Technical University of Civil Engineering

124 Lacul Tei Str., sector 2, Bucharest, Romania
Tel: +(40-21)-2421208

Study on seismic performanceof existing buildings in Romania
Authors:
Prof. Dr. Eng. Tudor Postelnicu
Lecturer Eng. Viorel Popa
Assist. Prof. Eng. Dragos Cotofana
Assist. Prof. Eng. Basarab Chesca
Eng. Raluca Ionescu
Assoc. Prof. Dr. Eng. Radu Vacareanu - responsible

Report prepared by: Technical University of Civil Engineering,
Bucharest
Report prepared for: Building Research Institute, Tsukuba, Japan

MARCH 2005

Study on seismic performanceof existing buildings in Romania
Table of Content
1.

Introduction............................................................................................................42.

Current enforced procedure for buildings evaluation............................................8

3.

2.1.

General background .......................................................................................8

2.2.

Enforced evaluation methods.........................................................................8

2.2.1.

Qualitative evaluation method ...............................................................92.2.2.

Quantitative evaluation method .............................................................92.2.3.

Level of seismic protection and seismic risk classes ...........................10

2.2.4.

Annex: seismic design force according to P100/92.............................11

2.3.

Retrofitting solutions ...................................................................................14

2.4.

Conclusions..................................................................................................16

Evaluation methods proposed to be used in Romania .........................................17
3.1.

Performance Objectives ...............................................................................17

3.2.

Level 1 methodology ...................................................................................18

3.2.1.

Principle of the method........................................................................19

3.2.2.

Evaluation of equivalent lateral seismic forces ...................................20

3.2.3.

Computation of the shear stresses in vertical elements .......................21

3.2.4.

Verification conditions.........................................................................22

3.3.

3.3.1.

Principle of the method........................................................................22

3.3.2.

Linear static analysis............................................................................23

3.3.3.

Linear dynamic analysis ......................................................................24

3.3.4.

Verification relations ...........................................................................24

3.4.

4.

Level 2 evaluation methodologies ...............................................................22

Level 3 evaluation methodology..................................................................28

3.4.1.

Principle of the method........................................................................28

3.4.2.

Method based on non-linear static analysis .........................................29

3.4.3.

Method based on the non-linear dynamic analysis ..............................29

3.4.4.

Verification relations ...........................................................................30

Current retrofitting techniques used in Romania.................................................32

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

2

4.1.

Retrofitting of reinforced concrete frame structures....................................32

4.1.1.

Interventions that do not involve the alteration of the structural system
32

4.1.2.

Interventions that involve the transformation of the reinforced concrete

structural frames...................................................................................................37
4.2.

Interventions on reinforced concrete wall structures ...................................40

4.2.1.

Interventions that do not involve the alteration of the structural system
40

4.2.2.

Interventions that involve the transformation of the reinforced concrete

structural walls .....................................................................................................42

5.

4.3.

Interventions for masonry structures ...........................................................44

4.4.

Interventions on slabs ..................................................................................45

Case study of building evaluation and retrofitting ..............................................48
5.1.

Short description of the analyzed building ..................................................48

5.2.

Building behavior during past earthquakes..................................................48

5.3.

Seismic evaluation based on the Romanian P100/92 seismic design code .50

5.4.

Checking of existing structure lateral stiffness............................................51

5.5.

Retrofitting solutions ...................................................................................52

5.5.1.

Retrofitting by reinforced concrete jacketing on the central span .......52

5.5.2.

Retrofitting by introducing reinforced concrete shear walls................56

5.5.3.

Retrofitting by introducing steel braces...............................................58

5.6.

5.6.1.

Retrofitting by beams and columns RC jacketing ...............................61

5.6.2.

Retrofitting by RC walls ......................................................................62

5.6.3.

Retrofitting by steel bracing.................................................................62

5.7.
6.

Comments on the selected solutions ............................................................61

CONCLUSIONS..........................................................................................62

References............................................................................................................63

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

31. Introduction
Romania is one of the earthquake prone countries in the Eastern Europe. The first
generation of Romanian seismic design code was issued in 1963, as P13/63. The
buildings build before the first issue of the seismic design code and also the buildings
designed before 1978, when P100-78 earthquake resistant design code was preparedand enforced, should be evaluated and most of them retrofitted in order to comply
with the current code provisions.
The vulnerable buildings in Romania can be ranked in two major categories:
1. Mid rise and high rise buildings built prior to 1945
2. Mid rise and high rise buildings built after 1945 and prior to March 4, 1977
earthquake
The two categories of vulnerable buildings have very distinctive structural featuresand consequently the approach for improvement of retrofitting techniques andmethods must be different.
Some explanations for each building categories follows.

1. Mid rise and high rise buildings built prior to 1945
No seismic design is incorporated in the structural system of these buildings. The
layout of the buildings and of the structural systems is different from one building tothe other.
There are two building typologies in this category:a. Unreinforced masonry buildings with wooden slabs (most vulnerable) or with
RC/masonry vaults slabs
The main vulnerability of this building typology comes from the lack of any
reinforcement to improve the behavior of masonry walls to lateral loads. The
situation is worsening if the slabs are made of wood. The main failure
mechanisms include in plane shear failure of the walls and/or out of plane
failure of the masonry walls. In 1977 earthquake, out of 28 buildings built
before 1940 collapsed in Bucharest 5 were unreinforced masonry buildings.
The main drawbacks of this building typology are:•Lack of shear strength of structural walls•

Lack of capacity of horizontal structural system to transfer lateral loads

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

4•

Lack of proper connections between the masonry walls in different
directions and between the masonry walls and the slabs.

The usual retrofitting solution consists in the provision of RC jackets on one
side (or both sides) of masonry walls and provision of RC slabs after
demolishing the wooden slabs or provision of RC slabs cast atop the masonryvaults slabs. The new vertical RC elements run along the height of thebuilding and new foundations are provided for them.
b. RC columns and beams
The RC columns and beams are not providing earthquake resistant frames.
This is because of lack of continuity of structural system in vertical andhorizontal directions and because the joints are not able to transfer the lateralloads. The concrete is of very poor quality and strength (<10 MPa) and thegravity axial loads in the lower columns are very high. In 1977 earthquake, outof 28 buildings built before 1940 collapsed in Bucharest 23 were in thisbuilding typology.
The main drawbacks of this building typology are:•Lack of strength (low capacity in shear and bending)•Lack of stiffness for the overall structural system•

Lack of ductility.

The most common retrofitting solution consists in provision of RC jackets forcolumns and for adjacent beams and provision of new RC shear walls. The
new vertical RC elements run along the height of the building and new
foundations are provided for them.
The main disadvantages of the previously presented retrofitting solutions suchas:•Long period of construction•

Major disturbance for the residents of the buildings.

2. Mid rise and high rise buildings built after 1945 and prior to March 4, 1977
earthquake
Some earthquake resistant design was used for the development of these buildings
after 1963 and some ductility rules were enforced and used after 1970. The mainbuilding typologies within this category are:Study on seismic performance evaluation of existing buildings in Bucharest, Romania

5a. RC framesb. RC shear wallsc. RC soft story buildings (more precisely, soft and weak groundfloor
buildings)

a. RC frames
The major vulnerabilities of this building typology come from:•

Insufficient shear capacity in columns•

Insufficient overall stiffness•

Insufficient ductility

The usual retrofitting solution consists in jacketing of RC vertical elements
and, sometimes provision of RC shear walls.
b. RC shear walls
The major vulnerabilities of this building typology come from:•

Insufficient shear capacity•

Insufficient bending capacity (not so often)

The usual retrofitting solution consists in jacketing of RC vertical elements.
c. RC soft and weak groundfloor buildings
This building typology consists of a dual structural system in the verticaldirection, i.e. RC columns in the groundfloor and RC shear walls in the upper
stories. The major vulnerabilities of this building typology come from:•

Concentration of most of seismic lateral displacement in the
groundfloor•

Concentration of most of seismic induced energy in the
groundfloor•

Insufficient ductility for RC columns in the groundfloor•

Insufficient shear capacity for both RC columns and RC upper
shear walls•

Insufficient overall stiffness.

One of the new buildings collapsed in Bucharest during March 4, 1977
earthquake was of this typology.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

6

The usual retrofitting solution for this building typology consists incontinuation of upper RC shear walls in the groundfloor and sometimes in theprovision of new shear walls along the height of the building.
For mid rise and high rise buildings built after 1945 and prior to March 4, 1977
earthquake, new/alternative retrofitting techniques are adequate since the existing
structural system is more regular and more engineered, the quality of the concrete isgood and the seismic capacity is definitely higher than of the buildings built before
1940. The structural system of the buildings built after 1945 and prior to March 4,
1977 earthquake is appropriate to be integrated with the new retrofitting systems suchas steel bracings, exterior mega frames, seismic dampers and so on. This category ofbuildings gives the opportunity to the designer to implement with fewer disturbances
and in a short period of time the modern retrofitting techniques.
Content of the Report
Chapter 2 of this study summarizes the principle and procedures of seismic evaluation
methods of existing buildings in Romania according to chapter 11 of P100-92 code.
In chapter 3 some evaluation methodologies, proposed to be used in Romania, arepresented.
Chapter 4 presents the current retrofitting techniques used in Romania for existing
vulnerable buildings. The most widely used retrofitting techniques consist in inserting
RC elements in the existing structures or members jacketing as it will be detailed in
Chapter 4.
Chapter 5 presents a case study of the evaluation process of one existing building andalso three solutions that may be used to retrofit that structure.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

72. Current enforced procedure for buildings evaluation
2.1.

General backgroundThe existing buildings are subject, according to the legal provisions in force, tosurveys aiming to assess the level of protection against seismic actions. According to
the regulations in force, only certified expert engineers (“technical experts”) assess
the level of protection of the existing buildings. The technical expert is responsible forthe way in which the protection level was evaluated, as well as for the proposeddecision of intervention. The responsibility for the carrying out of the interventions
falls upon the owner(s) of the building.
By their nature, the operations of assessing the protection level of the existingbuildings, of establishing and carrying out the intervention works require higher levelsof education, experience, professional and moral integrity of the engineer who carries
out such operations, than those necessary for the current design of new buildings.
Working in the field of the “building pathology”, the expert engineer has to facecompletely different, unconventional situations generated by the necessity to correctly
understand the notions related to the conception, design, maintenance, preservation,
response to seismic motions or other kinds of actions (gravity actions, soil settlements,
temperature variations, corrosion etc.) of the building.
The enforced provisions concern the investigations to be carried out only between
seismic events.

2.2.

Enforced evaluation methods

The enforced evaluation methods employed in P100/92 in order to asses the level ofprotection against seismic actions of the existing buildings may be classified asfollows:•

E1 – qualitative assessment method;•

E2 – analytical assessment methods (based on calculations); they are divided
into three categories:•

E2a – current calculation methods,•

E2b – static post-elastic calculation methods,•

E2c – dynamic post-elastic calculation methods

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

8As a rule, the combined application of the E1 qualitative method and E2 current
calculation method is compulsory.
When the strength and ductility characteristics of the materials or of the foundationsoil of existing buildings are not available, or if it is necessary to identify the areas
where discontinuities, degradation or uncontrolled bonds have occurred, the expert
may decide whether or not to apply non-destructive tests.

2.2.1. Qualitative evaluation method
The qualitative evaluation is based on the inspection of the building as a whole and indetail and on the examination of:-

the structural and architectural design of the building;-the plotting of the building, its members and details considered significant
for the assessment of the protection level, whenever the original design is
not available or when the building of the structure fails to comply with thedesign, or when the building has suffered structural transformations during
its service life without any purpose-made documentation;-

the designs and documentation on which previous interventions werebased, as well as other information on the history of the structure;-

information on the building behavior during previous seismic events;-

considerations related to the norms on which the design of the building
was based or, if necessary, related to the date practices compared with thepresent specifications in force;-the plotting of the possible damage and deterioration;-

the building as a whole and in detail.

The object of the qualitative assessment is the structural system as well as the nonstructural internal or external members that are likely to cause accidents during
seismic events (partition walls, parapets, ornaments, blind walls, chimneys, etc.).

2.2.2. Quantitative evaluation method
The quantitative evaluation method aims at:-determining the conventional capacity to bear the seismic loads – ”Scap” –of the inspected building;-identifying highly vulnerable members/areas of the structure;

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

9-checking up the compliance with the criteria of ductility and brittle crush
prevention;-

determining the structure’s stiffness to lateral displacement.

2.2.3. Level of seismic protection and seismic risk classes
The value of the nominal level of protection against seismic actions – “R” – isdetermined on the basis of the conventional capacity to bear the seismic load “Scap”;“R” is given by the capacity to demand ratio:
R=

S cap
S required

The conventional capacity to bear the seismic load “Scap” is the value of the seismic
load that leads, together with the gravity loads, to the achievement of the resistancecapacity in the critical sections (areas) of the structure. The critical sections (areas)
shall be indicated by the expert on the basis of engineering judgment.
In order to determine the value of “Srequired” in case of existing buildings, the values ofseismic force reduction factor “Ψ” is established by the technical expert on the basisof the analysis of the characteristics of the building.
Based on the sectional stress under conventional capacity to bear the seismic load
“Scap” the fulfillment of the criteria of ductility and brittle crush prevention shall bechecked up using the procedures for new constructions.
The fulfillment of the conditions of stiffness to lateral displacement shall be checked
up by comparing the structure displacements under the conventional capacity to bear
the seismic load “Scap” with the allowable displacements for new constructions. This
checking has an informative character, aiming to quantify the general performance ofthe structure to seismic events.
The entire survey activity is synthesized by the ranking of the building into classes ofseismic risk taking into account the seismic zone where the building is located and thefollowing criteria regarding the type of structure, the behavior of the building inoperation and under seismic actions:-the category of the structural system;-

the general conformity of the building from the point of view of the
expected seismic response;-

the nominal level of protection against seismic actions (index R)

Study on seismic performance evaluation of existing buildings in Bucharest, Romania10-

the presence of weak zones, from the point of view of the resistancecapacity in relation to the requirements, in the structural members playing
a major role in taking over the seismic strain;-

the probable nature of the yield of the main structural members that arevital for the stability of the building: ductile, with limited ductility, brittle;-

the method of solving the constructive details of sections (for instance: thecross reinforcement with cross-ties in the potential plastic zones,
reinforcement bars’ anchorage, their splicing, etc.);-

the age of the building (the year of erection);-

the number of significant earthquakes to which the building was subject;-

the structural damage experienced after earthquakes;-

the condition of non-structural members;-

the height and the mass of the building, etc.

Four classes of seismic risk are established as regards the seismic risk, i.e. the possible
effects of certain earthquakes, characteristic for the site, on the existing buildings onthat site:
Seismic Risk Class I, RsI, comprising buildings with high risk of collapse incase of earthquakes with intensities corresponding to the design seismic zones (design
seismic rank);
Seismic Risk Class II, RsII, comprising buildings for which the risk ofcollapse is low, but for which major structural damage is expected on the occurrenceof the design seismic rank;
Seismic Risk Class III, RsIII, comprising buildings which are expected tosuffer structural damage which does not significantly affect the structural safety, but
the damage of their non-structural members can be significant;Seismic Risk Class IV, RsIV, comprising buildings for which the expected
seismic response is the same with that of the new buildings, designed on the basis of
the design codes in force.

2.2.4. Annex: seismic design force according to P100/92
In order to assure a better understanding of the seismic evaluation method we provide
here the formula for computing the design shear force for a new building.
The relations further provided are used in determining the equivalent static
loads, used in engineering analysis, required by the common design method. These

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

11

loads take into account, in a simplified and implicit way, the effects of dynamic
behavior and post-elastic deformation phenomena.
The horizontal seismic loads, acting on a structure, are determined for each naturalmode of vibration. If the natural vibrations occur in one plane, the resultant ofhorizontal seismic loads (base shear force), corresponding to the ground motion
direction and to the „r”-the vibration mode, is determined using further relations:S r = cr G where: cr = α ⋅ k s ⋅ β r ⋅ψ ⋅ ε r where:1) cr, the overall seismic coefficient, corresponding to the „r”-the vibration mode;2) G , resultant of gravity loads for the whole building;3) α , importance coefficient of the building according to the classes of importance.
This factor differentiates the protection level of the building depending on theimportance classes, Table1. Based on the performance criteria the buildings are
divided into four importance classes, Table 2.
Table 1: Importance coeficient for classes of importance for buildings
Classes of importanceClass I

I

II

III

IV

1.4

1.2

1.0

0.8

Table 2: Classification of buildings according to importance classes
Buildings of vital social importance, whose functionality during and
immediately after earthquakes should be fully granted:

Class II

Very important buildings requiring a limitation of damage, keeping inview its potential consequences:Class III

Normal importance buildings (not falling into classes I, II, or IV)

Class IV

Reduced importance buildings4) “ks” coefficient represents the ratio between the peak ground acceleration of theseismic motion (with an average recurrence period of about 50 years), corresponding
to the seismic zone, and the gravity acceleration. The “ks” coefficients are supplied inTable 3, according to the seismic zones described on the code map.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

12

Table 3: ks coefficient
Seismic zone
ks
A

0.32

B

0.25

C1

0.20

D

0.16

E

0.12

F

0.08

5) amplification factor, Figure 1 (“βr”) is determined according to the natural
oscillation periods (“Tr”) of the building and to the local seismic conditions
characterized by corner periods (“Tc”) using the following relations:βr = 2.5

forTr ≤ Tc

β r = 2.5 − (Tr − Tc ) ≥ 1 for

Tr > Tc

βr

Tc=1.5 sec

2.5

Tc=1.0 sec
Tc=0.7 sec

1.0

0.7

1.0

1.5

2.2

2.5

3.0

Tr(sec)Figure 1: Amplification spectra6) coefficient of reduction of seismic forces (“ψ”) due to the structural ductility, to theredistribution of the efforts and to the damping effects, other than structural isprovided in the following Table 4.
7) The equivalence coefficient between the actual system and the system with one
dynamic degree of freedom for the vibration mode „r”, „ ε r ” is determined using thefollowing relation;

1

Bucharest belongs to seismic zone C.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

13

⎡n
⎤
⎢∑ Gk ukr ⎥⎦ε = ⎣ k =1n

G ∑ Gk u
k =12

where:

2kr- ukr the „r”-th eigenvector component corresponding to the „k”-th freedom degree;n- Gk the resultant of gravity loads at ‚k” level G = ∑ Gk ;k =1

The „ukr” eigenvectors, as well as the „Tr” natural periods are determined using
structural dynamics methods.
Table 4: The values of ψ coefficient according to structural type
Structure typeψ

coefficient
Reinforced concrete structures
1 Multistory frame structures:- with infill walls designed as structural members............................

0.25- the infill walls are not considered structural members...................

0.20

2 Industrial halls and other one story structures:- with stiff beam-columns joints........................................................

0.15- with hinged joints............................................................................

0.20

3 Buildings with structural walls............................................................

0.25

4 Structures made of walls, columns and flat-slabs (no beams).............

0.30

6 Elevated tanks......................................................................................

0.35

Masonry structures
1 Structures made of masonry structural walls with reinforced
concrete boundary elements (spandrel beams and columns:.................

0.25

2 Structures with plain masonry structural walls...................................

0.30

2.3.

Retrofitting solutions

When substantiating and deciding the intervention, as a rule, two solutions shall be
brought forward: a minimal one and a maximal one.
The minimal solution will focus on preventing the collapse of the building aswell as other phenomena that can cause serious injuries or human life loss in case an

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

14

earthquake occurs that has the same characteristics as the one considered in theseismic design according to the Code P100-92. For this purpose, the intervention will
provide the adoption of all necessary measures that can prevent the total or partial
collapse of the building, as well as the raising of the nominal level of building
protection against seismic actions.
Table 5: Recommended minimum values of the minimal level of protection against
seismic actions Rmin for the existing buildings
Importanceclass of the

I

II

III

IV

0,70

0,60

0,50

0,50

building
Rmin

The retrofitting method shall be substantiated by means of general calculation in
relation to the way the structural system withstands and transfers the loads to theground.
The compatibility of the associated bearing capacity (bending moment-axial force,
bending moment-shear force) shall be checked, as well as the correlation of thestrength capacity of the superstructure, the infrastructure and the foundation soil.
The retrofitting methods shall be applied and the addition of new structural members
shall be done such as to prevent any structural sensitivity (non-uniform distribution of
the stiffness along the horizontal and vertical axes, floor overloading, etc.).
Sudden modifications of the longitudinal sections of the structural members shall be
avoided; they could lead to concentrations of unfavorable internal stresses, such as,
for instance, unfavorable changes of the ratio “bending moment/shear force” or “shortcolumn” effects in the reinforced concrete structures.
This solution is described in the survey report and detailed in sketches and drawings.
The expert engineer (who drew up the survey report and the decision of intervention)
certifies the fact that the intervention works design fully complies with the concept in
the intervention decision. The expert engineer is, finally, responsible for the entire
development of the process of assessment/survey/ choosing the intervention
solution/drawing up the intervention works design including the execution details

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

15

The minimal intervention measure concerns the prevention of the total or partial
collapse of the existing buildings, rather than ensuring functionality similar to that of
new buildings.
The maximal measures are those that ensure – under the specific technical andeconomic conditions of a particular building – the enhancement of the protection levelto attain values similar to or even higher than those provided for the new buildings.

2.4.

Conclusions

The current enforced evaluation procedure gives a lot of decision freedom to theevaluation engineer (technical expert). Although it presents the basic idea ofanalytical evaluation procedures they are not at all detailed in establishing Scap and
Srequired. In the next chapter some evaluation procedures are proposed.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

16

3. Evaluation methods proposed to be used in
Romania
In Romania the evaluation of the building is usually made under the provisions of thecode P100/92. For detailed evaluation procedures Technical University of Civil
Engineering of Bucharest, Reinforced Concrete Department prepared a draft version
of “Methodologies for the seismic assessment of buildings”. This chapter will present
the main features of this document.

3.1.

Performance Objectives

The performance objective is determined by the level of structural and nonstructural
performance of the analyzed construction for a certain level of seismic hazard. The
performance levels describe the expected performance of the building, in terms ofdamage level, economic losses and interruption of its functioning, given a certain
level of seismic hazard.
Association of the performance level with a certain earthquake characterized by acertain mean recurrence interval is made function of category of importance and thebuilding exposure to seismic hazard:•

1st category, corresponds to those buildings whose activity must not beinterrupted for the duration of the earthquake or immediately following it.

In this category there are buildings that ensure essential functions during and after theearthquake.

•

2nd category, generally corresponds to very important buildings requiring adamage limitation•

3rd category, corresponds to current type of constructions•

4th category, corresponds to constructions of small importance and/or low
degree of occupation.

Seismic hazard level depends on the location of the building with respect to theseismic source, on the geological features of the region where the building is situatedas well as on the level of hazard selected for the seismic motion within theperformance objective.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

17

The current methodologies have in view 3 levels of performance or limit states,
namely, Figure 2:
1.

The immediate occupancy performance level, associated to the limit state ofservice (IO).
2.

The life safety performance level (LS).

3.

The collapse prevention performance level, associated to the ultimate limit

state (CP).
For the common buildings it is considered that the life safety limit state covers also
the collapse prevention limit state while the immediate occupancy limit state is
mandatory only for 1st category of buildings.

IO LS CP
Q

Q

Q

∆
a

∆
b

∆
c

Figure 2: Performance levels for ductile and brittle structures
Figure 2: The three limit states and building behavior (ductile/brittle)
The current document classifies the assessment methodologies into 3 categories:•

level 1 methodologies (simplified methodologies);•

level 2 methodologies (usual methodologies for common buildings types);•level 3 methodologies (methodologies used for complex or very importantbuildings and which use advanced methods of investigation).

3.2.

Level 1 methodology

The level 1 methodology is applied to the following building types:•

regular buildings having reinforced concrete frames, with or without infill

masonry walls up to 3 story high, located in seismic areas from D to F (low andaverage seismic activity);

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

18

•buildings with non-reinforced masonry structural walls with concrete slabs up

to 3 story high or with masonry walls reinforced with reinforced concrete cores
located in seismic areas – D ÷ F;•

buildings with reinforced concrete structural walls, made by cast in place orprecasted concrete panels, up to 5 story high, located in any seismic area;•

any type of building located in the F seismic area for any performance level, as

well as for any buildings with any characteristics located in the E seismic area, for the
performance level of life safety.
These conditions of applying the level 1 methodology refer only to categories 3 and 4of importance and exposure to the seismic hazard. Level 1 assessment methodology
may be optionally used for more complex buildings as well or buildings located in ahigh seismic area, with the aim of getting preliminary information.

3.2.1. Principle of the method
Level 1 assessment methodology entails the checking of certain well-conformed
conditions, grouped into several lists of requirements, structural or non-structural,more developed or less extensive, function of the requirement system and the level ofthe performance pursue.
The verification relations (compliance criteria) are expressed in forces. In order to becompared with the capable force values, the values of the forces produced by theconventional seismic force are reduced by coefficients, whose values depend on
behavior of structural elements, ductile or non-ductile. Since the checking of thestructure is done element by element, this approach has the advantage that a structure
is not defined by means of only one reduction coefficient (ψ in the case of the CodeP100/92), but by means of different reduction coefficients for the elements of thestructure, in accordance with the deformation capacity of the element.
In the case of level 1 assessment, this checking is applied only to elements vital forthe stability of the building (columns, walls), by using several values of the ψ
reduction factor considered appropriate by the certified designer. The values of the ψ
reduction factor are established based on the performance level and element type forevery element of the structural system.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

193.2.2. Evaluation of equivalent lateral seismic forces
The evaluation force in a horizontal direction of the building is computed by means of
the following relation:
S = ks β G
where:S= the equivalent lateral force (the pseudo-lateral force);

kS= the coefficient corresponding to the seismic area intensity from P100/92(PGA/g);β= the coefficient of dynamic amplification, according to P100/92;

G= the weight of the entire building. We use the values of the loads for the

special load combination.

The story shear forces
In the case of multistory buildings, the conventional lateral force is distributed on theheight of the building on the basis of the approximate relation:Sj =n+ j Gj
Sn +1 G

where:
Sj= the j storey shear force;n= the total number of storey above ground level;j= the number of storey, up to the level under consideration;

Gj= the total of gravitational loads for the storey above level j;G= the total weight of the building;S= the conventional seismic force;

Fundamental period of vibration
The fundamental period of vibration of the building in the considered directionneeded for the establishment of the values of the β spectral factors is usuallycalculated on the basis of the following expression :T = k T H3 / 4
where:T= fundamental period in seconds;

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

20

kT

= coefficient which has the values 0,07 for structure made of reinforced

concrete frames and 0,045 for structures with reinforced concrete walls and masonrywalls.
For reinforced concrete frame structures with up to 12 stories the approximate
assessment of the fundamental period can be made alternatively with the following
relation:
T = 0,10 n

where n is the number of storey above the base.

3.2.3. Computation of the shear stresses in vertical elements
The average unit stress τm in the columns of the reinforced concrete frames isdetermined by the expression:τm =ψ ⎛ ns ⎞ S j⎟⎜
q ⎜⎝ n s − n c ⎟⎠ A Ss

where:
nS= the total number of columns;

nC= the number of frames in the direction of the calculation;

ASS= the sum of the areas of the sections of all columns at the level underconsideration;
Sj= the shear force at the level under consideration, j;ψ= modification (behavior) factor for the frame structures,

ψ is taken to have the value 0,4 for the LS performance level and 0,55 for the IO
performance level.
The average value τm in the structural walls is calculated with the relation:
τm = ψSj
A Sp

where:
Sj= the storey shear force;ASp= the sum of the net areas (subtracting the openings) of the sections ofstructural walls oriented on the direction of calculation;ψ= behavior factor, Table 6.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

21

Table 6: Behavior factor values for walls structures (ψ)
Type of wall
Performance level
LS

IO

Reinforced concrete

0,25

0,5

Unreinforced masonry

0,50

It is not the case

Masonry reinforced with RC

0,33

0,5

elements

3.2.4. Verification conditions
The verification conditions express the conditions that a structure should respect inorder to be considered safe.
The verification implies checking of the conditions regarding the configuration of thestructure and conditions for the structural system resistant to lateral forces. One of themost important checking for the first level evaluation is the existence of horizontal
load carrying system in the structure. If the evaluation engineer cannot find thehorizontal load carrying system he usually proceeds to the retrofitting solutions rather
than making further analyses.
The average tangential stress in the concrete, determined by previous procedures,
should be lower than 0,8 N/mm2 for the LS and the IO performance objectives.

3.3.

Level 2 evaluation methodologies

Level 2 evaluation methodologies are usually applied to all buildings where level 1
assessment methodology cannot be applied. This methodology exclusively uses
methods of linear, static and dynamic analysis.

3.3.1. Principle of the method
The effects of the earthquake are approximated by a set of conventional forces(pseudo-forces) applied to the building. The size of the lateral forces is established sothat the deformation obtained as a consequence of the linear analysis of the structure
to the action of lateral forces approximates the deformation imposed on the structureby the design earthquake.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

22

S
Sel

∆t

∆

Figure 3: The elastic evaluation of seismic force for the building
The verification relation depends on the ductile or brittle failure of the structuralelement considered with different types of forces (M, N, Q).
Because in the case of ductile failure, the analysis relations indirectly express
deformation conditions, the capable forces must be computed on the basis of the most
likely values of strength, so on the basis of the mean values.
In the case of brittle failure, reaching the strength capacity cannot be admitted. The
checking consists of the comparison of the force resulting under the action ofgravitational and lateral forces, associated to the yielding of ductile structuralelements of the structure, with the minimum element strength. These types ofmembers are called “controlled by forces”.

3.3.2. Linear static analysis
The value of the basic shear force is determined according to the relation:
S = C1C 2 αk s β r ε r G
The α, ks, βr, εr, coefficients are determined in accordance with P100/92 code:α- importance factor of the building (0.8; 1; 1.2; 1.4)

ks

- ratio between peak horizontal ground acceleration (PGA) and gravitational

accelerationβr- dynamic amplification factorεr- equivalence coefficient between real structure and the SDOF structure

C1- coefficient that takes into account the amplifying of local deformation in the

non-linear domain compared to the deformation obtained from the elastic analysis.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

23

This takes into account the large extent of the degradation on certain levels of thestructure and for regular buildings we suggest the value 1.25.
C2- coefficient which takes into account that in the domain T<Tc (Tc, corner

period) inelastic displacements are superior to elastic ones. In the T>Tc domain
inelastic displacements may be approximated by means of the displacement of elastic
systems. The value of the C2 coefficient is suggested along the following relation:⎧2.0 for T < Tc / 3⎪
C 2 = ⎨linear interpolation for the Tc / 3 − Tc domain⎪1 for T > Tc⎩

3.3.3. Linear dynamic analysis
The linear dynamic analysis is recommended in the cases of tall buildings (no. ofstories > 10) or in the case of those buildings that have pronounced irregularities ofthe mass, stiffness or geometry.
In the case of modal analysis, the response of the structure is determined on the basisof elastic spectra of acceleration multiplied by C1 and C2 coefficients, respectively.
The response forces and displacements will be computed as the square root from thesum of the squares of individual spectral response, or by using CQC (complete
quadratic combination) if the period of the modes in the same direction differs by lessthan 25%.
It is advisable for the time-history linear dynamic analysis to be used in the case oftall buildings whose predominant period exceeds the corner period. In this case, thevalues of the forces will be multiplied only by the C1 coefficient.

3.3.4. Verification relations
The structural members are deformation-controlled or force-controlled elements
according to Table 7.
Table 7. Failure types and associated sectional forces
Structural elements

Reinforced

Loading
Deformation-controlled

Force-controlled

Beams

M

Q

Columns

M

Q,N

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

24

concrete frame

Joints-Q

Reinforced concrete walls

M

N, Q

structures

Deformation controlled structural members
The member forces for deformation-controlled elements is determined according to
the relation:
NC = NG ± NS

NC - total generalized force
NG - generalized force due to the gravitational forces from the special load
combination
NS - generalized force resulting from the application of the seismic force defined inprevious paragraphs.
The verification relation is:

N cap ≥ ψ N C
where
Ncap - strength of the structural element. In the case of the forces that trigger ductile
behavior, the strength capacity is computed on the basis of the mean strength of thematerials;
NC - generalized force;Ψ - reduction coefficient owing to the ductile behavior of the element under theaction of the force to be considered, Table 8.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

25

Table 8: The values of the reduction coefficient Ψ
Structural element
LS

IO

Beams
Ductile behaviour1)(p-p’)/pmax2) ≤ 0; Q ≤ 0.5bh 0 R t

0.125

0.35

(p-p’)/pmax2) ≤ 0; Q ≤ 2bh 0 R t

0.25

0.40

(p-p’)/pmax2) ≥ 0.5; Q ≤ 0.5bh 0 R t

0.25

0.40

(p-p’)/pmax2) ≥ 0.5; Q ≤ 2bh 0 R t

0.35

0.60

0.40

0.65

n3) ≤ 0.1

0.15

0.35

n3) ≥ 0.4

0.35

0.65

n3) ≤ 0.1

0.35

0.65

n3) ≥ 0.1

0.5

0.65

ξ4) ≤ 0.1

0.2

0.35

ξ4) ≥ 0.4

0.35

0.65

ξ4) ≤ 0.1

0.35

0.5

ξ4) ≥ 0.4

0.5

0.65

0.4

0.65

Non-ductile behavior
Columns
Ductile behaviour1)

Non-ductile behavior

Structural walls
Ductile behavior

Non-ductile behavior

Structural walls yielding by means of shear force
Notes:
1)

The ductile behavior means that the beam, the column, the structural wall fulfill all

the reinforcement detailing conditions for the designing of new buildings, specific forthese types of structures. Furthermore, all the conditions regarding the developing of afavorable energy dissipation mechanism must be met;2)p

- the reinforcing percentagep’

the compressed reinforcing percentage

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

26

pmax - the maximum reinforcing percentage corresponding to the balance point3)n- the adimensional axial force4)ξ- the adimensional height of the compressed area

Force-controlled members
The member forces that lead to a brittle failure of the structural elements must be
associated with the structural failure mechanism. For the computation of the shear
forces and the axial forces capacity for the structure with reinforced concrete frames
or with structural walls the use of relations for the design of new buildings describedin specific codes is recommended.

N min
≥ NC
cap
min
N cap
- the minimum strength of the structural element. In the case of forces thattrigger brittle failure it is recommended to compute the strength on the basis of thedesign strength of the materials.
NC- the force determined according to previous paragraph

Checking of the deformation for the IO performance level∆ OI =∆s≤ ∆admOI
C1 C 2 C

where:∆ OI- interstory drift at the level corresponding to the conditions of immediate

occupation;∆s- interstory drift at the level computed upon the seismic forceC- reduction factor which takes into account the fact that the earthquake

associated to this performance objective has a mean recurrence interval smaller thanthat corresponding to level LS;∆adm
OI- allowable interstory drift;∆adm
OI = 0.035 Hstory, if the partitions are rigidly connected to the structure;∆adm
OI = 0.1Hstory if the partition deforms independently of the structure.

In the case when the partitions are attached to the structure, the stiffness of thestructural elements corresponding to the non-cracked sections is used in order to take
into account their contribution to the stiffness of the building on the whole. If the infill
Study on seismic performance evaluation of existing buildings in Bucharest, Romania

27

walls deforms independently, the stiffness is taken half of the one corresponding to
the uncracked concrete sections.

Checking of the deformation for the LS performance level∆ LS = ∆ s ≤ ∆adm
LS

where:∆ LS- interstorey drift corresponding to LS limit state;

∆adm
LS

- allowable drift; ∆adm
LS = 0,025Hstory, with the aim of avoiding total breaking

of non-structural elements, thus reducing human life lost risk and limiting structural
damage.
It is admitted to take half of the computed values of the uncracked concrete sections,for all elements.

3.4.

Level 3 evaluation methodology

Although according to the proposed methodologies it is not compulsory to be applied
it is sometimes preferred by the evaluation engineers because it predicts the failure
mechanism of the structure and because of the accuracy of the results.

3.4.1. Principle of the method
Level 3 evaluation methodology uses computation methods that take into account theinelastic behavior of structural elements. This method is recommended in those cases
where level 2 evaluation method has application restrictions and/or when a more
precise analysis of the seismic performance of the structure is needed.
It is necessary that in the case of a level 3 evaluation method the original design of theanalyzed building should be available, due to the need of a much more precise
information of the execution details.
Two analysis methods may be used, namely:• the method based on non-linear static analysis (push-over).
• the method based on non-linear dynamic analysis (time-history).

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

28

3.4.2. Method based on non-linear static analysis
For the application of this method the base shear – lateral displacement curve of thereference point is needed. Any level of the building may be chosen as a reference
point of the structure, but the mass centre at top story of the structure is usuallychosen. The curve is obtained by performing a non-linear static analysis with thesupport of adequate software using fixed distributions of the horizontal forces applied
to the structure.
The target displacement, in case the assessment is performed for the life safety
performance level, is set by using approximant non-elastic displacement spectra
function of the structural period and the strength of the structure.
For the comparison with the seismic requirements by using the response spectra of theseismic response, the values of the base shear – top displacement curve are changed
using a MDOF to SDOF equivalence.
The checking is performed in terms of rotations for ductile behavior elements and interms of forces for brittle behavior elements at the point of target displacement.

S
Seq

∆t

∆

Figure 4: The Push-over curve of a building

3.4.3. Method based on the non-linear dynamic analysis
The method consists of performing a time-history analysis in the non-linear domain.
The seismic action is directly applied, by means of accelerograms, at the base of thestructure. The use of at least 3 significant accelerograms consistent with the site
conditions of the building is needed.
Although it is the most rigorous method from the theoretical point of view, thisapproach has important limitations due especially to the large amount of time required

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

29

for the analysis in the case of big structures, as well as the high level of technicalknowledge needed for design engineers. Since the available structural analysis
programs perform only a plain analysis, the method can not be applied without
additional modifications unless the buildings dominant vibration modes correspond to
the translations in the two main directions of the structure.
This assessment method is suitable only for the buildings with a ductile behavior.

3.4.4. Verification relations

Forces producing ductile failure
The maximum deformation of the element resulting from the non-linear static analysis
should not exceed the limit deformation admitted for the performance levelconsidered:θ adm ≥ θ cwhere:
θadm- allowed plastic rotation admitted of the plastic hinge, see Table 9;θc- plastic rotation resulted from the non-linear analysis.

Table 9: The values of the admitted plastic rotation θadm (radians)
Structural element
LS
OI
Beams
Ductile behaviour1)(p-p’)/pbal2) ≤ 0; Q ≤ 0.5bh 0 R t

0.025

0.005

(p-p’)/pbal ≤ 0; Q ≤ 2bh 0 R t

0.015

0.005

0.02

0.005

0.01

0.005

0.01

0.005

n 3) ≤ 0.1

0.02

0.005

n 3) ≥ 0.4

0.015

0.005

0.01

0.005

(p-p’)/pbal ≥ 0.5; Q ≤ 0.5bh 0 R t(p-p’)/pbal ≥ 0.5; Q ≤ 2bh 0 R t
Non ductile behavior
Columns
Ductile behaviour1)

Non ductile behaviorn3) ≤ 0.1

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

30n3) ≥ 0.4

0.005

0.000

ξ4) ≤ 0.1

0.015

0.005

ξ 4) ≥ 0.4

0.01

0.003

ξ4) ≤ 0.1

0.008

0.002

ξ4) ≥ 0.4

0.005

0.001

Structural walls
Ductile behavior

Non ductile behavior

Notes:
1)

The ductile behavior means that the beam, the column, the structural wall fulfill all

the reinforcement detailing conditions for the designing of new buildings, specific forthese types of structures. Furthermore, all the conditions regarding the developing of afavorable energy dissipation mechanism must be met.
2)p

- the reinforcing percentagep’

the compressed reinforcing percentage

pmax - the maximum reinforcing percentage corresponding to the balance point3)n- the adimensional axial force4)ξ- the adimensional height of the compressed area

Forces producing non-ductile failure
In the case brittle failure the minimum strength of the elements should not be lower
than the forces produced by the seismic action superposed over the gravitational ones.

N min
≥ NC
cap
N min
cap

- minimum strength of the structural element. In the case of forces producing

non-ductile failure it is advisable to compute the capacity using the design stress ofthe material;NC- design force resulting from the non-linear static analysis.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

31

4. Current retrofitting techniques used in Romania
The retrofitting of the reinforced concrete structures usually has the objective of
increasing the strength, the stiffness and/or the capacity of post-elastic deformation of
the existing structural elements (collar beams, columns, joints) or the transformationof the entire structural system.
The improvement of the performances of structural elements in frames is usuallyaccomplished through the jacketing of columns, of the beams and/or of the joints. The
jacketing technique of the columns and of the existing beams in a reinforced concrete
mixture may be applied in order to enhance the stiffness and ductility, with or without
an increase of the bending resistance and/or of the shear force.

4.1.

Retrofitting of reinforced concrete frame structures

4.1.1. Interventions that do not involve the alteration of thestructural system
The enhancement of the structural performances of the reinforced concrete frames
might be obtained through interventions that do not involve an essential change in thecharacteristics of the initial building. These interventions mainly rely on jacketing
techniques of the frames units, which pursue the enhancement in the resistance,
stiffness and/or the capacity of post-elastic deformation of the existing structuralelements.
The technique of jacketing the existent columns and beams with reinforced concrete
represents an advantageous solution, depending on the needs, for increasing either
stiffness only or both resistance and stiffness, and it is frequently used for the
retrofitting of the damaged buildings in Romania. Localized interventions in areas thatexhibit degradations of the elements are considered repairs if their scope is limited to
the restoration of the initial situation without implying an enhancement of the level of
performances.
The interventions may aim at increasing the strength of the elements in terms of shear
capacity, bending moment or axial force, an increase in stiffness or in the post-elastic
deformation capacity.
Structural deficiencies of the reinforcing detailing of concrete sections, having
negative effects on the performance of the frames (insufficient shear reinforcing, lack

Study on seismic performance evaluation of existing buildings in Bucharest, Romania32

of overlapping length, joints without shear reinforcement, etc.) usually require
generalized interventions based on the technique of jacketing the elements.
The intervention solution by jacketing of the frame elements increases also resistanceand stiffness of the substructure with smaller foundations compared to the techniquesof structural walls additions. The jacketing of the elements of reinforced concrete
frames may seriously affect the non-structural elements of the building
The technique of jacketing the existent columns and beams with reinforced concrete
represents an advantageous solution, depending on the needs, for enhancing either
stiffness only or both resistance and stiffness, and it is frequently used for the
retrofitting of the damaged buildings in Romania. Localized interventions in areas thatexhibit degradations of the elements are considered repairs if their scope is limited to
the restoration of the initial situation without implying an enhancement of the level of
performances.

Increasing shear capacity
Increasing the capacity in shear is many times necessary for the columns, beams or
joints of the existent buildings. The capacity in shear is increased mainly throughjacketing of the reinforced concrete elements.

Figure 5: Local intervention details with regard to the capacity in shear of the frame
reinforced concrete beams

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

33

Jacketing, and mainly the reinforced concrete jacketing, leads at the same time to theincrease in rigidity and deformability and, in some cases, also to increasing thecapacity in bending and shear.
In the situation when the concrete from the existing beam does not play an importantpart, it is also necessary to jacket the beams. Generally, in these cases, it is also
necessary to increase the capacity in bending, Figure 5.
Increasing the capable shear force in the columns can be achieved also throughreinforced concrete jacketing, as presented in Figure 6.

Figure 6: Jacketing of the columns

Increasing the capacity in bending
The intervention technique used in most of the cases consists of jacketing thereinforced concrete frame members, this solution ensuring a good cooperation withthe existent element and the possibility to achieve the continuity of the reinforcements.
The capacity in bending and in shear and in axial force can be controlled dependingon the thickness of the jacketing concrete layer, of the quantity and of the position of
the longitudinal and transversal reinforcements.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

34

Figure 7: Beam jacketing for increasing flexural and shear capacity
In the situation when the jacketing of the columns cannot be done on all their sides (asin the case of some columns situated on the facade of the building), the details of
performing the jacketing consider the bonding failure of the sleeve from the existentcolumn. For this purpose, connectors with expanding heads may be used orconnectors glued with epoxy resins that are introduced in the holes drilled in theexistent concrete, Figure 8.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

35

a. Favourable behaviourb. Acceptable solutionc. Incorrect solution

Figure 8: Column jacketing on three sides

Interventions on beam-column joints
The main deficiencies that frame joints can exhibit refer to their low resistance inshear or to the inadequate anchoring of longitudinal reinforcements consisting of theelements that bind together in a joint.
In the case of the retrofitting by concrete jacketing, lateral reinforcement on thevertical line of the joint involves one of the following measures to be taken, Figure 9:- drilling holes through the beams, fixing hoop and injecting the voids;- breaking of the heads of the beams, by adequately supporting them, execution of
the reinforcing of the joints and concreting.

Figure 9: Beam-column joint RC jacketing

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

36

4.1.2. Interventions that involve the transformation of thereinforced concrete structural frames

Introduction of infill structural walls
Filling in the empty spaces of the frames with reinforced masonry walls or reinforced
concrete leads to a significant increase of the stiffness and strength of the structural
system. The second solution involves the attachment of the wall near the beam of theframe, and the columns are coated thus becoming the bulbs of the new wall. The
solution can be applied also without coating of the columns and/or of the beams, if theelements of the frame can take over the efforts which result from the association with
the introduced panels, including the local compression consequences.
If the reinforced concrete frames are completely filled, the specific deformation of theframe can be inhibited through the stiffness of the infill wall. Stiff infill walls (madeof reinforced concrete or of reinforced masonry) will mainly resist to the lateral forcesin the same proportion like the structural walls, while the un-retrofitted frames will
have a relatively low level of loading. The reinforced concrete frames filled with lessrigid walls (un-reinforced masonry) will tend to resist to the lateral forces as systems
with compressed diagonals that are formed in the filling.
The existent frames can be filled with different types of masonry: solid brick,perforated brick etc. These types of fillings can be reinforced, partially reinforced or
un-reinforced. At the same time, the filling may be strengthened with coatings ofconcrete mortar reinforced with wire meshes. These meshes will not be able toprevent the breaking of the filling, but can prevent the unfolding of different parts
from the filling.
The possible situations of intervention on framed structures through building fillingwalls are presented in Figure 10.
The forces between the elements of the existent frame and the concrete or reinforced
masonry infill panel can be taken over by connection elements that are madeaccording to Figure 11. Such measures are necessary especially in those cases when
the filling panel is made of reinforced concrete or of a good reinforced masonry, andin this case the filling frame acts overall as a structural wall.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

37

Filling wall connected only with the frame beams
gapa.

Filling wall connected with the columns
and frame beams
Existent or new jacketed wallb.

Filling wall in window void
new wallc.

existent wall

Figure 10: Filling of the frame openings

Figure 11: Connecting the concrete wall with the frame units

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

38

Introduction of new structural reinforced concrete walls
The addition of reinforced concrete structural walls to the initial framed structure
triggers an important increase of both the strength and the stiffness to lateral drifts. In
this way, the cooperation of the existent frames with the reinforced concrete structural
walls may give the ensemble a specific behavior similar to dual structures or to a thestiff-walled structure, Figure 12.
The structural reinforced concrete walls may be placed both on the exterior, Figure 13
and/or the interior side of the building. The making of interior structural walls
involves some difficulties (such as the change in functioning or of the existentequipments) and higher manufacturing costs.

Exterior reinforced concrete structural
walls

Figure 12: Adding shear walls to an existing RC frame structure

Structural walls andreinforced

concretecores on the exterior

Figure 13: Reinforced concrete structural walls, situated on the exterior part of thebuilding
Figure 14 presents the structural walls that are made by placing the core outside theplane of the beams and attaching it to the coating of the columns, which act as wall

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

39

bulbs. This solution allows for the continuity of the vertical reinforcements and it isuseful when a substantial enhancement of the stiffness capacity of the lateral force isrequired.
By wedging the new elements among the already existent units of the frame during
the process of deformation under seismic action, we can ensure the engagement of thegravitational forces of the structure.
overcasting

existing colum
beam

structural reinforced

structural reinforced

concrete wallconcrete wall

Figure 14: The connection between the existing frame and new added shear wall
In the case of a solution similar to the one described in Figure 13, when the cores areattached to the exterior side of the building, it is necessary to tie the cores to the mainpart of the structure (drifting reinforcements, collectors etc.) that can ensure theentrainment of the building mass by the cores.

4.2.

Interventions

on

reinforced

concretewall

structures
4.2.1. Interventions that do not involve the alteration of thestructural system
The retrofitting of structures with reinforced concrete walls usually has the following
targets:•

Increasing the shear capacity of the walls;•

Increasing the bending capacity;•

Increasing the post-elastic deformation capacity of the walls;•

Increasing the capacity in shear of the connecting beams;

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

40•

Increasing the capacity in bending of the connecting beams;•

Increasing the post-elastic deformation capacity of the connecting beams;

The available techniques for fulfilling the above-mentioned targets are:•Wall thickening by adding several layers of reinforced concrete (for instance,
through injection of concrete) which work together with the existent wall ;•Thickening the connecting beam by adding several layers of reinforced
concrete which are anchored in the existing beam ;•Building reinforced concrete bulbs which are anchored on the cross-sectionsof the wall;•

Re-building the connecting beam with increased concrete sections andreinforcement.

Retrofitting the reinforced concrete walls with the technique of thickening thereinforced concrete cross-section favors the simultaneous achievement of several
different targets, through adequate dimensioning of the longitudinal and transversal
reinforcement and of the thickness of new layer of concrete.

Increasing the shear and flexural capacity of the shear walls
The retrofitting of the reinforced concrete wall structures with the purpose of having
an increased strength in shear for the walls and increased flexural capacity can beachieved through:•Wall thickening by adding several layers of reinforced concrete which work
together with the existent wall, Figure 15;
Existing wall

Mandrelfastenedthroughepoxy resin adhesion
Reinforced concrete wall(coating)

Figure 15. Jacketing of reinforced concrete walls
Figure 15: Wall retrofitting by using thickening
ofofthereinforced
concretecross-section
Study on seismic performanceprocessevaluation
existingbuildings
in Bucharest,
Romania41

•Filling in the empty spaces such as doors or windows with reinforced concrete.

Increasing the of post-elastic deformation capacity of the walls
The retrofitting of the reinforced concrete walls’ structures that targets the increase of
the post-elastic deformation capacity can be achieved through:•Wall thickening by adding several layers of reinforced concrete;•

Placing bulbs in the compressed areas of the cross-section, igure 16.
Brokensectionfrom the existent
Reinforced
concrete bulb
Horizontal
reinforcement in theexistent wall
Figure 16: Wall retrofitting by using bulbs
Generally, connecting the bulb with the wall is done with horizontal reinforcements.
If these are not sufficient and if it is intended not to break the wall on too long
distances, there can be added bolts attached with epoxy resin in holes from theexisting wall. The provision of the bulbs also implies interventions on the foundations.

4.2.2. Interventions that involve the transformation of thereinforced concrete structural walls
The interventions that mean the transformation of the structure with reinforced
concrete walls consisted of:•Introduction of new structural reinforced concrete walls in the existing system;
the procedure is similar to inserting structural wall to reinforced concrete
frames;•Filling in the empty spaces such as doors or windows, Figure 17 and Figure 18.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

42Figure 17: Transformation of structural reinforced concrete wall by filling the empty
spaces such as windows; interventions on masonry structures

Figure 18: Details regarding the process of filling in the empty spaces of the structural
walls with reinforced concrete

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

43

4.3.

Interventions for masonry structures

Unreinforced masonry was used as material for structural walls in pre ‘60s buildings.
The main deficiencies of the structures made of un-reinforced masonry walls are:•Low capacity in shear;•

Low resistance capacity to out of plane forces;•

Weak connections between the perpendicular oriented walls.

Interventions that are used in Romania to remove deficiencies of structures made of
unreinforced masonry are:•Introduction of reinforced concrete elements at appropriate distances;•

Coating masonry walls with reinforced concrete;•

Interlocking the walls arranged on these two directions, using reinforced
concrete elements that are capable to take over the efforts of tension andcompression;•

Introduction of additional interior walls.

Interventions that involve the increase of masonry walls shear
capacity
Failure of the masonry walls can occur in two situations: inclined cracking or joints’
failure. The capacity of unconfined masonry walls can be increased using thefollowing measures:
1. Masonry confinement through adding vertical reinforced concrete
components, situated at distances that take into consideration the level
height, Figure 19.
2. Local reinforcement of the masonry around the larger empty spaces
through the introduction of a consistent reinforced concrete frame,adequately anchored in masonry walls has also the ability to increase
the resistance capacity of the masonry in the panel, Figure 20

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

44Figure 19: Masonry retrofitting by inserting concrete elements

Figure 20: Masonry anchoring to the RC frame

4.4.

Interventions on slabs

The interventions on slabs considered to be horizontal diaphragms can have thefollowing targets:•

Increasing the capacity in shear;•

Increasing the capacity in bending;•

Increasing the strength to failure at the interconnecting areas between verticalelements and floors (the transfer of shear force between vertical elements anddiaphragm);•

Increasing the strength of the slab to local loadings (in case of suspended
loadings);•

Increasing the strength of the slab in the areas with local weakening (areas that
contains openings).

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

45

Increasing the strength of the slabs to shear force is usually achieved byoverconcreting the floors. The connection between overconcreting and the existentslab can be achieved by using one of the techniques presented in Figure 21. The
overconcreting is reinforced with reinforcement meshes and with local, concentrated
reinforcements.

Figure 21: Slab overconcreting
1 – existing floor; 2 – overconcreting; 3 – gravel; 4 – epoxy adhesive; 5 – bolts
attached with epoxy resins; 6 – connectors (metallic profile – angle); 7 – expandable
bolts
Increasing the capacity in bending is achieved through the introduction or increase of
the tensile reinforcement which plays the part of tie rope (truss). Additional necessary
reinforcements can be placed as bars included in over concreting (if the intervention
involves overconcreting), Figure 22.
The weakening of the diaphragm caused by the large openings usually imposesinterventions on the overall floor, Figure 23.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

46

Figure 22: Slab overconcreting with aditional flexural reinforcement

Figure 23: Interventions on horizontal membranes through planting the openings

Study on seismic performance evaluation of existing buildings in Bucharest, Romania47

5. Case study of building evaluation and retrofitting
In this chapter is shown a case study of current Romanian evaluation and retrofitting
technique. The study was conducted based on the design project of a hospitaldesigned and built in 1960.

Given the design procedures at that time it wasconsidered necessary to evaluate the seismic capacity.

5.1.

Short description of the analyzed building

The hospital is in use since 1963. The building is “Y” shaped. It consists out of 10
blocks of approximately 40m, separated by narrow gaps. Only block 2 evaluation andretrofitting is discussed hereinafter.
The building has 1 underground floor, and 10 upper stories. The structural system
consists of moment resisting reinforced concrete frames aligned on two orthogonal
directions. In transversal direction the frames have three spans of 5.80, 2.60 and 3.50,
respectively. In longitudinal direction the spans are uniform at 3.60m, Figure 24,
Figure 25 and Figure 26.
The columns and beams sections have been designed only to accommodate thegravity loads, based on the code available at that time, STAS 1546-56. The columns
and beams sections are gradually reduced over the building height. The main façade
columns have rectangular sections of 80cm height in transversal direction.
The floor slabs are made of reinforced concrete. They are reinforced in both directionsin the outer spans and on a single direction in the central span.
The underground floor is enclosed by a weakly reinforced concrete wall and a general
mat.

5.2.

Building behavior during past earthquakes

The hospital staff reported no major damage of the structural and nonstructuralelements during the past earthquakes. It is worth mentioning that from theconstruction time the building was hit by several Vrancea epicentered earthquakes.
However, the building response to these earthquakes can be considered satisfactory.
The reasons of this behavior will be explained in the following.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

48

Figure 24: Plan view

Figure 25: Transversal section

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

49

Figure 26: Longitudinal Section

5.3.

Seismic

evaluation

based

on

the

Romanian

P100/92 seismic design code
The required seismic force is:
S = cGc = α ks β ε ψ
c – seismic coefficient
α − building importance factor 1.4
ks – seismic intensity factor, depends on the site seismicity 0.12
β − dynamic amplification factor (Tc=0.7), Figure 27

βr = 2.5

forTr ≤ Tc

β r = 2.5 − (Tr − Tc ) ≥ 1 for

Tr > Tc

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

50

βr

Tc=1.5 sec

2.5

Tc=1.0 sec
Tc=0.7 sec

1.0

0.7

1.0

1.5

2.2

2.5

3.0

Tr(sec)Figure 27: Amplification spectraψ − reduction (behavior) factor (0.2)
G – building weight

5.4.

Checking of existing structure lateral stiffness

After the modal analysis the following values have been obtained:• the fundamental vibration period in the longitudinal direction
T1x=

1.75

s

• the fundamental vibration period in the transversal direction
T1y=

1.29

s

The global seismic coefficient in the longitudinal direction:c=

α ks β ε ψc=1.4 * 0.12 * 1.45 * 0.75 * 0.2

=

3.65%

where:

β=2.5-T+Tc

β=

1.45

β=

2.5-T+Tc

β=

1.91

The global seismic coefficient in the transversal direction:c=

α ks β ε ψwhere:

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

51

c=

1.4 * 0.12 * 1.91 * 0.75 * 0.2

=

4.81%

StoreyTable 10: Drift control for existing structure
Direction Level∆X∆/Ψ∗1000∆Y∆/Ψ∗1000

11

38.26

0.000637 3.2

0.000907 4.5

10

34.86

0.001031 5.2

0.001104 5.5

9

31.46

0.001336.7

0.001244 6.2

8

28.06

0.001556 7.8

0.001277 6.4

7

24.66

0.001758 8.8

0.001231 6.2

SEISMX

5.5.

6

/

21.26

0.001792 9.0

0.001197 6.0

5

SEISMY

17.86

0.001861 9.3

0.001174 5.9

4

14.46

0.001949.7

0.001126 5.6

3

11.06

0.001763 8.8

0.001028 5.1

2

7.46

0.001039 5.2

0.000769 3.8

1

3.86

0

0.000401 2.0

0.0

Retrofitting solutions

5.5.1. Retrofitting by reinforced concrete jacketing on thecentral span
(Checking of the building lateral stiffness)

Figure 28: Plan view: RC jacketing

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

52

After the modal analysis the following values have been obtained:• fundamental vibration period in the transversal direction
T1x=

0.78

s

• fundamental vibration period in the longitudinal direction
T1y=

0.97

s

The global seismic coefficient in the transversal direction:c=

α ks β ε ψc=1.4 * 0.12 * 2.42 * 0.75 * 0.2

=

6.10%

where

β=

2.5-T+Tc

β=

2.42

β=

2.5-T+Tc

β=

2.23

The global seismic coefficient in the longitudinal direction:c=

α ks β ε ψc=1.4 * 0.12 * 2.23 * 0.75 * 0.2

=

5.62%

where

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

53

Figure 29: Transversal section: RC jacketing

Table 11: Drift control for reinforced concrete jacketing:
Columns 75x105
Long. beams 60x45transv. Beams 55x75
StoreyLoading

Level ∆ X∆/Ψ*1000∆Y∆/Ψ*100011

38.26 0.000146 0.7

0.000715 3.6

1034.86 0.000233 1.2

0.000731 3.7

931.46 0.000326 1.6

0.000773 3.9828.06 0.000409 2.0

0.000792 4.0

724.66 0.000483 2.4

0.000791 4.0

6

SEISMX /21.26 0.000544 2.7

0.000777 3.9

5

SEISMY

17.86 0.000595 3.0

0.000753 3.8

4

14.46 0.000636 3.2

0.000702 3.5

3

11.06 0.000662 3.3

0.000632 3.2

2

7.46

0.000486 2.4

0.000497 2.5

1

3.86

0.000016 0.1

0.000283 1.4

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

54

Figure 30: Beam jacketing retrofitting solutions

Figure 31: Transversal view: RC Jacketing

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

55

5.5.2. Retrofitting by introducing reinforced concrete shear
walls(Checking of the building lateral stiffness)

Figure 32: Plan view: RC walls
The modal analysis resulted as following:• fundamental vibration in the transversal direction
T1x=

0.79

s

• fundamental vibration in the longitudinal direction
T1y=

0.83

s

Global seismic coefficient in the transversal direction:c=

α ks β ε ψc=1.4 * 0.12 * 2.41 * 0.75 * 0.2

=

5.70%

where

β=

2.5-T+Tc

β=2.41

Global seismic coefficient in the longitudinal direction:

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

56

α ks β ε ψc=where:

β= 2.5-T+Tc
β= 2.37c=

1.4 * 0.12 * 2.37 * 0.75 * 0.2

=

5.84%

Table 12: Drift control for reinforced concrete shear wall
Level

∆X

11

38.26

0.000637 3.2

0.000718 3.6

10

34.86

0.000637 3.2

0.000729 3.6

9

31.46

0.000658 3.3

0.000748

28.06

0.000654 3.3

0.000729 3.6

7

24.66

0.000653 3.3

0.000711 3.6

Story

Load

∆/Ψ*1000∆Y∆/Y*1000

3.7

SEISMX
6/

21.26

0.000636 3.2

0.000679 3.4

5

SEISMY 17.86

0.000587 2.9

0.000636 3.2

4

14.46

0.000521 2.6

0.000566 2.8

3

11.06

0.000431 2.2

0.000455 2.3

2

7.46

0.000291 1.5

0.000321 1.6

1

3.86

0.000005 0.0

0.000161 0.8

Figure 33: Transversal view: RC walls

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

57

Figure 34: Longitudinal view: RC walls

5.5.3. Retrofitting by introducing steel braces(Checking of the building lateral stiffness)

Figure 35: Plan view: Steel braces
After the modal analysis the following results have been obtained:

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

58• fundamental vibration period in the transversal direction
T1x=

1.01

s

• fundamental vibration period in the longitudinal direction
T1y=

1.02

s

The global seismic coefficient in the transversal direction:c=

α ks β ε ψ

β= 2.5-T+Tc

Where:

β= 2.19c=

1.4 * 0.12 * 2.19 * 0.75 * 0.2

=

5.52%

The global seismic coefficient in the longitudinal direction:c=

α ks β ε ψc=1.4 * 0.12 * 2.18 * 0.75 * 0.2

=

5.49%

β= 2.5-T+Tc

Where:

Table 13: Drift control for steel bracing retrofitting∆X∆/Ψ*1000 ∆ Y
Storey Load
Level∆/Ψ*1000

11

38.26

0.000623 3.1

0.000806 4.0

10

34.86

0.000722 3.6

0.000858 4.3

9

31.46

0.000804 4.0

0.000918

28.06

0.000861 4.3

0.000922 4.6

7

24.66

0.000899 4.5

0.000905 4.5

4.6

SEISMX
6/

21.26

0.000896 4.5

0.000877 4.4

5

SEISMY 17.86

0.000873 4.4

0.000839 4.2

4

14.46

0.000823 4.1

0.000776 3.9

3

11.06

0.000756 3.8

0.000659 3.3

2

7.46

0.000541 2.7

0.000506 2.5

1

3.86

0

0.000284 1.4

0.0

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

59

Figure 36: Longitudinal view: RC walls

Figure 37: Transversal view: V braces

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

60

5.6.

Comments on the selected solutions

The seismic evaluation of the existing building underlined the following structural
deficiencies:•

lack of strength for beams and columns•

lack of ductility for beams and columns•

a reduced lateral stiffness of the RC frame•

detailing deficiencies of the reinforcement – insufficient anchorage or splices,
low percentage of transversal reinforcement .

Therefore the retrofitting strategy was selected to accommodate some of the following
requirements:- increase the lateral stiffness of the structure.
- increase the strength of some beams and columns- increase the overall structural strength- increase the ductility of the structural members
Three methods of retrofitting have been proposed and commented hereinafter.

5.6.1. Retrofitting by beams and columns RC jacketingFirst solution consists of RC jacketing of- internal columns- transversal across the central span
- longitudinal beams along the central span
Basically a strong RC frame was intended for the central span. Such a solution meetthe functionality requirements for a hospital, given that most of the working space has
not been affected.
The jacketing width have selected based also on the functionality requirements:- columns with constant section of 105x75 cm- transversal beams of 75x55- longitudinal beams of 45x60
This procedure was used to mitigate the following deficiencies:-

strength-

stiffness-ductility-

reinforcement detailing

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

61

5.6.2. Retrofitting by RC walls
Second solution consists of introduction of RC wall as infilled panels in some of thetransversal and longitudinal frames. The connection between the infilled panels andthe existing frames can be made by chemical anchors.
The new structural system can be considered a dual system since both the RC shear
walls and RC frames will resist together the seismic forces. In the longitudinal walls
door openings had to be introduced in accordance with the functionality of thebuilding.
In this case special care has to be paid to the anchorage of the reinforcement. Local
retrofitting work has to be used to improve the behavior of the anchorages and thereinforcement in the splices regions.

5.6.3. Retrofitting by steel bracing
The third solution consists of introducing steel braces in the frames openings. To meetthe functionality requirements two types of braces have been selected:•

for the transversal direction “X” shaped braces since no doors are needed•

for the longitudinal direction “V” shaped braces to accommodate doors.

The connection between the braces and the existing RC frames is made through alayer of non-shrinkage mortar and by using steel headed anchors on the steel braces
and chemical anchors in the reinforced concrete elements.

5.7.

CONCLUSIONSThis study showed that all the proposed solutions are feasible for the selectedstructure. Advantages and disadvantages can be concluded from the previous
paragraph.
The authors opinion is that RC jacketing of the frames members in the central span(solution 1) is the best option of three presented here. Installing new RC shear walls
or steel braces requires a major intervention in the underground floor and in thefoundation system. Also, the anchorage deficiencies are difficult to be solved withlocal interventions.
However, any retrofitting strategy should take into account the functionality
requirements, the overall intervention costs, the availability of the retrofitting
materials and the technical know-how for design and construction.

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

62

6. References
1. Romanian Code for earthquake resistant design of buildings - P100/92,
MLPAT, 1992, 1996
2. Methodologies for the Seismic Evaluation of Existing Buildings (Draft
Version), Technical University of Civil Engineering Bucharest, Reinforced
Concrete Department, 2001
3. Guidelines For Seismic Rehabilitation (Draft Version), Technical Universityof Civil Engineering of Bucharest, Reinforced Concrete Department, 2001

Study on seismic performance evaluation of existing buildings in Bucharest, Romania

63