October 1, 2011
3d for Residential Structural Design
This is one of my more recent projects, the architect was dencity Design in Atlanta. It’s a very difficult house – look at the cantilevers:
This kind of strucutre is difficult to engineer because you have to visualize a very complex 3 shape from a 2 drawing. To make things easier, the Architect, Staffan Svenson, sent me a rendering done in Google Sketchup so I could visualize what was happening. I then built a 3 model in RAM Elements software to cover all the different forces we would encounter. The problem you get is not only vertical forces in a complex structure as this, but the wind action. How do you brace it? I used fixed connections to the foundations, which required me to spend time engineering anchor plates, anchor bolts, and very large foundations. I could do this by hand (and I have) would be very time consuming, and you run the chance of what I call “calculation fatigue” – you do so many calculations you get blind to the errors due to simple mental fatigue.
Here’s a view of the rendering from RAM Elements:
Working this way, I was able to model all of the effects of the structure – note how I put in concrete walls in the basement and OSB (Oriented Strand Board) walls on the first and second floors. All of this could factor into the design. Once the structural calculations are done, it’s time to do what the illustrious professors I had as an undergraduate didn’t think was important, but is actually critical – translate it into a drawing that can be understood and constructed. This is a pet peeve of mine, I run into engineers that can’t seem to understand how to develop their ideas into drawings. In such case you may as not have any ideas.
I personally had two choices for drawing this, well I guess three:
1. I could send the drawing out to a CAD service with hand sketches of what I wanted. We could go back and forth for a week or so until I got what I wanted. Maybe two weeks. Well, really four weeks.
2. I could draw it myself in AutoCAD – there is another type of fatigue you encounter when drawing. After working so hard to do the calculations, now you are drawing all these boring details, and repetitious joists, and then trying to make it all work. I could do this drawing in about 40 man hours.
3. Draw the drawing in Softplan, and use exported details from RAM Elements for the connections, foundations, and baseplates. RAM Exports details in DXF (Drawing Exchange format), so it’s easy to import into Softplan. Softplan generates drawings in 3d from your floor plans, and automates a lot of stuff like drawing columns, foundations, walls, and joists. The beauty is you can have a 3d model that constantly updates as you create your drawing. That way you can catch things you might overlook. I was able to make sure I had load bearing walls stacking to the floor, and that I had foundations placed properly under all walls. Also I was able to show the Architect, Staffan, what I was trying to do. To do this, I shared the model over GoToMeeting with Staffan, and he did point out a few changes I needed to do. I modifed the drawing, showed the model to Staffan again on GoToMeeting, and finished it. It worked out really well.
As you can see, I chose Option 3 above. My father was an engineer also, and he generally did his own drawings. He could draw well and very fast, and he said the time it took to explain his ideas to a draftsman (women weren’t in the business in his day), he could the same drawing several times over. I have the same issue. I can’t hand draw like my father could, I never had to put the time into it to learn the skill like he had. However, I’ve taken a lot of courses on AutoCAD, and I think I’m pretty good at drawing on the computer. I’m also pretty fast, so like my father, it’s not worth it for me to use a CAD person. I do use my son on many jobs because we’ve worked together enough he knows what I want, but some jobs like this I really feel like only I could do right.
Anyway, the design of this project worked out pretty well. The next stage is construction. I hope to intimately involved in the construction. My contract requires the client to contact me for a minimum of two site visits. I also explained to the builder that I want to go over everything with him to make sure there are no misunderstandings. The builder told me he has a great steel supplier, and he understands how critical this structure is. I have great hopes for this job, the key to making any job work is close communications between all parties.
Here’s a final view of my model in Softplan: