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.

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Figure 1. Out-of-plane collapse mechanism (Giuffre, 1993).

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

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 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

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1

I \ \

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 THE USE OF METAL TIE-BARS IN MASONRY STRENGTHENING

3.1 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

lU llllll l!l lU Jlllll lU jj ll!ll J! J! J!J JJ JJ

- -Figure 5. Section and plan ofthe tie bars constraint against seismic actions.

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.

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 THE CASE STUDY

4.1 The si/e

The studied building belongs to a dismantled indus­trial complex located in an outlying zone next to the

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Figure 7. The main entrance ofthe factory.

Figure 8. lron and Ii ttle brick vaults floor.

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 f irst 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

Figure 9. North elevation of the former boss pa lace accord­ing to design plan.

interposed little brick vaults (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 f irst 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) .

4.2 The design 01 strengthening 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 horizontal 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).

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. . . . "

::::Jíl[~::m~::~::m~::m<!::m~::~::" ~ . U": " " 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.

Figure 12. Location of metal tie-bars into a masonry pane\.

5 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

Table I. Mechanical characteristics of cakareous stone masonry.

Value Un i!

Modulus of elasticity E 660 MPa Poi sson 's ratio 0, 15 Density 2400 kg/m3

Friction angle 30° Cohes ion 0, 1 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 realization ofa 50 mm lhick reinforced­concrete slab, appropriately anchored to the walls.

8ased on the results of the analysis, the structural elements prior to strengthening 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 Non-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 horizontal 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

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"

" . " /'

/ / r--

/' /

LOId Factor'h Olsplacemenl

.-/' ./

>,10 >." ,00 <.:o I~ >," <!lO t ," '.00 'Sl •. m

' ..... ot>!K_ ont lnlml

Figure 13. Seismic load factor vs top di splacement, with left-right direction of se ismic action (ac = 0,250 - a y = 0,235).

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 (ac = 0,245 - a y = 0,225).

Figure 15. Co llapse of the wall without tie-bars under left-right seismic action.

of the 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 elastic limit is avai lable. The collapsing wall is represented in Figures 15 and 16, where it is possible to observe that the fai lure ofmasonry occurs along sloped sliding planes.

The analysis carried out on the wall re inforced by vertical and horizontal tie-bars proved a signifi cant

Figure 16. Collapse of the wa ll without tie-bars under right-Ieft se ismic action.

Figure 17. The strengthened wall under left-right seismic action.

Figure 18. The strengthened wa ll under right-Ieft seismic action.

increasing ofits resistance. The wall can bear horizon­tal actions greater than 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

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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.

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