Basic Structural Analysis

But before any repairs begin, in-shop or out, a thorough analysis of the vehicle's damage must be completed.

Terms of Reference

To properly analyze structural damage, a few terms and processes are required. When measuring a board, a carpenter starts at one end and measures to the other end. A geometry teacher would call this a two-dimensional measurement. To measure the volume of a packing box, three measurements are taken: length, width and height. These are two-dimensional measurements, but when used in combination they describe the box well. It is understood that the shape is a uniform rectangle.

The system we use works much like when you measure a box. The difference here is that each measurement has a reference point.

The first item measured is height. This is measured up from a datum plane, the term for the imaginary plane under the vehicle from which all height measurements are taken. This arbitrary plane is needed to provide a common reference to measure from any of the irregular points along the vehicle. Datum is the first line drawn on the designer's drafting table. An early example of datum from outside of the automotive industry is the 1929 National Geodetic Vertical Datum, which was established so that an absolute measurement of sea level could be made anywhere on the coasts of the United States or Canada.

Length must also be referenced to a common point. On ships and aircraft, this is also an arbitrary line, often in front of the vehicle. Automotive measurements, however, are typically taken from factory control points. Control points are places used to hold parts in place as the vehicle is constructed. They control the accuracy of the structure. The control points used for measuring the zero point on unibody vehicles are usually located in the area of the torque boxes, front and rear. All length measurements are taken forward or rearward. Width is measured from the center of the vehicle. Both length and centerline width measurements are taken parallel to the datum.

The process of measuring a vehicle starts in the center of the vehicle. Just as construction of a building starts with a foundation, manufacturing a vehicle starts with the frame. For full frame vehicles, this is literally the frame. On unibody vehicles, this is the center section of the vehicle followed by the frame rails, which are attached to the center section.

When measuring a vehicle, the center of the vehicle should be measured first. This establishes the datum to which all measurements are referenced. It also verifies that the center of the vehicle is undamaged. With the datum established, length and width measurements can be taken parallel to it. Build tolerance must also be considered. Vehicles are manufactured within a certain tolerance. For many years, a common build tolerance was 3 mm. I-CAR's Uniform Procedures for Collision Repair (UPCR) call for following manufacturer recommendations on tolerance. If no tolerance recommendations are given, then follow plus or minus 5 mm. (3¼16-in.).

As manufacturing technology has improved, many manufacturers build to a zero tolerance. This means that any of the control points must be precisely positioned during repairs. No longer can the fit of the sheet metal be used as an accurate measure of the repairs.

Comparative and Absolute

How exact does a measurement need to be? Determining if any damage is present that will need to be repaired requires only a relatively crude measurement. Installing a frame rail necessitates a precise set of measurements. Different tools are needed for each. Structural analysis begins when the first person looks at the vehicle after the collision and ends when the final measurement is taken verifying that all repairs are complete. The initial estimate should indicate at least that structural damage is present and whenever it will need attention or not. This can be viewed as a go/no-go gauge. Simple visual indicators may be all that are needed at this stage. Do the doors, hood, and deck lid fit? Are the wheels all in about the right place in the wheel well? Is the pickup bed aligned with the cab? Taking a few simple measurements helps to verify damage. Comparing the "X" measurement under the hood will indicate about how much the front rails are moved. This measurement is not an absolute measurement taken off published dimension guides, but only a comparative measurement. Comparing measurements up from a flat floor to the molding or bodyline on the bumper can be an indication of structural parts being up or down. Precise measurements can only be produced with precise tools and dimension guides.

The first level of more precise measurement is absolute point-to-point measurements. Published in various dimension guides, and some crash guides, point-to-point measurements are used to establish whether a part has moved. Common sense and visual indicators can help determine how a part has moved, but a point-to-point measurement will only indicate one direction. A length measurement along a frame rail that is greater than it should be certainly indicates that the rail has moved either forward or upward. Since it is all but impossible for a rail to move forward, it is safe to deduce that the rail is up. How much, if any, that rail has moved to the side requires a different measurement. In addition, that measurement may well be affected by the change in height. At one point in the development of collision repair processes, this was the best tool available. These measurements can produce reasonably good results, but much time is spent re-measuring the same places to build a complete picture.

Three axes measuring is a much more precise technique. Datum is established and height is measured up from that. Length is measured along the datum to various points along the structure. Control points used in the construction of the vehicle are used, as well as reference points, which are additional points determined by the dimension chart provider. Width is also measured along the datum, from the center of the vehicle. The tools used to do this can be mechanical, such as the early centerline gauges, which established datum and centerline, but required a tape measure to get length measurements.

Mechanical, three-axis measuring systems with sliding components and pointers to take the height, length and width were developed later. Systems using lasers and sound waves are the current state of the art. Some of these incorporate computer readouts as well.

Editors Note: This is the first of a two-part series. Next month, "Advanced Structural Analysis," will examine how these tools are used to critically analyze structural damage before and during the repair process.