Boathouse Design Questions 2

[djrdn]

Does anyone have experience designing a building (boathouse) supported on flotation, i.e. a dock. The boathouse will be located on a lake and is approximately 54'-0" x 36'-0". There are three boat slips with roll up doors on the long side of the boathouse.

The building loads will be transferred to the flotation through columns spaced throughout the interior of the boathouse to distribute the load to the flotation as evenly as possible. The boathouse will not be finished on the interior. The exterior walls will be sheathed with plywood and the roof will be skip sheathed and covered with wood shingles.

1. I'm concerned about building movement when subjected to wave action. How stiff or flexible should the building be so that it can withstand the cyclic nature of the wave action?

2. I'm also concerned about the side of the building that has the roll up doors, and in particular the two short walls that separate the three doors. Could there be a load case during extreme weather when the short walls and tributary roof loads are not supported on the column and flotation but rather the flotation and column are hanging from these walls?

If anyone has comments or insights they would be appreciated.

Thank you,

JSE

 

[MikeHalloran]

Does that lake freeze in winter?



Mike Halloran
Pembroke Pines, FL, USA

 

[msquared48]

I would skip the skip sheathing and use CDX ply on the roof for one thing.

Mike McCann
MMC Engineering

 

[djrdn]

No the lake doesn't freeze in the winter.

 

[racookpe1978]

Spend the weekend going to 5-8 other floating sheds.

Make sure they are in an equal part of the country (ice/wind loads/storm loads/nearby waves from boats, and expected surges from floods. (many marinas limit boat speed specifically because of erosion and wear damage to bridges, piers, and floating docks!). If a drought, will it "land" evenly or at an angle?

Frankly - steal a few ideas, find out how others are made. It won't be enough for your calc's - but it will tell what sort of questions to ask.

This, certainly, is far past my level of experience to answer.

 

[SkiisAndBikes]

There is a system for engineered floating docks where the dock floation is designed so that the entire dock can be drawn slightly down into the water via a system of chains connected to large concrete blocks that have been lowered onto the lake bed. Drawing the structure down into the water can increase the stability of the structure significantly such that it won't move due to wave action. The more the structure is drawn down, the more stable it becomes (but also the more counterweights are needed) Several of these floating docks that I use in the summer are so stable, one would think they were on a piled foundation.

Large steel tube sections are typically used for flotation with a steel/wood frame being constructed ontop for the dock/boathouse.

 

[RockEngineer]

I designed one about 10 years ago about 38 ft tall and 80 ft long for a guy to pull his 50 ft boat into for service. It's currently on Lake Union in Seattle, WA. The builders were carpenters so they insisted on wood. Here is what I looked at.
1. The roll up doors will often bind and can't be opened in moderate or high wind. There's just too much pressure. Some of the other boat sheds used them but we used a canvas cover with stiffened ribs. It opened easy and we put a brace across the opening after the boat came in and out to stop some of the relative motion.
2. You need to really stiffen your trusses which go over the top and connect the floats on both sides of the boat because even if you use heavy beams in U-shaped frames under the water my analysis showed that about 80% of the forces due to relative movement of the floats goes thru the overhead trusses. I used glued and nailed plywood gussets on the trusses just because I didn't trust any regular truss connectors with the constant movement. My trusses went all the way to the outside edge of the floats. The vertical posts went down on the inside edge of the floats and connected to the trusses to keep the floats level.
3. You need to have your floats adjustable because it is easy to get the front walkway crossover too buoyant or cause stress related problems due to lifting in the front-center if you can't adjust. I used fiberglass coated wood boxes for floats and filled them with gravel to the level needed to even out the ballast. Foam floats can work but you need to figure out your buoyancy accurately or you will cause problems.
4. For longitudinal stiffness along the sides of the floats we used double 2x12 built-up beams inside and outside and then later ended up bolting on 10" channels. It's hard to calculate the longitudinal stress between the floats because you can't keep it stiff like the hull of a boat unless you are using steel pontoons. You just don't have the strength. You have to allow for movement but not too much. It's a hard judgement call.
5. You loads are higly dependant on how you are able to moor. The boatshed will act like a battering ram to the pier in high winds due to the large projected area so hopefully your pier is adequate. If you can tie off on both sides to some structure with spring lines you are much better off.

Good Luck. I hope this helps even though it is kind of vague.

 

[racookpe1978]

Could you go the other direction?

Be very flexible, very limber: come up from the separate floats on each side to a ball-joint, then from the ball joint to each sidewall, overhead, and back section independently so, while the "wall" and "roof" are stiff panels, but each panel can move with respect to each other?

 

[RockEngineer]

It may be possible to make each panel flexible relative to the floats but I sure wouldn't know how to do it.

Just as a side note on this, the City of Seattle last year decided that all the owners of these types of structures within the city of Seattle need building permits even though they have been tied to the same pier for over 10 years. (Previously they only had to have US Coast Guard certificate as a floating structure.) The Boat Sheds need to be engineered to the 2006 Seattle Building Code (SBC). The SBC requires a structural engineering license for engineering on any building except a single story metal building within the Seattle City Limits. They won't except any of my original engineering or anything from me since I only have a civil PE license and almost 30 years experience engineering naval ships, cranes, industrial buildings, framed houses, pole barns and log homes. They had to hire a licensed structural engineer to re-engineer everything including all the glued and nailed joints. The city also wanted to see the "special inspections" for the glued & nailed joints from 10 years ago and plus full scale tests of the trusses since they didn't use normal "pressed truss plates". They wanted the "seismic base shear" and ground motion coefficients put on the drawings for these floating structures because it is an SBC requirment. I have to laugh even though it is a sad state of affairs. At least I'm not having to deal with them.

 

[racookpe1978]

Some "earthquake": Did they ask about the tidal wave forces on the floating structures that would have survived easily the minnor rocking forces of the initial earthquake itself?

It appears that the city is only capabble of reading the spec, not of thinking about WHY earthquake forces on a floating object "just might" be "a little bit" different than on a solidly-attached building.