South Arm Bridge, Brunswick Heads

The two layered deck structure was one of the major areas of decay on the bridge. The two layers, one running along the length of the bridge and one perpendicular, were laid directly atop one another. This technique was used in bridge construction to strengthen the deck with thinner section timbers, allowing a longer span and more load limit.

Once the upper parallel boards were lifted by the restoration team, it became obvious that the bottom perpendicular decking was severely damaged by termites and decay. The vertical coach screw connection between layers meant there was a pathway for moisture to penetrate and settle between the two layers, with no air circulation to dry it out. Over the years this had a significant effect on the integrity of the bridge deck. The best solution to this system is to have a single layer of perpendicular decking boards with a gap between to allow air to circulate, debris to fall through, and to keep the timber dry.

The original timber piles were encased with 60mm concrete shells and packed out with sand. This was an innovative way to protect the timber piles from marine borers, and overall proved quite effective. However, over time the salty environment corroded the reinforcement mesh, causing concrete cancer, which cracked the concrete. The cracked shell allowed moisture to enter, and slowly, the concrete fell apart. Once the tide had washed out the sand, the marine borers were able to enter the timber piles. Interestingly, the piles that were still completely encased in concrete had become soft with decay, as the concrete had locked moisture in that had made it through over time and kept the timber moist.

The team at Wood Research and Development used a stress wave timer (SWT) to accurately assess the quality of existing timber members without having to cut them open. The results identified that the timber piles were indeed very compromised below the water line. The structure above the water line was less subjected to borer attack, and could be salvaged. The SWT equipment takes the guesswork and assumptions out of assessing the damaged timber, and makes it simple and effective to find the timber with integrity remaining. 

The timber pile was cut above the borer line, plumbed, and marked for a jointing plate to a new lower foundation. Two holes are vertically drilled into the new timber pile end grain to insert shear pins, and copper naphthenate (CN) oil is applied prior to jointing to prevent mould growth. To structurally reinforce the pile and protect it from further borer damage, TRS triple-wrapped the pile to weather protect the join. All timber member drill holes, end cuts and notches are also coated with the CN oil and sealed with a paraffin-based sealant.

A further major issue was with the timber girders, which had developed re-entrant stress cracks at the notches due to the abrupt change in the stiffness at the notch zone. The stiffness of the girder is directly related to the timber depth, and as the timber girder changes depth at the notch, it is a joint that frequently has issues. The two parts move very differently as a load travels over them, causing differential movement, which generates tension stress separating the fibres at the notch, causing the reentrant crack to occur.

The solution to this problem is to introduce a sloped notch, which reduces the differential stiffness effect, by tapering it off. Through decades of experience and observations, TRS recommends that slope cuts be set at 1:6 to reduce the tension stress checks. By creating a more gradual slope cut, the differential stiffness effects are less pronounced and the likelihood for re-entrant cracking to occur is very low.