The REDi Rating System (v1.0) was published in October. 2013 as a guideline ...... guide rail supports designed to OSHPD requirements. These requirements ...
Practical Insights from Project Implementation of the REDi Rating System Ibrahim Almufti, SE and Nicole Paul Arup San Francisco, CA Andrew Krebs, SE and Eric Long, SE Skidmore, Owings, and Merrill Los Angeles, CA Nabih Youssef, SE and Owen Hata, SE Nabih Youssef and Associates, Los Angeles, CA Professor Gregory Deierlein, PE Stanford University Abstract The REDi Rating System (v1.0) was published in October 2013 as a guideline for implementing resilience-based earthquake design; an enhanced design, contingency planning, and verification approach for targeting "beyondcode" resilience objectives in new construction. The current version focuses on seismic hazard, but a working draft for flooding is being finalized and will soon be extended to other hazards. Since its publication, the REDi guidelines have been incorporated into several RFP’s and have been adopted for the design of high-profile projects, such as the 181 Fremont Tower in San Francisco (designed by Arup) and the Long Beach Civic Center (designed by Nabih Youssef and Associates and SOM). In addition, the REDi downtime methodology (built upon the FEMA P-58 repair time method) has been implemented in the commercially available computer software SP3, allowing engineers to quantify and improve the expected recovery time of various design alternatives. The REDi downtime assessment methodology has also been utilized for a number of existing building projects and its utility for assessing the recovery times associated with existing building portfolios is illustrated through a case study of the University of British Columbia, Vancouver (UBC). The momentum behind REDi has also been fueled by recognition from the sustainability community, which has
increasingly taken up the cause of resilience. The US Green Building Council (USGBC) has recently adopted REDi as a means to obtain new Resilient Design Pilot Credits for LEED projects. A new LEED Resilience Working Group has been formed to expand these credits to reward resilience-based design. Future versions of REDi will further align the synergies between resilient and sustainable design, such as by incorporating more specific guidance on passive survivability (the ability of people to comfortably occupy a building in the absence of utilities via passive design). This and other outreach efforts has spurned interest in the adoption of REDi from across the globe, including Japan, China, Europe, and Mexico. In this paper, the designers will share their experiences and lessons learned from incorporating the REDi framework in their projects. These include communication with their respective clients, influence on their design approach, the adoption of specific design and/or planning enhancements, and reflections upon the overall design and peer review process. They will share proposed modifications to future versions of the REDi guidelines based on these experiences. The purpose of this paper is to reveal to other engineers, designers, and risk analysts how REDi is currently being implemented on actual projects in order to minimize the barrier to entry and to address common perceptions and misconceptions about resilience-based design including costs, complexity, and breadth.
1
Introduction Often, our role as a structural engineer is to design or retrofit new or existing buildings to address life safety concerns caused by physical building damage. In this capacity, our focus is to design or retrofit the structural components of the building such that it has a low probability of collapse and non-structural components and anchorage to prevent falling hazards. However, the emerging role of resilience - the capacity of an entity to bounce back, adapt, and even grow from shocks and stresses - in the design process has the potential to shift and augment our traditional role. First, we must reevaluate our role and responsibility for communicating vulnerability and risk to owners and the general public to allow them to make more informed decisions. Earthquakes around the world, even in regions with relatively modern building codes (e.g. Christchurch), have caused devastation that has shocked the public. This indicates that we have not done a good job of explaining the risks deemed acceptable by the building code, allowing a false sense of security that new buildings will be “earthquake proof”. In other words, stakeholders are generally unaware that there is a gap between the building performance they expect and the performance modern building codes target. As a result, there is less demand for “beyond code” or resilience-based design. As structural and earthquake engineers in a competitive business environment, it can be perceived as risky to raise these issues to a client, as it may take them out of their comfort zone and raise the specter of increased complexity, cost, and schedule. It is against this fragile backdrop that the issue of resilience must be broached with an owner or developer. This is beginning to change as more engineers recognize the importance of promoting awareness. In addition to the REDi Rating System, several other recent initiatives have made progress in helping to make conversations around risk more commonplace. The US Resiliency Council aims to communicate risk to the public through a star rating system. And the performance-based methodology (FEMA P-58) and enabling software such as PACT and SP3 have greatly aided in the quantification of earthquake performance in terms that stakeholders can understand. We also have the opportunity to expand our role beyond that of building designer by recognizing that resilience does not refer to the capacity of the building itself to bounce back. Resilience refers to the ability of an individual, owner, family, or organization – those who occupy buildings - to do so. This includes tenants re-occupying their homes with normal comfort conditions restored and businesses resuming their normal operations. By this definition, the performance of the building itself is only one of several key components we must consider when assessing resilience and good building performance is not necessarily sufficient for an entity to
2
recover after a natural hazard. In this paradigm, the attributes of an entity (such as organizational structure or financial health) and preparedness of the entity itself (to physical and non-physical vulnerabilities such as decreased customer demand or supply chain issues) are perhaps the key predictor of its ability to recover. As structural engineers who have studied the aftermath of earthquakes and learned the causes of slow recovery, we are uniquely placed to help organizations prepare for the worst. The REDi Rating System was developed as an alternative to the traditional structural engineering approach in recognition that to achieve specific recovery objectives, the design process and mitigation measures must revolve around the building stakeholder and not the building alone. It is a holistic resilience-based design tool that identifies, to the extent possible, all risks that may hinder the ability of an entity to recover quickly from a natural hazard and provides the basis for addressing them. In the REDi framework, this includes the physical performance of the building itself (i.e. building resilience), the preparedness of the entity to deal with shocks and stresses (i.e. organizational resilience), and an understanding of threats outside of the entity’s direct control (i.e. ambient resilience). It combines the roles of several traditionally independent disciplines (structural engineering, risk consulting, business continuity planning) and attempts to integrate them for the benefit of increasing confidence that an entity will recover quickly after an earthquake. This provides the opportunity to engage with stakeholders at their level on items that directly affect them, elevating our already important role as structural engineers. There are several impediments to incorporating resiliencebased design into building projects. As described above, the primary impediment may be that owners and stakeholders do not yet recognize the need for resilience-based design. Assuming that an organization, once educated as to the risks associated with minimum building code requirements, strives to achieve higher resilience objectives, the main issues are “how” to achieve them and at what cost. As for the “how” (the design basis for which to achieve the agreed-upon resilience objectives) – a recent informal survey of architects at the 2016 AIA annual convention found that if they incorporated resilience at all in their building projects, 60% did it through an ad-hoc manner and 25% believed they were doing it through application of the building code. As far as the costs, the real world necessitates that these must be kept within a negligible premium for "beyond-code" design to pencil out. The REDi Rating System is useful in addressing these impediments. One of the most important features of REDi is that it outlines specific and prescriptive design and preparedness measures. Firstly, this provides the owner and design team, who may otherwise be unfamiliar or have only some experience with resilience-based design (i.e. a holistic understanding of what
contributes to resilience or on the other hand causes downtime), a starting point and detailed roadmap for incorporating and implementing it. Secondly, since it identifies all (to the extent possible) the design enhancements and preparedness measures to be considered, it allows insight into the potential cost premiums early in the project. This prevents the accumulation of resilience-related costs throughout the design process (which may be value engineered out) or exponentially higher costs due to late additions or modifications to the design (which can be considered pricegouging). The impediments above are not insurmountable. As structural engineers, we have a significant role to play in not only make our buildings safer, but our communities more resilient. We achieve this by redoubling our efforts to communicate the true risks associated with code-based design and promote the holistic philosophy of resilience-based design. The case studies provided herein serve as examples of when resiliencebased design was successfully communicated and achieved through collaboration of the entire project team. Figure 1: Construction photo of 181 Fremont Tower Case Study: 181 Fremont Tower Overview The 181 Fremont Tower is a 56-story mixed-use high rise located in downtown San Francisco, adjacent to the new Transbay Transit Center. The lower two-thirds of the building are office space, while the upper third is residential. It is an allsteel building generally comprised of an external mega-brace system with perimeter steel moment frames at the office levels and internal braced core at the residential levels. It is currently under construction and will be the second tallest building in San Francisco when complete in 2017. The Structural and Geotechnical Engineer of Record is Arup and the owner is a developer, Jay Paul Company. The project has recently been awarded a REDi Gold rating, confirmed via an independent peer review process (described below) and is the first REDi rated building.
As a building that exceeded the height limitations of the building code, the project had already adopted a performancebased design approach as mandated by the San Francisco Building Code Administrative Bulletin 083. The primary objective of such an approach for the vast majority of buildings that adopt it is to verify that the minimum code performance objectives (i.e. low probability of collapse at MCE) have been satisfied. Arup, in collaboration with Stanford University, have undertaken significant research into the expected repair cost and downtime associated with new reinforced concrete tall buildings designed via the performance-based approach in San Francisco and found that the time required to achieve functional recovery is on the order of 1-1/2 to 2 years in a 475 year return period shaking (Almufti et. al, 2016). This is not surprising, given that significant damage is allowed by current building codes at the design level earthquake. Broaching the subject of resilience Arup broached the subject of resilience with Jay Paul Company by hosting an educational presentation communicating the expected losses associated with codebased and performance-based design of tall buildings. This discussion is preferably held very early in the design stages, such as concept or schematic design. The communication of the expected downtime was found to be the most effective tool, as it is difficult and non-intuitive for an owner to appreciate the financial losses associated with expected repair costs, often expressed as a fraction of the total building value. For new buildings (tall or otherwise), it is not uncommon for the losses to be on the order of 10 – 15% of the total building value. To
3
an owner, this can seem an insignificant amount and thus a measure of a successful design. On the other hand, the time (in days, months, or years) required to achieve specific recovery states such as re-occupancy, functionality, and full recovery is a direct measure of resilience and is easily understood. For the same buildings that experience 10 – 15% loss, the downtime is often on the order of months or years. This often shatters the misperception of owners that structural engineers are designing buildings their buildings to be “earthquake-proof”. For Jay Paul Company, the notion that a new tall building designed with the highest regard for sustainability and quality was not going to be useable for months or years after an earthquake was unacceptable. Once this was recognized, the discussion turned to enhanced resilience objectives. Ultimately, Jay Paul Company elected to pursue a REDi Gold rating because it corresponded to immediate re-occupancy (which was of utmost importance for the residents of the condominiums) and functional recovery once utilities were restored (an acceptable timeframe for the commercial and residential tenants). The resilience objectives corresponding to each of the REDi rating tiers (Platinum, Gold, Silver) provided an enormously useful framework to facilitate this conversation. Like LEED before it, the designations could be easily invoked by a client or stakeholder without necessarily re-stating the associated quantitative objectives repeatedly. Once Jay Paul Company elected to pursue REDi Gold, the entire design team (including Heller Manus Architects, the general contractor Level 10, steel subcontractor Herrick, and the mechanical engineer WSP) were receptive to the proposed enhancements and committed to achieving higher performance. From that point, the REDi requirements for the project were tracked and discussed often at internal design team meetings and coordination meetings with the owner, architect, and contractor. Implications on cost For the 181 Fremont Tower, Arup had the advantage of previous experience and research on tall buildings (in collaboration with Stanford University), which showed that the cost premium associated with enhancing the performance of a tall building designed to meet the minimum code objectives to a REDi Gold designation is relatively small. That research showed that for new concrete buildings, it may be on the order of 1 to 2% (assuming that base-isolation is a feasible solution for a concrete core wall building, which remains unproven). Other studies have shown that the cost to achieve “beyondcode” performance varies from 0 to 5%. The REDi Platinum and Gold ratings specify that structural and non-structural components should remain essentially elastic and operational for a 475 year earthquake. The fact that 181 Fremont is a tall building with an all-steel solution made it practical to achieve these objectives. That is because floor accelerations in tall
4
buildings are not particularly significant, so it was relatively straight-forward to show that acceleration-sensitive equipment and components would be operational and to design anchorage to remain essentially elastic. The “essentially” elastic seismic demands from the 475 year earthquake were comparable to the design wind demands since it is a relatively light building, so only some “deformation-controlled” structural components were governed by the REDi requirements. In addition to hard costs, the owner must consider the premium on design fees associated with resilience-based design. While nonlinear response history analysis (the basis for performancebased design) is not explicitly required by the REDi guidelines for all circumstances, it would be difficult to prove that the REDi resilience objectives have been met without it. Since performance-based design was a requirement of the 181 Fremont Tower project to meet minimum code objectives, the associated costs for REDi did not represent a premium. For that reason, resilience-based design may be a more natural extension for projects which already require performancebased design (such as tall or base-isolated buildings). In addition to the structural design and analysis requirements, there are design fees associated with the loss assessment (performed using in-house software by Arup) required by REDi to achieve a rating, for documentation of the methods used to satisfy each of the REDi criteria, and the peer review process. Since the REDi guidelines are based on a holistic and multi-disciplinary approach, Arup took responsibility for coordinating the building resilience requirements with the architect and engineering sub-consultants and to guide the owner in implementing contingency planning measures. This role in the project team can be referred to as a resilience or “REDi” consultant (similar to the role a sustainability or “LEED” consultant may play). There are potential opportunities for off-setting these costs. For the 181 Fremont Tower, the owner is pursuing reduced earthquake insurance costs. At the time of this writing, the process is still underway. In addition, the developer is significantly featuring the earthquake resilience aspects of the building in its marketing strategy. This is not uncommon in Japan, where base-isolated buildings command a price premium. In the United States, this may be one of the first instances that a developer has done so. Besides these, the costs could be further off-set by policies enacted by the local government to incentivize resilience-based design via tax breaks, flexible zoning policies, or other benefits. Implications on structural design and analysis Arup incorporated the REDi criteria into their standard basis of design, which had been developed for the performancebased design approach. This included an additional hazard intensity level (475 year return period) and corresponding
resilience objectives (measured in terms of repair costs and downtime). This required the development of additional ground motions at the 475 year return period (part of Arup’s Geotechnical scope) and a suite of nonlinear response history analyses to verify that the structural components remained essentially elastic. REDi provides some commentary for essentially elastic design (for ductile or “deformationcontrolled” elements, they are to be designed using elastic demands but expected strength capacities and a strength reduction factor of 1) but no guidance on the acceptance criteria for verifying that structural components achieved essentially elastic performance in the nonlinear response history analysis. In the next version of the REDi guidelines, we propose to provide more explicit guidance on an appropriate design philosophy and acceptance criteria for defining essentially elastic performance. The REDi criterion does state that “cracking is allowed” but this is in reference to concrete elements. The intent of the REDi criterion is to allow some localized and limited yielding (on the order of 2x the yield strain) in steel reinforcement and at the extreme fibers of steel members. Thus, the recommendation to use elastic design demands should also be relaxed accordingly, by either applying an R factor (in the range of 1.5 to 2) or more explicitly through displacement-based design. For the 181 Fremont Tower, Arup verified that the mean component demands (including moment frame beam rotations and brace strains) from the suite of 475 year ground motions remained in the elastic range (i.e. no plastic rotations or strains exceeding the yield strain). Building resilience The REDi Gold resilience targets and prescriptive design criteria had a profound influence on the structural design, particularly the criteria that requires the structure remain essentially elastic. In order to achieve this performance, Arup incorporated a number of seismic design features. These included: •
Incorporation of 32 viscous dampers into the external mega-brace system; these generate approximately 8% critical damping to the system and thus significantly reduced both seismic and wind force demands.
•
Uplifting mega-columns at the base of the structure to limit seismic demands in the foundation and tension demands in the mega-columns themselves.
•
A façade system that can remain weather-tight up to drifts of 1.2%, which exceeds the calculated mean demands at the 475 year intensity level.
•
One elevator that services every floor in the building was upgraded to achieve California Building Code standards for elevators in hospitals (CBC, 2013). It should be noted that REDi criterion 2.5.3 in the published guidelines is in error and should instead reference the CBC requirement.
•
Stairs are designed for a higher displacement tolerance, including the ability to maintain dead and live loads with minimal damage at MCE level. This was achieved by providing a slotted hole connection at the base of each scissor stair that could accommodate displacements associated with 1.5x MCE.
•
Anchorage of architectural, mechanical, and electrical components is designed to remain essentially elastic at the 475 year intensity level. This was achieved by limiting the Rp factor to less than 2 for the force demands associated with the peak floor accelerations measured in the nonlinear response history analysis.
Organizational resilience In the category of Organizational Resilience, REDi outlines contingency planning and preparedness measures to be undertaken by the owner to ensure that the re-occupancy and functionality objectives are satisfied. For developers, REDi makes some concessions in this category in recognition that they are responsible for the building core and shell but not necessarily long-term operation of the building. Jay Paul Company took the following actions to meet the REDi criteria: •
Sign up for the Business Operation Resumption Program (BORP, 2013) which allows qualified professionals to be pre-deputized by the City of San Francisco to undertake post-earthquake inspections within hours of an earthquake.
•
Commission Arup to develop an Owner’s Resilience Manual, which provides recommendations to future tenants on how to fit-out their space to be consistent with the REDi Gold objectives. This included enhanced partition details, recommendations to anchor heavy and mission-critical building contents, and storage of food/water.
•
Coordinate with Chief Building Engineer of 181 Fremont to locate sledgehammers in select facility
Other non-structural design measures that were implemented to satisfy the prescriptive design criteria in REDi, included:
5
storage areas in the event that doors are jammed postearthquake. •
Coordinate with the Chief Building Engineer of 181 Fremont to train designated facility personnel to restart elevators that may become halted due to automated shake-actuation.
The last two items are not explicitly required by the current REDi guidelines, but our research indicates they are important and thus are proposed to be included in future versions. Since these contingency planning measures are difficult to implement until after the building is completed and tenants are occupying the building, the owner signed “affidavits” that they will be undertaken at that time. We propose that verification of these items in a “commissioning” phase should be a part of any formal future certification program. Several other items could be included in such verification, including the REDi criterion for observation of non-structural component installation. In addition to the REDi criteria, Jay Paul Company also undertook a preparedness self-evaluation via the Red Cross Ready Rating Program, which is a requirement for achieving the new LEED Resilient Design Pilot Credit IPpc98.
Through the 181 Fremont project, loss assessments on existing buildings, and collaborative research with Stanford University, Johns Hopkins University, and University College London, Arup has identified several improvements to the original methodology which have been included in the in-house version and used for the assessment of 181 Fremont. This includes the following: •
Assessment of loss and downtime To achieve a REDi Gold rating, a risk assessment must be performed to show that the median repair costs are less than 5% of the total building value, the building can be reoccupied almost immediately (per the REDi guidelines, the median reoccupancy time is only due to post-earthquake inspection, which is expected to be 1 day with the BORP provisions) and the time required to achieve functionality is less than a month assuming utilities are restored. For the financial loss assessment, the procedure and consequence functions outlined in FEMA P-58 are adopted directly. For the downtime assessment, the FEMA P-58 procedure (which calculates repair time) has been modified significantly to allow the estimation of intermediate recovery states like re-occupancy and functionality. In addition, the REDi downtime assessment accounts for “impeding factors” that delay the initiation of repairs and utility disruption. The REDi downtime assessment methodology was published in the v1.0 REDi guidelines and has been implemented in SP3, a commercial loss assessment software (www.hbrisk.com). In v1.0, the downtime methodology was supplemented by a worked example, which for simplicity assumed only one realization using median estimates for repair time from the FEMA P-58 assessment. While not explicitly stated in the v1.0 guidelines, the intent of the original REDi downtime method was to follow the probabilistic framework outlined in FEMA P-58. Arup has developed an in-house loss assessment tool to facilitate the Monte Carlo simulations and the SP3 tool does the same.
6
•
•
One of the primary improvements of the original REDi downtime method relative to the FEMA P-58 repair time procedure was the introduction of Repair Classes, which relates the severity of damage and criticality of specific building components to whether it would cause a specific recovery state to be hindered (e.g. damage to elevators or MEP equipment would hinder functionality). In the new version, the extent of this damage was also considered in the assignment of the Repair Class. This is critical, since the stacking of lognormal distributions to characterize the damage (fragility) functions and the probabilistic framework (i.e. Monte Carlo simulation) means that it is highly likely that for each realization, at least one individual component in the building would trigger reoccupancy or functionality disruption despite being an unlikely event to occur on its own. This creates disproportionately increased downtime estimates for tall buildings or large buildings because there are many more components and the likelihood that at least one is damaged is significantly increased. The “impeding factors” associated with Contractor Mobilization were refined to be more explicitly dependent on the types of repairs required and what type of equipment (mobile cranes, elevators, or derricks) would be required to convey building materials up to higher floor levels. The estimates were based on an extensive survey and responses from Level 10 and various sub-contractors. Note that the “impeding factors” associated with Contractor Mobilization are only triggered if damage occurs; by definition, no damage occurs in more than 50% of the realizations performed. The Repair Classes of some building components were re-assigned relative to those published in REDi v1.0. This included partitions, where Damage State 3 was originally assigned to Repair Class 3 (indicating that it would hinder re-occupancy) because it was thought that DS3 corresponded to damage that could cause a life-safety risk. In re-examining the description of DS3, it is unlikely that it corresponds to such severe damage. At the drifts which trigger DS3, it is our opinion that it corresponds to a 1” vertical separation at corners (personal comm.,
Siavash Soroushian) but not out-of-plane buckling and warping of partitions (which could pose lifesafety and thus re-occupancy issues). However, partitions in this damage state (DS3) are likely to be considered as compromised if they are acting as fire barriers (i.e., allowing hot gases and/or flames to be transmitted that would fail the British fire test standard criteria). It is unlikely that partitions in DS1 or DS2 (which corresponds to roughly ¼” separation) would be compromised as far as fire protection. Ultimately, partitions in DS3 were assigned to Repair Class 2 (hinder functionality, but not re-occupancy). This is based on two assumptions: 1) in the aftermath of an earthquake, an inspector will not prevent re-occupancy because a fire barrier is compromised and 2) the 1” separation would likely compromise privacy, acoustics, and security – thus hindering return to normal occupant comfort conditions (which is required to achieve functionality). In general, it is likely that the performance of partitions would affect different occupancy types in a different manner. For example, while the above may hold for residential occupancies, in hospitals, even slight damage may hinder functionality of operating rooms. In future versions of REDi, updated Repair Class assignments will be provided. Presentation of all of the details of the improved downtime methodology is beyond the scope of this paper, but Arup plans to document it in the near future. The median repair costs for the 181 Fremont Tower for the 475 year earthquake intensity level are approximately 1 – 2% of the total building replacement value. The median time to achieve re-occupancy is less than the 24 hours allowed for the time required for the BORP-designated professional to complete the inspection, and the median time to achieve functionality is directly related to the restoration of utilities. In San Francisco, published studies have indicated that electricity would be disrupted for a few days, water for a few weeks, and natural gas for several months. This is consistent with the estimated utility disruptions provided in the REDi guidelines, which were based on research of utility outages from several past earthquakes which occurred in developed countries. This meant that residents could shelter in place, but in the short term would have to rely on bottled water and flashlights and be without gas stovetops until the natural gas was restored. Importantly, manual operable windows are provided in the residential floors to provide air flow in the event of power outage. For commercial tenants, business could resume within a few weeks (once electricity and water were restored) but be without electrical hot water or gas heating until the natural gas is restored. Arup worked with the owner and the MEP
engineers WSP to determine the specific effects of utility disruption. It should be noted that any downtime assessment procedure, no matter how robust, has inherent limitations. Unlike the estimates for repair costs, which generally increase predictably and continuously as the confidence levels are increased, downtime is characterized by a bi-modal distribution. In other words, the building is either re-occupiable or not. If it is reoccupiable, the expected downtime is on the order of hours. If it is not re-occupiable, the downtime could be measured in months or years. This “cliff-edge” effect is amplified by "impeding factors" similar to reality, but is extremely difficult to predict through a loss assessment. There are other limitations in any downtime procedure: •
The results have never been robustly validated mainly because no comprehensive data exists that provides all the required elements to facilitate a validation study for a single building (i.e. the measured building demands, severity and location of structural and non-structural damage, the type of “impeding factors” and corresponding quantified estimates of delays, and the repair times which are defined by number of and allocation of workers and repair sequencing).
•
The assessment procedure relies on the accuracy of fragility and consequence functions, many of which are based on judgment or insufficient data. In addition, the consideration of specific design details within the corresponding fragilities for each component are tenuous (e.g. the effect of enhanced design details on the standard or default fragility function is difficult to quantify). For that reason, it is not clear if the standard fragilities represent what is actually constructed in a building. In addition, many fragilities use non-explicit engineering demand parameters (such as interstory drift) when explicit demand parameters like strain or beam rotations would likely be more representative of the actual demands experienced. The product of all these assumptions is that the results are not reproducible.
•
Perhaps most importantly, a downtime assessment of an individual building cannot quantify all threats that may hinder re-occupancy or functionality. There are too many uncertainties and unknowns to consider. For example, in the downtime of some buildings in Christchurch were governed by where they were located (within the cordoned zone) even if little or no damage was sustained.
7
It is for the reasons stated above that the downtime assessment should be treated as one part of a holistic resilience-based design approach. It alone is insufficient in predicting whether a building or organization is resilient. The REDi guidelines were developed such that enhanced design, preparedness, and verification through loss assessment are all equally important and required to obtain the highest confidence that the preidentified resilience objectives are likely to be satisfied. Certification There is currently no established third party entity to award formal certification of a REDi rating in a traditional manner, such as the role GBCI plays for LEED. However, a simpler and more cost effective path does exist if so desired. The REDi guidelines require that besides a formal structural peer review (which is already a requirement for projects that pursue the alternative performance-based design approach), an independent peer reviewer must verify that the designers have successfully demonstrated that the REDi criteria are satisfied. In the case of the 181 Fremont Tower, Jay Paul Company commissioned Professor Gregory Deierlein (Stanford University), who was already familiar with the building as a member of the structural design review team on behalf of the San Francisco Department of Building Inspection. Arup provided Professor Deierlein 1) a report which documented how each of the REDi Gold criteria were satisfied (similar to a calculation package), 2) supporting technical documentation including nonlinear response history analysis results, specifications, owner’s “affidavits”, and façade testing report and 3) a report which documented the assumptions and results for the loss assessment. Professor Deierlein reviewed the documentation and provided formal comments in a similar manner to a typical peer review process. The comments were resolved by providing additional information, analyses, and clarification. At the conclusion of the process, Professor Deierlein wrote a letter to the owner documenting the review process and stating that based on his review of the documentation provided by Arup the project satisfied the REDi Gold criteria. It should be noted that similar to a typical structural peer review process, some items (including assumptions or results of the loss assessment) required engineering judgment to resolve since the REDi guidelines were written such that the criteria were not overly prescriptive. The general philosophy, which guided these discussions, focused on whether the intent of the resilience objectives were met for the building as a whole. The review process began during construction and lasted approximately 2 months from start to finish. This included approximately 40 hours of peer review and approximately 80 hours for Arup to prepare the required documentation and resolve the peer review comments.
8
In addition to the independent peer review process described above, the introduction of the new LEED Resilient Design Pilot Credits (LEED, 2016), which explicitly reference REDi, provide a tacit but formal approval process by a worldrecognized third party. At the time of this writing, the entire LEED package for 181 Fremont project is about to be submitted. There are three LEED points that may be awarded: IPpc98 (Assessment and Planning for Resilience), IPpc99 (Design for Enhanced Resilience), and IPpc100 (Design for Passive Survivability). A qualitative multi-hazard assessment is required for IPpc98; the requirements in REDi for seismic hazard development would automatically qualify for this point. In San Francisco, other natural hazards besides drought pose little risk. Once the hazards are identified, IPpc99 provides guidance on which mitigation strategies should be used to address them. For earthquakes, one of two provided options is to satisfy a REDi Silver rating or better. The documentation requirement to achieve IPpc99 in relation to REDi is a signed “executive summary” from a licensed design professional that describes how the project met or will meet the REDi Silver or better designation. This was purposely meant to be a fairly flexible requirement in order to prevent barriers to submit for the points. For the 181 Fremont project, the review letter from the independent peer reviewer constituted the sole documentation for the IPpc99 point. It is likely that a letter from the engineer of record would also suffice in lieu of that from a peer reviewer (personal commentary, Almufti and USGBC Resilience Working Group). The third credit (IPpc100) requires that a building is designed to maintain reasonable living conditions in the event of utility disruption by satisfying two out of three options described below. This includes specific design and analysis requirements for thermal variations and natural ventilation (Option 1), back-up emergency power (Option 2), and access to potable water (Option 3). The REDi criteria provide generally similar requirements and recommendations for passive survivability (though the LEED credit provides specific modeling and performance criteria), back-up systems (for Platinum buildings), and food/water storage. While the 181 Fremont project is likely to satisfy Option 1 because there are manual operable windows and San Francisco is a temperate climate, the rigorous analysis required to prove it were beyond the scope of the project. Ultimately, two additional LEED points (IPpc98 and IPpc99) will likely be awarded to the project due to the implementation of REDi. Case Study: Long Beach Civic Center Overview The Long Beach Civic Center consists of a New City Hall and New Port Headquarters constructed over a combined two-level subterranean parking garage (Figure 2). The City Hall and Port Headquarters buildings each consist of 11-story office towers
162 ft tall above grade with matching typical floor plate sizes of approximately 275 ft x 85 ft. The structural system for each office tower utilizes post-tensioned concrete flat slabs supported by reinforced concrete columns and two C-shaped special reinforced concrete shear wall cores. Internal stud rails are designed to strengthen the slab-to-column connections, and upturned PT beams support the four corners of the floor plate. A raised access floor system conceals the upturned beams and MEP systems, revealing exposed concrete ceilings in the typical office space.
4.
Experience less than 5% financial loss (as compared to replacement value)
At the kick-off meeting, NYA and SOM presented the REDi resilience-based design approach and specific REDi design and planning criteria that the project was aiming to incorporate to the owner, contractor, and peer reviewers. This accomplished several things: it involved the owner with the design process for achieving their higher resilience objectives, provided baseline design parameters for contractor costing purposes, and provided confidence to the peer reviewers that the resilient design strategies were being implemented as part of the basis of design. Implications on structural design and analysis
Figure 2: Rendering of the Long Beach Civic Center The Structural Engineer of Record for the Long Beach Civic Center is a combination of Nabih Youssef and Associates and Skidmore, Owings & Merrill, LLP, with Arup providing structural and business continuity peer review and Haselton Baker Risk Group providing consulting for business continuity. Clark Construction and Plenary-Edgemoor complete the Design-Build team for the Civic Center, with official ground breaking kicking off shoring and excavation on site during July 2016. Foundation construction is planned to start at the end of 2016.
In order to design the project to satisfy the additional resiliency objectives, a baseline linear elastic model for a typical code based building was first established for analysis using ETABS. Parametric code design studies were then performed to evaluate building performance with respect to resiliency performance measures in order to identify critical response parameters. The initial enhanced design approach was code based with a reduced R-value to limit yielding (damage) of the structural system, a more stringent drift limit to reduce nonstructural damage, and consideration of floor accelerations. A design strategy using displacement based design to limit drift and wall rotations was then implemented. The results of the initial studies indicated that it was not feasible to meet the resiliency objectives if conventional coupling beams, as fuses, were used. Thus, the architectural layout of the core and configuration of the walls were modified to eliminate coupling beams by staggering the openings from floor to floor, resulting in a punched tube lateral system behavior, as opposed to traditional coupled shear walls (Figures 3 and 4).
Broaching the subject of resilience As the owner of the Civic Center, the City of Long Beach was interested in their short term and long term seismic risks. Over the course of a short phone call, we educated them on the expected performance of typical code designed projects. They were not content with the expected downtime of code buildings following a design level (roughly 500 year) earthquake. The City therefore elected to pursue REDi Gold objectives and embedded resilience requirements into the RFP for the project. The RFP specified that the Civic Center is not required to be an essential facility, but should be designed to a 50% confidence level that after a design earthquake occurring, the City should be able to: 1. 2. 3.
Figure 3: Typical Tower Framing Plan
Experience few or no casualties; Re-occupy the new facility within a week; Re-gain full functionality within 30 days; and
9
accommodate large inter-story drifts without damage. In fact, employment of horizontal slip tracks may focus more damage at the intersections. Since losses associated with partition damage seem to be a common, further research into enhanced detailing should be undertaken. Building resilience
Figure 4: Typical Tower Elevation showing Staggered Shear Wall Openings without Coupling Beams Nonlinear response history analyses at the 475-year return period intensity level were performed using PERFORM-3D to determine global and element responses which were used to verify “essentially” elastic structural performance (as required by the REDi criteria) For example, steel reinforcing strain was correlated to residual crack width based on research tests of concrete walls. In addition, the ASCE 41 Immediate Occupancy acceptance criteria for plastic rotations in shear walls were adopted to prove the intent of the REDi criterion to minimize structural damage. As noted above in the 181 Fremont case study, the current version of REDi does not provide a specific design approach or acceptance criteria for proving that “essentially” elastic performance is achieved. The ASCE 41 acceptance criteria provide a useful purpose in this regard and we recommend referencing them in the next version of REDi. It should be noted that the FEMA P-58 fragility curves for shear walls provide a median wall rotation associated with discrete damage states. However, these were deemed inappropriate because the first damage state corresponded to spalling of cover concrete which seemed to indicate greater than “essentially” elastic structural performance. Performance criteria for the maximum inter-story drift ratio was established as 1% (half the code limit of 2%), with an average residual inter-story drift ratio of less than 0.5% (as specified in REDi). Both downtime and financial loss objectives are sensitive to inter-story drift, since drift sensitive structural components such as slab-column connections and non-structural components such as dry wall partitions take substantial time and money to repair. Even with vertical and horizontal slip tracks utilized at the tops of partition walls, it is difficult to detail the intersections of partition walls to
10
The strategy for minimizing structural damage was described above. In order to achieve the REDi requirement to minimize non-structural damage, non-structural components, anchorage and bracing designs were designed to exceed minimum code requirements (Figure 5) as follows. • Anchorage and bracing must be designed to resist forces due to the mean peak floor accelerations from the nonlinear response history analysis. • Rp (response modification factor) shall not exceed 2.0 for all mechanical and electrical components. This was done to keep the anchorage essentially elastic. • Ip = 1.5 for selected equipment and components (for those components deemed critical or would otherwise require a long-lead time to procure) including: o Cooling Generating Systems Chillers Cooling Towers Pumps & Compressors o HVAC distribution – AHU o Electrical Service and Distribution Control Panels & Switchgear
Figure 5: Comparison of Code vs Project Anchorage Demands (Flexible Equipment)
Non-structural component performance criteria also includes required shake-table testing of mission critical equipment and components, and long-lead time equipment and components to ensure the equipment remains operable after a 475-year earthquake. In order to achieve the REDi requirement to prevent air and water intrusion, the curtain wall detailing allows for connections to remain elastic at the 475-year earthquake interstory drift (1%) and to ensure no catastrophic failure at twice the design level inter-story drift (2%). Egress stairs are designed to accommodate 2% inter-story drift (2x DE demand), and elevators are designed with an Rp = 1.0 with guide rail supports designed to OSHPD requirements. These requirements have been incorporated into the General Notes of the structural drawings.
evaluation. SP3 is a commercial software which runs on a cloud-based platform and includes embedded USGS soil/hazards, simplified structural response estimates, and prepopulation algorithms for building contents. The software can be used by engineers, with a modest amount of training, and is currently being used by many companies for new design, retrofit, due-diligence analysis, and insurance risk assessments. The loss assessment was conducted in SP3 with the results of the initial nonlinear analysis. This process was repeated in an effort to meet the target criteria established for key building performance objectives with a focus on inter-story drift, peak floor acceleration, wall rotations, and residual drift. Design decisions were made to improve performance, nonlinear response history analysis was used to validate the structural response, and SP3 evaluation was used to validate performance (Figure 6).
Organizational resilience At the time of this writing, the project is still under design and detailed conversations with the owner regarding the organizational resilience requirements and recommendations in REDi are to be held at a future date. In contrast to the 181 Fremont Tower, the owner of the Long Beach Civic Center will be a long term occupier of the building. In many respects, their role as the city government prepares them better than most organizations for quick recovery. While the building is designed such that damage is limited, it is likely that impeding factors such as the time required to achieve post-earthquake inspection or permits would be mitigated since city staff are responsible for them. Assessment of loss and downtime In order to evaluate the resilience objectives for the project, FEMA P-58 methodology was used to assess building performance measures in terms of casualties and repair cost, and to establish supplemental structural and nonstructural design criteria. REDi downtime assessment methodology was used to estimate repair times for assessing re-occupancy and full functionality objectives. This methodology is able to evaluate both re-occupancy and full functionality (rather than full recovery), consistent with the resilience objectives for the project. The methodology also appropriately captures impeding factors, which are delays due to the time it takes to complete inspections, secure financing for repairs, mobilize engineering services, obtain permitting, mobilize a contractor, and time required for long-lead time items, which can significantly affect the repair time of a building. The Seismic Performance Prediction Program (SP3) software was used to run the FEMA P-58 and REDi based analyses, combine the results, and provide a comprehensive quantitative
Figure 6: Performance Design / Evaluation Flow Chart Certification The resilience-based design approach and loss assessment results are subject to an independent peer review, as required for all REDi buildings. This consists of a review of drawings, calculations, specifications, performance-based analysis reports, and the loss assessment report. As noted above, this process is voluntary as no formal REDi certification process exists. Key lessons The resilience-based approach ultimately inspired the design team to select a staggered wall structural system rather than a traditional coupling beam solution. A general take-away for the 11-story mid-rise Civic Center buildings is that designing robust, stiff concrete shear wall buildings with a carefully
11
selected opening pattern reduced damage and downtime by limiting inter-story drift and structural yielding/cracking. The stiffer office towers alleviate damage to drift sensitive components such as slab-column connections and partition walls. The trade-off is that floor accelerations may increase; however, this was addressed by augmenting anchorage and design provisions to limit damage. Limiting residual drift is another key component towards successfully designing a resilient building. Finally, mitigation of impeding factors are addressed through planning and coordination with the Owner and City during the design process to ensure inspection, funding, engineering, and contractor services are all prepared to react quickly following a major seismic event. Case Study: Vancouver
University
of
British
committed to integrating resilience within their forwardthinking approach to sustainability and have asked Arup to identify synergies between the two. At the project inception, a workshop was held to develop UBC’s mission statement around seismic resilience with involvement from both UBC and Arup. At this workshop, Arup presented the seismic hazard, potential vulnerabilities to the building stock, and the potential consequences of an earthquake in hindering UBC’s teaching and research mission. The ultimate mission statement underscored UBC’s commitment to life safety but also highlighted the importance of the continuity of research and instruction and protection of valuable and invaluable assets.
Columbia,
Overview As a part of University of British Columbia, Vancouver's (UBC) re-evaluation of its seismic mitigation plan, Arup was commissioned to assess the seismic risk of the existing building stock and to develop a resilience strategy for the campus. This included investigation of the seismic hazard, quantification of campus exposure (including building replacement values, populations, locations of critical equipment or specimens, etc.) through comprehensive surveys and interviews of university faculty and staff, quantification of vulnerability (via on-site building inspections and structural analysis), and ultimately quantification of their risk in terms of casualties, repair costs, and downtime. The mitigation strategies may include business continuity planning, seismic retrofits of buildings and non-structural components, and adoption of more stringent new design guidelines with the aim of achieving their resilience objectives.
Assessment of loss and downtime Since one of the main objectives is to fulfill the University’s teaching and research mission in the aftermath of an earthquake, the expected downtime of individual buildings was important to understand as it could impact the amount of available classroom or laboratory space. The REDi downtime assessment methodology was applied to the existing building stock (approximately 350 buildings) as opposed to a more conventional method (i.e. HAZUS) in order to get a more building-specific and component-based view of the drivers of loss, downtime, and casualty rate. This more detailed risk assessment was made possible through the development and use of an in-house portfolio tool that enables the REDi downtime calculations. This also required estimates of engineering demand parameters for each building for each floor level. For a large stock of UBC’s buildings, this was facilitated by results from non-linear analysis (of simplified models representing buildings of various typologies, strengths, and ductilities) conducted for the British Columbia School Retrofit project (Ventura, 2013).
Broaching the subject of resilience In the case of UBC, life-safety was their top priority prior to the commencement of the project. They were acutely aware of the seismic risks to their building stock, particularly in light of the recent media coverage of the Cascadia Subduction Zone. The project manager for UBC was also well versed in the topic of resilience-based design, having attended a session at the Greenbuild Conference which focused on resilience and featured REDi. Thus, in the RFP issued by UBC, the REDi Rating System and LEED Resilient Design Pilot Credits were both explicitly referenced. This underscores the importance that structural engineers attend and participate in design industry events to communicate the benefits of resilience-based approaches. While UBC is focused on their seismic risk, they also commissioned Arup to perform a multi-hazard assessment following the requirements of LEED IPpc98. UBC is also highly
12
In order for this detailed seismic loss and downtime assessment to be successful, collaboration with the client (UBC) is essential. A significant effort was undertaken by Arup and UBC to gather exposure data. This was largely through the development of a comprehensive and detailed survey which aimed to capture the locations, value, criticality, lead times, and vulnerability of building contents including lab equipment, research specimens, data, artwork, artifacts, animals, servers, and supercomputers. Other important data for the risk assessment included building information such as year of construction, floor areas, heights, building typologies, and year of retrofit (if applicable). An investigation of the vulnerability of the utility network is also being undertaken by Arup. Guiding retrofit and mitigation strategies
The evaluation of risk is still underway. Future phases will include cost-benefit analysis for targeted retrofits and other resilience strategies including continuity planning. The REDi conceptual framework and guidelines can be leveraged for these tasks. For existing buildings, the REDi downtime assessment is important and the criteria associated with organizational resilience can be incorporated. For new construction, the REDi guidelines can be used to set resilience objectives for specific types of buildings and provide the method for achieving those objectives. Conclusions From these case studies, the designers have identified opportunities for improving future versions of the REDi guidelines. The following is a summary of those described above and additional provisions to consider for achieving the enhanced resilience objectives set out by REDi: •
In taller buildings which are comprised of internal cores, the racking drifts may be significantly higher than the story drifts. This should be considered in the loss analysis and for drift limits (see below)
•
The considerable experience Arup has gained from performing numerous loss assessments on various buildings suggests that it would be difficult to achieve REDi Platinum or Gold resilience objectives if the drifts exceed 1% for the 475 year earthquake. In fact, NYA and SOM arrived at this conclusion independently. Thus, a drift limit of 1% may be incorporated for Platinum and Gold buildings. The drift demands should be taken as the greater of the story and racking drifts.
•
•
It is clear that there is little guidance in the published literature for the design and detailing of nonstructural components to achieve functionality-level performance. This is one of the greatest opportunities to significantly increase building resilience. The current version of REDi provides some guidance but a future companion guideline which provides examples of best-practice detailing and other requirements (based on lessons learned from comprehensive testing programs of partition walls, ceilings, and piping systems for example) is envisioned. Example specifications for non-structural components to achieve enhanced resilience objectives may also be beneficial.
relatively low levels of drift and residual drift. This is outside the scope of REDi, and relates to the damage assessment. Nevertheless, improved characterization and understanding of door performance would improve the resilience assessment. Including resiliency objectives in the design process does not radically change the typical design process. The REDi guidelines provided a valuable tool for informing the initial design criteria and an additional step of checking performance (via loss assessment) using REDi and FEMA P-58 (and enabled with software such as SP3) is required using the output from traditional structural analysis models. Several iterations are likely required until the resiliency objectives are met. Downtime was the governing resiliency requirement for design on this project. References Almufti, I., Tipler, J., Merrifield, S., Carey, B., Willford, M., Deierlein, G. (2016), “Performance and Cost Implications of Resilience-Based Earthquake Design of Tall Buildings in High Seismic Zones Using the REDi Rating System”, submitted for review, J.Struct. Engr., ASCE. BORP (2003). Building Occupancy Resumption Program. Retrieved from Structural Engineers Association of Northern California: http://www.seaonc.org/public/all/borp.html California Building Code (CBC 2013), California Building Code, California Code of Regulations, Title 24, Part 2, Volume 2 of 2, California Building Standards Commission. LEED (2016), Resilient Design Pilot Credits, available from: http://www.usgbc.org/credits/assessmentresilience http://www.usgbc.org/credits/enhancedresilience http://www.usgbc.org/credits/passivesurvivability REDi™ (2013). Almufti, I. and Willford, M., “Resiliencebased earthquake design (REDi) Rating System”, Arup, Version 1.0, September 2013. Available from: . Ventura, C. et. al (2013), “Structural engineering guidelines for the performance-based seismic assessment and retrofit of low-rise British Columbia school buildings”, 2nd Ed., UBC, Vancouver
There is little consensus on how doors will perform following an earthquake. There are concerns that they will be jammed and become inoperable, even for
13
