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Research Article
Evaluation of Behavior and Seismic Retrofitting of RC Structures by Concrete Jacket

Hamidreza Nasersaeed
 
ABSTRACT
The aim of this study is to offer appropriate solutions for improving strengthening methods of concrete structures by concrete jacket. Although, there are numerous conducted laboratory studies for strengthening structures reported in the literature, there is still an apparent need for the detailed analysis and more executive and practical methods to provide further understanding of concrete jacket. Thus, in this study, in addition to review previous studies, performance of epoxy resin for achieving adequate adhesion between connector bar to old concrete is tested. Test results on 12 samples indicate that, with respect to braced length of bars planted in concrete, samples behavior are predictable by design methods. Design and calculation of connectors are offered and proper techniques for executing designing model are proposed. Results indicated that using reinforced concrete as a method of strengthening can be cheaper than other methods has been used in concrete frame. In addition, results shows for establishing complete bond of beam-column, the new longitudinal reinforcements in columns should be passed through floor and they should be surrounded in concrete jacket too. And For improving Inappropriate function arising from inadequate development length and slip of reinforcement in the connection of extra reinforcement to the column base, this length can be designed by using of epoxy. Results also propose the use of designed connectors bars for transfer interlayer shear flow instead of welded bent down connection bars.
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Hamidreza Nasersaeed , 2011. Evaluation of Behavior and Seismic Retrofitting of RC Structures by Concrete Jacket. Asian Journal of Applied Sciences, 4: 211-228.

DOI: 10.3923/ajaps.2011.211.228

URL: http://scialert.net/abstract/?doi=ajaps.2011.211.228
 
Received: April 29, 2010; Accepted: August 13, 2010; Published: October 13, 2010

INTRODUCTION

Components of concrete structure, depending on desired seismic resistance, amount of damage and type of connections can be repaired or strengthened by using of resin injection, replacing separated parts, attaching sheets on surface and embedding reinforced concrete jacket or steel cages. However, successful operation for repairing and strengthening attain by reaching to high degree of bond between old and new concrete. Connecting jacket reinforcements to foundation bar, with enough braced length and passing longitudinal reinforcements from beam-column joints can help the increasing of resistance and bend stiffness in frames.

Each reinforcing strategy is useful for a structure with special characteristics. Increased resistance is particularly useful for strengthening hard structures. Retrofitting ductility is also suitable for brittle components. Increasing the stiffness is only appropriate for strengthening flexible structures, because it causes the decreasing of relative displacement of story. Of course in practice, the increasing of stiffness is usually accompanied with increasing strength. It has shown in Fig. 1.

Fig. 1: The relationship between the moment-curve in different retrofitting approaches (Rahaee and Nemati, 2004)

The choice of the repair and/or strengthening method depends on the structural behavior objectives. Strengthening strategies may be divided into increasing the resistance to lateral loads, improving the ductility and an association of both (Sugano,1981). Basically, the strengthening techniques for reinforced concrete structures can be divided into addition of new structural elements and strengthening of the existing structural elements.

Two general techniques are considered in the US for retrofitting designs. First strategy is overall reform of structural system with increased structural walls, steel bracing and base isolator. The second is partial reforming of structural and unstructured components. Increasing components sections (sections of components) for reducing displacement of story and reducing adjustability are common methods for seismic retrofitting. It usually reduces the displacement (Mohebbimoghaddam, 2008).

Although, several methods are available for Structural rehabilitation, little information is available and insufficient code guidelines are accessible. In fact, most repair and strengthening designs are based on the assessment of engineers only and, often, empirical knowledge and current practice have an important role in the decisions to be made (Julio et al., 2003).

Thus, the objective of this study is to review one of the most commonly used retrofitting techniques: jacketing of reinforced concrete columns and beams. This method is evaluated according to different characteristics and, in order to help structural engineers to choose the most appropriate solutions, recommendations are given based on published experimental research and real case studies.

REINFORCED CONCRETE JACKET OF COLUMNS

Addition of a concrete jacket is used to enhance flexural strength, ductility and shear strength of columns. In the usual when jacket is performed around the column section, sometimes the combined function of old and new concrete exclusively occurs due to natural bond between the materials. As shown in Fig. 2 this bond can be strengthen by roughing old levels and welding a series of bent reinforcement between new and old reinforcement bars (Rahaee and Nemati, 2004).

Fig. 2: Connection of reinforcement jacket to old reinforcement (Rahaee and Nemati, 2004)

Jacketing procedure of RC columns step by step: This strengthening technique, unlike other methods where steel elements are used, does not have a specialized work demand. Its simplicity of execution makes any construction company, capable of building with quality new RC structures, also competent to execute structural rehabilitation using RC jacketing. In the following paragraphs, this method is assessed according to different aspects: anchoring and slab crossing of the added longitudinal reinforcement, interface surface preparation, spacing of added stirrups and addition of new concrete (Julio et al., 2003).

Added longitudinal reinforcement
Anchoring to the footing: One advantage of RC jacketing strengthening is the fact that the increased stiffness of the structure is uniformly distributed, in contrast to the addition of shear walls or steel bracing. In fact, for these procedures, it is usually necessary to execute new foundations or, at least, to strengthen the existing ones. Generally, in the case of RC jacketing, the steel longitudinal reinforcing bars of the added jacket can be anchored to the original footings. Although there are several commercial products, very effective to bond the added steel bars to the RC footing, attention must be taken when executing this operation. Actually, the quality of the bonding can only be ensured if some details are considered.

Crossing the slab: When continuity between floors of the RC jacketing is required, holes must be provided in the slab to allow the longitudinal bars of the jacket to pass through. In the case of slab-column structures, this procedure is easy to perform and has no drawbacks. In the case of beam-column structures, to avoid interrupting the middle bars, the longitudinal reinforcement must be located at the corners, which can lead to excessive bundling of bars (Jara et al., 1989).

Interface surface treatment: It is well known that the success of jacketing to strengthen Reinforced Concrete (RC) columns is dependent on good bond between the original column and the added jacket. To achieve this purpose, the treatment of the interface must be carefully choosen. The common practice consists of increasing the roughness of the interface surface and applying a bonding agent, normally an epoxy resin. Steel connectors are also occasionally applied (Julio et al., 2005).

As previously mentioned, two-component epoxy resin is most commonly used. However, when an effective method to increase the surface roughness, such as sand-blasting, is used, in this latter situation, the subsequent application of an epoxy resin can even produce the opposite result and therefore should not be used (Julio, 2001).

Spacing of added stirrups: The monolithic performance of jacketed RC columns can be achieved if a higher percentage of transverse reinforcement is considered in the repaired solution. Thus, it has recommend that half the spacing of the original column transverse reinforcement be adopted for the jacket transverse reinforcement.

Added concrete: Normally the added concrete has a maximum aggregate dimension of about 2 mm because of the lack of space in the jacket. This is due to its diminished thickness associated with the volume occupied by the added steel reinforcement (Julio et al., 2003). Concerning the added concrete mixture and due to the reduced thickness of the jacket, the option is usually a grout with characteristics of Self-Compacting Concrete (SCC) and Highstrength Concrete (HSC) (Julio et al., 2005).

REINFORCED CONCRETE JACKET OF BEAM

The Reinforcement Concrete (RC) jacket can be executed by adding concrete to 3 or 4 faces of beams. We can also reinforce tensile and compressive zone of beams by concrete coat. Reinforced coating underneath face cause to increase the flexural capacity. Executing of concrete jacket on each four faces of beam is more effective style. The concrete thickness that is added to upper face of beam is in a way that can be lost in the thickness of ceiling (50-70 mm). In this way the stirrup will be controlled in upper side properly. Executing the jacket on the four faces of a beam has shown in Fig. 3.

In Diyab study (Rahaee and Nemati, 2004) by analyzing loading-displacement curve in reinforced beams with concrete jacket, the final loading of all reinforced beams was about double as primary specimen. Figure 4 shows the details of steel reinforcements of experimented beams, Xanthakos (1996) drew attention to the necessity of dowels of sufficient length to develop the required connection strength. He indicated when new concrete is mechanically bonded to the old concrete, the new concrete will be prevented from shrinking and therefore may crack. This cracking in turn may cause weakness in integrated behavior of section (Coskun, 2002).

Surveys show concrete coatings or jackets are effective strategies for repairing or strengthening structures. If the cases are followed, the additional layers during the whole loading span act integrally until failure. Reduction resistance in repaired beam change 8 to 15% compare to resistance of entire beam.

Fig. 3: Executing the jacket on the four faces of a beam 1-connect of welded reinforcement

Fig. 4: The details of steel reinforcements of experimented beams

Literature reviews on repair and strengthening RC structures: Aguilar et al. (1989) performed a statistical study on the repair and strengthening methods of 114 RC buildings damaged after the 1985 earthquake in Mexico City. According to this work, the most commonly used techniques were the addition of shear walls and the RC jacketing of columns.

Julio et al. (2001) performed tests on RC columns strengthened by jacketing. The steel bars of the added Longitudinal reinforcement were anchored to the footing of the original column by a commercially available two-component epoxy resin. The models were submitted to monotonic tests, consisting of a constant axial force combined with an increasing bending moment and shear force. Initially, failure of all steel bars of the longitudinal reinforcement of the original column and slipping of all the corresponding steel bars of the added jacketing were observed. Pull-out tests were performed to analyse the problem and it was concluded, without any doubt, that the bars slipped because the holes drilled on the footings had not been adequately cleaned. So, the use of a vacuum cleaner was enough to guarantee the change from slipping failure to tension rupture of the added steel bars.

Julio (2001) performed slant shear tests and pull off tests on specimens, considering different interface surface situations and also considering the application of a commercial two-component epoxy resin. The values of the shear and tension strength of the interface reduced when the epoxy resin was applied on the sand-blasted surfaces, contrarily to what happened when other roughening methods were used.

Bett et al. (1988) and Arturo (2001) studied three models of columns (models of 1-1, 1-2, 1-3). After primitive test, specimen 1-1 was repaired again by jacketing and then retested as specimen 1-1 R. The remaining two specimens were strengthened prior to testing by the same jacketing system. This Jacketing system consisted of a reinforcement cage, not attached to the column end abutments, covered with a layer of shotcrete. Additionally specimens 1-1 R and 1-3 were reinforced with cross-ties introduced in drilled holes made in the original column, then cemented with epoxy adhesive gel and tied to extra longitudinal bars placed on the middle points of the reinforcement cage. Model of reinforced column, examined at the University of Texas has been shown in Fig. 5. Under a constant axial and lateral load, specimens showed displacements. This displacement was gradually increasing until failure. Figure 6 shows test set up in Texas University.

From the test results, the following conclusions were made:

The model 1-1 performed poorly under moderate earthquake loading, exhibiting a shear-dominated failure
Shear resistance of specimens increased by retrofitting columns with shotcrete jacketing prior to lateral loading and ductility was greatly improved compared to that of specimen 1-1

Fig. 5: Model of reinforced column which was examined at the University of Texas (Arturo, 2001)

Fig. 6: The test set up of axial and lateral loading in Texas University

The use of cross-ties as a part of the strengthening or repair technique, was not significantly affect the columns stiffness or strength under monotonic loading, but it improved the maintenance of such stiffness or strength under lateral displacements exceeding from allowable limits
The repaired specimen 1-1 R using the same jacketing technique was proven to withstand almost the same lateral loading and drift ratio imposed to the strengthened ones. Its stiffness and strength were almost equivalent to those of specimens 1-2 and 1-3
Improved behavior of the specimens would have been achieved, in terms of ductility, if the longitudinal bars of the reinforcement cage were fixed to the column end blocks since these bars never yielded (Bett et al., 1988). Envelope of lateral load and displacement drift of specimens has been shown in Fig. 7.

Studies on shear transfer between old and new concrete were done by bass and his colleagues (Bass et al., 1989; Mohebbimoghaddam, 2008). Bass and his colleagues found such a shear strength which was increased by increasing of connector steel, the depth embedment and strength of concrete. Moreover the preparation of common surface didn’t give any distinctive affect as a connecting factor with results. The performance of jackets becomes more important when the columns are the boundary component of frameB (Mohebbimoghaddam, 2008). The improvement techniques of present splice is including removal of concrete coat, using welded overlap bar and surrounding coating splice with reinforced concrete jacket and new reinforcement in jacket.

Fig. 7: Envelope of lateral load-displacement drift of specimens

According to test results (Bett et al., 1988; Tonkabonipour, 2005; Beer et al., 2006). The lateral capacity of reinforced column can be predicted carefully that it’s supposed the concrete jacket and main column are completely agreeable. Of course the results for shotcrete jacket, is slightly lower than gained result for cast-in-place jacket.

As workshop conditions are not ideal it is better the compressive strength of new concrete be 5 MPa greater than member’s concrete strength. Otherwise, a correction coefficient of modal ø = 0.8 have to be introduced for the calculation of stiffness and strength of repaired component.

And is monol has been used for regarding all components consisting the new and old jacket.

Valluan et al. (1993) presented the result of retrofitting of column splice by meld and cross-steels. The results indicated the bond in the zone of splice by melding longitudinal reinforcement decreased the risk of buckling for this bars in order additional layer can show integrated behavior until failure.

Ersoy et al. (1993) ran two series of tests to study the behavior of strengthened and repaired concrete jacketed columns. The first series compares the behavior of jacketed columns with a monolithic reference specimen under monotonic axial loading. All the concrete for the monolithic specimen was cast with the base column and retrofit reinforcement in place, to provide a specimen with perfect interaction and bond between the base column and retrofit material. Hoop reinforcement is used in the base column and retrofit reinforcement. The jackets are applied under two conditions: after the compression loading was applied and removed, as well as while the axial load is still applied. It is determined that columns jacketed after unloading performed well, reaching 80 to 90% of then strength of the monolithic reference specimen. Repair jackets applied while the column is still under load did not perform as well and only reached 50% of the axial load carried by the monolithic specimen. The second series of tests study the effectiveness of concrete jackets with columns tested under combined axial load and bending. Both repair and strengthened jackets behave adequately under monotonic and reversed cyclic loading.

Rodriguez and Park (1994) conducted further testing on rectangular columns repaired and strengthened by concrete jackets under compressive axial loading as well as lateral loading. Rebar hoops are provided as the retrofit reinforcement for the concrete jackets. Concrete jackets increase the strength and stiffness of the as-built (unretrofitted or base) columns by up to three times. It is also shown that damage before the retrofit has no significant influence on the performance of the jacketed columns. Overall, concrete jackets with rebar reinforcement significantly improve stiffness, strength and ductility of typical reinforced concrete columns, but construction is very labor-intensive.

Lehman et al. (2001) used concrete jackets to repair severely damaged columns. Three repair methods are considered and implemented for the damaged columns, which were built to modern seismic specifications. Initial damage to the columns includes crushing of concrete, buckling and fracture of longitudinal reinforcement and fracture of the spiral reinforcement, which was the result of axial and lateral loading. Concrete jacketed columns are reinforced with spiral transverse reinforcement. Loose concrete is removed from the cover as well as the base column. The concrete jacket retrofitted column displays increased stiffness and strength, comparable to the original column before damage.

LABORATORY ACTIVITY

In this study, 12 bars with numbers 8, 10, 12 were planted in concrete section by the use of epoxy resin. For any number of reinforcement, four samples were tested. Length of reinforcement coverage distinguishes them from each other. In order to investigate test results in meaningful way, nominal strength of concrete (fc) and yield strength of steel bars (fy) was kept constant.

Concrete: Since, the situation of concrete placing has significant influence on concrete resistance, all samples had depositing concrete at the same time and with same mix design. More details about mix design and mechanical characteristics of concrete have shown in Table 1.

Reinforcements: Round steels used in this study, were kind A II steel bars with 8, 10, 12 mm diameter. Mechanical characteristics of round steel have shown in Table 2.

Glue: The glue used in this study was two-component epoxy resin with mix ratio of A:B = 3:1. Its Characteristics have shown in Table 3.

Table 1: Amount of existing materials in one cubic meter (m3) of normal concrete (kg)

Table 2: Mechanical characteristics of round steel

Table 3: physical details of two -component epoxy resin

Table 4: Braced length of reinforcements in concrete section

Table 5: Minimum braced length required for bond stress

Test method and results: Since, determining of adequate braced length and adhesive resistance of epoxy resin are very important, steel reinforcements in three sizes and different lengths were placed in concrete section by the use of epoxy resin. characteristics has shown in Table 4.

After placing reinforcing steel in concrete sections, tension test was conducted. Table 5 offers the minimum length which is necessary to provide bond stress on the basis of yield strength of steel.

Bond stress which is created between bars and concrete by the use of epoxy resin, is recommended according to the following equation:

(1)

where, L is minimum braced length for providing bond stress. Thus, Epoxy resin bond stress between u = 3-3.5 Mpa is estimated According to above equation.

MATERIALS AND METHODS

Designing of connectors: The execution of this project was prolonged about one year, 2009, in Mazandaran and Mohaddes university laboratory. When jacketing is used for increasing the flexural strength and when combined function is needed, In order to provide shear transfer between new and existing materials, applying roughening of concrete surface before the concrete, using of shear connector and root reinforcement are recommended.

Shearing stress that occurs on the faying surface of new and old concrete, is figured out by following relation according to strength of mechanics principles:
When contact surface is in tensile zone:

(2)

(3)

Figure 8a and b show shear stress between the old and new concrete.

Fig. 8: Shear stress between the old and new concrete: (a) strengthening in tension zone and (b) strengthening in compressive zone

Resin layer including higher strength could be used to achieve adequate adhesion between new and old concrete. Amount of resistance obtained from following relation on contact surface should be compared with the shear strength of concrete base.

(4)

Total shear flow (T = τ0b) should be transferred through welding new and old reinforcement.

τRd is the base shearing strength design, Eq. 4.

In Eq. 5, the amount k is equal 1 when 1.6-≥1 (d is according meter):

(5)

τ0 is th axial stress because of axial loading.

Contact surface should be in compress zone: The amount of shear stress created on contact surface between new and old concrete is presented by following equation with a proper approximate according to classic theory of materials strength.

Fig. 9: Shear uncuses between the old and new concrete in compressive zone (Rahaee and Nemati, 2004)

Fig. 10: Strain Changes of composite and noncomposite beam (Shapoor, 1999). (a) noncomposite action and (b) Composite action

If established tx1, is larger than tRd1, the total shear flow (T = tx1b) has to be transfer by shear uncuses. The final shear is tolerated by these uncuses is equal to following amount, see Eq. 6:

(6)

wher, d is the diameter of shear Uncus, fcd is the strength of concrete design and fyd is the strength of shear design.

Figure 9 shows shear uncuses between the old and new concrete in compressive zone (Rahaee and Nemati, 2004).

Designing of shear connectors for anchoring of concrete coating of composite section: In general method, for designing of jacket and its connectors with present concrete, when the connectors between old and new materials is proper, the nominal shear strength could be figured out like combined zone method.

Designing in this method is similar to concrete slab on steel beam. In mentioned method the affect of friction between layers has been ignored. Figure 10 shows strain Changes of composite and no composite beam (Shapoor, 1999).

Fig. 11: Shear in beam (horizontal section) (Beer et al., 2006)

In Eq. 7, the amount of total horizontal shear force that has to be transfer between the maximum point of positive moment and zero moment is equal:

(7)

Where:

Vh = Final horizontal shear power between the maximum point of positive moment and zero moment
N1 = The number of required shear connectors from the point of maximum flexural moment to the point of zero moment
q = Shear strength of shear connector.

The method of establishing longitudinal shear in a section of a beam: Figure 11 shows shear in beam (horizontal section). as shown, we separate the component CDD'C' with the length of Δx from the length of member and this component goes through two sides to desirable curve surface. The force striking to this component consisting of shear force Vc and VD, slight horizontal force CdA and DdA. Longitudinal shear force ΔH that is the resultant of slight longitudinal shear forces striking on the curve surface, reaches to balance point according to following Eq. 8:

(8)

By integration on the shaded surface, the longitudinal shear striking to component of beam is identified by the Eq. 9:

(9)

The horizontal shear to length unit or shear flow and the distance of connectors figured out according to the following Eq. 10:

(10)

Fig. 12: Tridimensional model and the plan of retrofitted building

Where:

V = Vertical shear in section of beam
Q = The first moment of shaded surface
F = The force of each connector
L = Distance of connector

The model of design: The modeling building, with concrete frame, containing seven story has built in 2007, it had much less seismic strength expected in present construction code but by the idea of using concrete jacket has been treated in seismic. It has shown in Fig. 12.

For estimating the structure and the method of treating it, by regarding the limitation of liner method (TODCMPO, 2002), linear spectral analysis and the software of Etabs 9.2 were used.

The present building with residential application is including regular plan, joists and filler block ceiling. Approximate size for each story is 368 m2. To improve the seismic behavior of the existing building, Idea of using lightweight materials has been used. The studied option for seismic treating is the usage of concrete jacket. The selection of strengthened members, after many different computational analyses have been considered very impressive. Beam Plan has been shown in Fig. 13.

The result of analysis, after strengthening of Strengthened members, according to Fig. 13 showed the increasing of sections accompanying with increasing in the stiffness of frame can resist against maximum moment. When the structure is treated for the affects of little fluctuation, with decreasing of usual period time the stiffness of structure increase and its displacement decrease.

RESULTS

As shown in Fig. 14, three different strengthening techniques of columns with jacket were presented by Vandoros and Dritsos (2008) and the specimens were studied in following states:

Welding the jacket stirrup ends together (Model N)

Fig. 13: Beam plan (a strengthened section is defined with hatch)

Fig. 14: The structure of axial and lateral loading test (Vandoros and Dritsos, 2008)

Using of connection bars, jacket stirrup end welding and poured concrete (model E)
Using bent down steel connector bars welded to the original column longitudinal reinforcement bars and using of concrete (model W)

After axial and lateral loading of specimens, results indicated that usage of concrete jacket has an impressive affect in increasing strength and damping capacity of energy of strengthened RC columns as regards primary specimen. Although, in comparison with composite specimen (model M), decreasing strength and stiffness occurrence, were not much, but they must be included in calculations. Load against displacement curve for each specimen has been shown in Fig. 15.

By studying characteristics of designing model and according to experimental results, we can improve the efficiency of this method by doing a series action outlined below:

Surrounding present concrete section by concrete jacket and passing additional longitudinal bars around current section, in a way that related code is regarded, in order to facilitate working on this method (Fig. 16)
Using bent down dowel for connecting jacket bar to the original column longitudinal bar (Moosavi and Abbasnia, 2007; Frangou et al., 1995)

Fig. 15: Load against displacement curve for specimen (a) sample N, (b) sample E, (c) sample W and (d) sample M (Vandoros and Dritsos, 2008)

Fig. 16: Strengthening beam and column by concrete jacket (Vandoros and Dritsos, 2008)

Welding the jacket stirrup ends together, for compensating the loss of inevitable seismic function of construction and fulfilling the executive needs of constructions according to present building code (Moosavi and Abbasnia, 2007; Frangou et al., 1995). Figure 17 shows geometry of stirrup end welding
The least thickness of concrete jacket for poured concrete is about 100 mm and for shotcrete concrete, 50 mm is offered
Minimum strength of concrete should be considered at least 20 Mpa. The least thickness of additional bars coating is considered as followings: beam 35 mm, column 40 mm, foundation 75 mm and foundation 50 mm (Mohebbimoghaddam, 2008)

Fig. 17: Geometry of stirrup end welding

Fig. 18: Creating integrated close circle from two pieces

In studying of special rules for flexural members in earthquake zone and in many cases that a ring is needed, each longitudinal bar should be located on the corner of stirrup and its interior angle should not be more than 135°. The free distance of longitudinal bar should not exceed more than 15 cm (Shapoor, 1988). Figure 18 shows creating integrated close circle from two pieces

DISCUSSION

The results of this study showed that using concrete jacket is affective method in increasing strength and stiffness in component of a frame and structural frame as well. Although sometimes supplying the anchorage bond of extra reinforcement concrete jacket, passing these bars from connection joint of beam-column, drilling of zones in executing stirrup and also using bent down connection bars with melded bars, cause difficulties in performing of this method, in this study, by summing up the results and assessment the requirements of this method, proper techniques for executing designing model are offered:

In line with earlier studies by Bass et al. (1989), Bett et al. (1988), Valluan et al. (1993), Ersoy et al. (1993) and Vandoros and Dritsos (2008), using of reinforced concrete as a method of strengthening has suggested. It can be cheaper than other methods has been used in concrete frame. This is because of availability of materials and familiar implementation of this style
For establishing complete bond of beam-column in a way that moment could be transferred to frame components corresponding to increased resistance, the new longitudinal reinforcements in columns should be passed through floor and they should be surrounded in a concrete jacket too. Similar methods like adding stirrups or continuous reinforcements across the connection on beams can also establish bond on the floor
For improving Inappropriate function arising from inadequate development length and slip of reinforcement in the connection of extra reinforcement to the column base, this length can be designed by using of epoxy
The study of epoxy resin bond tests on concrete samples with a fixed nominal resistance showed that by providing minimum length required, epoxy can be used for connecting old and new concrete Instead of passing cross-stirrup through section of old concrete
Since, the lack of space and method of cross-reinforcement limited volume of extra concrete and buckling of longitudinal bars, consistent with Vandoros and Dritsos (2008) study, designed connectors bars can be used for transfer interlayer shear flow instead of welded bent down connection bars
Studies by Xanthakos (1996) and Bett et al. (1988) showed, In designing of computational models of structures that don’t have adequate seismic strength, the reduction of stiffness and resistance for reinforced section should be considered

ACKNOWLEDGMENT

I would like to profoundly thank Ommehani Alizadeh for his support throughout this project. Dr. Ghasempouri also contributed valuable additional guidance before, during and after the completion of this project. It is greatly appreciated.

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