-Conventional vs Lightweight Construction - a primer…
"Building Construction for the Fire Service", Third Edition by Frank Brannigan
We need to be able to differentiate between “conventional” construction and “lightweight” construction. In conventional construction, you have solid structural elements -- 2x6, 2x8, 2x10 or larger rafters, floor joists, as examples. The distinction is important for the simple reason that solid structural elements will result in a longer burn time, flame exposure time before yielding to the stresses they are undergoing and failing, resulting in either a localized or more extensive building collapse.
In the case of lightweight types of construction, this is not the case. Engineered materials such as metal gusset plate trusses, plywood “I” beams, open web
bar joist trusses, unprotected structural steel elements, will yield under direct flame
exposure or fire exposure much more quickly because in the old days of conventional
construction, if the architect wasn’t quite sure if a structural element would carry the
anticipated load, they would simply move to the next larger size. If they weren’t quite sure a 2x6 would cut it, they would put in a 2x8 or a 2x10 just for the additional safety factor.
Nowadays, computer aided design systems engineer structural elements literally down to the exact safety factor that the building codes require for the anticipated loading the
structure will be expected to support. As a result, if the computer tells them that a
plywood “I” beam will suffice where they used a 2x8 before, they’ll use that plywood “I”
beam and it will fail much more quickly than a conventional 2x8 would. This is due to the plywood “I” beam having less mass than the conventional beam/rafter had, and elements of less mass that carry the same load are less fire resistant. Engineered components usually have a higher surface area-to-mass ratio, resulting in heat conduction into the structural component occurring more quickly than it would in a solid element.
In order to determine if you are dealing with lightweight construction it is necessary to examine the structural elements used in the floor and roof systems. If you find any extensive use of 2 X 4 or 2 X 3 materials, or unprotected structural steel, or any engineered components which use these lightweight elements, it should be considered lightweight construction. For the purposes of evaluating older wood frame structures, this would include those buildings with 2 X 4 roof rafters, even though they were conventionally built, as they will fail in the same rapid time frame that contemporary lightweight construction styles will fail in.
Modern Home Construction Methods Pose Problems for Firefighters…
Lightweight construction materials used in recent decades for home construction are generally sturdy, safe and meet code requirements… but are they safe under fire conditions?
Modern townhouses, residences, and other buildings are collapsing at alarming rates across the country when subjected to what many firefighter’s often think of as routine fire conditions.
When a townhouse on fire quickly collapsed recently, neighbors were shocked. Firefighters there called it routine and an example of the problem with modern construction techniques.
Lightweight construction materials used in recent decades for home construction are generally sturdy, safe and meet code requirements. But according to many firefighters, when exposed to fire, the components quickly break down and can lead to a rapid
collapse of the structure. Many in the fire service believe this is a safety issue for firefighters and the public.
In that recent fire, the collapse occurred minutes after firefighters were ordered out of the burning townhouse. Firefighters are taught that generally, interior firefighting efforts are more effective than attempting to extinguish a fire from the exterior.
However, with lightweight construction, conditions for firefighters generally deteriorate much more rapidly than homes built prior to the 1970s.
Most homeowners have never heard of the term "lightweight construction". They are generally shocked to see the relatively rapid collapse of roofs, floors and other interior spaces in burning homes.
This is not a new issue, but it is one that is a good lesson for homeowners and firefighters alike. Many fire chiefs in today’s fire service think a combination of smoke alarms and sprinkler systems are the best way to ensure your safety and to protect your property.