Performance-based Analysis of Code-base Designed Structures in the Philippines

Article information

J. Korean Soc. Hazard Mitig. 2017;17(3):289-296
Publication date (electronic) : 2017 June 30
doi : https://doi.org/10.9798/KOSHAM.2017.17.3.289
* Member, 1 Ph. D. Student, Department of Civil and Environmental Engineering, Kongju National University, South Korea 2 Faculty Member, Department of Civil Engineering, University of Santo Tomas, España, Manila, Philippines
** B.S. Civil Engineering Graduate, Department of Civil Engineering, University of Santo Tomas, España, Manila, Philippines
*** Faculty Member, Department of Civil Engineering, University of Santo Tomas, España, Manila, Philippines
Member, Professor, Department of Civil and Environmental Engineering, Kongju National University, Republic of Korea (Tel: +82-41-521-9300, Fax: +82-41-568-0287, Email: smjeong@kongju.ac.kr)
Received 2017 April 10; Revised 2017 April 12; Accepted 2017 May 02.

Abstract

The ability of code-base designed structures to meet predictable performance requirement in an event of an earthquake was investigated in this study using nonlinear static pushover analysis. This was performed by developing the structural framework of Manila Science High School (MSHS) building using SAP2000. Subsequently, a nonlinear static pushover analysis according to the provisions of ATC-40, a code provided by the Applied Technology Council (ATC) on Seismic Evaluation and Retrofit of Concrete Buildings was performed. The analysis generated a pushover curve, a graph that displays the total drift of the structure versus base shear and the possible location and condition of the plastic hinges at every step of the analysis. Results were compared with the acceptable criteria provided by ATC-40 for Immediate Occupancy (IO) performance level since most school buildings in the Philippines are commonly used as evacuation centers. By analyzing the results, it was found out that the MSHS building will behave at Immediate Occupancy after an earthquake event. This study can contribute in verifying if the parameters used in code-base designed structures satisfy the requirements for IO performance level. Also, this study can estimate the possible post-earthquake damage effects on the MSHS building by using the nonlinear static analysis procedure.

Trans Abstract

본 연구에서는 비선형등가정적해석법(nonlinear static pushover analysis)을 사용하여 지진 발생시 예측 가능한 성능 요건을 충족시키는 코드 기반 설계 구조의 기능을 조사하였다. 이는 SAP2000을 사용하여 Makati Science High School (MSHS)건물의 골조 구조를 개발함으로써 수행되었다. 이후, Applied Technology Council (ATC)에서 제공된 콘크리트 건물의 내진평가 및 보강에 대한 코드인 ATC-40 조항에 따라 비선형등가정적해석법이 수행되었다. 이 분석을 통해 pushover curve를 얻었으며, 이 그래프는 모든 분석단계에서 구조물의 변위에 따른 소성힌지의 위치 및 상태를 나타낸 것이다. 필리핀의 대부분 학교건물은 일반적으로 대피소로 사용되기 때문에 Immediate Occupancy(IO)성능기준을 만족하기 위해 ATC-40에서 제공된 허용 기준과 pushover curve를 비교하였다. 비교결과 분석을 통해, MSHS 건물은 지진발생시 비상대피소로의 기능을 수행할 수 있는 것으로 평가되었다. 본 연구는 코드기반으로 설계된 건물의 변수와 IO 성능기준의 관계를 이해하는데 기여할 수 있다. 또한, 비선형등가정적해석절차를 이용하여 지진발생시 MSHS 건물에 미치는 손상 결과를 예측할 수 있다.

1. Introduction

The West Valley Fault, one of the major fault systems in the Philippines, had four major earthquakes in the last 1,400 years. It has a recurrence interval of 400 to 500 years. The last major earthquake originating from the fault was recorded in 1658 or 357 years ago (Sabillo, 2015). With the seven major earthquakes recently hitting the Pacific Region in just 96 hours (from April 13 to April 20, 2016) (Austin, 2016), there is a high possibility that the anticipated “Big One”, a magnitude 7.2 earthquake striking the West Valley Fault, a 100-kilometer fault that runs through six (6) cities in Metro Manila and nearby provinces, may occur any time soon (Adel, 2015).

The structural design of the buildings in the Philippines is governed by the “National Structural Code of the Philippines” (NSCP), developed by the Association of Structural Engineers in the Philippines (ASEP). The most recent version of the code was released 2010 incorporating most of the recent developments. However, many important provisions of the code, in particular, those related to the seismic design and safety, could not be made in line with the international standards due to the lack of proper definition of seismic hazards and mapping (Sy et al., 2012). The earthquake provisions set by NSCP 2010 and maps currently available in the Philippines were developed to support the Uniform Building Code 1997 (UBC 1997). It takes into account a lateral load produced by an earthquake of 10% exceedance probability within 50 years. Moreover, conventional elastic design analysis method cannot capture many aspects that affect seismic performance of the building. The ability of a building to undergo inelastic deformations determines the structural behavior of building during seismic ground motions. For that reason, the evaluation of a building should be based on the inelastic deformation applied demanded by an earthquake, besides the stresses induced by the equivalent static forces as specified in seismic regulations and codes (Giannopoulus, 2009).

In contrast to prescriptive or code-based design approaches, performance-based design (PBD) provides a systematic methodology for assessing the performance capability of a building, system or component. ATC (1996) defined PBD as a methodology in which structural criteria are expressed in terms of achieving a performance objective. PBD explicitly evaluates the response of the building under the potential seismic hazard, considering the probable site-specific seismic demands as well as the uncertainties in the post-yielding response and behavior of the building under seismic events.

Studies regarding the performance of facilities after being subjected to seismic forces has been gaining interest. In Korea, development and application of a mobile-based post-earthquake safety evaluation system for buildings in order to support related local government tasks was found to be useful through verified field tests (Kim et al., 2015). Moreover, to satisfy national seismic performance objective, a scheme for national earthquake-risk management was proposed in advance (Kim et al., 2013). In the Philippines, PBD procedures have been utilized in the design of most of the tall buildings since late-2000s. Most of the buildings are 40 to 70-storey tall, reinforced concrete residential buildings (Sy et al., 2012).

The primary objective of this study was to determine whether conventionally-designed or code–base designed structures in the Philippines meet predictable performance requirements using nonlinear static pushover analysis. In particular, Makati Science High School (MSHS), a 10-storey public school building was assessed whether it will satisfy the Immediate Occupancy (IO) performance level since most school buildings in the Philippines are commonly used as evacuation centers. Results from this study verified that the importance factor of 1.50 (seismic importance factor for Essential Facilities such as public school buildings and evacuation centers) used for the seismic analysis of MSHS building met the acceptability criteria for IO performance level. Also, this study determined whether the MSHS building can be used immediately after an earthquake event or some maintenance on the structure should be done before using it.

2. Pushover Analysis

According to Priestly et al.(1996), pushover analysis is a method for determining the ultimate load and deflection capability of a structure. Local nonlinear structure effects, such as flexural hinges at member joints, are modelled and the structure is deformed or “pushed” until enough hinges form to develop a collapse mechanism or until plastic deformation limit is reached at hinges.

Pushover analysis basically includes the capacity spectrum method (CSM) that uses the intersection of the capacity (pushover) curve and reduces response spectrum to estimate maximum displacement. The intersection of the two spectra is called the performance point. It verifies if the structural and non-structural components are not damaged beyond the acceptable limits of the performance objective for the forces and displacements implied by the displacement demand (ATC, 1996).

The Applied Technology Council (ATC) established a set of representative damage description of elements and components for concrete. It represents the post-earthquake effect damage to a certain structure and its degree is dependent on the performance level of the structure which are illustrated in Fig. 1.

Fig. 1

Load-deformation Curve

3. Methodology

3.1 Development of Structural Model

The MSHS Building is a 10-storey reinforced concrete building made up of special moment resisting frames. The building is a public educational institution and is located in Poblacion, Makati as shown in Fig. 2a, Fig. 2 wherein the soil profile type is of Type SC (very dense soil and soft rock). It is approximately 3.5 km away from a Type B (maximum moment magnitude is between 6.5 to 7.0) seismic source, valley fault line. The building consists of 14 bays @ 5m in the X-direction, 3 bays @ 9.5m in the Y-direction with a total height of 44.1m. The structure does not have typical floor plan and contains different areas of opening per floor therefore it has vertical irregularities. According to Table 208–9 of the NSCP 2010, the structure falls under Irregularity Type 1–Stiffness Irregularity (Soft Story) (ASEP, 2010). The structural model is composed mainly of columns, girders, slabs, moment released intermediate beams, fixed supports to represent the footing and applied dead load for the structure’s self-weight and live load. The material properties such as compressive strength of concrete and design tensile strength of reinforcing bars were derived from the structural plan and are listed in Table 1.

Fig. 2

Makati Science High School

MSHS Building Material Properties

The structural model was developed using SAP2000 and is shown in Fig. 2. Cantilever slabs at the perimeter of MSHS building was modeled into distributed loads along the perimeter beams. The first floor plan was not modeled because of the assumption that the structure is fixed on the ground. Thus, the main floor will not support lateral forces on the structure. The stairs were replaced by a point load applied at the stair beams. Finally, the metal deck of 10th floor and the arch roof of 11th floor were replaced by point loads to the supporting concrete columns and beams of the structure.

Federal Emergency Management Agency’s report on Prestandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA 356) was used to define the nonlinear hinges assigned to the beams. Plastic hinges were observed at 5% of the member length from the support (FEMA, 2000). Although plastic hinges occurs at the end of the beam, this value is needed to distinguish if the hinge formed is from a beam or a column.

3.2 Load Definition

In defining the loads for the MSHS Building, four load patterns were considered namely DEAD, LIVE, EQX and EQY for dead, live and earthquake loads along x and y axis directions, respectively. A Self Weight Multiplier of 1 was used for the dead load indicating that the full weight of the structure was considered in the analysis. For the earthquake load pattern, Uniform Building Code 97 (UBC 97) was used as an Auto Lateral Load Pattern since the earthquake provisions of NSCP 2010 were adopted from it. An eccentricity ratio of 0.05 was used for Load Direction and Diaphragm Eccentricity which indicates the axis where the elastic dynamic lateral load will be applied. Other parameters derived from Section 208 of NSCP 2010 were as follows: Eccentricity Ratio = 0.05; Overstrength Factor = 8.5 and Importance Factor = 1.5.

Table 2 summarizes the eight different load cases that were used. These load cases define how loads are to be applied to the structure and how the structural response is to be calculated. SPECX and SPECY uses a Response Spectrum load case, an elastic dynamic analysis which was defined using UBC 97 function and whose parameters such as Seismic Coefficients Ca = 0.4 and Cv = 0.672 were based from NSCP 2010. NONDEAD was analyzed using Nonlinear Static load case type which signifies that this load case will be used in the inelastic analysis of the structure. NONDEAD takes into account the full self-weight of the structure and 10% of the live load. Nonlinear Static was also used for PUSHX and PUSHY load cases. The Initial Condition used for PUSHX and PUSHY was “Continue from State at End of Nonlinear Case: NONDEAD” signifying that this load case will start only after NONDEAD load case has come to an end. The load combinations used were based from the NSCP 2010 and is presented in Table 3.

Definition of Load Cases

NSCP 2010 Load Combination

3.3 Pushover Analysis

In this study, pushover analysis is carried out using the SAP2000. Capacity spectrum method (CSM) was used to estimate the performance point of the structure in the pushover analysis. It uses the intersection of the capacity curve and the reduced response spectrum to estimate the maximum displacement. A Damping Ratio of 0.05 was used since MSHS is a concrete structure. In addition, Structural Behavior Type B was utilized which was based on ATC–40 for existing building with short shaking duration (Nv ≥ 1.2). Displacement Control was used as the Load Application Control for the pushover analysis of the inelastic behavior of the MSHS building. The FEMA 356 rule, which is a predefined rule within SAP2000 with the IO, LS, and CP limits for hinge rotation was used for the acceptance criteria.

After the analysis, the program generated the following: (1) capacity curve and capacity spectrum; (2) global minimum displacement of roof; (3) global base shear; (4) performance point of the structure; and (5) localized plastic hinges formation on the structure. These outputs were compared to the acceptable criteria provided by ATC–40 for IO performance level. Fig. 3 illustrates the deformations limits for each performance levels. Therefore, for MSHS, a target displacement not exceeding 0.4m was allowed in order to satisfy IO performance level.

Fig. 3

Deformation Limits (ATC-40)

4. Results and Discussion

In order to check the ability of MSHS to perform under IO performance level, a pushover analysis was carried out for a seismic source Type B (maximum moment magnitude is within 6.5 to 7.0) which is approximately 3.4 km away, using SAP2000.

4.1 Pushover Curve

The pushover curve presented in Fig. 4 shows the relationship between the base shear reaction and its corresponding displacement for PUSHX (Fig. 4a) and PUSHY (Fig. 4b) case. Fig. 4a signifies that the base shear and displacement at yield point were 11959.32 kN and 0.11 m, respectively. At its point of failure, the peak base shear was 17633.90 kN and the displacement was 0.57 m. As for the PUSHY case shown in Fig. 4b, Fig. 4 at its yield point, the base shear was 12955.9 kN and the corresponding displacement was 0.14 m. Consequently, at point of failure, the peak base shear and displacement were 20572.9 kN and 0.64 m, respectively.

Fig. 4

Pushover Curves

4.2 Capacity Spectrum

Fig. 5 shows the performance point observed for both PUSHX and PUSHY cases, coordinates of which are summarizes in Table 4. The elastic period of the structure was estimated at 1.8 sec while the inelastic period at performance point was 3.0 secs for PUSHX case. The performance point occurs with a base shear of 16651.1 kN with a global displacement of 0.37 m, which is still within the target displacement of 0.4 m for IO performance level. As for the PUSHY case, the elastic period of the structure was estimated at 1.9 sec while the inelastic period at performance point was 2.8 sec. The performance point occurs with a base shear of 18210.7 kN with global displacement of 0.38 m, which is also within the target displacement for IO.

Fig. 5

Capacity Spectrum Curve

Performance Point Coordinates from the Capacity Spectrum Curve

4.3 Plastic Hinges

Possible local structural failures were also checked in order to determine if the structure’s performance was within the IO performance level. In this section, the maximum hinge formed was at IO level. This step occurred at period T = 2.831 sec (the closest step to the Teff = 2.962 sec. at performance point) for PUSHX, while for PUSHY it occurred at period T = 2.9 secs. (the closest step to the Teff = 2.8 secs. at performance point). Fig. 6 presents the deformed shape of MSHS for PUSHX and PUSHY, respectively while Table 5 summarizes the plastic hinging for PUSHX and PUSHY cases at different damage levels.

Fig. 6

Deformed Shape of MSHS at Grid 1 and Grid

Summary of Plastic Hinging for Pushover Analysis at Different Damage Levels

5. Conclusion

The Makati Science High School was used to assess the ability of code–based design buildings to meet the acceptability criteria of performance-based design after being subjected to earthquake. Its lateral displacements for the PUSHX and PUSHY cases were 0.37 m and 0.38 m, respectively. These displacements are less than 0.4m (1% of the total building height of 44.1m), which is the required maximum total drift of the building under IO performance level thus, making the MSHS Building fall under this performance level.

The possible locations of the plastic hinges at performance point were also identified. On the deformed shape of the structure at PUSHX and PUSHY cases, the beams from second to fifth floors yielded under IO plastic hinge. The beams from sixth to eighth floors started to yield, while the remaining beams did not. The columns at the first floor began to yield, while the majority of the columns at other floors did not. No structural member yielded under LS or CP plastic hinge.

In the execution of the analysis, it was found that the code–based design of the MSHS building, which is an essential facility with an importance factor of 1.5, passed the IO Performance Level. The building’s global displacement and localized plastic hinge behavior at Performance Point met the acceptability criteria provided by ATC–40.

The minimum earthquake design loads for essential facilities with high importance factor subjected to earthquake load of seismic source Type B (maximum moment magnitude is within 6.5 to 7.0) provided by NSCP 2010 gave significant and conservative outputs. As a result, it complied with the acceptability criteria set by ATC–40 for the IO performance level.

Acknowledgments

This research was supported by a grant (13SCIPS04) from Smart Civil Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport (MOLIT) of Korea government and Korea Agency for Infrastructure Technology Advancement (KAIA).

References

Adel R. 2015. June. 15. What we need to know about the Valley Fault System The Philippine Star; from http://www.philstar.com. (Retrieved November 10, 2016).
Applied Technology Council (ATC). 1996. Seismic Evaluation and Retrofit of Concrete Buildings Volume 1. ATC-40 Report, Applied Technology Council Redwood City, California:
Association of Structural Engineers of the Philippines, Inc (A). 2010. National Structural Code of the Philippines Volume 1 – Buildings, Towers and Other Vertical Structures (6th Edition, 4th Printing)th ed. Association of Structural Engineers of the Philippines, Inc.
Austin J. 2016. April. 20. Fears of the ‘THE BIG ONE’ as SEVEN Major Earthquakes Strike Pacific Region in Just 96 Hours Express, from http://www.express.co.uk. (Retrieved November 16, 2016).
Federal Emergency Management Agency (FEMA). 2000. Prestandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA 356) Federal Emergency Management Agency. Washington, D.C:
Giannopoulos P.I. 2009. Seismic Assessment of RC Building according to FEMA 356 and Eurocode 8. Proceedings of the 16th Conference on Concrete TEE, ETEK. 21-23 October 2009.
Kim I, Sun C, Park K, Seo H. 2013;Establishment Scheme of Seismic Performance Objectives of Facilities for National Earthquake – Risk Management. J. Korean Soc. Hazard Mitig 13(No. 6):299–304. 10.9798/KOSHAM.2013.13.6.299.
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Priestly M.J.N, Seible F, Calvi G.M. 1996. Seismic Design and Retrofit of Bridges John Wiley & Sons. New Jersey: 10.1002/9780470172858. 8993611.
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Article information Continued

Fig. 1

Load-deformation Curve

Fig. 2

Makati Science High School

Table 1

MSHS Building Material Properties

a. Concrete Compressive Strength
Member f’c (MPa)
Beams 28
Columns (1st to 3rd floor) 35
Columns (4th and above) 28
Slabs 28
b. Reinforcing Bar Tensile Strength
Diameter fy (MPa)
12 mm and below 280
16 mm and above 420

Table 2

Definition of Load Cases

Load Case Name Load Case Type Load Case Name Load Case Type
DEAD Linear Static SPECY Response Spectrum
MODAL Modal NONDEAD Nonlinear Static
LIVE Linear Static PUSHX Nonlinear Static
SPECX Response Spectrum PUSHY Nonlinear Static

Table 3

NSCP 2010 Load Combination

Load Case Load Combination Basis
Dead Load 1.4DEAD NSCP 2010 Eq. 203–1
Dead Load & Live Load 1.2DEAD + 1.6LIVE NSCP 2010 Eq. 203–2
Dead Load & Earthquake Load 1.2DEAD + (0.5CaI)*DEAD + SPECX + f*LIVE NSCP 2010 Eq. 203–5 and Section 208.5.1.1
1.2DEAD + (0.5CaI)*DEAD + SPECY + f*LIVE
Dead Load, Live Load & Earthquake Load 0.9DEAD + (0.5CaI)*DEAD + SPECX NSCP 2010 Eq. 203–7 and Section 208.5.1.1
0.9DEAD + (0.5CaI)*DEAD + SPECY

where: Ca = seismic coefficient from NSCP 2010

f = 0.50, for other loads (Section 208.5.1.1)

I = 1.50, for Essential facilities

Fig. 3

Deformation Limits (ATC-40)

Fig. 4

Pushover Curves

Fig. 5

Capacity Spectrum Curve

Table 4

Performance Point Coordinates from the Capacity Spectrum Curve

PUSHX Case PUSHY Case
Base shear, V (kN) 16651.104 18210.697
Global displacement, D (m) 0.368 0.375
Spectral acceleration, Sa 0.135 0.151
Spectral displacement, Sd (m) 0.298 0.298
Effective time period, Teff (sec) 2.962 2.806
Effective damping, Beff (%) 0.246 0.217

Fig. 6

Deformed Shape of MSHS at Grid 1 and Grid

Table 5

Summary of Plastic Hinging for Pushover Analysis at Different Damage Levels

Hinge damage state Number of Plastic Hinges
PUSHX Case PUSHY Case
Did not yield 2248 2287
B to IO 329 296
IO to LS 335 329
TOTAL 2912 2912