Timber options to build a footbridge

[Couldyajust]

I was planning on buying an Alaskan Mill in the summer as I will be taking on around 40 acres of woodland (10 acres of douglas & sitka the rest assorted hardwood) which has been neglected for the best part of 50 years. Initially I will just clear the fallen & dangerous trees which the landowner will sell off as firewood then I will look at the best way to manage the site. Due to location & access being able to mill the timber in situ will make extraction easier & it's certainly preferable to paying someone else £28 per hour to bring his bandsaw up as I usually have to when I need milling done.

 

Getting to the point I am carrying out some riverbank work for the same landowner. Usually there is barely 6" of water in most places along this stretch of river but recently it's been constantly in flood making crossing very difficult anywhere along the mile of river he owns & I am almost out of trees to prune or fell on the side I can access.

Discussed the issue with the landowner & the solution we have agreed on is that I will build a footbridge so I have at least got one point I can cross safely on foot when it is not usually possible to do so either on foot or in a tractor due to flooding, he then gets the benefit of the bridge in the future.

 

It will be a 20' span, the bank is approx 6' higher than standard water level approx 2'-3' higher when in flood. rather than bringing in timber it makes sense to bring forward my Alaskan Mill purchase & use what is already there. The trees which are long/straight/thick enough for the full span in the vicinity are Alder, Ash, Sycamore & Wych Elm. There is also about a dozen 80' Douglas firs around which ideally want to be left alone but can be used if required. Shorter thinner sections available are as above & crack willow, goat willow, hazel, hawthorn, oak, silver birch.

 

I should probably mention I am looking at purchasing the 30" Alaskan & will initially be using my Echo CS-8002 with a 28" bar. May upgrade in the future.

 

Part of me is tempted to use the douglas for convenience as it will give nice long straight lengths simplifying the job but as said ideally those will not be dropped at the present time. I can't extract timber from the woods yet either as I have yet to take a digger up there to make a track up the hill so i can access the woodland on something bigger than a quad I did consider taking the 4wd dumper up there but my sense of self preservation keeps kicking in preventing me from doing so.

 

Any advice on which of these trees would be best suited to build this bridge would be appreciated.

[Brushcutter]

Most of those timber will rot out really quickly exposed to the weather. The other thing you need to consider is the strength over a 20' span. I think Oak is your best bet. Alder might be Ok.

 

If i was doing it i'd use Elm for anything in contact with water and sweet chestnut or oak for the structural work.

[agg221]

Likewise I wouldn't use any of the species you have mentioned in permanently damp conditions - expect a lifespan of a year or less.

 

Elm is good below the water (permanently submerged) and SC/oak heartwood are good permanently out of the water. If you have to cross the waterline, robinia is the best bet.

 

If you can design the bridge so that the timber is not in direct contact with the ground and sheds water well with no ponding, of the species you have available I would use the Douglas fir, maybe with oak heartwood slats if the oak you have available is wide enough (a foot or more) but in shorter lengths (as wide as the bridge).

 

If it's normally very shallow, can you put a concrete/blockwork pier in the middle, maybe by shuttering up or dropping in a slab which is thick enough to be above water level when the water is down? This would drop the span to 10ft, which would let you use thinner sections that would be easier to handle.

 

Alec

[ArthurBottlesworth]

Have seen old artic trailer used

Remove axles etc and then drag frame across as bridge

Surface with recycled railway sleepers

Strong enough for most stuff

 

Local council just put bridge over river for bridleway

About 20 feet

£80,000

Pity they have no right of access on one side of river so it is bridge to nowhere

 

Bad news is we're getting another one

So thats £160,000

 

But bridalways So overgrown too dangeros for horses

[Couldyajust]

No sweet chestnut available.

There are only two big oaks on the list to come down one is dead & completely hollow so not much use for anything. The other I intend to dismantle due to location, 11k lines close by in the field & it is right on the edge of the river with a 15 foot drop about 6 inches from the trunk on one side with most soil washed out underneath it. There is a sheer rock face on the other side of the river so if it was dropped intact the only option would be to drop it into the river but it would be a nightmare to get out.

Plenty of small diameter oaks around 14"-16" but relatively short straight sections not much I could get anything over about 7 foot as a straight piece of timber. They will do for slats but I wont get the primary beams out of them.

 

The bridge will not be in direct contact with water it will be level with the field which is approximately 6 foot above the typical water level at the intended site for the bridge, the water never rises more than 4 foot even in extreme flooding so even then it will still have a couple of feet of clearance but a lot of debris washes down primarily trees & branches which have fallen in further upstream. The ends of the bridge will either sit on concrete pads or I may just skim off the clay so it sits directly on the rock which is only about a foot down in that location.

[pycoed]

How about using two electricity poles for the main beams. Mill up some nice timber for the deck & clad the visible sides of the poles too perhaps? That'll be fine for foot traffic, perhaps even a lawn tractor.

[Pete Bannister]

Importing steel concrete or treated timber may be an option but I like your idea of building it with what’s on site

If the Alder on site is big enough, I'd consider that for piles. It tends to rot off at ground level but is renowned for it natural durability in anaerobic conditions i.e. below water and in waterlogged soils

With the species available I think Id be looking at using oak (heartwood only) for the spans and alder for the stuff in the ground and using short spans with many piles/posts. I’d thinks about a design that can easily be refurbished as elements rot out, e.g. lashed connections.

If you can build in plenty of ‘redundancy’ by using many smaller members and short spans rather than large spans and tall collumns, there's not so much invested in the thing when it needs repair.

Sounds really interesting. Good luck

[forestgough]

Sounds obvious,but are there any trees you could drop across the river .Then just take a slab off the top to make a deck and joint some uprights in to hold a handrail.

[forestgough]

where are you in Powys by the way,i could show you a tree bridge I made in llangurig