Masonry Construction: Fire Risks, Earthquake Retrofits, and Structural Hazards

Masonry buildings, especially older ones, present unique fire and structural hazards. Many cities have undergone seismic retrofits following major earthquakes. For example, the 1933 Long Beach CA earthquake led to widespread structural upgrades in California.

There are a variety of features, which will help to identify a building as masonry type construction. Upon approach, you may actually see masonry walls. If so, we should look for features, which would indicate whether it is pre or post seismic retrofit construction. Pre-33 buildings in Long Beach CA will have seismic upgrades, which in most cases, will be visible, most often on the side and rear walls. Seismic upgrade features will not be present on post-33 Long Beach masonry buildings. These seismic upgrades consist of concrete caps on top of the parapet walls, bond beams wherever trusses contact the exterior bearing walls, and rafter tie plates wherever floor joists or roof rafters (in those buildings with flat roofs) contact the exterior bearing walls. Other indications of pre-33 masonry buildings are the presence of sand-lime mortar and “King Rows” a.k.a. “Header Courses”. King rows (header courses) are commonly placed between every seventh to eleventh row of stretcher courses of brick. In King rows, only the end of each brick is visible. In stretcher courses, only the sides of the bricks are visible. In pre-33 construction the bricks in the king rows (header courses) connected the inner and outer withes of stretcher courses together. The 1933 Long Beach earthquakes clearly demonstrated the insufficiency of this connection between the inner and outer withes.

Other features common to masonry type construction (both pre- and post-seismic retrofit) are very thick walls which result in a deeply recessed doors and windows, and lintels over the door and window openings. These lintels may be either straight or arched brickwork or a steel plate. A lintel in masonry construction is the equivalent of a header in wood frame construction, as it serves as a transfer beam, spanning the top of the door or window opening, carrying the weight of the structure above it horizontally to a vertical load carrying element on each end.

The presence of sand-lime mortar significantly increases the hazard of some older buildings, to both occupants and firefighters in an emergency. Sand-lime mortar contains no concrete, and has lost its adhesion ability over time. Hence, the walls of these buildings are essentially a loose stack of bricks that are no longer glued together. Therefore, they may collapse from even a minor eccentric or lateral load. Firefighters may dislodge large sections of these walls by improper application of large hand line or master streams. Due to its age, sand-lime mortar will often appear badly eroded. An easy way to determine whether mortar is sand-lime or concrete based is by scraping. Sand-lime mortar will scrape away quite easily, concrete based mortar will not. Any building with sand-lime mortar is automatically a pre-33 structure.

Fire personnel MUST understand that seismic upgrades increase the buildings resistance to earthquakes ONLY! They do NOT increase the building's fire resistance! In fact, there is some concern that these seismic upgrades increase the risk of fire related collapse in masonry structures. Older structures often had what were called fire cuts on the ends of the floor joists and roof rafters. The ends of the joists and rafters were cut at a slope before being inserted into the exterior bearing walls. If the joists or rafters were to collapse inward as a result of a fire, the sloped cuts at each end theoretically prevented the joists or rafters from acting as a lever causing the masonry wall above the insertion point from collapsing inward. Although fire cuts reduced the likelihood of inward wall collapse, once a floor or roof system collapsed inward, it frequently exerted enough outward lateral force to cause the exterior bearing walls to collapse outward, resulting in significant damage not only to the fire building but to neighboring occupancies (exposures) as well. As a result, the practice of fire cutting the ends of joists and rafters ceased in later construction.

Rafter tie plates reinforce the connection between exterior bearing walls and the ends of floor joists or roof rafters. They decrease the risk that the exterior bearing walls will peel away from the building in an earthquake, resulting in roof and floor collapse. They occur at regular intervals along nearly the entire length of the bearing walls, at the same height(s) that the floor(s) and, or roof system connect to the bearing walls. However, due to the increased strength of connection between the exterior bearing walls and the floor and roof systems, due to the seismic upgrades, an inward collapse of the floor(s) or roof from fire related damage, increases the likelihood of an inward exterior bearing wall collapse.

The stigma of seismic instability attached to pre-33 buildings in California has resulted in the publics avoidance of these structures. Hence, many building owners attempt to disguise the true nature of the construction by having the exterior walls stucco’ed over. This makes the determination of the exact construction of the building more difficult. Due to the cost of this “facelift”, usually only the front and sides of the building are stucco’ed. Frequently, the rear of the building is left uncovered, allowing us to correctly determine the true construction style.

Fire personnel must be able to distinguish between the exterior appearance of rafter tie plates and the end of “Tie Rod and Turnbuckle” assemblies. As mentioned earlier, rafter tie plates are placed at regular intervals along nearly the entire length of the bearing walls at floor and roof heights. The ends of “Tie Rod and Turnbuckle” assemblies will exhibit the same flat stock steel plate with a threaded nut through the center, as seen on rafter tie plates, however they will occur at irregular intervals in the bearing walls, and will almost always be located at heights other than floor or roof levels.

Unless associated with a true bowstring truss (serving as the bottom chord-tensile member) or a Lamella (a.k.a. Summerbell) style roof system (resisting the outward (lateral) thrust of the roof system), the presence of tie rod and turnbuckle assemblies is a significant indicator that the building has already begun to collapse, albeit in extremely slow motion. In some buildings, the bearing walls have begun to lean outward due to age related weakening, or present or past overloading. A contractor will drill through the entire building from one exterior bearing wall to the other, place a tie rod and turnbuckle assembly inside the structure, fasten the wall plates on the outside of the exterior walls and turn the turnbuckle until the exterior bearing walls are brought back plumb (vertical).

In so-called “taxpayer” type structures, with residential units over commercial or mercantile occupancies, the first floor street frontage usually consisted of large glass display windows. The lintel over these openings was usually a small steel “I” beam carrying the weight of the wall above it. Should a fire in the first floor commercial occupancy burn through the display windows the I beam will overheat, and possibly buckle, causing collapse of the wall above it. If fire personnel must operate in this area while fighting fire, applying cooling streams to this beam should be a tactical priority if the beam is being heated.

This beam will be supported by two or more posts or columns, and as a result, it may collapse outward in a 90-degree collapse, rather than the curtain fall pattern normally associated with unreinforced masonry. We may also see an inward-outward type collapse pattern, in which a portion of the wall begins to fail in one direction and exerts a lateral load on the wall directly below it, causing it to fail in the opposite direction. The establishment of collapse zones may require working from flanking positions, which severely limits the effectiveness of operations, and usually signifies a change to a defensive strategy.

The doorway leading to the residential units is normally located in front of the building in these type occupancies. If it were blocked by fire, residents would be limited to a rear exit or rear fire escape. If these do not exist or are blocked somehow, fire personnel may be forced to resort to laddering the front of the building to rescue occupants through windows. While this may allow rescue, caution should be exercised, as it may also draw the fire further into the exit corridors.

Fire personnel should be alert to the pre-collapse indicators that may be present. Creaking or rumbling sounds, cracked or bulging walls, cracks which continue to grow, water or smoke escaping through cracks, twisted or warped columns or beams, floors or the roof sagging or floor joists or roof rafters pulling out of their attachments are signs that a collapse may be imminent. If operations must continue in an area where collapse indications are found, they should be ceased until shoring can be placed.

Due to the age of many of these structures, they frequently have been remodeled one or more times. One of the cheapest methods to dress up a building is by installing a new drop ceiling. In some cases you may find more than one. Each of these installations adds additional weight that the building was not originally designed to support. They also create additional void spaces increasing the risk of internal extension, which may be difficult to detect.

As a result, if a truck crew cuts a heat hole, and reports heavy fire in the attic, but interior crews have opened the ceiling and have not found the fire, they should suspect multiple drop ceilings and continue opening upward until they reach the underside of the roof. Conversely, if interior crews report heavy fire in the attic, but a heat hole in the roof yields little or no heat or smoke, the truck crew should suspect multiple drop ceilings and continue to breach downward until the fire is found.

When originally built, masonry apartment buildings normally had conventional flat roofs due to the presence of interior bearing walls, which allowed short rafter lengths. In most cases, all other types of masonry occupancies utilized some form of truss roof construction in order to achieve the large expanses of open space inside the building. In most cases, these trusses were built with unprotected structural steel, heavy timber or a combination of wood and metal materials. Fire personnel should understand the characteristics of each type of roof support.

This shot shows solid sheeting over conventional rafters spanning between unprotected structural steel trusses. So although the roof is conventionally built using materials, which would normally provide an approx. 20 minute burn time if properly supported, the presence of the unprotected structural steel trusses makes this an unsafe roof for extended operations. As opposed to the next shot showing a conventional heavy timber truss supporting the same roof construction, yielding a true 20-minute burn time before crews should consider leaving the roof.

One of the most common types of heavy timber truss (and the most prevalent of the three types of structures that yield an arched shape to the roof) is the ribbed arch truss. If made of heavy timber, this truss also provides approx. 20-minute burn time. In almost all cases, roofs supported by ribbed arch trusses will have a small “hip” section at each end of the roof, running from the truss nearest the end wall to that end wall. This is important, as if the roof is seriously compromised due to age, rot, fire damage or overloading, or in the rare instances of either a bowstring roof which will normally have hip ends or a lamella roof with a small hip section, an alternative method of ventilating is to place openings in the “hip” sections at each end, and positive pressure ventilate horizontally across the interior of the roof. This minimizes the placement of personnel on a risky roof structure.

Personnel operating on a truss roof should place a heat hole over the fire if possible, but should back up one truss bay before cutting a hole if the decision is made to “go defensive”. They should also remember that the safest course of action, should a truss begin to fail, is to travel perpendicular to the trusses, back toward their escape route, rather than toward a sidewall. This is due to the fact that if personnel sense a truss weakening and move toward a sidewall in line them down with it. Traveling perpendicular to the failing truss shortens the distance traveled (since most trusses are no more than 20-30 feet apart, whereas the distance from the top of the roof to the side wall may be 35-40 feet) to a temporarily safe point over the adjacent truss.

The second type of roof support, which may yield an arched roof, is the true bowstring truss (many individuals mistakenly refer to a ribbed arch truss as a bowstring due to its shape — this is incorrect, and the mistake is an important one, since the two behave very differently under fire conditions — a critical distinction). This truss will have a heavy timber top chord, but has a Tie Rod and Turnbuckle (unprotected structural steel) bottom chord (tensile member). At approx. 850 degrees Fahrenheit steel begins to elongate. This elongation reduces the resistance to the outward (lateral) thrust created by the weight of the roof on the top chord, and may eventually result in failure of the truss. Since even class “A” combustibles produce temperatures well in excess of 850 degrees very quickly, this type of truss should be considered high risk, with a failure time of approx. 10 minutes. This type of roof normally will have hip sections at each end. Cutting ventilation openings in these hip sections and horizontally cross ventilating should be given serious consideration in this type of roof.

The third type of construction that will yield an arch shaped roof is a Lamella (a.k.a. Summerbell) roof, in which the truss construction is replaced by an “egg crate” style interconnecting series of 2X materials, which forms the support for the sheathing. The outward (lateral) thrust created on the walls is resisted by tie rods and turnbuckles. Hence, this style of roof construction usually has similar limitations as the true bowstring truss roof. Personnel must remember that according to Professor Frank Brannigan, if 10-20 percent of the roof is damaged in a fire, you should expect a complete roof collapse! As a rule (with some exceptions) if a building has an arched roof without hip sections at each end, it is normally a Lamella roof. There are seven Lamella roofs that we know of in Long Beach.

The roofs of masonry structures may also be supported by steel “I” beams, with conventional rafters spanning between them. Unfortunately, these qualify as unprotected structural steel, with the same shortcomings. If interior operations are required, placing cooling streams on these beams should be a tactical priority, just as it was with the steel lintels over display window openings in “Taxpayer” occupancies. Upon overheating (which happens very early in the average fire), these beams will elongate. If the walls that the beams attach to are unable to resist this elongation, the ends of the beam will punch through the walls. If the walls are strong enough to resist this outward (lateral) thrust, the beam(s) will relieve the heat stress by twisting or buckling (torsional loading). If this happens, it may result in at least a localized collapse.

Due to the size of some occupancies, there may actually be several different rooflines, or pitches in an occupancy. Should roof ventilation operations be necessary, truck crews should repeat their “diagnostics”, in order to insure they know what type of roof they are working on. This is due to the possibility that the building may have been built in stages, with different types of roofs for each stage. We also have several buildings that have suffered a fire, destroying the original roof in one part of the building, and a lightweight roof being built in its place.

Reinforced masonry structures are significantly stronger than their unreinforced counterparts. By code, cells of concrete block bearing walls are to be concrete filled, and steel reinforcement bars (rebar) are required in at least every third cell. Because of their strength and resistance to damage from heat, they are frequently used to form firewalls, or fire division walls. True firewalls have a parapet that extends at least two feet above the roof surface, and one foot out from any sidewalls, if those sidewalls are of combustible construction. They are required to be built substantially enough to withstand collapse on one side of the wall, without affecting the structural stability of the structure on the other side of the wall. Fire division walls subdivide an occupancy into separate fire zones, from foundation to roof, but lack parapet walls above the roofline or sidewall extensions. The strength of these walls allows fire personnel to make defensive stands, or anchor an offensive attack on the fire.

A fire rated assembly that is rated the same as the wall itself must protect any openings in these fire rated walls. If fire attack teams need to operate through one of these doors, they should insure that the door is blocked open enough to prevent their hose from being pinched off if the door is activated and closes. There are documented cases of firefighters being trapped when fire doors closed, shutting off the supply to their hose lines, and preventing them from being able to escape back through the door!

Whenever fire personnel inspect any occupancy with firewalls or fire division walls, they should insure that the occupancy hasn’t been remodeled, with new openings created in these rated assemblies, which are not adequately protected. They should also inspect the fire doors, if so equipped, to insure they are properly maintained, and operable.

The necessity of keeping costs of construction down has lead to non-combustible walls (usually reinforced concrete block) with lightweight “built-up” roofs. These roof structures are usually lightweight open web parallel chord trusses, or open web bar joist trusses. These trusses are then covered with a corrugated sheet metal pan, which has the built-up roofing placed over it.

The problem with these built-up roofs is the multiple layers of tar paper, which are weather sealed by “hot-mopping” molten tar between each layer. If a small fire starts inside the structure, and burns to the underside of the roof, it heats the tar to its vapor point. The tar releases large amounts of vapor, which, due to the multiple layers of paper above it, are forced back down into the structure feeding the fire with additional fuel. Eventually this vapor production becomes a self-propagating phenomenon, and contributes more fuel to the fire than the original source did. Even if the original seat of the fire is knocked down, the roof fire will continue unless water is applied directly to the underside of the roof. If smoke conditions seem inordinately heavy for the amount of fire found inside such a structure, you should suspect that the built-up roofing is burning. If, while making an interior attack, you notice hot molten tar droplets landing on you, direct the hose stream toward the roof to cool the vapor production. If this stops the tar droplets, you should suspect a built-up roofing fire.

The other roof system, which is becoming increasingly common over the corrugated sheet metal pan, is lightweight concrete. There is no rebar in this concrete, although there probably will be expansion wire (to resist cracking caused by weather related heating and cooling). This concrete may contain expanded shale aggregate, or no aggregate at all. This will significantly increase the difficulty of vertical ventilation operations. If a lightweight concrete roof is encountered, a carbide tipped blade may be more effective in cutting it than a masonry blade on the partner saw.

Another type of structure with non-combustible walls with a lightweight combustible roof is the “tilt-up”. Concrete walls are poured and cured on top of the foundation, and then raised and braced in their final upright position, until the roof is attached and the braces removed. The roof is “panelized”, heavy “glue-lam” beams and purlins with 2X4 rafters in lightweight sheet metal hangers and 3/8-inch plywood covered by rolled roofing.

There is a misconception that this is a completely lightweight roof, when in fact the glue lam beams and purlins are substantial enough to allow crews to operate for extended lengths of time during fire suppression operations. These roofs also have the advantage of burning through the plywood and self-venting relatively quickly.

There are actually four generations of tilt-up structures. The first can be recognized because they had spaces between the wall panels that were approx. twelve inches wide, which were then filled with concrete. This was deemed too expensive, so in the second generation, the slabs were poured immediately adjacent to one another, and each panel had a steel plate at both top inside corners. Once the panels were raised and braced in position, scrap steel was welded between adjacent steel plates to secure the panels together, and then the roof was placed. So second-generation tilt-ups can be differentiated from first generation by a narrow gap between the wall panels. This was also deemed unnecessary, and the steel plates were eliminated. So third generation tilt-ups were simply raised, braced in position and the roof placed. Quite literally, the roof secures the walls in place and the walls secure the roof in place. If either fails, so does the other! It should be thought of as a massive house of cards!

The weakness of wall panel-to-wall panel and wall-to-roof connections in second and third generation tilt-ups was demonstrated very clearly during the Northridge earthquake, when a number of these structures sustained significant collapse damage. As a result the codes have been improved and we now have fourth generation tilt-ups. Fourth generation start out with the same steel plates at both inside top corners of each wall panel. When the wall panels are raised and braced in position, substantial welds are placed between adjacent panel plates, improving the wall-to-wall connections. The wall-to-roof connections were strengthened by the equivalent of rafter tie plates securely attaching the roof to the ledger boards at the top inside of the wall panels. Unfortunately, it is very difficult to tell second, third and fourth generations from one another, as outward appearances are the same. You may be able to see the heavy welding across steel plates of the fourth generation from the interior, however.

If it becomes necessary to ascend to the panelized roof, ground ladders should be placed eight feet from any corner, as this location should place the weight of the ladder against either the end of a glue lam beam, or a purlin. This assumes that the panelized roof is built with these features. Recently we have begun seeing purlins replaced with either large wooden “I” beams or open web trusses, leaving only the glue lam beams as the only safe structural element to walk on, or ladder against. Hence, diagnostics, and target hazard inspection become critical to insure you know what type of roof you are working on. It is only a matter of time before the engineers figure out a way to replace the heavy, expensive glue-lam beams with a lightweight component!

These reinforced masonry structures are essentially monolithic, and hence, as a general rule, will normally collapse outward (due to the floor and roof systems providing resistance to collapsing inward) a distance equal to the wall height, which should serve as a guide for the establishment of collapse zones. You should evaluate whether you want to bet your life on this general rule, as quite literally that is what you’ll be doing. The Music Recital Hall at CSULB sustained a roof collapse due to improper roof renovations, and several of the wall panels were pulled inward in that incident!

Inspection of the interior of buildings with panelized roofs, reveals huge open expanses of the underside of the roof. Several points should be noticed. First, many of these roofs were lined with a “Kraft” paper which was designed to be a fire retardant barrier. Unfortunately, this multi-layer paper was glued together with a flammable adhesive. While the barrier reflects heat away from the roof, once the heat rises enough that the layers begin to delaminate, the glue ignites and contributes to even more rapid flame spread! But, because the paper keeps interior temperatures lower, reducing air conditioning costs, the manufacturer stopped advertising it as a fire barrier, and began advertising it as a cost reduction aid!

Another point which should be noticed are the corrugated curtains suspended from the roof. These serve as heat curtains. Without them, the heat from a fire would spread throughout the roof area, and not build up sufficiently to set the sprinklers off until the fire had built up to a point that the sprinklers would be unable to contain it. By installing heat curtains, the heat is contained in an area small enough to activate the sprinklers while the fire is still small enough to be contained. It should be noted that most sprinkler systems are only engineered to have a maximum of six heads flowing. If many more heads open, the system frequently isn’t engineered large enough to supply the necessary flow to support all of the open heads. Therefore, none of the heads creates a proper pattern, and the fire is able to expand.