Revit Architecture: Using Dimensions to control model geometry


In the realms of architectural drawings, dimensions are traditionally used to convey information (a distance or angle for instance) to somebody viewing those documents. However in Autodesk Revit, dimensions have an additional role- and that is that they can control elements. I like to think of this as a two-way relationship. You can place your elements into your project and Revit dimensions will display the respective data- ie the distance between two walls for example. OR you can use Revit dimensions to place constraints on those elements- in this scenario you are using dimensions as a sort of notation, to form relationships between different Revit elements.

Let’s take a look at this in practice. First of all let’s look at the “Equals” constraint. Let’s take a wall with 5 windows elements in it…..


You can see in the image above that all five windows are spaced unequally. We can actually use Revit dimensions to not only space them equally apart but then keep them equally spaced. First of all I need to add an Aligned dimensions across all five windows. The aligned dimension tool can be found on the Annotate menu, on the Dimension panel…..




The crucial thing to note here is that you must add these dimensions all in one go– do not place each dimension individually. They must be placed as a string of dimensions for this tool to work. Just hover over the window centers for Revit to pick up the geometry of each element…..


In the above image, you can see that Revit has found the centre of the window element- denoted by the short blue vertical line running across the element. In the image below , you will see that I have added my string dimension across all five windows….


OK. Now there’s two important things to note in the above image. The first is the small blue padlocks under each dimension segment. And the second is the small symbol which is made up of the letters “EQ” with a red diagonal line running through them. In the above image you can just see it hiding on the right hand side of window number 3.

Let’s start with the blue padlocks. If you move your cursor over them and click, you will find that you can lock and unlock each one. Quite simply, when you chose to lock a padlock in Revit, you place a constraint on certain elements- be they 3D model elements or 2D detail elements. In the example above, if we chose to lock the padlock under the dimension between windows 1 and 2, we would be telling Revit to always maintain an absolute distance (2110mm in this case) between these two elements. Once constrained we could move either window and the other one would automatically follow in order to keep the constraint. This is useful were we need to keep the start of a door opening a certain distance from the corner of a room….



Or if we need to maintain a corridor width……



In the above image, we could now move either of the corridor walls to the left or right and the other wall would follow accordingly. The two walls would also be spaced 2200mm apart.

So let’s go back to the funny little EQ symbol on our string dimension…..


The EQ stands for Equal and the red line through it tells us that the spacing of the segments are currently not equal- which we did already know to be fair! The clever part comes when we click on the symbol itself- which tells Revit to make the segment spacing all equal…..


Notice how Revit has actually moved the window elements so that they are all equally spaced. The red line has been removed from the EQ symbol because they are in fact now all equal. You will also note that the values on each segment have been replaced by “EQ”. If you wish to display the numeric value instead of the EQ symbols- just select the dimension and toggle the instance parameter “Equality Display” from “Equality Text” to “Value”…..


The above operation has placed an “Equality” constraint on our five windows. With this constraint in place, the five windows will always be equally spaced- irrespective of what the actual spacing is. For example, I could select window number 5 and drag it off to the right….


Note how all the intermediate windows have spaced themselves out over the increased distance. After you placed constraints on your model using the above techniques, you can go ahead and delete the dimensions if you wish. However, when you go to delete a dimension that has created a constraint, you will receive the following warning…..




If you select Unconstrain (highlighted in red), your elements all behave independently of each other- just as when you first created them. If you select OK (highlighted in purple), the dimension itself is removed but the constraint remains intact.


If you’re new to Revit Architecture, you may be interested in my freeComplete Beginner’s Guide to Revit Architecture84 part video tutorial course

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