A 24 foot long 2x6 treated wooden beam. (This construction used the beams for a walkway on a play set)
Treated 2x6 8' long lumber (6)
Galvanized 1/2" carriage bolts 4" long (13)
Galvanized 1/2" Nuts (13)
Galvanized 1/2" Flat Washers (13)
T25 2.5" long exterior screws (1 box)
Drill (1/2" drill bit and T25 Screw Driver)
Step 1: Lumber Prep.
Take one of the full size boards and cut it in half. For most cases this will be 48.5" due to the 2x6 having and actual length of 8' - 1".
Since the boards are treated I leaned them upright against a fence and faced them south the day before assembly to help dry them out. The added water weight makes moving the bream hard as well as cutting and drilling through the pieces.
Step 2: Laying Out the Boards for Assembly
Lay three or four additional 2x6's or any other type of lumber to create a flat surface.
On top of this flat surface lay three 2x6 by 8' long boards end for end.
Starting at one end of them place a 48.5" board on top and make sure the exterior edges are flush.
Follow it with two 8' long boards and lastly the remaining 48.5" board.
Now you have two layers of 2x6 members with their exterior edges aligned. (The 48.5" board on each end gave a displacement for one layers ends to be located at the midpoint of the other layers members)
Step 3: Attaching the Layers
I recommend using a pad or something else to kneel on for the attachment process.
Begin with one screw at the farthest edge of the 48.5" board on top. After inserting a screw, make sure the member is aligned with the one below it and insert another screw at the opposite side of the 48.5" board. This member is now fixed in place, insert screws along the top edge every 12 inches. Along the opposite edge start at the midpoint of the top edge screws and insert screws every 12 inches as well. See photo. Now screws hold the two members together every 6 inches.
Follow the above process for the next two full size members and lastly the 48.5" member.
Once completed, with assistance of one or more individuals flip the beam up on its edge.
Insert screws from the opposite side along the middle, starting at the edge and continuing every 12" down the board.
From this same side, after the screws are inserted switch to the 1/2" drill bit and drill a hole 6" from the edge on each side at the midpoint of the beam as well.
Using a tape measure mark the beam at 2 foot intervals starting from either edge. (11 marks). Starting from one side drill a 1/2" hole at the first mark 2.5" from the top. On the second mark drill a hole 2.5" from the bottom. Alternate top to bottom continuously along the marks until you reach the other end. Off setting these holes creates varying points so all the tension isn't put on one plane across the board, causing it to crack.
Step 4: Install the Bolts
While the beam is still on edge use the hammer and pound all 13 of the carriage bolts through the drilled holes. I would recommend pounding them all through on the same side so that the connections look uniform. Also you may want to hide the side with all the protruding bolts, nuts, and washers.
On the opposite side place a washer on each bolt (13).
Then start the nuts on all 13 bolts. Continue with your fingers to ensure the threads are crossed or damaged. Once the nuts have threads coming out the other side of them continue with hand or ratchet until the nut feels tight.
After the nut is tonight continue for one revolution, sucking the carriage bolt into the beam but not to far. To many revolution could cause damage to the wooden beam.
You now have a 3" wide, 5.5" tall, 24 foot long beam.
(I used three of these in a play set that I will have an instructable for later)