Below is the OptiTruck Frame Strength Module. On the left is the single frame option, on the right is the triple frame option. The pictures update with the frame option chosen.
OptiTruck’s frame strength module provides a means for calculating the strength of the most common truck frames, using basic knowledge about the steel from which the frame is made and measurements that can be made easily with a tape measure. OptiTruck uses your measured and input data to calculate section modulus for single, double, triple and L-reinforced frames, as well as estimating the frame’s resistance to bending moment (RBM). The calculations can be used for estimating the strength of the frame in an existing chassis, and also for evaluating changes to the frame – for instance, the addition of an inner or outer wrap – to increase frame strength to adequate levels for your equipment.
Many equipment manufacturers require that the chassis on which their equipment will be installed is equipped with a frame of adequate strength. Most often, frame strength requirements are given as either section modulus accompanied by yield strength, or by Resistance to Bending Moment (RBM). The three quantities are related and once you know two, you can calculate the third.
If you know section modulus and yield strength, you can calculate RBM by multiplying the section modulus (in3) by the yield strength (psi).
Section modulus is a geometric term that describes the ability of the frame section to resist bending loads. It is only related to the frame’s dimensions, and can be easily determined by measuring a truck frame, entering the dimensions into OptiTruck and hitting the ‘Calculate’ button. OptiTruck does all of the hard work for you and gives the frame’s section modulus based on your dimensions.
The frame’s yield strength cannot be determined quite as simply. However, any truck dealer will be able to tell you the strength of the frame material used to make a particular truck’s frame rails. Yield strength is a measure of how resistant the material itself is to bending. High-strength steels are more resistant to bending than low-strength steels, given the same section shape. Common yield strengths in heavy trucks (class 7-8) are 100,000psi, 110,000psi, and sometimes even higher. In lighter trucks, lower strength steels are commonly used, ranging from 36,000psi up to 110,000psi. You must check with published data or with a qualified truck dealer to obtain this information.
A frame’s actual strength is affected by a variety of things that can happen to it after the truck leaves the factory, none of them good. Frame strength can be adversely affected by corrosion, welding to heat-treated steel frames (almost all of them are heat treated), holes drilled in inappropriate areas near the frame flanges, accidents which bend truck frames even if they are set straight again, and a host of other factors. Be wary of the things which could compromise your frame strength and take steps to compensate for them if necessary.