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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)
GEM-SEA Workshop on Seismic Vulnerability of Buildings Nanyang Technological University, Singapore
1st July 2013
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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 1900-2006, the historical records show 115 earthquakes with
M=4-4.9, 17 earthquakes with M= 5-5.9 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.
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Seismic design:
- From 1954 - 1976: Most buildings were low-rise and not designed
considering the seismic resistance.
- From 1976 - 1986: 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 1986 - 1997: 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:1991.
2. Earthquake resistance of existing buildings
in Vietnam – general remarks
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- 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 : 2006.
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: 2006. However, through the
investigations conducted in Hanoi, Ho Chi Minh city, Vung Tau and Dien
Bien Phu in 2009 – 2010, it was found that the seismic detailing were
not strictly followed.
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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: 1997.
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(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”.
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(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.
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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;
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(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.
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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.
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(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.
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(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.
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(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.
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Procedure (based on EN 1998-3:2005):
- Determination of the seismic risk at the site (PGA etc.).
- Building and site investigations.
- Assessments.
- Select the intervention solutions.
4. Guidelines for assessment and
retrofitting of existing RC buildings regarding
to earthquake resistances
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Tcl - the remain life service of the existing structure = Tds – the age of
the building
kgh – parameter depending on the applied limit state (recom. kgh =1 )
IgRg aa
3
1
50
clgh
I
Tk
Determination of PGA value:
agR - referenced peak ground acceleration
I - importance factor, determined by the following formula:
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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
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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: dd RE
Conclusions of structure assessments.
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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;
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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.
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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; FS2;
- 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).
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Block D5, Giang Vo district, Hanoi
- 5 storeys and 2 blocks, prefabricated RC building
constructed during 1960-1970
- 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).
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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.
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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!
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Thank you very much !
