Using the circular saw, cut three of the timber beams into 12 equal lengths measuring 24 inches each. These will become the side rails of the bench.
Using the electric drill and 1-inch bit, drill through holes centered 6 inches from the top and 6 inches from the bottom of each of the 12 previously cut side rails. Use a 1 1/2 inch counter-sink bit to make a counter-sink cut to a depth of 1/2 inch in both holes of two of the side rails.
Lay the side rails on a flat work surface with the previously drilled holes positioned parallel to the work surface. Measure 9 1/2 inches from each end and draw a pencil line across the width of each side rail at both of these points.
Set the circular saw blade cutting depth to 2 inches and cut across the width of the pencil line previously drawn. Make multiple additional cuts by repositioning the saw 1/16 inch from the previous cut and in the direction of the second pencil line until the final cut is made on the second pencil line. Use the wood chisel to remove any wood remaining between the pencil lines to finish the mortise cut in side rail.
Repeat Steps 3 and 4 for each of the (12) side rails pieces.
Measure 12 inches from each end of each of the six remaining timbers and make a mark in the center of the timber. Make a third mark centered 48 inches from each end. Using an electric drill and a 1-inch bit, drill through holes at each of these three marks.
Repeat Step 6 for the remaining five timbers.
Using 1 1/2 counter-sink bit, make 1/2-inch deep counter sink on the entry side of the three drilled holes on two of the full length pieces in Step 6.
Lay the six side rail pieces side by side on a work surface with the previously drilled holes parallel to the work surface and the drilled counter sunk pieces facing the outside. Insert the steel rods through the previously drilled holes and place a 1-inch washer and a 3/4-inch nut on each end and tighten until washer seats firmly against the bottom of the counter sink, locking the six side rails together. Repeat this process for the second set of six side rails.
Position one 96-inch timber perpendicular to one set of side rails and insert the timber into the mortise cut aligned with the second side rail in the six piece set. Use an assistant to hold the timber in vertical position and use two 2-inch wood screws to attach one "L" clamp to join the timber rail piece to the second side rail in the set of six.
Carefully lift the set of side rails to a vertical position lowering the inserted timber rail to a position horizontal to the ground. Hold the timber rail parallel to the ground and raise to vertical the second set of side rails and insert the opposite end of the timber rail into the mortise cut aligned with the second side rail. Using an assistant to hold the side rail in position, attach the timber rail to the second side rail using one "L" clamp and two 2-inch wood screws.
Position additional timber rails into the mortise cuts on both sides adjacent to side rails one, three, four, five and six.
Using the 2-inch wood screws attach "L" clamps to each end of the fifth rail to join the timber rail to the side rails. See photo
Using the holes drilled in Steps 6, 7 and 8, insert the 32 1/2-inch threaded steel rods through the previously drilled holes and place a 1-inch washer and a 3/4-inch nut on each end and tighten until washer seats firmly against the bottom of the counter sink, locking the timber rails firmly together.
Add an optional back support by using three 3-inch wood screws on each side to fasten an 8-inch by 2-inch by 96-inch board flush with the top of the rearmost side rails.