In the last two articles we looked at concrete and steel, two of the basic building blocks of commercial construction. This month we will discuss wood and how it is used in commercial construction. As we know, wood is the primary material used in residential construction and it's use in commercial buildings can be different.
I feel most firefighters would be surprised at how often wood is used as the primary structural component in commercial buildings. There are also many misconceptions about the use of wood as the main structural component of buildings; many firefighters believe that all wood-frame commercial buildings need to be sprinkled, which is not the case. This month we will discuss some of the common practices of wood-frame construction and some of the building code requirements surrounding it.
Many commercial buildings that use steel and masonry as their main structural component also use wood for non-load bearing interior framing. Wood was used extensively for interior framing of partitions and walls until the last couple of decades when metal studs became the framing material of choice. This article will not discuss wood used in masonry and steel buildings; it will be limited to buildings that use wood as their main structural component.
The use of wood framing in building construction has existed for hundreds of years in the United States. Wood is generally available in all parts of the country and was used to construct many buildings that exist in towns and cities today. This material has once again become an acceptable method for constructing commercial buildings. With the construction industry being more competitive than ever and the cost of concrete and steel increasing, wood has become a popular alternative. The continued development of engineered wood has increased its use as a structural component. Engineered wood, trusses, and beams are now available in lengths that span large distances and have excellent load carrying capabilities.
Many firefighters incorrectly think that all wood-frame commercial structures are sprinkled. According to the International Building Code wood-frame structures that are of the VB construction type can be two stories high with a total area of 9,000 square feet and type VA can be up to three stories high and 17,000 square feet of area without sprinklers. Once either the fire areas or heights are exceeded, sprinkler systems can be used to allow for additional height and square footage. This additional height and square footage can be determined based on a formula which is in the building code. It is very important to know your response area and the codes in your area; you need to know what your jurisdiction allows and what types of buildings you have in your response area. Recently, many jurisdictions have passed more stringent sprinkler requirements.
By now I am sure that many of you are thinking, well, this is all well and good but what does any of this have to do with firefighting. Wood-frame buildings are inherently dangerous under fire conditions. These types of buildings are completely constructed of wood including all interior and exterior walls and all structural components. The stability of the structure will deteriorate quickly if the component is exposed to fire. When you have a building that is completely framed of wood the potential for fire extension is very great. If the fire extends into the space between the ceiling and the floor above, or plenum space you are adding more fuel to the fire. The fuel is the wood structure itself, which is also holding up the building.
If you remember back to previous articles I discussed ceiling types, voids, and openings in fire-rated partitions and how they effect the growth of the fire. In many commercial buildings there are lay-in ceiling tiles. Under fire conditions these tiles may fail, which exposes the wood structure to fire. There will most likely also be horizontal extension of the fire, decreasing the stability of the building. There are many buildings that have no ceiling, which is probably the worst case because the building structure is immediately exposed to fire. If a penetration is made in a partition for a pipe or conduit you will have the same problem.
When dealing with steel or masonry building and the fire extends into the plenum space, you generally have a limited amount of combustibles. However in a wood-frame building that is all you have. The void space is between the ceiling and the floor above and it is a place for the products of combustion to build and gather heat just waiting for the correct mixture to ignite. We all know the dangers of lightweight-wood trusses, I-beams, and the many other lightweight wood products that are in use today. The most important is that under fire conditions they fail quickly. In many wood-frame buildings the lightweight wood products are what is holding up the floor, the trusses in turn are tied into the wall, making an entire assembly. Like steel, the fire resistance of wood is proportional to the size of it. The larger the wood beam or truss the longer it will take to burn. In today's construction practice with cost being a driving factor on many projects, the wood will be as small as possible.
Knowing what happens in a wood-frame building is the easy part. If we know we have a wood-frame building we know we have a limited amount of time to perform interior firefighting operations. The difficult part is actually determining that the building is of wood-frame construction. As we have discussed in past articles the fire department needs to have a good working relationship with the code enforcement or building department, and the building owners or facility managers in your response area. Although the building is wood-frame the building may have a variety of different exterior finishes including; siding, vinyl, cementious, wood, or masonry.
Collapse Concerns
Masonry exteriors on wood-frame buildings are of particular concern to me due to the potential for collapse if there is damage to the structural members the brick is tied into. If there is failure of an exterior wall due to fire, it is the high likelihood the masonry veneer will fail without notice. Your overall firefighting tactics will be completely different if you know the building is wood-frame. Accurate pre-planning of facilities in your response area is critical to effective firefighting operations. If you make the assumption that a commercial building's structural members are steel or masonry and don't find out that it is would frame until you are fighting the fire your, pre-plan is useless.
After reading this article you are probably thinking that wood-frame buildings are dangerous from a firefighting stand point and I would say there are. We need to keep in mind that the majority of the fires we fight are residential which is wood-frame construction. Wood-frame structures can be made much safer with the addition of a properly designed, installed, and maintained sprinkler system.
This is just a brief overview and discussion of some of the important issues with wood-frame construction. There is much more to this subject and I urge you to further research this topic.
MATTHEW STIENE, a Firehouse.com Contributing Editor, is a project manger for the Mecklenburg County Real Estate Services Department, and a firefighter with Robinson Volunteer Fire and Rescue, in Charlotte, NC. He is a licensed professional engineer in North Carolina, New York and Pennsylvania, and is a certified facility management professional. To read Matthew's complete biography and his archived articles, click here. You can reach Matthew by e-mail at [email protected].
