Beach Tower

[medeek]

Call me crazy but I want to build something like this on my 10 acres so I can see all the way to the beach.

Given my wind speed (155mph ult.) and seismic zone (D2) I am probably looking at some serious holdowns at the first level (HDU 11). I would have to run the numbers but it would probably work. I would probably also sheath it with 5/8" plywood inside and out for some serious shearwall action, at least on the the first two floors. I would probably also frame the first two floor with DF No.2 2x8 studs or 2x6 studs at 12" o/c. I'm thinking spiral staircases between levels to save as much floor space as possible and to make it more interesting. Minimal windows on the first three levels and then the top level would have a lot of windows for the view. 11-7/8 TJI 210 for the floors with 3/4 sheathing T&G, 16" o/c.

The structure is 16'x16' with grade to top plate height of 38'8".

The roof is 6:12 pyramid hip with 2x8 rafters, I may go with a steeper pitch though.

On a structure like this overturning is a serious threat, I would need to check the dead weight of the structure and see if the seismic or wind forces could potentially lift the foundation right out of the ground. I think the solution would be to to increase the dead weight of the structure especially at the base, by upsizing the footing and stemwall. You will notice my stemwall is 10" thick, 36" deep and a 24"x12" footing, even this may not be enough. I'm also thinking about some larger spread footings at the corners where the shearwall studs drop down.

Anyone design something goofy like this before. I don't know if my county building dept. would give it an approval though even if its engineered (stamped).

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

At least you should be able to engage most of your dead load for all of the shear walls. There might be some logic in constructing the non-observation levels using rod braced post and beam framing. Pneumatic tube elevator of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Yes, I have done a similar thing...in reinforced masonry.

 

[XR250]

There might be some economy in using (4) helical piers for overturning restraint. They are only about $500 a piece for new construction (in my area).
Don't know how that would compare to the additional concrete required - probably a wash when all is said and done.

 

[oldestguy]

Have you given any thought to a platform with railings at 40 feet on 4 poles and a temporary canvas roof? Most of the structural concerns are taken care of. After all you won't be thee during big storms anyhow.

 

[MikeHalloran]

How about an outside staircase, spiraling up to a platform around the top floor,
just to relieve the boredom of those big plain shearwalls?

... or supergraphics?
Something that suggests a V2 launcher?
Or something to suggest a platform on poles, as suggested.





Mike Halloran
Pembroke Pines, FL, USA

 

[canwesteng]

If by helical piers XR means screw piles, I like that idea. To me, I'd be worried about overturning with something tall and skinny like that, due to construction not being plumb and uneven frost heave.

 

[JStephen]

Any issue with storm surge or ground water? (Where I grew up on the Gulf coast, it's flat, so if you can SEE the beach, it means you're at about 2' above sea level, ground water is about 2' below the surface, and storm surge is gonna flood several feet deep if you have a hurricane blow in. You tend to have pile foundations and/or disposable structures.)

 

[medeek]

Probably cheaper to build. I was thinking outside of the box on this one, maybe too far outside of the box.

Both my wife and I would like a secondary storage structure and my wife was wondering if I could add some sort of perch onto the house so I naturally thought why not combine the two ideas...

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

With 16' x 16' I would like a very tight spiral staircase, something like this:

External would deteriorate in our environment too quickly.

You can view the model here in 3D:

https://3dwarehouse.sketchup.com/model.html?id=e79a19aa-5f90-482f-8104-ed19a91cd442

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

On this same note I've also been giving some thought to a vertical evacuation tower in case of a tsunami. The cascadia subduction zone is overdue for the big one and I doubt we would be able to get out of harms way if it ever did happen. We are 45 feet or so off of sea level but I'm thinking we probably need to be another 30 or 40 feet up to guarantee our safety.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jdgengineer]

Medeek, on a non-engineering note, would your planning department allow you to build this? Are you exceeding max building height or # of stories?

 

[medeek]

Very likely, max. stories is probably three, not four. I would have to check the zoning and restrictions. I'm not near an airstrip if that counts for anything.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

I'm sure some other permitting issue would be a show stopper as well, the building height is in the path of a migrating spotted owl or some other ecological sensitive item. Its hard to do anything these days. I get the life safety stuff but a lot of the EPA and other environmental permitting seems way over kill in my opinion.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[oldrunner]

At 155 mph I think your structure will probably sorta fold over per you sketch just showing studs. If you enclose it, it becomes a building and not a tower. I've done some stealth towers that are sheathed and they of course catch a lot of wind. Probably best to start with a geotech report as to what you are going to anchor the foundation to and then a person who does regular structural engineering can design you a structure - more or less like you have except with the proper connections including stability members. Most places will require a building permit. You will have to be careful that it doesn't come apart in a big blow. I don't like to climb circular stairs.

 

[dik]

And use sub-floor adhesive with your studs and sheathing...

Dik

 

[medeek]

My wife took one look at the thing and put the kabosh on it, one word: "ugly".

However, out of pure curiosity I threw it into my shearwall program just to get a better idea on what kind of numbers we are talking about. As I expected the wind loads far out govern the seismic. I got it to pass if I used 15/32" sheathing (6 edges,12 field) interior and exterior on level 1 and level 2 shearwalls.

If I was to frame this I would probably use 2x8 lumber on the 1st level, with 8x8 shearwall studs at the four corners and a (2) HDU14 holdowns at each corner, this is probably a bit overkill but I would rather be conservative on a structure that is sure to take a beating. The second level, I would use 2x6 lumber but use 12" o/c stud spacing. The shearwall studs at the corners would be 6x6 with (2) HDU11 holdowns at each corner. The third level I would use 2x6 studs at 16" o/c with 4x6 shearwall studs at the corners and (2) HDU8 holdowns. At the 4th level (2) 2x6 studs at the corners and (2) HDU2 holdowns.

I would probably also run interior shearwall sheathing 7/16" on level (3) just for good measure but the numbers show that one exterior sheathing should be sufficient.

My soil is quite expansive in my estimation (I'm not a geotech) so I would probably install 42"x42"x12" spread footings at each corner of the building, integrated with the 24x12 footing. The one thing I haven't checked is the total overturning of the structure.

On the lower windows and doors I would probably running some strapping for force transfer around those openings.


So yes, I think one could platform frame this thing and actually make it work. The only thing that I don't have a good handle on is the overall deflection of the structure, it probably does not comply with the ASCE7-10 in that regard. Take a look at the numbers below:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

My wife want's it octagon shaped and tapering up like a lighthouse, how do you actually build that and how would you analyze it and make it work.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jdgengineer]

Don't forget to add in deflection caused by shrinkage of lumber. This will increase the deflection from your holdown. For multifamily residential we would use a shrinkage compensating continuous tiedown system. Probably a bit overkill for you here, but the deflection should be evaluated considering shrinkage.

 

[MikeHalloran]

She probably meant "too phallic".

I agree that an octagonal lighthouse could look better, if you get the proportions right.
I would start with a photo of an actual lighthouse that looks about right, and start overlaying a structure on that.

Maybe you could find an unused actual lighthouse and move it.

Or put up a slender guyed tower, and put a pan/tilt head on the top with a high resolution camera, and feed the image to a widescreen TV in your house.



Mike Halloran
Pembroke Pines, FL, USA