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ARCH 367/667 Working Drawings

Notes, week 4

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Drafting conventions: lines and dimensions

Based on National CAD Standard 4.0

Lines

Lines on a drawing have meaning, and their relative "weight" can confuse or clarify the information to be conveyed. Guidelines for line weights follow:

Line typeWidth (mm)Typical uses
Extra fine0.13Detail too fine to be rendered with 0.18 line
Fine0.18Material rendering, patterns
Thin0.25Small text (3/32" - 3/8"), dimension lines, leaders, break lines, dotted lines, grid lines, etc.
Medium0.35Medium text (5/32" - 3/8"), object lines, property lines, door/window elevations, accent lines in schedule grid
Wide0.50Larger text (7/32" = 3/8"), titles, edges of elevations, profiling, drawing block borders
Extra wide0.70Largest text (1/2" - 1"), match lines, large titles, title block or sheet borders,schedule outlines
XX wide1.00Major title underlining
XXX wide1.40Border sheet outlines
XXXX wide2.00Border sheet outlines (children under 17 not permitted to use this thickness without parental consent)

Some definitions of important line types:

Dimensions

The idea is to indicate the size and position of everything that is to be built. To repeat: indicate size and position of all building elements. The usual strategy is to tie each object to something relatively fixed, such as structural column centerlines, or walls, or existing elements. These, in turn, are located to some absolute site feature, such as a property line (in plan) and a vertical datum point (in section).

The other basic strategy is to show each dimension only once, so that changes can be made without running into contradictory dimensions that one forgot to change. In an intelligent building model, on the other hand, such redundancy would not necessarily be a problem. In any case, this rule is violated with column grid lines (which are dimensioned on every floor plan) and in other circumstances where the redundant dimensions are judged to be more useful than dangerous.

Try to place dimensions on the outside of the drawing, on the top and right side of plans (but to the left of sections), where possible. Where parallel dimension lines occur, space then 3/8" apart.

Use consistent "terminators" at intersection of dimension lines and so-called "extension lines" (which run from dimension line up to, but not touching, the object being dimensioned -- 9/16" should separate extension lines and object lines): either use short slanted lines (slashes, with positive or negative slope for horizontal or vertical dimension lines, respectively), or filled arrowheads (subtle; not too large).

Arrange dimensions in continuous strings, both for clarity, and to avoid errors.

Text size of dimensions should match the other text in notations.

Parallel strings of dimensions are useful, as they permit quick readings of small and large dimensions. To avoid extension lines crossing dimension lines, it is natural to place the smallest dimensions at the bottom, with the largest at the top. It is not always possible to avoid some line crossing, as column grid lines invariably cross the overall building dimension lines, but avoid unnecessary line crossing.

A typical plan dimensioning scheme is based on column centerlines, with an additional dimension at each end from the corner column centerline to the outside face of the building skin. Where the "skin" face is ambiguous, one needs a notation somewhere to clarify what the dimension actually signifies. Sometimes, the corner columns are dimensioned to their outside faces, in cases where those faces remain constant while the centerline moves (this happens more often with concrete than steel columns).

There are at least 3 options for dimensioning to walls and partitions:

  1. Dimension consistently to the centerline;
  2. Dimension to the face of the finish (e.g., the face of drywall in a drywall partition);
  3. Dimension to the face of the stud, or other internal structuring unit (e.g., CMU or concrete covered with some other finish material).
There are different reasons for choosing one or another approach:

Note that where existing construction is part of the project, do not use dimension strings to indicate the existing dimensions; rather, just show the new construction in relation to the existing. Think of the intent of the drawings, which can never be to build what is already there. Where uncertainties are present, note VIF (meaning "verify in field").

Where different drawings show the same thing at different scales, attempt to avoid redundancy by showing the detailed dimensions only on detail drawings. Exceptions always are possible, if redundancy is justified by some overarching concern. See site plan for advice on datum for vertical (and horizontal) dimensioning.

Sections and elevations always have the same dimensioning scheme, on the right side if possible. This is an exception to the rule about redundancy that is justified by the need to relate each drawing to the grid of vertical dimensions. The "elevation mark" relating to the fixed datum point is generally shown on one floor (the same floor) only, and all other dimensions relate to it through dimension strings which pass through its extension line. There may well be instances where other "elevation marks" are useful (e.g., high point of roof), especially where establishing a datum clarifies what would otherwise might be confusing. Use judgement.
elevation mark and datum point

In general, leave a zone around the drawing for graphics and notations, outside of which the dimension lines are drawn. The column grid circles are the farthest items (in plans), just beyond the last dimension line.

Wall sections typically have at least two parallel dimension strings: the inner string for rough openings and top of slab (or subfloor); the outer string for floor-to-floor dimensions (i.e., top of slab or subfloor).

Ceiling height changes are awkward to show in section or elevation; they often show up in reflected ceiling plans, or finish schedule remarks.

Accuracy: try to avoid small fractional dimensions, especially those less than 1/16". Even that is pushing it a bit. Use the EQ EQ trick to subdivide areas, especially those with strange dimensions to begin with. The EQ dimension also has the advantage of displaying an intention, which is sometimes more important than the actual proposed dimension (which might end up disappearing within the reality of construction tolerances. The notion of using dimensioning to show your intention is important, and helps in ruling out (and ruling in) many dimensioning decisions:

dimensioning example
In the sketch above, assume that a cabinet (dotted line, NIC) must fit into the space shown. In that case, dimensioning the partition using "A" would not be as effective as dimensioning the same partition using "B."


dimensioning example
In the sketch above, the EQ-EQ dimensioning trick is used, since it unambiguously indicates the intention. A dimensioning string with actual proposed dimensions is dangerous in this context, since any inaccuracies in the overall conditions might result in a non-symmetrical result.
dimensioning example
In the sketch above, the "ALIGN" notation is used to indicate that the two partitions should align. This is safer than showing independent dimension lines for each partition.

Use stable "datum points" for dimensioning. Do not tie some dimension string to a sloping roof beam, for example. Use the constant "top of parapet" or some other reference point.

Show dimensions to the side where the available space (for very short dimensions) cannot accommodate the notation.

NTS (not to scale) should be used in the drawing title where appropriate.

Use +/- after any dimension that is not intended to be precise (for example, because it includes some existing condition that cannot be accurately determined). Sometimes, the +/- note is combined with the VIF if your intervention or advice may be needed.