Sawn timber bridge

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What is it?

 

A 7.5m span x 1.2m wide sawn timber bridge which can be used in many path situations. Like the Glentrool bridge, this bridge was originally designed and developed by Forestry Commission's Forestry Civil Engineering. The design uses timber throughout and is suitable for a range of spans. A fundamental principle of this bridge design is that no drilling or nailing into the main beams is carried out, helping to minimise timber decay.

sawn timber bridge oatridge college

How does it work?

 

A sawn timber bridge is assembled from the following components: timber beam and transom frame, timber decking and parapet. In between the handrail support posts vertical timber slats provide a more secure bridge side. Timber nailing strips are attached to the beams with tied galvanised fencing wire. Timber decking boards are screwed to the nailing strips, and can be prefabricated off-site prior to transporting them to site in sections. This can make decking installation on site easier, quicker and safer. The main assembly work for this bridge is carried out on site regardless of how good or poor access to the bridge site is. The beams would be lifted into place first using the easiest and safest method of installation, and all timber components (timber deck sections, parapet etc.) would be fitted last.

Is it suitable for?

 

A sawn timber bridge is suitable for all path users and a range of spans from 3m to 9m.

How much will it cost?

 

Depending on the span and width, the height of the bridge parapet and other factors like access, the sawn timber bridge may cost £5,000 - £15,000.

How do you install it?

 

The construction sequence is different for off-site fabricated and on-site fabricated sawn timber bridges.

Off-site fabricated bridge:

For this bridge design the only component that may be prefabricated off site and transported to site ready for fitting in place once the main beams are secure are the decking sections: decking boards are pre-fitted to nailing strips to provide easy-to-handle decking sections. All other components like beams and handrail support posts would be individually installed or fitted on site.

On-site fabricated bridge:

The main beams, handrail support posts and decking sections are fabricated off site elsewhere and transported to site in kit form ready for building. The main beams would be placed in position and secured first, and then the decking sections and parapet components etc would be fitted last. These components would be progressed together across the bridge from one side to the other.

  • Construct bank end seats and approaches on site ready for bridge installation.

  • Lift in and bolt down the bridge's main beams only on to the bank end seats.

  • Starting from one end of the main beams, fit the first two transoms and handrail support posts to the beams in their required position. Drill transom and handrail post connecting holes in situ to ensure a tight fit around the beams, and then tighten transom clamping bolts to refusal.

  • Fix first and second decking sections on to main beams using galvanised fencing wire to secure them tightly down.

  • Fit the second two transoms and handrail support posts to the beams in their required position. Drill transom and handrail post connecting holes insitu to ensure a tight fit around the beams, and then tighten transom clamping bolts to refusal.

  • Screw on top and inner handrails and inner toe boards to first and second handrail support posts.

  • Screw in vertical timber slats to inner handrails and toe boards between handrail support posts, and then attach outer toe boards.

  • Continue above assembly sequence for remaining bridge length.

  • Screw down anti-slip strips or flat sheets to the decking boards. Alternatively, install decking boards with pre-fitted anti-slip strips.

sawn timber bridge end view oatridge college

What variations are available?

 

Timber nailing strips are prone to decay where decking board nails or screws are driven in. Instead of timber, consider using recycled plastic for nailing strips. It will last longer and will not rot.

Under trees, timber decking boards can become very slippery from algae growth caused by continuously wet timbers or decaying leaf litter. Again, instead of timber, consider using recycled plastic for a bridge deck that inhibits algae growth and has a rough surface texture, providing some grip. However, recycled plastic is heavier than timber, a little more expensive and more flexible – for wider bridge widths, recycled plastic decking boards would need extra strengthening.

For comprehensive information about all elements of path bridge planning, design and construction check out the icon Path Bridges Guide. The manual also contains standard designs for other bridge types such as Log Bridge, Glentrool Bridge - Timber.

Detail drawings - Sawn timber bridge

 

If you like the look of the sawn timber bridge at Oatridge College as a potential bridge design for your project, download the various detailed drawings below:

icon Sawn Timber Bridge - General Bridge Layout Plan & Elevation View

icon Sawn Timber Bridge - Concrete Abutment & Transom Fixing Elevation View

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