Detailed Design — How To Design A Structure

After the architectural design was complete, the next step was to actually figure out how to construct it. My plans needed to be detailed enough for the city building department to approve them. At first blush, one might think that pushing out a bedroom by seven feet is no big deal. Throw together a few trusses, use the existing flooring, build some walls with holes in them for windows, and you’re done.

Well, not really. The short story is that building codes have advanced, and when you build an addition, you are actually going to build a carefully engineered structure. If you’re really not a die hard DIY and/or don’t have any background or training in structural engineering or construction, then your best bet is to hire a designer to do the work. They aren’t cheap (I got a quote for $7,500 minimum), but it may be worth it, depending on the complexity of your project. Having said that, you don NOT have to be a structural engineer to design an addition , or any other structure for that matter. All you have to do is follow the prescriptive  methods contained in the applicable codes. This is essentially a “cookbook” method of designing a structure that includes a number of safety factors such that a design using these methods will withstand loads and stresses (people, wind, earthquake, etc.) that are expected for a residential home in a specific location. Here is where you can benefit from my experience. DO NOT try to get all fancy and design something that is not clearly specified in the codes and deviates from the “cookbook recipe”, then you will have to get a sign-off by a licensed Professional Engineer (P.E.). Again, this is expensive, and probably not worth it for a smaller project. So if you’re willing to spend some study time (and maybe even learn something!), a DIY solution awaits!

The best place to start is the building code that is applicable to your jurisdiction. The California Residential Code is actually reasonably easy to follow, but I found the American Wood Council Wood Frame Construction Manual (WFCM) a better resource for my purposes. Since the California Residential Code allows it, that is what I used. What really made a difference and put it all together for me was the WFCM Workbook, which has an example home design that steps you through the process. You will also have to determine the environmental conditions that your structure will need to withstand. This includes maximum wind conditions, seismic design category, whether or not you’re in a flood zone, maximum and minimum temperatures, termite infestation likelihood, and other factors. These are usually spelled out in the code and it makes sense to put together a little table for yourself so that you can refer to it when bouncing back and forth between the various parts of the code to get your numbers.

The approach that I used, which was taken directly from the WFCM workbook, was to start at the top and work my way down from the roof to the foundation. At each step you not only have to specify the materials (trusses, roof underlayment, studs, joists, etc.), but you ALSO need to show how these elements are connected. The code gives a table of fasteners (mainly nails) for fastening framing and sheathing, but when it comes to connecting major assemblies to each other, you typically have to use engineered specialty connectors (for example, roof truss to wall top plate). You have to be able to show, step by step, that the loads from each element are transferred through successive elements all the way to the foundation. So, roof to wall, wall to floor, floor to wall, wall to foundation.

A quick word about fasteners. The common nail is a very nuanced component. There are many types of many materials, and it is IMPORTANT that you use the right nail for the right purpose. The tables in the code tell you what to do, but therein are requirements for not only nail type, but spacing, and orientation, e.g., toe nail vs. face nail. Bigger and more is not necessarily better because you risk splitting the underlying wood member. So follow the instructions! In general, nails are better than screws, especially for framing. This is because they have significantly higher shear strength, and have some ductility which means that they will “give” a bit in a storm or an earthquake whereas screws tend to be brittle. Not that screws are bad. Just don’t use them for framing or shear walls. An exception to this are structural wood screws (SWS). These are larger screws made of heat treated steels that have higher quality control than your run-of-the-mill screw. The manufacturers of these screws have data sheets which detail their application. I used them in some places, a ledger board for example, but in general, I stuck with nails when I could.

One thing that was scary for me at the last was trying to figure out how to retrofit concrete anchors to bring the addition into compliance with seismic requirements. Fortunately, the folks who make these connectors also provide a method of anchoring these connectors with special epoxy into existing concrete. The only “downside” is that I have to have a certified inspector sign off on the installation. Well, maybe not a downside as it really has to be right. Just additional expense.

Although I spent many months getting to this point, and went down a few “rabbit holes”, I can definitely say that the effort was worth it, especially as a DIY’er. The process of designing showed me how to build it with all of the right materials, methods, and references. It’s going to be really pro!

Here is a link to my detailed construction plans.

1370 BFD Detail Views

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s