The arch, a centuries old construction technique that combines the support function of columns and beams, has been incorporated into some of man’s most beautiful creations (cathedrals, castles, etc.). Its aesthetic appeal has made the arch a popular means of spanning windows and doorways, and since the arch is under compression along its entire length, it is also capable of spanning longer distances. However, not all arch construction features are visible to the casual observer, and it’s these features that are the hidden enemies of firefighters. Even the loveliest work of masonry can become a deadly pile of bricks and concrete.

As Frank Brannigan notes in BUILDING CONSTRUCTION FOR THE FIRE SERVICE, if the arch is properly built, connections are not important to its strength. However, the removal or destruction of any part of an arch will cause the entire arch to collapse.

The engineering principle of the arch can be easily understood and demonstrated by bending a 3 X 5 card to form a U shape. Notice that as long as the bottom sides are compressed inward, the card will stand and even support additional weight. Now, remove pressure from one side and instant collapse results.

There are basically two different forms of arches, the segmental or curved arch and the flat arch, which is constructed by tapering the masonry units. The arches addressed here involve those used for supporting floors.

The segmental arch may or may not be visible from below the floor it’s supporting, depending on its location in the building. Arches in a basement or those that have been aesthetically designed (usually constructed of brick) to support the floor above may be open to inspection. Although a single arch may support an entire floor, usually there are several arches supporting a segment of a floor. A filler is used above the arch to make a level base for the actual flooring.

In the perfect arch, all members are under compression, such as can be seen in the stone arches which have stood for centuries. However, the side compression of some arches may be supplied not by the same masonry that forms the curved portion of the arch, but by unprotected steel or wooden beams that are subject to elongation and deterioration under fire conditions. The failure of a compression member will result in the collapse of the arch supporting the floor. To resist the outward thrust of the arch, tension rods, commonly of steel, may be tied at the base in lieu of or in conjunction with compression members. If unprotected, these rods are subject to failure from heat. If cast into concrete floors, they can be easily mistaken for reinforcing rods and may be cut during breaching.

Tile arch (shown from the floor below) is found in high-rise construction. Terra-cotta tiles form a supportive arch to the flooring material above it.

Photos by Allen Clark

The existence of arches and their supporting systems must be discovered during pre-planning. Be especially suspicious of late 19th and early 20th century “fireproof” construction such as that found in public buildings, hotels, and churches. These structures may have been remodeled, their arches covered over, or refurbished with a material that may resist fire more or less than the original construction. Check with the building’s owner and plans department if in doubt.

When opening up a floor and bricks are encountered, stop and make sure that you are not destroying the integrity of an arch. The same holds true for cutting rods or cables in floors, as they may be an arch tie.

The second type of floor supporting arch is the tile arch. Flat terra cotta tile arches, usually found in high-rise construction, are designed to fit into place, and a thin layer of concrete or other material form a level floor or segment of a floor.

As with the segmental arch, different flooring materials may be applied over the top of these tile arches. While this type of flooring gives the appearance of monolithic construction, improper masonry work can create combustible voids under the floor.

Tile compression may come from the sides of the building (in which case the building’s walls will usually be buttressed), the outside ground, or a grid of beams and girders. This arch type construction may not be obvious, even when visible. One clue to its presence is water seepage. Tile arch floors were often manufactured at the factory to specifications that were frequently incorrect. The floor was then laid in an improvised manner. Even if tightly compressed, many arch tile floors are not water tight.

The danger with tile construction lies in the fact that breaking out only one tile may cause the entire arch to lose its integrity and the complete floor or a large segment of floor to collapse. And firefighters can be working on top of or underneath these tile arch floors when the collapse occurs. Another concern is that the pressure wave created by the floor collapse may push large amounts of fire upward.

Here again, the key to safe operations in such a structure lies in the knowledge of a building gained during pre-planning and inspections. Although the tile arch, like the segmental arch, is a feature of late 19th and early 20th century “fireproof” construction, do not discount the possibility of its existence in any building. People will often use a construction feature that has struck their fancy, and tile arches could appear anywhere, even in modern structures.

Knowing and pre-planning your district cannot be stressed enough. Construction knowledge too is vital. Never interrupt the integrity of an arch while working above or below it. If arches must be breached, take extreme caution. Know what to look for and beware of these hidden dangers.

Typical arch forming a lintel spans openings for windows and doors supporting the spandrel wall structure directly above.

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