Struetural Analysis of Historieal Construetions - Modena, Lourenço & Roea (eds)
© 2005 Taylor & Franeis Group, London, ISBN 04 1536 379 9
Masonry strengthening by metal tie-bars, a case study
G. Spina, F. Ramundo & A. Mandara
Department ofCivil Engineering, Second University of Naples, Aversa, Italy
ABSTRACT: The requirement of preservation of aesthetical and architectural characteristics of historical
naked masonry walls implies the need to define compatible intervention techniques. To this purpose, a traditional
strengthening practice, applied by means of horizontal and vertical metal tie-bars inserted into masonry panels
is analyzed in the paper. The aim ofthe method is to ensure a global box-type behavior ofthe structural complex,
in order to avoid ali the collapse mechanisms due to turnover or rotation of walls. The case study deals with a
residential building belonging to a historical brick factory placed in Campobasso (Italy), whose skeleton inc1udes
calcareous stone walls and brick columns. The above technique has been proposed for improving the building
behavior under seismic actions. The results show that a significant performance upgrade in both strength and
ductility can be achieved.
fNTRODUCTION
Masonry structures represent the most of historical
Italian construction heritage; they present different
characteristics in each geographical area, depending
on the quality and shape of the constitutive material, execution techniques, mortar function, so that
the mechanical behavior, greatly influenced by these
features, exhibits sensible differences in each particular case. In the south-centre Apennine area traditional
masonry is made of ca1careous stones of different size,
almost knobble or rough-shaped, sometimes chaotically arranged, connected by low quality lime mortar.
Recent seismic events have emphasized the vulnerability of this rubblework, due to the lack of internai
cohesion, as well as to the low effectiveness of both
panels and floor-to-panel connections.
In the recent past, improving the safety of these
structures has represented a priority need that often
involved the execution of interventions that changed
radically the identity of the original strucnlre and
sometimes proved their incompatibility and ineffectiveness. On the other hand, the need of absolute
conservation of historical constructions can represent an obstac1e for the achievement of a satisfactory
safety levei; so it is necessary to develop an operating
method capable of satisfying both the needs of safety
and conservation. For this reason, a correct intervention methodology has to be set out preserving the
original character of the strUCture and eliminating, at
the same time, the intrinsic causes of vulnerability.
As a consequence, techniques compatible with both
original materiais and aesthetical and architectural
characteristics have to be developed.
The case study dealt with in this paper refers to
a residential building placed within a historical brick
factory located in Campobasso (Italy). The building
behavior under seismic actions has been analyzed
by non linear static analysis in order to evaluate the
improvement involved by the use of both vertical and
horizontal ties. Results show that a significant performance upgrade in both strength and ductility can be
obtained.
2
POSSIBLE COLLAPSE MECHANISMS
OF MASONRY BUILDfNGS
Seismic action causes the building to be loaded by horizontal forces which in most cases have not been taken
into account in designo In certain circumstances, this
can produces damage that can develop until collapse.
Failures in masonry constrllctions can be classified
into two main categories: (I) out -of-plane mechanism,
that shows up with turnover of panels (Figure I) or
local buckling of compressed members with material ejection (Figure 2); (2) in-plane mechanism, with
local cracking and overall wall rotation or large cracks
spread ali over the panel (Figure 3). The onset of a
given mechanism depends on many factors, among
which the cOlmection effectiveness between panels or
between floors and panels, as well as the stonework
arrangement and material qllality play a major role.
1207
Figure 1.
1993).
Out-of-plane collapse mechanism (Giuffre,
Figure 3.
In-plane collapse mechanism (Giuffre, 1993 ).
can take place because of the combination between
excessive verticalloads and horizontal seismie action.
These eollapse meehanisms are the most dangerous,
as they can produee the destruetion ofthe whole building as a eonsequence ofthe loss ofstability. In addition,
they are not able to dissipate any input seismie energy.
They can be avoided earrying out interventions that
make the panels and floor-to-panel connection more
effective and increase the internai transverse eohesion
of the walls.
2.2
Figure 2. Buckling of a masonry layer and ejection of
material (Giuffre, 1993).
2. 1 Out-of-plane collapse mechanism
Current historical masonry buildings often lack care
in the execution of effective structural connection
between vertical panels or between panels and floors.
This usually involves lack of an efficient constraint to
out-of-plane kinematic motion ofpanels.
The first effect of earthquake on this type of structures is the separation between panels and, as consequence, their turnover; the collapse can be either
global, if the connection is completely ineffective, or
local when the lack of link is localized in some parts
of the structure, only.
Another kind of out-of-plane collapse can be activated by the lack of a transverse monolithic wall.
This behavior takes place in panels made of pebble
masonry or two-leafmasonry with or without an interposed incoherent filling. In these cases a buckling of
the externai leaf with ejection of compressed stones
In-plane collapse mechanism
Once that the out-of-plane eollapse meehanisms have
been prevented, the structure reaction to the seismie action is entrusted to the in-plane masonry panel
strength. Compared to the out-of-plane resistance, this
is much higher beca use the wall is loaded in the plane
ofmaximum stiffness. The overcoming ofthe in-plane
wall resistance gives place to a type of damage consisting of diagonal cracks spreading throughout the
panels.
When this kind of damage embraces the whole
masonry panel without loss of stability, a significant
energy dissipation can be obtained, achieving the most
ductile collapse meehanism for masonry struetures.
There are two different types of in-plane eollapse
of masonry walls: the first one concerns rotation phenomena, the seeond shear failure. In the first case, if
stonework has a good internai cohesion, the rotation
of the element is preceded by a local cracking that
allows the formation of a hinge at the base of the wall
(Figure 4, left). Otherwise, the rotation involves only
a part of the element, whose contour depends on the
stonework arrangement (Figure 4, right). This kind of
local collapse has to be avoided because of its very
1208
1
lU llllll l!l lU Jlllll lU jj ll!ll J! J! J!J JJ JJ
- -
I \ \
Figure 5. Section and plan ofthe tie bars constraint against
seismic actions.
Figure 4. Left: failure with rotation and local cracking in
cohesive masonry; right: failure with rotation in not cohesive
masonry (Giuffre, 1993).
reduced amount of dissipated energy, due to cracking
mostly produced by tension.
Shear failure mechanisms exhibit higher performance in terms of dissipated energy. They can be
distinguished according to whether horizontal slipping, especially in elements subjected to low vertical
loads, or diagonal cracks ali over the element height
occur. The latter collapse mechanism takes place when
masonry elements can behave like compressed sloped
struts and failure starts when the tension resistance in
the orthogonal direction is overcome.
3
3.1
THE USE OF METAL TIE-BARS IN
MASONRY STRENGTHENING
Figure 6. Plan, section and front view of typical tie-bars
and end plate.
structure and let the failure take place with large energy
dissipation.
General features
The use of metal tie-bars represents a simple, old
and widespread intervention technique, mostly used
to eliminate the horizontal thrust of arches, vaults and
roofs. It is particularly suitable in the cases ofnot effective connection between walls or between walls and
floors.
By means oftie-bars it is possible to obtain an better connection between structural elements at the floor
levei, ensuring a box-type behavior ofthe entire structure. If properly applied, this technique also allows
to avoid ali the out-of-plane turnover mechanisms of
masonry walls (Figure 5).
As a further option respect to traditional horizontal ties, strengthening by vertical tie-bars is gaining
a greater popularity in recent applications. Vertical
ties have the same effectiveness in avoiding every inplane rotation ofmasonry elements, as horizontal ties
have in preventing out-of-plane wall displacements.
In particular, they cause the resistance of the structure under seismic action to be effectively entrusted to
shear walls.
The combined use of horizontal and vertical metal
tie-bars noticeably increase the resistance ofthe entire
3.2
Execution techniques
Some considerations on material durability tum out
necessary about the use of steel in masonry retrofit
interventions, because carrying out maintenance operations is often very difficult or even impossible. [n this
view, it is fundamental to protect the metal element
against corrosion by means of a suitable covering or
galvanization zinc plating or, in extreme cases, using
stainless steel elements. Depending on constructional
needs, the tie-bars can be installed inside or outside
of masonry elements. In the first case the housing is
realized drilling the wall , whereas in the second case
they are placed near the walls or in grooves cut on the
wall surface. The anchorage is guaranteed by metal or
concrete end plate that allow the pre-stressing of the
bars, toa (Figure 6).
4
4.1
THE CASE STUDY
The si/e
The studied building belongs to a dismantled industrial complex located in an outlying zone next to the
1209
Figure 9. North elevation of the former boss pa lace according to design plan.
Figure 7.
The main entrance ofthe factory.
Figure 8.
lron and Iittle brick vaults floor.
interposed little brick va ults (Figure 8), the roof is
made of wooden beams and boarding, covered by
plane tiles. The "L shape" appendix is built with naked
brick masonry and iron beams with interposed hollow
brick fioors.
The refurbishment project proposed for the entire
factory aims at creating a multifunctional complex
including conference, business and meeting centre,
expositions, industrial archeology museum, hotel,
restaurant and swimming pool area. According to the
design plan, the ground fioor ofthe forme r boss palace
will be used as reception and hall of the entire complex, whereas some meeting rooms will be placed on
the first floor.
The existing roof structure will be replaced with a
new bearing skeleton made of glued laminated timber beams with interposed plane bricks. In addition,
walled arches at the ground fioor will be opened in
order to allow for a new internai distribution (Figure 9) .
trading estate of Campobasso, south centre of ltaly
(Figure 7). The factory was used from 1899 to 1986 for
the production ofbrick and calcined gypsum and represented one ofthe most important industrial plant of
the region. The whole complex extends on a 21400 m2
land surface, 6850 m2 ofwhich roof covered. Tt is made
of different buildings, both of industrial and dwelling
type, comprising a brick kiln, driers, warehouses, an
artistic workshop, an engine room and some residential constructions like the boss palace and the workers '
houses.
The structure analyzed in this work is the palace,
that develops on a rectangular plant with a "L shape"
appendix added afier the construction of the first set
up. The rectangular part is organized on two leveis,
of which the ground fioor was used as brick drier
room and the first fioor as residence. The two leveis
are connected by two stairs. The added part inc1udes
ground and mezzanine fioor and is covered with a
terrace.
The bearing structure is made of plastered calcareous rubble-work walls in the externai zones and
brick-work pillars in the central part; the pillars support brick arches (now walled) along the externai
facades and iron and brick beams along the central
alignment. The first floor is made of iron beams with
4.2
The design 01strengthening intervention
Different types of intervention are foreseen to satisfy safety requirements, namely the strengthening
and stiffening of the fioors with the execution of a
reinforced concrete slab, the casting of a reinforced
concrete top beam, the insertion of horizontal and
vertical tie-bars in masonry walls.
The tie-bars intervention technique is based on the
execution of horizontal and vertical drilled holes of
about 60 mm diameter. Metal bars are inserted in the
holes, which are then injected by cement grout. The
procedure foresees before the injection the holes to be
washed by metal nozzle in order to eliminate powder
and soak the masonry, so as to favor the setting of
the mixture. The use of corrugated steel bars turns out
more appropriate, in order to increase friction. Each
bar should be zinc coated and properly centered in the
holes in order to ensure the highest protection against
corrosion. The hori zontal tie bars are disposed at the
first fioor levei into perimeter walls, whereas the vertical ones are mostly concentrated at the ends of each
masonry panel between openings (Figures 10 and I I)
and develop continuously from the foundation to the
top of the building where possible (Figure 12).
1210
::::_ ::::m:::Dl:::::m:::EIl::::mtt::::m: : :' n.
R.
.
.
"
"
"
"
.
::::Jíl[~::m~::~::m~::m<!::m~::~::"
Table I. Mechanical characteristics of cakareous stone
masonry.
.
"
~
U":
. .
Modulus of elasticity E
Poi sson 's ratio
Density
Friction angle
Cohes ion
II
.
k:m--B--m_---_m_---_m_m~--~--.
____ ..-w ___
~
___
~
___ ..l:AMI ___ ..-w ___ ..LW:1. __ LlM:L __
Figure 10. Plan ofthe ground floor with localization ofthe
interventions by vertical metal tie-bars.
fJT~~~~~~0n
~
::
::
~
U::EE::=::=::=::=::=::EIEI::l,:J
Figure 11 . Plan of the first floor with locali zati on of the
interventions by vertical metal tie-bars.
5
Location of metal tie-bars into a masonry pane\.
SEISMIC ANALYSIS OF THE STRUCTURE
The seismic study of the structure was carried out by
the non-linear static analysis method. The obtained
results and the following check of bearing elements
put into evidence the need of strengthening inlerventions aimed at increasing lhe resistance of lhe walls
against horizontal actions. After designing the required
interventions a careful assessment ofthe performance
improvement was carried out by means of a refined
FE.M. non-linear static analysis of the more stressed
masonry wall.
5.1
Structural modeling and seismic input
The building examined is placed in Campobasso, town
belonging to the second category zone according to the
Un i!
660
0, 15
2400
30°
0, 1
MPa
kg/m 3
MPa
new Italian seismic territory classification. A reference
value ofPOA = 0.25g is allowed for this category. The
soil where the building is founded belongs to category
D. For a fundamental vi bration period TI = 0.22 s, this
leads to an acceleration value Se = 0.844g obtained
from the elastic spectrum given in the annex 2 of the
seismic code (Ordinanza PC.M. n. 327420/03/2003).
The analysis ofthe structure was carried out by considering a rigid floor 3-D model. Such constraint is
ensured by the reali zation ofa 50 mm lhick reinforcedconcrete slab, appropriately anchored to the walls.
8ased on the results of the analysis, the structural
elements prior to stre ngthening result inadequate to
satisfy both Ultimate Limit State and Damage Limitation State as defined in the code. This led to design
a reinforcing intervention based on the use of vertical
16 mm diameter tie-bars arranged in the extreme parts
of each masonry panel and horizontal 24 mm diameter
tie-bar in the bearing walls at the first floor. Eventually, a reinforced-concrete top beam has been fitted at
the roof leveI ofthe building.
5.2
Figure 12.
Value
No n-linear static analysis of a masomy pane!
The wall more stressed by seismic action was the
one indicated by logo I Y, represented in figure 12.
A non-linear static analysis before and after the intervention was carried out for this wall , based on a 2-D
FE.M. model ofthe wall running on the code Slraus 7.
The masonry elements were reproduced by 8 node
plate elements with membrane behavior, whereas the
non-linear response of the material was interpreted
considering the Drucker-Prager failure criterion. The
parameters used to represent calcareous stone masonry
are reported in Table 1.
The seismic action was applied through hori zontal
forces distributed at floor leveis according to a preliminary linear analysis ofthe entire structure, which
led to evaluate the ratio between the intensity ofthese
distributed actions.
The analysis was carried out by applying first the
vertical service load according the seismic code. Then,
the seismic action was applied in a step-by-step procedure. In the case of not reinforced wall the elastic
limit is reached for a value of the seismic load facto r
a y = 0.225Se , corresponding to an acceleration value
of 0.190g . The structural collapse occurs for a value
1211
LOId Factor'h Olsplace me nl
"
.-/'
./
.
"
/'
/
"
/'
/
/
r--
>,10
>."
,00
<.:o
I~
>,"
<!lO
t ,"
'.00
'Sl
•.m
' ..... ot>!K_ ont lnlml
Figure 13. Se ism ic load factor vs top di splace ment,
w ith left-ri ght direction of se ismic acti on (a c =
0,250 - a y = 0,235).
Figure 16. Collapse of the wa ll without tie-bars under
right-I eft se ismic acti on.
Load Faclo. Ve OI.plltCe ment
~
.
"
"
"
.~
1/
otJJ
/
' ,!lO
/
1,00
/
/
t loO
/
1,1)11
/
l,loO
..--
./""
3,00
l ,!IO
' 00
' .loO
' OIIcNopIK_t-1
Figure 14. Seismic load factor vs top displacement,
with right-Ieft di rection of seismic action (a c =
0,245 - a y = 0,225).
Figure 15. Co llapse of the wall without tie-bars under
left-right seismic action.
of th e load factor a c equal to 0.245Se , corresponding
to an acceleration of 0.206g (Figures 13 and 14). The
ratio between these two factors is ac / a y = 1.09, showing that a very poor extra-resistance beyond the elasti c
limit is avai lable. The collapsing wall is represented
in Figures 15 and 16, where it is possi ble to observe
that the fai lure of masonry occurs along sloped sliding
planes.
The ana lysis carried out on the wa ll re inforced by
vertical and horizontal tie-bars proved a signifi cant
Figure 17. The strengthened wall under left-right seismic
action.
Figure 18.
action.
The strengthened wa ll under right-Ieft seism ic
increasing ofits resistance. The wall can bear horizontal actions greater tha n the ones given in the seismic
code, corresponding to Se = 0.844g. This is possible
because of the optimal exploitation of the resistant
mechan ism in the masonry wall. The masonry panel
arranges itselflike a series of compressed sloped struts
(Figures 17 and 18), which can etfectively work due
to the confi ning action of the tie-bars that avoids the
risk of turnover and sliding collapse. As a resul t, the
1212
wall can resist to seismic loads until the struts do not
collapse by crushing.
6
CONCLUSIONS
Typical collapse mechanisms of masonry structures
have been presented in the work, emphasizing that the
most frequent are the ones activated by loss of stability
under wall turnover or buckling.
The strengthening technique examined in the paper
is the most ancient used for masonry buildings and, for
this reason, its reliability is largely proved. The work
has demonstrated, by means of a case study, that the
more dangerous collapse mechanisms can be avoided
bya proper design oftie-bars. Also, the masonry wall
resistance can be largely increased by the insta lIation of both horizontal and vertical tie-bars suitably
arranged into the panels .
Another benefit ofthis intervention is the very low
visual impact that turns out very appropriate for preserving the aesthetical characteristics of walls. This
results in this technique to be very convenient when
operating on historical and monumental buildings.
REFERENCES
1999. Manuale per la riabilitazione e la
ricostruzione postsismica degli edifici. Regione
dell'Umbria. Roma: DEI.
AA.VV
Benedetti, D. & Tomazevic, M. 1984. Sulla verifica sismica
di costruzioni in muratura. Ingegneria Sismica, vo/. /.
Beolchini, G. C., Grillo, E, Ricciardulli , G. & Valente, G.
1996. Comportamento di una pare te in muratura di
pietrame rinforzata con iniezioni diffuse di malta di
cemento soggetta a forze cicliche nel piano. La muratura
tra teoria e progeflo. Messina , 18-20 sellembre 1996.
Bologna: Pitagora.
Calderoni , B., Marone, P. & Pagano, M. 1987. Modelli per la
verifica statica degli edifici in muratura in zona sismica.
Ingegneria sismica n. 3.
Ca lderoni, B., Lenza, P. & Pagano, M. 1989. Attuali prospettive per I'analisi sismica non lineare di edifici in muratura.
40 Congresso Nazionale ANIDIS. Milano.
Como, M. & Grimaldi, A. 1985. An unilateral model for the
limit analysis of l11a sonry walls. Internatiollal Congress
on Unilateral Problems in Strllcrural Analysis. Ravello:
Springer.
Gambarotta, L. & Lagomarsino, S. 1996. Sulla ri sposta
dinamica di pareti in muratura . La Meccanica del/e Murature tra Teoria e Progeflo. Messina, 18- 20 setlembre 1996.
Bologna: Pitagora.
Giuffré, A. (ed) 1993. Sicurezza e conservazione dei centri
storici. fi caso Ortigia. Bari: Laterza.
Giuffré, A., & Carocci, C. (eds) 1999. Codice di pratica per la
sicurezza e la conservazione dei centro storico di Palermo.
Bari: Laterza.
Heyman, 1. 1997. Coulomb memoir on statics. An essay in
the history orcivil engineering. London: Imperial College
Press.
s
Lagol11arsino, S., Podestà, S., Risemini , S., Eva, C.,
Frisenda, M., Spallarossa, D. & Bindi, D. 2004. Terremoto
dei Molise: correlazione tra il danno agli edifici monumentali e le caratteristiche dello scuotimento sismico. Xl
Congresso Nazionale ANIDIS. Genova.
Liberatore, D. 2000. Prageflo Catania. Indagine sul/a
risposta sismica di due edifici in mura/ura. CNR-Gruppo
Nazionale per la Difesa dai Terremoti.
Magenes, G. & Calvi , G.M. 1996. Prospettive per la ca librazione di metodi semplificati per I'analisi sismica di
pareti murarie . La Meccanica del/e Murature tra Teoria e Progetto. Messina, 18- 20 setlembre 1996. Bologna:
Pitagora.
Magenes, G. , Bolognini, D. & Braggio, C. 2000. MefOdi
semplificati per I 'analisi sismica non Iineare di edifici
in muratura. CNR-Gruppo Nazionale per la Difesa dai
Terremoti.
Mandara, A. 1992. L'uso dell'acciaio nel consolidamento di
elementi murari verticali. L'Edilizia li. De Lettera.
Mandara, A. & Mazzolani , EM. 1998. Confining ofmasonry
walls with steel elements. Proc. ofthelnternationallABSE
Conference on Savings Buildings in Central and Eastern
Europe. Berlin.
Mandara, A. 2002. Strengthening Techniques for Buildings,
in Refurbishment of Buildings and Bridges In EM.
Mazzolani & M. Ivanyi (eds) Refi/rbishment olBuildings
and Bridges. CISM publications, Chapter 4.
Mazzolani, EM. & Mandara, A. 1991. L'acciaio nel consolidamento. ASSA.
Mazzolani , EM. & Mandara, A. 1992 . L'acciaio nel restauro.
ASSA.
Mazzolani, EM. & Mandara, A. 2002 . Modern trends in the
use of special metais for the improvement ofhistorical and
monumental structures. Journal oi Eng. Structures n.24.
EIsevier.
Mazzolani , EM. 2002. Principies ofrehabilitation and design
criteria. In EM. Mazzolani & M. Ivanyi (eds) Refi/rbishment oi Buildings and Bridges. CISM Publications,
Chapter I.
Modena, c., Pineschi , E & Valluzzi , M.R. 2000. Valutazione
del/a vulnerabilità sismica di alcllne classi di strullure
esistenti. Sviluppo e valutazione di metodi di rinlorzo.
CNR-Gruppo Nazionale per la Difesa dai Terremoti.
Tomazevic, M. 1999. Earthquake-resistant des ign of masonry
buildings. Series on Innovation in Stmctures and Construction, Vo/. I. London: Imperial College Press.
M.LL.PP. D.M. 2 Luglio 198 1. Normativa per la riparazione
ed il rajJorzamento degli edifici danneggiati dai sisma
nel/e regioni Basilica/a, Campania e Puglia.
M.LL.PP. Circolare 30 Luglio 198 1 n" 21745. Istruzioni
relative aI/a normativa tecnica per la riparazione ed il
rajJorzamento degli edifici in lIIuralUra danneggiati dai
sisma.
M.LL.PP. D.M. 20 Novembre 1987. Norme tecniche per
la progellazione, esecuzione e col/audo degli edifici in
muratura e per illoro consolidamento.
PCDM ord. N 3274 dei 20/03/2003 a11.2. Norme tecniche per
il progetto, la valutazione e I 'adeguamento sismico degli
edifici.
CEN - ENV 1996-1-1 Eurocode 6. Design oi masonry
Slructures.
FEMA 273. 1997. NEHRP Guidelineslor the Seismic Rehabilitation oi Buildings.
1213