1

Characterization and Seismic Assessment of a Mixed

Masonry-Reinforced Concrete Building

João Pedro Silva

Instituto Superior Técnico, Universidade de Lisboa

October 2016

Abstract

‘Placa’ buildings represent a mixed typology constructive transition between the masonry and the

reinforced concrete (RC) buildings of Lisbon’s city, result of the urban expansion on the decade between

30 and 60 of the twentieth century. Due to the fact that they were constructed before the actual seismic

design regulations, it is important to analyze and assess seismically these types of buildings.

From a representative building of this typology, a three-dimensional model in BIM (Building

Information Modelling) representation about the building under consideration and other in 3D GIS (3D

Geographic Information Systems) representation about its geographic context was performed, in order

to improve not only the perception of its geometric characteristics and surrounding area, but also to

provide relevant building’s data storage and accessibility.

Afterwards, it was intended to evaluate the building’s seismic performance through non-linear static

analyses from a structural modeling of it, taking into account the effect of the adjacent buildings and

using the program 3MURI/TREMURI.

Concerning the seismic analysis performed, it can be concluded that the required safety verification for

the pseudo-triangular load pattern was achieved. However, structural safety was not verified for the

longitudinal direction of the building in case of the uniform lateral load distribution. Moreover, visible

significant damages in most reinforced concrete elements for all the cases analysed was presented,

revealing an inappropriate design of these elements for seismic actions and leading to the need of

implementing structural strengthening techniques.

Keywords ‘Placa’ building, ‘Bairro de Alvalade’, Building Information Modelling (BIM), 3D Geographic

Information Systems (3D GIS), Seismic Performance Assessment, Non-Linear Static Analysis

1 Introduction

Lisbon building stock is represented by a large

number of old buildings that shows seismic

vulnerability issues and in need of structural

intervention (Monteiro & Bento, 2012b). Taking

into account the seismic potential associated to

this city and ensuring the structural safety and

also the preservation of this building heritage, it is

important to assess their seismic behavior and, if

needed, to adopt strengthening solutions.

In agreement with Monteiro & Bento (2012a),

‘Placa’ buildings represent a significant stock of

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Lisbon’s city and they are characterized as the

first masonry buildings where the reinforced

concrete structural elements were introduced,

which, in most of the cases, do not fulfill the

necessary specific design features to resist

seismic actions, as the actual earthquake-

resistance’s regulations recommend.

Thus, in this work is intended to structural

characterize and assess the seismic-base

performance of a representative building of

‘Placa’ typology. In order to support this seismic

vulnerability study, it was developed two three-

dimensional models, one in BIM (Building

Information Modeling) representation and other

in 3D GIS (3D geographic information systems)

representation that aim the improvement not only

in terms of visualization of geometric

characteristics and geographical context of the

building, but also providing a database,

management and reading tools of its structural

properties.

Using 3MURI/TREMURI (S.T.A. Data, 2005)

(Lagomarsino et al., 2013) program, static non-

linear analysis was performed to evaluate

the seismic performance of the building under

consideration, from the structural modelling and

after calibration of it, for the specific building and

taking into account the effect of the two adjacent

buildings.

2 Characterization and territorial

context of ‘Placa’ buildings

Lisbon building stock includes different

constructive typologies, derived of structural

proprieties changes throughout the history

of this city. ‘Placa’ buildings represent the end of

the period of “Gaioleiro” buildings, and resulted of

the urban expansion of the city between the

decades of 30 and 60 of the twentieth century

(Milošević et al., 2014). This type of construction

is characterized by the removal of wood structural

elements and adoption of reinforced concrete

elements and is predominantly in two areas of

Lisbon, namely ‘Bairro de Alvalade’ and ‘Bairro

dos Actores’.

2.1 Characterization of ‘Placa’

buildings

‘Placa’ constructive typology shows relevant

geometric and structural changes from ‘Gaioleiro’

buildings. In terms of exterior walls, they maintain

to be made of rubble stone or brick masonry, but

it a replacement of air lime mortar to hydraulic or

cement mortar. The thicknesses are between 0.4

m and 0.7 m and, in some cases, with a decrease

along the height (Milošević et al., 2014).

Regarding the interior walls, there was a

substitution of wood as principal material and use

of concrete blocks or brick masonry, with

thicknesses between 0.15 m and 0.25 m which

may also decrease along the height of the

building (Monteiro & Bento, 2012a).

Foundations are composed of very stiff stone

masonry and with a hydraulic mortar, working as

a continuous wall with a thickness superior to the

walls they support. Some buildings may also

have reinforced concrete foundations (Monteiro

& Bento, 2012a).

As mentioned before, there was an introduction

of reinforced concrete structural elements,

namely concrete slabs with a thickness between

0.07 m and 0.12 m, initially applied in back

balconies and wet areas (kitchens, bathrooms

and balconies) and, later, extended to the whole

floor. Afterwards, it was also implemented in

‘Placa’ building structure, as reinforced concrete

columns and beams (Monteiro & Bento, 2012a).

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Regarding the geometry in the plan, these

buildings have two main configurations: (i)

rectangular and (ii) ‘Rabo de Bacalhau’,

presenting a rectangular format with a salient

shape in the back of the building

(Milošević et al., 2014).

2.2 ‘Bairro de Alvalade’

‘Bairro de Alvalade’ represents the area where

the study building is located. This neighborhood

was built to solve the problem of shortage in

housing supply registered during the second

quarter of the twentieth century, providing, in this

period, 45 000 inhabitants spread over 12 000

homes (Costa, 2010).

The area was divided in eight cells defined by the

main existing road structure, as seen on

Figure 1.

Figure 1 – ‘Bairro de Alvalade’ cells’ division layout (Costa, 2010)

All cells had a habitational purpose, with

affordable rented housing and cell 3 had also a

commercial use, being constructed with buildings

with double function (commercial ground floors

and residential upper floors). The most

representative building’s configuration in plane

for cell 3 is “Rabo de Bacalhau” (Costa, 2010)

and a specific building from this cell was chosen

in order to study its seismic behavior.

2.3 Building’s study

The building under study is the most

representative of the “Placa” buildings, type

‘Rabo de Bacalhau’, in terms of structural

proprieties and geometry. The selected building

is presented in Figure 2 and is located in a block

with buildings of the same type. Due to the fact

that adjacent structures have influence on a

building’s seismic behavior, the analyses were

performed taking into account the presence of the

two buildings adjacent to the study building.

Figure 2 - Study building's front facade

The structural and geometric data about the

building was obtained through consultation of

documents found in the Municipal Archives of

Lisbon of the building under consideration.

Additional information can be found in

Silva (2016).

This building has four floors, where the ground

floor has a commercial purpose and the three

upper floors are for habitational use, without any

major geometric and structural modifications.

The wall foundations are continuous and made of

stiff stone masonry with cement mortar and the

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column foundations are constituted of reinforced

concrete.

The main and back facade walls are

characterized with a reinforced concrete frame

with the empty spaces filled with brick masonry

with 0,40 m of thickness. The side wall is

described as reinforced concrete wall with 0,20 m

and the interior walls with two possible structural

solutions, namely hollow brick and solid brick

masonry, with thicknesses between 0,15 m and

0,25 m.

The stairs and pavement structure are made of

reinforced concrete with 0,10 m of thickness.

A pine wood’s structure with tiles of Marseille type

characterize the roof of the building.

Regarding the existing reinforced concrete

beams and columns, they are placed in the main

and back facades, along the height of the

building, and in the interior of the ground floor.

3 Geometric characterization

In this work, it was adopted two information

representation approaches, namely a 3D

modelling of all the features of the study building

in one single information model through a BIM

representation and also a 3D modelling of the

surrounding e geographic context of the building

under consideration, using a 3D GIS

representation.

3.1 Study Building geometric

characterization

In order to contribute to a better understanding of

the geometric characteristics of the study building

and provide a data base of all the relevant

information of it, a BIM representation of this

structure was developed.

For the execution of the BIM model it was used

the Archicad (Graphisoft, 2015) software and it

was based on the documents collected in the

Lisbon Municipal Archive such as floor plans,

elevations, specific drawings, etc. For the model

conception was followed the procedure

described in Silva (2016)

The Figure 3 presents two views of the BIM

model produced for this study.

Figure 3 - BIM model (a) front facade view and (b) back facade view

Regarding the management and visualization of

the relevant data of the buildings, was attached a

set of links to specific existing objects in the

model, that directs to an online storage and

sharing service, as mentioned with more detail in

Silva (2016).

3.2 Geometric characterization of the

surrounding area

It was intended to study the surrounding area of

the building under consideration, thus a 3D GIS

model through CityEngine (ESRI, 2013) software

was made. The main feature of this program is to

automatically generate numerous buildings, in an

urban area, as 3D elements and with different

possible levels of detail.

(a) (b)

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For the conception of this 3D urban

representation, data collected from Lisbon

Municipal Council, namely geographic data and a

digital elevation model of Lisbon’s city, and,

afterwards, the application of a set of shape

grammar rules to the geographic data was used.

The Figure 4 presents the 3D GIS representation

of the urban area ‘Bairro de Alvalade’, cell 3,

where the building is placed. It is possible to

observe that it was assigned higher levels of

detail to the block where the building belongs, to

enhance the buildings with the same geometric

and structural proprieties.

Figure 4 - 3D GIS model of study area

Regarding the accessibility of the different

attributes linked to each building representation,

the procedure is quite simple and intuitive, as it is

only needed to select the 3D element wanted and

automatically a window appears with all the

existing information related to that building, as

shown in Figure 5.

Figure 5 - Object attribute list

4 Structural modelling

The seismic assessment of the building case

study can be perform with different possibilities of

analyses, such as linear or non-linear, static or

dynamic (CEN, 2010).

For this study the non-linear static analyses were

chosen, which is considerate a suitable case to

model properly the non-linear behaviour of

structures under seismic actions, representing a

simplification of non-linear dynamic analyses, but

with similar results.

The software 3MURI/TREMURI was used to

model and assess the study building’s seismic

behaviour. This program was developed to

perform non-linear analyses of three-dimensional

models of masonry buildings with the possibility

to integrate other types of structural elements

such as reinforced concrete, wood or steel

elements. To take into account the masonry

wall’s behaviour under seismic actions, this

software uses the FME (Frame by Macro

Elements) assuming that masonry walls can be

divided in three distinct elements (macro-

elements): (i) Pier, vertical element that supports

the gravity and seismic load; (ii) Spandrel,

horizontal element defined between two vertically

aligned openings; (iii) Rigid node, element of non-

damaged masonry, confined between piers and

spandrels (S.T.A. Data, 2000).

This equivalent frame idealization of masonry

walls allows an easily integration of reinforced

concrete structural elements, due to the fact that

piers, spandrels and RC elements are modelled

as 2D elements, defined between two end nodes,

with non-linear behaviour, as the Figure 6 shows.

6

Figure 6 - Equivalent Frame Idealization of a mixed reinforced concrete-masonry structure (Cattari &

Lagomarsino, 2013)

About RC elements, the non-linear behaviour is

regarded as elasto-perfectly plastic with limited

resistance and plasticity concentrated at the end-

element (Lagomarsino et., 2008). Shear and

compressive/tensile failures are considered as

brittle failures while combined axial-bending

moment, modelled by plastic hinges at the end of

element, are assumed as ductile failure

(Lagomarsino et al., 2013). Once the collapse is

reached, for both failures, the element influence

to the global strength is only associated to its

capacity of carrying vertical loads, as well as in

the case of masonry elements (Lagomarsino et

al., 2008).

4.1 Numerical modelling and

calibration

It was intended that the computational model

represented all the relevant features of the study

building and the two adjacent buildings, as close

to reality as possible. Therefore, all material and

geometric proprieties of the structural elements

were set and the relevant vertical loads applied

to the structure defined.

Due to the inclination of the ground, the buildings

are not at the same level. However, 3MURI is not

designed to correctly represent the macro-

elements mesh this way; thus it was assumed the

three buildings have the same level of foundation,

representing a possible case in which this may

happen (Figure 7).

Figure 7 - 3MURI/TREMURI model

About the mechanical properties adopted for

each existing material, the values were based on

the results related to experimental tests from

buildings of the same typology, taking into

account the suggestion from Italian code for the

similar typology (NTC, 2008). However, in the

process of calibration of the numerical model,

using some dynamic in situ vibration tests of the

building under consideration as reference, it was

evident that the side walls may not be of

reinforced concrete, due to the high frequencies

of the building shown in the direction of these

walls. Since it was not possible to do in situ

analyses for the building’s characterisation,

existing literature on such buildings was checked

and it is possible that those walls could be made

of concrete blocks instead. For this case, the

modal frequencies were closer to the in situ tests

results, so it was assumed this material for the

side walls in this study.

Table 1 and 2 resumes all the material’s

proprieties adopted for this numerical model.

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Table 1 - Masonry material proprieties

Masonry

Stone Hollow Brick

Solid Brick

Concrete Blocks

fm

(MPa) 2.33 1.66 7.19 5.81

𝜏

(MPa) 0.077 0.277 0.277 0.24

E (GPa)

0.82 2.95 5.73 2.15

G (GPa) 0.27 0.98 1.91 0.89

w (kN/m3) 21 15 18 14

Table 2 - Reinforced concrete material proprieties

Concrete C16/20

Steel S235

fm

(MPa) 32.40 171.10

𝜏

(MPa) - -

E (GPa)

29 210

G (GPa)

12.08 80.77

w (kN/m3)

25 79

𝜐 0.2 -

5 Seismic assessment

The seismic performance-based assessment

comprehends the determination of the

performance point of the building, computed from

the intersection between the capacity curve of the

structure and the seismic demand (in terms of

response spectrum).

As it is described in Standard NP EN 1998-3

(CEN, n.d.), for existing masonry buildings, Limit

State of Significant Damage should be

considered.

5.1 Non-linear static analysis

The non-linear static or pushover analysis,

characterizes the buildings’ seismic resist

through force-displacement curves (capacity

curves). These curves are defined applying

incremental static lateral forces to the structure,

measuring the displacements of a relevant point

of it and the corresponding base shear forces

(S.T.A. Data, 2000)

In this study, these analyses were performed in

TREMURI, for each main direction of the building

(𝑋 and 𝑌 in both directions) considering two load

patterns: (i) uniform, proportional to the mass;

and (ii) pseudo-triangular, proportional to the

product between the mass and height

(CEN, 2010). In Figures 8 and 9, the capacity

curves for 𝑋 and 𝑌, longitudinal and transversal

direction of the building are presented,

respectively.

Figure 8 - Capacity curves for 𝑋 direction

Figure 9 - Capacity curves for 𝑌 direction

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0,005 0,01 0,015 0,02 0,025 0,03

Bas

e Sh

ear

Forc

e (k

N)

Displacement (m)

Uniform X +Triangular X +Uniform X -Triangular X -

0

2000

4000

6000

8000

10000

12000

0 0,005 0,01 0,015 0,02 0,025

Bas

e Sh

ear

Forc

e (k

N)

Displacement (m)

Uniform Y +

Triangular Y +

Uniform Y -

Triangular Y -

8

The ultimate displacement was reached for 20%

decay of the maximum base shear force for all

cases (red cross representation), in accordance

with NP EN 1998-1 (CEN, 2010). From Figures 8

and 9, it is possible to observe that, for all cases,

the uniform load has a higher value of the base

shear force, which means that the pseudo-

triangular load is the most demanding load

pattern. However, in 𝑋 direction, the uniform

distribution leads to a less ductile behaviour, so

was assessed the seismic performance for this

case as well.

Observing Figure 10, it is evident that the 𝑌

direction presents a more rigid and resistant

behaviour than the 𝑋 direction, explained by the

contribution of side walls and some interior walls

without openings aligned along 𝑦, in the contrast

with the large number of walls with openings in 𝑥

direction.

Furthermore, there is an identical behaviour in

terms of ductility for both directions. The fact that

the building is inserted in a block, allows a better

distribution of stresses along 𝑋 direction. In case

of 𝑌 direction, sufficient transfer of inertia forces

between walls was observed, since that these

walls do not have openings.

Figure 10 - Capacity curves for pseudo-triangular pattern

5.2 Target displacement and safety

verification

The N2 Method, described in the NP EN 1998-1

(CEN, 2010) was adopted, to define the target

displacement. The performance point was

obtained through the intersection between the

capacity curve and the seismic response

spectrum, for each case. The seismic action was

defined in correspondence to the building’s site

and regarding the NP EN 1998-1 (CEN, 2010).

Table 3 presents the values of ultimate and the

target displacement obtained for the far-field

seismic action, Type 1 (most demanding

situation), for a multiple degree of freedom

system and for each load pattern. The ultimate

displacements obtained were multiplied by a

factor of ¾, according to the Limit State of

Significant Damages (CEN, 2004).

Table 3 – Seismic Performance Displacements

Pseudo-Triangular Uniform

𝑋 + 𝑋 - 𝑌 + 𝑌 - 𝑋+ 𝑋-

𝒅𝒖(m) 0,019 0,018 0,017 0,018 0,007 0,006

𝒅𝒕(m) 0,011 0,011 0,006 0,008 0,008 0,008

Figure 11 shows the ratio between the ultimate

displacement (𝑑𝑢) and target displacement (𝑑𝑡)

for each case. The safety verification is ensured

when 𝑑𝑢/𝑑𝑡 >1 (Milošević et al., 2014).

Figure 11 - Results from the seismic performance-based assessment

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0,005 0,01 0,015 0,02 0,025 0,03

Bas

e Sh

ear

Forc

e (k

N)

Displacement (m)

Triangular X +

Triangular X -

Triangular Y +

Triangular Y -

0

0,5

1

1,5

2

2,5

3

X + X - Y + Y - X + X -

du/d

t

Pseudo-Triangular Uniform

9

It is possible to conclude that the pseudo-

triangular distribution verifies the safety for all

directions, whereas for the uniform load pattern

this safety criterion is not satisfied.

Although the uniform distribution, in 𝑋 direction,

presents higher base shear forces in relation with

pseudo-triangular distribution (Figure 8), this load

pattern is the most demanding case for this

direction.

In Figure 12 the damage pattern for the ultimate

displacement of the uniform distribution for 𝑋

direction is presented.

Figure 12 - Damage pattern of the ultimate displacement for Uniform Distribution in x direction for

(a) front facade (b) back facade and (c) an exterior wall align along x direction

Observing Figure 12, it is evident the presence of

relevant damages in the structure, specifically

shear collapse in some masonry elements and

flexural damages on all columns on ground level.

This displacement does not constitute a situation

which causes a structural collapse, but it should

be to highlighted the presence of shear collapse

(orange outline) and flexural collapse (red

outline) in the reinforced concrete beams in the

front façade (Figure 12 (a)).

Further analyses have been made, presented on

Silva (2016).

6 Conclusions

‘Placa’ building represents a mixed constructive

typology with masonry and reinforced concrete

structural elements. Due to its widespread

availability and lack of seismic vulnerability

studies about this type of buildings, in this work a

characterization and a seismic performance-

based assessment through non-linear static

analyses of a representative ‘Placa’ building set

within a band of its block was presented.

Regarding the geometric and surrounding area

characterization, it was performed two three-

dimensional models in BIM representation and

3D GIS representation, which improved the

detailed understanding of the geometry

characteristics of the study element and allowed

an integrated management of the building’s

information and its territorial context, through the

possibility to access, manipulate and visualize

the collected data.

The non-linear static analyses were performed

using 3MURI/TREMURI software and the

capacity curves were defined for both main

directions and for uniform and pseudo-triangular

load pattern.

About the results of the seismic assessment, the

safety verification was satisfied for the pseudo-

triangular load pattern, but not for the case of

uniform load distribution in 𝑥 direction for the far

field seismic action.

Observing the damage distribution for the

ultimate displacement of the uniform load pattern

(a)

(b) (c)

10

in 𝑋 direction, it was noticeable the existence of

significant damages and even collapse of

masonry elements and reinforcement concrete

beams. This fact led to the conclusion that there

is an inappropriate design of RC elements to

resist seismic actions and, therefore, a need to

adopt solutions of strengthening and retrofitting

of these elements.

As mentioned before, the structural model

developed does not represent the real existing

situation, because of the building’s foundation

level alignment assumed, which do not consider

the relevant action of the concrete slabs over the

side walls that separate two buildings. Also it

would be beneficial for the improvement of this

study, the execution of in situ material

characterization.

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