Use of Cables in Rehabilitation of Reinforced Concrete Buildings Reza Hassanli*, Sina Hassanli**

Transcription

1 Use of Cables in Rehabilitation of Reinforced Concrete Buildings Reza Hassanli*, Sina Hassanli** * School of Natural and Built Environments, University of South Australia, (reza.hassanli@unisa.edu.au) ** Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran (sina.hassanli@aut.ac.ir) ABSTRACT "Cable" as a high nonlinear element, has a unique characteristic of having high flexibility and loadcarrying capacity. Cables are widely utilised in strengthening and upgrading of bridges. The successful experience in application of cables in bridges can be extended to building. The main purpose of this paper is to investigate the efficiency of using cables in RC structures for retrofitting purposes. Different methods for rehabilitating individual elements (simplified rehabilitation method) and the whole building (systematic rehabilitation method) are discussed and the effectiveness of application of cables in the structural stiffness is investigated. In order to reach this goal, a reinforced concrete frame designed first. This frame was vulnerable to damage under ground excitation of a high seismic zone. The frame was then modelled and systematically rehabilitated with cables applying three different strategies. After analysing the models, the effects of each retrofitting strategy in structural behaviour, plastic hinge formation, performance point and storey displacement was investigated. Based on the results, cables can be used to improve the behaviour and upgrade the stiffness of the system. Especially, it could be used temporarily for building during the construction or for temporary structures. KEYWORDS Cable; Rehabilitation; Nonlinear; Pre-stressing; Pushover Analysis INTRODUCTION Application of cables in bridge (Figure 1) is a practical way to improve the behaviour and increase its capacity. This paper aimed to extend this idea and apply it to buildings for rehabilitational purposes. As usually the cost of replacement of vulnerable structures is high, the development of new rehabilitation and strengthening techniques is required to increase the life of many existing structures. Although there are number of repair and strengthening techniques for reinforced concrete structures, unfortunately, the majority of them are too expensive, time consuming and require the interruption of use of the structure whilst works are carried out. Hence, there is a pressing need for the development of improved, low cost and less disruptive techniques, which will make necessary interventions in many structures economically viable. It should be kept in mind that the cost of retrofitting buildings is the primary factor which deters many private owners from executing essential works.(frangou and Pilakoutas, 1995; Moghaddam et al., 2007) Pre-tensioning provides the RC member with an immediate and active load-carrying capability. Tensioning of an external cable is an appropriate technique to prestress the reinforced concrete member and is a convenient method to reduce the effect of low tensile strength in concrete. The main advantages of external restressing are reducing deflection, providing high fatigue and impact resistance and crack free members and the ability to restress, distress and exchange any external prestressing cable. On the other hand main disadvantages of external prestressing are: - Applicable for members with a considerable section depth. - Exposure to environmental influences (fire, vandalism, aggressive chemicals etc.).

2 - Difficulties in handling of the tensioning devices For both "Simplified Rehabilitation Method" and "Systematic Rehabilitation Method" ( FEMA ), it seems that cables are able to improve the member and structure. Figure 1. Rehabilitation of a bridge with cables, DYWIDAG-systems international Bracing of a tree SIMPLIFIED REHABILITATION METHOD In simplified rehabilitation method the focus is on the rehabilitation of structural members individually. Rehabilitation of beams Cable as shown in Figure 2 is capable of enhancing the flexural capacity of a beam. This method is common in rehabilitation of bridges but is rarely used in buildings, due to small dimension of beams depth in ordinary buildings. Pre-tensioned cables impose a compression force to the beam, which is favourable because of low tensile strength in concrete. Figure 2 shows an appropriate way of increasing positive and negative capacity of a beam by using external cables. Figure 2. Rehabilitation of beams with cables Rehabilitation of columns In moment resisting frames, cables can be used to increase flexural capacity, the same procedure as mentioned for the beams. Another technique is to confine the concrete columns by post- tensioning high-strength packaging straps around the column (by application of standard strapping machines used in the packaging industry) and subsequently locking their ends in metal clip. (Moghaddam et

3 al. 2007) The low cost of strip and speed and ease of application of the strapping technique make this method efficient for use as a repair and strengthening technique for RC structural members. An RC column would normally require six man days work to be jacketed whilst a maximum of two days work is required for external strapping, which clearly demonstrates the cost saving when using the proposed technique.( Frangou and Pilakoutas, 1995; Moghaddam et al. 2007) Rehabilitation of footings Generally rehabilitation of footing is highly expensive and requires extensive and time-consuming operation. A frequent technique of strengthening the flexural capacity is to raise the height of the footing. This method causes reduction in the storey effective height, which might results in architectural deficiencies and serviceability problems. It seems that in single, strip and mat footings, cable could effectively improve the flexural capacity. The advantage of the method is that it does not reduce the storey effective height and also could be used to increase both negative and positive moment capacity (Figure 3) Figure 3. Rehabilitation of foundations strip footing mat footing (c) single footing (c) Rehabilitation of slabs In all standards there are some limitations to prevent excessive deformation and probable shaking of slabs. (Usually by limiting the slab thickness) If excessive sagging and vibration is observed, slab should be repaired. Raising the height of the slabs is a common method but as discussed before it has some disadvantages. Cable can be used in slabs as well as footings. The procedure includes: punching two side of the slab, passing a cable, pre-tensioning and anchoring the cable properly to the surface of the slab. The number and diameter of cables required, depend on dimensions and thickness of the slab and magnitude of loads applied to it. In two-way slabs, this method can be used in two main directions. SYSTEMATIC REHABILITATION METHOD In order to do systematic rehabilitation, a building should be regarded as a unit and the objective is to improve the behaviour of the building in general rather than individual members. If the structure can not undergo the displacement equal to the displacement of performance point without extensive damage, the lateral stiffness and strength of the structure is not adequate. The required capacity can

4 be obtained by strengthening the lateral stiffness of the system. One of the most common methods to enhance lateral stiffness is bracing the building with inclined steel members. Although this method is widely used in steel frames, its advantages made it practical in reinforced concrete frames (Khaloo and Mohseni, 2008). Cables, as well as steel, can also be used as bracing elements inside (internal bracing) and outside (external) the concrete frames. This paper investigates the effectiveness of cables for systematic rehabilitational purposes and its ability to enhancing the structural lateral stiffness. It could be used temporarily for building during the construction or for temporary structures. The main idea of the this method is taken from a method of bracing of trees (Figure 1) MODELIING In order to investigate the effectiveness of of using cables, the behaviour of a 4-story frame is investigated before and after rehabilitation with cables. In the first step the initial model was designed based on the Iranian Building codes. The mass and vertical loads were evaluated and applied to the structure according to Iranian building codes. Vertical loads consisted of line loads and point loads applied on the beams and joints respectively. This frame was designed for moderate lateral load, therefore it was vulnerable to damage under ground excitation of a high seismic zone The designed model was a 4-story three-span frame with 3 meters story height and 5 meters span length. Rectangular concrete section of 40cm*40cm and 30cm*60cm was used for columns and beams, respectively. The dimensions and details of reinforcement of beams and columns were the same for all stories. The initial weak frame then systematically rehabilitated with three strategies: - Rehabilitation with x-bracing cable elements (Figure 4) - Rehabilitation with inclined-bracing cable elements (Figure 4(c)) - Rehabilitation with both inclined-bracing and x-bracing elements (Figure 4(d)) The adequacy and performance of each strategy was then investigated by pushover analysis. (c) (d) Figure 4. Initial frame, frame with X-bracing, (c) frame with inclined-bracing, (d) frame with inclined- bracing and X-bracing

5 Bases shear (kg.f) Displacement (m) ANALYSIS In order to perform non-linear pushover analysis, hinge properties of the components determined first. Hinge properties contain the plastic rotation values that a component s end can carry and the acceptable plastic rotation varies depending on the structural performance level (i.e. immediate occupancy, life safety, collapse prevention and etc). For this research hinge properties were taken from ATC-40 and FEMA-356 based on the component type and failure mechanism. (ATC , FEMA ) Failure mechanism of all components is assumed as flexure failure and all components were assumed to be properly reinforced against shear failure. Hoop spacing of all components was considered to be sufficient to provide conforming section features, so that all failure mechanisms were controlled by flexure. Four and two types of hinge properties were assigned to the columns and beams respectively base on FEMA-356. Five-precent of the component length from the edges assumed as hinge locations. Cables assumed to be force-controlled members. Table 1 shows acceptance criteria for plastic hinges, defined for columns and beams. Table 1. Plastic hinge properties Member a b c IO LS CP Column Beam After adding required cables in the model, hinge parameters and properties were assigned to the elements and pushover analysis ran to obtain the capacity curve of the structure. Frames were subjected to gravity and lateral loads and the distribution of lateral load assumed to be linear. In static nonlinear analysis, the behaviour of the structure is characterized by capacity curve. (Base shear versus roof displacement) The obtained capacity curves of investigated models versus spectral displacement coordinates are illustrated in Figure 5. Table 2 also presents the tabulated displacements and consequent base shears corresponding to different methods. For the determination of performance point and building capacity level, spectrum method was used. In this method the capacity and demand spectrum of the earthquake were plotted together in Acceleration Displacement Response Spectrum (ADRS) format. Table 2. Displacement and base shear of the performance point Frame Figure 5. Capacity Curve and Performance point Initial Frame X-bracing Incline Bracing Inclined+ Xbracing

6 Beyond E D to E C to D CP to C LS to CP IO to LS B to IO A to B Comparing performance point of four frames, the frame retrofitted with both inclined-bracing and X-bracing methods exhibits higher strength. The base shear changed from 1063kN in the initial from to 1679kN in the frame with incline and X bracing. Figure 5 shows that as it was expected, the lateral stiffness of the frame with both incline and X bracing was considerably greater than the initial frame. The inclined-bracing was more efficient than x-bracing in strengthening the lateral stiffness. The distribution of plastic hinge in the expected performance level is shown in Figure 6. It demonstrates the stage in which the displacement of the control point of the roof reaches the performance point displacement. Table 3 presents the number and type of plastic hinge for different performance level. (c) (d) Figure 6. Hinge distribution at performance level initial frame, frame with x- bracing, (c)frame with inclined-bracing,(d) frame with inclined- bracing and X-bracing (e) hinge specification colour (e) Table 3. Comparison of hinges at the final step Frame Initial Frame X-bracing Incline Bracing Inclined+ X-bracing

7 At the performance point there are 8, 6 and 5 plastic hinges between C to D level (Figure 6.e), in the initial, x-bracing and inclined-bracing frame respectively. For the frame retrofitted with both inclined-bracing and x-bracing members, there is not any plastic hinge beyond LS level. Step-by-step pushover analysis and comparing the frames shows that: Generally, first plastic hinges formed at the end of the middle span beam of the first storey. The plastic hinges in columns formed firstly in the middle span columns of topmost storey followed by side columns in the first storey. None of the columns exhibit force-control hinges and brittle behaviour. Sequence of formation of hinges was almost the same in all models. The effective time period of initial frame, frame with X-bracing, frame with inclined-bracing cables and frame retrofitted with both inclined-bracing and X-bracing members, was 2.22, 2.03, 1.57, 1.40 respectively. Figure 7 compares the story displacements and inter-story drifts in different models. In the model with the inclined and X-bracing, the roof displacement and drift was reduced by 38% compared with the initial frame. It shows the ability of retrofitting strategies to decrease the storey drift. Figure 8. Story displacement and Comparative displacement CONCLUSIONS The research concluded that cables are capable of enhancing the lateral stiffness of structures and could be applied temporarily for building during the construction or for temporary structures. Performance based analysis done for different rehabilitation strategies confirmed that as bracing members, cables can increase the structural lateral stiffness. However the effectiveness of each strategy is different. Roof displacement in frame retrofitted by x-bracing, inclined-bracing and both inclined-bracing and x-bracing was 10%, 27% and 38% fewer than that of initial frame, respectively. REFERENCES ATC, (2000), Evaluation and Retrofit of Concrete Building, ATC-40, Volume 1and 2, Report NO.SSC Badous, M. and Burdet O., (2000), Comparison of Internal and External Prestressing for Typical Highway Bridges, 16th congress of IABSE, Lucerne. BHRC, (2005), Iranian code of practice for seismic resistance design of buildings: Standard no.2800 (3rd edition), Building and Housing Research Centre. FEMA 356, (2000), Prestandard and commentary for the seismic rehabilitation of buildings. Washington (DC): Federal Emergency Management Agency.

8 Frangou, M. and Pilakoutas, K., (1995), structural repair/strengthening of RC columns, Construction and building materials, 9, Khaloo A.R and Mohseni M. Mehdi, (2008), Nonlinear seismic behaviour of RC frames with RC braces, Asian Journal of Civil Engineering (Building and Housing), Vol. 9, No.6, PP Sofi A. and Muscolino G. B., (2002), Dynamic Analysis of Prestressed Cables with Uncertain Pretension, Meccanica 37: Structural Engineering Association of Buildings, Sacramento, California. California (SEAOC), (1995) Performance Based Seismic Engineering of Moghaddam H., Samadi M. and Mohebbi S. (2007), RC Members Strengthening by lateral post-tensioning of external metal strips, International Earthquake Samposium, Kocaeli, PP Sap-2000 tutorial (October 2006), Linear and Nonlinear Static and Dynamic Analysis and Design of Three- Dimensional Structures, Version 11.0 Wu, I. W. J., Cai C. S and Chen S. R. (2004), Experiments on reduction of cable vibration using MR dampers, 17th ASCE Engineering Mechanics Conference