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· Registered
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Discussion Starter · #1 ·
Has anyone built any shorter bridges or boardwalks with metal vertical posts and / or metal stringers instead of wood? The decking could still be wood, or maybe a composite or polymer material. But knowing that most pressure treated wood bridges in my area only last 10-15 years before they start to rot, it would be nice to build something that lasts much longer, if I'm going to put in the effort to build it in the first place.

Here is an example of what I am sort of picturing, but not so swoopy and fancy, and sitting on dirt instead of rock.
http://instagr.am/p/CTchmofnq0I/
Maybe the simplest compromise is to isolate the wood from the ground to keep it drier and wick less water. I keep looking at the metal post supports like these to not need to dig post holes and carry in concrete.
Screw Amazon.com : American Ground Screw U-Model Premium No Dig Ground Anchor - Screw in Post Stake - 27", Fits Standard 4x4 (3.5" X 3.5" Inch) Wooden Posts : Patio, Lawn & Garden
Spike Amazon.com: MTB Fence Post Anchor Ground Spike Metal Black Powder Coated 36 x 4 x 4 Inches Outer Diameter (Inner Diameter 3.5 x3.5 Inches), Pack of 1 : Patio, Lawn & Garden

Any thoughts, or been there done that lessons learned?
 

· Dirt Monkey
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293 Posts
Was ground contact rated treated wood used in the previous bridges that rotted out at the 10-15 yr mark? That stuff is usually good for 20-40 years. A lot of 2x material at the big box stores is only treated and rated for above ground use, which actually means ~12" above ground or higher, and definitely won't last long when touching the ground. I posted about the differences a while ago here: bike / foot bridge

There is one local feature where a spike ground anchor was used and it seems to be holding up fine, but only 5-6 years old at this point.
 

· Elitest thrill junkie
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Was ground contact rated treated wood used in the previous bridges that rotted out at the 10-15 yr mark? That stuff is usually good for 20-40 years. A lot of 2x material at the big box stores is only treated and rated for above ground contact, which actually means ~12" above ground or higher, and definitely won't last long when touching the ground. I posted about the differences a while ago here: bike / foot bridge

There is one local feature where a spike ground anchor was used and it seems to be holding up fine, but only 5-6 years old at this point.
Will it hold up in a forest with all the fungi and organic elements constantly trying to attack? Seems that might be different than standing in a residential area? Rotten wood features suck, but they seem to always happen after a certain amount of years, which seems to vary by environment.
 

· Registered
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Discussion Starter · #4 ·
Was ground contact rated treated wood used in the previous bridges that rotted out at the 10-15 yr mark? That stuff is usually good for 20-40 years. A lot of 2x material at the big box stores is only treated and rated for above ground contact, which actually means ~12" above ground or higher, and definitely won't last long when touching the ground. I posted about the differences a while ago here: bike / foot bridge

There is one local feature where a spike ground anchor was used and it seems to be holding up fine, but only 5-6 years old at this point.
To my knowledge all previous bridges were built with typical big box store PT lumber, so the 2x material is not GC rated, but the 4x material is. In many cases the 2x6 decking is rotting away in 10 years or less. 4x4's seem to be holding up ok at the 10 year mark, and for the last 5-6 years we use 4x6 now to ensure longer life. I actually wasn't aware of the minimum distance above ground requirement which might explain some of the boardwalk issues when one edge can be only a few inches away from, and occasionally touches, the ground (at least until it can be dug out).

One example in the same park is a boardwalk that was built over a wet area in 2013. It is made of PT ground contact 4x4 footers and stringers, with non GC PT 2x6 decking. A few months ago a tree fell on it and cracked a 4x4 stringer. I was able to sister the stringer back together with another 4x4 under it, but I had to replace a dozen deck boards that just rotted away. I'm in Pittsburgh which is basically a rainforest most of the time and it seems like in wet places the wood just doesn't dry out and/or wicks moisture from the ground.

I read through your linked post and I'll have to see if any of the local non big box lumber stores have GC PT 2x lumber. That might solve my issues. I'll report that the bridges I posted in that thread are holding up well so far, it's some of the bridges built before my time that have had issues.

It's good to hear the spike anchor is working out well. We have a project coming up where a bridge over a small gulley transitions into a short boardwalk on a steep hillside, and I think digging post holes may be impractical.
 

· Dirt Monkey
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Will it hold up in a forest with all the fungi and organic elements constantly trying to attack?
Yes, to a point. The chemicals are to prevent insects, fungi, bacteria, etc. from attacking/eating the wood; they are poisons to these organisms. The concentration and depth of penetration of the chemicals into the wood is what matters most for longevity; more is better.

Using steel in wet environments presents a challenge as well since it will need protection from corrosion in order to exceed the longevity of GC treated wood. Especially if touching the ground in a wet forest type environment. Hot dipped galvanization (HDG) is the best means of protecting steel exposed to a harsh environment and can easily last 50+ years. However, this is not always a great solution for a small batch of custom parts since it is somewhat expensive and can be difficult to find a place that will even accept smaller batches of parts. We made some custom beam brackets for a large bridge and had them HDG in 2017. The closest place that would accept our small batch of eight brackets was ~2hrs away and had a minimum batch fee of around $150. Industrial corrosion inhibiting paints are also an option but I don't have much experience with them; they will be less durable and higher maintenance than HDG though.

If treated wood and steel are touching, galvanizing the steel is usually required by building codes to ensure the treatment chemicals won't cause accelerated corrosion. This also holds true for load critical fasteners.


If you look closely you can see the HDG brackets: beam cradles and post bases from 1/4" steel.​
 
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