The Ties That Bind
BUILDING CONSTRUCTION
Are you well-connected – the buildings in your district, that is? An understanding and awareness of building connections may be the only thing that stops gravity from winning the battle.
IN MANY fire departments, connections-political, fraternal, family, religious, or personal – are integral, sometimes essential requirements for promotion or desirable assignments. Such connections are important, but there are others that may be more important to your very life.
The force of gravity is the eternal enemy of all structures Twenty-four hours a day, seven days a week, gravity is trying to pull the building down to the earth. By one method or another, the builder assembles a system to defeat the law of gravity. Except for the very simplest structures, connections, which transfer the load from one structural element to another, are a vital part of the gravity-resistance system. The connection system must be absolutely 100 percent complete. It is only as strong as its weakest (to fire) link. A single failure can be disastrous.
Before we can discuss connections, we should consider the structure as a whole.
THE TIES THAT BIND
The overwhelming percentage of fires start in the contents. As long as the fire is confined to the contents and affects only the contents, it is not necessary to consider the stability of the structure as a most important factor in fire suppression. When the fire starts to affect the structure, either by igniting structural elements or by physically affecting them, the stability of the structure, in whole or in part, becomes a factor in every decision made by the foreground commander.
For this reason, 1 have long recommended that the fire service make some changes in terminology. We should, for example, dispatch apparatus to a “building fire.” When the fire starts to affect the structure, an announcement should be made: “This is now a structured fire.” All would be alerted that the stability of the structure should now be closely watched, and tactics adjusted accordingly. Be aware that live fire training in fire schools does not address this situation, since the training building is built to resist fire collapse.
like so many topics in building construction, a discussion of connections does not lend itself to the precise outline. Some backtracking is unavoidable.
MASONRY WALLS
Mortar Joints: Mortar is used to bind the masonry elements, brick, stone, tile, or block together. Before 1880, all mortar was sand-lime, which is water-soluble. After 1880, Portland cement mortar, which is not water-soluble, came into use gradually. In some cases it was mixed with sand-lime for ease of working. Buildings constructed with sandlime mortar will very probably be restored with the same mortar. Sand-lime mortar can be washed out by hosestreams during a fire, or by a water leak, causing collapse of the structure, as happened in the Broadway Central Hotel in New York and the Empire apartments in Washington, D.C.
Paying close attention to this type of construction could well be worth it. A probationary firefighter who had heard me lecture on this subject noted the washout of sand-lime mortar from cellar walls. He notified the lieutenant, who in turn ordered the company out. The building collapsed about five minutes after it had been cleared.
Wood: The continuity of a substantial number of apparently solid masonry walls is broken by wood, inserted during construction. Wooden lintels across windows and doors are common. They are not apparent from the exterior. They can be detected only by competent prefire inspections. Wood planks are sometimes laid into the wall so that floor beams can rest on them, providing a level floor.
When any of this wood burns out, the effect is the same as if a slice was cut into the wall, and the wall collapses. This is an unrecognized defect in conventional discussion of wall collapse, or in reports of disastrous collapses. In my opinion, it is much more significant than the sometimes offered “cold fire streams hitting hot bricks.”
Cross Walls: Any masonry wall at a right angle to another is a cross wall that connects the two walls together and provides stability. The joints in many such connections are deteriorated, often due to the difference in expansion between two different materials, such as brick and stone.
SAWN WOOD BEAMS
Fire Cut: Years ago it was learned that wooden floor beams inserted into a masonry wall could act as a series of levers that pulled down a masonry wall when the floor fell. Some codes require that the end of the beam be cut off at an angle so that it can slip out of the wall easily. This is the so called “fire cut.” It is intended to make the floors release from the walls, to save the walls. In truth, the floors are designed to collapse. When the beam is cut, there is often very little wood left where the beam rests on the wall. In some heavytimber buildings the floor girders (a girder is any beam that supports other beams) are inserted into a “beam box,” a cast-iron box placed in the wall as it is erected, so that the beam can fall out without affecting the wall. This prevents weakening of the beam by removing wood for a fire cut.
The problem of preserving the walls from floor collapse is solved in yet another way. The wall is corbelled out (that is, successive courses of brick are stepped out to create a shelf) and the floor beams rest on the corbel. No material is removed from the beams, and collapsing floors do not pull down the walls.
Note that entirely apart from the effect on the wall of the floor beam “levers” is the effect of the undesigned lateral impact of a collapsing floor, which may be enough to collapse the wall.
Self-Releasing Floors: Many old, heavy-timber buildings were built with “self-releasing” floors. The floor girders are not tightly connected to the columns, but merely sit on a shelf that sits on the column. The concept is that one section of the floor can collapse without affecting the rest of the floor or the column. These floors also are literally “designed to collapse.” Some contemporary designers opposed this idea because it deprives the building of stiffness. This was particularly true on the West Coast to resist the many relatively small earthquake shocks. It would be a good idea to examine such buildings and determine which school of design the architect followed.
Heavy-Timber Girders and Cast-Iron Columns: Heavy-timber girders (as big as 24 inches square) are attached to cast-iron columns in a particularly dangerous manner. A shelf is cast onto the column. The girders are cut away in a half moon in order to fit the tw o opposite girders snug around the column. Only the two “ears” of wood are resting on the shelf. Failure of the shelf or burning of the wood causes collapse. An old building handbook points out this hazard and recommends that the big cast-iron column be reduced to a solid “pintle” of much smaller diameter to pass through the wood. This leaves much more wood in the connection. I have never seen such a pintle used in a cast-iron-columned building. I think the heavy-timber girder/cast-iron connection is far more likely the cause of collapse than the poetic “red-hot castiron struck by cold fire stream.”
Overhanging and Drop-In Beams: Structural design is always intended to be as economical as possible. The economy may be in material or in the work of erection. Consider a space that’s three bays wide. There are tw’o masonry w alls and two lines of columns supporting girders. Sometimes it is most economical to let the two outermost beams overhang the girders by two or three feet. The gap is then closed by a beam that’s “dropped in” and simply nailed to the overhanging ends of the outer beams. As a result, the “drop-in” beams are connected only by the nailing. They have no support underneath.
Spliced Beams: Trees that are tall and straight enough to make long wooden beams are rarely available. Shorter lengths are often spliced together with metal connectors to produce the desired length. The resultant beam will carry its design load, but the connectors will fall out when heated sufficiently, causing collapse. In some buildings these connectors may have been made to look decorative. Take a second look!
Some years ago, the Jai Alai Fronton at Daytona Beach, Florida was destroyed by fire. The roof was supported on laminated wood arches. In the subsequent lawsuits, the owners of the building were suing the supplier of the foam plastic insulation on the roof, alleging that the flammable plastic had destroyed their lovely heavy-timber building. Pictures of the fire (taken by Assistant Chief Franklin of the Silver Spring. Maryland Fire Department) clearly showed that the arches had fallen apart at the connections. When the plaintiff’s learned that the defendants had these convincing pictures, they withdrew their suit.
The Hidden Wall: Most multistory brick-and-wood-joisted buildings along “Main St. USA” are 25 feet wide with brick bearing walls on each side. To make a larger store, it is necessary to open up or entirely remove the firstfloor brick walls between the buildings. One popular way is to support the brick walls above on steel or wooden beams resting on cast-iron columns. These columns are rarely connected to the beam. They depend on gravity to keep the column in place. This makes it vulnerable to any lateral thrust. Nine firefighters died in Boston in 1972 in the Vendome Hotel collapse. Years before, a brick wall had been removed to make a larger space. The wall above was supported by a steel beam resting on an unconnected cast-iron column.
THE TIES THAT BIND
Gravity Connections: In 1938, a deadly, surprise hurricane hit New York and New England with devastating force. Thousands of houses were destroyed because they were moved a few feet off their foundations. Building codes were amended to require that houses be bolted down to the foundation. Many times this sound practice is omitted or poorly done because the reason for it has long been forgotten. In too many cases, gravity is relied on as a sufficient connection.
STEEL
Steel connections are important not only in steel buildings, but in buildings of all types.
Tie-Rods: Steel tie-rods extend from one wall to another to resist the outthrust of the roof and to stabilize the walls. They may be part of the initial design or installed after defects develop. The stars and other spreaders seen on brick walls are almost invariably a sign of a wall in trouble. The sole exception is some buildings in which the designer wanted a good, tight building, resistant to vibration. The ties were installed as part of the original construction. An indication of this design feature is regularity in the arrangement of the spreaders on the face of the building. Again, this is more common on the West Coast because of the earthquake problem.
Tie-rods in wooden walls can fail by the burning of the wood at the connection to the wall. In any wall, the tie-rod can fail by elongation of the steel, which becomes significant at around 1,000°F.
Steel Cables: In place of tie-rods, cold drawn steel cables are sometimes used. These totally fail at 800°F, a very low temperature. They have also been used to tie panel walls to steelor concreteframed buildings when the original structure failed and the wall panels are in danger of falling off the building. Even though the building is required to be fire resistive, the building department often allows cables to remain unprotected.
Suspended Loads: Designers are becoming more aware of the advantages of suspending loads rather than supporting them by columns underneath. A thin rod can carry a load that would require a four-by-four or larger wooden post. Loads may run the gamut from a simpledecorative grid of two-by-fours (as seen in some Roy Rogers restaurants, for example) to an entire part of a structure, such as a balcony. If the fire burns away the connection to the overhead, the load will fall.
Joist Hangers: The best way to transfer the load from a beam or joist to a girder is to place it atop the girder. This has the maximum inherent fire resistance. However, this method adds height to the structure and makes the walls more costly.
Various devices are used to connect the beam to the girder so that the tops are level. The simplest method is to toenail them together. Examine a country building. You will often see the floor joists just toenailed. If the wood around the nail burns, the connection fails.
Joist hangers were originally of heavy steel or wrought iron. Today, joist hangers are made of light, galvanized steel. The steel can act as a heat collector, heat the nails, and cause pyrolytic decomposition of the wood holding the nail.
Gusset Plates: Gusset plates or gang nails are used to connect the parts of lightweight wood trusses. Like the joist hangers, they can deliver heat to the prongs, which are imbedded a much shorter distance into the wood than nails. This destroys the wood, causing failure. It’s been claimed that the gusset plate protects the wood. I have never seen this, nor has any one of my confreres with whom I have discussed this problem.
Note that the bottom chord of a lightweight wood truss of any substantial length is made up of several pieces connected together by gusset plates. The bottom chord of a truss is under tension like a rope. Like a rope, it is necessary to cut it in only one place to cause failure.
CONCRETE
Some concrete buildings are monolithic (from the Greek: monos, meaning “one” and lithos, meaning “stone”). As the sections are cast, reinforcing rods protrude from the concrete. These are incorporated in the next section and so on, so that the entire building, when finished, can be thought of as one piece of stone. Such buildings redistribute the load, and general collapse is rare.
Concrete buildings built of precast sections are often pinned. Such buildings are connected with bolts and nuts, or with welds. These connections can fail, and collapse is likely. The recent collapse of a lift-slab concrete building under construction in Bridgeport, Connecticut was due to a failure of a connection from a floor to a column.
Many concrete buildings are “mix and match.” Some concrete is cast in place, while some is precast. Precast sections may be joined to the cast-inplace concrete and become part of the monolith. Others are merely pinned with bolted or welded connections.
Concrete is not inherently fire-resistive. Most concrete is merely noncombustible. It will not burn, but it has not been fabricated to meet fire-resistive standards. Therefore, it can fall apart when exposed to fire.
We should understand the term fire resistance. All structures have some resistance to fire-caused collapse, however limited that resistance may be. We can call this “inherent” fire resistance, as distinguished from “legal” (that is, “code-specified”) fire resistance. The “inherent” fire resistance of one structure may be absent in another similar structure. Experience, whether personal or organizational, is not necessarily transferable from one structure to another.
ASTM El 19 sets the standard for fire resistance. The structural element is tested in a furnace by a fire which follows the standard time-temperature curve. The material’s fire rating is determined by the length of time the test structure resists collapse, passage of fire, or excessive heat. Thus if a floor survives the test for one hour, the assembly would get a one-hour rating. This tells us nothing about how long the floor would last in a real fire. For a more complete discussion of this important, generally poorly understood matter, see Chapter 6 of Building Construction for the hire Service, 2nd edition, published by the NEPA.
When I was teaching in college, I would send the students out to ask builders the question, “How do you overcome the Law of Gravity?” One oldtime carpenter threw his hammer down and exploded, “They got so many !@#$%¢ laws now, you can’t get nothing done!!”
When you look at a building for enforcement or preplanning purpose, take a hard look at the connections so that you will not be in the collapse path when the connection fails.