Many people are unaware that there are no industry standards for setting the weight capacity of low-bed trailers — those commonly used to transport large loads, such as precast concrete culverts, pipe, and structural bridge elements. In fact, not all trailers with the same stated capacity can safely haul the same loads. The determining factor is load concentration, which refers to how much of the trailer deck length is needed to support the weight of a load, and what form that load will take.

A 60-ton block load, for example, concentrates the weight on a short span of the trailer deck and requires maximum-strength main beams, while a 60-ton self-supporting load with two contact points that covers the length of the deck can be hauled on a trailer with smaller main beams. It's up to you to find out what type of trailer is best suited for your particular needs.

When choosing a trailer, first consider what equipment will be hauled. If, for instance, the trailer will only be used to haul a wheel loader with contact points that are 12½ feet apart, the stress on the trailer deck will be spread over its full length so a 60-ton capacity trailer of lighter construction will be sufficient.

However, if the trailer will be used to haul a variety of loads with shorter load concentrations, it must have stronger main beams. The increased strength will come at a higher price, but choosing a trailer with lighter construction can result in cracked frames and shortened trailer life — particularly if the equipment is hauled over rugged off-road terrain.

A trailer with a higher load concentration, designed to carry a block load, will benefit a customer who wants a long-lasting trailer that performs well no matter what load is carried. These trailers can cost thousands of dollars more than those designed for self-supporting loads. However, the investment will pay for itself in the longer life of the trailer, and will eliminate the expense of lost time and money due to costly breakdowns and repairs.

The trade-off is in the empty weight of the trailer. A trailer with a higher load concentration tends to be heavier because its beams are larger. Trailer manufacturers have addressed this problem by using higher-strength steel (up to 130,000 psi) and cambering the main beams to increase structural strength and to offset the additional frame deflection present in lighter deck constructions.

Before you make a final decision, it's important to spend time evaluating your hauling needs and examining the options. The bottom line: asking the right questions about trailer load concentration will help you find the trailer that works best for your equipment.

— Becky Weaver represents Rogers Brothers Corp., Albion, Pa. (


Buy, rent, or customize?

You may want to consider renting a trailer rather than purchasing, to solve the problem of having the correct trailer for differing loads - particularly if you only have occasional need to transport a block load, in which the weight will be concentrated on a short span of the trailer deck. Renting also provides a chance to evaluate different trailer brands for durability and ease of use, in case you decide to purchase one in the future.

It may sometimes be necessary to have a trailer custom-engineered for the task at hand, as was the case when the Pennsylvania Turnpike Commission decided to build twin bridges across the Susquehanna River in Harrisburg, Pa.

Completed in 2008, the bridges were the first major vehicular crossings in the state to be constructed of precast concrete segments: 57x13 feet and 8½ feet tall, weighing approximately 100 tons each.

The 100-ton capacity trailer used to transport these segments was built with main beams and side beams fabricated with 100,000 pound-per-square-inch materials to withstand 1½ years of continuously hauling bridge segments on a steep downgrade that led to the construction site.

The fixed gooseneck trailer was designed with five axles to provide adequate carrying capacity for the immense segments. The first four axles were mounted to the trailer using a heavy-duty combination spring/walking beam suspension; the final axle used an air lift suspension that allowed it to be lifted off the ground when the trailer was being transported or unloaded, for better maneuverability. The trailer's deck size was 35x10 feet with pullout outriggers on the sides to handle the extra width of the bridge segments.

Many trailer manufacturers are willing to work with customers to engineer a trailer to meet such specific project needs. PW