Sunday, July 28, 2013

Footbridge near Rt 7 (Sheffield)

Day 1 (July 27th)

One of 3 truckloads headed to the site.

Before today, we had made several trips to the job site to ferry materials from the Tool Shed on Mt Greylock to a location near the job site.  For the largest pieces, we used the power wheelbarrow to carry many at a time.
A full load for the wheelbarrow

Despite a hot, wet summer we had nice weather for the first day of construction.  Our first tasks were to set the sills that the bridge would be resting on and to assemble the various pieces of lumber needed to create the three large timbers necessary to span the banks of the stream.  How big are they you ask?  They need to span a 28ft wide ravine with enough left over to rest securely on the banks--36ft in total.  In addition, they need to be strong enough to carry a reasonable number of hikers.  Using data from the US Forest Service, we found beams 4" wide and 14" deep would be required.

Early in the planning stages (over a year ago) we looked into the possibility of purchasing timbers of this size.  While they could be ordered, the logistics of transporting something this long and heavy (600lbs) to the job site were more than we could support.  So the plan is to laminate plywood and 12ft long 2x14's together at the site set them onto the sills with a bit of rigging.

Today we got the sills started by using a water level (a clear plastic tube filled with water) to make sure the sills on both sides were at the same elevation, we did not want the bridge to be sloped. 
Dave and Jesse use the water level to set the sill height
While the south end of the bridge could be set on sills dug directly into the bank, the north side  required about 10" of excavation to provide a level bearing surface.
North sill is set deeper into the ground

While one team installed the sills, the other began on the beams.  Because we had no workbench available, these would be assembled on the ground next to the trail.  Fortunately,  the AT crosses into a farm field at this location and there was relatively flat open ground to work on.  We laid out the first layer of 2x14's and blocked them up so they sat flat and straight along a taught string we set for that purpose. 
Don and Christine use a string to lay out the beam pieces.
After spreading a liberal amount of construction adhesive, 14" wide pieces of 3/4" plywood were screwed to the 2x14's.  Then another application of adhesive, and the outer later of 2x14's were screwed into place. 
Adhesive is applied
The lengths of all the pieces were arranged so that butt joints were always overlapped by two other pieces.  Finally, at each joint, bolts were installed to reinforce these locations. 
All joints reinforced with bolts.
While theoretically, this should work, we won't be completely certain until everything is in place.  We'll know more after the next project day.

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