case study: cost delta for achieving higher structural performance levels · 2018. 4. 2. · the...

2017 SEAOC CONVENTION PROCEEDINGS

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Case Study: Cost Delta for Achieving Higher Structural Performance Levels

Kamalpreet Kalsi, Project Engineer

Daniel Zepeda, Principal Degenkolb Engineers

Los Angeles, CA

Abstract Major cities in California are now mandating that existing buildings with the highest seismic risks be retrofitted in conformance with local ordinances. As a consequence to the recent engineering community’s push for cities and building owners to become more resilient against seismic events, many owners are now more conscious about their buildings’ anticipated seismic performance. Owners are now beginning to ask engineers what it means to design above the minimum code standards. For an owner to make an educated decision on building design, engineers need to convey the increased cost of a higher structural performance in simple terms. This paper covers a scenario in which an existing Pre-Northridge Steel Moment Frame building was evaluated for three different performance objectives under California’s hospital building standards. This paper highlights the differences in structural scope between each performance level as well as the expected percent increase in construction costs. Engineers can use this case study as an example when speaking to their clients about relative costs between different seismic performance levels.

Introduction and Background The case study project is a hospital located in California and falls under Office of Statewide Health Planning and Development (OSHPD) jurisdiction. Under the state code, any essential facility (in this case an acute care hospital) requires the importance factor “I” to be 1.5 when designing a new building. Similarly, when evaluating or retrofitting an existing building, a higher performance criteria is utilized. The higher seismic criteria result in more extensive retrofits compared to similar buildings under the same hazard. California has always been in the forefront of seismic protection by demanding stricter regulations to protect lives and property. Essential facilities, such as hospitals, came under regulation after passage of the Alfred E. Alquist Hospital Seismic Safety Act post 1971’s Sylmar earthquake. Senate Bill (SB) 1953 and subsequent regulations have provided timelines and performance objectives that hospitals should meet to comply with California State’s overall goal of ensuring the

improved seismic performance of acute care facilities after a major seismic events. These Performance Objectives are categorized into five categories, SPC-1, SPC-2, SPC-3, SPC-4 and SPC5 (See Figure 1c). SPC-1 is not considered safe under a seismic event, while SPC-2 and above will have different levels of safety under a seismic event. Figure 1a and Figure 1b show the timeline of major milestones associated with California legislation for hospitals regulated by OSHPD.

Figure 1a: Timeline of Hospital Upgrades in California

Note: Figure re-printed from webinar by California Hospital Commission on May 2015.

Figure 1b: Timeline of Hospital Upgrades in California

contd.


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Note: Figure re-printed from webinar by California Hospital Commission on May 2015 Table 2.5.3 of the California Administrative Code (CAC) defines the Structural Performance Categories (SPC) in more detail (Figure 1c). It is worth noting that an existing building could only be upgraded from an SPC-1 to an SPC-2 or SPC-5. However, in recent years OSHPD has defined an intermediate category SPC-4D that can be used to retrofit an existing building. This new category is expected to yield a similar seismic performance level as SPC-4. The retrofit example is a six story steel moment frame building, which was erected in 1972. OSHPD designated the building as an SPC-1 due to significant structural deficiencies. The project’s general contractor provided pricing to upgrade the building to structural performance categories: SPC-2, SPC-4D and SPC-5. See Table 1 for reference. What follows is an analysis of project’s potential seismic upgrades and their associated costs. This type of analysis is recommended for assisting building owners, as they make critical safety and financial decision to make seismic upgrades.

Table 1: Snapshot from CAC Table 2.5.3 for SPC

description

Note: Reprinted from California Administrative Code 2016. Performance Levels and Hazards As indicated in the table above, in 2016 California Hospital Building Safety Board included a new performance category (SPC-4D) under California Building Code (CBC) to allow acute care hospitals another seismic upgrade option. The SPC-4D performance category is meant to be equivalent to the minimum prescriptive requirements of the 1980 CBC. Prior to the addition of this new performance category, an existing building (SPC-1 or SPC-2) was required to upgrade to SPC-5. Buildings not upgraded to these stringent SPC-5 requirements would have to be removed or rebuilt before 2030. The SPC-4D

category provides an alternate path to compliance beyond 2030. American Society of Civil Engineers (ASCE) 41-13 defines the different performance levels and CBC 2016 outlines the objectives for each performance level. As part of this case study these objectives and criteria were performed for three different performance categories: SPC-2, SPC-4D, and SPC-5.

• SPC-2: Life Safety structural performance level in accordance with § 2.3.1.3 of ASCE 41-13 at BSE-1E hazard. BSE-1E hazard corresponds to a return period of 225 year.

• SPC-4D: Damage Control structural performance level in accordance with § 2.3.1.2.1 of ASCE 41-13 at BSE-1E hazard and Collapse Prevention structural performance level in accordance with § 2.3.1.5 of ASCE 41-13 at BSE-2E hazard. BSE-2E hazard corresponds to a return period of 975 years

• SPC-5: Immediate Occupancy structural performance level in accordance with § 2.3.1.1 of ASCE 41-13 at BSE-1N hazard and Life Safety performance level in accordance with ASCE 41-13 § 2.3.1.3 at BSE-2N hazard. BSE -1N corresponds to a return period of 475 years, and BSE-2N corresponds to a return period of 2,475 years.

ASCE 41-13 defines the post-earthquake damage state for Life Safety as the state in which a structure has damaged components but still regains a margin against the onset of partial or total collapse. In addition, the definition of damage state for a Damage Control performance objective is the midway point between Life Safety and Immediate Occupancy. It is intended to provide a structure with a greater reliability of resisting collapse and being less damaged than a typical structure but not to the extent required of a structure designed to meet the Immediate Occupancy Performance Level. It is important to note that, for this performance level, repairs may be required before the building can be re-occupied after a design level earthquake. The post-earthquake damage state for Collapse Prevention is such that the building is on the verge of collapse with significant portions of the structural and non-structural components damaged beyond repair. A building designed to this level of performance will likely not be able to be reoccupied after a maximum credible earthquake. Figure 2, shows different damage states for buildings under a seismic event.


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Figure 2: Different damage stages for buildings

Note: Figure re-printed from webinar by California Hospital Commission on May 2015 Existing Building Description The project building is an existing six-story structure built in 1972 that is triangular in plan. It is located in San Bernardino, California. The lateral force resisting system (LFRS) of the existing structure consists of pre-Northridge steel moment frames that extend to the roof level of the structure. The moment frames are positioned such that one 6-bay moment frame is located at each of the three sides of the structure along the perimeter, and one 4-bay moment frame is located 20 ft. directly inward and parallel with each 6-bay moment frame. This structural configuration provides 6 frames total (three 6-bay frames, and three 4-bay frames) within the building. The moment frames were designed using the prescriptive pre-Northridge welded unreinforced flange (WUF) moment connection. The gravity system of the structure consists of a flat one-way concrete slab that spans between wide flange steel joists. These joists span between larger wide flange beams & girders and are in turn supported by the wide flange columns. Above the roof level, there is an existing penthouse and elevator machine room. The penthouse and elevator machine room areas consist of un-topped metal deck over wide flange steel beams supported by steel columns. The penthouse floor plan is mostly triangular in plan as well. The lateral system for the penthouse structure consists of diagonal steel plate-straps in selected beam-column frames. At the foundation level, the structure consists of a slab-on-grade system with cast-in-place drilled piles and grade beams. Existing Building Deficiencies As part of the initial assessment, a Tier 1 evaluation was conducted in accordance with Chapter 6 of the CAC. The following deficiencies were identified by performing a analysis:

• Soft Story per § 3.3.2

• Adjacent Building per § 3.4

• Drift Check per § 4.3.2

• Strong Column Weak Beam per § 4.2.8

• Pre-Northridge Welded Moment Frames per § 4.2.10

• Untopped Diaphragms per § 7.3.1 and 7.3.2 Building Retrofit Scheme The building retrofit consists of new primary external damped 3-bay moment frames at all three sides of the building. The external frames are connected to the structure through horizontal trusses. The existing perimeter beams are used as drags and minor strengthening was required to increase the drag capacities. Deep pile foundations support the new lateral elements. Each new external frame is supported on a pile cap. The new piles consist of a battered, proprietary pile system to resist seismic demands. Figure 3a shows a typicalplan view of the building and Figure 3b shows a typical retrofit elevation of the external damped moment frame with foundation.

Figure 3a: Overall Plan of the Building


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Figure 3b: Elevation of External Damped Moment Frame

- Retrofit Scheme

Cost Comparison Although the evaluation of multiple performance objectives was not the initial intention of the project, changes in state legislation (i.e. introduction of SPC-4D) opened the opportunity to explore them. In addition the owner was attracted to the idea of having a better structural performance with an extended life cycle. As such the design team was requested by the owner to explore various performance categories. The various analysis provided the team with an opportunity to understand the overall construction costs associated with different upgrades. Although in a structural retrofit project, the structural costs are a major portion of the overall budget, they are not the only deciding factor of the project scope. This is because there are other non-structural costs associated with a retrofit that require a large percentage of the overall budget and can also be the determining factors when deciding how to proceed with a project. Major costs associated with the structural retrofit explored in this paper include structural upgrades, non-structural upgrades, make ready projects, ADA projects, and patch back work costs. The case study does not include costs the owner incurs for special inspections, and operational downtime. This paper refers to make ready projects as site preparation and relocation of staff in and around the construction area to make space “ready” for the retrofit. Patch back work refers to the costs associated with

returning Architectural-Mechanical-Electrical-Plumbing (AMEP) fixtures back after necessary removals (if any) to conduct the retrofit. Finally, ADA projects are upgrades that are triggered by code for seismic upgrade projects. Figure 4 shows the direct cost associated with the structural upgrade of the building for three different structural performance categories: SPC-2, SPC-4D, and SPC-5. As illustrated, the incremental structural cost difference between SPC-4D and SPC-5 is less than that of SPC-2 to SPC-4D. In this case study, the low relative structural cost difference between SPC-4D and SPC-5 is atypical, and related to the selected retrofit configuration, specifically the use of external dampers. Additionally, when selecting an upgrade option based on costs, it is important to note that SPC-2 would still require future upgrades before 2030 per current legislations (implying additional costs in the near future).

Figure 4: Direct structural cost of retrofit for three

different structural performance categories.

Figure 5 shows the cost associated with removing and re-patching architectural, mechanical, electric and plumbing. For this project, this cost also includes the mandatory ADA upgrade requirements per California Building Code totaling to 20% of the overall project. The total project cost differences between incremental upgrades from SPC-2 to SPC-4D to SPC-5 are nominal as compared to the structural cost differences (see Figure 4).

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SPC-2 SPC-4D SPC-5

$192 /sf

$222 /sf$234 /sf

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STRUCTURAL COST


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Figure 5: AMEP (patch back) cost of retrofit for three


Figure 6 shows make ready project costs. Similar to the AMEP cost differences between upgrade scenarios, the make ready costs differences between SPC-2 to SPC-4D to SPC-5 upgrades are minimal compared to the structural cost comparisons (see Figure 4).

Figure 6: Make Ready projects cost of retrofit for three


Figure 7 compares the overall (total) cost for the three different structural upgrades and an approximate cost of a similarly sized new building. Whereas Figure 4 shows a large structural

cost jump between SPC-2 and SPC-4D upgrades (over $3.5 million), the total cost difference between SPC-2 and SPC-4D is not much greater (just $4 million). Although the AMEP and make ready costs are substantial, these costs do not increase significantly between each upgrade scenario the way the structural costs increase between SPC-2 and SPC-4D. This holds true for the overall cost differences between SPC-4D and SPC-5. In addition, the total cost was approximately half of what a new building would cost in today’s market.

Figure 7: Overall (total) project cost of retrofit for three

different structural performance categories and new

construction comparison.

Conclusion As seen in the cost comparison study of different structural upgrade performance levels, the total upgrade cost consists of many different aspects apart from just upgrading the structural components. A 15% increase in structural only upgrading cost from SPC-2 to SPC-4D results in only 8% increase in total cost of retrofit. A 5% increase in structural only upgrading cost from SPC-4D to SPC-5 results in only 4% increase in total cost of retrofit. A 22% increase in structural only upgrading cost from SPC-2 to SPC-5 results in only 13% increase in overall cost of retrofit. As illustrated in Figure 5 and Figure 6, the AMEP and make ready costs are substantial to the project; however, the cost differences between the three upgrade scenarios are not large. Therefore, when looking at the overall costs differences between upgrade scenarios in Figure 7, the AMEP and make ready costs dilute the larger structural cost differences between each upgrade scenario. Having the cost data available for different performance categories, the owner ended up choosing

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SPC-2 SPC-4D SPC-5

$137 /sf

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AMEP COST

$8.8

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SPC-2 SPC-4D SPC-5

$73 /sf

$74 /sf

$75 /sf

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MAKE READY COST

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$402 /sf$438 /sf $456 /sf

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TOTAL COST


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SPC-4D performance level for upgrade. Even though cost delta was small to get SPC-5 performance level, the existing adjacent buildings limited the size of new foundation. SPC-5 foundation size would have resulted it more disruption and downtime. We hope this study can be used to encourage building owners to do a cost study on various performance levels prior to proceeding with the retrofit design. A reasonable cost increase can result in having a higher performance building.

References

2016 CBC, California Building Code, California Code of Regulations, Title 24, Part 2, Volume 1 & 2 (based on the 2015 International Building Code), including Supplements, California Building Standards Commission, Sacramento, CA. 2016 CAC, California Administrative Code, California Code of Regulations, Title 24, Part 1 2015, California Hospital Association (CHA) webinar on new seismic performance category (SPC-4D)

case study: cost delta for achieving higher structural performance levels · 2018. 4. 2. · the...

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