BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

4.2.2 Deck
NBIS Condition Rating: 5 (Fair)
Condition Description: The timber structural deck planks, which include the sidewalk areas,
exhibit visual evidence of moisture retention and decay with staining and mildew on the
underside between the timber stringers. Much of the preservative chemicals in the deck planks
appear to have leeched from the timber and thus do not provide significant protection from
decay. Although, the top surface of the structural deck beneath the timber wearing surface and
the underside of the deck above the stringers is not accessible for visual inspection, the timber
structural deck has experienced structural failure at one location in Span 4, which indicates that
deterioration of the structural deck may be more significant than the visual evidence reveals.
As described above for the timber wearing surface, moisture enters the small gaps between the
timber wearing surface planks and is retained between the top of the structural deck and
underside of the wearing surface, especially where the nails for the wearing surface planks have
loosened. The underside of the deck exhibits signs of moisture retention and decay in varying
severity throughout. Open nail holes in the top of the structural deck from attachment of timber
wearing surface planks previously replaced, as well as splits and checks, provide opportunities
for moisture and fungal spores to enter the structural deck planks to a depth beyond the
preservatives, which promotes decay. As the top surface of the structural deck beneath the
wearing surface and the underside of the structural deck above the stringers is not visible, it is
not possible to visually confirm the condition of these areas. However, based on the observed
condition of the accessible surfaces, it is likely that the condition of the non-visible surfaces is
likely to be more severe due to the greater potential for retained moisture.
The top surface of the structural deck in the sidewalks is visible and exhibits moderate
deterioration with early signs of decay and significant splits and checks throughout. Similar to
the timber wearing surface, the splits and checks in the top (horizontal surface) of the wood
contribute to increased water absorption by providing easier access of the moisture to enter
grains of the wood. The increased water absorption results in greater volumetric expansion of
the wood when it is wet and increased shrinkage of the wood as it dries. The increased shrinkage
promotes growth in the number and size of splits and checks in the wood, which further
increases moisture absorption and provides an avenue for fungal spores to enter the wood. The
significant volumetric change in the wood also acts to work nails loose. Increased water
absorption also promotes decay by providing the source of moisture required by fungi that feed
on the wood. Larger splits and checks will retain a greater amount of moisture.
The ends of the timber structural deck planks exhibit early signs of decay. The cut ends of the
structural deck are not sealed, and although the cut ends are not located where water typically
ponds, water on the sidewalks drains over the cut ends as it drains through the openings in the
pedestrian railing curbs, which permits water to absorb into the ends of the planks. Water
absorption in the cut ends of the structural planks promotes decay near the ends of the planks.
Based on the above conditions, it is expected that the deterioration and decay of the timber
structural deck will continue and may accelerate.
Repair/Rehab. Feasibility Study March 10, 2011
Bridge No. C-07-001 (437) 16 Final Report