Know Your Enemy #48

We continue with our pictorial discussion of trusses which started with Know Your Enemny #44, Previous units are archived.

Photo By Frank Brannigan

Photo R252 - This is a wooden scissor truss that is about 300 years old. This type of truss was used for a high clear ceiling. A fire could cause failure to any part which will cause the truss to fail in a short amount of time. When a roof collapse is anticipated go defensive AND set up a collapse zone. The walls are vulnerable, assign an officer to police the collapse zone boundary.

Photo R262 - Truss roofs can be complex and create the hazard of massive collapse from a ONE POINT FAILURE. A heavy truss girder (a girder is a beam which supports other beams) supports a large number of smaller trusses. The bottom chord of the truss, which is under tension is constantly pulling on the gusset plate connectors, the bottom chord consists of a number of two inch planks connected end to end by gusset plates. All the gusset plate connections are located at the same point, no overlap. A fire attack at that point could cause A FAILURE OF THE GIRDER AND THUS THE COLLAPSE OF THE ROOF. I Have noted DENNY's restaurants in the US with truss on truss construction but do not know how their truss girders are connected. In other cases the girder is unprotected steel I urge you to consider a DOA (defensive on arrival) SOP

Photo By Frank Brannigan

Building deficiencies like these should be explained to the owner occupants, tell them that it is their best interest to sprinkler the building and be very pro active in fire safety. Also be sure to inform these occupants to call 911 on smoke, not to waste time looking for the source, while the fire grows

In 1971 I wrote an article in NFPA's "Fire Command". It was about telling owners of a property structurally unsafe for fire fighting, that a fire on the property would be fought defensively unless lifesaving was involved. The article included an example letter. Ask me for a copy of "Do you know your buildings" by emailing me at