ASSESSMENT AND RETROFITTING OF EXISTING RC BUILDINGS IN VIETNAM IN TERMS OF EARTHQUAKE RESISTANCES

Transcription

1 GEM-SEA Workshop on Seismic Vulnerability of Buildings Nanyang Technological University, Singapore 1 st July 2013 ASSESSMENT AND RETROFITTING OF EXISTING RC BUILDINGS IN VIETNAM IN TERMS OF EARTHQUAKE RESISTANCES Dr. Nguyen Dai Minh PhD PEng Deputy-General Director, Vietnam Institute for Building Sciences & Technology (IBST) 1

2 1. Seismicity in Vietnam - Vietnam Seismic Zone map (500 year RP) shows that in the country the earthquake intensity of VI-VII in MSK-64 scale will generally possibly happen. There is only in the North-West part of Vietnam seismic intensity up to VIII in MSK-64 scale. - From , the historical records show 115 earthquakes with M=4-4.9, 17 earthquakes with M= and 2 earthquakes with M>6. - Previously, there were no records of earthquakes with M>4 in the South of Vietnam but in 2005 earthquake with M>4 rocked tall buildings in Vung Tau and HCM city. - In 2012, there were a series of reservoir induced earthquakes in Song Tranh hydro power plant, the Middle part of Vietnam, causing the worries to local people and government. 2

3 Seismic design: 2. Earthquake resistance of existing buildings in Vietnam general remarks - From : Most buildings were low-rise and not designed considering the seismic resistance. - From : There were widely used the prefabricated RC buildings based on the East-German and the former USSR standards in Hanoi and Nghe An. These buildings were designed to resist the earthquakes with intensity of VII - VIII (MSK-64 scale). - From : Many buildings and structures were not designed considering seismic loads. However, key-important government structures and some buildings as requested by the investors were designed considering the earthquake loading. The applied codes were the Russian SNiP II -7-81* and UBC:

4 - From 1997 until now: Many buildings (since 2006, most buildings) were designed considering the earthquake with intensity VII. The applied codes were SNiP II-7-81*, UBC:1997 and TCXDVN 375 : Seismic resistance of buildings: - All most old residential buildings and houses are very poor in terms of seismic resistances. - Most high-rise buildings constructed in recent 5 years were designed taking into account the earthquake loading either based on SNiP II-7-81*, UBC 1997 or TCXDVN 375: However, through the investigations conducted in Hanoi, Ho Chi Minh city, Vung Tau and Dien Bien Phu in , it was found that the seismic detailing were not strictly followed. 4

5 Comments: (1) On the codes and regulations: There have been the regulations for the buildings regarding to earthquake resistances. Seismic zoning maps including the earthquake intensities (MSK-64 scale), PGA values etc. have been issued for design and constructions. (2) High-rise buildings and structures built since 2000 up to now generally have been designed considering the earthquakes, and based on one of the: TCXDVN 375:2006, SNiP II-7-81* and UBC:

6 (3) Residential buildings, especially the prefabricated RC structures, having the below signs, will possibly be collapsed if earthquakes with I=VII-VIII occurred: - Large-prefabricated RC panel buildings with the connections joints significantly damaged or fully damaged; - Buildings that have the additional loadings such as the water tanks on roofs, extending the rooms in the apartments, adding stories etc. but not calculated and designed regarding to the seismic loads; - Buildings where the walls were removed or columns were cut-off to create the larger space for shopping or other commercial services; - Buildings classified by D class according to TCXDVN 373 : 2006 Specifications for evaluations of the dangerous of the building structures. 6

7 (4) Normal houses with following signs will be dangerous if earthquakes with I = VII-VIII happen: - Settlement with inclination > 1 %; - Very thin buildings; - Very old masonry buildings with footing and brick walls significantly deteriorated; - Brick buildings with the walls or columns having the vertical cracks with crack widths > 2mm and crack lengths > 1/2 the height of the story. 7

8 Recommendations: (1) For buildings (prefabricated buildings) that are evaluated in D class based on TCXDVN 373 : 2006, recommend to demolish and to replace with new buildings; (2) Enhance regulations and code system, prepare technical guidelines in terms of seismic resistance construction; (3) Propagate to people to be aware of the construction to follow the regulations; publish, spread and teaching the construction methods regarding to earthquake resistances; guide people the actions needed for human safety, evacuate and rescue when earthquakes occur; 8

9 (4) Teaching in universities and training for engineers and architects the seismic analysis and detailing; (5) Continue to update the seismic data; develop, adopt and transfer the effective earthquake resistance technologies in Vietnam. 9

10 3. Causes for damages or collapses of existing buildings by earthquakes (1) Irregularities in plan of the buildings: The large difference between the mass center and the rigidity center in plan shall cause the torsional effect when earthquakes occur. (2) Irregularities along the height of the buildings: The very in-uniform arrangement of the lateral stiffness and mass along the height of the buildings shall lead to local damages at the locations where the stiffness or mass changed. (3) The unsuitable changes of the existing structures: Changes or upgrading such as making the floor holes, columns removed, increase the stories, wall removed or adding the separating walls in the upper floor etc. => soft story or basements. 10

11 (4) The construction errors: Columns are not in vertical, the eccentricities between beams and columns, large errors in construction materials (properties), foundation problems (very soft foundations, foundations that are easily in liquidations) etc. (5) Big difference between the old applied codes and the present effective codes: the safety of buildings is even critical under the normal loading and becoming more critical under the seismic loading combinations. 11

12 (6) Construct or install the structure components or equipments (such as water tanks, mobile phone antennas ): possibly cause the main structure damages when earthquake occurs. (7) Influence of the structure modeling and the calculation data input: Because of the complicated geological conditions or the difficulties of the structural modeling due to the architecture (like transferring slab with many stories above in high-rise structures), the earthquake analysis may not be accurate. (8) Progressive collapse: the vertical seismic loads influence to the structure because of removing of the columns in the basement or lower stories of the buildings for car parks or shopping areas or office for rents etc. 12

13 (9) Brittle or very low-ductile structures: most old masonry or concrete structures (without reinforcement) are belonged to this type. These structures are dangerous in terms of earthquake resistances. (10) Influences of the adjacent buildings: Due to the adjacent buildings mostly do not ensure the seismic joints, and easily collide each other during vibrations caused by earthquakes. (11) Poor quality of the construction joints. 13

14 4. Guidelines for assessment and retrofitting of existing RC buildings regarding to earthquake resistances Procedure (based on EN :2005): - Determination of the seismic risk at the site (PGA etc.). - Building and site investigations. - Assessments. - Select the intervention solutions. 14

15 Determination of PGA value: a g a gr I a gr - referenced peak ground acceleration I - importance factor, determined by the following formula: I k gh T 50 cl 1 3 T cl - the remain life service of the existing structure = T ds the age of the building k gh parameter depending on the applied limit state (recom. k gh =1 ) 15

16 Investigations: - Investigations on the life and imprortance class as well as the present functions of the buildings - Foundations and footings investigations - Structure investigations - Evaluations of the knowledge levels of the existing structures Knowledge level (KL): KL1, KL2 and KL3 Confidence factor CF KL CF KL1 1,35 KL2 1,20 KL3 1,00 16

17 Assessments: Determine the ground type based on TCXDVN 375:2006. Structure modelling analysis: based on the information collected to model structures in which the mean values of the material properties shall be used in the analysis. Earthquake loading and seismic load combinations: following TCXDVN 375:2006. Methods of analysis: Linear methods, non-linear method and q- factor method. Safety check: Ed R d Conclusions of structure assessments. 17

18 Structure retrofitting/interventions: a) Local or overall modification of damaged or undamaged elements considering strength, stiffness and ductility of these elements; b) Adding new structural elements (bracing, brick in fill, RC columns and even shear wall etc.); c) Modification of the structural system (elemination of some structural joints, widening of joints; modification to more regular and more ductile structures etc. ); d) Addition of new structural system to take some or all of the entire seismic loads; e) Change the existing non-structural elements to be structural elements; 18

19 f) Introduction of passive protection devices through either dissipative or base isolation; g) Mass reduction; h) Restriction or change of use of the building; i) Partial demolition; j) Intervention into the footing and foundations if the foundation is not uniform (eg.: building on both pilling and shalow foundations); k) Combinations of the above methods. 19

20 5. Cases of studies Rach Mieu res. building - designed in 1985, located in Phu Nhuan district, HCM city - 1 basement + 16 storeys - Present situations: there are not found any damages. Comments: - The structure used RC frames-shear walls; regular in plane but irregular along the height; - Pile footing; ground type C; FS 2; - Importance factor I = 1,0; - The building has not shown the defects on materials; - Detailing following TCXDVN 375:2006 was not applied; - The structure can sustain the earthquake in HCM city (PGA = 0.08g). 20

21 Block D5, Giang Vo district, Hanoi - 5 storeys and 2 blocks, prefabricated RC building constructed during Existing situations: degrading, construction joints are in corrosion, rebars corrosied and rusted Comments: - This is a prefabricated structures with longitudinal and crossing bearing walls, the ductility is very low; - The regularities in plan and along the height are not satisfied because of modifications, extensions etc.; - Ground type: D, using shallow foundations; - KL1 - limited; - Can not sustain the seismic loads (PGA = 0.1 g). 21

22 Building B3, Nghĩa Tân district, Hanoi - 5 storeys, built in 1982; - Present situation: degradtion. Comments: - Prefabricated structures with bearing walls systems; - Construction joints rae very deteriorated; - There were no records on the buildings (including the structures and foundations). KL is very poor; - Cannot sustain the earthquake with PGA=0.1g. 22

23 THAT ARE THE INFORMATION ON THE ASSESSMENT AND RETROFITTING OF EXISTING RC BUILDINGS IN VIETNAM REGARDING TO EARTHQUAKE RESISTANCES. Thank you very much for your attentions! 23

24 Thank you very much! 24