By Sean P. Clifton, PE, SE, and Kevin P. Aswegan, PE, SE
The University of Texas at Austin often touts the phrase, “What Starts Here Changes the World.” That is precisely what is happening with ATX Tower—the world’s first high-rise tower to incorporate the American Society of Civil Engineers (ASCE) Prestandard for Performance-Based Wind Design (PBWD)—under construction in Austin, Texas. PBWD will likely revolutionize how tall buildings are designed for wind loads, creating more reliable, lower-carbon, and less costly buildings.
Rising 58 stories and 671 feet above downtown, ATX Tower is located at 321 West 6th Street. It will offer 561,000 square feet of office and residential space for tenants and residents of the city’s historic entertainment district. The project is expected to top out this summer and be completed in 2025.
Innovation doesn’t happen overnight. Magnusson Klemencic Associates (MKA) has been developing PBWD for much of the past decade through funding research at universities nationwide, participating in professional committees, and testing designs through numerous case studies of actual buildings. Since the ASCE Prestandard for PBWD’s release in 2019 (see sidebar below, “ASCE Prestandard for PBWD Primer”), MKA searched for the right project on which to implement PBWD, and the stars finally aligned on ATX Tower with a supportive ownership partnership of Tishman Speyer Properties and Ryan Companies and a collaborative architectural team of Handel and Page Southerland Page.
Stakeholder Buy-in
Before incorporating PBWD into the project’s design, MKA and project wind engineer CPP Wind Engineering Consultants (CPP) had to convince the ownership team and the City of Austin’s Development Services Department, which is in charge of building permitting, that there was value in applying this first-in-the-world approach.
Wind tunnel testing began in May 2021 under the guidance of CPP Senior Principal Roy Denoon, whose team conducted multiple test regimens to evaluate wind loads on the structure, cladding, and pedestrian environment for ATX Tower. A 1:350 scale physical model was used to consider the tower’s geometry and the impact of adjacent buildings by testing wind influences from 36 different directions to identify the controlling combinations of wind speed and building response.
From these early test results, MKA and CPP discovered several key factors that made ATX Tower a good candidate for the new methodology:
Shape—The tower’s slender, square shape produced a significant dynamic response caused by wind vortex shedding.
Location—The surrounding towers proved to buffet ATX Tower as wind was redirected.
Test Results—Wind loads predicted by wind tunnel testing revealed loads 20-30% higher than building code estimates—particularly from the east and northeast directions.
The tower’s design needed to accommodate the increased wind demands, and MKA felt PBWD could be an excellent alternative to increasing the sizes of structural members. The firm tapped into its extensive experience in developing Performance-Based Seismic Design (PBSD) to investigate the feasibility of implementing a PBWD methodology to mitigate the impacts of higher-than-expected wind demands. Stakeholders agreed, proving to be open-minded and supportive of PBWD.
Design: Three Key Steps
With the stakeholders’ preliminary approvals in place, a peer review team made up of wind engineers and practicing structural engineers was assembled to collaborate and review the design. The design process involved three key steps:
- The team needed to verify the residents would be comfortable during common wind events. CPP confirmed that predictions of occupant comfort were within industry-standard acceleration limits without any modifications to the building massing or structural layout.
- A more detailed methodology, as outlined in the ASCE Prestandard, was then employed to assess story drift. Typically, story drift is calculated as the difference in displacements between adjacent levels. For ATX Tower, a more detailed assessment of racking drift specific to the exterior walls and internal partitions was pursued. MKA compared these estimates to recommended limits for exterior glazing, interior drywall partitions, and masonry. Movements proved to be within acceptable limits for the various non-structural materials.
- Finally, MKA’s team designed and verified the strength performance, which proved complex and challenging. The stated performance objective for the main wind force-resisting system—“Continuous Occupancy, Limited Interruption”—allows specific structural system elements to respond inelastically. In ATX Tower, these elements were limited to the shear wall coupling beams, shear wall flexural response, and outrigger column tension action. Following the ASCE Prestandard provisions, these elements were designed to a 1.25 demand-to-capacity ratio, using expected material strength and phi factors of 1.0.
Wind Non-Linear Response
With the design completed, MKA’s team analyzed and verified the performance by developing a detailed analysis model using non-linear properties where elements may experience inelastic behavior. Engineers calibrated those properties to physical testing completed at several universities. The analysis model was then subjected to a time history loading protocol involving five unique windstorms. Each one-hour storm represented wind from a different direction that controlled an aspect of the design.
The analysis model ultimately took an impressive one month of runtime to complete the simulation of all five storms. This runtime was significantly more than the typical PBSD analysis due to earthquakes being short-duration events compared to windstorms. However, when engineers reviewed the terabytes of output data, the building exhibited excellent performance. The results showed limited yielding in a small number of coupling beams and little yielding in the shear walls or outrigger columns. This result was exactly what the team was looking for, demonstrating that acceptable performance can be achieved with more rigorous engineering using PBWD.
Lower Costs and Reduced Embodied Carbon
As ATX Tower was the first high-rise to utilize the PBWD process, the team also completed a parallel code-prescriptive design as a “fallback” in case of trouble securing the permit. The PBWD design was approved and permitted, but the parallel design proved valuable. The prescriptive design allowed the project team to compare the savings in quantities and materials between the novel PBWD design and a code-prescriptive design. The PBWD design offered many project benefits:
- 350 tons of reinforcing steel, 125 tons of structural steel, and 1,800 cubic yards of concrete were saved.
- A 5% reduction in the structure’s total cost was realized.
- Floor plans were improved because shear wall thickness was reduced.
- The construction schedule was improved through the reductions in materials.
- A 6% reduction in embodied carbon was achieved.
Remarkably, the PBWD process didn’t slow the project’s aggressive schedule. The structural permitting process was rapid and smooth as the city officials participated throughout the design process, rather than simply reviewing after the design was complete. The first drilled shaft was installed a mere 12 months from the start of Schematic Design. This efficiency was achieved thanks to a collaborative approach among the design team, client, general contractor, and city officials.
A Bright Future for PBWD
PBWD has sparked an industry that historically struggles to innovate and improve upon many years of status quo. Akin to PBSD, which has become the standard of care when designing high-rise buildings in America’s earthquake-prone regions, PBWD could become the norm for designing tall buildings in regions vulnerable to fierce wind events.
Successfully incorporating PBWD into the ATX Tower’s design has spurred MKA to implement this approach in other Austin projects, though a sluggish real estate market has paused that activity. Elsewhere, PBWD research and advancements are ongoing at universities worldwide, and the American Concrete Institute is finalizing guidelines to implement PBWD for concrete members. When it comes to PBWD’s future, the wind is at the industry’s back. ■