Tolerances for Civil Engineers

1. Tolerances for Civil Engineers Kurt McMullin CE20 – San Jose State University

2. Tolerance • The total amount a specific dimension can vary. • Tolerances are assigned so that any two mating parts will fit together. • Highly accurate parts are extremely expensive, so tolerances should be as generous as possible while still maintaining proper function for the part.

3. Why Consider Tolerances • Tolerances are required because it is impossible to construct engineering projects to exact dimensions. • Tolerancing is the engineering of drawing blueprints. • Tolerances are seldom determined by drafting personnel.

4. Individual Civil Engineering industries determine suitable tolerances for common measurements. These are usually defined in some Standard or Code written by the industry. Varying these tolerances should only be done in extreme situations! Industry Tolerances

5. When a tolerance is difficult to achieve during construction it is referred to as TIGHT. LOOSE refers to tolerances that allow lower quality work. TIGHT tolerances result in high quality projects but are usually expensive to build. LOOSE tolerances make cheap projects that have low quality. Loose and Tight Tolerances

6. Tolerance for the location of piers for the Bay Bridge built in 1937? Tolerance for the amount a building’s column can be out of plumb? Reasonable Tolerances

7. Tolerances may be given in industry standards (text). Tolerances specific to a project may be stated in project specification (text). Tolerances may be included in the project plans. Methods of Stating Tolerances

8. Dimensional Tolerances • Most tolerances are used with a lineal dimension. • The nominal dimension is given on the drawing and then some acceptable variation (the tolerance) is added.

9. Plus – Minus Tolerances • Tolerance is given as some dimension added and/or subtracted from the nominal dimension. +5” -3” 10’-0”

10. Plus – Minus Equal Tolerances • Tolerance is given as the same dimension added and/or subtracted from the nominal dimension. +5” -5” 10’-0”

11. Plus – Minus Equal Tolerance • Tolerance can be written using the plus/minus symbol. ±5” 10’-0”

12. Plus – Minus Unilateral Tolerance • Tolerance is given as a dimension added and/or subtracted in only one direction from the nominal dimension. +5” -0” 10’-0” +0” -5” 10’-0”

13. Graphic Layout • Tolerances should be: behind the dimension they cover smaller font above the centerline of the dimension text

14. Control of Tolerances • Engineers must think about the effect of a chain of tolerances on the final product. • The use of baseline or continued tolerances can affect the total length of an object.

15. Baseline vs. Continuous • Total length may vary significantly. +5” -0” +5” -0” 10’-0” 10’-0” +0” -5” 10’-0” +0” -5” 20’-0”

16. Geometric Dimensioning & Tolerancing • Most tolerances are assigned to dimensions (such as the ones we have just talked about). • Tolerances can be assigned to Geometric shapes, and these are called Geometric Tolerances. • For example: we may be more interested in the variation of the weight of a cube, not the specific gravity or the length of the side.

17. How would this work? • The specific density of some material may vary from 1.6 to 2.1. • As engineers we may buy a truck that can carry 12 cubic yards of the material.

18. Continued • When we want to send this truck on the road, we are probably more concerned about the weight (because that is what the weigh scales are going to check). • We could allow more than 12 cubic yards if the material was light in specific density.

19. Continued • Hence, the tolerance we give on the length of the area excavated may depend upon the specific density of the soil removed.

20. Geometric Tolerances for Civil Engineering • We may be interested in: Positional Tolerances Flatness Straightness Cylindricity

21. Positional Tolerances • Tolerances of the dimensions that locate an item may allow for a different position than what the engineer desires. • Usually we are more interested in the total distance from the intended position rather than the direction in two orthogonal directions.

22. Illustration of Positional Tolerance Tolerance Zone defined if dimensional tolerances are used. Region acceptable to engineer. Location intended for object.

23. Flatness • This refers to how close to a plane is a surface. • Flatness is a measure of the difference between the closest point and farthest point from the reference plane. REFERENCE PLANE OBJECT SURFACE FLATNESS

24. Reference Plane • To measure flatness a reference plane must be designated. • A common reference plane for civil engineering projects is a horizontal plane parallel to the mean sea level.

25. A concrete slab is specified to have an average elevation of 112’-0” with tolerance of ± ¼”. The flatness is to be 3/8”. Do we meet the tolerances if the actual elevations measured are: 111’ – 11-7/8” 112’ – 0-1/8” 112’ – 0” 111’ – 11-15/16” 112’ – 0-3/16” Example of Flatness

26. Straightness • Measures whether the edge of an object varies from a straight line. STRAIGHTNESS 11-1/4” 11-1/4” 11-1/4” 11-1/4” ALL TRANSVERSE DIMENSIONS MAY BE EQUAL, BUT THE 2x12 LUMBER IS NOT STRAIGHT.

27. Measuring Straightness • First draw a straight line along one edge STRAIGHTNESS

28. Measuring Straightness (Con’t) • Offset that line to touch the extreme opposite edge. STRAIGHTNESS

29. Measuring Straightness (Con’t) • Now offset a line at the closest surface. STRAIGHTNESS Measure the distance between the lines.

30. Straightness vs. Nominal Dimension • Note that the value for the straightness is independent of the magnitude of the original dimension. • The straightness of a 2x2 may be the same as a 2x12.

31. Cylindricity • Cylindricity is similar to straightness but is used for a cylindrical item, not a flat plane. Red object does not fit inside of acceptable tolerance zone. Tolerance zone is between blue cylinders.

32. Tolerances are used to state acceptable variations in geometry from the nominal dimension. Dimensional tolerances are applied to lineal dimensions. Dimensional tolerances may be Plus-Minus, Equal, Uniaxial or Symmetric. Tolerances may be baseline or continuous. Summary

33. In civil engineering most common tolerances are stated in standards. Tolerances for a specific project may be written in the project specifications or may be shown on engineering blueprints. Summary (Con’t)

34. Tolerances may be given to geometric configurations (not simple lines) and are then called Geometric Tolerances. Common Geometric Tolerances for Civil Engineers include: Position, Flatness, Straightness and Cylindricity. Summary (Con’t)