Tips and Tricks for Modeling Existing Conditions
Updated: May 26
When modeling existing conditions, it's a question of orthogonal or real-world.
I may make some readers upset, but I contend that modeling existing conditions is exponentially more difficult to do than modeling for design. After all, there is no need to define a level of accuracy with design models. If you need to represent a structural grid you just lay out your design grid spacing, place your columns at the intersections and you are done.
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However, with existing conditions there is so much more to consider. If you have ever had to set a structural grid on a point cloud, you probably noticed that the center of the columns is often not in precise alignment with one another. This may be caused by many reasons such as imprecise construction methods, settling of the building, and columns that are not perfectly plumb just to name a few.
Take, for example, the following illustration.
We have two steel columns on the same column grid. The first step to place the columns is to locate the grid on the column centers. Given the steel is exposed with no fire proofing, we can take a plan view slice at cut line A-A and C-C to determine the placement of the column grid line by choosing the center of each column at this position. We can also confirm that our column family matches up closely with the real-world representation shown in the point cloud.
Now let’s take a slice a little higher on the same columns at cut lines B-B and D-D
In this instance we notice that the point cloud and the modeled column no longer match. Given our point cloud data is level, this illustrates that the columns are leaning slightly since our modeled columns are orthogonal and are perfectly plumb.
The question then becomes, how do you model this condition? Where do you place the column grid? Do you adjust your modeled column to be non-orthogonal and follow the point cloud? If you are contracted to a certain level of accuracy, this may play into your decision of whether to show the modeled element non-orthogonally. But you still need to decide where to place the column grid and whether you will maintain the intended design grid spacing.
Most recipients of an as-built model will prefer things to be modeled orthogonally. Doing so will introduce more error into your representation of the existing conditions which in turn, may put you outside your contracted accuracy tolerance. As you can see, modeling of existing conditions can create many more challenges than are encountered simply by modeling design intent. There are a number of best practice workflows that may be employed to address the concerns outlined above. If you have any favorite workflows, I would love to hear from you in the comments below.
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