Dimensional Tolerances in Construction and for Surface Accessibility

Measuring instruments and accuracy

There are many instruments that are currently available for measuring distances and angles as well as the surface roughness of accessible elements. These range from inexpensive, moderately accurate measuring tapes and carpenter's levels to extremely accurate, automated electronic devices costing tens of thousands of dollars. The problem is not a need for a good measuring instrument but an agreement on which instruments to use and a protocol for using them to check for accessibility compliance.

Generally, a measurement device should read one unit more accurate than the required tolerance reading (one more decimal place or fractional graduation).

The Precast Concrete Institute recommends that the precision of the measuring technique used to verify a dimension should be capable of reliably measuring to a precision of one - third the magnitude of the specified tolerance.

Metal measuring tapes are the most commonly used tools for measuring distances. They are inexpensive, easy to use, and are available in English or metric units. Most tapes used in construction are graduated in units of 1/16 inch or millimeters. Accuracy depends on the quality of manufacturing, how they are maintained, and correctness of use.

The National Institute of Standards and Technology publishes tolerances for metal tapes in its Handbook 44, Section 5.52.

From NIST Handbook 44, Section 5.52, p. 5 - 12.
http://ts.nist.gov/WeightsAndMeasures/h44 - 07.cfm.

The NIST Handbook 44, Section 10.3 gives the following rules for the reading of indications on graduated scales if it is desired to read or record values only to the nearest graduation. If the indicator is between two graduations, but is closer to one graduation than it is to the other, the value of the closer graduation is the one to be read or recorded. "In the case where, as nearly as can be determined, the indicator is midway between two graduations, the odd - and - even rule is invoked, and the value to be read or recorded is that of the graduation whose value is even." In most cases readings can be no more accurate than the smallest graduation.

Carpenter's levels are used for setting level and plumb only. To determine angles the level must be used with a measuring tape to determine slope. This introduces several sources of possible errors, but uses inexpensive and readily available tools.

Digital inclinometers (SmartTool®), while slightly more expensive than standard levels, are easy to calibrate and use and can measure slopes in degrees, percent, and fractions per foot. They have an accuracy of 0.1 degree and come in 2 - foot and 4 - foot lengths. The individual electronic module can also be mounted on other devices to create customized measuring instruments.

Transits and construction lasers are useful for setting or measuring overall elevation points to determine a total slope. Most construction lasers have an accuracy of ±1/16 inch in 100 feet (1.6 mm in 30.5 m) and even greater accuracies in shorter distances. While these instruments have the necessary accuracy to determine distances and elevation points, they are not as well suited for measuring local variations of slope over small distances.

Electronic instruments have been developed to measure floor flatness. Originally created to measure the flatness of concrete floors in critical applications such as narrow - aisle warehouses, these devices can be used to measure slope flatness. Their disadvantages include a high initial cost and training needed for their proper use or the employment of a testing agency. These devices were developed to more accurately and easily measure floors according to the F - number system and the waviness index, which are described later in this paper. Electronic instruments include the F - meter, Dipstick®, and FloorPro®. Other devices are listed in the appendix.

Laser scanners use laser beams to automatically develop a three - dimensional image of a space. These types of instruments could be used to measure floor flatness and level but they are very expensive, require training, and give accuracies in excess of what is needed for accessible design.

The SmartWheel is a wheelchair - mounted device for measuring propulsion parameters that can be used to compute forces, acceleration, rate of rise, velocity, stroke length, and other aspects of wheelchair use. The SmartWheel has been accepted as a measurement tool for the ASTM PS 83 - 97/F1951 Standard on Playground Surface Accessibility.

In addition to the instruments discussed above, during the course of this project the city of Bellevue, Washington developed a Segway - mounted device to survey the city's sidewalks and curbs to determine if they met accessibility standards. It used an ultra - light inertial profiler (ULIP) manufactured by Starodub, Inc. in Kengington, MD. This instrument gives a very small sampling interval of about 0.5 mm at maximum speed. Although providing very accurate data and the ability to extrapolate to longer intervals, there still needs to be a standard for slope, flatness, and smoothness at longer intervals.