BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

SUMMARY OF TIMBER ELEMENT SERVICE LIFE
Element
Est. Remaining
Life (years)
Est. Overall
Life (years)
Anticipated Governing
Failure Mode
Wearing Surface 1 0 10-20 Abrasion/Wear
Structural Deck 0-10 30-40 Decay
Curbs 0-10 30-40 Decay
Railings 10-20 40-50 Decay
Sheave Poles/Masts 10-20 40-50 Decay
Lifting Beam 2 20-30 30-40 Fatigue/Decay
Stringers/Blocking 10-20 40-50 Decay
Cap Beams/Sills 10-20 40-50 Decay
Bracing 3 0 20-30 Marine Borers/Decay
Piles 4 0 20-30 Marine Borers/Decay
Fender System 3 0 20-30 Marine Borers/Decay
1 – The existing wearing surface effectively required replacement 5 – 10 years prior and
has required significant repair over this period.
2 – The lifting beam was replaced in 2007 due to fatigue failure.
3 – Many of the existing bracing and fender members required replacement 5 – 10 years
prior.
4 – The long service life of many of the existing piles is due to the use of heavy
creosote oil-based preservative not permitted for use today.
The above estimates assume that the timber will be replaced with commonly available treated
timber and installed following current best management design and construction practices. The
estimates do not include the extension of the service life by way of periodic in-place preservative
treatment, pile jackets, pile wrapping, etc. Recent use of tropical timber on similar bridges and
environments in Massachusetts (e.g. Powder Point Bridge, Duxbury) has not demonstrated a
significant improvement in the service life of the timber and thus are not considered here.
Similarly, there is insufficient experience with Accoya wood, glass infused wood and other
recent advances in timber products to support that this material can provide longer service life on
bridges in this environment.
As identified earlier, decay of timber can be slowed and the service life extended with the use of
in-place preservative treatments (see Section 4 above.) However, for various reasons, the service
life of these treatments is relatively short, typically only 5 to 10 years depending on a number of
factors including the type of treatment, chemicals used, timber condition, exposure conditions,
type of timber, etc. As such, these treatments require frequent reapplication, which can
significantly increase the cost of maintenance. Furthermore, many of the toxic chemicals used in
these preservative treatments raise concerns regarding human health. A number of the timber
elements are readily accessible to human contact including the bridge railings, sidewalks, curbs,
wearing surface and sheave poles and thus certain chemicals are not recommended for these
locations. Many of the more effective chemicals in preventing decay are the same products not
suitable for direct human contact. There are available preservative chemicals that prevent decay
that are suitable for human contact (e.g. boron and sodium fluoride) however, research on the
Repair/Rehab. Feasibility Study March 10, 2011
Bridge No. C-07-001 (437) 50 Final Report