Study of Seismic Retrofitting of Reinforced Concrete Buildings

---------------------------♦-----------------------------

Seismic retrofitting of constructions vulnerable against quakes is a present issue of awesome political and social pertinence. A large portion of the Italian building stock is helpless against seismic action regardless of the possibility that situated in ranges that have for quite some time been considered of high seismic peril. Amid the previous thirty years direct to extreme quakes have happened in Italy at interims of 5 to 10 years. Such occasions have plainly demonstrated the weakness of the building stock specifically and of the manufactured condition when all is said in done. The seismic risk in the zones, where those tremors have happened, has been known for quite a while on account of comparative occasions that happened before. It is consequently authentic to inquire as to why constructions vulnerable against quakes exist if individuals and foundations knew about the seismic peril. A few causes may have added to the making of such a circumstance. These are related to recorded occasions, blurring memory, ravenousness, insatiability, neediness and obliviousness. Among recorded occasions especially applicable are wars, scourges, and cataclysmic events, which may constrain, essentially, the accessible assets of a nation. In such conditions there is an inclination to work with poor materials and without an excess of regard for good construction methods and wellbeing edges. A circumstance of this kind happened in Italy and in Japan after the Second World War and comparable circumstances has happened in Italy commonly before. In such a circumstance it is conceivable that the marvel of blurring memory happens and past recollections are effectively deleted. In Italy business benefits frequently result from the work of poor material and workmanship instead of the ideal usage of the creation factors. The discouraging circumstance of low quality control and material acknowledgment likewise falls into this building, which, as a rule, comes about just in printed material without substantive esteem. Minimal affinity to use now and again guarantees that even the proprietor inclines toward a low quality item to spare assets for more prompt needs. Among causes emerging from obliviousness there might be both a lacking learning of the seismic peril and plan mistakes because of inadequate information of the tremor issue; likewise the vulnerable to effectively display the auxiliary reaction to the seismic action. While the exploration group in managing the above issues has gained significant ground as of late, it has turned out to be harder to exchange the outcomes to

Late changes in the educational program of building schools are prompting a general impoverishment of the fundamental learning and operational abilities of our designing graduates. A last reason for defenselessness is associated with the support of constructions; clearly if a construction is not frequently kept up, much as occurs for a motorcar, the mechanical properties of the materials may experience neighborhood and worldwide debasement with a huge loss of resistance of the basic individuals and of the whole construction. Additionally, changes in benefit conditions, regularly made self-assertively, may prompt generous changes in the basic conduct bringing about a corruption of the auxiliary reaction to the normal stacking conditions. On the premise of what has been displayed up until now, it is not amazing that in ranges long known to be liable to the seismic danger it is not rare to discover constructions helpless against tremors. These constructions should be retrofitted to enable them to withstand the impacts of the seismic tremor ground movement expected at the site considered. In the accompanying segments a few systems utilized for the assessment of the seismic resistance and helplessness of reinforced concrete buildings will be depicted together with customary and creative strategies of seismic retrofitting of similar buildings. The paper closes with a depiction of the seismic retrofitting of two reinforced cement private buildings in the town of Solarino, close Syracuse, in Sicily. The buildings have a place with the Institute Autonomo Case Popolari (IACP) of Syracuse. As will be clear from following contentions the point of the paper is not to talk about inside and out the cutting edge of seismic retrofitting, but instead to give a general outline. The point is likewise to concentrate on a couple of particular methods which may enhance the best in class rehearse for the assessment of seismic defenselessness of existing reinforced concrete buildings and for their seismic retrofitting by methods for inventive procedures, for example, base separation and vitality scattering.

Seismic vulnerableness is not a flat out idea but rather is unequivocally identified with the occasion being considered. A similar construction may not be defenseless against one class of quakes but be vulnerable against another. In this manner, before endeavoring a seismic defenselessness assessment of a given construction, the seismic action that will influence that construction must be completely indicated. manufactured and quantitative portrayal of the seismic action which, other than relying upon the attributes of the ground movement, relies upon some inherent qualities of the building, for example, the principal method of vibration and its vitality dissemination limit.

An review of the current EC8 hone in repair and fortifying of concrete buildings in Europe (Elnashai and Pinho, 1998) talked about the requirement for the outline reasoning supporting the appraisal and reinforcing of buildings to be reliable with that for new buildings. While diverse plan target execution breaking points might be took into consideration new and existing construction, the fundamental outline theory ought to be reliable. It was presumed that there is a need to expressly incorporate distortion related execution goals in retrofit outline rules in perspective of the pattern towards twisting based seismic plan of new buildings. Therefore, the parallel floats and basic distortions announced for concrete casings and brick work infill are particularly. Considering first the concrete edge all alone, it might be normal that it will withstand a sidelong float of the request of 1.5% to 2% preceding the pillar segment joints as well as sections fall flat (Beres et al, 1992a). The greatest base shear quality for the concrete edge will probably be of the request of 15% of its weight and happen at approximately 1% float (Bracci et al, 1995a,b). With respect to the brick work, a definitive quality of the workmanship infill might be assessed utilizing an incentive for the greatest shear quality of URM of τ u = 0.4MPa ± 0.2MPa . There is a wide range in the qualities announced in the writing for the shear strain (or parallel float point) at which the most extreme shear stretch happens. By and by, in view of the writing it gives the idea that this most extreme anxiety might be assessed to happen at a parallel float point of roughly 0.3% (see for instance Pires and Carvalho, 1992; Valiasis et al, 1993; Fardis and Calvi, 1995; Zarnic and Gostic, 1997; Schneider et al, 1998). Swinging to disappointment modes, a current survey by Dyngeland (1998) found that the most widely recognized disappointment systems for concrete buildings because of seismic stacking are: (1) pillar segment joint disappointments; (2) segment disappointment because of deficient flexural or shear quality; (3) shear divider disappointment; or (4) infill divider disappointment because of lacking shear quality or insufficient out-of-plane flexural quality. Bruneau (1994) gives a comparable outline of the seismic helplessness of brick work (block and concrete square) buildings and their most regular

this venture are those due to in-plane strengths. A hefty portion of the auxiliary disappointments amid quakes in the mid 1970s were because of deficient shear quality as well as absence of constrainment in concrete segments. Henceforth, early segment reinforcing systems commonly included expanding the concrete segment's cross-segment. The primary issue with this approach is that it regularly unsatisfactorily expands the measurement of the segment, rendering the retrofit unreasonable. The utilization of thin carbon fiber composite sheets maintains a strategic distance from this issue and has therefore picked up acknowledgment in the course of recent years. It was noticed that steel and composite jacketing was especially valuable for amending insufficient lap graft issues and that grapple jolts can be utilized to enhance constrainment far from the edges of rectangular segments (eg, Priestley et al, 1994; Saadatmanesh et al, 1997). In any case, concrete jacketing of concrete sections has been appeared to be exceptionally compelling in enhancing quality and malleability and changing over concrete shaft vulnerable segment buildings into buildings with a concrete segment frail pillar instrument (Choudhuri et al, 1992; Rodriguez and Park, 1994; Bracci et al, 1997; Bush et al, 1990). Where the workmanship infill is helpless, it can be retrofit in a wide assortment of ways. Split infusion grouting is frequently used to restore a brick work divider to its "unique" condition though the utilization of purported "jacketing" methods adds both quality and concreteness to the infill. In this unique situation, jacketing comprises of encasing the current component by an extra auxiliary segment. For brick work infill dividers, jacketing can appear as: • Shotcreting – the application by splashing a thin layer of cement onto the substance of the brickwork. Support could possibly be connected to the brickwork before showering; • Prefabricated reinforced concrete boards connected, ordinarily, with dowels through the brickwork; • Steel plates or fiber composite sheets stuck/fortified onto the brickwork; or • Steel strip propping joined to the brickwork utilizing either through-catapulting or some type of compound holding specialist. While late research has focused on the utilization of cutting edge fiber composites, vitality dispersal and seismic segregation gadgets for the seismic retrofit of buildings, the more customary techniques ought not be disregarded while considering which system(s) to utilize. Practically speaking, the ideal plan will rely on for example, feel and the level of interruption to tenants (Jirsa, 1994).

3. DESCRIPTION OF ELSA TEST BUILDING

The ELSA test building is a 4-story, 3-sound reinforced concrete edge with unreinforced block brick work infill dividers. The concrete edge was intended for gravity loads and an ostensible parallel heap of 8% of its weight, W (Carvalho, 1998). The support points of interest were determined to be illustrative of buildings built more than 40 years back in European Mediterranean nations, for example, Italy, Portugal and Greece. In the accompanying segments, the building is portrayed and the aftereffects of a preparatory evaluation of its probable seismic conduct are displayed. It can be found in the rise and plan drawings that the story statures are 2.7m and there are two 5m traverse narrows and one 2.5m traverse cove. Block brick work infill (200mm thick) is contained inside each narrows. The left-hand straight contains a window (1.2 x 1.1m) at each of the 4 levels. The focal straight contains an entryway (2.0 x 1.9m) at ground level and window openings (2.0 x 1.1m) in each of the upper 3 levels of the building. The right-hand (2.5m traverse) cove contains strong infill. The shaft fortification subtle elements are appeared in Figure 2. It ought to be noticed that the longitudinal strengthening steel comprises of smooth round bars which are ended with 180° twists. All shafts toward stacking are 250mm wide and 500mm profound. The transverse shafts are 200mm wide and 500mm profound. The concrete section thickness is 150mm. The segment fortification subtle elements are appeared in Figure 3. The segment stirrup detail with a 90° abridgement twist ought to be noted specifically. Preparatory counts have been done with a specific end goal to build up which disappointment instruments are well on the way to happen under seismic stacking. With a specific end goal to do this, the mean esteems for the particular material qualities appeared in Table 1 were utilized.

Figure 3 – Column reinforcement details Frame Sidesway Potential and Column Shear Strength The ultimate moment capacity, Mu , was first calculated for each beam and column cross-section using conventional rectangular stress-block theory and the mean values for the respective steel yield and concrete compression strength properties shown in Table 1. In order to assess whether a column sideway mechanism was likely to occur, the sum of the moment each level using equation (1). Even though the slab contribution to the beam capacity was ignored, the value given by equation (1) was still less than one for each level of the building, indicating that the building is highly susceptible to column sidesway collapse. (Note: in practice, the value given by equation (1) should be markedly greater than 1, say 1.4 for example, to ensure that a column sidesway mechanism is not likely to occur.) The shear capacity of each column, Vu , was then estimated to determine whether the columns were likely to suffer shear failure before reaching their maximum flexural strength. These calculations indicated that no columns are expected to suffer premature shear failure.

Next, the relative shear quality of the brick work infill dividers were evaluated and contrasted with the assessed extreme shear quality for every account of the exposed concrete edge. The qualities recommend that a definitive quality of the block dividers is around four times that of the uncovered casing. Obviously, the segment story shears won't accomplish their most extreme at an indistinguishable horizontal float from will the brick work since the sidelong firmness of the brickwork is likewise considerably more prominent than that for the casing. The retrofit conspire in this way should be equipped for obliging the distinctions in both quality and concreteness. It ought to likewise be noticed that the shear quality of the concrete edge above level 2 is just 60% of the edge's shear quality beneath level 2.

There are two principle parts of worry for the bar segment subtle elements appeared in Figures 2 and 3. The first is identified with the likelihood of untimely joint disappointment because of poor jetty of the base shaft steel that ends in the pillar segment joint district. The conduct of smooth round bars with 180° curves in the joint district is probably going to be superior to anything the conduct seen by Beres et al (1992) who tried joints with distorted bars which ended in the joint area with no twists. In those tests, outside joints fizzled at parallel floats of in the vicinity of 1.5% and 2% of the story stature. Inside joints fizzled at parallel floats in the vicinity of 2% and 2.5%. Subsequently, it

enough, in any event up to floats of 2% to 2.5%. In any case, the utilization of the 90° stirrup "covering points of interest" (Figure 3) in the decrease of the shear fortification for the pillars and, all the more especially, for the segments is another issue. Experience has demonstrated that sections will crumple if their stirrups are not found adequately near bind the concrete center and counteract clasping of the longitudinal steel. The stirrup dividing utilized as a part of this venture is 150mm in all segments ( s = 10db or 12.5db ) and either 100mm or 200mm in the shafts ( s = 8.3db or 16.7db ). These are strikingly close spacings in perspective of the age of the plan. By and by, the stirrup detail appeared in Figure 3 is not anticipated that would withstand rehashed vast cycles of sidelong stacking once concrete pounding has happened.

4. RETROFIT STRATEGIES

The retrofit decision will depend to a huge degree upon the seismic execution level that is required amid the plan premise quake (DBE). Predominately versatile reaction so the brickwork infill dividers are secured amid the DBE will require a very different retrofit than if the dividers are permitted to neglect to allow malleable minute or supported edge conduct. Subsequently, a few retrofit alternatives were considered for this venture.

In this choice, the URM infill would be supplanted with K-propping in the 2.5m traverse narrows at each level of the building. The supporting would concrete vitality scattering gadgets that would help diminish the seismic requests from the levels relating to the 5% damped plan range for the DBE. With the expansion of the damped propping, the casing can be intended to give more uniform story stiffness‘s and qualities and the vitality scattering gadgets can be intended to "yield" at suitable constrain amplitudes. Along these lines, bendable propping can confine the powers that they draw in and help restrict the greatest base shear response. By and by, any retrofit arrangement that expands the base shear response will acquire extra costs because of the need to enhance the building's establishment.

Composite jacketing can be utilized to fortify the sections and infill dividers and to enhance the malleability of the segments and the post-breaking conduct of the URM infill dividers. In any case, this retrofit alternative will cause an unobtrusive increment On the off chance that the results of this are adequate (e.g. establishment fortifying) at that point the composite coats ought to be effectively equipped for tending to the potential section stirrup shortcoming and the poor post-breaking conduct of the brick work infill. Since the jacketed infill would have the capacity to convey sizeable loads in the wake of splitting, the adjustment in quality and firmness of the concrete casing itself would not be basic and most likely need not be particularly altered.

In this alternative, just the concrete casing components would be repaired. The workmanship infill would basically be overlooked. The presumption being that the misshapenness in the working amid the DBE would be in the vicinity of 1.5% and 3% float and that the URM would have totally fizzled by then. The imaginable greatest reasonable float for the concrete edge was assessed to be roughly 2%. On account of this, the section pivot zones would should be kept (composite or cement jacketing) to keep up repression amid vast inversions of dislodging (in overabundance of 1.5% float). Moreover, the adjustment in quality and firmness at level 2 must be tended to in this choice.

In light of this survey, it was reasoned that buildings having points of interest normal of construction of over 40 years back in Mediterranean European nations are probably going to have greatest parallel distortion limits relating to around 2% horizontal float. The unreinforced brick work infill dividers are probably going to start breaking at substantially littler parallel floats, of the request of 0.3%, and to totally lose their heap conveying capacity by floats of in the vicinity of 1% and 2%. In this paper, the particular points of interest of a 4-story, 3-narrows fortified concrete casing test building with unreinforced block brick work (URM) infill dividers were depicted and gauges of its imaginable shortcomings as to seismic stacking were delineated. The concrete casing was appeared to be basically a "frail segment concrete bar outline" which is probably going to show poor post-yield hysteretic conduct. To exacerbate the situation, there is an abatement in quality and firmness of the concrete edge of the request of 35-40% at level 2. This specific issue is not basic as long as the URM infill holds its heap conveying limit since the quality and firmness of the URM infill is evaluated to be substantially bigger than that of the casing. Be that as it may, regardless of the possibility that the infill were intended to react flexibly in the DBE its malleability is much more regrettable and in case of a bigger than anticipated seismic tremor the infill quality is probably going to be

adequacy of the retrofit plots in the end chosen from among the alternatives examined here will be tried utilizing full-scale pseudo-dynamic tests at the European Laboratory for Structural Assessment (ELSA) at the European Commission's Joint Research Center in Ispra, Italy. The aftereffects of the itemized examinations and tests will be accounted for in future productions.

Beres, A., White, R.N., Gergely, P., Pessiki, S.P. and El-Attar, A. (1992). ―Behavior of existing non- seismically detailed reinforced concrete frames,‖ Proceedings of the 10th World Conference on Earthquake Engineering, Madrid, A.A. Balkema, Rotterdam, Vol. 6, pp. 3359-3363. Bracci, J.M., Kunnath, S.K. and Reihnorn, A.M. (1997). ―Seismic performance and retrofit evaluation of reinforced concrete structures,‖ ASCE Journal of Structural Engineering, Vol. 123(1), pp. 3-10. Bracci, J.M., Reinhorn, A.M. and Mander, J.B. (1995). ―Seismic resistance of reinforced concrete frame structures designed for gravity loads: performance of structural system,‖ ACI Structural Journal, Vol. 92(5), pp. 597-609. Bracci, J.M., Reinhorn, A.M. and Mander, J.B. (1995). ―Seismic retrofit of reinforced concrete buildings designed for gravity loads: performance of structural model,‖ ACI Structural Journal, Vol. 92(6), pp. 711-723. Bruneau, M. (1994). ―State-of-the-art report on seismic performance of urm buildings,‖ ASCE Journal of Structural Engineering, Vol. 120(1), pp. 231-251. Bush, T.D. Jr., Talton, C.R. and Jirsa, J.O. (1990). ―Behavior of a structure strengthened using reinforced concrete piers,‖ ACI Structural Journal, Vol. 87(5), pp. 557-563. Carvalho E.C. (1998). Personal communication to ICONS – Topic 2 participants. CEN (1998). ―Eurocode 8, Part 1-4: Strengthening and repair of buildings,‖ Draft, prENV 1998 – 1-4, Comite Europeen de Normalisation, Brussels. Choudhuri D., Mander J.B. and Reihnorn A.M. (1992). ―Evaluation of seismic retrofit of reinforced concrete frame structures: Part 1 – experimental performance of retrofitted sub- 92-0030, State University of New York at Buffalo, 136p. Dyngeland, T. (1998). ―Retrofitting of bridges and building structures – a literature survey,‖ European Laboratory for Structural Assessment, JRC-ISIS, Special Publication No. I.98.33, Ispra, 19p. Elnashai, A.S. and Pinho, R. (1998). ―Current status and future developments for repair and strengthening of RC structures in European seismic design practice,‖ Proceedings of the 11th European Conference on Earthquake Engineering, Paris, A.A. Balkema, Rotterdam, 12p. Fardis, M.N. and Calvi, M.C. (1995). ―Effects of infills on the global response o reinforced concrete frames,‖ Proceedings of the 10th European Conference on Earthquake Engineering, A.A. Balkema, Rotterdam, Vol. 4, pp. 2893-2898. Griffith, M.C. and Pinto A. V. (1999). ―Seismic retrofit of RC frame buildings with masonry infill walls – Literature review and preliminary case study,‘ in press, European Laboratory for Structural Assessment, JRC- ISIS, Special Publication, Ispra, 72p. Jirsa, J.O. (1994). ―Divergent issues in rehabilitation of existing buildings,‖ Earthquake Spectra, Vol. 10(1), pp. 95-112. Pires, F. and Carvalho, E.C. (1992). ―The behaviour of infilled reinforced concrete frames under horizontal cyclic loading,‖ Proceedings of the 10th World Conference on Earthquake Engineering, Madrid, A.A. Balkema, Rotterdam, Vol. 6, pp. 3419-3422. Priestley, M.J.N., Seible, F., Xiao, Y. and Verma, R. (1994). ―Steel jacket retrofitting of reinforced concrete bridge columns for enhanced shear strength – Part 2: test results and comparisons with theory,‖ ACI Structural Journal, Vol. 91(5), pp. 537-551. Rodriguez, M. and Park, R. (1994). ―Seismic load tests of reinforced concrete columns strengthened by jacketing,‖ ACI Structural Journal, Vol. 91(2), pp. 150-159. Saadatmanesh, H., Ehsani, M.R. and Jin, L. (1997). ―Repair of earthquake-damaged RC columns with FRP wraps,‖ ACI Structural Journal, Vol. 94(2), pp. 206-215.

(1998). ―Lateral strength of steel frames with masonry infills having large openings,‖ ASCE Journal of Structural Engineering, Vol. 124(8), pp. 896-904. Valiasis, T.N., Stylianidis, K.C. and Penelis, G.G. (1993). ―Hysteresis model for weak brick masonry infills in r/c frames under lateral reversals,‖ European Earthquake Engineering, Vol. 7(1), pp. 3-9. Zarnic, R. and Gostic, S. (1997). ―Masonry infilled frames as an effective structural sub-assemblage,‖ Proceedings of the International Workshop on Seismic Design Methodologies for the next generation of codes, Bled, Slovenia, A.A. Balkema, Rotterdam, pp. 335-346.

Student