Most trucks are built on a ladder-type frame that includes side rails and cross members. These components come into play while the truck is moving and they are subjected to three types of physical forces: vertical, torsional, and sideways.
Frame side rails provide support for vertical and side loads, such as the engine, transmission, fuel tanks, battery box, suspension, body and cab, cargo box and all that stuff that is in it.
The frame cross member provides rigidity against torsional, or twisting, forces. Cross members also provide support for the engine, transmission, and radiator. They also prevent the side rails from twisting with side loads like the fuel tank and battery box.
An article in Trailer-Body Builders notes that when a frame section is supported at each end and is properly loaded in the middle, the top is under compression and the bottom under tension. There is also a neutral axis where the frame is not stressed.
For purposes of identification, the vertical section of a frame rail is called the channel or web while the horizontal section is called the flange and it can be either the top or bottom of the rail or both. In a frame rail cross-section, the central area is called the centroid and the shear center is that point of a frame rail that can support a vertical load without collapsing.
Resist Bend Moment
Weight is considered a force when applied to the frame. Multiplying the weight times the distance from one point on the frame to another is used to determine the resist bend moment (RBM) and can be used to determine proper load positioning and the correct location for attachments or mounts. On most cargo trucks, the high-moment point is directly behind the cab while on a semi-tractor, for example, the RBM is closer to the forward drive axle.
When repairing a truck frame, take care to avoid bending, scratching, or otherwise damaging the exposed frame while lifting or moving. Cushioning hoists or slings with a rubber hose or similar material against the frame is advised.
Drilling holes or welding along the neutral axis will not affect frame strength to a great degree. However, frame manufacturers restrict and advise service techs against drilling holes or welding near the flanges because that could lead to frame failure. Likewise, not more than two vertically aligned holes should be drilled in the channel while cuts and welds should always be on a diagonal.