Watch tower frame design suggestions?

[okwantegls]

I'm planning on builing a small watchtower which is approx. 4 meters high, 2 by 2 meter width.

After a few iterations i came up with the following frame design:


The big beams have size 9x9cm and the smaller ones 4.5x4.5 cm.

All wood connections will be done with mortise and tenon joints or variations.

The cross braces are there for overall stability. The knee braces on below first floor are there to support first floor. The knee braces at the top of the structure are for support because i might want to sit on the roof sometimes.

Ground level will be open, the first floor (second level) will get light wood walls and light windows.

Does this look like a steady frame design and would you think that it can survive hard storms (its quite windy over here)?

Suggestions are of course also welcome!

[Dave Shepard]

Welcome to the Forum! Even without knowing the exact loading on that frame, dead, live, wind, etc., I can tell you that those dimensions are way too small. I would think the posts should be in the neighborhood of 20x20cm. In timber framing, you not only have to plan for the loading, but also the joinery that must go into the timbers, and the wood that is removed for the joinery.

[Ljohnsaw]

And not to mention the 3-way joints you have going on for the "roof".  Those will need to be staggered.  There has been quite a number of threads dealing with that particular issue.  Cool idea, though!

[Heartwood]

I suggest tapering the tower by canting the legs inward. It makes the joinery trickier (unless you scribe) but much sturdier.

[Don P]

There are some designs here;
https://www.fs.fed.us/database/acad/lt/
I seem to remember another set of forest service drawings, oldtimers is setting in  :D

[okwantegls]

Cheers for the thoughts on the subject!

@Dave Shepard, i had the understanding that posts would keep their strength as long the gap for joints was replaced with other wood. But now i think about it it does make sense that i looses some of it strength. I looked through the wood list and found that 15x15cm for the post is more pricey but still affordable. When going to 20x20cm the price quadruples, which is a bit out of my price range. Would you think it would work with 15x15 posts?

@ljohnsaw , i did a bit of googling/searching about staggered joints, but couldn't find much about the subject, mostly staggered floors and such, although i have a suspission about the meaning, is it correct that with staggered joints two or more joints are not at the same height on a post? If that's true i did already take that in account, maybe its not very visible in my screenshot, but the posts are of different sizes so that they don't occupy the same space in the post.

@Heartwood @Don P, tapering is indeed something that is very common with wooden towers, i considered it in my early stages, but as you noted it makes everything very difficult when it comes to joints. In addition making a foundation for such a structure without letting the wood touch the ground is difficult. Most of the old watch towers start to go bad at the posts because of this, although this also depends on ground conditions. But i'm still going to do a bit more research on this one, maybe i did put this idea to the side too quickly.

In the meantime i made a new version of my design:


This one has 15x15cm posts and all other wood is 5x10cm. Should i increase the size of the 5x10cm wood (for the knees and horizontal beams) to for example 7x10 or maybe even larger?

[Don P]

When you remove part of the wood for a joint and replace it with wood, first that fiber is broken so bending strength is reduced considerably. It creates a stress riser at the corners of that joint that concentrates the bending stress on that re-entrant corner and a split tends to form on those inside corners. A post still bends in the wind and under load. Replacing that wood with wood running the other direction might seem to "fill in" the missing wood in compression, but, the cross grain wood soon shrinks away from the notch and doesn't support what is above or beside it. Really wood in end grain compression is several times stronger than in side grain so it wouldn't be a good replacement anyway, in other words, banish that thought 

Staggering the joints; look at the floors, if you drop the beams supporting the joists underneath the joists then the joists and those girders are in different locations on the post. I probably wouldn't notch the joists into the posts just run them alongside.

Your long X braces pass each other, think about that detail. The knee braces don't really attach to anything, think about making them thicker and notching the ends to accept the joists and girders. Also remember the girders supporting the joists are carrying multiple joists, they will not be the same dimension as the joists, they need to be heavier, size these members based on load, species, grade and span.

Boy that sounded picky, not meant to be, just stuff to think about 

[Dave Shepard]

I can't say if the 15cm posts would  work. My suggestion of 20cm posts was a "gut" feeling. Someone would have to actually start engineering the frame to tell you that.

[florida]

I'm not a timber framer  but I am a builder. Your 15cm (6 x 6")  posts are still way to small. By the time you cut out the mortises for framing members what you're left with is a 10 cm (4 X 4) which isn't even big enough for a 1 story deck. By the time you tack on another story the bending stresses on those floor joints are going to be dangerous. 20 cm  (8" x 8) is the minimum I'd use and I'd prefer 25 cm( 10 X 10) for a 2 story structure.

[okwantegls]

Cheers again for the helpful replies & suggestions!

@Don P the strength of the post and how it is related to the joints makes much more sense to me now. I adjusted my design by reducing the amount of holes (caused by joints) in my posts to keep as much strength in my posts as possible. I moved the x braces to the outside and inside and will just bolt them on. Still need to do bit research on how to connect them in the middle.
I also increased the size of my knees and girders to 9x9cm.
Thumbs up for such a detailed reply, it helped me improved the design a lot. I don't mind those at all.

@Dave Shepard, @florida I'm convinced, in my new design i replaced the posts with 20x20cm posts. Although the steady increase of the post sizes does introduces a problem because the weight of a frame size increases and this causes trouble with raising the frame, i can't get much heavy equipment on side let alone a big crane (i'll need to do a bit of research on how they did that back in the old days).
But in the meantime i split my structure in 2 parts, the bottom box (the foundation) and the upper box (the sitting area). I'm not sure if this is the way to go.

My new design:

Things i changed:
- increase the height from 4 meters to 4.5 meters to make the cross braces fit on the bottom box (foundation)
- increased the post size width from 15cm to 20cm
- increased the knee and girders from 5x10cm to 9x9cm
- moved the x braces outside the post, to not unnecessary reduce the strength of the post.
- split the whole frame in 2 parts, bottom (2.5m high) and the top (2 m high). I'll likely just bolt the top box onto the bottom box through the girders. The initial thought behind it was that i could make the top box lighter reducing the stress on the bottom box, it also solves the "no crane" problem (i'm not sure about this whole decision though)

Any suggestions are of course welcome.

I'm also going to do research on strength, stress and load subject.

[Ljohnsaw]

I don't think you want to split the design - attaching the post on post will be a significant weak point.  Also, you didn't get the alternate the corner joints correct.  Without a drawing, let me try to explain. 

There would be two beams, opposite sides of the frame - call them North and South beams.  Those would be jointed to your posts lower then the top, equal to the height of your floor joists.  Then, lay your floor joist on TOP of these beams spanning the gap from north to south.  The extreme East and West joist would be jointed into the posts while the other joists could either sit on top of the beams (attached with timber screws - toenailed in) or have slight pockets but account for with the initial setting of the beam heights.  The East and West "joists" could be as large as the North and South beams for aesthetic reasons.

Clear as mud?  Hope this helps.

[okwantegls]

First my apologies for the late reply. I had too much things to do last week.

@ljohnsaw cheers for the suggestions. The split of the entire frame has now been undone. The height of the frame is also back to 4 meters high. Thanks for the extensive explanation on alternate corners, i first didn't get it, but after drawing it on paper it started to make more sense. Now the girders (or beams) don't occupy the same level on the post, increasing the strength of the post which is a good thing. I hope i did understand it right though.

It also makes the joinery much simple! And it also gives me space for the floor and ceiling isolation.

Last evening i did an quite extensive redesign because of the alternate corners and the decrease height:



Things that changed this time:
- alternate corners (thanks @ljohnsaw )
- reduced height from 4.5 to 4 meters
- moved first floor downwards to get a better ceiling height on first floor

It sure is a big difference when compared to my the the first version i posted. It looks way more sturdier!

[Don P]

My mind doesn't work well in metric, 4 meters, ~12' tall. I think we're all looking in human scale relations at the frame, more like 6 meters maybe. Then my gut looks at aspect ratio, height to width, that is what brings in the bracing. I see redundant bracing in the lower section. I would probably not notch the joists at the posts into the posts, rather fasten them to the outer face of the posts. That can then form the "stop" for the upper end of the long braces.

A little terminology, horizontal members that take load in bending are all beams, so that is the general term that covers joists, rafters, girders. Girders are beams that carry other beams. So your dropped beams under the joists are girders, they are beams that carry the joists 

[okwantegls]

The models are indeed a bit weird, but that might be because i try to get a angle that shows most of the frame. I made a new image which includes 2 men (one sitting on top and one standing on the ground, man about 188 cm, normal height). I also added the ground to illustrate what's in the ground.




@Don P, the joist that go directly in the posts (on the frame sides) are actually beams that have the same dimension as the girders (9x9cm), my joists are a bit smaller (5x10cm). The thought behind this was that otherwise the tenon on the knee braces would become really small.

But indeed you raise an interesting question are these knee braces on the outer joists even necessary now we have alternate corners?

I can put the outer joist on the outside of the frame, then i would have no use of the knee braces underneath the outer joist. Would bolting the outer joist to the outside of the frame be strong enough?
I'm a bit worried when i sit on the edge (on the outer joist) the floor would not be strong enough. (that's why i currently use the knee braces underneath the outer joists)

[Dave Shepard]

Where do you live that 6'2" is normal height?

[Ljohnsaw]

Where do you live that 6'2" is normal height?

:D  Funny you should bring that up.  I'm 5' 10" and my son is the same and just turned 13.  Googling, it looks like he is gonna be 6'4" to 6'5"!

Okwantegls:

This discussion is "what I would do" and I don't mind if you ignore it!

I'm sure this is just your quick modeling but just to be sure, where the beams intersect the posts, it is just a small tenon, not a "through beam" penetrating the entire post.

The "beams" holding the floor joists on the outer edge on both levels:  I would make them a little more stout for an aesthetic improvement as well as allowing the attachment of the braces.

If you want the really long braces, I would attach the top end even with the floor joists and keep them all on the outside or inside, not mixed.  Where they cross, do a half-lap joint.  Run a bolt through that joint and you will be good.  Once you do that, you will not need the knee braces on the lower section.

So to get upstairs, you will have an external ladder or stairs?

[okwantegls]

@Dave Shepard the average male height is 183.8 cm (6 ft 12 in) according wikipedia in The Netherlands, we are on the third place in the world when it comes to height, i was not aware of that

@ljohnsaw cheers, i don't mind any "what i would do", it helps me a lot!

The model indeed does not include the actually wood joints yet, mainly because when changing dimensions also modelling the joints is a lot of work.

I've increased the size of the  girders (the beams holding the floor joints) from 9x9cm to 15x15 cm, it indeed make sense to make them a bit bigger, they do now carry the entire floor and ceiling.

As you suggested i also moved the long x braces back to the outside, which solves my open problem of those braces sticking through the floor. I'm afraid i can't move them downwards: the metal stands which connect the wood with the concrete columns will use bolt to connect the metal to the wood. Bolting the long x braces also on the same level will reduce the strength of the post (cheers for the gained knowledge on this subject)

There will indeed be an outside ladder, a simple vertical ladder straight to the roof. On the backside without the long x frame will get a small platform to "climb" from the ladder to a first floor. In the future i will likely design a nice looking stairs that goes along 3 sides of the structure. But designing a stable pair of stairs is quite a challenge, that's why a simple vertical one will do the trick until then.



On a side note my first appointment with the local authorities (for permits and such) is next week. Hope it goes smooth!

[Ljohnsaw]

The model indeed does not include the actually wood joints yet, mainly because when changing dimensions also modelling the joints is a lot of work.

Ah, yes, but good practice  ;)  Don't forget to clone objects.  For instance, you only need 2 braces (left and right) with the joints defined.  Any changes you make to one propagates through all.

I've increased the size of the  girders (the beams holding the floor joints) from 9x9cm to 15x15 cm, it indeed make sense to make them a bit bigger, they do now carry the entire floor and ceiling.

To a certain extent, width does not matter.  The depth of the beams is the big thing.

As you suggested i also moved the long x braces back to the outside, which solves my open problem of those braces sticking through the floor. I'm afraid i can't move them downwards: the metal stands which connect the wood with the concrete columns will use bolt to connect the metal to the wood. Bolting the long x braces also on the same level will reduce the strength of the post (cheers for the gained knowledge on this subject)

Don't get stuck on 45° bracing.  You can make the X a little more squashed.

[Brian_Weekley]

I don't know anything about towers, but I would think setting the plates directly on top of the posts and trying to stagger the joints would help keep it strong.  Maybe something like this...

[okwantegls]

@ljohnsaw don't praise me too much  ;) , i already designed everything else (wall, windows, doors, roof, floor and so on) in the same model, every time something changes i have to resize and move a lot of things around. But i don't mind, because most of the time i see a thing that could have done better.

I didn't know the X braces could also be squashed, i gave it a try but wasn't really happy about the way it looks. It just doesn't line up that nicely with everything (likely because the knee braces are 45 degrees). On a side note: i will likely leave the X braces off and see if everything is stable without them, if everything wobbles or starts to wobble in a storm i can always add them. One of the advantages of having them outside of the frame and on the outside of the posts.

@Brian_Weekley thanks, that looks great! I've always been a fan of roofs with a bit of a overhang. I've seen the staggering of beams, like in your picture, in many images of barns and similar structures. It sure looks strong. One of the side benefits is that it raises the roof a bit, this did mean i could raise the floor a bit, which isn't a bad thing.

After a bit of dragging things around i came up with a new design: