Dear John
|
|
|
|
This was sent to an engineer friend of Matt's
who uses AutoC*D. Subsequently I received an interrogatory from Peter (comments added) |
Dear John,
Drafting principles:
(I realize some of these comments may be due to the .dwg translation, but bear with me)1. Lay out the job before beginning drafting, organize each sheet. For example, the foundation plan for this job should be on one sheet, at 1/8" = 1'-0". The mezzanine framing should be at the same scale. We try to modularize our sheets and details as much as possible, to make the pieces easier to re-use on future projects.
2. Create an obvious hierarchy of information. There are two types of information on the drawings: the material (what is to be constructed - primary) and the requirements (descriptive information - secondary) they should be obvious and distinct from each other -- at first glance. Also, more important information should jump out from less important information, like flowers among weeds. Zoom to the drawing extents and look at the sheet from there once in a while.
Inquisitor: I have not heard it discussed this way. Can you point to sources to learn more? Or can you give some more concrete examples of how this is reflected in drawings?
Good question. Heirarchy of information is created by Consistency and Contrast. For example, when you use a large, bold font for room names and a smaller, finer font for notes. When you look at a drawing done this way, the more important, more fundamental information is more readily apparent, and more refined or detailled information is only apparent after further study.
If you have ever seen a drawing done using only one line weight, you will immediately understand the benefit of multiple line weights -- a bolder line gives primacy and emphasis to certain things, and a finer line casts its element in a supporting role.
A heirarchy of information provides a framework for the eye to explore the document, familiar elements which have familiar meanings. It reduces the amount of puzzling-it-out that your mind has to do, allowing you to focus on the content rather than the format of the document.
3. Differentiate between things, such as [dimension lines and centerlines] as opposed to [beams, joists, and slab edges]. Use the linetypes of ________ , _ _ _ _ _ _ , & ____ _ ____. Vary line weights if possible. We used to use the AutoC*D colors for line weights, Magenta very heavy, cyan very light, whatever works for you is fine; thick polylines always seemed to cause more problems than they solved.
4. Tie your drawings together. Show north arrows, slab edges, building envelopes; it should be easy to tell how each part relates to the whole. Don't extend subordinate column centerlines beyond their useful length...for example if you have a 200' x 200' building with a row of columns 15' in from the exterior wall, there's no reason to extend their centerlines through the body of the building - it can be confusing.
5. Draw at appropriate scales. Most plans are fine at 1/8". Details at 3/4" are fine for structural items (of course larger is fine for special plates, etc.). 1/2" details are usually fine for foundation/footing stuff.
6. Schedule and Key things as much as possible. Try to avoid hatch patterns for keying purposes. For example footings can be scheduled to save time. This avoids tedious and redundant noting, and also avoids the ambiguity of un-described elements left to be assumed by the builder (which is the case when you say "C footing, typical for 12" but don't point to each of the twelve).
Inquisitor: I'd expect a post size to be in the structural plan or a schedule. I find it more confusing (and often wrong) when it is given in details. The exception might be steel frames where it is more clearly shown in section or elevation of the frame.
My experience exactly. More often than not, sizing called out will be erroneous, represents an impossible-to-keep commitment for coordination, and turns simple changes during construction into a hopeless and endless nightmare.
7. Use appropriate lettering sizes: most drawings notes and dimensions are done about 1/8" high -- maybe 5/32" or 3/16". Drawing titles larger than 3/8" are going to be too big...1/4" BOLD letters usually pack enough punch for drawing titles. Don't let dimension numbers float too far away from the dimension line they're associated with (when I draw by hand, if my lettering is 1/8" high, the numbers are usually at the most 1/8" away from the line).
8. Avoid the seductive and hazardous practice of repeating infomration needlessly. It is repetitive and redundant too; for example the mezzanine plan says " 3" round column extra strong steel (wall thickness: 0.300", wt/ft 10.23 lbs" and this information is repeated verbatim on the column connection detail. It belongs on the detail (or in the specs) but not on the plan, because in all likelihood somebody is going to call you up and ask if he can substitute 3-1/2" regular columns and you'll say OK, and correct the detail but miss the plan and then there will be a problem later. Redundant information does not increase the certainty that the requirement will be fulfilled.
Inquisitor: I have begun, in major sections, to describe assemblies under major titles like "Roof/Ceiling Assembly" because these are general descriptions but important.
Similarly, I suppose my notes can have a de facto heading by starting the wording, where possible, with the general term following with more detailed information.
We have been trying to move the standard (often no-brainer but required) comments off the face of the drawings, but have them well referenced. Some (more old school) people I've worked for prefer important notes to be right on the plans near the related objects.
The point is not to put so much in the drawing it becomes too much to follow or hard to sort out. But the other extreme is ConDoc concept where there are few direct comments on the plan at all.
It is sometimes tricky to find a balance. We typically take this approach, to have a text block that says:
TYPICAL ROOF CONSTRUCTION
- trusses at 24" o.c.
- 1/2" plywood sheathing
- 30 lb bldg paper
- fiberglas shingles
then near the eaves a note that says
- eaves flashing per code
etc etc
so we note the TYPICAL stuff once (or as few times as possible), and don't try to put arrow heads to each component in the assembly then elsewhere in the drawing we note the things that DEVIATE from the typical, are special conditions, or otherwise exceptional in some way.
That way, again, we are presenting a heirarchy of information...the unique elements aren't camouflaged among repetitive descriptions of the typical elements.
9. Never cut a dimension string -- it loses its meaning if it doesn't clearly extend to what it is dimensioning. I like engineering drawings to use arrows or dots for the dimension strings, not hash marks, but do what you like. The hash marks look lousy to me when the string doesn't extend past the mark, but whatever.
Inquisitor: What practice is cutting a dimension string referring to? For some elements like windows I have taken the practice from others of just tying it to the corner or centerline of wall, but not going on to connect it to other elements. Also sometimes it seems best to stop a string when it is no longer related to elements
Correct. I was referring to the use of cutlines within a dimension string. Seems to me to defeat the whole purpose of dimensioning.
Now, in an ideal world, I see no reason to put any dimensions in a CAD drawing (just give them the file, for crying out loud, let them ask it)....but we ain't there yet. In the meantime, I think since it is so easy to put dimensions on a drawing, we should do it thoughtfully, and without presuming that the people using the document a) are starting from the left or right and b) know what the dimension is taken to(!) (face of framing, face of finish, control line, whatever) (it burns me up to see a dimension like 17' - 5 7/16" and no description of what the heck it is taken to!!!).
We also don't need to insult the fabricators by over-dimensioning things, basically, I think you should be able to explain why you want any particular dimension in a drawing to your grandmother. If you can't, then you probably don't need it. Cut them some slack...
I think dimensioning is a neglected art, and it is worth it's weight in gold in terms of keeping a good relationship with the rest of the construction team. It is worth doing thoughtfully and well.
the Inquisitor sends a supplemental comment:
Another thing on dimensions. I have had it pointed out to me that dimensions should start (at least in your head) from one point-- so all dimension strings can be traced in that direction.
For elements other than walls--if one trade lays out one way and one from the other direction, you could have a conflict. As you probably do, I find I am often accounting for a number of things in a dimension (including fudge factors) that are never mentioned in the drawing.
It is a good point to note somewhere near an odd dimension (5'-0 5/8") , why you show that--as in the case that you want to be sure they run the wall board down the end of the bath tub by a fire wall.
10. Number each plan, detail, section, whatever. It's much easier on the phone to say "on sheet 4, look at detail 3. OK, now ...." It's easier to key, and it avoids the problems which occur when you have detail A on more than one sheet (instead, the key will be unique, as in: "5 / S4" or "6A / S3" no room for doubt). It also forces you to define your target for the project (lay out the sheet, list the details you're doing) before you get too far along.
Matt
Go back to The Bull Pen
