Jacketing is the most popularly used method for strengthening of building
columns. The most common types of jackets are steel jacket, reinforced concrete
jacket ...
International Journal of Advanced Engineering Research and Studies
E-ISSN2249 – 8974
Research Article
MATERIALS AND JACKETING TECHNIQUE FOR RETROFITTING OF STRUCTURES Shri. Pravin B. Waghmare
Address for Correspondence Acharya Shrimannarayan Polytechnic,Pipri(M), Wardha –Maharashtra ABSTRACT Seismic protection of buildings is a need-based concept aimed to improve the performance of any structure under future earthquakes. Earthquakes of varying magnitude have occurred in the recent past in India, causing extensive damage to life and property. Some recently developed materials and techniques can play vital role in structural repairs, seismic strengthening and retrofitting of existing buildings, whether damaged or undamaged. The primary concern of a structural engineer is to successfully restore the structures as quickly as possible. Selection of right materials, techniques and procedures to be employed for the repair of a given structures have been a major challenges. Innovative techniques of the structural repairs have many advantages over the conventional techniques. Some guidelines regarding selection of materials for repair work such as steel, fiber reinforced polymer, has been discussed in the present paper. The selection of materials and techniques to be used depend on many aspects that may be viewed from different prospectives i. e. requirement and availability of financial resources, applicability and suitability of materials for the repair of damaged structures. Use of standard and innovative repair materials, appropriate technology, workmanship, and quality control during implementation are the key factors for successful repair, strengthening and restoration of damaged structures. KEYWORDS: confinement, jacketing, fiber reinforced polymer jacketing, steel jacketing, beam jacketing.
INTRODUCTION Jacketing is the most popularly used method for strengthening of building columns. The most common types of jackets are steel jacket, reinforced concrete jacket, fibre reinforced polymer composite jacket, jacket with high tension materials like carbon fibre, glass fibre etc. The main purposes of jacketing are: 1. To increase concrete confinement by transverse fibre reinforcement, especially for circular cross-sectional columns, 2. To increase shear strength by transverse fibre reinforcement, 3. To increase flexural strength by longitudinal fibre reinforcement provided. They were anchored at critical sections. Transverse fibre should be wrapped all around the entire circumference of the members possessing close loops sufficiently overlapped or welded in order to increase concrete confinement and shear strength. This is how members with circular cross-section will get better confinement than member with rectangular cross-section. Where square or rectangular cross-sections are to be jacketed, circular/oval/ elliptical jackets are most oftenly used and the space between the jacket and column is filled with concrete. Such types of multi-shaped jackets provide a high degree of confinement by virtue of their shape to the splice region proving to be more effective. Rectangular jackets typically lack the flexural stiffness needed to fully confine the concrete. However, circular and oval jackets may be less desirable due to: (i) Need of large space in the building potential difficulties of fitting in the jackets with existing partition walls, exterior cladding, and non-structural elements and IJAERS/Vol. I/ Issue I/October-December, 2011/15-19
(ii) Where an oval or elliptical jacket has sufficient stiffness to confine the concrete along the long dimension of the crosssection is open to question. The longitudinal fibers similar to longitudinal reinforcement can be effective in increasing the flexural strength of member although they cannot effectively increase the flexural capacity of building frames because the critical moments are located at beam-column ends where most of the longitudinal fibers are difficult to pierce through to get sufficient anchorage. Technical Considerations The main objective of jacketing is to increase the seismic capacity of the moment resisting framed structures. In almost every case, the columns as well .as beams of the existing structure have been jacketed. In comparison to the jacketing of reinforced concrete columns, jacketing of reinforced concrete beams with slabs is difficult yielding good confinement because slab causes hindrance in the jacket. In structures with waffle slab, the increase in stiffness obtained by jacketing columns and some of the ribs, have improved the efficiency of structures. In some cases, foundation grids are strengthened and stiffened by jacketing their beams. An increase in strength, stiffness and ductility or a combination of them can be obtained. Jacketing serves to improve the lateral strength and ductility by confinement of compression concrete. It should be noted that retrofitting of a few members with jacketing or some other enclosing techniques might not be effective enough to improve the overall behaviour of the structure, if the remaining members are not ductile. Jacketing of Columns Jacketing of columns consists of added concrete with longitudinal and transverse reinforcement around the
International Journal of Advanced Engineering Research and Studies existing columns. This type of strengthening improves the axial and shear strength of columns while the flexural strength of column and strength of the beam-column joints remain the same. It is also observed that the jacketing of columns is not successful for improving the ductility. A major advantage of column jacketing is that it improves the lateral load capacity of the building in a reasonably uniform and distributed way and hence avoiding the concentration of stiffness as in the case of shear walls. This is how major strengthening of foundations may be avoided. In addition the original function of the building can be maintained, as there are no major changes in the original geometry of the building with this technique. The jacketing of columns is generally carried out by two methods: (i) reinforced concrete jacketing and (ii) steel jacketing.
REINFORCED CONCRETE JACKETING: Reinforced concrete jacketing can be employed as a repair or strengthening scheme. Damaged regions of the existing members should be repaired prior to their jacketing. There are two main purposes of jacketing of columns:
E-ISSN2249 – 8974
(i) Increase in the shear capacity of columns in order to accomplish a strong column-weak beam design and (ii) To improve the column's flexural strength by the longitudinal steel of the jacket made continuous through the slab system are anchored with the foundation. It is achieved by passing the new longitudinal reinforcement through holes drilled in the slab and by placing new concrete in the beam column joints as illustrated in figure 1. Rehabilitated sections are designed in this way so that the flexural strength of columns should be greater than that of the beams. Transverse steel above and below the joint has been provided with details, which consists of two L-shaped ties that overlap diagonally in opposite corners. The longitudinal reinforcement usually is concentrated in the column corners because of the existence of the beams where bar bundles have been used as shown in figure 1. It is recommended that not more than 3 bars be bundled together. Windows are usually bored through the slab to allow the steel to go through as well as to enable the concrete casting process.
Figure No.1: Construction Technique for Column Jacketing Table No. 1:Details For Reinforced Concrete Jacketing. Properties of jackets
• •
Minimum width of jacket
• • • •
Minimum area of longitudinal reinforcement
•
• •
Minimum area of transverse reinforcement
• • • •
Match with the concrete of the existing structure. Compressive strength greater than that of the existing structures by 5 N/mm2or at least equal to that of the existing structure. 10 cm for concrete cast-in-place and 4 cm for shotcrete. If possible, four-sided jacket should be used. A monolithic behaviour of the composite column should be assured. Narrow gap should be provided to prevent any possible increase in flexural capacity. 3Afy, where, A is the area of contact in cm2 and fy is in kg/cm2 Spacing should not exceed six times of the width of the new elements (the jacket in the case) up to the limit of 60 cm. Percentage of steel in the jacket with respect to the jacket area should be limited between 0.015and 0.04. At least, 12 mm bar should be used at every corner for a four sided jacket. Designed and spaced as per earthquake design practice. Minimum bar diameter used for ties is not less than 10 mm or 1/3 of the diameter of the biggest longitudinal bar. The ties should have 135-degree hooks with 10bar diameter anchorage.
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International Journal of Advanced Engineering Research and Studies
Shear stress in the interface
•
Due to the difficulty of manufacturing 135-degree hooks on the field, ties made up of multiple pieces, can be used.
• •
Provide adequate shear transfer mechanism to assured monolithic behaviour. A relative movement between both concrete interfaces (between the jacket and the existing element) should be prevented. Chipping the concrete cover of the original member and roughening its surface may improve the bond between the old and the new concrete. For four-sided jacket, the ties should be used to confine and for shear reinforcement to the composite element.
• •
Connectors
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• • •
Connectors should be anchored in both the concrete such that it may develop at least 80% of their yielding stress. Distributed uniformly around the interface, avoiding concentration in specific locations. It is better to use reinforced bars (rebar) anchored with epoxy resins of grouts.
Steel jacketing Local strengthening of columns has been frequently accomplished by jacketing with steel plates. A general feature of steel jacketing is mentioned in Table 2. Table No. 2: Details Of Steel Jacketing. Steel plate thickness Height of jacket
• • • •
Shape of jackets
Free ends of jackets bottom clearance.
• •
Gap between steel jacket and concrete column Size of anchor Number of anchor bolts
• • • •
At least 6 mm. 1.2 to 1.5 times splice length in case of flexural columns. Full height of column in case of shear columns. Rectangular jacketing, prefabricated two L-shaped panels The use of rectangular jackets has proved to be successful in case of small size columns upto 36 inch width that have been successfully retrofitted with %" thick steel jackets combined with adhesive anchor bolt, but has been less successful on larger rectangular columns. On larger columns, rectangular jackets appear to be incapable to provide adequate confinement. Welded throughout the height of jacket, size of weld1” 38 mm (1.5 inch), steel jacket may be terminated above the top of footing to avoid any possible bearing of the steel jacket against the footing, to avoid local damage to the jacket and/or an undesirable or unintended increase in flexural capacity. 25 mm fill with cementations grout. 25 mm in diameter and 300 mm long embedded in 200 mm into concrete column. Bolts were installed through pre-drilled holes on the steel jacket using an epoxy adhesive. Two anchor bolts are intended to stiffen the steel jacket and improve confinement of the splice.
Figure No.2: Construction Technique for Steel Jacketing
IJAERS/Vol. I/ Issue I/October-December, 2011/15-19
International Journal of Advanced Engineering Research and Studies
E-ISSN2249 – 8974
causes beam hinging. The location of the beam critical section and the participation of the existing reinforcement should be taken into consideration. Jacketing of beam may be carried out under different ways, the most common are one-sided jackets or 3and 4-sided jackets. At several occasions, the slab has been perforated to allow the ties to go through and to enable the casting of concrete. The beam should be jacketed through its whole length. The reinforcement has also been added to increase beam flexural capacity moderately and to produce high joint shear stresses. Top bars crossing the orthogonal beams are put through holes and the bottom bars have been placed under the soffit of the existing beams, at each side of the existing column. Beam transverse steel consists of sets of U-shaped ties fixed to the top jacket bars and of inverted U-shaped ties placed through perforations in the slab, closely spaced ties have been placed near the joint region where beam hinging is expected to occur (figure no. 4). The main features of reinforcement details of beam jacketing are given in table 3.
FRP JACKETING E x istin g C o lu m n
C a rb o n fib re / stra p d ip p e d in to e p o x y re sin
C a rb o n fib re w in d in g
Figure No. 3:Carbon Fibre winding Several researchers have investigated the possibility and feasibility of fibre reinforced polymer composite jackets for seismic strengthening of columns winding them with high strength carbon fibres around column surface to add spiral hoops (figure no. 3) The merits of this method are: • Carbon fibre is flexible and can be made to contact the surface tightly for a high degree of confinement; • Confinement is of high degree because carbon fiber is of high strength and high modules of elasticity are used; • The carbon fibre has light weight and rusting does not occur. Limitations: There are some disadvantages associated with the column jacketing techniques well, • In some cases the presence of beams may require majority of new longitudinal b to be bundled into the corners of the jacket; • With the presence of the existing column it' difficult to provide cross ties for new longitudinal bars which are not at the corners of the jackets; • Jacketing is based mostly on engineering judgment as there is a dearth of guidelines. Figure No. 4: Construction Technique for Beam BEAM JACKETING Jacketing. Jacketing of beams is recommended for several Although those guidelines can give a rational basis purposes as it gives continuity to the columns and for practical design, research still needs to address increases the strength and stiffness of the structure. critical aspects in the behavior of jacketed elements. While jacketing a beam, its flexural resistance must The change in behaviour in jacketed elements, whose be carefully computed to avoid the creation of a shear span/depth ratios are significantly reduced, due strong beam-weak column system. In the retrofitted to their jacketing, needs to be clarified. structure, there is a strong possibility of change of mode of failure and redistribution of forces as a result of jacketing of column, which may consequently Table No. 3: Reinforcement of beam jacketing. Minimum width for jacket Longitudinal reinforcement
• •
Shear reinforcement
• •
Depth of jacketed beam
• • • • •
8 cm if concrete cast in place or 4 cm for shotcrete Percentage of steel on the jacket should be limited to 50 of the total area of the composite section. Ignore the effect of existing shear reinforcement New reinforcement should have 135 hooks and at each corner of the tie there must be at least one longitudinal bar. The bar used for the tie should have at least 8 mm diameter Multiple piece ties can be used, as discussed before for columns. Span/depth ratio Storey height Ductile behaviour
IJAERS/Vol. I/ Issue I/October-December, 2011/15-19
International Journal of Advanced Engineering Research and Studies REFERENCES 1.
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Pankaj Agrawal & Manish Shrikhande,”Earthquake Resistant Design of Structures”, Printice Hall of India Private Limited, New Delhi, 2006, pp 256-289. UNDP/UNIDO Project RER/79/015,”Repair and strengthening of Reinforced Concrete, Stone and brick Masonry Buildings”, Building Construction Under Seismic Conditions in the Balkan Regions, 1983, United Nations Industrial Development Programme, Austraia, Vol.5. Rodriguez. and Park R., ‘Repairing and strengthening of Reinforced Concrete Buildings for Seismic Resistance”, Earthquake Spectra, 1991,Vol. 7, No. 3. Katsumata, H.and Kobatake, Y., “Seismic Retrofit with carbon fibers for Reinforced Concrete Columns”, Eleventh World Conference on Earthquake Engineering, Paper No. 293,1996. Alcocer, S. M., “rehabilitation of RC Frame Connections using Jacketing”, Tenth World Conference on Earthquake Engineering, Madrid, Spain, 19-24 July, 1992 IS13945, Repair and Seismic Strengthening of Buildings –Guidelines, Bureau of Indian Standards, New Delhi, 1993.
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E-ISSN2249 – 8974
