Redefining High-Performance Concrete...

Concrete international NOVEMBER 2012 23

Redefining High-Performance Concrete StructuresDesign and construction of the San Francisco Public Utilities Commission Headquarters

by Leo Panian, Phillip Williams, and Mike Donovan

The San Francisco Public Utilities Commission (SFPUC) Headquarters (Fig. 1) in San Francisco, CA, provides a case study of an innovative structural design that

helped achieve an aggressive balance between stringent seismic criteria and ambitious sustainability goals. Unlike a building designed to minimum seismic code requirements, the SFPUC structure is designed to survive major earth-quakes with little damage, allowing it to be immediately reoccupied and easily repaired. In addition, the building is on track to be certified as LEED Platinum by the U.S. Green Building Council (USGBC). All of this was achieved at a cost comparable to that of a conventional office building, setting a new standard for seismic and sustainable design of concrete structures.

Innovative Structural SystemThe basic objective of any building code is to protect

the safety of occupants. Although the concept of seismic durability is not explicitly addressed within the framework of current building codes or green rating systems, it was a central performance goal for the structural design of the SFPUC building. We sought to reduce or eliminate the potential for lengthy building closures needed for repairs following an earthquake, thereby preserving the investment in materials and energy required for the construction of the new building.

To reach this goal, we chose to avoid the use of sophisticated technologies, such as hydraulic dampers or base isolation systems. While these technologies can be very effective in controlling the response of a building and minimizing earthquake damage, they command significant price premiums and tend to introduce design and construction complexities that limit their applicability and practicality.

The challenge for the engineering team was to develop a structural system comprising conventional components, yet: • Capable of withstanding intense ground shaking with

little damage; • Superior to more exotic engineering solutions in terms

of construction cost and schedule; and • Well-suited to the architectural and sustainability goals

for the project.

Fig. 1: Architectural rendering of the completed SFPUC building. The glass curtain wall was designed with operable windows and shading devices to enhance ventilation and daylight. The design of the façade features wind turbines integrated within a glass airfoil tower. The curtain wall system is designed to accommodate earthquake movement with minimal damage (Photo ©KMD Architects)

24 NOVEMBER 2012 Concrete international

Breaking the height barrierThe architectural design for the SFPUC building

required it to be 180 ft (55 m) tall. This created a significant design obstacle, as the International Building Code1 imposes a 160 ft (49 m) height limit for buildings that rely solely on concrete shear walls for seismic resistance. For taller buildings, only moment-resisting frame systems or dual systems (systems with moment-resisting frames and shear walls) are permitted for reinforced concrete structures. But, a moment-resisting frame system would have been inadequate to satisfy our seismic performance objectives, and a dual system would have imposed significant additional costs. Further, if the moment frames required with either structural system were located at the building perimeter, the amount of natural light available to the interior of the building would be decreased, thereby increasing the demands for electrical lighting and energy use.

We therefore employed a performance-based design approach to explicitly demonstrate an alternative structural system that would satisfy the basic building code provisions and provide the enhanced level of seismic protection we sought. This method relied extensively on detailed nonlin-ear seismic shaking analyses and a rigorous peer-review process for validation.

The key parameters for performance-based seismic design are typically displacement-based. The basic quantity of interest is the interstory drift, defined as the ratio of horizontal deformation over the story height for a given floor. The interstory drift can be correlated to the level of damage that can be expected to occur in structural or nonstructural elements. For a conventional building design, the maximum drift under the design basis earthquake (DBE) is typically limited to 2.0%. For the SFPUC building, the maximum drift was set at 1.0% to limit damage sufficiently to allow the building to be reoccupied shortly after an earthquake.

Another important goal was to prevent permanent post-earthquake deformations. In a conventional setting, such deformations in a structure can be significant, adversely affecting the functioning of elevators, interior doors, and other systems. Although not specifically addressed by the building code, these residual deformations can be very expensive to rectify.

Our solution relies on an innovative system of self-centering concrete core walls incorporating unbonded vertical post-tensioning and composite link beams. The system (Fig. 2) is capable of controlling building move-ment during intense ground shaking and restoring the building to its original position after the event. The vertical post-tensioning tendons, which impart the strength and elasticity needed to recenter the structure, extend over the full height of the towers. Tendon bundles are routed from the top of a wall to a saddle within the mat foundation (Fig. 3) and back to the top of the wall. Each of the two towers contains eight tendon bundles that are about 400 ft (120 m)

Fig. 2: Schematic cutaway view of the structural system. The shear walls are vertically post-tensioned using multistrand tendons that pass through saddles within the mat foundation. The tendon was stressed by jacking simultaneously on the two terminations at the top of each wall. The central prestressing force is designed to bring the walls back to the vertical after a seismic event (Photo ©Tipping Mar)

Fig. 3: Mat foundation reinforcing. The curved ducts behind the worker are saddles for the walls’ post-tensioning tendons and will anchor the tendons to the foundation system (Photo ©Tipping Mar)


in length—roughly twice the height of the building. Each tendon comprises up to 28 continuous 0.6 in. (15 mm)

diameter strands (Fig. 4). The vertical post-tensioning allows a reduction of almost 50% in the quantity of vertical mild-steel reinforcing in the cores, reducing labor and minimizing congestion (Fig. 5).

Link beamsThe composite link beams incorporated into the system

also represent an innovative departure from conventional design practice. In typical reinforced concrete construction, coupling beams formed over doorways in shear walls may be deep, heavily reinforced, and include diagonal reinforcing bars. As a result, conventional coupling beams can be particularly difficult to construct. Because of their stiffness, diagonally reinforced coupling beams are also susceptible to significant cracking and damage under seismic movement. Because coupling beams tend to be numerous and distributed throughout a building, post-earthquake repairs can be costly and disruptive to ongoing operations.

For the SFPUC building, composite link beams were fabricated with a 3/8 in. (10 mm) thick plate shell designed as stay-in-place formwork and beam reinforcement (Fig. 6),

Fig. 4: Workers lower a hydraulic ram over a wall’s post-tensioning strands (Photo ©Tipping Mar)

Fig. 5: Ironworkers install reinforcing bars in the shear walls. Corrugated steel ducts placed in the center of the walls will house post-tensioning strands (Photo ©Tipping Mar)

Fig. 6: Rendering of the reinforcement for the composite link beam and shear wall. The beam’s steel jacket was fabricated from a single plate and serves as both stay-in-place formwork and beam reinforcement. Bolts (3/4 in. [19 mm] diameter) extend transversely through the link beam for confinement (Photo ©Tipping Mar)


resulting in a shallower, more flexible beam. It also simplified construction by eliminating the typical diagonal reinforcing bars and reducing the required number of stirrups. Under the influence of the horizontal prestressing in the floor system, the composite link beams are also designed to contribute to the recentering behavior of the structure. The link beams for the project are 30 in. (760 mm) wide and from 20 to 36 in. (500 and 900 mm) deep.

The external steel plate alters the behavior of the link beam in a fundamental way (Fig. 7). Under reversed seismic loading, the plate forces a single flexural crack to form at

Fig. 7: Construction of composite link beam assembly showing debonded horizontal reinforcement and shear brackets (Photo ©Tipping Mar)

Fig. 8: Detail elevation and cross section of composite link beam. The external steel jacket includes horizontal and vertical shear studs to develop composite action. At the face of each wall, steel brackets are provided to transfer shear forces. Bond breakers on longitudinal reinforcing bars allow ductile behavior (Photo ©Tipping Mar)

the face of the wall, rather than allowing the distributed cracking that would be expected in a plastic hinge region of a conventional beam. Furthermore, the steel jacket relieves the compressive strains on the concrete and allows for a more ductile response with less degradation in strength and stiffness. These effects required several special-ized design considerations.

The longitudinal reinforcing bars in the link beams were debonded from the surrounding concrete with waxed sleeves at the critical regions to allow an adequate plastic strain length to prevent premature tensile fracture (Fig. 8). In addition, embedded steel brackets were provided to mechanically restrain the link beam, ensuring a direct, reliable transfer of shear at the interface with the walls. The external steel jacket eliminates the cracking and spalling damage that would be otherwise expected with a conventional coupling beam, thus minimizing the need for extensive post-earthquake repairs. The result is a more cost-effective and damage-resistant structural component that helps achieve a higher standard of performance than simple life safety.

This combination of innovative components forms a structural system that provides a highly resilient backbone to stabilize and protect the building and its critical systems from earthquake damage, allowing the building to be quickly reoccupied and put back into service.

Floor structureIn addition to the challenges of seismic design, it was

important to also devise a floor framing system optimized for cost, speed of construction, and overall depth. While post-tensioned, two-way flat plate floors would generally meet these constraints, the building footprint and architectural program called for an asymmetrical grid arrangement requiring a series of long spans (nearly 41 ft [12 m]) in one direction and short spans (20 ft [6 m]) in the other. With this column arrangement, a constant thickness flat plate would have been inefficient and impractical.

We devised an optimized floor structure incorporating shallow post-tensioned beams and one-way slabs (Fig. 9). The beams were typically 36 in. (900 mm) wide and 16 in. (400 mm) deep, and the slabs were typically 6 in. (150 mm) thick. The overall depth of the selected floor system was


12 in. (300 mm) less than a steel-frame design alternate, allowing an additional floor within the overall height envelope and adding significant value to the project.

Sustainable ConcreteUltimately, the overall goals of the project would not be

effectively met without addressing the environmental impact of concrete. Our goal was to develop concrete mixtures that would limit carbon emissions without compromising strength or workability requirements. It was known that producing high-strength, low-cement concrete was feasible; however, it wasn’t clear if such mixtures could be cost-effectively produced at the scale needed for this project.

Low-cement mixturesBased on previous project experience, we set a goal of 70%

portland cement replacement with Class F fly ash and slag cement. Several refinements were required to ensure that the specified strength would be met without compromising the finishability of the slabs or delaying set time.

Our specifications listed the following requirements for the mixture designs:

• Mat foundation slab: 8000 psi (55.2 MPa) at 90 days. Maximum portland cement content: 200 lb/yd3 (119 kg/m3);

• Columns and shear walls: 8000 psi (55.2 MPa) at 90 days. Maximum portland cement content: 225 lb/yd3 (133 kg/m3); and

• Post-tensioned slabs: 4500 psi (31 MPa) at 3 days and 6000 psi (41.4 MPa) at 56 days. Maximum portland cement content: 350 lb/yd3 (208 kg/m3).The strength requirements were set at 56 and 90 days in

acknowledgment that design strength wasn’t required until the building was occupied. The concrete supplier worked with the design team to develop a set of custom mixture designs comprising high-range water-reducing admixtures, allowing each to have a low water-cementitious material ratio (w/cm). While the current LEED framework provides strong incentive to use locally sourced materials,2 it was necessary to balance that objective with the overall goal of reducing the carbon footprint of the concrete. Preliminary testing showed that many of the locally available fine aggregates were unsuitable for high-performance concrete with significant cement substitutions. Using the inferior

Fig. 9: The post-tensioned floor system consisted of 6 in. (150 mm) thick post-tensioned, one-way slabs spanning between 36 in. (900 mm) wide by 16 in. (400 mm) deep post-tensioned beams. Typically, two levels of reshores were placed below the formwork. The longer cantilevered slabs pictured on the left were supported by three levels of reshores (Photo ©Tipping Mar)

Fig. 10: Aerial view showing placement of concrete for the mat foundation. Almost 5000 yd3 (3800 m3) of concrete were placed in a single continuous operation, using a mixture with 70% cement replacement and a maximum cement content of 200 lb/yd3 (119 kg/m3), with a required strength of 8000 psi (55.2 MPa) at 90 days (Photo ©SFPUC)


aggregates would also require a relatively higher proportion of portland cement. We therefore allowed the use of higher- quality aggregates sourced over 500 miles (800 km) from the job site, effectively reducing the overall cost and carbon footprint of the in-place concrete.

The USGBC is currently reviewing the LEED frame-work to incorporate methods that account for the true embodied energy of construction materials in the context of a comprehensive life-cycle assessment (LCA). While these methods have not yet been formalized for broad use, we used them to inform the selection of the appropriate concrete mixtures. The “Hybrid Economic Input-Output” carbon accounting method was applied to rapidly, accurately, and transparently analyze design options based on consistent metrics.3

The method provided a highly reliable and flexible approach for formulating and assessing the environmental impact of the concrete mixtures. While the low-cement mixture designs were unquestionably green, the diligent and systematic design-stage assessment of carbon content demonstrated a 49% reduction in equivalent CO2 relative to a standard mixture design.

The hybrid LCA carbon-accounting method combines the completeness of input-output LCA with the bounded specificity of process LCA. Process LCA consists of methodically analyzing material and energy flows at every stage of the life cycle to understand precise consumption and emission values. This method is time-consuming and limited in scope for practical application.4 In contrast with process LCA, where the analysis of interrelated flows is built from the bottom up, input-output LCA starts with a top-down model of the economy. This method is more complete than process LCA because the economic input-output tables capture the interrelations of all inputs all the way back to mining of raw materials. However, input-output LCA has the problem of providing only aggregated industry scale data. The hybrid approach combines the two methods to minimize the weaknesses of each and take advantage of their strengths.5,6 This hybrid method was implemented for the SFPUC project using proprietary software developed by Climate Earth, Inc. (www.climateearth.com) to consistently and reliably assess the environmental impact of the concrete mixtures.

Concrete placementWith the specifications finalized, the designs were put

to the test as construction began and progressed. We learned a number of important lessons and will develop improvements for future applications.

The mat foundation mixture used the lowest amount of portland cement. Its plastic performance was exceptional, with almost 5000 yd3 (3800 m3) placed in a single continuous operation to construct the 10 ft (3 m) thick mat (Fig. 10). Hard troweling was completed within 4 hours after final concrete placement. Temperatures within the mat slab remained well below the 150°F (66°C) limit set in the specifications, and no significant cracking was subsequently observed. For quality control, 48 sets of cylinders were tested in the laboratory. Each of the test cylinders exceeded the minimum strength requirement, and the mean value for all tests was 8560 psi (59 MPa) at 90 days.

For the columns and shear walls, a very similar mixture was used. Usually, forms were stripped within 24 hours, and the surface finish was exceptional—uniformly smooth with very limited surface blemishes. However, some of the strength test results for the column and shear wall mixtures were lower than specified. Core samples were taken and tests indicated that some of the column and shear wall placements were short of the specified strength.

Top-Down Hybrid Life-Cycle Assessments

Two methods have been widely used for LCA: Process LCA and Input-Output LCA. A Process LCA consists of methodically analyzing material and energy flows at every stage of a product’s life cycle to precisely quantify consumption and emission values. In practice, this method is time-consuming to implement. To accommodate time and budget constraints, practitioners must ignore portions of a product’s life cycle.7 Even so, because a bottom-up LCA requires development of quite exact measures of standard building assemblies, the approach does not efficiently apply to new products and new methods.

While a Process LCA requires an analysis of interrelated flows built from the bottom up, an Input-Output LCA starts with a top-down model of the economy. This method is more complete than process LCA because economic input-output tables capture the interrelations of all inputs, all the way back to extraction of raw materials. However, an Input-Output LCA has the problem of providing only amassed, industry-level data.8,9

The hybrid approach combines the two methods to minimize the weaknesses and take advantage of the strengths of each. A top-down hybrid LCA starts with a good environmental sketch and then substitutes physical measures to refine high-impact areas of the building (such as concrete). Large portions of an LCA can be automated, reducing assessment time from months to days. Climate Earth has built software and database technology to use top-down hybrid LCA for very large-scale projects. In addition to the SFPUC Building and many other projects, the company has used the top-down hybrid method to assess the environmental impacts of supply chains of multiple Fortune 100 companies.


The in-place strength in some locations could be as low as 6500 psi (44.8 MPa).

Once the deficiencies were pinpointed, the engineering design of the columns and core walls was reassessed for gravity and seismic loading using lower-bound strength values determined through testing. The evaluation indicated that the overall design of the shear walls was adequate and the seismic performance of the system would not be detrimentally affected by the localized deficiencies in the strength of the concrete. Nevertheless, the evaluation of the columns indicated that the combination of seismic and gravity loads would exceed the design capacity at several locations. As a result, one of the columns was strengthened by enlarging it in one direction over the height of the first two levels.

Our assessment also indicated that the concrete mixtures were very sensitive to sampling, storage, and testing meth-ods. These were modified, and the sampling rate, which initially required test cylinders only for every 100 yd3 (76 m3) of concrete placed, was increased.

The mixture design for the elevated slabs proved to be more challenging due to a number of additional constraints. To maintain the required cycle time for floor construction, concrete strength needed to reach 4500 psi (31 MPa) at 3 days to allow for stressing and stripping. To reliably meet the schedule and minimize the potential for problems, in-place maturity testing was used to monitor the concrete strength under field conditions. Ultimately, the concrete supplier developed a specialized mixture design that was able to achieve the strength requirement using the maximum allowed content of portland cement of 350 lb/yd3 (208 kg/m3). The mixture pumped and finished well and allowed the contractor to maintain a normal construction schedule, despite some unusually cold weather for the San Francisco Bay area.

Light reflectanceAnother significant challenge for the concrete mixture

design for the elevated slabs was meeting the specified light reflectance criterion. The specifications required a minimum solar reflectance index (SRI) value of 70 for the slab soffits. This very light-colored surface was selected to allow significant reductions in the required electric lighting and additional finish materials (Fig. 11). The supplier relied on prior experience and research10 showing that concrete using elevated levels of slag cement could reach the required reflectivity.

A full-scale mockup was used to test the reflectivity of the slab soffits and identify construction issues that could affect the results in the field. While the tests verified that the high-slag cement mixture would provide a significantly lighter color than conventional concrete made with straight cement or cement plus a nominal amount of fly ash, the mockup did fall short of the ambitious target. Reasons included the fact that the soffit was not exposed to direct

sunlight (which would result in a lighter color). Other issues included discoloration caused by the form release agent and variations in the time of form stripping.

Collaborative Solutions The innovative solutions and synergies among the

building systems were the result of close collaboration within the project team. Productive interaction among the owner, design, and construction teams produced a unique set of solutions that are also replicable for future projects. Within this context, the design solutions that evolved were robust, reliable, and constructible.

As a result of working closely with the contractor and concrete supplier, the design team was able to develop and test a suite of green high-strength mixture designs, each tailored for different components of the structure and capable of meeting the diverse project requirements. The resulting mixture designs provided for up to 70% cement replacement using a blend of slag cement and fly ash with an upper limit portland cement content as low as 200 lb/yd3

(119 kg/m3)—roughly a quarter of the quantity used in a conventional mixture. The project clearly illustrates that the ambitious goals for high-performance green concrete can be cost-effectively and practically met.

Fig. 11: Slab soffits in the building are typically left exposed to take advantage of the high reflectivity of the slab and minimize the use of additional finish materials (Photo ©Tipping Mar)


ACI member Leo Panian is a Principal at Tipping Mar, Berkeley, CA. He is a licensed structural and civil engineer in California with more than 18 years of experience, specializing in the design of steel and concrete structures and with particular expertise in seismic engineering. He is a graduate of the University of California, San

Diego, and received his master’s degree in structural engineering from the University of California, Berkeley. Panian received the 2011 ACI Design Award with coauthor Mark B. Stevenson for their paper “Sustainability through Strength,” published in the March 2009 edition of Concrete International.

ACI member Phillip Williams is a Vice President at Webcor Builders, LP. He heads Webcor’s Sustainability and Systems Engineering groups and serves as the Chairman of San Francisco Mayor’s Green Building Task Force. A licensed professional mechanical engineer in California and a LEED AP, he is an active member of the USGBC and the

Business Council on Climate Change. He is a member of ACI Committee 130, Sustainability of Concrete, and the Strategic Development Council’s Accelerated Technology Implementation team for Building Information Modeling.

ACI member Mike Donovan is Director of Quality Assurance and Research at Central Concrete Supply, a Division of US Concrete. A licensed engineer, he specializes in engineering and implementation of low CO2 concrete mixtures. He received his BS in civil engineering from the University of Texas at Austin, Austin, TX. He is a member of

ACI Committees 232, Fly Ash and Natural Pozzolans in Concrete, and C610, Field Technician Certification.

By taking an innovative approach to the structural design and fundamentally linking the idea of seismic durability and sustainability, the project demonstrates how the selection of the structural system and materials can make a crucial and positive contribution to the broader goals of sustainable design. In doing so, the structure represents a new benchmark for the design of high-performance concrete buildings.

Furthermore, the collaboration required for this project has spurred ongoing research. Two projects have been funded by the Charles Pankow Foundation:

Users Guide to Green Concrete in Building Construc-tion—Principal Investigator: Ward Malisch, American Society of Concrete Contractors. Testing is being performed at Central Concrete’s San Jose, CA, laboratory to develop a more complete understanding and better characterization of green concrete properties. A guide for owners, architects, and engineers will be developed. Plans also include development of correlations between the strength results of cylinder samples and in-place core samples for concrete with high proportions of cement replacement.

Post-Tensioned Cast-in-Place Concrete Walls for Seismic Resistance—Principal Investigators: Steve Pessiki and Richard Sause, Lehigh University, Bethlehem, PA. The purpose of the research project is to obtain test data to confirm and codify a design protocol for a new type of cast-in-place concrete shear wall system that incorporates vertical post-tensioned tendons. The primary research product will be a complete design guide ready to be employed on actual commercial building construction projects in all seismic zones.

More information on these and other projects supported by the Charles Pankow Foundation can be found at www.pankowfoundation.org/grants.cfm.

References1. 2003 International Building Code, International Code Council,

Inc., Washington, DC, 2002, 672 pp.2. LEED Reference Guide for Green Building Design and Construction,

U.S. Green Building Council, Washington, DC, 2009, 645 pp.3. Treloar, G.J.; Love, P.E.D.; Faniran, O.O.; and Iyer-Raniga, U., “A

Hybrid Life Cycle Assessment Method for Construction,” Construction Management and Economics, V. 18, Issue 1, 2000, pp. 5-9.

4. Panian, L., and Stevenson, M., “Sustainability through Strength,” Concrete International, V. 31, No. 3, Mar. 2009, pp. 34-39.

5. Panian, L.; Steyer, M.; and Tipping, S., “Post-Tensioned Concrete Walls for Seismic Resistance,” PTI JOURNAL, V. 5, No. 1, July 2007, pp. 7-16.

6. Panian, L.; Steyer, M.; and Tipping, S., “Post-Tensioned Shotcrete Shearwalls,” Concrete International, V. 29, No. 10, Oct. 2007, pp. 39-45.

7. Hendrickson, C.T.; Lave, L.B.; and Matthews, H.S., “Environmental Life Cycle Assessment of Goods and Services; An Input-Output Approach,” Resources for the Future, Washington DC, 2006.

8. Joshi, S., “Product Environmental Life-Cycle Assessment Using Input-Output Techniques,” Journal of Industrial Ecology, V. 3, No. 2/3, 2000, pp. 95-120.

9. Gibbons, J.C.; Wolsky, A.M.; and Tolley, G., “Approximate Aggregation and Error in Input-Output Models,” Resources and Energy, V. 4, No. 3, 1982, pp. 203-230.

10. Marceau, M.L., and VanGeem, M.G., “Solar Reflectance Values for Concrete,” Concrete International, V. 30, No. 8, Aug. 2008, pp. 52-58.

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