Friday, October 24, 2008

Dimensions - They Tell a Story!

Ask someone how a floor plan should be dimensioned. You might be surprised at the range of responses you'll get. Answers will depend on what is important, what is in the view, who you ask, if they have field experience...but the answers you get will vary. Just as you'll get varied responses to where dimensions should reference, face of stud, face of finish, wall center line....

Take a look at this picture and "tell me" what's wrong dimensionally. Now I'm sure that you could come up with something other than what I'm trying to show but I've made ONE mistake that is important to me (hint, yeah it's the one in red).
This is just a simple wall layout, no column grid lines, no columns...just a simple rectangular layout yet it is wrong. It is wrong because I couldn't stop "talking", I said too much. By talking I mean using dimensions. Dimensions are a dialogue that we, the documenter, has with the reader/listener. They are NOT just information or something we do without intention or purpose.

My mistake is that I closed the dimension string. By closing the string I haven't told the person in the field what I think is important. What is important changes. An exit corridor has different criteria from a room in a line of rooms. A laboratory for highly technically equipment will have its own needs. My simple drawing should not have included the middle dimension, something like this instead, not much difference but an important one.
This image tells a better story, that I want the room on the left to be a specific size and the room on the right to be a specific size as well. The space in the middle is LESS important than those because it is NOT dimensioned. This is the story I intended to tell but I failed in the first version. I was ambiguous, they could come to their own conclusions. (Yes I realize that I didn't dimension the other wall offset...I stopped talking too soon!)

Worse my dimensions might not actually add up. By the time they are ready to put up those partitions the exterior walls have been there awhile. If they can't make the center room the correct size, what then? If they work from the left and make the left and center room the correct size and the right one is the one I really cared about...now it is not the size I wanted. Maybe we are only talking about inches or less...but maybe not.

Purpose - Why am I using a dimension and who is my audience? A presentation drawing has a different purpose than a construction document. Dimensions on one drawing compared with another therefore can have a different purpose. They speak with a different dialect, the one that the intended audience will understand and appreciate.

Lessons learned...years ago I worked for J.R. Clancy, a theater equipment manufacturer. I was their dealer sales person. I wore several hats. One was shop drawings for custom components our dealers needed for their projects. This deceptively simple looking drawing was a lesson learned and the same as the one above. (Note: I used a drafting view to mock up the critical mistake I made so the bill of materials and all the other information it would technically have is missing here)

I closed the dimension strings in this case too (yes in red). The problem I created by doing this is, in this case, consistency. The person who spends his day punching the "circles" out of the steel angle cut 300 pieces first and then spent the rest of the morning making the first 150 or so. He set up his stops on the punch press assuming measuring from the left end. He finishes the job after lunch and sends them off to assembly. No worries.

The next job comes along and I reuse this drawing since it is just like that last one they ordered. Off to the shop it goes but this time "that guy in the shop" decides to set up the press from the right end. Why not? I gave him all the numbers to do it easily didn't I? Trouble is that this part is used with others, a pair of which a computer makes in the thousands by an outside firm. These angles are the only custom part of the job, done on a job by job basis. If I did this on standard part drawings the effect would be more dramatic.

When he measures from the "wrong" end these parts don't fit as well because the cutting process for the steel angle isn't that precise, it's a band saw and well human beings are easily distracted. When these pieces are used later to assemble the final component the finished product looks "crappy" because of the slight misalignment.

There are any number of things the person running the press could have done to avoid the situation. But if I didn't provide the closing dimension at the end it would have "told him" that it was more important to me that the holes were cut relative to the left than from other end. It starts with me and my dimensions, I need to enunciate when I "speak".

The lesson I learned then, and reinforced in many other situations since, is that dimensions are not something pretty that a drawing should have to look correct. They tell a story, they tell someone else what I WANT! If a dimension doesn't tell your story then you don't need it. Dimensions should speak with your voice but in the language of the reader/listener. If a dimension doesn't tell your reader/listener what you want then it isn't doing its job and neither are you. Know when to stop talking, don't say too much. Wait to say something else on a different drawing when the right audience is listening.

With Revit it is my recommendation that you use the full depth of Revit's precision, 1/256" as your project units (Settings menu > Project Units) or whichever units are involved in your situation. This means that temporary dimensions will display "funny, messy" numbers when they occur. They WILL occur because construction and physical materials have "funny/messy" dimensions. Dimensions will also show this if they are set to use Project Units, as the default styles are.

You can decide when to be abstract, just how abstract or real by using dimensions that use different rules for tolerance. All manufacturing drawings discuss tolerance so the person doing the work can determine what tolerance is appropriate for the task at hand. So should ours.

Now I'll take my own advice and stop talking!

2 comments:

Malleristic-Revitation said...

Certainly a hot topic over the course of the last few days :)

While i do agree that in a string as youve shown (across a full building) we need to make good on our attachment to reality by offering an *out* or a *hierarchy* by leaving dimensions out in insignificant places.

Still, i contend that leaving out dimensions in some situations should be choice, not necessity, as was the premise of the thread that started on AUGI.

Consider the example of the architectural column enclosure that is 2'-6" out to out, and a Stl column in it at 1-2" from CL to ouside of the outside wall. I may even dimension that wall from the back of stud, to the CL so they can lay the track down. Having given the Hierarchally Significant dimension of the Out to Out, it stands to reason i shouldnt give the dim to the back of the INSIDE walls track. That would be my *opening*.

But at this point, is architecture becoming more about covering our behinds? If i give the dimension, and something is off, a conversation has to happen, maybe an RFI, and people are upset. But if i DONT give it, all im really doing is makng the poor guy in the field take 2'-6", and subtract the thickness of the wall type. And if it doesnt fit, im STILL getting an RFI. I just cover it with a blanket statement that "The pier is to be 2'-6", beginning with outer walls placement" and i leave him to fend for himself.

I say we all have to push forward, ESPECIALLY with the new draft Documents like E202 that are coming out to stipulate that MODELS will supercede DRAWINGS in the near future. Because if that happens, folks.... guess what:

EVERY dimension string is closed.

Great post Steve! :)

Jimmy Bergmark - JTB World said...

At a company I worked for we introduced kind of mech-standard for dimensioning even though it was for arch/struct, etc. We used baseline dimensions but without any baseline spacing. That both informed about from what side to start measuring and eliminated sum errors. If there where rooms for example further into the building that needed a fixed dimension it was added to say that this room needed to be this or that size. When needed we also used tolerances.

Later on we combined that with using laser and coordinates.