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Glue vs. Gusset Plates...The Danger Surrounding Us!

It has been quite a few years since I have graduated college with a degree in building construction design. As I was growing up, I was always swinging a hammer high up on the roof trusses.  Little did I know, how valuable the knowledge and experience on building construction materials, elements, and structural members would so relevant to my life long dream of firefighting.  So many debates on lightweight construction and the hazards we face on the simplest run, make you wonder what reaction a little heat will have on you as a human being.  We must not forget that as buildings are our worst enemy when fighting a fire, so is our "lack" of knowledge and first hand experience with the enemy.  It is unbelievable how far building construction has come since graduating in 2000.  As times and economies change, what do we have to look forward too as emergency responders?

           Building materials have changed drastically over the years in regards to the types of materials used in building construction.  In today’s building industry, fire and emergency services personnel are introduced to light weight construction materials which are engineered products assembled in a factory.  Pre-engineered trusses are manufactured to save time during the construction process, creating hazards for firefighting operations.

            Lightweight building construction has been known to be a firefighter’s enemy and has been coupled to factors associated with firefighter deaths and injuries.  When firefighters look at the products being built in the manufacturing industry, it is a common principal that lightweight materials have been introduced to save money for the builders and homeowners.  Lightweight construction materials have been challenging for the fire and emergency services due to the exposure of materials burning hotter and quicker.  Pre-engineered trusses that are glued at the finger joints were introduced in 1989 and are used primarily to support floors.  Pre-engineered trusses do not use gusset plates, and are called “Open Joist 2000” (Bracken Engineering, 2012), which are manufactured in Canada and distributed through the Universal Forest Products, Inc.   

            The “Open Joist 2000” (Bracken Engineering, 2012) truss system is a finger jointed joist that uses glue to adhere the pieces of wood, saving time during the build process.  Gusset plates, connection plates with small teeth used to penetrate the wood products covering three (3) cross members, have been used for a long time.  Tests at the National Institute of Standards and Technology (NIST) have been conducted that show the plate actually chars slower than the wood surrounding the plate.  However, the gusset plate’s behavior can react in a negative manner when heat is applied.  Since the gusset plate does not penetrate the wood deeply, heat causes the gusset plate to pop off the lumber causing the bracing members to disconnect themselves. This process occurs at temperatures as low as 880 degrees Fahrenheit (Bracken Engineering, 2012). 

            Glued truss systems use an adhesive called “Phenol – Resorcinol Formaldehyde” (Bracken Engineering, 2012) to hold the finger joints together.  As cross members are fed through a machine creating the finger joints, adhesive is immediately applied on a jig to form the truss.  “Phenol – Resorcinol Formaldehyde” (Bracken Engineering, 2012) is a thermosetting adhesive that hardens up when exposed to higher temperatures, which begin to char at 1130 degrees Fahrenheit.

            Although researchers from various firms have conducted studies on the adhesives used in glued truss systems, firefighters must not take this construction element lightly.  According to Gallagher (2009) in his article in Fire Engineering, The Dangers of Modular Construction, tests were conducted by researchers at Cone Calorimeter on adhesives used in modular home construction.  During the research, results of heat energy released from structural adhesive was seven (7) times more than polyurethane foam plastic; which was a major factor in The Station Nightclub Fire.

            When the glue heats up in the finger joints, there is no gusset plate to protect the joint; immediately exposing it to the fire and heat.  As the finger joint loses structural integrity, it will create a risk of collapse or sag (Bracken Engineering, 2012).

            Firefighters must be able to recognize warning signs that relate to structural compromise when engaged in firefighting operations.  Having a good Incident Safety Officer (ISO) working in the command organization is crucial, but a strong attribute to firefighter safety is self-imposed.  

            As firefighters begin to work on interior operations, there are some key indicators that can be recognized while performing fire attack and overhaul operations.  The most recognized indicators are:

  1. Sagging Floors

    1. Missing cross members

    2. Burned joints 

  2. Collapse

    1. Missing cross members

    2. Separation of the glued joint

    3. Broken web members

  3. Shrinkage

    1. Results from the joint being heated

    2. Fingers in the joint become disconnected

  4. Charring

    1. Exposed web members

    2. Fire travel along voids in the cross members

The indicators listed above, give firefighters the basic concept of what to look for during their operations.  When a group of firefighters start to make a push inside for fire attack, recognizing a failure in the floor system should result in an evacuation of the structure. 

            Fire departments all over the country should use preplanning to their advantage.  This tool can be utilized by personnel to get a visual perspective on the construction features that are being introduced in the first due area.  Having a solid set of pre plans can assist in reducing firefighter injury and Line of Duty Deaths associated with rapid fire spread, flashovers, collapse, and entanglement issues.

            When firefighters conduct pre plans, a note should be annotated on what type of truss system is being utilized for research and training knowledge.  When responding to a fire in a structure which has been pre planned, the firefighter has the knowledge of performing a size up during the response.  This allows the firefighter to have a mindset of what might be encountered while operating inside of a structure and what indicators to look for.

            Based off the information in this study, I hope you see the differences in the building industry and the need for knowledge on construction principles.  It has been determined that firefighters are being subject to hazards at the cost of builders and homeowners.  Firefighters must be able to recognize indicators of structural failures early in the incident, reducing injury and death.  Having a good set of pre plans can assist the crew performing a scene size up before arrival, having a positive mitigation of the incident.

~Jeremy Rebok~

 

 

 

References:

Bracken Engineering. (2012). Fire Load Performance Characteristics of Metal Gusset Plate Trusses vs. Finger Joint Glued Trusses. Retrieved May 17, 2014 from: www.brackenengineering.inc ;

U.S. Fire Administration and National Fire Academy. (2006). Coffee-Break Training. Retrieved from: www.usfa.dhs.gov/training/nfa/coffee-break/.

Gallagher, K. A., (2009). The Dangers of Modular Construction. Fire Engineering. Retrieved from: www.fireengineering.com/articles/print/volume-162/issue-5/features/...

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