Boat House Design for Ice 2

[stickmotion]

I have a client that would like to construct a boat house in approximatley 5-10 feet of water. They would like to use concrete or steel for the base of structure to minimize maintanence of the structure. This would be in a lake that would freeze over in the winter. My biggest concern with the design of the structure is the forces that would be applied to the structure by ice in the winter. Has anyone designed a structure like this? What kind of forces would the ice apply to the structure? Are there any firms that specialize in this type of project? I'd be willing to refer my client to an experienced firm if I am not able to find adequate design information.

 

[ishvaaag]

Never built one of these. AASHTO standard has I think pressures exerted by ice. Maybe there is a WSDOT or FDOT Mathcad worksheet that portraits ice action for piers of bridges. Arctic offshore use to show aprons to deflect the push from normal to the inner wall faces, but it may be impractical at your scale, or not. Maybe someone has experimented with some compressible interface able to deal effectively and durably along the expected life of the structure. Compressible and closed pore? Inflatables? Outer fencing, impermeable structures?

Look in AASHTO. As well go to page 41 at this publication

http://research.iarc.uaf.edu/nx2020/slides/SpringISO19906Final.pdf

 

[JAE]

On a lake you get ice pushing up against the shore and this is the issue with boat houses - especially in the Minnesota area or in areas with cold winters.

Link to article on this effect: Link

Here's someone who designed a boat house for ice forces: Link to boat house design

 

[JAE]

You might be able to come up with a standard concrete base and then "protect" it from ice forces by covering it with a compressible closed cell foam material as a guard against the ice pressures from the lake. This might be hard to do and also hard to maintain each winter. Another alternative would be to use concrete piers (isolated) and have the walls of the boat house start just above lake level. Thus minimizing lateral ice forces.

 

[concretemasonry]

Much depends on the size of the lake, orientation ans sweep of the wind.

In an early "ice-out" situation with a long sweep for the wind, the wind can pile up sheets of ice, since the sheets of ice slide easily over each other and pile up. In that situation above the lake level can easily be totally destroyed by the ice. I have seen ice sheets accumulating 15' deep going over a state highway (MNDOT), closing it for days. With that exposure, any normal guidelines mean little.

I had a lake home on a lake 20 miles away withe the same temperatures and 3'-4' ice thickness, but different exposure and I left my 120' dock and two boat lifts in several winters with minimal damage, although I usually took them out if time permitted.

Obviously, it was not ice thickness and the freezing action, but the effects of the wind speed and duration. There are situations where the ice does move just due to the combination of the expansion or shrinkage and the surface wind effects to move the ice ridges 50' feet or more gradually.

Just a couple of anecdotal comments on the wide range of effects depending on where you are and the situations. - Do a local survey with people that may offer some historical information, since mother nature is reasonably consistent over a time span that a boat house my last.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[MiketheEngineer]

Where you at ??

I am in Missouri and it would be a small issue - but Anchorage would be something else.

I would suggest a "floating" type structure that wouldn't be affected. -- At least not much!!

 

[ishvaaag]

AASHTO LRFD 2007 SI

has section 3.9 on ice loads, several pages long (3-43 to 3-51)

 

[JAE]

ishvaaag - I believe that AASHTO ice loads are specifically for flowing ice forces in a river/stream condition - not a lake. Lake ice pressure is a different effect.

 

[stickmotion]

Thanks for all the good information. The boathouse would be located in Minnesota. I have seen images of ice sheets 6-8ft tall on shore of large lakes around the area, which is what concerns me from a force/design standpoint. The boathouse will be on a smaller lake in a bay so it should be somewhat protected which will help. I think isolated columns/piers with the frame of the structure above the water level is a what I am looking at for a design.

 

[ishvaaag]

JAE, I have not examined the question in depth since not practicing by AASHTO but it seems to me rare that some grade of coverage for still waters is not included in the AASHTO standard, since bridges also span lakes and still waters. Thinking along this line maybe Canada has formalized in some document some straightforward approaches to ice effects in lakes and shorelines, I think that would be worth to look after.

 

[dhengr]

Stickmotion:
You might want to check with the MN DNR and the local zoning and lake assoc. people, etc. before you get to deep into the design. Can you even do that any longer; build out over the water or within some setback distance, back from the OHWM? I’m not real sure about the MN DNR rules, but in WI you couldn’t do that even if you were sleeping with the DNR commissioner.

Some misc. observations without any hard numbers.... JAE is right about a difference btwn. flowing ice forces and ice forces on large bodies of water which have large wave action too. The ice tends to get broken up, but into some pretty large pieces nonetheless. On a river the driving force is the water flow, while on the large lake it can be wave and wind driven. Our winds in MN & WI are predominantly from the W to the N, and in that quadrant. Thus, on average, you will see fewer ice problems if you live on the W or N shore of an inland lake, but I’d never say never. On inland lakes, the ice melts around the shore line first, because it is only a few inches thick there and the sun hits the dark lake bottom. You end up with 6 or 8' of open water at the shoreline. Then that damn NW wind comes up and starts moving a 2' thick block of ice a mile in diameter. Once it gets moving it is a pretty effective bulldozer for hours after the wind subsides. The ice doesn’t really pile up until it hits the shoreline, which offers the first real crushing resistance. Then it’s just pushed up onto a flat shoreline into a windrow. Obviously, a steeper bank will finally stop it, but really get chewed up in the process. Any driven piles, or whatever, have to have sufficient lateral resistance to cause a splitting and crushing force on that moving block of ice. You can’t stop it, so you want to split/crush it so it moves around the pier or pile. If you can do it, design it so the real framing (top of pile) is 3' or so above the OHWM, and allow that the bottom 3' may have to be replaced from time to time. If the boat house is sufficiently out in the water, the ice will probably just tend to pile up behind hit, if your splitting piles work as intended.

 

[concretemasonry]

dheng offered an accurate description of the action of the ice-out forces that vary year-by-year depending on the conditions.

With wind grabbing the surface of a large sheet of ice riding on a film of no friction water, there is little that can be done (within economic reason) to resist Mother Nature's forces that can be highly variable. When the above lake level structure goes, the supports are usually disturbed and weakened. That is the reason for the decreasing number of boat houses surviving and the restriction on the construction of new future "disaster zones". It may just be a regional situation, but can be similar elsewhere with less frequency and strength in other areas.

I can remember driving on a state highway that skirted a large lake (20 miles across) and had to take a 25 mile detour to get to my lake home. About 6 weeks later, the same highway was closed because there were not enough plows available to plow the frogs off the highway.

It is all localized and not usually subjected to codes and standards, but a local evaluation of the oldest residents may provide some valuable guidance for design once permission to build a boat house. Construction of structures is difficult to get approved because many states have riparian rights based on the Roman common law that treats the land between the record high water and the water surface as "borrowed", so a lake with 100% private ownership is really not private if a person can get on the lake, he can access the existing "borrowed" lake shore.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[shobroco]

I live on the north shore of Lake Erie, and have lots of experience with it, Lake Ontario & many points further north. Ice & frost heave are always an issue with a structure in this latitude built on the ground improperly, however they seldom get the opportunity to be your problem with a structure in the water (not that they wouldn't do it if they could). The issue as many others have said is going to be the movement of the ice, whether because of wind piling it up, regulatory authorities changing water levels after freeze-up, or water currents pulling it down the lake on you. Boathouses can and are built successfully every day, but the deeper the owner's pockets, the better you can do. Bubblers are the best. Sleeves & robust construction are next, & a floating structure will work fine but is unlikely to please all but the most impecunious buyer.