from Aug. 1999
Small Bridge Construction - One Method
Scott Hanson

I n 1994, I purchased 92 acres of cut over timberland east of Silveron near Drakes Crossing. The topography of the property is a steep, V-shaped canyon. Bridge Creek flows through at the bottom of the canyon and splits my property into two pieces. Roughly 60 acres lies to the south of Bridge Creek and 30 acres to the north. To do forest management activities on the northern piece of property I need to someday construct a small bridge. For the past couple of years I have been gathering information, ideas and viewing bridges constructed by tree farmers.

Since I operate my tree farm on a shoestring budget, I was interested to learn that John Belton, CCFFA member, had constructed a 12 foot by 12 foot bridge on his Sandy property for $1400. The remainder of this article details how John constructed his bridge. My sincere thanks to John for letting me interview him and share this information with you. I make two disclosures before reporting the details of this bridge construction. One, I am not a structural engineer, so I will not address the technical issues of load carrying requirements. Two, I will not address the math calculation for size of culvert (bridge) necessary to accommodate the size of the surrounding watershed. I will leave both of these topics to the professionals.

The Belton bridge used the following materials:


• Eight cement blocks from Estacada Rock Products at size 5’ x 5’ x 2.25’
• Seven utility poles sections from discarded utility poles tops at 8-11” diameter, 12’ -14’ in length.
• Fourteen pressure treated planks from a lumber yard in Boring at size 4” x 12” x 12’

John had one operator (Gene Copher, Sandy) handle the purchase, transportation and placement of the blocks. Estacada Rock Products was chosen as the supplier of the cement blocks, however there are likely many ready mix companies that make this product from their overruns. A driver brought a backhoe to the site on a lowboy, unloaded, and went to pick up the cement blocks. While Gene waited for the lowboy to return with the cement blocks, he lifted out old damaged logs from the crossing and carefully set them a safe distance from the stream.

The stream opening between the two banks is 5.5 feet. The site was cleaned and prepared for placement of the cement blocks. The creek is medium-sized, but without a strong current. Eight cement blocks were brought to the site, however only four blocks were used to directly support the bridge. The remaining four blocks were used to armor the bridge site. Two blocks were placed on the inflow side and two on the outflow side. These blocks fan out slightly from the bride structure. See figure below. The blocks on the exit side of the bridge are important to prevent back eddies during high water that may wash out backfill material around the bridge. Concrete footings may be beneficial at the edge of the creek to give the cement blocks a foundation, but they were not used on this bridge. The backhoe placed cement blocks on edge: so visually, the length was five feet, the width 2.25 feet, and height 5 feet. Two cement blocks lying end to end make a 10 foot foundation area on each side of the creek.

Gravel trucks brought in backfill material to go between the bank and the cement blocks. The driver must use care at this step, to prevent the blocks from moving or toppling over. Temporary jacks may be a solution here, to keep cement blocks correctly placed on each bank of the creek. The seven utility poles are placed on top of the cement blocks to span the 5.5 foot opening. The utility poles are not attached to the cement blocks, but the weight of the poles and planks make it unlikely that the deck would float off its foundation in high water. Finally, the pressure treated planks are nailed to the utility poles. Small cables (1/4 inch) were not used on the bridge, but may add to further stabilize the cement blocks together, or “tie down” the utility poles to the cement blocks.

The bottom of the bridge sits 4.5 feet above the water level. The bridge construction took place in the summer to take advantage of low water level. John Belton recommends that tree farmer bridge builders work closely with Oregon Department of Forestry Forest Practices forester to review your bridge design before the bridge is constructed and inspect the structure upon completion.

If other tree farmers have experience in small bridge construction, I would be interested to view their bridge design and understand the steps they went through to complete it. It is great to share ideas among tree farmers!

Editor’s Note: CCFFA member, Scott Hanson, is an Associate Editor for the Forest-Tree Leader. The May 1996 issue of the Forest-Tree Leader also had an article by CCFFA member, Gilbert Shibley, titled “Do-It-Yourself Bridge”. Contact John Foster at 503-630-7333 if you want a copy of that article mailed to you.