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In general, engineered wood construction refers to specially designed wood members used in applications where normal dimension lumber either would not work or would be prohibitively expensive. They can include beams, columns, arches, trusses and just about any other building component you can think of. This article will deal specifically with one increasingly common type of engineered wood beam, wood I beams. There are many other types of engineered wood beams available and many outstanding sources of information available on building construction and collapse and I hope that you take the time to become familiar with them. They include "Collapse of Burning Buildings" by Vincent Dunn, "Building Construction for the Fire Service" by Francis Brannigan and the "Know your Enemy" series by Brannigan in Firehouse Magazine and on firehouse.com.

Wood I beams may be the wood truss of the 21st century, in more ways than one unfortunately. A wood I beam resembles a steel I beam in that it consists of 2 small wood beams for the flanges connected by sheets of Oriented Strand Board (OSB). From a design standpoint they work the same way as a steel I beam. When a beam bends, the top surface shortens (compression) while the bottom surface stretches (tension). Portions of the beam on the top and bottom edges are stressed more than ones in the center because they are being compressed or stretched the most. The elements in the middle portions of the beam are stretched less and carry less of the load. Someone smarter than me figured out a long time ago that if you add material to the top and bottom of the beam, you can carry more load with the beam without overloading the elements in the center. In fact, you can also take away material in the middle portions of the beam since it doesn't carry much of the bending load. This is why an I beam is shaped the way it is, the flanges carry nearly all of the bending stress in the beam while the vertical web carries little bending. The web of the beam does have to resist the vertical loads in the beam and acts to hold the flanges together.

These beams offer a number of advantages over regular wood beams. They are much stronger than sawn lumber studs so they can span much greater distances, sometimes as much as 30' or more. In residential construction, this can sometimes eliminate the need for a steel support beam in the middle of the floor; the floor will span from the front to the back wall. They deflect less than regular wood beams so they don't "bounce" as much when people walk on them. You can also pre-engineer openings in the beams for HVAC or other penetrations.

The disadvantage that we are concerned with is that these beams have historically performed poorly under fire conditions and tend to collapse earlier in a fire and with little warning, just like a truss. In my opinion, there are a couple of reasons for this. First is the obvious lack of mass of the beam, particularly the OSB web. A study was done by the US Department of Agriculture that tested the charring rate of OSB boards. The boards were found to char at a rate of about 1.6 inches per hour when exposed to a 572°F fire. While this isn't much different than a rate for solid wood, you need to consider the thickness of the web. If the web is ½" thick and is being exposed to heat on both sides, the web will char through in about 10 minutes.

Another reason is something that these tests fail to address. The tests are done to look at the rate a fire will penetrate the wood. One thing they don't consider is that the OSB in the web is under a variable load. The resins used in making the OSB have significant amounts of water in them, and when heated they will render the water out. When you put a load on the beam, like you walking on it, the beam tries to deflect. Even though the web is carrying only a small amount of the load, it is enough to pull the particles of the web apart. Since the resin has failed, the OSB simply falls apart. No more web, no more beam, and whatever was on the floor is now in the basement, including you. Now the engineered wood apologists of the world will cite all kinds of wonderful test data about how well OSB performs in flame spread tests. That's all well and good for flame spread, too bad their product falls apart when exposed to heat and a load is placed on it.

Other reasons this type of construction performs poorly in fire are fewer members are often used and they are typically end connected to their supports. Since the beams are stronger than normal floor joists, designers can often place them at larger spacings, often at 24" spaces or more. They also are normally designed using a joist hanger connection rather than sitting on top of the support. Joist hangers have problems under heat similar to the plates used to hold trusses together. The connections can fail in a fire even if the beam is still somewhat intact.

I found a good example of a wood I beam floor failure at www.firefighterclosecalls.com. This incident occurred in Baltimore County, Maryland. The floor failed about seven minutes after the first arriving engine got on scene. Here is the link to the PowerPoint presentation of this fire: http://www.firefighterclosecalls.com/FIREBOX14-7.ppt.

There are a couple of interesting things to note about this incident. Look at the photos taken at the first floor level. There is little, if any, heat or smoke damage to the walls. The walls in the basement don't show a lot of evidence of very high heat either. This fire does not appear to have been burning for long or have been particularly large. This appears to have been a fairly routine incident that went downhill in a hurry.

There are important lessons to be learned from an incident like this, both for rapid intervention and for basic firefighting. You need to know what is being built in your district and how it is being built. You need to preplan, not just the high hazard commercial property in your district but everything. You need to develop tactics and SOPs to fight fires in this type of construction without exposing your people to undue risk (it might be time to dig out your cellar and piercing nozzles from the storage cabinet in your station and put them back on your engines). You need to think before you act, and look at the building before you charge in.

From the RIT and safety standpoint, you can't be complacent about keeping your people safe on "routine" incidents. Would you have assigned a RIT on your first alarm for this incident? How about a safety or accountability officer? Would you have even put your accountability procedures into place for a routine incident like this?

The type of building construction is just as important to the RIT as it is to everyone else on the fireground, and in some ways perhaps more so. During your size up you need to look at the type of construction and identify collapse hazards. You need to consider what actions you are going to take if there is a collapse. Are you going to need a shoring operation? Do you have alternate access to an area that has collapsed? Look at the incident above. After a major collapse of a large, interior portion of the floor would it be safe to send your team in on the remaining floor, or do you have to create alternate access into the basement? With this type of construction, it is likely that a large portion of the remaining floor has also been weakened.

The saying "They don't build them like they used to" unfortunately is very true, and firefighters are getting hurt and killed because of it. Take the time to educate yourselves and your fellow firefighters on building construction and then apply what you learn in the field.

About the Author

Thomas Anthony, P.E.
Structures Specialist, Pennsylvania Task Force 1 and PA Strike Team 1 Urban Search and Rescue
Rescue Captain, Adamsburg and Community Volunteer Fire Department