Seismic Risk Assessment of Heritage Buildings in Iloilo City, Philippines

The 5th Asia Conference on Earthquake Engineering October 16-18, 2014

SEISMIC RISK ASSESSMENT OF HERITAGE BUILDINGS IN ILOILO

CITY, PHILIPPINES

Kirk Kennedy U. Yu1 and Andres Winston C. Oreta, D. Eng.2

ABSTRACT

Heritage buildings are symbols of our culture and heritage as a people. These buildings stand as a testament to who we are and tell the great story of our past—our very identity. Through time, these heritage buildings run great risks of being destroyed due to their vulnerable state and the hazards surrounding them. With limited budget and resources, it is a problem to preserve a great number of buildings simultaneously. Through the developed methodology for seismic risk assessment, heritage councils can prioritize which buildings should be subject to deeper investigations and retrofitting methods on the basis of its cultural value and level of risk. The City of Iloilo (Philippines) is one of the oldest cities in the country. In its Central Business District (CBD), there are 29 identified heritage buildings, excluding churches and temples. The City Government has declared these buildings as ‘heritage buildings’ and has demarcated the CBD area as a cultural hub. The developed method has been applied in the CBD area of Iloilo City. The buildings have been ranked for preservation and their critical seismic parameters identified. Among others, this included mass irregularities, unmapped framing system and degradation of structural material. Courses of action to address the risk condition have also been recommended. Keywords: rapid screening, heritage buildings, prioritized preservation, risk assessment

INTRODUCTION

Heritage buildings are structures constructed in the early times. During this period, seismic detailing for the structural design of buildings has not yet been established. This results to the lack of proper seismic detailing for most heritage structures (Vicente, Mendes Da Silva, & Varum, 2005). In the Philippines, the post-benchmark for seismic design is the year 1992. The lack of seismic detailing places heritage structures among the most vulnerable class of buildings (D'Ayala & Speranza, 2002). In addition to this, the prevailing construction practices and construction material may pose higher risks. Since then, the wear and tear of the materials also leads to aggravate the structural integrity of the structures. This keeps with the basic principle that as buildings age, their usefulness and structural strength lessens in capacity. To this end, the case for the seismic risk assessment of heritage buildings is unique. These structures must be preserved and not simply demolished.

Presently, risk assessment methodologies do not account for the special condition of heritage

buildings. The current rapid assessments being made rarely account for the material condition, and more importantly, the cultural heritage value of the building. This can be particularly observed in seismic assessment procedures made by the Federal Emergency Management Agency (FEMA 154), Architectural Institute of Japan, and the National Research Council of Canada. The gap between disaster risk reduction and cultural heritage must be bridged. Useful knowledge from both fields may richly benefit the preservation of both culture and life. In response, a methodology is developed that allows cultural heritage councils to actively preserve heritage buildings by accounting for its value and condition. The developed methodology is a rapid screening method that permits an efficient and

1 Graduate Student, Civil Engineering Department, De La Salle University, Manila, Philippines, [email protected] 2 Full Professor, Civil Engineering Department, De La Salle University, Manila, Philippines, [email protected]

Kirk Yu

ISBN 9789869120104


economic prioritization of building preservation against seismic risks. Central to the procedure is a qualitative, semi-quantitative type of assessment where key parameters to the seismic risk of a heritage building can be swiftly rated and subsequently ranked according to the level of risk. The multi-risk assessment methodology of heritage buildings is applied at Iloilo City as a case study.

METHODOLOGY FOR HERITAGE BUILDINGS

Risk is defined as a ‘convolution of hazard and vulnerability’ (D'Ayala, et al., 2008). Similarly,

the United Nations Development Program (UNDP) defined risk as the probability of harmful consequences resulting from interactions between hazards and vulnerable conditions (United Nations Development Programme, 2010). These definitions give us the two key principles that define risks. Hazards pertain to the event, where vulnerability pertains to the capacity of the object in study to sustain damage before total loss. Taking the perspective of preservation, the aspect of the object’s importance becomes crucial. Ostensibly, the preservation must also be prioritized based on the object’s importance aside from the structural risks. The object’s importance is the heritage building’s asset value. The asset value will gauge the relative importance of the heritage building in terms of architectural, historical, commercial, tourism and adaptive reuse values. All in all, the risk is calculated using the equation below, !! = !×! ×! (1) where R, H, V and, A are indices corresponding to the risk, hazard, vulnerability, and asset value, respectively; the latter three indices are referred to as sub-indices. Vulnerability is modified by applying aggravating factors, such as the material conditions of the structure. These modifiers increase the calculated risk index by a certain percentage, but not exceeding 90-percent. The listed parameters have been adopted and modified from previous studies made by (Architectural Institute of Japan, n.d.) and (Brizuela & Oreta, 2013). Qualitative/Semi-quantitative Risk Assessment The sub-indices are each defined by a cluster of key parameters, rated through a rapid visual screening (RVS) procedure. RVS is a first level screening method that involves a quick visual inspection of key critical parameters associated to the risk (Federal Emergency Management Agency, 2002), which has strong potential to develop into actual disasters. The key parameters for each index are rated high, medium or low risk, corresponding to a score of 3, 2 or 1, respectively. The weighted sum of the relevant parameters will return the value of the sub-index in study. Equation 1 is then used to compute for the main risk index.

The proposed methodology uses the conventional technique of risk assessment (Vicente, Mendes Da Silva, & Varum, 2005). This technique returns a vulnerability index without estimating the corresponding damage. Hazard Parameters The strength of the seismic event is measured through three key parameters: (1) distance of the study location to the nearest fault, (2) seismic source type and (3) soil conditions. Altogether, the three aims to encapsulate the possible magnitude of ground shaking the study location may sustain during seismic events. The strength of the ground shaking will correspond to the magnitude of force the structure bears. Vulnerability and Modifier Parameters The vulnerability parameters, including its modifiers aim to measure the relative capacity of a structure through identification of critical structural makeups or “weak points.” The amount of weak points will determine the structure’s capacity. The key vulnerability parameters are (1) age of structure,


(2) plan irregularity, (3) vertical irregularity, (4) provision for seismic gap, (5) number of floors, (6) material, (7) number of bays in the transverse axis and (8) column-to-column spacing. These are then further modified through time deterioration and structural modifiers (M). The modifiers adjust the vulnerability index through consideration of cracks, deflections, building age, continuity of the force load path and material deterioration among others. Hence, the sub-index vulnerability, V, is multiplied by a factor, (1+M). Asset Value Parameters The United Nations Educational, Scientific and Cultural Organization (UNESCO) described heritage buildings as a “group of buildings which, because of their architecture, their homogeneity or their place in the landscape, are of outstanding universal value from the point of view of history, art or science” (Harun, 2011). In table 1, these definitions were translated into five key values for the asset value index. The asset value index is a key parameter determining the importance or significance of the heritage building.

Table 1. Asset Value Index Parameters.

Cultural Heritage Cluster Economic-Tourism Cluster Architectural Value

Historical Value Commercial Value

Tourism Value Adaptive Reuse Value

To ascertain an index for each type of value, several descriptive statements are rated agree-neutral-disagree, corresponding to numerical scores of 3, 2 and 1. The average of all numeric scores under each type of value is the prevailing index. The index for each type of value forms the asset value index through a weighted sum. The relative scores for architectural and historical values are combined to obtain the Cultural Heritage Value (CHV); whereas the commercial, tourism and adaptive reuse values are combined to obtain the Economic-Tourism Value (ETV). The asset value (A) is then obtained by combining the cultural heritage and economic-tourism values, as: !! = (!!!!!!"#) !+!(!!!!!!"#), where the appropriate weights (w1 and w2) are determined through an Analytic Hierarchy Process (AHP).

Table 2. Sample descriptive statements for different value types of heritage buildings.

Architectural Value Historical Value Commercial Value Tourism Value Adaptive Reuse Value

The architectural style is able to capture the prevailing style during its era.

I am able to identify with Iloilo’s culture because of the history the building represents.

The commercial establishment(s) in this building helps stimulate the city’s economic welfare in a sustainable manner.

The tourism industry in the city is improved because of the building’s existence.

The use is able to adapt with the city’s needs without sacrificing the culture it represents.

Analytic Hierarchy Process Survey Corresponding weights for each key parameter under hazard, vulnerability and asset sub-indices, can be determined through the conduct of an Analytic Hierarchy Process (AHP) survey. The AHP survey is a pairwise comparison of all possible parameter combinations. Experts in the field are asked to choose the relative weight of each pair of parameter over a nine-point scale. The expert’s opinions of each respondent are aggregated by geometric mean prior to the calculation of weights. This method is known as aggregation by individual judgments (Angiz, Mustafa, Noraida, & Kamil, 2012).


PILOT STUDY IN ILOILO CITY The City of Iloilo is situated in the central part of the Philippines. In the country, it is one of the oldest cities, having witnessed one of the first establishments of the Spanish colonial rule. The downtown central business district (CBD) of the city hosts a total of twenty-nine identified heritage buildings. These structures were houses of the illustrados (educated Filipino class) and trading bazaars of the entrepreneurial Chinese during the era. Recent efforts from the city government has demarcated the CBD as a cultural hub and regulated the preservation of such structures. The city has also instituted a cultural heritage conservation council to oversee preservation regulations (Yu, Oreta, Ibabao, & Hechanova, 2013). Through the council, several renovations of the heritage buildings have been undertaken to preserve the buildings. In line with these efforts, the identified heritage buildings were subjected to the developed methodology for heritage buildings. Seismicity of Iloilo City The Philippine Institute of Volcanology and Seismology (PHIVOLCS) lists the city under seismic zone 4 in the seismic map of the Philippines (National Structural Code of the Philippines, 2010). A fault map from the PHIVOLCS shows that there are around three active faults in the near proximity of the city. Historical earthquakes of the city with magnitudes six and above are plotted on a map; see figure 1a (Geoscience Australia, 2012). The red star is the location of the city. In the figure, the 1948 magnitude 8.2-earthquake is worthy of note. This seismic activity is the second largest in the 500-year history of Philippine seismic activities (Geoscience Australia, 2012). Historical reports have shown that the event has damaged about fifty churches and brought down seventeen of these. The seismic event is believed to originate from the Western Panay Fault line.

Figure 1. (a) Documented seismic activities in the Western Visayas Region. (b) Annual Spectral Acceleration Exceedance for Seismic Activities in the Vicinity of Iloilo City.

A hundred-year seismic activity occurring from a 300 km radius from the city, with magnitude 4 and above was collected from the PHILVOLCS. Over the 100-year seismic data collected, it showed that the nearest seismic activity occurred about 6.7 km from the city, with a magnitude of 5.2 and a depth of 22 km. These correspond to an attenuation of 0.09g and an annual probability of exceedance of 3-percent (figure 1b). On the other hand, the largest seismic activity in the data is a magnitude 7.7- earthquake, located 66 km from the city and 33 km in depth—translating to 0.12g attenuation. The derived Guttenberg-Richter equation showed that a magnitude 4-seismic activity might occur every 12 days and a magnitude 8, every 115 years. In a seismic modeling and simulation study by (Geoscience Australia, 2012), showed that a magnitude 8 earthquake, similar to the 1948 Panay earthquake, would leave about 26-percent of buildings completely damaged (Geoscience Australia, 2012).


Seismic Rapid Assessment for Building 02-005 This building ranks eleventh most risky heritage building among twenty-nine inspected heritage buildings. It has a normalized risk index of 0.571 out of 1.000. The building has sustained several structural damages, including rebar exposure and cracks in slabs and beams. Figure 2 shows some of the photo documentation of the building inspection. The building has risky structural conditions but ranked eleventh due to its low asset value index, 2.153 (ranks 28th among twenty 29 heritage buildings). Discussed in the following sections are the hazard, vulnerability and modifier assessment conditions.

Figure 2. Photo documentation of seismic rapid assessment for building 02-005.

Hazard Conditions

The nearest fault line to the city is more than 15 km away, able to generate more than a magnitude 7 earthquake. The study location has a stiff soil profile. Table 3 shows the assessment form and the corresponding calculations.

Using a weighted sum, the hazard returns a 1.878 index. The corresponding weights are shown in the same figure. These weights are taken from an Analytical Hierarchy Process survey.

Table 3. Hazard condition assessment form for building 02-005.

Vulnerability and Modifier Conditions This building has been constructed prior to the 1972 post-benchmark for seismic design standards. The two-floor building is irregular in plan due to non-parallel system as it is triangular in


shape. A possible soft-story is also present due to varying ceiling heights. The building exhibits mass irregularity because of mezzanine floors used as storage of goods sold by the tenants. The building exhibits no seismic gap to the adjacent buildings. The framing system utilizes reinforced concrete material and have less than three bays in its transverse axis. A full mapping of the framing system was not possible due to limited access in the second floor and dropped ceilings. Maximum column spacing is found to be 6.5 meters. Using the listed weights in table 4, the weighted sum returns a vulnerability index of 2.306. The table also shows the calculation of the vulnerability index. Building condition can be described as poor. It is greater than thirty years old and material degradation is severe in the building (see figure 2). As per inspection, several shear cracks were found in the beams, often penetrating to expose rebars. Rebar exposure was also observed in the slab. Seepage of water through the structural elements was also observed. These beams have also deflected due to sustained loads. Load discontinuity was assumed due to limited access to account for structural uncertainties. Architectural features are considered manageable. Ordinary framing system is assumed due to lack of deeper structural information. Seen in table 5, the lowest value for time deterioration modifier is 0.7 while the strucutral modifier has a ratio of 0.778. The sum of both modifiers, with a value of 1.478, exceeds the 0.9 limit. In this case, a building condition modifier of 0.9 is adopted. Thus, the vulnerability sub-index is modified as: !! = 2.306 1 + 0.9 =4.3814.

Table 4. Vulnerability condition assessment form

for building 02-005.

Table 5. Modifier condition assessment form for building 02-005.


Asset Valuation of Iloilo City Heritage Buildings

Figure 3. (a) Heritage Building Asset Index of top, median and last-ranked heritage buildings out of 29. Highest index is 2.750 out of 3.000. (b) Building 03-001.

A Focus Group Discussion (FGD) was attended by a total of seven stakeholders coming from the academe, historians, architects, civil engineers and members of the Iloilo city heritage council. The AHP survey revealed that the Cultural Heritage cluster has a relative weight, w1, of 0.667 and the Economic cluster has a relative weight, w2, of 0.333. Using these weights, the 29 heritage buildings were ranked according to heritage building asset index. Figure 3a displays the bar chart of the indices and their respective values for the different value types. It shows that building 03-001 is the most valuable building and building 02-007, the least valuable. 03-001 is a residential structure owned by a prominent family in the city. The structure has a Beaux Arts type of architecture, figure 3b. Seismic Risk of the Heritage Buildings

Figure 4. Normalized seismic risk indices of top five of twenty-nine inspected heritage buildings. Structural inspection of the heritage buildings revealed that the level of risk among the buildings do not vary by a great range. All of the seismic risk indices fall under the “medium risk” classification (normalized score ranging 0.334 to 0.666), except for the lowest ranked building, under the “low risk” classification. Medium risk buildings score a fair amount of high-risk vulnerability parameters, while low risk buildings score several low-risks in its vulnerability parameters. Second


priority is given to the former in terms of preservation but a detailed inspection of the building’s capacity is recommended. Shown in figure 4 is a bar chart of the seismic risk indices of the top 5 most risky heritage building, considering asset index. The highest risk index attained is 0.615 (building 01-008) and the lowest is 0.322 (01-003). Building 01-008 is horizontally and vertically irregular and provides no seismic gaps. On the other hand, building 01-003 is regular in plan, a uniform lateral framing system and has six bays in its transverse axis.

CONCLUSION

The proposed methodology is able to bridge the gap between fields of disaster risk reduction and cultural heritage. The risk assessment procedure is also able to account for the special nature of the heritage building, specifically, its asset value or importance and material condition. The first level screening procedure (rapid visual inspection) technique is able to allow savings on time and financial resources of heritage councils and local government units through prioritization. The method is effective in identifying risk-critical and culturally-important buildings and subject these to deeper investigations and prioritized preservation. The flexibility and basic nature of the method allows for easy application to multi-hazard scenarios and execution even by laypeople. This stands to benefit local government units in urban planning and creating disaster-prepared communities—with premium in saving both lives and cultural heritage.

REFERENCES

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Seismic Risk Assessment of Heritage Buildings in Iloilo City, Philippines

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