Super Heavy Load Analysis

Superheavy loads are defined as loads that exceed 254,300-lbs. gross vehicle weight, exceed the maximum permissible weight on any axle or axle group, or exceed 200,000 pounds with less than 95 feet of axle spacing. However, these loads have approached 2,000,000 lbs. gross. Due to the magnitude of these loads, there is a risk of causing severe pavement damage in one pass. Typically, movement of loads of this magnitude must be permitted through appropriate State agencies. The number of these permitted loads has increased dramatically in recent years; empirical information obtained during field studies of superheavy load moves provides the basis for screening criteria of 500,000 lb GVW / 6,000 lb transporter tire load to conduct more detailed analyses. If a superheavy load exceeds 500,000 lbs. regardless of tranporter tire load, or if any superheavy load has a transporter tire load exceeding 6,000 lbs. regardless of GVW, an analysis of the superheavy load route should be conducted to determine potential for load related damage. The permitting agency should notify appropriate engineering staff of the proposed move. Notification should include proposed movement dates, routes, and load configuration diagrams. The engineer will provide recommendations on the proposed route, possible alternate routes, or methods to minimize potential damage (e.g., add axles to reduce wheel loads, mat pavement surface, etc.).

Figure 1. The general relationship in Gross Vehicle Weight and increased transporter tire load based on loading diagrams. Super heavy loads with GVW above 500,000 lbs and/or transporter tire load above 6,000 lbs. should be analyzed by a licensed engineer.

The GVW load limit screening criteria was established based on the observed occurrences of pavement distress on Texas highways , tempered with practical limitations (time, manpower) regarding the number of analyses that can be performed. Emphasis has been placed on analyzing superheavy loads with a greater potential for pavement damage. As depicted in Figure 1, the general observed trend is that as the gross vehicle weight of the superheavy load increases, the load per tire increases. A study of movement configurations shows that it is uncommon to have transport vehicle tire load exceeding 6,000 lb. when the GVW is less than 500,000 lbs.

Figure 2. This figure shows the stress distribution within a flexible pavement structure for a low and a high tire load.

Flexible pavements typically are constructed with higher quality, stiffer materials over weaker materials at lower levels within the pavement structure. If these weaker materials are subjected to high stresses due to high tire loads, there is a greater chance for permanent damage in the form of rutting or cracking. As can be seen in Figure 2, as tire load increases, the stress within the pavement structure increases throughout the structure including the lower levels. For this reason, higher tire loads (lower threshold of 6,000 lbs) are evaluated for superheavy loads regardless of the gross vehicle weight.

As mentioned previously, the transport vehicle wheel load threshold of 6,000 lbs was established based on empirical information related to observed distress occurring during superheavy load moves in Texas. The 6,000 lbs threshold approximates the manufacturer’s recommended highest single tire load limit (observed to date) for standard truck tires used on superheavy loads. Although a superheavy load with a standard single truck tire loaded to its capacity has yet to be observed, this arrangement could pose a high potential for damage, especially on thinner pavements. Also, as tire loads increase above 6,000 lbs., super single tires may be employed by the carrier since these tires are rated to carry maximum loads exceeding 9,000 lbs. Super single tires have been shown to cause more damage to a pavement than standard dual truck tires based on tests conducted by the FHWA and CALTRANS.

Superheavy loads can damage (by peeling up) pavement surfaces that have been overlaid or chip sealed for less than 5 weeks.

The exact date and time of the superheavy load movement is difficult to know due to numerous factors. Agency personnel may take the proactive role to contact the carrier to obtain the dates of movement.

It’s the responsibility of the carrier (permit holder) to check their total load and wheel load. The axle information should be provided on the axle load diagram sent to the Permit Coordinator along with a copy of the permit. If required, an engineer or maintenance personnel can check the actual axle configuration to ensure that the load matches the permit. Only state or local law enforcement are authorized to verify individual tire loads. The permitting authority should be the point of contact that notifies law enforcement when a load needs to be weighed.

References

Original article content and pictures contributed by TxDOT.