An icon deconstructed: The demolition of the Georgia Dome
Home to the NFL’s Atlanta Falcons for 25 years, the massive Georgia Dome was unique among stadiums. It was the only venue ever constructed to host the Olympics, the NCAA Men’s Final Four and the Super Bowl (which it did twice). Its Hypar-Tenstar dome structure was also the first of its type. And on the morning of Nov. 20, 2017, with the help of 240 tons of explosives, it all came crashing down.
Retained by a joint venture of Atlanta-based Holder-Hunt-Russell-Moody, New York-based Thornton Tomasetti provided structural engineering services to assess the viability of imploding the stadium. Pettigrew Inc., a subcontractor for the Detroit-based Adamo Group, developed the master plan for demolition.
Thornton Tomasetti was also the original structural engineer for the stadium, which took 30 months to design and construct before it opened in September 1992. Now playing a key role in its demolition, the team at Thornton Tomasetti was charged with identifying vulnerabilities and risks while optimizing a plan to mitigate damage to the newly adjacent $1.6 billion Mercedes-Benz Stadium as well as other Georgia World Congress Center Authority assets.
Various technical challenges came with the demolition of the unique oval-shaped stadium, which was approximately 770 feet long, 610 feet wide, 275 feet tall and topped by the world’s largest cable-supported dome. The roof featured a webbed structure made of triangulated tension cables and floating compression posts combined with stretched fabric membranes. The network of cables and posts were anchored at 26 locations along a concrete cast-in-place compression ring beam that was about 2,300 feet in circumference, 26 feet wide, 5 feet tall and featured a hollow cross section. The structural frame featured beam and column bays along 52 radial grid lines and seven circumferential grid lines.
The large compressive forces in the ring beam required careful consideration. Sudden release of the energy stored in the beam could result in unpredictable behavior of the roof as well as the ring beam itself. Therefore, it was essential to understand how the beam would behave upon explosive concrete removal before the demolition could move forward.
Given the structure’s size and complexity, Thornton Tomasetti adopted a dual-modeling approach to balance the accuracy and efficiency of the engineering analyses. Based on Pettigrew’s plan and existing structural drawings, the company first built a detailed quarter-symmetry model of the ring beam to study its response to the explosive demolition of the material in critical locations, validate the proposed demolition methodology and identify any possible drawbacks. The results of the analyses from the detailed ring beam model were used to inform the global progressive collapse of the roof and lower part of the superstructure.
Thornton Tomasetti used its proprietary NLFLEX software—typically used by the team’s engineers and analysts to analyze nonlinear dynamic events—to carry out the engineering analyses.
Demolition analyses and findings
Ring beam analysis
In Thornton Tomasetti’s model of the ring beam, all the concrete, steel rebar, cable anchorages and sliding bearings were represented to accurately simulate their responses during the collapse.
The company performed the calculations to examine the ring beam failure due to buckling upon explosive concrete removal in the hollow bays of the ring beam between cable anchors. After performing the analysis, the crew determined approximately 24 inches of concrete were required to be removed at corner points to initiate buckling of the ring beam rebar. Near the 50-yard line, approximately 36 inches of concrete were required to be removed due to higher compressive stresses.
In addition to analyzing the ring beam behavior for the explosive failure between roof anchorages, the team also performed calculations to investigate the ring beam response to explosive failure at specific anchorage points. If both the concrete and rebar were removed around the anchors, the cable anchors would pull out immediately and would impart no inward pull to the ring beam as it disintegrated. However, if only concrete was removed and the steel rebar left intact, the cables would pull out of the anchors at a slower rate, thereby pulling the ring beam inward as the exposed rebar buckled and severed.
Thornton Tomasetti also performed a comparison of explosive failure at hollow bays between cable anchors and explosive failure at heavily reinforced anchorage points. Compared to the latter scheme, the former, where the cables were not impacted, ensured the ring beam debris fell within the stadium’s footprint and presented less risk to the adjacent Mercedes-Benz Stadium. The team concluded severing the ring beam at locations between column lines, instead of at cable anchor points, was the preferred strategy. The findings in the model not only informed the representation of the ring beam behavior in the second model, but also resulted in modifications to Pettigrew’s master demolition plan.
Stadium progressive collapse analysis
In close collaboration with Pettigrew, the second model of the stadium was exercised with more than 30 different analysis cases to arrive at the final demolition plan. Subsequently, Pettigrew’s plan included three methods to displace structural concrete and steel mass, namely hinging and rotation, softening and 100 percent mass displacement. The demolition sequence was read into the progressive collapse analysis and material was eroded per the delays to mimic explosive failure.
In its entirety, the analyses involved displacing the concrete and steel in the ring beam as well as in the load-bearing columns, as per the proposed plan, and simulating progressive collapse of the structure under its own weight. The analyses consisted of a preload phase and the dynamic analysis phase. In the preload phase, pre-stressing of the roof structure, along with application of dead load, was undertaken to achieve static equilibrium.
Once the structure was in static equilibrium, the dynamic demolition phase could begin, wherein structural elements were removed as per the final demolition plan. During the collapse process, all critical interactions—like the impact between structural members and the impact with the ground—were accounted for to ensure the dynamics of the collapsing structure were accurately represented.
This showed that as the concrete in the ring beam was removed in critical locations, rebar buckled and released the compressive energy in the ring beam upward. As the collapse progressed under the pull of the intact cables, the ring beam disintegrated into big pieces and fell within the boundaries of the stadium’s footprint.
Meanwhile, the hinge motion and the softening in the columns caused them to rotate and collapse inwards. Thus, the lower portion of the superstructure below the roof would also collapse inwards, though some debris outside the original perimeter was to be expected. In addition, the cable constraints between the ramp columns and the stadium columns and beams would provide effective connection, ensuring the ramp framing was pulled inward during the collapse as well.
Thornton Tomasetti’s analysis results indicated that Pettigrew’s final demolition plan for the dynamically induced collapse of the Georgia Dome was feasible. (The collapse progression is illustrated in a series of figures on pages 30-31.)
Most of the debris was expected to fall within the original footprint, with some likely spreading beyond the original footprint. This highlighted the importance of having temporary fencing around the structure to prevent damage to the new stadium—a measure that was recommended and implemented into the final demolition plan.
At 7:30 a.m. on Nov. 20, 2017, the stadium came crashing down during a 12-second implosion in a feat that belied the event’s painstakingly methodical, careful preparation. For Thornton Tomasetti, it was a moment of pride to watch the demolition unfold in a fashion that largely mimicked our analyses.
This article originally ran in the Sept.-Oct issue of Construction & Demolition Recycling magazine. Mahesh Raju Bailakanavar is an associate at Thornton Tomasetti. He can be contacted at MBailakanavar@ThorntonTomasetti.com.