There are a number of forces and loads applied to a structure at any time. Forces applied are compression, tension, torsion, and shear. Loads include Dead, Live, Impact, Wind, Static/ Repeated, Concentrated, Axial, Eccentric, and Suspended. We won’t attempt to go over every single type of force or load within one post, but we’ll start off with Shear Forces because they are often not considered opposed to others yet play a significant role the collapse of building elements. Shear Forces occur within a building member when opposing forces pull the member in opposite directions. When a passenger bus stops short, the bolts that attach the seats the floor undergo shear forces. How do Shear Forces impact your job as a firefighter? Building materials that are affixed to buildings as dead loads such as steel beam-columns, decks, and roll down gates are all under sear forces, and their failures can yield collapse of the structural member and other building components. We have to first accept that buildings are under constant stress and strain, gravity is always acting upon a building to bring it down. When you drop a pen to the ground, the speed that you are witnessing is gravity’s force, and that is the acceleration (9.8m/sec2) that is always acting upon the structures we operate in or on.
Roll down gates are dead loads attached to a building with shear forces acting upon them. They are either attached to wood or masonry structural members. In the commercial setting we see this often. Firefighters attempting to force entry into a burning structure are tasked with attacking roll down corrugated metal doors to gain access to the fire area and suppress. What firefighters often overlook is how these doors can collapse, and how many firefighters have been struck by roll downs due to the fires burning within or behind them. In the commercial setting, let’s say with an Ordinary (Type III) structure, a roll down is attached in either two ways. Wood dimensional lumber can be affixed to both sides of the header/lintel with carriage bolts, and the housing for the barrel assembly is subsequently affixed to that. Another way includes large threaded rods with bolts often seen in old roll downs that are drilled through the masonry and affixed with bolts. Other methods of attachment can be found obviously depending on the installer. When fires in commercial buildings begin to mushroom and roll across the ceiling, in the direction of the entrance to the building they reach the connections of the hood assembly that are interior. The wood components begin to burn, and will either release the bolts or the connectors themselves being cold drawn steel (Failure at 800 deg) will expand and release. This can cause the up to 1000lb housing and roller to let go causing serious injury or worse to firefighters beneath. The Shear force acting upon the bolts connected to the building aid in the demise of the assembly.
Steel frame structures utilize hot rolled rivets to connect beams to columns. These are called
shear connections. We are concerned with unprotected steel elements. The release of a shear connection in a steel framed building can be catastrophic. If a steel beam is sheared from the column, it can cause subsequent collapse of floors. This doesn’t exclude fire resistive applications. Shear force does not subside because of applied fire resistance; with fires that are prolonged, applied fire resistance can be compromised. It should also be noted that the impact load of a collapsing floor can cause the rapid failure of shear connections. This was seen in the world trade center disaster, when floor assemblies released causing progressive collapse and subsequent failure of the shear connections.
One of the more recent notable collapses involving a shear connection was a deck collapse in Wildwood, NJ on September 14th 2019 injuring more than 20 people. The Deck was affixed to the building via shear connection and supported via ascending columns on the outer portion. When excessive live loads were applied to the deck the shear connection failed, releasing the deck and sending it to the ground. If the building had been under attack by fire, the old building and its shear connections could have failed sooner with only the addition of a few firefighters
Shear forces and connections are only one of the many points of the structure where if weakened can cause deadly effects. Firefighters need to be cognizant of these connections and understand that both fire load and active live loads in the form of firefighters create an acceleration of compromise. We cannot get tunnel vision and forget that even without fire, the building is under stress and wants to come down.