seismic upgrade of a

2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT
OF RESIDENCIAS GALILEO
Caracas, Venezuela
Submitted by B.R.S. Ingenieros, C.A.
Abstract
Residencias Galileo is a ten-story building located in Caracas, Venezuela. It has a
basement for parking, one level for office and commercial space and eight levels for
residential apartments. The building was constructed in 1950. Initially, it had been used for
rent for a period of 40 years. Then, it had been abandon until 2006 and finally, the owner
decided to refurbish it for subsequent selling.
At the time the original structure was designed and constructed, seismic provisions in
building codes were underestimated. Current codes have at least triplicate seismic loads. In
addition, the new codes emphasize the need for retrofitting the structure of any building
that has done mayor repairs, refurbishes or modifications in use. This was what motivated
the seismic upgrade of Residencias Galileo.
The project was part of a multidisciplinary work, involving not only the structural
repair and retrofit, but also a complete refurbishing of the building. The project involved a
site inspection, non destructive and destructive tests, dynamic analysis and design of the
rehabilitation.
Lateral stiffness, strength and ductility deficiencies were detected in the original
structure. The main repair and strengthening methods applied were: section enlargement of
some columns and beams, construction of additional reinforced concrete beams,
strengthening of columns and joints using carbon fiber reinforced polymers (CFRP) and
some minor corrosion repairs.
Some adjustments to the retrofitting project were necessary. A challenging team effort,
among the owner, the designer and the contractor, was necessary to deal with the
modification without affecting execution time and costs. Finally, the project was executed
with less than 10% deviation from the original budget cost and the estimated time schedule.
Building restoration proved to be much more profitable and cost effective than demolishing
the old structure and constructing a new equivalent building.
Owner:
GRUPO IMALCA
Project Engineer/Designer:
EDISISMO
Repair Contractor:
B.R.S. INGENIEROS, C.A.
Material Supplier:
SIKA VENEZUELA
2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT
OF RESIDENCIAS GALILEO
Caracas, Venezuela
Residencias Galileo is a ten-story building located in Caracas, Venezuela. It has a
basement for parking, one level for office and commercial space and 8 levels for residential
apartments. The building was constructed in 1950. Initially, it had been used for rent for a
period of 40 years. Then, it had been abandon until 2006 and finally, the owner decided to
refurbish it for subsequent selling.
A condition survey was performed, as well as a project for seismic update of the
structure. The job involved a site inspection, non destructive and destructive tests, dynamic
analysis and design of the rehabilitation.
Lateral stiffness, strength and ductility deficiencies were detected in the original
structure. The main repair and strengthening methods applied were: section enlargement of
selected columns and beams, construction of additional reinforced concrete beams,
strengthening of columns and joints using carbon fiber reinforced polymers (CFRP) and
some minor corrosion repairs.
The Need for Seismic Upgrade
Most of the population of Venezuela lives in regions with a high seismic hazard. Some
recent earthquakes caused major property damages and life loss. The most significant in the
last fifty years have been The Caracas Earthquake (1967) and The Cariaco Earthquake
(1997). The last one, with magnitude Mw=6.9, caused mayor damages in Cariaco town
located 370 km (230 miles) east from Caracas, including the collapse of four school
buildings.
The seismic risk in Venezuela has been recognized in building analysis and design
codes since 1939. Such codes have been continuously improved due to worldwide
experiences on structure’s performance during severe earthquakes. Every new code
incorporates sterner regulations for seismic analysis and structural design.
The current seismic code was enacted after The Cariaco Earthquake in 1997. This code
emphasizes the need for retrofitting the structure of any building that would have mayor
repairs, refurbishes or modifications of its use. This was what motivated the seismic
upgrade of Residencias Galileo.
It is important to highlight that Residencias Galileo building was constructed five
decades ago. Back then, seismic provisions in codes were underestimated in contrast to
current codes, which have at least triplicate seismic loads.
Structural System of the Building Prior to Retrofitting
The original property was a nine-story building, including a basement and eight floors
above the ground level. An additional story was added after retrofitting.
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The building dimensions were 41 m (135 ft) length by 13 m (43 ft) width, considering
its maximum dimensions in plan. The typical floor plan had 480 m2 (5,170 sq-ft) and the
complete building had about 5,000 m2 (53,800 sq-ft).
The main structure comprises reinforced concrete frames in transverse direction and
ribbed slabs with no beams along the longitudinal direction. This was a typical
configuration of many old buildings in Venezuela.
The columns had a 30 cm x 60 cm (12x24 in) cross-section in the first floor, gradually
reducing until 30 cm x 30 cm (12x12 in) at the top of the building. All columns were
oriented in transverse direction.
Transverse beams were 30 cm width x 60 cm depth (12x24 in). The concrete ribbed
slabs were 20 cm (8 in) thick with 10x20 cm (4x8 in) joists along longitudinal direction
spaced at every 50 cm (20 in). Beams and slabs spans varied from 3.35 m to 6.60 m (11 to
22 ft).
Foundation system was located approximately 4.20 m (14 ft) below ground level. It
comprises a one-way 30 cm (12 in) thick slab (longitudinal) and 50 cm x 130 cm (20x51 in)
inverted beams (transverse). The perimeter concrete walls of the basement were 30 cm (12
in) width, connected to the slab-beam system of the foundation.
Condition Survey
A detailed investigation was performed to determine the real condition of the structure.
The survey included reviewing available documentation, on site inspection, non-destructive
and destructive tests and damage evaluation.
The first step was checking the available documentation (original structural drawings,
previous reports, etc.). Afterwards, an in situ inspection was conducted to confirm the
accuracy of these documents.
To evaluate the quality of concrete, a series of tests were performed. First a Schmidt
Hammer Test was done to obtain a preliminary idea of the uniformity and quality of the
concrete in slabs, beams, and columns. Then, core-drill samples were taken in places
selected upon the rebound test results to determine the range of compressive strength in
structural elements. It was established a correlation between compressive strength vs.
rebound results with a high level of significance.
The reinforcing steel layout, size and quality were determined by magnetometric and
direct explorations. Foundations were also explored and a geotechnical survey completed to
evaluate foundation capability.
During the on site inspection, some corrosion damages in slabs and minor shear cracks
in beams were detected and evaluated. Fortunately, the shear cracks were inactive and less
than 0.3 mm width. Those were assumed to be previous seismic damages. Regardless of
damages founded, the structure was in a good condition in contrast to most buildings with
the same construction date.
The results of this survey were used in the foreseen structural analysis and design of
repairing and retrofitting works.
2/4
Structural Analysis and Design
Based upon the results of the previous survey, the structure was modeled using a
computer program. The model considered the real sections of the structural elements
measured during the inspection. For the quality of materials, after a statistical analysis of
test results, there were selected fc=210 Kgf/cm2 (2,990 psi) for the concrete and Fy=2,800
Kgf/cm2 (39,825 psi) for the reinforcing steel.
The dynamic analysis allowed quantifying the lack of stiffness, strength, and ductility
that were previously supposed. Since there were no beams in longitudinal direction, the
excessive flexibility of the structure was of major concern. In transverse direction, there
was also lack of stiffness and the columns showed low strength and ductility.
In its original condition, the structure had a high probability of having serious damages
considering the expected seismic loads. The current seismic loads in Venezuelan codes, for
residential and commercial buildings, are presented as spectral accelerations having 10%
probability of being exceeded in fifty years.
The retrofitting work was designed to: i) increase the stiffness in longitudinal direction
by adding ductile beams; ii) enhance stiffness, strength and ductility in transverse direction
by section enlargement of columns and beams in selected frames; iii) improve overall
ductile behavior of columns, beams and joints (in frames not being enlarged) by shear
strengthening using carbon fiber reinforced polymers. The reinforcement of the structure
allowed gaining an additional story.
Rehabilitation and Retrofitting Works
This project was part of a multidisciplinary work, involving not only the structural
repair and retrofit, but also a complete refurbishing of the building. The time schedule was
closely monitored by weekly site meetings for coordination with all contractors involved in
the rehabilitation. The description of the different repair techniques applied and the benefit
to the overall performance of the structure are as follow:
• Columns and beams strengthening by section enlargement and construction of new
beams: This was the main part of the work. Four frames in transverse direction were
selected to enlarge columns and beams sections. Two frames in longitudinal direction
were chosen for placing new beams. Those enlarged and new elements were designed to
carry most of the lateral stiffness and strength of the structure.
The cross-sections of enlarged and new structural elements are: 50 cm x 100 cm (20x40
in) for columns, 35 cm x 80 cm (14x31 in) for transverse beams and 30 cm x 80 cm
(12x31 in) for longitudinal beams.
The construction process involved: i) vertical and lateral shoring of the building; ii)
drilling and epoxy anchoring of the main new reinforcing steel to the existing
foundations; iii) section enlargement of the foundation in coincidence with reinforced
columns iv) partial concrete removal in columns and beams using low energy chipping
hammers; v) installation of a new reinforcing steel, including anchoring transverse bars
to the existing columns; vi) surface preparation by high pressure water blasting, allowing
the concrete surface to be firm and saturated-dry-surface before adding the new material;
3/4
vii) formwork and placing Portland cement-based mortar and concrete to build up the
new sections. It is important to highlight that magnetometric and direct explorations were
made before any drill to avoid cutting the existing reinforcing steel.
• Shear reinforcement of columns and beam-column joints using CFRP: The carbon fiber
reinforced polymer (CFRP) was presented by the contractor as an alternative for a
previously proposed reinforcement using steel shapes anchored and fixed to the existing
concrete members using epoxy mortar. Using CFRP instead of steel shapes reduced the
time of the work at a competitive cost.
The objective of the shear reinforcement was to provide enough strength as well as
ductility to the structure, favoring flexural behavior of the elements avoiding fragile shear
damages in case of earthquake loads. Possible short column effect was particularly
attended with this method.
To promote technical awareness with the state-of-the-art methods in concrete repair,
guided visits to the work site were organized for civil engineer students. One of the
techniques that was more attractive for students was the use of Fiber Reinforced
Polymers. The use of this system is under development in Venezuela, it was first used in
2001. This technique is gaining recognition and acceptance by other project designers and
contractors, after seven years of sharing worldwide experiences and promoting
investigations at national universities, conferences and guided visits to work sites. The
Residencias Galileo project has contributed in that sense.
• Additional repairs: Some corrosion damages in structural elements were repaired by
partial depth removal of concrete, passivating of corrosion activity of steel and replacing
the concrete section using the dry-packing method. Beams showing minor cracks were
also repaired using CFRP.
Time and Cost Effectiveness
Just before the refurbishing works started, most of the apartments had been already sold
in order to get financial resources, so the apartment prices were previously established. Due
to that, this project was financially tight.
Even though it was performed a careful condition survey, some adjustments to the
retrofitting project were necessary. A challenging team effort, among the owner, the
designer and the contractor, was necessary to deal with the modification without affecting
execution time and costs.
Finally, the project was executed with less than 10% deviation from the original budget
cost and the estimated time schedule. The work ended up in a reasonable business for the
building owner and a safe place for the buyers.
Building restoration proved to be much more profitable and cost effective than
demolishing the old structure and constructing a new equivalent building.
4/4
2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT OF RESIDENCIAS GALILEO
Fig. 1: Building before retrofitting
Concrete frames in transverse direction and absence of beams in longitudinal direction
Fig 2: Building appearance after refurbishment
2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT OF RESIDENCIAS GALILEO
Fig.3: Foundation reinforcement for section enlarged columns
Fig 4: Partial concrete removal for placing new beams using low energy chipping hammers
2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT OF RESIDENCIAS GALILEO
Fig 5: Drilling columns for placing the new reinforcing steel of beams
Fig 6: Surface preparation of section enlarged columns
2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT OF RESIDENCIAS GALILEO
Fig 7: Shear reinforcement using CFRP in columns not to be section enlarged
Fig 8: Civil engineer students in a guided visit to the job site
2008 ICRI PROJECT AWARDS
REHABILITATION AND SEISMIC RETROFIT OF RESIDENCIAS GALILEO
Figure 9: Detail of new steel reinforcement in section enlarged columns and beams
Fig 10: Retrofitting in progress