STRUCTURAL DIMENSIONING

Dimensioning procedures on structural drawings vary somewhat from other drafting areas. The following are basic structural dimensioning guidelines:

I. Dimensions should be placed far enough away from the view of the object to avoid crowding the drawing itself.

2. Dimensions should build from the object drawing, with smallest dimensions closest and longest, overall dimensions farthest away.

3. Dimensions should usually be referenced from the centerlines of beam shapes, the backs of angle shapes, the backs of channel shapes, and the centerlines of round shapes.

4. Dimensions should be given to the point on the beam that is designated by level (or elevation). In most cases this means dimensioning to the top or bottom of the beam, rather than the top and bottom both.

5. Dimension lines on structural drawing run unbroken for their full length, with dimensions placed above the dimension line near the center.

6. Dimension figures generally carry the mark for feet, but the Inch mark does not appear: thus 8' - 8 stands for 8 feet - 8 Inches.

7. Unlike many other systems, structural dimensioning for units less than 1 foot uses only the number to designate Inches. The inch is assumed, so that 0' - 5 becomes simply 5.

8. The reverse is true for dimensioning an even number of feet where no Inches occur; thus 8 feet even appears as 8' - O.

9. All dimensions over one foot are given in feet and Inches except the width dimension of plates and the depth of structural members. These are in inches.

1O. As with any dimensioning system, dimensions should be provided for all items of significance, and crossed dimension lines should be avoided, wherever possible.

I I. Dimensioning systems for holes are generally of two types. One system utilizes

extension dimensions starting from the centerline of hole groups. The other system gives extension dimensions to the first line of holes In each group, progressing from left to right.

12. Lettering used on structural drawings can be slanted or vertical, depending on office standard.

I 3. 5hop drawings should be dimensioned so those making the members do not have to add or subtract dimensions.

14. Details, such as the size of fasteners, hole diameters, and painting Instructions, are given as notes

It should be noted that dimensions in relation to the depth of beams, end connections, cuts and spacing of holes are drawn to scale. It is the practice of structural drafter to draw the depth dimensions to scale so that the relationship of detail is correct and that the shop man can interpret the relationship of holes to bolts and holes more readily. It is a rule in the fabricating industry that the shop man is not to scale a drawing; the drafter must have a dimension or note to describe any operation that is required In fabricating the member.

The length of beam and dimensions in relation to the length can be drawn to scale but are usually foreshortened with break lines. The reason that the length is usually shortened is that the scaled length would, in most cases, be of such length that It would take more space on a drawing than is economical, and would be of no practical value to the shop man. However, foreshortening the length so much that the holes in the web or some of the detail will appear crowded or ambiguous, should be avoided.

The structural drafter does not always draw to the exact scale, but exaggerates the drawing to clarify the object. An example of this exaggeration is the two lines that would show the thickness of the web of a beam. If an attempt were made to draw to the exact thickness, depending on the scale of the drawing, the line weight of the two lines would run together, and would print as one heavy line. To avoid this running together of two parallel lines, the lines are separated more than the actual thickness,

In order that the lines clearly define a change in the shape of the object. DO NOT OVER EXAGGERATE. Any part of a structural member that is exaggerated should not appear thicker than those parts that are represented to an accurate scale.