Dolphin+Towers+Condominiums+-+Transfer+Slab+Failure

Dolphin Towers Transfer Slab Failure //Eric Cook, BAE Penn State - Structural 2012 //
 * Sarasota, Florida - 7/2010 **

Introduction
toc Dolphin Tower, as shown from Google maps in figure 1, is a 15 story condominium building located in Sarasota, Florida built in 1974 which consists of 116 residential units. In late June 2010, two residents of Dolphin Tower returned home to discover a significant crack in their residence which was accompanied by buckling walls and broken floor tiles. Residents were given until July 1st to completely vacate the building which was deemed unsafe by local building officials. Reports as to the cause of this failure were initially unclear due to the fact Dolphin Tower stood for 36 years before any sign of failure became prevalent. Further investigations revealed structural issues with the transfer slab on the 4th floor of the building at columns causing a significant loss of strength due to spalling and voids/delamination.

Background Information
Built in 1974 Dolphin Tower stands in downtown Sarasota, Florida overlooking Sarasota Bay causing it to be a highly sought real estate location. The design of Dolphin Tower includes a 3 story above ground parking garage with 12 additional residential stories rising from the middle. The building was designed and constructed entirely of cast in place reinforced concrete. Due to the difference in space requirements from the parking structure on level 3 to the residential units on level 4 the columns were designed to be dis-joined as shown in figure 2 and therefore required the use of a transfer slab in order to direct load forces from above down to the foundation.

Residents of Dolphin Tower were initially given two weeks to evacuate the building with July 1st bringing a mandatory 11 a.m. evacuation. At that time, city officials closed the building and posted "Do Not Occupy" warning signs for the public's safety (Martin 2010, 2).

Furthermore, there have been problems related to funding for the repairs of Dolphin Tower. One issue "is that coverage exists only where a building or component actually collapses, rather than being merely at risk of collapse," (McQuaid 2010, 1). Confidentiality agreements were attempted to be imposed on owners through federal court if they wished to know the monetary value of the damage but was later denied stating "they would have to do so in state court" (Pollick 2012, 2).

Investigation & Causes of Failure
Karnis Engineering Group responded immediately on the day of the failure and worked with Structural Preservation Systems to stabilize the structure and reduce the risk for continued damage and settlement. Soon another structural forensics company was brought in to examine the structure more in-depth. Examinations began at the 4th floor transfer slab which revealed several issues including voiding/delamination, honeycombing, and exposed reinforcement bars.

Several different techniques have been used to analyze Dolphin Tower including visual inspections and non- destructive testing such as ground penetrating radar (GPR). Visual inspections revealed a number of cracks at the surfaces of the slab and coupled with hammer tests began to determine areas of voids within the concrete its self. GPR was used to determine the location and lengths of rebar contained in the slab (Hill et al 2011).

Cracking of the transfer slab was primarily located around the garage columns below and most visible from the top of the slab with the widest crack measuring about 3/8 of an inch in width with the largest vertical offset measuring about 1 -3/8 of an inch as shown in figure 3. With respect to the top side of the slab, the bottom was relatively crack free which indicates the negative moment due to the columns above may have been too great.

As previously mentioned, there are several factors when combined may be the reason the strength of the transfer slab at the fourth floor was not adequate to resist the forces applied by the floors above. The design strength for the concrete slab was specified to be 5000 psi. However, after core testing 7 samples, the average strength resulted to be only 4150 psi (Hill et al 2011, 11).

The low strength of the concrete used for the transfer slab can primarily be traced to the aggregate which was limestone but not uncommon for the period and location of Dolphin Tower. Voids were found to be included in the aggregate which caused a lower density as well as a below normal amount of coarse aggregate. Coarse aggregate may have been minimized during construction in order to increase workability around the complicated reinforcing layout. None of the 7 core samples achieved the minimum specified strength through laboratory testing and the aggregate choice may be the leading cause of the low compressive strength however is only one of the contributing causes of the failure. Another major concern with the transfer slab was punching shear due to the garage columns below. Punching shear is due to localized forces on slabs at supporting columns which attempt to punch a hole upwards through the slab. Due to the close offset nature of several of the garage columns and tower columns at the fourth floor transfer slab, these localized forces became great enough to cause concern. Typically reinforcing is used in these circumstances in order to resist the stresses caused by punching shear and in fact was used for the transfer slab at the top of the columns. The reinforcement was found to be sufficient to resist the stresses caused by the garage columns but after inspection it was found that the failure plane appeared to extend past the limits of the longitudinal reinforcement as shown in figure 4. Due to this, the longitudinal reinforcement ratio would effectively be 0 causing no resistance of the tensile forces for the negative bending moment in the transfer slab at the column. The only resistance of these forces would have been due to the concrete which as stated above was already reduced in strength. It was found at these locations, the cut-off point appeared to be measured from the center of the column rather than the face of the column as specified in the code. About half of the bars in the east-west direction on the east side of column 13 were cut at 41 inches while the remaining were cut at 72 inches equating to a length of approximately 2d and 3.5d. A cut-off distance such as this would be satisfactory for a much thinner slab but are not significant for the 24 inch slab used in Dolphin Tower. On contrary, the bars on the west side of column 13 were continuous over column 21 and did not exhibit any signs of similar failure (HIll et al 2011).

For additional information on punching shear issues see also Harbour Cay Condominiums and 2000 Commonwealth Avenue - Boston

Long term deflections and stresses due to creep have been shown to be significantly lower than that of short term tests performed on concrete members and slabs. This is increasingly amplified in lower strength concrete. Since Dolphin Tower was created as a completely cast in place structure, the dead loads are much higher on the columns and transfer slab than that of the live loads. Given that the concrete used in the transfer slab is classified as low to medium strength concrete and the high ratio of sustained loads, it is highly possible this could have contributed to the failure of the transfer slab (Hill et al 2011, 12)

Additional Failure Concerns
Additional evaluation by TRC Worldwide Engineering revealed potential larger problems to the structure that could cause additional failure but did not contribute to this instance. Being located on the west coast of Florida, Dolphin Tower is vulnerable to collapse in the hurricane prone region. 3D models showed "the buildings shear walls were overstressed by more than 200 percent of the allowable load and that the overall building drift would be in the order of magnitude of 30 inches" (Pollick 2012, 1). Due to the lack of stability and over stressed shear walls, a major storm could have the potential to cause "extensive damage" to the condominium which is already weakened due to the defected transfer slab. Additional wind loads could cause increased bending moments on the transfer slab at the fourth floor level which would have difficulty resisting such. However, reports stop short of stating that there is potential for Dolphin Tower to overturn. The temporary post shoring (figure 5) which have been installed to stabilize the structure while repairs take place are significant to prevent a collapse due to wind loading though the design for permanent repairs should take this loading into account.

Further analysis of floors above the 4th floor transfer slab also revealed issues with punching shear and the shear walls. The shear walls have been found to be "overstressed under the maximum, wind loading prescribed by the original design code" (Hagood 2010, 1). Several other columns on the upper floors are subject to punching shear issues due to gravity loads alone but have not currently shown any signs of failure but could cause future failures.

Building Repair
Due to the fact building repair is still underway at the time of this case study, there is limited information on the repair procedures due to confidentiality requirements. Initial repair reports require jacking each floor back into place and designing additional support points to transfer the load to the existing foundation of the structure (McQuaid 2010, 1).

Currently there is a good amount of research on carbon fiber reinforcement which has already been used in the industry to repair concrete beams that have been over stressed and are the cause of concern. Since the transfer slab in this case is cracked primarily on the top surface, this could be one possibility to repair the building with minimal additional damage or demolition. In this sense, the slab would be hydraulically jacked back into place (already done as stabilization measures) and sheets of carbon fiber reinforcement would be epoxied to the slab (many layers may need to be used to gain the required strength). This method would allow quicker repairs and less damage to the structure, however would not completely solve the punching shear issue since it would only assist with the negative bending moment associated within the slab.

Conclusion
It is still unclear as to why Dolphin Tower remained unharmed for over 36 years of occupancy however one probable trigger is due to the sustained loading which over time can weaken concrete to 70 - 80 percent of its original strength (Hill et al 2011, 12). The 1971 code did not fully account for sustained loading. This reduction over time on concrete that had a much lower compressive strength than specified in the drawings may have been just enough to where it reached its critical point 36 years after its construction. The primary cause of failure was due to punching shear created at the 4th floor transfer slab which transitioned the building from wide bays below to more frequent column spacing above in the residential area. This is combined with the fact that codes during the time of design and construction were not as developed as they are today and did not adequately portray the proper techniques to deal with these issues.