Editor's note: This article finishes summarizing a small portion of the 79-page Chapter 2, "Principles of Construction," of the 667-page third edition of Building Construction For The Fire Service, by Francis L. Brannigan. Part 1 was published in Firehouse® in February 1996, part 2 in July...
To access the remainder of this piece of premium content, you must be registered with Firehouse. Already have an account? Login
Register in seconds by connecting with your preferred Social Network.
Complete the registration form.
A mechanically fastened membrane is loose between fasteners. In high winds, it can blow like a sail and throw firefighters or equipment off the roof. White membranes make roof edges difficult to see in blinding glare or ice and snow.
There are a number of different compounds used for the membrane. All should be expected to emit toxic gases, so self-contained breathing apparatus (SCBA) should be worn, even through a roof fire might seem to permit otherwise. Membranes are slippery when wet. Check for fire traveling under the membrane. Insulation may mask the possibility of a structural collapse. Installation is a hazardous operation due to the use of flammable compounds which emit heavy vapors. There may be a large quantity of hazardous flammable materials on the job site.
The simplest building might have walls of mounded earth and a roof of tree trunks. All other structures must have connections which transmit loads from one structural element to another. They are a vital part of a structure's Gravity Resistance System. The system is only as strong as the weakest link. Any failure lets gravity take over and collapse occurs.
When examining buildings, pay particular attention to connections. Watch out for overhead loads, which must have multiple connections. Potential connection failures will be cited throughout this series but some are summarized here to get you started looking intelligently and warily at buildings. It's your life.
- In older heavy timber buildings, look at the connection of wooden beams to cast iron columns. They are probably very dangerous (pages 174-176). Cast iron columns in older buildings often have only gravity connections; they just depend on the weight of the structure to keep the support column in place. Any lateral thrust will displace the column. Nine Boston firefighters died when a gravity connection failed.
- In newer buildings, very often unprotected steel columns carry heavy laminated wood girders.
- In many older buildings, the connections may be adequate as long as the building is axially loaded. However, an eccentric or lateral load or shifting wall, floor or column alignment may cause collapse even after the building has been standing for many years. Undesigned changes in loading are dangerous and frequently cause collapse.
- Be especially wary of buildings in areas where there was, or is, no building code supervision, such as rural areas that suddenly become urbanized. A Los Angeles City firefighter died in a roof collapse where once there had been a vacant lot between two brick buildings. To provide a roof over the vacant area, mortar had been removed from between bricks. Pieces of shingle were hammered into the gaps. A ledger board nailed to the shingle pieces supported a wood joist roof. The makeshift assembly survived for a number of years until a fire destroyed it.
- Buildings of the same age probably have the same defects
- Sand lime mortar was used exclusively in masonry work until about 1880, and for many buildings after that date. Sand-lime mortar is water soluble. A firefighter operating with his unit in the basement of a building noticed that a hose stream had washed the mortar out from the bricks. He alerted the officer. The building was evacuated and shortly thereafter collapsed.
- Steel connections enter into the construction of almost every building. Steel is non-combustible but it has poor fire characteristics. The characteristics of steel ar described later in "Characteristics of Materials."
Unprotected steel columns supporting a concrete floor failed in a fire and plunged four Pennsylvania volunteers to a fiery death.
It was noted earlier that a load can be suspended on a thin tension rod as contrasted with the bulky column required to support it in compression. However, the load must be changed to a compressive load and delivered to the ground. This requires a series of connections.
Illustration by Christopher J. Brannigan
Many heavy timber buildings or timber interiors of masonry buildings were built under the concept of "self-releasing floors." Each of the girders can collapse without bringing down the others. A "dog iron" gives some slight degree of lateral stability but will not prevent a collapse. Other builders did not like this concept and chose heavily bolted connections. It is fair to characterize a building with self-releasing floors as "designed to collapse."