, they tend to be difficult to inspect and maintain. In my opinion, they are designed more for temporary or emergency use over longer spans, and are not particularly suited for short spans that require a long service life.

What is the site access like? Can a crane be brought in? What is the climate for the bridge? Do you foresee excessive salt use on the trail? Are there any hydrotechnical concerns? Are there soffit clearance issues that would necessitate a shallow superstructure?

A steel structure with either a timber or concrete deck might be a good alternative. Typically, the most cost-effective approach is a two-girder system, but if clearance is an issue, you can increase the number of girder lines, which often reduces the required girder depth. At a 16 m span, the superstructure can be either craned into place or launched. You won’t need a launching nose at this span if you have a 300-series excavator on the far side. As for weathering steel versus plain steel, it really depends on the site conditions. We’ve seen poor performance with weathering steel when there’s heavy salting or poor joints. One option would be to paint the girders completely or at least the first few meters near the approaches.

Regarding the deck, you could consider a precast concrete deck (composite or otherwise) or a timber deck. I prefer timber decks for pedestrian structures because they appear less industrial, although they do have a shorter service life compared to concrete. Concrete decks are prefabricated, but timber decks can also be prefabricated in short sections and installed quickly.

Another point to consider is the foundation, which is often the greatest uncertainty in construction—particularly if deep foundations are required. A high contingency is recommended for this portion of the work.

Finally, if the bridge is located in a high seismic zone, there will be additional costs to consider. Seismic demands could influence many of the design decisions.

Hope this helps!

 

[BridgeSmith]

As for weathering steel versus plain steel, it really depends on the site conditions. We’ve seen poor performance with weathering steel when there’s heavy salting or poor joints. One option would be to paint the girders completely or at least the first few meters near the approaches.

Finally, if the bridge is located in a high seismic zone, there will be additional costs to consider. Seismic demands could influence many of the design decisions.

Using fully integral abutments typically alleviates both of those concerns, but is typically only feasible on a deep foundation on steel H-piles. If a shallow foundation is required, a semi-integral abutment type can be used, which would still alleviate the concern with leaky joints, but requires special consideration for the anchorage if in a high seismic category (C-D or 3-4, depending on whether the design is per the AASHTO design spec or the seismic guide spec).

 

[BridgeSmith]

If vertical curvature (camber) is required/desirable, welded plate girders can accommodate pretty much any vertical profile that is desired. They're typically not significantly more expensive than rolled beams, and not a significant driver of the overall cost of the bridge.