It would appear that our public officials are trying to scare the hell out of their constituents and stampede them into replacing 78 bridges that may not need to be replaced.
Instead of hysteria-based decisions, a modest engineering review and the use of common maintenance practices — some of which were developed at LSU years ago — can inexpensively preserve and/or improve deteriorated timber substructures.
This would lead to preservation and repairs, and avoid the millions in demolition and replacement costs that include the substantially more expensive precast concrete superstructures, most of which are probably in fair to good structural condition with more than 25 years of service life remaining.
Timber pilings do not significantly deteriorate from traffic load as was implied. They deteriorate from environmental conditions such as the presence of water, weather and bugs. To suggest that every treated timber piling field rated in poor structural condition is a serious threat to public safety is nonsense.
Structural components are field-rated relative to their original structural condition. Because the size of timber pilings that are large enough to be driven provides much more structural capacity than needed, they are usually structurally adequate when rated in poor structural condition. This is probably the case with most of the 78 bridges proposed to be replaced.
When the remaining sound portion of a deteriorated timber piling becomes insufficient to carry legal truck loads, it is time to strengthen the piling, place weight limits on the bridge or close the bridge, depending on the remaining structural integrity. This probably explains the need — for safety reasons — to close one of the 78 bridges proposed to be replaced.
The precast concrete slab span superstructures with treated timber substructures constructed between the 1950s and the early 1970s are known to those who live with them as the “upside-down” bridges. These bridges replaced bridges constructed entirely of treated timber. The experience with timber bridges was that the superstructure deteriorated and became structurally deficient first. The more durable precast concrete slab replacing the timber superstructure left the remaining timber substructure (piling and pile caps) to become structurally deficient first due to deterioration, hence the moniker the upside-down bridges.
In the 1970s it was proposed that the timber substructure of the upside-down bridge be replaced with precast concrete. This met strong opposition. An investigation exposed the problems causing the premature deterioration of the treated timber. The timber being used was less durable than the slow-growth timber used in the 1930s and earlier, and the quality of the creosote preservative treatment was questionable. Considering the costly experience with treated timber, it was replaced with a precast concrete substructure at a modest increase in cost.