What's on the news

"There was a fiery explosion at an apartment complex in Silver Spring, Maryland." (source: USAtoday). According to a recent investigation, the explosion was caused by accidentally cutting the gas line.

Why it matters:

This apartment incident highlights a significant difference in residential buildings permitted before 2009.

Making a sprinkler system a requirement for construction is nothing new. There was an optional sprinkler provision in 2006 edition of the International Residential Code. ICC members voted to make the sprinklers system mandatory in IRC's 2009 edition(1). Thus, now there is a footnote under IBC table 504.3 states, "New Group R occupancies are required to be protected by an automatic sprinkler system in accordance with section 903.2.8"

IBC Table 504.3. (Source: ICC) For best use of this chart please check the latest IBC edition at your local jurisdiction.

Other than changes in sprinkler requirements in multifamily homes, this type of construction is an ideal example for our discussion regarding the safety aspect of our built environment.

Dive deeper:

Lightwood frame construction

These three to four stories garden-type apartments are a typical solution for many multifamily homes; you probably have seen them everywhere in a populated suburban area. And most of these buildings were designed based on type 5 construction.

This type 5 construction is commonly known as "lightwood frame" construction. There are two sub-categories of type 5 construction: type 5A and type 5B.

The distinction between these two is based on their primary structure's fire rating. Type 5A has a one-hour fire-resistance rating on primary structure frame, exterior, and interior bearing walls. On the other hand, type 5B does not have any fire-rating requirement on those building elements.

Based on the aerial photo I looked up on Google Maps, I speculated this building complex was designed based on type 5A because the building meets the following limitations.

(Pause here, you might wonder why some elevation of this building complex looks like a four-story building, but it meets the 5A criteria? I will explain this in another post)

Fire protection from a Building Code standpoint

Based on the damaged area, we can tell: 1. the fire burned most of the exterior walls at west end. 2. the burned area seems to stop at the edge of the central staircase (that leads to the public way).

"So why does this type 5A which has a more restricted fire rating requirement than type 5B, still damage a significant portion of the building during a fire accident?" if you have such a question. That has to do with two other aspects of the building code: the "fire separation distance" and "fire barrier requirement on exit enclosures."

Site condition excerpt from Google Map (version 2018). The orange hatched area indicates the building portion that burned down by this fire incidence. (The distance marked on the map is approximate.)

“Why did the fire burned most of the exterior walls at west end?“

The term "fire separation distance" defined in chapter 2 replaced terminology related to "location on property" with a more general idea describing the spatial relationship of buildings on the same site.

In this table 602, we can tell that there is no fire-resistance rating requirement for buildings (of use group R) separate more than 30 feet apart. (in the case of Friendly Garden Apartment, two buildings were at least 50 feet from each other.)

Therefore, it is no surprise that most exterior walls turned to ashes.

Table 602 Fire-Resistance Rating Requirement per Fire Separation Distance. (Source: ICC)

"Why did the damage seem to stop at the this wall?"

Though I am not the designer of this building, I speculate this two hours fire resistance requirement at mean of egress saved the other portion of the building.

"Interior exit stairway and ramp enclosures shall have a fire-resistance rating of not less than 2 hours where connecting four stories or more..." (IBC 2015/ 1023.2 Construction) ICC members regulated this provision because they wanted to ensure all the occupancies could evacuate during a fire event. Making the evacuation path with a more robust and continuous fire-resistance rating will assure this goal.

A better picture for more affordable constructions in the future

Many opinions on why these lightwood frame constructions are so popular are because of their comparative-low construction cost. A shorter construction time is also a factor why developers like to use this type of construction. Since this type of buildings does not require 28 days of curing period compared to other concrete construction.

There is no doubt that installing automatic sprinkler system increases the cost of construction due to its extensive piping, maintenance and labor to the project. and often time that increased cost reflect on people’s rent or overall housing price.

Accidents happen. While many building technologies have evolved to provide us with quicker, more affordable housing choices, the safety issue will still requires our attention. Adding an automatic sprinkler system would probably be the cost-effective way to protect our property and life safety before reaching the ultimate economical housing solution.

Notes and references:

“Home fire sprinklers are a code requirement”, NFPA Fire Sprinkler Initiative
Prior than 2009 the International Residential Code regulated residential dwelling that has less than four units. The principle has changed since. If you read the current definition of Residential Group, it defined this use group as such, “Residential Group R-2 occupancies containing sleeping units or more than two dwelling units ….“
The current building code using in Maryland is IBC 2018. The adaptation of the IBC 2021 is currently under review. The ICC only offers IBC 2018 basic without local amendment. If you would like to understand the local amended version, please check UpCode. For Montgomery residents, pleas look up at Department of Permitting Service for additional information: Commercial Building Code, and Residential Construction Code.
Another side note, for explaining this building condition, I am using IBC 2015 as basic example. I have no access on when Friendly Garden Apartment was built, but suspected it built before the major changes in the code.
Generally speaking, the states require a routine safety inspection. Those inspections cover several safety units, such as boiler and pressure vessel, elevators, etc. Now the Montgomery County also offers "Fire Prevention and Code Compliance" Courtesy Inspections; you can schedule the inspection by contacting DPS (Department of Permitting Services).