Legal Load Limits, Overweight Loads and Pavements and Bridges

and increase e/ ciency.<br><br> Competitio n and changing market conditions continue to put pressure on /reight-dependent industries to lower costs and increase service quality. Transp ortation costs and fexibility /or load size can have a signi cant e//ect on economic sustainability, particularly /or heavy/bulk commodities a nd highly priced sensitive goods, such as agriculture, lumber/timber, construction, etc. It 9s important to us and to the economic vitality o/ the state that we maintain an e/ cient /reight transportation system and s upport /reight- dependent industries.<br><br> WSDOT 9s mission is to keep people and business moving by operating and improving the state 9s transportati on systems vital to taxpayers and communities. To do this, we must manage the resources entrusted to us /or the highest possible return o/ value and to protect the citizens 9 investment in Washington 9s transportation in/rastructure. It is vital that WSDOT, decision makers and the public understand the trade-o//s between economic bene t and increased in/rastructure costs that occur when considering increasing loa d limits.<br><br> Allowable load limits have increased over the years and bridges designed /or lighter loads have not been strengthened. Partly because o/ repeated heavy loads, many bridges are showing signs o/ structural distress. Signs o/ distress include /atigue cracking o/ str uctural steel (Figure 9) and corrosion o/ the rein/orcing steel in concrete members (Figure 10).<br><br> This can lead to bridge closure or collapse (Figures 11 and Figure 12). Figure 9. Fatigue Crack in Structural Steel Beam (I-5, Nisqually River Bridge) Figure 10.<br><br> Corroding Rein/orcing Steel Bars in Concrete Beam (SR-2, Ebey Island Viaduct) Figure 11. Bridge Failure due to Truck Loading (old SR-106, Skokomish River, circa 1984) Figure 12. Bridge Failure due to Truck Loading (old SR-106, Skokomish River, circa 1984) To maintain sa/e bridges /or everyone, WSDOT engineers need to reduce allowable loads as bridges su//er /rom /atigue damage.<br><br> A bridge with /atigue damage may rst be cload restricted d, making it illegal /or any overloaded truck to use the bridge. As the /atigue damage progresses, the bridge 9s capacity to carry heavy loads decreases and the bridge is cload posted d. This restricts the allowable weight o/ trucks below typical legal weight limits.<br><br> The nal step /or a seriously /atigued bridge is total closure. Since bridges have a limited load capacity, increasing the maximum allowable load would accelerate the need /or load restrictions on a//ected bridges as they become /atigued. These bridges could also need to be entirely closed, decreasing mobility o/ people and business.<br><br> Alternatively, the bridges would need to be strengthened to accommodate heavier loads or replaced be/ore the expected li/e o/ the structure; both would require signi cant costs dependent on the a//ected structure. What are the choices and related costs for bridges? Concrete and structural steel bridges exposed to overweight loads, or increased legal load limits, most o/ten su//er /rom /atigue.<br><br> Fatigue results /rom repetitive stress, much like bending a paper clip back and /orth repeatedly, eventually the metal /atigues and breaks. Structural steel /atigue cracks continue until the carrying capacity o/ the a//ected structure is reduced to the point that it will no longer support a load. In steel rein/orced concrete bridges, /atigue cracks the concrete and allows water or other contaminants to a//ect the steel rein/orcing bars.<br><br> The bars corrode and cause expansion, which breaks o// the concrete cover and creates more exposure /or corrosion. This process continues until the carrying capacity is reduced to the point that the bridge can no longer support a load. The heavier and more /requent the loads, the /aster these /atigue cracks will grow in size and length.<br><br> Bridges consist o/ several di//erent structural elements, combining together to /orm the complete bridge. Loads greater than the current legal loads a//ect these structural elements in di//erent ways. Bridge decks must trans/er the wheel load to the main support beams, which in turn trans/er the load to the /oundation supports.<br><br> Each o/ these elements can experience /atigue and /atigue damage /rom larger than legal loads (Figures 6, 7 and 8). Figure 7. Beam cBending d Failure Figure 8.<br><br> Beam Shear Failure What happens to bridges exposed to loads that they were not designed to handle? Figure 6. Concrete Roadway Deck Section and Fatigue Location Load limits restrict how much weight can be carried on an axle, a single tire or pair o/ tires, and on the vehicle or vehicle combination in total.<br><br> Concerns over the impacts o/ tire load and gross vehicle weight on a /ragile in/rastructure were rst addressed in the 1913 and 1915 Legislative sessions, respectively. Tire loads began at 400 pounds per inch width o/ tire and a gross vehicle weight limit was established at 24,000 pounds. In almost every subsequent legislative session, through 1975, load limits have been re ned to address changes in in/rastructure design and observed e//ects o/ vehicle loads.<br><br> In 1975, /ederal laws were implemented to provide protection to the highway in/rastructure and uni/ormity among the states /or interstate use. The Washington State Legislature adopted the /ederal weight limits /or all state highways. What are weight limits and how are they established?<br><br> Tire and axle limits are imposed /or a number o/ reasons; /oremost, is to ensure that loads carried by trucks are transported sa/ely. Having de ned load limits allows engineers to design pavements that will hold up under anticipated truck tra/ c with minimal maintenance required /or xing cracks, ruts, and potholes. Load limits are also necessary /or protecting bridges /rom structural weakening or /atigue, preventing unsa/e conditions and early replacement o/ bridge structures.<br><br> Current in/ormation shows that even slight changes in load limits have major impacts on pavement and bridge per/ormance. Both the axle and tire load a//ect pavements and bridges. Total axle loads a//ect large areas o/ a pavement or a bridge, while tire loads a//ect smaller, more localized areas.<br><br> Narrow width tires concentrate the vehicle 9s weight on a small area, while wider width tires distribute the weight over a larger area and cause less stress on a single spot. As the total load carried by an axle increases, so does the total load on the pavement or bridge. An axle carrying 20,000 pounds puts the same total weight on a bridge or a pavement whether 6-inch wide or 12-inch wide tires are used.<br><br> The total load may cause damage or /ailure, even i/ the local point stresses under the tires are not large. Why are weight limits placed on axles and tires? Photo: SR 153 Note: The American Association o/ State and Highway Transportation O/ cials (AASHTO) designates standard truck loadings /or designi ng highway structures.<br><br> Type cH d is /or Highway Truck, cHS d /or Highway semi-trailer and cHL d /or Highway Lane loading. What are the current tire and axle load limits? Loads are typically de ned according to the type o/ axle as well as the number o/ tires per axle.<br><br> Legal load limits /or the var ious axle con gurations in Washington State are shown in Figure 1 and Table 1. Maintenance varies by pavement type: " The surface of asphalt pavements wears out and needs to be replaced on a regular cycle, about every 15 years, but the pavement below the worn sur/ace remains. Replacing just the sur/ace is much less expensive than replacing the /ull depth o/ the pavement structure.<br><br> " Concrete pavements are designed to handle the weight o/ legal loads and last /or up to 50 years. These pavements need to be cground d smooth about every 25 years to remove wear caused by studded tires. This is much less expensive than replacing cracked and broken concrete.<br><br> Washington State has an inventory of 2,975 bridges that have a length of 20 feet or more: " 16 percent are designed for trucks (H10, H15, H20) weighing 20 tons (40,000 lbs) or less. " 6 percent are designed for trucks (HS15) weighing 27 tons (54,000 lbs). " 70 percent are designed for trucks (HS20) weighing 36 tons (72,000 lbs).<br><br> " 8 percent are designed for trucks (HS25 or HL93) weighing more than 36 tons (72,000 lbs). Most o/ the current state highway system, and all new state highways, are designed using these load limits. Some o/ the older highways were not built to current design standards and require work to upgrade to today 9s standards.<br><br> As designed, these highways can w ithstand current legal loads without damaging the pavement structure. Figure 1. Tire-axle combinations Table 1.<br><br> Current Washington State tire and axle load limits Tire / Axle Limit Tire Load 600 lb/inch of tire width Single Axle 20,000 lbs Single Axle with two tires (carrying more than 10,000 lbs) 500 lb/inch of tire width Tandem Axle 34,000 lbs Gross Vehicle Weight 105,500 lbs Single Axle & Single Tires Single Axle & Dual Tires Tandem Axles & Single Tires Tandem Axles & Dual Tires Increasing the axle load on a dual single tire axle by 10,000 pounds would result in an immediate need o/ $900 million to ensur e additional pavement damage does not occur. Under the same scenario, an increase o/ 10,000 pounds on the dual tandem axle would require ju st under $500 million. There would also be associated societal costs and inconveniences due to the closure o/ highways /or repairs or u pgrades.<br><br> I/ legal load limits are increased, there are two basic choices /or preserving current pavement in/rastructure: " Spend more money to handle the increased load by increasing the pavement thickness o/ existing roadways and changing designs /or new pavement, or " Spend more money repairing the damage caused by the increased loads. What are the choices and related costs for pavements? For asphalt pavements, which constitute the majority o/ roadways in Washington State, WSDOT calculates that the increased pavem ent depth needed to handle larger truckloads are as shown in Figure 4 (dual single axle) and Figure 5 (dual tandem axle).<br><br> Figure 4. Impact o/ Single Dual Axle Load Increase on Pavement Thickness Figure 5. Impact o/ Tandem Dual Axle Load Increase on Pavement Thickness Figure 2.<br><br> Cracking due to Heavy Loads (SR-532 near I-5) Figure 3 . Rutting due to Heavy Loads (I-90, Spokane, Weigh Scale) Repeated overweight loads, or an increased number o/ legal loads, damage asphalt pavements by overstressing the pavement structure, causing cracking and eventually potholes. Concrete pavements also break and crack under repeated overweight loads, or an increased number o/ legal loads, making them rough and decreasing the li/e o/ the pavement.<br><br> The relationship between axle weight and pavement damage is not linear, but exponential. For example, a single axle loaded to 40,000 lbs (twice the legal load) causes 16 times more damage than a single axle legally loaded to 20,000 lbs. Many highway pavements around the state do not have su/ cient thickness to carry heavy loads and without load restrictions would su//er pavement /ailures, as shown in Figure 2 and Figure 3.<br><br> Unlike bridges, a single overloaded truck rarely causes a spectacular pavement /ailure. Many repetitions o/ trucks beyond the current legal load limit, or above the original estimated number o/ trucks, must occur be/ore you see damage in the /orm o/ extensive pavement cracking or potholes. Un/ortunately, by the time this damage is visible the pavement structure may have been damaged to the point where it must be replaced 3 which is an expensive and time consuming process.<br><br> What happens to pavements exposed to loads they were not designed to handle?

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