After the Mission Bay fire: Construction types explained

The Mission Bay construction site fire on March 11, 2014 as seen from a BART train in West Oakland (photo by author).

Yesterday’s huge fire on a construction site in San Francisco left people with a lot of questions, some of which were circulating on Twitter as the fire was still burning. The six-story, eighty foot tall uncompleted structure burned out of control for several hours as nearly 150 firefighters fought to contain the flames and keep the fire from spreading to occupied buildings nearby. The apartment building across the street faced heat so intense, 30 windows facing the blaze cracked from the heat.

Many people were surprised that a 180 unit apartment building would be constructed out of wood. This is actually the most common way to build apartments in California, in anything but a high rise. As I’ve pointed out before, housing construction costs are very high and wood construction is usually the most economical way to build condos or apartments up to six stories tall. In some cases, with very large buildings, economies of scale will dictate concrete construction with metal stud walls, but in most cases developers choose to build in wood over a concrete “podium,” which is the platform structure the wood building goes on top of. It usually contains parking, retail, common spaces and sometimes additional housing units. In California, wood construction also has the benefit of performing very well in earthquakes. It is lightweight and its resistance to lateral movement (side to side wind or seismic forces) can be easily increased with plywood shear walls. Unreinforced masonry (brick or concrete block) on the other hand is a disaster in earthquakes– those are the buildings that are required to have warning labels next to the front door (you can’t build those buildings any more under current codes, for obvious reasons).

What is the building code?

The California Building Code (CBC) is based on the International Building Code, which is used all over the United States. It is concerned with prescribing safe methods of designing buildings, and is particularly concerned with safety and accessibility (although there is also a Fire Code that buildings must comply with). There used to be a larger variety of  “model codes,” which are codes that the state codes are based upon, but in the late 1990s the various codes were phased out and the IBC was promoted as a national standard. California lagged behind and took longer than the rest of country  to decide to adopt it, but the current CBC is based on it with some differences. It is worth pointing out that building construction is also regulated at the local level, for instance San Francisco has its own amendments that are published online. Additionally, there are many other codes that effect construction. The printed versions fill an entire bookshelf or more, covering plumbing, electrical, mechanical, energy use, wildland/urban interface zones, etc. Every building built within a particular jurisdiction has to comply with all of them.

So what are the various types of construction that can be used under the building code?

Every building is designed to fit the requirements of a construction type, of which there are five. The building can be designed out of the materials of a higher type for other reasons, but be evaluated based upon a lower type of construction if desired (i.e. a concrete building evaluated as Type V). There are also subtypes of each of these types:

Types I and II. Types I and II construction are those types of construction in which the building elements  are of noncombustible materials, which means steel or concrete.

Type III. Type III construction is that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of any material permitted by the code. Fire-retardant-treated wood framing is usually used for exterior walls, but traditional brick buildings with wood framing inside are also of this type.

Type IV- (Heavy Timber) Type IV exterior walls are of noncombustible materials and the interior building elements are of solid or laminated wood without concealed spaces. This is typical for old mill buildings, it is based upon the concept that once wood building elements are larger than a certain size they will char and not burn all the way through. Large wood beams and columns of this type are made out of laminated pieces of smaller wood today.

Type V. Type V construction is that type of construction in which the structural elements, exterior walls and interior walls are of any materials permitted by the code. This is the most common type of construction for residential building in the United States. It is usually wood studs covered with sheathing of plywood and gypsum board (drywall) and finish materials. Type VA is common for larger buildings, which requires 1-hour rated load bearing and exterior walls, and 1 hour rated floors and ceilings. Hour-ratings are based on the amount of type an element can withstand sustained fire. New York City has banned Type V construction in Manhattan, the Bronx, Brooklyn, and some portions of Queens and Staten Island. This article helps to explain how codes have evolved over time.

Construction types and compartments can be combined to make up a larger structure. There are area limitations in the code for each construction type and uses that are only allowed in certain types. Fire rated walls must be used to enclose things with higher fire risks, like parking or transformers. The code allows for a ground floor parking garage to be built out of concrete (Type I) with a wood (Type V) building on top of it. This can allow for each portion of the building to be treated as a separate structure. The code allows maximum sizes (areas and heights) for each construction type, and the area get larger as fire resistance goes up (a wood building has to be smaller than a concrete building).

Bonus area is given for certain factors. Including sprinklers and having fire department access on all sides of a building are two things that would allow for additional area of a building under the code. This is done to encourage safer buildings.

Type V construction is actually very safe, once it is finished.

All residential buildings are now required to have sprinklers in new construction in California, even private homes. Large fires are basically unheard of in sprinklered buildings, assuming the system is working. Fire treated walls with drywall on them are very good at stopping the spread of flames. However, when a  building is under construction (like we saw yesterday) there is still a big risk of fire until the sprinkler system is installed. There will always be a lag because the building obviously needs to be framed before pipes can be put in.

Fire safety is very important on construction sites, and despite all of the construction that has gone on in San Francisco in the past ten years there has been very few fires (the Santana Row fire in San Jose in 2002 comes to mind). Often, construction site fires are the result of arson, as we saw in Oakland a few years ago. Fire departments are heavily involved in the permitting process and it would be good to get more feedback on how to increase fire safety on construction sites without drastically increasing cost. An innovative new construction type, cross-laminated timber, holds a lot of promise (and that’s something I’ll cover in a future post).

5 thoughts on “After the Mission Bay fire: Construction types explained”

  1. I know the investigation still underway, but for the sake of discussion, what are the key things to watch out for?

    What are the requirements and/or best practice for welding steel (seismic braces / moment frames) or cutting rebar / studs / other steel on a largely wood frame structure? Are work-area drop-mats, portable fire extinguishers, or other low-cost preventive measures required/common/rare?

    With the cost savings of bolting pre-engineered elements (reduced on-site weld inspection), what would be the cost premium of eliminating this risk factor?

    Other things to watch out for?

  2. There are many safety precautions in place for welding near a wood frame structure or for applying hot roofing, which include fire extinguishers etc. There are very few fires that start as a result of construction activity as a result (also, there is little structural steel used in the wood portion of a building like this).

    What is important to remember in this case is that the fire started after the workers left for the day. Because of how quickly it burned, and thoroughly, we may never find out how it started (which is the case with the Santana Row fire I mentioned).

  3. At 80ft that was going to be a pretty tall for Type VA over Type I podium, wasn’t it? Do you have a guess at how that worked with the building code?

  4. I suspect that the building was not actually 80 feet tall, I think the reporters were guessing. I don’t know enough about the project to comment on the specifics of the construction but you are right that 80′ would be very tall for that type of construction.

  5. news outlets are saying suspected start was on 8th or 9th floor; so that’s getting pretty close to 80’… and if it did start so high, makes arson seem unlikely. My question was, regardless of how rare such an event is, how can we make it even rarer?

    I’ve seen structural steel mixed with wood above a concrete podium (e.g., s. side of 16th just west of Bryant), but not high up. But there could be welding (and certainly cutting) aside from structural steel – brackets, etc; connections between fire-stair cores and wood framing, other points?

    Anyway, we’ll have to wait for the investigation to conclude (though it may be inconclusive); but important to be vigilant and aware of the risks…

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