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General Forestry => Timber Framing/Log construction => Topic started by: ChugiakTinkerer on May 10, 2018, 05:39:26 PM

Title: Small temporary bridge - timber or log construction?
Post by: ChugiakTinkerer on May 10, 2018, 05:39:26 PM
I'm working on a project to walk a mini-excavator out to my remote property for a week or two of brush clearing and trail making.  There are plenty of logistical challenges, but one of them is posed by this creek:


(http://forestryforum.com/gallery/albums/userpics/43616/tolsona_creek~1.jpg?easyrotate_cache=1525986267)


Where I plan to cross this creek is further upstream and it is much smaller there, about half the width of this channel.  I'm looking to move a piece of equipment that is at most 10,000 lbs.  I'll probably rent much smaller, more like 6,500 lbs.  There are fish in this creek so I'm thinking a temporary bridge will provide a good crossing with minimal impact.

That's my snowmachine windshield in the foreground, to give some sense of scale.  The snow trail is about 4' wide, and the creek looks to be 6' to 8' wide.  Where I plan to cross it's generally no wider than 4'.

I'm considering a skidder bridge (http://forestryforum.com/board/index.php?topic=78930.msg1198212#msg1198212) or a log stringer bridge (http://www.extension.umn.edu/environment/trees-woodlands/forest-management-practices-fact-sheet-crossing-options-series/timber-bridges/).  There is plenty of black spruce in the area, but I'm favoring dragging in a pre-built skidder bridge rather than felling and building a log bridge.  I'll be paying for the mini-excavator time and I don't need to add delay by taking a day or more to build a bridge.  So here's my thinking, appreciate any insight anyone can offer.

Using 6x6 cants, bolt four of them together to make a 24" wide panel, ten feet long.  Since I'm working with a small excavator I can't handle anything much bigger.  I'm hoping I can get by with just two panels, spaced with a gap between them.  Each panel will be supporting half the weight of the machine, so roughly 3,250 lbs.  I'd rather err on the side of overbuilding and would like each panel to support at least 5,000 lbs.

A single 6x6 beam of Spruce/Pine/Fir #2 can support up to 2750 lbs when spanning 60".  Is it reasonable to assume that a panel contructed of four of such beams ought to be able to hold up 5,000 lbs?  Construction method will be threaded rod and spikes, consistent with the UMN cooperative extension
Vermont design (http://fpr.vermont.gov/forest/your_woods/harvesting_your_woodlots/skidder_bridge).

Anyone have experience with a project along these lines, or some advice for how to make it go smoother?
Title: Re: Small temporary bridge - timber or log construction?
Post by: Don P on May 10, 2018, 07:21:01 PM
If it is shallow putting a log midstream in the direction of stream flow cuts the span in half.
Are the banks firm? In other words do you also need to think about swinging mats as you go.
Logs are nearly twice as strong as timbers because the grain is unbroken.

Title: Re: Small temporary bridge - timber or log construction?
Post by: ChugiakTinkerer on May 10, 2018, 07:42:58 PM
I've only been at the crossing location in the winter so I am making some assumptions.  I may need to walk the route first get some firsthand knowledge of where I'd like to cross.  It's at a place where the stream gradient is greater, rather than the low spot shown in the picture.

I've sent an inquiry to our fish & game habitat division that does the stream crossing permits.  A local log might be okay, but I don't know about the depth or the streambed firmness.
Title: Re: Small temporary bridge - timber or log construction?
Post by: ChugiakTinkerer on May 12, 2018, 04:48:17 PM
So I think I'm going to go with a timber bridge like this:


(http://forestryforum.com/gallery/albums/userpics/43616/TimberBridge_3DView.jpg?easyrotate_cache=1526157460)
 

It's much like the log stringer bridge design but instead of 6" spruce logs I will be using 4x8 spruce timbers.  I'm shooting for a 78" span and this configuration allows me to do that and still support a load upwards of of 15,000 lbs.  I don't plan on needing anything that heavy but it gives me a reasonable degree of comfort.

One thing I will need to do is carve out a seat for the timbers in the abutment logs.  I'm sure a chainsaw will suffice for that, but I wonder if it's better to do it with an adze.
Title: Re: Small temporary bridge - timber or log construction?
Post by: TimFromNB on May 14, 2018, 07:50:30 AM
Cool design. I may use it as inspiration for a similar sized bridge for a stream crossing on the property here.
Title: Re: Small temporary bridge - timber or log construction?
Post by: ChugiakTinkerer on May 14, 2018, 12:16:58 PM
It was suggested to me that the abutment logs should be longer by a couple feet on each end to ensure that soft ground wouldn't result in a corner of the bridge dropping.