the important points seem to have gone unnoticed.
I didn't write this, the South Dakota Department of Transportation did. It is something to think about, perhaps something that should be addressed sooner than later now that there may be evidence that gives this concern some credence.
http://www.state.sd.us/Applications/HR19ResearchProject...Bridges in South Dakota are designed at an inventory stress level and permitted overloads are allowed up to an operating stress level. The Department uses the load factor design method and the operating stress level allowed is five thirds of the inventory stress level. Some of these non-standard vehicles are possibly exceeding the operating stress level on some bridges but are not considered to be overloaded according to the bridge weight formula. This may be detrimental to the life of some bridges. There are approximately 1200 bridges on the State trunk highway system. The four common bridge construction types include: reinforced concrete slab, steel girder, prestressed concrete girder and trusses.
It is known that non-standard vehicles exist but it is not known to what extent. To further complicate this, vehicles having more than thirteen axles are not counted by weigh-in-motion systems and traffic counters since these systems only accommodate upto thirteen axle vehicles.
A study is needed to determine the truck configuration limits (i.e. -- restriction on gross weight, restriction on number of total axles or number of axles in a group, weight of axle groups, or a modification to the bridge weight formula) that should be set so that the life of bridges in South Dakota are not prematurely shortened. To limit the gross weight of trucks or the number of axles that can be used would require legislation. Strong evidence is needed to support any legislation recommended.
Taiwan:
http://trb.metapress.com/content/a477666106407493 /
After data collection and analysis it was found that the average truck load factor for combined heavy vehicles computed from the WIM data collection was 2.7 times higher than the original design value, which already took into account 30 percent truck overloading. It was also found that computed axle load ratios for various types of heavy vehicles were dramatically different from the ratios given in the bridge design standard specification. Bridge deck designs for a simply supported bridge were studied. It was concluded that the current bridge design standard specification will result in a 28 percent underestimation of steel volume in bridge deck design.
http://www.fhwa.dot.gov/policy/otps/truck/wusr/chap05.h...Types of Loads
Trucks affect bridges in several ways. When moving across a bridge, they produce static live loads and dynamic live loads. These loads result in the bridge experiencing bending, shear and fatigue stresses. The weight of the vehicle causes the live load stresses; its movement across the bridge, in conjunction with its weight, causes the dynamic stresses; and the movement, weight and the number of repetitions cause the fatigue stresses. When designing bridges, engineers typically increase the static load by a fixed percentage (about 10 to 30 percent) to account for the dynamic load.
Additionally, the bridge must withstand dead loads (the weight of the bridge itself, including the weight of future overlays), wind, thermal, earthquake, and other loads. The AASHTO bridge design manuals provide procedures to account for all these stresses.
Critical Stresses for Analysis
This analysis concentrates on bending moment stresses for several reasons. Generally a bridge designed to accommodate the bending moment stresses caused by the live, dead and dynamic loads, will also accommodate the fatigue and shear stresses. Thermal, wind and seismic stresses are not a function of vehicle weights and dimensions. If the bending stress is excessive, the other stresses usually are excessive as well. This is one reason that bridge replacement often is the best solution for an overstressed bridge. Another important reason is that highway agencies often must improve safety features, alignment, lighting, utilities, and other level of service characteristics if they strengthen a bridge. When costs of these other improvements is added to the cost of strengthening, total bridge replacement often is found to be more cost effective. Strengthening is possible for only some bridge types. Steel girder, some truss and even some prestressed concrete beam bridges can be economically strengthened if they meet all other stress and level of service criteria, but reinforced concrete slab and several other bridge types cannot be easily strengthened.
Bridge analysis for nationwide policy studies must rely on readily available nationwide data. The FHWA's National Bridge Inventory (NBI) is the only such dataset that meets this objective. Unfortunately, the NBI does not contain any detailed data describing the bridge geometry, location of details and the like which effectively rules out the analysis of fatigue, shear or other stresses that require this level of detailed data on the individual bridge design elements. However, the NBI does contain sufficient data describing the bridge length, support type, design type, material, etc., that permits the accurate estimation and computation of the live load and total bending moments. This is an additional reason why previous studies of national TS&W policy issues have either ignored fatigue and other less critical stresses or have handled them in a very simplified manner. But, as noted above, little is gained by considering fatigue or other stresses, since the bending stress is a reasonable proxy for all stresses.
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