Private Pedestrian Bridge

[volcomrr]

I'm in the very early discussions with a GC about designing a pedestrian bridge over a stream. The bridge will serve the members of an HOA for a multifamily complex. My question, and this can even be broadened to private residential bridges:

How do you handle hydrology? Given the resent Hurricane Helene disaster, do you look at published flood data through FEMA and local jurisdictions to determine flood elevations and flood velocity, or do you team with a hydro engineer to provide a study for the design.

How do you handle geotech? Do you assume some values and state as such for your abutments / bents or do you outsource to a geotechnical engineer?

I ask because, especially for a single family residence, they don't always have a ton of money to spend and can price a GC out of work. In NC, private residential bridges are only mandated to follow AASHTO if the bridge is serving two or more residences. Thoughts?

 

[jayrod12]

I feel based on the "...follow AASHTO if serving two or more residences..." and the "...serve the members of an HOA for a MULTIFAMILY complex..." means you must do this properly. Engage a civil/hydro engineer to give you the data you need for water levels, engage a geotechnical engineer for foundation design guidance, and design the thing to the AASHTO bridge code.

Condo/Multifamily residences are high liability clients because they've generally got some money to back them, and a lot of people to please. Don't want to screw around with designing something less than clearly necessary.

 

[phamENG]

I agree with jayrod, but you also need to figure out what design criteria you are going to use. This is going to be a conversation with the AHJ to determine where they stand on the matter, and also with the owners to see what they want to pay for.

Helene was terrible, but it was nothing new. Go about 250 miles north east up the spine of the Blue Ridge to Nelson County and about 55 years into the past, and you'll see the same thing. Worse, in many ways. Hurricane Camille hit the gulf coast, went north, and hit the mountains and dumped all its rain, washing away whole communities and killing 160 or so people.

 

[STrctPono]

The first and most important question that we tackle when designing a bridge over a stream is what the 100 year flood elevation is. For public works projects, you need to maintain 2ft of freeboard to the soffit of the bridge. I've seen the effects of bridges that are too low or bridges with center piers that cause debris rafts and flooding of adjacent properties. How confident are you that you will not be held liable if a bridge you design does not provide the adequate hydraulic capacity?

One thing that comes up quite often when inspecting and retrofitting old bridges is the issue of footing scour. It's a real thing. Sometimes it's minor and other times it's critical. Many times the long term performance of the foundation design is not purely bearing values but more an in depth look at the RQD values of the rock if the hydraulics engineer notes in their analysis that there is a potential for scour.

 

[gte447f]

You definitely need proper hydrology and geotech. Even then, this will be quite a challenge if you do not have bridge design experience. Maybe you do.

I would have a frank conversation with the owner about their budget for design and construction. If it isn't realistic, I would be very cautious. Designing and building bridges to proper standards is not cheap.

 

[JoshPlumSE]

I worked on a residential bridge over a small stream. I came it late on the project.... I believe the original engineer had a health issue that prevented him from completing the project.

The job had to be submitted to two different jurisdictions. The city in which it was built and the jurisdiction that own the water way. The water department required a certain height over the water way so that it couldn't be blocked by debris or cause a flood or such. They had another requirement that there had to be a plan to quickly remove the pedestrian bridge if necessary. So, we had to have anchorage that could be easily unbolted or cut if they needed to do so for some reason.

I can't remember all the details, but I've got some records somewhere that I can look up if you think it would be helpful.

 

[JAE]

All good comments above - one other thought.

In some cases a "private" development of homes will have on its future radar a possibility of being annexed into an adjacent and larger city.
That city may have distinct provisions regulating their "in-city" pedestrian bridges and to be annexed, the city may require that your new bridge meet all of their criteria that are on the books now. Might be good to pretend that your bridge is in the adjacent city and design accordingly ... this assuming you are even near a city.