Revit - Residential Foundations, Pads, and Phasing Topo Surfaces

 All right, Revit junkies.  This will be the first in a series of posts talking about making a full BIM model of a traditional residential project in Revit.  Today, I'm starting with the foundations.  For this house, we've already gone through the design phase, and I have a completed Revit model from schematic design that I will be using for reference in the Construction Documents phase.  Typically, in order to avoid any awkward wall joins between schematic walls (generic 6" walls are the ones I typically use), and CD walls (which will include all the wall layers like studs, plywood, sheathing, etc), I use autocad files that I've exported from my SD model as a reference, and import them into my new file.

Topography

When starting any CD project, I try and start with a topo map of at least 2' contours, ideally 1'.  I like to draw the surrounding topography of a few blocks and put that in the existing phase.

Just as a side note, when drawing the topo surface, I like to draw a box just inside the topo lines to use when picking points, so I can avoid areas where points start making funky slopes along the edges of your surface.  Another way of avoiding this is to draw a toposurface slightly bigger than the toposurface you are given, and then just trim off the excess by using the split toposurface tool.  Then, delete the surface on the outside, and you are left with a perfect square.  The thing is, you can now use that new surface to create boundary points, which can be helpful, but I digress...

Then, I split the toposurface using the property lines of my site, using the pick line tool, to give me my site's toposurface independent of the surrounding toposurface.  You should now have two surfaces, both on the existing phase; one is the context and one is your site.

Now is a good time to make any grading adjustments to your site that you need to.  To do this, you should use the graded region tool to make a new surface of JUST YOUR SITE, and make whatever changes you need to topography.

Pads, Foundations, and Phasing

*SPECIAL NOTE* for sites that have existing building pads.  Unfortunately, existing building pads that need to be demolished do not seem to work correctly.  They screw up the phasing as well as the new toposurface.  This is one reason that I always try and avoid pads at all cost.  There are others, but I won't go over them now.  For now, this is for a project that has no existing building or foundation on the site.

Ok, so to start off, I would avoid using building pads at all cost.  Instead, I would use the split surface tool, as it gives you much better control over your topography, and a foundation is a much better tool to work with than a pad, because you can add extended edges where you need two foundations to lap together, like where you might have a garage and a floor slab separated by a step up of a few inches.  I'll be going over that a bit later.

First thing I would do is split your new topo surface at any point that you are now going to be having a foundation.  Also, anywhere you will have a change in floor elevation, split the surface.

Here is a 3D View.  The elevations shown represent where the bottom of my 4" Slab will be hitting the ground.

Here is the plan view that I used to draw the split lines.

After you've split your surfaces, just add foundation pads (found under the structure tab) over your topography surfaces where your building is going to be, and then add foundation walls for your footers, and then footers using the foundation tool again, and clicking on the walls.  For this house, the foundation walls went around the perimeter of the house, including the garage, which left a gap in the middle of the house where the house goes from the garage to the finished house floor.  To fill in that gap, you can use the slab edge tool, which snaps to the foundation pad.  If you had been using a crappy toposurface pad, you wouldn't have been able to add that fancy pad footing, which is a necessary detail when doing the construction detailing.

There are a lot of customization options when you get away from using a site pad, and instead split your surface when doing work.  I have a suspicion that if you have an existing pad on your site and need to model it and keep track of the cut/fill, that this would also be the best way to go rather than try and use pads.  However, I haven't done much research into this.

  The next series will start to go over wall constructions and how they join with the floors.  Stay tuned...