Cost for a large pontoon

[caprice67]

I was elected to figure out the pontoon size and cost to support a floating building. I have determined that the cross sectional area for each pontoon will have to be 40 square feet. The pontoons will be about 75' long. I have estimated many bridge costs in the past, but this is slightly out of my area. I have searched the web, but mostly come up with smaller pontoons for pleasure boats or extremely large pontoons for floating bridges. Any suggestions on where to find a cost?

 

[dpa]

Since you mentioned boats I assume you are talking about metal pontoons. Since you said building not houseboat I also assume we are talking about a fairly major structure where a lot of people would be onboard.

It would be impossible to figure out a cost without detailed structural plans. Then you would have to get estimates. I have used aluminum pontoons for lifting steel into place on a steel pier project and they worked nicely but public safety was not a concern in that project since the pontoons were removed after completion.

Flexure can be a killer with pontoons subjected to wave action. Welds can easily crack under all those repeated moving loads. The amount of structural foam required to fill them would have astronomical costs so better to make them plenty stiff and compartmentalized like a ship. Also structural foam will eventually (over a period of years) become waterlogged if the pontoon leaks. Be sure one or two flooded pontoons adjacent to each other would not cause the building to sink or heel over. In other words redundency. You must take extreme care to determine the waterborne stability of the structure - both intact and damaged stability. This means almost anal retentitiveness with regard to the weight and location of every piece of material. You also need to consider the effects of wind loading on stability. If you don't know how to calculate metacentric height I can help or you could contact a naval architect in your area.

If you are talking about concrete pontoons my experience with troubleshooting houseboats is that concrete pontoons alway leak. Concrete absorbs water and also cracks. One houseboat we worked on had settled in one corner as the water gathered there and none of the doors would close. In addition, there was a spiral staircase built around a 6 inch steel pipe and it had visibly deformed. We fixed the floatation, the doors worked again but the staircase was permanently warped.

On the other hand don't rule concrete out since there are quite a few concrete boats - even commercial fishing boats - in service worldwide. Some of them have withstood collisions with much larger ships. A great deal of care goes into the construction of ferro cement boats. I have a hunch that they are much more carefully constructed than the average pontoon.


You will need an alarm system as well as bilge pumps with float switches and a backup power supply.

Eventually you may need to remove and repair or replace a pontoon. You should account for stability and structural support with one pontoon missing.
If wave action is a problem you will have to insure that the pontoons are rigidly connected to each other so that they cannot move in counterrotation to each other. You could end up with two halves of the building that way.

I would love to know more about this project. It sounds fascinating. Could you tell us the overall size, number stories, square footage, occupancy etc?

 

[dpa]

One more thought, if this project is near a seaport you might look into buying surplus barges. Just have them carefully inspected by a marine surveyor or better yet two independent inspections plus have a look for yourself.

 

[caprice67]

The building is a large covered mooring for rescue boats, so that radar,etc. does not become frozen. It is located in a harbor of the Atlantic ocean. The building will be subjected to waves, wind loads and snow loads. It will be about 75' long by 50' wide by 25' high. I have looked into the pontoons for floating bridges which are concrete and have compartments. This is just for a preliminary estimate to see if the idea is feasible.