Measures of the Seismic Vulnerability of Reinforced Concrete Buildings in Haiti

Asif Mohammed Fahad &

Sabbir Rahman Khan

BACKGROUNDA survey of 170 buildings following the 12 January 2010 Haiti earthquake (Port-au-Prince and Le´ogaˆne) provided the opportunity to evaluate the relevance to Haiti of a practical screening methodology developed by Hassan and So¨zen (1997). The methodology prioritizes structures according to the Priority Index (PI), which is a weighted ratio of column and wall cross-sectional areas to total floor area.

• 12 January 2010

• Mw=7.0, according to the U.S. Geological Survey)

• 22,570 people were killed, 300,000 people were injured, 1.3 million people were displaced, 97,294

homes were destroyed

• Haiti experienced strong earthquakes in 1701, 1751, 1770, 1842, 1860, 1953, and 2010

• 73% of the Haitian building stock is single-story structures

• Haitian infill walls are constructed using concrete masonry units

Facts and Figures

In June 2010 researchers from Purdue University, the University of Washington, and the University of Kansas collaborated with researchers from the Universite´ d’Etat d’Haı¨ti to collect basic properties and damage information from 170 reinforced concrete (RC) frame buildings. Four teams were formed with three or four members per team and were composed of professional engineers, graduate students, and civil engineering faculty. Over a period of one week, 170 buildings were surveyed from randomly selected streets in the Port-au-Prince (132 buildings) and Le´ogaˆne areas (38 buildings).

The

Research

KEY

Findings

• As had been done in Turkey (Hassan and So¨zen 1997, Gur et al. 2009), “priority indices,” which are weighted ratios of column and wall cross-sectional areas to total floor area (Hassan and So¨zen 1997), were calculated from the collected data to determine whether the priority index was an appropriate tool to estimate the seismic vulnerability of buildings in Haiti. The results from Haiti are compared here with the results from the surveys done after the earthquakes that took place near Du¨zce, Turkey in 1999 (17 August 1999, Mw=7.4 and 12 November 1999, Mw=7.2, according to the U.S. Geological Survey).

KEY Findings

(CONTINUED)

• For each building, teams recorded the dimensions of building plans and the cross sections of vertical elements at the ground story; documented damage to the RC and masonry elements; recorded the number of stories; the presence of captive columns and noted geographical coordinates. Top stories with lightweight corrugated metal roofs were counted

as half stories. Masonry walls with large openings and walls detached from the RC structure were not considered.

• Damage to the RC structural elements was classified as “light,” “moderate,” or “severe.” Damage to the RC elements was rated as “light” if only hairline inclined and flexural cracks were observed. A structure was said to have “moderate” RC damage if cracks were wider or if spalling of concrete was observed.

• Damage to the masonry infill walls was also classified as “light,” “moderate,” or “severe.” Damage to the masonry was rated as “light” if hairline cracks were observed. The damage to the masonry was categorized as “moderate” if wider cracks or flaking of large pieces of plaster were observed. Masonry damage was classified as “severe” if collapse or wide and through cracks were observed

Methodology Followed and Applied

PRIORITY INDEX

• Followed a practical method to rank low-rise RC buildings according to their seismic vulnerability. The

method consisted of classifying buildings as vulnerable or not vulnerable depending on the wall area

index, a ratio of the cross-sectional areas of RC walls to total floor area, and the nominal shear stress in

columns and walls. This method requires simple information that can be obtained through rapid

field inspections.

• A building is vulnerable to earthquakes if its PI (for the direction with the smallest wall area) is less than

0.4%.

• The method is not effective for mid-rise and high-rise buildings but perfect for the Haitian low-rise

structures with small areas of RC wall, masonry infill walls, and poor reinforcement detailing.

• The data collected in Haiti provided the opportunity to determine the extent to which the index is

sensitive to local construction practices and properties of local materials.

Methodology Followed and Applied

(CONTINUED)INSPECTED THE CHARACTERISTICS OF SURVEYED BUILDINGS• The majority of the buildings surveyed were RC frames with CMU infill walls.

• In housing units (typically one or two stories high) the depth and width of the column cross sections were

equal to the thickness of the infill walls (6 to 10 in).

• In schools and commercial buildings the columns had proportions similar to the proportions of columns

found in regions of the United States with low seismicity.

• In ordinary residential construction, however, material quality and detailing were poor.• Out of the 170 surveyed buildings, most were short, with an average of 2.5 stories: 12% (20 buildings) had one

story, 57% had two stories (67 buildings), and only 5% of the buildings had more than four stories.

• The average PI for the surveyed buildings was 0.24%, which is far below the threshold value of 0.4%

• As the number of stories increased, the average PI decreased, indicating that the column and wall cross-

sectional areas did not increase in proportion to the number of stories.

• Structures with the lowest range of PI (0.0%–0.09%) had a 77% likelihood (10=13) of suffering severe masonry

damage.

GRAPHICAL PRESENTATION OF THE FINDINGS

One Story

Two Story

Three and Four stories

More than four stories

0% 10% 20% 30% 40% 50% 60%

Number

Number

DAMAGE STATISTICS

Priority Index range

Light Damage Moderate Damage

Severe Damage

Total

0.00–0.09 5(3/03) 0 (0) 7 (4.24) 12 (7.27) 0.10–0.19 28 (16.97) 8 (4.85) 31 (18.79) 67 (40.61) 0.20–0.29 19 (11.52) 9 (6.45) 17 (10.30) 45 (27.27) 0.30–0.39 13 (7.88) 3 (1.82) 09 (5.45) 25 (15.15) >¼0.40 7 (4.24) 6 (3.64) 03 (1.82) 16 (9.7)

Priority Index range

Light Damage Moderate Damage

Severe Damage

Total

0.00–0.09 1(0.6) 2(1.2) 10 (5.99) 13(7.78) 0.10–0.19 18(10.78) 6 (3.59) 44(26.30) 68(40.70) 0.20–0.29 14(8.38) 9(5.39) 23 (13.77) 46(27.54) 0.30–0.39 11(6.50) 2(1.2) 12(7.19) 25(14.37) >¼0.40 5 (2.99) 5 (2.99) 5 (2.99) 15(8.98)

OBSERVATIONS

1. Severe damage is higher in masonry walls (54%) than to the reinforced concrete houses (41%).

2. 19% of safe elements (PI>0.4%) were damaged while 43% of the unsafe structures ((PI<0.4%) ) followed the same route.

3. As the range of PI index increases observed damages in both reinforced and masonry elements decreases i.e in 0.0-0.09 range it is 77% for masons structures while it is 65* in 0.1-0.19 range.Results are insensitive to locations i.e Port-au-prince and Leogane and possible variations in practice.

4. Damage is sensitive to the presence of captive column. It is observed that at lower PI range in the presence of captive columns damages almost double.

5. Two Tier Screening process: Firstly to find out the low rise reinforced concrete buildings and secondly to identify buildings with captive column.

COMPARISON BETWEEN TURKEY AND HAITI

COMPARISON CRITERIA HAITI TURKEY Intensity IX VIII PGA 0.65-1.0g 0.4g maximum PGV 120cm/s 40cm/s Magnitude 7.0 7.2 Distance Within 15 km Within 15 km Structure on soil Soil type C and D Soil Type D Damage Comparison 50% more than Turkey Less than Haiti Structure marked safe as per PI 90%<0.4% 75%<0.4% Building Story Average 2.5 stories Average 3.5 stories

CONCLUDING REMARKS

• PI is effective to assess seismic vulnerability-therefore, it is required to attribute standard to apply universally.

• More budget and other financial resources.

• More training to engineers.

• Effective zoning and appropriate response strategy during emergencies and frequiencies.

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