Engineering Innovation and Environmental Leadership in Downtown Houston

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In the heart of Houston’s Central Business District, a bold new addition to the skyline exemplifies both structural ingenuity and an ambitious commitment to sustainability. The Norton Rose Fulbright Tower—formerly known as 1550 on the Green—is a 28-story, 375,000-square-foot Class A office building developed by Skanska and with Bjarke Ingels Group (BIG) as the design architect, Kendall/Heaton Associates (KHA) as the architect of record and Walter P Moore (WPM) as the structural engineer of record. Rising alongside Discovery Green park, the tower offers more than just prime office real estate; it serves as a case study in how complex urban development challenges can be addressed through innovative structural solutions, sustainable design, and forward-looking material selection.

Located on a uniquely shaped parcel in Downtown Houston, the Norton Rose Fulbright Tower occupies a quarter-circle block fronting the curved edge of Lamar Street and facing Discovery Green. The remaining quadrant is occupied by an existing 20-story hotel with multiple basement levels, creating a complex interface condition along the western edge. This constrained footprint prompted a highly coordinated architectural and structural response (Fig. 1).

Rather than view the irregular site geometry as a limitation, the design team leveraged its form to create a cohesive and efficient layout. Key planning strategies included:

In early design stages, multiple strategies for foundation design were evaluated. The three main frontrunners were as follows:

Option 1 offered key advantages: Deep foundations would have eliminated the need for complex, sequential excavation and significantly reduced the risk of destabilizing the adjacent hotel. However, the associated cost and schedule implications to drill and install the foundations coupled with the loss of valuable below ground parking levels made this option impractical, leading to its dismissal.

Option 2 presented a different opportunity. Excavation for a single basement level (approximately 17-feet of soil) would reduce the overburden pressure on the soil layer at the base of the proposed mat foundation, enabling the soil to better support the new tower’s loads—a common strategy with mat foundations in Houston. The primary challenge was the complex interface between the existing hotel foundations and the new structure. Careful coordination was needed to prevent either building from imposing loads on the other's foundation. This constraint led to deeper-than-necessary foundations in some areas, and ultimately, it was more efficient from a cost and planning point of view to add the second basement level, carrying a deeper foundation design across the entire project. This addition of an extra below-grade parking level without increasing the overall building height above grade benefited project efficiency and enhanced the overall ability to appeal to future tenants of the building.

The final solution featured a mat foundation bearing at 35 feet below street level, matching the bearing elevation of the neighboring hotel’s mat. To mitigate lateral soil pressures and prevent undermining of the adjacent structure during both construction and in the final condition, the mat was designed as three separate segments. This approach enabled a staggered excavation and pour sequence: the soil for the central section was removed and the concrete was placed first while the soil for adjacent sections was left undisturbed. Only when the central section had cured did excavation begin on the adjacent mat sections, allowing the central mat section to prohibit sliding of the existing hotel mat foundation.

In total, approximately 8,200 cubic yards of 6,000 psi concrete were used across all mat pours. The mix design used for the mats limited the maximum cement content to 300 lb/cy and implemented a 55% Class C flyash cement replacement to reduce heat of hydration. Thermocouples were embedded to monitor temperature gradients and ensure temperatures met project mass concrete placement requirements of ACI 301 Specifications for Structural Concrete.

The retention system selected for the project consisted of permanent, 30-inch-diameter drilled slurry retention piers spaced at 3 feet, 9 inches and extending 45 feet deep, tied back with a single row of anchors (Fig. 2). The closely spaced slurry retention piers, also known as soldier piers, were faced with a 9-inch cast-in-place concrete wall to provide a clean finish for the interior basement levels. In zones where tiebacks were infeasible—specifically adjacent to the existing hotel—larger soldier piers were used and internally braced during the excavation phase.
At one critical grid intersection, a pair of 42-inch-diameter piers extending 100 feet deep served as direct supports for a vertical building column, with the cap beam embedded into the excavation shoring system.

Working in collaboration with the geotechnical engineer, the structural team conducted a comprehensive foundation analysis. The mat foundation, ranging from 5 feet to 8 feet thick depending on tributary loading, was evaluated through an iterative soil-structure interaction model in CSI SAFE. Subgrade reaction modulus values ranged from 14 to 60 pci. Predicted settlements were within acceptable thresholds—ranging from ½ inch at the lightly loaded northwest end to a maximum of 4 inches beneath the offset core.

A defining architectural feature of this Houston high-rise was the use of an offset concrete shear wall core—a deliberate deviation from the conventional centrally located core typically favored for structural lateral efficiency and elevator access. Adding further complexity to the project was the tower’s distinctive massing. Its curved and stepped profile is made up of six vertical segments—or “towers”—each offset by 9 degrees and with facade setbacks at different elevations. This geometry not only generates a dynamic skyline presence but also creates elevated terraces that serve as highly desirable outdoor amenities. Nevertheless, the building’s articulated form presented challenges for framing and constructability.

The rotation and stepping of the tower segments required precise beam alignment and load path continuity, particularly through the moment frames and interior girders. To address this, the structural team implemented a 21-inch-deep wide module pan-formed normal weight concrete framing system with a 4 5/8-inch thick concrete slab, designed to repeat across rotated bays. This approach allowed for the standardization of formwork—a key cost and schedule advantage within the Houston market—while preserving architectural consistency across the varied elevations.

A signature structural element used to create the tower’s unique profile was a “forked” cantilevered, post-tensioned concrete beam. This innovative beam relied on a single continuous back-span to support the desired horizontal layout. Both its analysis and detailing were critical, requiring careful attention to constructability and serviceability.

Walter P Moore developed a standardized detailing approach that clearly established reinforcing steel hierarchy and placement unique to each beam configuration. This clarity streamlined shop drawing review, field installation, and trade coordination. The layout also allowed for straightforward anchoring and stressing of the post-tensioning tendons—eliminating conflicts with mild reinforcement and contributing to a predictable, efficient construction process.

The implementation of the offset core meant that an eccentricity was introduced intentionally in the lateral force resisting system. To address this, Walter P Moore supplemented the lateral system by using concrete moment frames. The most noteworthy of these were two additional lines of circumferential moment frames wrapping the perimeter that were used to effectively collect and transfer lateral loads back to the offset core while providing the stiffness and torsional resistance needed for overall stability. This integrated system allowed the structure to align with the building’s unique architectural aspirations without compromising performance.
Given the building’s unusual form and low seismicity requirements, wind behavior was a critical consideration. Initial lateral analyses used ASCE 7-10 code-based Main Wind Force Resisting System (MWFRS) loading and an ultimate wind speed of 139 mph across multiple directions to identify critical load cases. As the design progressed into Design Development, CPP Wind Engineering Consultants conducted a wind tunnel study to refine pressure assumptions and reduce uncertainty (Fig. 7).

Using 36 synchronized pressure tap measurements and local climate data, the team captured wind behavior from all directions. These pressures were converted into equivalent static frame loads using the tower’s dynamic modal properties. The resulting data provided statistically reliable wind loads for both 700-year and 10-year return periods, forming the basis for the final lateral design (Fig. 8-9).

The building is clad primarily in a glazed curtain wall system supplied by Arrowall Co., requiring strict control of inter-story drift to prevent damage to the cladding joints and interior partitions. Although inter-story drift limits for wind loading are not explicitly defined by code, industry practice for buildings of this scale typically limits drift to H/400 under 10-year mean recurrence interval (MRI) wind events.

To quantify potential distortion, Walter P Moore modeled null area elements representing the curtain wall within the lateral system and calculated a Drift Measurement Index (DMI) for each panel. This index captured the average in-plane shear distortion across elements. Higher DMI values indicated greater potential for cladding or partition damage.

These values were then compared to Drift Damage Index (DDI) thresholds, defined as the maximum acceptable level of distortion before material damage occurs—based on racking test data from comparable materials given in “Serviceability Limit States Under Wind Loads” by Lawrence G. Griffis (AISC Q1,1993).

The following serviceability condition was enforced across the facade system:

Drift Measurement Index (DMI) ≤ Drift Damage Index (DDI)

This analytical approach provided a quantitative and materials-based framework for evaluating serviceability limit states under wind loading.

Skanska, the project’s developer, has a strong focus on reducing the carbon footprint of all their projects and has set an ambitious goal of achieving net-zero emissions across their operations and value chain by 2045. Walter P Moore shares this commitment to sustainability by supporting the net-zero goals outlined by SE2050, aligning with the broader industry shift toward low-carbon, climate-resilient design practices.

To benchmark sustainability goals on the project, the design team used the Embodied Carbon in Construction Calculator (“EC3”) tool co-developed by Skanska. EC3 is an open-source platform that allows users to assess embodied carbon in building materials including steel, concrete and architectural finishes. The team used EC3 to evaluate and compare the carbon impact of various concrete mix options, with the goal of reducing embodied carbon emissions by 20% from a benchmark building.

From the outset, the Walter P Moore project team adopted a data-driven approach by performing a Whole Building Life Cycle Assessment (WBLCA) during early design phases. This allowed the team to identify major contributors to embodied carbon, especially in the foundation and floor systems, and informed the structural design process. The insights from this assessment were crucial in setting performance targets for low-carbon concrete and achieving compliance with the LEED v4.1 Building Life Cycle Impact Reduction credit.

To support the use of low-carbon concrete, Walter P Moore included specific requirements in the construction documents for each mix, such as maximum cement content and limits on Global Warming Potential (GWP). Strength requirements for foundation and column elements were specified at 90 days rather than the conventional 28 days, allowing for lower-cement mixes with higher cement replacement and slower strength gain to be utilized. To maintain construction pace, Walter P Moore collaborated with the contractor by permitting maturity meter testing on all elevated floor framing, ensuring early strength targets required for post-tensioning operations were met without relying on the higher-cement mixes that are traditionally used. These combined strategies reduced cement content while maintaining structural integrity and meeting the project schedule.

In addition, Walter P Moore required the ready-mix supplier submit Environmental Product Declarations (EPDs) during construction for acceptance by the design team to ensure project goals related to sustainability were achieved. This was the first project in the Houston market with mix-specific EPDs.

As a result of these efforts, Norton Rose Fulbright Tower achieved an approximate 45% reduction in embodied carbon overall compared to standard baselines and achieved LEED Platinum V4 certification. This was made possible not only by the strategic selection of materials but also by requiring environmental criteria in the concrete specifications—something that structural engineers have growing influence over.

The tower demonstrates that even in markets where EPD infrastructure is still emerging, it’s possible to set meaningful carbon goals, engage with suppliers, and use tools like EC3 to make data-informed decisions. For engineers looking to design more sustainable structures, this project highlights the critical role they play in shaping both environmental outcomes and industry expectations.

The Norton Rose Fulbright Tower stands as a landmark project in Houston—one that demonstrates the power of integrated design to overcome site constraints, architectural ambition, and environmental imperatives. Through a combination of structural innovation, material transparency, and collaborative execution, the project sets a new standard for what is possible in high-rise office construction.

From its offset core and rotated bays to its forked cantilevered facade and low-carbon concrete mixes, every design decision was both intentional and performance-driven. The building not only enriches the urban fabric of downtown Houston but also charts a path forward for sustainable, resilient urban development. ■