BOOK DETAILS

Handbook of Construction Tolerances

Handbook of Construction Tolerances

by David Kent Ballast FAIA CSI

ISBN: 9780471931515

Publisher Wiley

Published in Calendars/Architecture

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Sample Chapter


Chapter One

Building Layout and Sitework

1-1 Horizontal Building Layout

Description

One of the first sources of inaccuracies in building construction is the establishment of horizontal and vertical referencing systems for the layout of a building and subsequent marking of lines and benchmarks for horizontal and vertical dimensions as construction proceeds. Layout is dependent on the accuracy of the instruments used, as well as on environmental conditions and the skill of the people doing the layout. This section includes some of the horizontal layout accuracies possible with various instruments, assuming the surveyor uses a normal degree of skill. This section also gives accuracy standards given in three publications. Refer to sections on specific materials for industry standard tolerances for each specific material. Refer to Chapter 17 for more information on the accuracy of measuring instruments and methods of measurement.

With currently available surveying equipment, it is possible to lay out a building and monitor construction with a high degree of accuracy, in many cases much higher than is generally required for most construction. Because of this fact and because there are few standards for general building layout, the architect and engineer should clearly state in the specifications what tolerances are required, recognizing that higher degrees of accuracy generally will result in higher costs.

Allowable Tolerances

Although there are no generally accepted standards that are widely used for building layout in the United States, one international standard and one U.S. model standard may be used to gauge what is realistically possible for most building construction and layout of site elements. These standards may be used to guide the development of specifications for individual building projects or to determine a reasonable standard in the absence of specific project requirements.

International standard ISO 4463-1, Measurement Methods for Building-Setting Out and Measurement, describes procedures for establishing a survey grid, relating it to a building site, and establishing building layout and control points based on property boundaries and major survey control points. It gives guidance on measurement methods and acceptance criteria (tolerances) for various stages of the process, which includes the primary system, the secondary system, and position points. The primary system is connected to the official control system (national, municipal, or other higher-order coordinate system) and normally covers the entire site. The secondary system is that structural or other grid reference system that is used for the erection of a particular building. Position points mark the location of individual elements in the building, both horizontally and vertically. The standard gives acceptance criteria for distance measurement, angle measurement, plumbing, and the establishment of levels. The ISO acceptance criteria are shown in Figure 1-1 for secondary and position points and in Figure 1-1.1 for primary positioning. For building construction, the accuracy of individual building layout (secondary points) and position points within a building are, in most cases, more important than the exact position of the building on the site. The acceptable values in ISO 4463-1 are for general layout and not specific materials. In this standard, the tolerance level is typically given in terms of the length being measured.

For the structural grid or reference grid of a building, ISO 4463 suggests that a reasonable linear dimension tolerance can be ?4.0 mm (?3/16 in.) for distances up to 4 m (13 ft.) and ?1.5 mm for distances over 4 m (13 ft.), where L is the length in meters. For position points, ISO 4463 suggests that a reasonable tolerance can be ?3.0 mm (?1/8 in.) for distances up to 4 m (13 ft.) and ?1.5 mm for distances over 4 m (13 ft.) where L is the length in meters. These numbers are very close to the linear accuracies published by the Institute for Research in Construction (IRC) in 1975 (Latta).

For right angle layout, ISO 4463 suggests that a reasonable angular tolerance can be ?degrees, where L is the length in meters of the shorter side of the angle. In a 30.5-m (100-ft.) length, this translates to a tolerance of approximately 1 minute of angle (0.0163?). This is well within the accuracy standards of transits as well as construction lasers. Viewed in terms of an offset over a length of 30.5 m (100 ft.), ISO 4463 suggests an allowable tolerance of ?8.3 mm (?3/8 in.). Again, this is well within the capabilities of a standard transit as published by the IRC in 1975 and is easily accomplished with construction lasers.

In the United States, the National Society of Professional Surveyors (NSPS) publishes model standards that are intended to be used as guidelines for those individual state associations, professional registration boards, state surveying agencies, and others who have the authority of set standards. Section D of these model standards is for construction layout surveys and the recommended positional accuracies are given in Table 1-2 in Section 1-6.

The NSPS standard for building offset stakes is ?10 mm (0.03 ft.) for horizontal positional accuracy. Positional accuracy in this standard is given at the 95 percent confidence level. This means, for example, that if a 200-ft. (61-m) distance is measured 100 times, the measurement will be between 199.97 ft. and 200.03 ft. (60.09 m to 61.0 1m) 95 times out of 100. Refer to Chapter 18 for a discussion of expressing the uncertainty of measurement.

Construction lasers and other electronic devices can measure distances, angles, and plumb with a high degree of accuracy. The exact accuracy level depends on the specific manufacturer's device, the calibration of the equipment, the conditions under which the equipment is used, whether or not a prism is employed, and, of course, the skill and diligence of the surveyor. Refer to Chapter 17 for a discussion of electronic distance measuring devices. The numbers given in Figure 1-1 are derived from several manufacturers' product literature and represent what can reasonably be expected when these devices are used correctly under ideal conditions.

While more precise construction laser equipment is commonly used for commercial construction, many contractors still use more traditional equipment for residential and small commercial projects. The expected degree of accuracy when using steel tapes and transits is also shown in Figure 1-1.

The dimensional accuracy for a 100-ft. (30.5-m) steel tape depends on the amount of sag, temperature, tension, and angle of use. The National Institute of Standards and Technology (NIST) sets tolerances for metal tapes as shown in Table 1-1. For typical situations and when sag is minimized, the tolerances shown in Figure 1-1 can be expected. If higher accuracy is required, a laser should be used or steel tapes with correction factors included for temperature and other variables.

Right angles for many buildings, such as houses and small commercial structures, can be laid out with a steel tape measuring a 3:4:5 triangle. When greater accuracy is required, a transit or construction laser should be used.

Related Sections

1-2 Vertical Building Layout 1-6 Grading and Sitework 17-1 Measuring Devices 18-2 Expressing Uncertainty

1-2 Vertical Building Layout

Description

After the horizontal form and dimensions of a building are established, the next source of inaccuracy is setting elevations, plumbing vertical elements, and maintaining levels. These are dependent on the accuracy of the instruments used as well as on environmental conditions and the skill of the people doing the layout. This section includes some of the layout accuracies possible in establishing elevations, plumb, and level with various instruments, assuming the surveyor uses a normal degree of skill.

The tolerances shown in other sections of this book for plumb and level for individual construction materials and assemblies are often more or less accurate than those shown in this section. Refer to individual sections for more specific information on industry standards for specific materials. Refer to Chapter 17 for more information on the accuracy of measuring instruments and methods of measurement.

Allowable Tolerances

As with horizontal layout, there are no generally accepted standards for vertical layout. The numbers given in Figure 1-2 for tolerances are based on ISO 4463 and research done by the Institute for Research in Construction (Latta). However, with construction lasers and laser levels, optical levels, and digital levels, higher degrees of accuracy are possible. For example, some manufacturers' digital levels can achieve an accuracy of ?1.5 mm per kilometer if used according to the manufacturer's recommendations. Single-measurement optical levels can be accurate to ?0.8 mm in 30 m (?1/32 in. in 100 ft.). The required tolerances should be clearly stated in the specifications based on the level of accuracy required by the project.

For less exacting applications, spirit levels and plumb bobs are often used for residential and small commercial buildings. The typical accuracies for these instruments are also shown in Figure 1-2.

For establishing slopes and checking for level, digital inclinometers are now commonly used. The accuracy for a digital inclinometer (SmartTool) is 0.1 degree (approximately ?1/16 in. in 4 ft. or1.6 mm in 1,200 mm). They are available in 2-ft. and 4-ft. lengths. Because local variations of flatness are possible, large slabs and ramps should be checked with a transit or construction laser to establish overall level or slope.

The NSPS standard for building offset stakes is ?10 mm (0.03 ft.) for vertical positional accuracy. Positional accuracy in this standard is given at the 95 percent confidence level, as described in Section 1-1.

Related Sections

1-1 Horizontal Building Layout 1-6 Grading and Sitework 17-1 Measuring Devices 18-2 Expressing Uncertainty

1-3 Concrete Paving

Description

Concrete paving includes drives, parking surfaces, and other site paving. In many cases, tight tolerances for exterior paving for vehicles is not critical. However, when existing building and site elevations require minimum slopes for drainage (usually 1/4 in. per ft. or 6 mm per 300 mm), the tolerances shown in this section should be specified. When ponding of water might create hazards in any situation, these tolerances should also be specified.

Industry Standards and Recommendations

ACI 117-06, Specifications for Tolerances for Concrete Construction and Materials and Commentary (Farmington Hills, MI: American Concrete Institute, 2006).

NCHRP Project 20-07/Task 167, An Analysis of the Draft ADA Guidelines for Accessible Rights-of-Way, David Kent Ballast, June (Washington, DC: Transportation Research Board, National Cooperative Highway Research Program, 2004).

Standard Specifications and Supplements (Washington, DC: American Association of State Highway and Transportation Officials, 1998) http://fhwapap04.fhwa.dot.gov/nhswp/servlet/LookUpCategory

UFGS Section 02752, Portland Cement Concrete Pavement for Roads and Site Facilities, and UFGS Section 02754, Concrete Pavements for Small Projects. United Facilities Guide Specifications, August 2004 (Washington, DC: National Institute of Building Sciences, 2004) www.ccb.org/docs/ufgshome.

Allowable Tolerances

The only industrywide tolerances for paving on a construction site are published by the American Concrete Institute (ACI) as shown in Figure 1-3(a), (b), and (c). For highway work, state and local departments of transportation (DOTs) often give allowable tolerances in their model specifications or contract requirements. The tolerances by ACI are for vertical deviations of surfaces below an unleveled straightedge resting on high spots. Slope tolerances are not included in the ACI document.

Guide specifications for concrete pavement published as part of the Unified Facilities Guide Specifications (UFGS) for military facilities state that roads and streets should have a tolerance of ?5 mm (3/16 in.) in the longitudinal direction and ?6.5 mm (1/4 in.) in the transverse direction when measured with a straightedge. Interestingly, in Section 02754, no length of straightedge is specified, and in Section 02752, a 4-meter (12-ft.) straightedge is required. For other surfaces, such as parking areas, the suggested tolerance is ?6.5 mm (1/4 in.) in both directions.

A research report for the Transportation Research Board suggested an allowable slope tolerance for concrete of ?0.5 percent, as shown in Figure 1-3(d). The same research report surveyed state and city departments of transportation and found that the allowable tolerances for roadway slope ranged from a low of 0 percent to a high of 5 percent, with an average of 0.6 percent. However, the suggested slope tolerance did not indicate the length over which the slope should be measured or in what locations slope should be measured. See Chapter 17 for discussions on measuring slopes and the flatness of surfaces.

For highway paving, the American Association of State Highway and Transportation Officials (AASHTO) publishes the AASHTO Standard Specifications and Supplements. In Section 501 for Portland Cement Concrete Pavement, the specifications require a surface test that limits surface defects to ?5 mm under a 3-m straightedge placed at random locations. (Section 501 [L]1). This is very close to the commonly used 1/4-in. Deviation under a 10-ft. straightedge.

When necessary, concrete road pavement, highway barriers, curbs and gutters, and sidewalks can be placed by specialized paving equipment with a high degree of precision. Slope tolerances of ?0.13 percent can be achieved for roads and sidewalks using automated equipment guided by stringlines, lasers, or global positioning system (GPS) devices. They are made by a number of manufacturers. However, this equipment is commonly used only for very long runs of highways or sidewalks.

Related Sections

1-4 Asphalt Paving 1-5 Pedestrian Paving 1-7 Right-of-Way Construction

1-4 Asphalt Paving

Description

Asphalt paving includes drives and parking surfaces. Like concrete paving, asphalt paving tolerances are not always critical if sufficient slope is built into the paving for drainage. However, when a minimum drainage slope of 1/4 in. per ft. (6 mm per 300 mm) is required, the tolerances shown in this section should be specified.

Recommendations

NCHRP Project 20-07/Task 167, An Analysis of the Draft ADA Guidelines for Accessible Rights-of-Way, David Kent Ballast, June (Washington, DC: Transportation Research Board, National Cooperative Highway Research Program, 2004).

SS-1: Model Construction Specifications for Asphalt Concrete and Other Plant-Mix Types, November (Lexington, KY: Asphalt Institute, 1984) Out of print.

Standard Specifications and Supplements (Washington, DC: American Association of State Highway and Transportation Officials, 1998) http://fhwapap04.fhwa.dot.gov/nhswp/servlet/LookUpCategory

UFGS Section 02742, Hot Mix Bituminous Pavement. United Facilities Guide Specifications, August 2004 (Washington, DC: National Institute of Building Sciences, 2004) www.ccb.org/docs/ufgshome.

Allowable Tolerances

There are currently no industry tolerances for asphalt paving. The Asphalt Institute (AI) previously had tolerances as part of its Model Construction Specifications. This document is no longer in print and the Asphalt Institute no longer establishes tolerances. However, in the last edition of the printed volume, the recommended tolerances were ?1/8 in. (3) parallel to the centerline of the compaction roller and ?1/4 in. (6) perpendicular to the centerline as measured under a 10-ft. (3-m) straightedge. See Figure 1-4(a). These previously published tolerances suggest what one trade organization deemed reasonable.

(Continues...)

Excerpted from "Handbook of Construction Tolerances" by David Kent Ballast FAIA CSI. Copyright © 0 by David Kent Ballast FAIA CSI. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher. Excerpts are provided solely for the personal use of visitors to this web site.
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