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  • Envelope #31: Cliff notes for “Type of Construction” and why you might care as a structural engineer

Envelope #31: Cliff notes for “Type of Construction” and why you might care as a structural engineer

Happy Thursday! Andy from Back of the Envelope here – this is where I share fun-ish SE-related things (plus a dad joke), all in 5 minutes or less.

(My goal is to, hopefully, help you learn something, have a quick laugh, and maybe share it with your friend/coworkers – in less time than it takes to open a Revit model!)

Today’s email is going to be about the “Type of Construction.”

As structural engineers, we could probably get by 97.5% of the time not knowing anything about it.

But by being somewhat informed, we can understand and speak “the language” a little better with clients.

Let’s dive in!

(Estimated read time: 3 minutes and 51 seconds)

Years ago, I remember someone asking me if I was working on a “Type Five” building, and embarrassingly, I had no idea what she meant.

“Uhm… five?”

“You know, Type V? Wood buildings?”

“Oh right, yup… 🙄”

Eventually, I figured it out…

Buildings can be categorized as type I, II, III, IV, and V based on IBC Chapter 6. It represents the fire resistance and fire performance of a particular building.

As structural engineers, we rarely venture out of Chapters 16-23 of the IBC (or CBC if you are in California), so this was pretty unfamiliar to me.

Recently, during one of our monthly roundups at my workplace, the topic of “fire-retardant-treated wood” (FRTW) came up, which inspired a slew of questions related to the Type of Construction.

I am going to share what I learned in a Q&A format below.

By the way, as with the rest of the building codes, there are exceptions, footnotes, and other nuances (surprise!). But the information here should be sufficient to at least get you started on becoming familiar with the subject.

What is FRTW, and why is it important for us (structural engineers)?

Fire-retardant-treated wood -- It’s pretty much what the name suggests: wood that has been treated with chemicals to resist ignition and slow the spread of fire.

It’s important for us because there is a reduction in design value when the wood is treated.

For example, here is a snippet from an ICC-ES evaluation report for one of the products (there are many): link

As you can see, the reduction can be substantial depending on the species and the usage. Therefore, we must consider it in design if we know that FRTW is required.

So when is FRTW required?

Short answer: when the construction type is III (both III-A and III-B).

It came from this code section:

What other types are there?

  • Type I: Building elements are non-combustible and fire-resistive (i.e., steel and concrete).

  • Type II: Building elements are non-combustible, but the assemblies have little to no fire resistance.

  • Type III: Exterior walls are non-combustible or FRTW; interior elements are combustible.

  • Type IV: Heavy timber.

  • Type V: Building elements are combustible (wood).

What are “elements”?

See the left column on Table 601:

What’s the difference between subtype A and B or C… etc.?

“A” is more restrictive – meaning the architect needs to detail and specify the assemblies that meet the required fire rating based on the table above.

For example, Type V-B has no fire rating in anything, so you could do whatever you want (figuratively speaking). V-A on the other hand, needs a 1-hour rating in various things, so certain things need to be covered with gypboard and whatnot based on the approved assemblies.

Now, the good stuff…

Why is the Type of Construction important to the architect or the owner?

Because it affects how tall, how many stories, and how big the building can be (based on the occupancy type – another topic).

Other factors like frontage size and sprinklers can also help increase the allowable height/stories/area.

If FRTW costs more than regular wood, and reduces design values, why would anyone want a Type III building instead of Type V?
(especially popular for “wood framing over podium” type of buildings)

The short answer is that since Type III allows more area and stories, the cost tradeoff can be offset by the value created with more space.

For example, for typical residential stuff (hotels, apartments, condos, etc.), Type III-A with sprinklers allows up to 5 stories above the podium. Whereas Type V-A only allows up to 4 stories.

The 5-stories allowance rivals the allowance for Type II, and is supposedly much cheaper to build.

Side note: Interestingly, the introduction of heavy timber types IV-A, IV-B, and IV-C in the 2021 IBC further allows more flexibility for taller buildings using heavy timber and wood. More on that in another email.

Cool… so how does that affect us?

Here are some ideas:

At the start of a project (or anytime really), check out the first few sheets of the architectural drawings (usually T or G) to see if you can find out what Construction Type you are dealing with.

If it’s Type III and a wood-framed building, automatically apply the reduction anticipating FRTW for exterior walls (usually in the ballpark of 10% or so, depending on the manufacturer and species). And also include sprinklers for the dead load.

If the construction type is not indicated on the architectural plans yet, at least now you know how to ask intelligently about it!

(e.g., if you are dealing with a 4 or 5-story wood building, ask the architect whether the building is Type III or Type V because if FRTW is used, it’ll affect the structural design.)

That’s all for now. Hope that helps!

Laugh or Chuckle (and/or Cringe)

(A colleague once suggested I include a dad joke in the Back of the Envelope because, first, I love those. Second, if you didn’t learn anything from these emails, at least you’ll get a quick laugh!)

This is one of my son’s recent favorites…

Have you heard about the movie ‘Constipation’?





Because it never came out! 😆

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