Building Construction: Structural Loads, Defined
Learn how buildings support loads for safer firefighting ops
This column supports Chapter 2 of my book, Building Construction for the Fire Service, Third Edition (BCFS3). This chapter, called Principles of Construction, is difficult for some to teach or even study because it introduces a new vocabulary and changes the definitions of some words as we know them.
Loads are imposed on buildings in three ways, and firefighters should learn to recognize load-support systems to help them develop preplans and avoid possible catastrophe during a structure fire.
The 1'-square wood column centroid rests right atop the granite block’s centroid, which is evenly placed on the brick pier. This is the most stable way to impose a load and provides the greatest carrying capacity.
This eccentric load is perpendicular to the column, but it doesn’t pass through the column’s centroid. As a result, the load pushes down on one side of the column (compression) and pulls up (tension) on the other. The column appears adequate; perhaps it’s filled with concrete, with reinforcing bars providing tensile strength on the pulling side.
The three types of loads include axial (or concentric), eccentric and torsional loads. An axial load’s force passes through the load-support structure’s centroid. (If you draw two diagonal lines in a square they cross at the centroid — only circles have centers.) Axial loads run perpendicular to the load-bearing structure’s plane, and are applied evenly to the load-bearing structure.
Eccentric loads run perpendicular to the plane of the load-bearing structure, but don’t pass through the structure’s centroid; that is, an eccentric load is concentrated on one side of the supporting wall or column’s center.
Undesigned shifts from axial to eccentric loading have caused many structure collapses. The “Garage” sign on the wall of this building is eccentrically loading the wall. No one is anticipating a collapse. In fact, firefighters are bringing up a handline for an interior attack, and a firefighter is on the top of a ladder at the wall.
Torsional loads are offset from the shear center of a load-support structure and therefore apply a twisting force to the structure.
During a structure fire, the damage inflicted upon a building’s structural supports might change the loading with disastrous results. Example: If one part of a steel-frame building fails during a fire, it could place an undesigned torsional or twisting load on another part of the building and extend the collapse area.
The wall collapses "without warning," injuring a number of firefighters. Why? The warning existed but wasn’t recognized. The Garage sign changed the load on the wall from axial to eccentric. The pull of such a load on the wall was countered by a rod or rods attached to the wall and the roof structure, but when the roof structure burned away, the rod released and the eccentrically loaded wall fell.
This steel floor-support structure was subjected to heavy, confined fire. As the steel heated, it accommodated the elongation by twisting and ultimately tearing off the connection. When the heat is confined, steel heats up faster than when the heat escapes.
In the past, many firefighters believed that applying water to hot steel caused such twisting. I refuted this idea with a letter from the fire protection engineer of the American Iron and Steel Institute. When I presented the letter to IFSTA, they included an errata sheet in all copies of Science in the Fire Service distributed thereafter. Take a look at the IFSTA statement on page 259 of BCFS3.
Fatal Collapse Reports
The National Institute for Occupational Safety and Health (NIOSH) investigates and reports on firefighter line-of-duty deaths. Use them for training purposes by finding reports on buildings similar to those in your response area. A suggestion: Start at the beginning and extract every report that involves structural failure.
NIOSH Report #2002-44
On Sept. 30, 2002, a 50-year-old male career captain died when a parapet wall collapsed on him at an autobody shop fire. Arriving units advised responding crews that the structure was fully involved. Crews began an exterior attack as the fire had self-vented through the side windows and the roof. Crews were operating in a defensive posture when the parapet wall and overhanging facade fell forward, landing on top of the victim. A lieutenant and an emergency medical technician were also injured by the collapsing parapet wall. Rescuers removed the victim from the debris and found that he did not have a pulse and was not breathing. The victim was transported to an area hospital where he was later pronounced dead.
The wooden, mansard-like facade attached to the brick wall was probably constructed after the building was built to improve its appearance. The eccentric load was countered by tying the wall to the roof, and when the connection burned out, the wall collapsed. The steel truss visible in Photo C played no part in the collapse.
This tragedy reinforces the recommendation that when a firefight goes defensive, all personnel should stay out of the collapse zone. Make this warning part of your defensive announcement. We would not have these pictures if the fire department had known of this hazard and preplanned with it in mind, and if dispatch had radioed responding units, "The preplan shows that the mansard at the front is tied to the roof."
In addition, you should establish a collapse zone immediately if a building is fully involved. Every overhanging sign or other structure hung on a wall eccentrically loads the wall and poses a potential collapse hazard. Get out and look at each one in your area. Determine how the eccentric load is countered, put the information into a preplan and make the preplan available to dispatch and responding units.
Note: Such hazards are not confined to masonry walls. Overhanging elements eccentrically load wooden walls, and the failure of a connection within the wall or to the roof can bring down the overhanging load and the wall.
When a collapse zone is announced the IC should designate an officer as Collapse Sector to police the zone, assisted by a police officer if necessary. All sorts of people believe they have the right or the duty to enter the zone for what they consider good reasons; keep them all out.
Francis (Frank) L. Brannigan, a fireground commander in the 1940s and a fellow of the Society of Fire Protection Engineers, was named one of the 20 most influential people in the fire service by Fire Chief Magazine. For 37 years, Brannigan has defined building hazards for firefighters. His book, Building Construction for the Fire Service, Third Edition, is available from the NFPA.