Playground equipment foundation 2

[m_struct]

I am designing the foundation for a timber constructed playground. It is an ocean front site. The challenges are corroion and rotting. There are two conditions - 6x6 timber posts and approximately 12" diameter log posts. Both require fixed base connections. The concrete will be embedded a few feet into the ground. Few option were looked at:
- Timber post embedded into concrete (risk of rotting?)
- A fixed base shoe connection such as the Simpson moment post base (MPBZ does not meet the corrosion requirements). Even if corrosion requirements are met, not sure teh architect will go with it.
- Stainless steel cruciform fixing embedded in concrete foundation and fixed to base of log with 4 bolts each face. Expensive. Works for the 12" diameter log but too big for the 6x6 post.

Any suggestions? It is supposed to be 50 year design life.

 

[rowingengineer]

Normally if timber is embedded you provide gravel to the base of the footing for the timber post to sit on. Concrete is only provide to the sides of the timber to get moment requirements. The gravel stops water sitting in the base of the timber in the footing, depending on the soil this gravel depth can be up to 600mm, I like to use 10mm gravel.

 

[dik]

and fill with pea gravel... can you look into a non-toxic preservative, too.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[kingnero]

Are you demanding a 50 yr service life out of wood, outside, near the ocean? Your climate must be very different than ours... Wouldn't last 10 yrs here (Belgium).
I've specced Hot dip galvanised square hollow profiles to be set in concrete, with tight fitting square timber posts, in order to facilitate periodical changing. As said by rowing engineer, provide adequate drainage. And keep the dogs out...

 

[phamENG]

Rather than a crucible, use a stainless steel angle. It can be on the outside of the 6x6. There was a picture posted on here a couple years back of a playground that used it. Posted by a user named JAE. He was prolific when he was around, so you may not have any luck scrolling through it...

For the Simpson moment post base, you can always cover it up with some kind of trim.

 

[kingnero]

That's also a good alternative. I've also seen small(ish) round steel tubes protruding the concrete, and wooden posts with corresponding holes, that go over the tubing, during a holiday in France some years ago. They had some amazing public playgrounds there, with wooden miniature ships and castles.

 

[phamENG]

Well, I couldn't find the original post, but I did find a post that had a screenshot of the original post...

thread507-462549

 

[CarlB]

Here's the original thread with JAE's post:

https://www.eng-tips.com/viewthread.cfm?qid=436868

(search function works great)

 

[m_struct]

There are some great ideas there. Thank you.

A key part to many of these suggestions is a design that allows free drainage at base and no ponding. For JAE's angle connection, is there something at the base of post to allow for drainage?

For a embedded timber pole, sitting on free draining gravel, and encased in concrete, is there any need for reinforcing foundation? What is the recommended minimum clear cover to timber post?

Kingnero - good point. The 50 year design is a concern for the embedded timber piles. Looking at another oceanfront playground, some for the timber is showing wear after 5 years. For best design life in these oceanfront conditions, would you be looking treated pine or a hardwood with timber protection such as CD50X Extreme Timber Protection.

 

[dik]

If there is no uplift on the posts, on one project, I had a HDG steel plate with a round bar going through it. This was cast into the concrete and a long longitudinal hole was drilled in the bottom of the post that was placed over the vertical bar from the plate... OK if no uplift. The owner didn't want to have a visible connection. The plate separating the concrete from the post is all that can be seen, no nuts, bolts, or nails...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

should have added, you might want a decay resistant timber... redwood or cedar?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[m_struct]

The free draining gravel at base seems like it will work well in granular soils.

What about in cohesive soils, such as clay? Would it be better to have to the post fully encased in concrete?

 

[DBronson]

m struct:

Embedding wood posts in concrete is a very bad idea. One is asking for trouble by creating an ideal environment for wood to rot by keeping it moist, and it creates a pathway for termites. I'd use knife plate connections for this application. Use HD galvanized (or better, stainless steel) plates embedded in the concrete footing and bolts to secure the timbers. The base of the wood members can be held up 2"-3" above the concrete to ensure drainage. Note that most areas around play equipment needs to have a soft surface material to lessen the chance of injury. Wood chips and sand are often used for this so you may need to hold the timbers up a little higher to keep the ends dry.

Check with the playground designer to see if exposed hardware (like bolt heads and threads) are a safety hazard. If so, you could switch to self-drilling dowels which are flush with the wood surface.

The wood absolutely needs to be treated to get any reasonable life out of it, but check the toxicity of the treatment - could be a problem around kids.

Regards,

DB

 

[kingnero]

Use HD galvanized (or better, stainless steel) plates embedded in the concrete footing

I'm not sure that stainless steel (I'm assuming you're talking the "regular" austenitics like 304 or 316) would be better in this case.
I'd stick to HDG (or directly go up to a lean duplex) before considering 304/316 in this case, but I'll leave this up to the metallurgists to comment further on this.

 

[XR250]

I'd use knife plate connections for this application

My experience with design of knife plates in this situation is that they work really well in one direction but not so great in the other direction (plate weak axis)

 

[jayrod12]

I've watched every episode of Timber Kings that they've produced multiple times. In a few of them they had cantilevered wood posts and they used a cruciform steel section attached to the concrete base that projected up into the post a fair bit (maybe 2 feet). Granted in that scenario they are using fairly substantial posts that likely would be hard to topple over without the connection. But the cruciform shape would allow for some good resistance in each direction.

 

[phamENG]

A cruciform works well in heavy timbers, but there's not enough meat in a 6x6. OP already brought it up (I'm assuming that's what was meant by "crucible").

 

[jayrod12]

Good call. Missed that. More just wanted to pump the tires of that Timber Kings show. Love log houses.

 

[EdStainless]

I have seen this done with a pair of angles legs in. The angles can be spaced up off of the concrete, and the post can be held up off of the concrete as well. And it keeps the major fasteners under the post.
Of course there will be uplift, have you never played with you kids at a park? Picture a 250# man swinging from this thing.
316 SS and fasteners stand a chance of making a 10 year life, if your site gets lots of rain to keep them rinsed off. Look at using 2205. The lit on SS in architecture is replete with tales of 316 rusting away in coastal applications.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TheDW]

Well, I couldn't find the original post, but I did find a post that had a screenshot of the original post...

thread507-462549: Wood light pole base

[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=436868[/url]