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Exterior of office towerOne Meridien Plaza
Philadelphia, Pennsylvania

At about 8 p.m. on Saturday, 23 February 1991, linseed oil-soaked rags left behind by a cleaning crew burst into flames on the 22nd floor of the 38-story One Meridian Plaza in downtown Philadelphia. The fire quickly spread, unimpeded by fire sprinklers, throughout the 22nd floor and then upward. Sprinklers were not required by the City's building code at the time of construction and were being added to the building only as opportunity presented itself.

The twelve-alarm fire burned for 18 hours. The extreme heat caused window glass and frames to melt and concrete floor slabs and steel beams to buckle and sag dramatically. Large shards of window glass fell from the facade, cutting through fire hoses on the ground around the building. Three firefighters were trapped on a fully engulfed floor, and efforts to rescue them failed.

The fire would not yield and there were increasing concerns about the stability of the structure. Fire officials called off the attack and allowed the fire to "free burn," concentrating their efforts on containing the fire to this building. When the fire reached the 30th floor, a tenant-installed fire-sprinkler system was activated, and the worst high-rise fire in U.S. history was finally brought under control.

Steel stabilizing poles Efforts to stabilize the structure and the facade began immediately and continued for six months. Over 2,000 steel poles were installed to shore the burned-out floors and to brace the steel girders supporting the concrete slab. During the night of the fire, one granite panel had fallen from the facade of the damaged middle section and other panels were suspect. Emergency belting was installed to lash the granite to the building until it could be safely detached and lifted down. Fabric netting covered the building to prevent debris from raining down on City Hall Plaza, the heart of Philadelphia's business and tourist activity.

Lawsuits and insurance claims, counter lawsuits, and counter claims were filed immediately. The challenge became one of determining the extent of structural and material damage and whether to repair or demolish the building.

To settle a vexing issue of the dispute, the owner's insurance company retained SGH several months after the fire to evaluate the condition of the building's facade. SGH was retained because of our experience in evaluating facade performance, both the materials which comprise facades and the structural systems which anchor facades to buildings. At the time of our initial walk-through, the owner was completing the emergency stabilization phase and was about to embark on the removal of the facade at the fire floors. The insurer was concerned that no one had yet evaluated the facade's condition in detail. While some areas of the facade suffered very severe damage, other areas at the fire floors exhibited much less damage. SGH was asked to conduct an in-depth investigation of the affected areas to determine whether repairs were feasible.

One Meridian Plaza's glass and granite facade consisted of granite-faced precast concrete spandrel panels, 8 in. thick granite column covers, and 4 in. thick splayed corner panels, surrounding 16 ft wide windows. SGH's investigation focused on the nine floors most affected by the fire by dividing the scope into three discrete yet interrelated pieces (windows, granite panels, and spandrel connections) the fire's dynamic effects can be discerned.

Damage At Window Openings

diagram of fire pathDuring the fire, flames lapped up the side of the building from window openings spreading the fire through to the window above. The windows were the least protected element of the facade and experienced the most intense fire and heat; consequently, they experienced the most widespread damage. At the fire floors, nearly all of the glass was broken from the windows. The glass that fell outward scratched the surface of the building on the way to the ground. Some of the aluminum framing and glass melted, indicating temperatures in the range of 1,000 degrees F to 1,300 degrees F.

The damage to other facade elements around the window openings varied widely. The granite panels surrounding the window openings, particularly those above the window head, experienced direct exposure to the flames and extreme heat levels. After the fire, some areas of the granite crumbled readily under hand pressure due to a phase change in the quartz crystals that occurs at temperatures of about 1,060 degrees F.

Temperatures were substantially lower in areas away from the window openings. The exterior granitetogranite joint sealant, which melts at less than 500oF in our laboratory testing, was not severely charred or melted in most areas away from the window openings.

During the fire, temperatures within the building along the interior of the facade varied widely. The plaster soffit above the windows and the plaster enclosure around the columns, along with the sprayed-on fireproofing, shielded much of the inner surface of the facade from the fire's heat. In some areas on the fire floors, we found uncharred cardboard, plastic shims, and rubber pipe insulation tucked into spaces next to the back of the facade panels. Our testing found that these materials disintegrate readily below 500oF. Concrete masonry unit walls behind the splayed corner panels shielded the corners, minimizing damage.

Granite Cracking

Although only one piece of the granite veneer actually fell during or after the fire, the granite facing on the spandrel panels contained numerous cracks. SGH investigated the extent and pattern of cracking to determine the dynamic forces of failure.

The granite cracks were vertical and coincided with the location of the hairpin anchor that connected the granite facing to its precast concrete back-up. Most of the cracks occurred in locations with large temperature changes, such as in the stones abutting the window head or on the west elevation where water had been sprayed on the facade during the fire. These temperature differences around and through the granite veneer created differential movement. Stresses built up as the hairpin anchors tried to restrain these movements, causing the granite to crack.

Most of the granite column covers contained one or more horizontal cracks typically coincident with the dowels that connected two pieces of the column cover beside the window jambs. The edges of the column cover were hotter and expanded more than the cooler central part. The resulting tensile stresses cracked the granite.

Failure of Spandrel Panel Connections

Although the cracking of the granite facing was pervasive, the granite generally held to the building. During the fire, many of the structural connections between the precast concrete spandrel panels and the building's steel frame had failed; typically by cracking or spalling of the concrete at the connection. In the one location where the structural connection of precast panel to the structural steel frame did fail, a seven-ton panel fell, landing on a roof below. The result was extremely sobering. It was fortunate other panels with broken connections did not fall from the facade.

At the broken connections, some of the spandrel panels shifted outward approximately 1 in. from the plane of the building. What pushed the panels outward? In many areas with failed connections, the edge of the concrete floor slab was in contact with the interior surface of the spandrel panel, and the slab was crushed around the exterior steel column. The concrete floor slab became very hot during the fire. On the 23rd floor, a firefighter reported that the carpet backing had melted and the wood nailing strip glowed red. The temperature of the slab rose faster than the fireproofed building frame. Consequently, the slab expanded more than the frame, closing the gap between the edge of the slab and the back of the spandrel panel. In moving outward, the slab crushed itself around the exterior columns. Eventually, the expanding slab pushed the spandrel panel outward, breaking the panel connections.

Repair Program

After our investigation, we recommended to the insurer that while some portions of the facade at the fire floors were damaged beyond repair (the window glass and frames obviously had to be replaced), other components could be restored and reused. In general, there was relatively little facade damage away from the fire floors.

SGH conducted field and laboratory testing of the remaining facade components. We tested the capacity of the hairpin anchors behind the granite veneer and the bending strength of the granite. We calculated the stresses in the facade components. Where obvious fire damage, such as cracking or granulation, did not exist, we found that the granite possessed sufficient strength to continue service.

Because the original facade was over-designed significantly, any minor reduction in material strength was inconsequential. Our testing of post-fire conditions found factors of safety on the order of 10 to 20 for the granite and the hairpin anchors, significantly higher than normal design factors of safety which are on the order of 5.

Following the completion of the investigation and analysis, SGH developed two sets of details and specifications: one for complete facade replacement at the fire floors; and one for a remedial program of spot replacement, repair, and inspection of the facade. The estimated cost for the remedial work nearly approached the cost of complete replacement at the fire floors. Based on this information, the insurance company agreed to the removal of the facade at the fire floors in late 1991.

The building, however, continued to be the object of dispute. Questions remained about the extent of structural damage and the need for further facade removal beyond the fire floors. Some parties to the matter claimed the building should be demolished entirely, others claimed at least the upper half of it should be razed, while still others wanted the building restored as it stood. The ensuing litigation has thus far prevented replacement of the facade. An undisclosed settlement reached in 1997 gives new hope for the abandoned building next to City Hall Plaza.

While the circumstances of the fire at One Meridian Plaza were extraordinary in their ferocity and intensity, there are several lessons to be drawn for individuals investigating the effects of fires on buildings and for design of buildings:

  1. Do not assume that a fire creates the need to replace a facade: evaluate the original design and test the postfire materials conditions before reaching a conclusion.
  2. Fire can spread easily from floor to floor via large window openings without adequate separation.
  3. Once again, One Meridian Plaza shows the efficacy and importance of sprinklers.
  4. Spray-on fireproofing and plaster provide substantial protection to key structural members and help shield much of the inside face of the facade from severe fire exposure, thereby reducing damage.
  5. Since floor slabs generally have lower fire resistance ratings than principal structural members, they can heat up and expand faster. Floor components need to be separated adequately from the facade to avoid damage from this expansion. Such gaps need proper fire safing.



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