Houston Walker, P.E., Tennessee Department of Transportation
It has long been recognized by many bridge engineers that the best expansion joint is no expansion joint at all. The AASHTO LRFD Bridge Design Specifications, 6th Edition, recognizes this opinion about joints in section C2.5.2.1.1 of the commentary: “Other than the deterioration of the concrete deck itself, the single most prevalent bridge maintenance problem is the disintegration of beam ends, bearings, pedestals, piers and abutments due to percolation of waterborne road salts though the deck joints. Experience appears to indicate that a structurally continuous deck provides the best protection for components below the deck.” Thus, the elimination of joints is one of the best ways to preserve the condition of bearings, beam ends and substructure components.
The first step in eliminating bridge joints is to utilize continuous spans, thus eliminating joints at piers. The second step is to build the girder and deck system integrally with the abutments. The Tennessee Department of Transportation (TDOT) has been utilizing this method of constructing bridges since 1964. Although other states, notably Ohio, used this method as early as the late 1920’s, TDOT has pushed the limits. TDOT has extended the use to longer and longer spans and currently is considered a national leader in applying this design concept. TDOT routinely builds jointless, integral abutment bridges up to 400 feet long in steel and up to 800 feet long in concrete. TDOT’s longest entirely jointless and integral bridges are 1175 feet long in concrete and 536 feet long in steel. TDOT has also constructed a concrete bridge 2,695 feet long with joints only at the abutments.
The benefits of jointless, integral bridges are many. The most obvious is the elimination of the initial cost of joints and expansion bearings, which can be quite expensive. The biggest benefit from the standpoint of bridge preservation is the reduction in the amount of water, which may be salt laden, that can leak through the joint and be deposited on the girders, bearings and substructures below. Water leakage can cause accelerated deterioration of both concrete and steel girders and rusting of metal bearings. Why do joints leak? Elastomeric glands can become filled with road debris, eventually causing tears, and mechanical parts can break under the pounding of truck wheel loadings. Some common problems which can be caused by expansion joints are:
| • | Bearings can seize due to corrosion |
| • | Bearings can tip over or ratchet out of position |
| • | Joints can be difficult to install and may need to be raised for future paving |
| • | Lubricated bearings often lose their effectiveness due to the buildup of grime and the loss of lubrication |
| • | Malfunctioning bearings can cause structural damage |
| • | Joints can be damaged by snowplows |
| • | Loose or damaged joints in traffic lanes may be a hazard to traveling public |
All of these conditions may be cause for future expensive repairs or replacement of expansion joints and bearings.
There are also inherent advantages to jointless, integral bridge construction. Some of those are listed here, but a detailed explanation is beyond the scope of this article:
| • | Substructure design is more efficient, since there is an increase in the number of supports over which longitudinal and transverse forces may be distributed |
| • | Adds redundancy for catastrophic events |
| • | Eliminates loss of seat support in seismic events |
| • | Increases damping capacity by absorbing seismic energy |
| • | Enhances live load distribution to girders at bridge ends |
| • | Promotes rapid construction of abutments |
| • | Minimizes construction tolerance problems |
| • | Enhances flexibility for end span ratios (less uplift concerns) |
| • | Reduces seat width requirements |
| • | Lessens expense of bearings |
Joints in bridges can be compared to a cut in the skin, with the expansion joint as a bandage. Over time, the bandage can become damaged, allowing foreign materials (salt and water) to enter the underlying tissue (beams and bearings), allowing infection (corrosion) to cause damage to the body (bridge). Eventually, medical treatment (bridge repair) is needed to correct the problem. If left untreated, severe illness can occur and perhaps debilitation (bridge closure) would result. This analogy might be a little “clinical”, but it illustrates the circumstances. The best remedy would be to avoid the cut in the first place.
There are of course limits on the amount of thermal movement that can be accommodated by jointless bridges either at piers or at integral abutments. Large thermal deflections and forces on stiff pier columns can be reduced by using expansion bearings. Integral abutments can accommodate movements of 2 inches or more. The use of jointless bridges with integral abutments has proven over many years to be an excellent strategy to help preserve bridges from the ravages of salt induced corrosion damage. Not only are jointless bridges effective, they are more economical and provide several inherent design advantages.