Dimensional Tolerance Guidelines from the US Access Board

Previously on the ArchBarrierBlog, I shocked the industry by writing about dimensional tolerances: New Accessibility Standards, Part 2: Dimensions

Now, the Access-Board has released a new report titled “Initiative on Dimensional Tolerances in Construction Dimensional Tolerances for Surface” and prepared by the amazing David Kent Ballast, author of noir thrillers such as the ARE Review Manual.

Personally, I credit Mr. Ballast for helping me to pass the ARE on my first attempt.

SPOILER ALERT: If you are a hardcore fan of technical documents , stop reading now.

Although, I haven’t read the whole document cover to cover here is the spoiler:

1.2 Suggested tolerances

1.2.1 Walks and other non – ramp surfaces. When overall running slope for walks is measured according to Section 1.1.3 a recommended tolerance for running slope is +1%. When overall cross slope for sidewalks is measured according to 1.1.4 a recommended tolerance for cross slope is +0.5%.

1.2.2 When flatness of running slope for an accessible surface other than a ramp is measured according to Section 1.1.5 no more than 20% (rounded to the nearest whole number) of the measurements should exceed ±1/4 in. in 10 ft (±6 mm in 3 m). When flatness of cross slope for an accessible surface other than a ramp is measured according to Section 1.1.6 at least 80% (rounded to the nearest whole number) of the measurements should not exceed a 2% slope. The remaining measurements should not exceed a 2.5% slope.

1.2.3 Landings. Both measurements of ramp landings as described in Section 1.1.15 should not exceed a plus tolerance of 0.5%.

1.2.4 When local horizontal discontinuities and vertical alignments are measured according to Section 1.1.9 a recommended tolerance is ±1/8 in. (3 mm).

1.2.5 Ramps. When overall running slope and cross slope for accessible ramps are measured according to Sections 1.1.11 a recommended tolerance for these slopes is +0.5%.

In the ideal case, planning for a 7.5% running slope allows for construction inaccuracies while still maintaining the required 1:12 slope. However, when a design slope of 1:12 is indicated a tolerance of +0.5% is reasonable.

Many accessibility experts consider a 2% cross slope to be the maximum. However, there is conflicting research concerning the need to have a 2% maximum cross slope and that the actual maximum depends on user type (wheelchair, walker, cane, etc.), length of travel, and other variables. It seems reasonable to allow a +0.5% tolerance for ramp slopes and cross slopes.

1.2.6 When local variations (flatness) in running slope of ramps are measured according to 1.1.13 at least 80% (rounded to the nearest whole number) of the measurements should not exceed an 8.3% slope. The remaining measurements should not exceed a 10% slope.

Allowing a small percentage of localized slopes to exceed 8.3% is based on the allowable slopes in ADA/ABA – AG (2004) for existing buildings of 1:8 (12.5%) for maximum rises of 3 inches and 1:10 (10%) for maximum rises of 6 inches. The 1980 ANSI A117 standard also allowed this with the additional provision that an existing ramp slope of up to 1:8 could have a maximum run of 2 feet (0.6 m). Allowing 20% of local variations to slope up to 10% seems reasonable for a distance of one foot. This would mean that localized dips and high points in a 2 – foot distance would be about ¼ in. (6 mm) or a little less.

1.2.7 When local variations (flatness) for cross slope of ramps are measured according to 1.1.14 at least 80% (rounded to the nearest whole number) should not exceed a 2% slope. The remaining measurements should not exceed a 2.5% cross slope. When four or fewer measurements are made, only one should not exceed a 2.5% cross slope, while the others should not exceed a 2% slope

1.2.8 Exterior stairs, cast – in – place. When cast – in – place exterior stairs are measured according to Section 1.1.17 the requirements of the local building code shall govern tolerances.

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