Dimensional Tolerances in Construction and for Surface Accessibility

Compilation of Instruments and Measurement Methods for Surface Accessibility

During the course of this project the following information was developed as background information to assist industry in developing their guidelines and standards.

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.

Some existing measurement protocols

Currently there are no generally accepted measurement protocols for determining the slope, flatness, or waviness of floor surfaces in terms of accessibility. Some people use spot elevations, some use a 10 - foot straightedge, some use a 2 - foot digital inclinometer, while others may use special devices that measure slopes in 1 - foot increments. ASTM E1155 and ASTM E1486 do prescribe the methods for determining floor flatness and waviness using the F - number system and waviness index, but these standards are not mandated for testing for accessibility, nor are there any standards for accessibility that can be tested other than overall slope of ramps or local slope. There is also no accepted standard method for using a digital inclinometer, lasers, or other surveying equipment to measure ramp slope and cross slope.

Some of the currently available methods of measuring flatness and slope are outlined below. Some of these methods are standardized and others are suggested methods.

10 - foot straightedge method
This is the classic method for determining the flatness of concrete and other types of finished floors. When used as a specification or requirement, a maximum deviation in length under the straightedge is given, such as "no point shall exceed 1/4 inch under a 10 - foot straightedge." However, there is no standardized protocol to measure deviations in lengths less than ten feet. The same problem exists when measuring slope. There is no way to measure a localized slope that may actually be greater than the overall slope of the straightedge.

The 10 - foot straightedge method has been a standard in ACI - 117, Standard Tolerances for Concrete Construction, for many years and is optional as part of ACI 117. When the 10 - foot straightedge method is used for random traffic floors, ACI 117 sets the minimum number of samples required at 0.01 times the floor area, measured in square feet. For ramps ACI 117 refers to RMS Levelness tolerance as defined in paragraph 4.11 in ASTM E - 1486.

Department of Justice method
In the tips and techniques section of "Survey Tools and Techniques," (www.usdoj.gov/crt/ada/ckstools.htm) the checklist states that slopes can be measured in three ways: with a land survey to shoot grades, using a digital level, or using a 24 - inch long builders level and tape measure.

The directions for using a level are as follows. "Using a builders level, place the level on the pavement at the steepest point parallel to the direction of the slope. While holding the uphill end of the level on the pavement, place a pencil under the other end and roll it toward the uphill end of the level until the horizontal air bubble shows level. Use the tape measure and measure the open gap at the downhill end of the level to establish the critical dimension. For a 1:50 slope this is 1/2 inch; for a 1:20 slope it is approximately 1 - 1/4 inch and for a 1:12 slope it is 2 inches."

F - number system
The F - number system, ASTM E 1155, Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers, was developed primarily to aid in the construction of superflat industrial floors. When tolerances are specified using the F - number system both the overall levelness and the flatness can be defined. The flatness number also gives an indication of the "bumpiness" of the surface.

The F - number system develops two number ratings, the FF and the FL. The FF defines the maximum floor curvature ed over a 24 - inch (600 mm) length computed on the basis of successive 12 - inch (300 mm) elevation differentials. The FL defines the relative conformity of the floor surface to a horizontal plane as measured over a 10 - foot (3.05 m) distance. Statistical sampling procedures are used to determine a floor’s [sic] F - numbers. F - numbers are reported as two numbers such as FF30/FL24. The higher the number, the flatter and more level the floor.

There are several methods given in ASTM E 1155 that can be used to measure a floor and develop the F - numbers. However, in practical terms, sophisticated electronic measuring devices developed specifically for this purpose are used. They are expensive and require some amount of training or a testing agency can be employed.

Although direct equivalents are not appropriate, an FF of 25 approximately correlates to a ¼ inch variation under an unleveled 10 - ft straightedge. An FF of 50 approximately correlates to a 1/8 - inch variation under a 10 - ft straightedge. Other approximate correlations are shown in the following table.

Concerning issues of vibration and rollability, the F - number system is probably a better measure than the straightedge because it takes into account local variations of flatness. Although it was developed to measure level floors, it can be used measure sloped floors.

For slabs - on - grade the F - numbering system works well. However, to determine the F - number for levelness of suspended slabs, measurements must be taken within 72 hours of floor installation and before shoring and forms are removed. For elevated slabs under current standards, the specified levelness and flatness of a floor may be compromised when the floor deflects when the shoring is removed and loads applied. However, local variations that could affect vibration and rollability would probably not be affected to any significant degree by slab deflection.

Additional limitations with the F - number system are that the measurements do not cross construction joints and only come within two feet from penetrations. Construction joints as well as other types of joints can affect vibration and rollability. The F - number system is optional as part of ACI 117 - 06.

Waviness Index
The Waviness Index, ASTM E 1486, Standard Test Method for Determining Floor Tolerances Using Waviness, Wheel Path and Levelness Criteria, was developed in response to the discovery that the F number system was not particularly responsive to floor deviation wavelengths between 4 and 15 feet. FF detects floor quality for wavelengths of 1.5 to 4 feet. FL detects variations when wavelengths are from 15 feet to 80 feet. The Waviness Index provides information about flatness in the wavelength range between 1.5 feet and 20 feet, which was deemed important to measure floor flatness as required by forklift trucks.

The Waviness Index measures the bumps and dips in a floor surface as the average of deviations up or down from the mid - points of 2 - , 4 - , 6 - , 8 - , and 10 - foot chords. In addition to providing a single Waviness Index number, the measurement method can also provide a computer - simulated deviation from a 10 - foot straightedge. This gives similar values as using a straightedge manually, but with the advantages of following a defined profile line according to the procedures in ASTM E 1486 and using an instrument that more accurately measures deviations.

As with the F - number system, determining the Waviness Index can be performed in a variety of ways, but practically, a sophisticated instrument must be used along with computer software that performs the calculations and reporting. The test method does NOT apply to clay or concrete unit pavers. The Waviness Index method is also optional as part of ACI 117 - 06.

Original research for ANSI A117.1 (1957 - 1961)
In the original research for ANSI A117.1 - 1961, American National Standard Specifications for Making Buildings and Facilities Accessible to, and Usable by, the Physically Handicapped, ramps were assessed for flatness based on measuring 18 - inch increments on both edges of the ramped surface. Both slope and cross slope were measured. These guidelines for measurement were deleted from later versions of the standard.

Unified Facilities Guide Specifications

In Section 02752, Portland Cement Concrete Pavement for Roads and Site Facilities, published by the National Institute of Building Sciences, it is stated that surfaces should be tested with a 4 - meter (12 ft) straightedge in both a longitudinal and transverse direction on parallel lines approximately 4.5 meters (15 ft) apart. The straightedge is to be held in contact with the surface and moved ahead one - half of the length of the straightedge for each successive measurement. The amount of surface irregularity is to be determined by placing the straightedge on the pavement surface and allowing it to rest on the two highest spots covered by its length and measuring the maximum gap between the straightedge and the pavement, in the area between the two high points.

Suggestion by Eldon Tipping
As published in Concrete Construction magazine, September 1998, Eldon Tipping made suggestions for a specification for sloped random - traffic floors such as parking decks, ramps, and other sloped surfaces, but not necessarily for accessible ramps.

Mr. Tipping suggested that each ramp should be evaluated independently as a Random - Traffic Test Surface. Slopes were to be measured with a Dipstick Floor Profiler (Face Construction Technologies) within 16 hours after completion of final finishing, and where applicable, before removal of any supporting shores. Sample measurement lines were to be parallel or perpendicular to slopes shown on the drawings. Measurement lines that were parallel to slopes were to connect elevation control points. At least two sample measurement lines were to be taken per bay and in perpendicular directions where slopes permitted. The minimum length of measurement lines perpendicular to the slope were to be one column bay. In the testing report, slope departures were to be calculated at 5 - foot overlapping intervals along each sample measurement line. His suggestions for tolerances are given later in this paper.

Suggestions by Jean Tessmer
As published in Concrete Technology Today, April 2001, Jean Tessmer, accessibility consultant for Space Options, suggested that ramp slopes be measured with a digital inclinometer mounted to measure 1 - foot increments. The line of measurement should be parallel to the long edge of the ramp. Longitudinal measurement lines should be spaced 3 feet apart, but in no case should fewer than two lines be used. For cross slopes, measurements should be taken every 6 feet.

Construction Specification Institute
While developing suggested tolerances for surface materials, the Construction Specifications Institute developed a suggested method for measuring ramp slopes. These have neither been adopted nor are they published.

CSI suggested taking measurements using a digital inclinometer with an accuracy of ±0.1 degree mounted on an aluminum beam with rotating ball joint on metal pads at 12 inches on center. For longitudinal lines, measurements were to be taken in minimum 5 - foot lengths running parallel to the long dimension, with one measurement line per 3 feet of width and within 2 feet of edges, spaced equidistant apart, with not less than two lines evaluated for each ramp.

For transverse lines, measurements were to be taken in minimum 2 - foot lengths along a line running parallel to the long dimension, with one measurement line per 6 feet of length and within 2 feet of ends, spaced equidistant apart, with not less than two lines evaluated for each ramp.