Dimensional Tolerances in Construction and for Surface Accessibility

Part I

Summary of the project

Throughout the course of the project representatives from various trade organizations that represent the following materials that may be used for exterior surfaces were invited and encouraged to participate to develop industry standard tolerances, installation methods, and measurement protocols unique to the individual materials their organizations represent.

• Asphalt

• Brick

• Concrete (poured)

• Concrete pavers

• Metal (solid and grating)

• Stone

• Tile

• Wood (dimensional lumber and panel surfaces)

In addition, the following professional organizations and private companies were invited to participate to develop best practices for design and construction.

• American Institute of Architects

• Construction Specifications Institute

• Master specification writing companies

Other interested parties were also involved in the project including contractors, building owners and managers, a landscape architect, a rehabilitation researcher, and representatives from the U.S. Army Corps of Engineers, the National Institute of Standards and Technology, and the National Institute of Building Sciences.

There was a two - phase process to discuss issues, share knowledge and information, and set direction for future work for individual industries and trade groups. Phase 1 was completed in spring, 2007 in a workshop that brought together 14 representatives of users of tolerances data - - design professionals, contractors, code officials, facility providers, and others. During this workshop (www.access - board.gov/news/tolerances - workshop.htm) the following conclusions were established:

a) measurement protocols for exterior walkways must be appropriate to need, material, and construction methodology;
b) rollability measures for walkway smoothness, flatness, and planarity may be adapted from roadway standards, although this may be problematic;
c) the use of the F - number system in exterior surface measurement may not be appropriate;
d) the effect of different tolerances for different walkway materials is a concern;
e) accumulated tolerances must be addressed;
f) repair, alterations, and maintenance of exterior surfaces are also issues;
g) training, availability, and skill levels of construction workers must be considered;
h) contractors and others responsible for compliance want a more clearly defined set of requirements.

Phase 2 of the project focused on the following:

1. to work with trade associations to encourage and support them in the development of materials - specific tolerances;

2. to encourage organizations such as the AIA, CSI, and master specification providers to develop and communicate "best practices" for the design, construction documentation, and specification of tolerance and measurement information for walking and rolling surfaces covered by accessibility standards;

3. to continue to coordinate with ACI on their efforts, already underway, to address tolerance standards and tolerance compatibility;

4. to pursue the development of measurement protocols for ramps and walkways in a range of surface materials; and

5. using information gained from this phase, to expand our efforts to include the development of industry tolerances beyond surfaces, where they currently do not exist.

Of the industries that were asked to participate only the Interlocking Concrete Pavement Institute (ICPA) and the Tile Council of North America (TCNA) developed and published guidelines for tolerances related to accessible surfaces. The Brick Industry Association is currently working to develop their standards. The American Concrete Institute (ACI) began development of guidelines as part of another document the 117 Committee (Tolerances) was working on but these guidelines were subsequently deleted from the document during its development. These efforts are summarized below and copies of the documents included in Part 3 of this report.

Throughout the course of the project it was very difficult to engage many of the trade groups. It appeared there could be several reasons for this. 

First, trade and professional groups may be reluctant to develop standards related to accessibility that could have legal consequences. 

Second, trade groups have certain priorities, such as the promotion of a specific material and technical assistance to contractors, architects, and other industry members. With limited staff and funding, they may not be willing or able to take on additional standards development. They may also feel accessibility standards are not within their realm of responsibility or interest.

Third, the typical structure of a trade group makes it difficult to complete standards development. Trade organizations vary in their size, membership, funding, and type of full - time administration support. Most trade associations rely on volunteer participation of their membership to develop standards and produce publications, which takes a significant amount of (generally unpaid) time. Leadership also varies, ranging from members elected to short - term positions (a president, for example) to full - time (and long term) administrators with extensive office support. A long - term project often lacks a continuity of leadership and commitment, especially in smaller organizations. A limited budget may also prelude participation. 

Current industry standard practices resulting from this project

Interlocking Concrete Pavement Institute (ICPI)
The ICPI, along with the Brick Industry Association and the National Concrete Masonry Association, funded research in 2002 to evaluate the vibration exposure during electric and manual wheelchair propulsion over several types of sidewalk surfaces. Studies were repeated in 2003 and 2004 to determine the effects of environmental factors and evaluate additional surface types. The research was conducted by the University of Pittsburgh Human Engineering Research Laboratory and was based on exposure limits of vibration to the human body established in ISO 2631, Evaluation of Human Exposure to Whole - Body Vibration.

Based on this research, it was determined that some types of pavers (both square edged and beveled) and installation methods were an acceptable type of exterior paving. With the encouragement of the Access Board project, ICPI developed recommendations for segmental concrete pavement and installation tolerances based on the research they funded. 

Refer to the ICPI document in Part 3 for the detailed recommendations and construction tolerances that were developed.

Brick Industry Association (BIA)
The BIA participated in the funding of the research conducted at the University of Pittsburgh and two brick paving patterns were tested along with the concrete pavers. The BIA agreed to develop surface tolerances as part of the Access Board project. These are currently under development.

Tile Council of North America (TCNA)
The TCNA participated in this project and developed guidelines for ramp slopes, changes in level, flatness and lippage. These were published in the 2009 edition of the TCA Handbook for Ceramic Tile Installation. Refer to the TCNA document in Part 3 for the full text of the guidelines. 

Areas for further work

In the process of assisting the various organizations that represent surface materials and professional groups the following issues were identified as parameters that might be considered in the development of any industry standards. Of course, it was not expected that all of these would be included in any one industry's standards, but they do indicate the range and complexity of issues related to construction tolerances and accessibility, both with surfaces and any other aspect of accessible construction. This list may be useful for any future work.

Design issues:

• Existing industry tolerances

• Units of measure

• Measurement instruments

• Accuracy of instruments/measurement uncertainty

• Use of significant figures

• Metric conversions or dual unit standards

• Measurement of dimensions that involve two or more trades/materials

Construction issues:

• How/where to take measurements/precision of measurement

• Accuracy of construction/tolerances for individual materials

• Cost/time implications

• Influence of accepted local practices for construction

• Inspection/measurement protocols

• Effects of weather, such as curing and freeze - and - thaw on outdoor surfaces

• Maintenance/durability of surfaces

• Workforce training

Usability issues:

• Planarity

• Maneuverability

• Rollability/rolling

• resistance

• Jointed surfaces/vibration

• Cross slope

• Gaps

• Flatness lippage

• Slip resistance

In the end, most of these issues were not considered by the trade and material organizations that participated in this study or reference was simply made back to the ADA/ABA Guidelines. This may be due the time and effort required to consider them all or to the reluctance of an organization to commit to the development of standards that could have legal consequences.