Engineering Analysis And Design
Kingdome Roof Repair
On July 19, 1994 four ceiling tiles dropped from the underside of Seattle’s Kingdome roof into the seats just two hours before a Mariners baseball game. The incident closed down the Kingdome for nearly four months and set off a chain of renovations and repairs that resulted in a $67 million price tag. Complicating the work was the high visibility and oversight that the project was receiving from local politicians, the sports world, and the media.
Early in the process, Q-Metrics was called upon to help determine the cause of the tile failure and to provide technical justification for the proposed repairs. A total of 40,000 tiles were installed on the ceiling at the time of the Kingdome’s construction in 1975 primarily to control the noise within the concrete structure. Examination of the transient heat transfer through the concrete roof and the generation of moisture within the dome from the fans, food preparation equipment, and even the ‘monster truck shows’ that were often held confirmed that a contributor to the failure of the tiles was years of condensation on the underside of the roof that was subsequently absorbed by the wood fibers in the tiles, softening the fibers and causing the tile to cup and warp. Excessive sagging caused the 26 pound tiles to pull away from the metal clips holding them against the ceiling and drop to the seats below.
The first part of the analysis consisted of developing a 3-D thermal model of the Kingdome roof structure. The unique design of the roof structure presented a problem in insulating the roof. Forty-one concrete ribs, each approximately 6 feet high by 2 feet wide divide the roof of the Kingdome like wedges in a pie. To insulate the entire rib structure would be difficult, expensive, and subject to leaks due the number of joints. Therefore, the 3-D thermal model was exercised in a transient mode to determine the effect of various insulation types, thickness, and coverage schemes. To drive the transient analysis, actual hourly weather data for the previous 10 years was located and scanned to determine a bounding profile for a one month period. Together with the typical operating schedule for the ventilation system, occupancy, and food preparation equipment use, this weather data was used to determine the optimal insulation design which would prevent condensation on the roof’s underside while minimizing installation costs.
During a review of the proposed insulation design, questions were raised by Seattle’s building authorities as to whether or not the 20 year old Kingdome should be made to comply with the state’s energy code. Since the energy code provides exceptions for facilities where conforming to the code would pose unusual economic or practical hardships or where it can be shown that it is not necessary to achieve the code’s conservation requirements, Q-Metrics was called upon once again to develop a computer model of the Kingdome structure, its heating/cooling systems, the lighting system, and its internal equipment. This energy conservation model was exercised for several insulation schemes and under several proposed facility operational schedules. The results demonstrated that the proposed insulation design was indeed the best approach since the facility switches from heating to cooling mode once the stadium fills with people. As such, increasing the insulation amount or coverage would simply trade heating savings for cooling costs and not achieve an overall energy savings. The energy conservation model did point out several areas for energy savings, including tighter control over the amount of outdoor air brought in during unoccupied periods, improved central plant efficiency through the optimal selection and loading of which boilers to run, optimal fan start to avoid pre-mature heating of the Kingdome prior to each event, and variable speed drives on the pumps used to circulate hot water and chilled water in order to match system capacity with the heating and cooling needs.
Following the recommendations developed by Q-Metrics and the other consultants assisting on the project, King County replaced the 40,000 wood tiles previously used for acoustical control with a spray on insulation that does not rely on mechanical devices for attachment and which can withstand condensation without damage. The exterior of the Kingdome’s roof was insulated in the manner determined by the Q-Metrics’ analysis. Subsequent operation of the Kingdome has not shown any repeat of water condensation or roof leakage.
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