Improving the Seismic Response of a Reinforced Concrete Building Using Buckling Restrained Braces

Transcription

1 Bauhaus Summer School in Forecast Engineering: From Past Design to Future Decision 22 August 2 September 2016, Weimar, Germany Improving the Seismic Response of a Reinforced Concrete Building Using Buckling Restrained Braces TRANDAFIR Alexandru Technical University of Civil Engineering Bucharest Abstract In the last years BRB were used in existing structures and in new ones as primary lateral force resisting elements. Retrofit of existing buildings in seismic areas can be made by using buckling restrained braces (BRBs), because they have the ability to sustain large inelastic deformations without important loss of strength. Therefore dynamic nonlinear analysis was performed on a reinforced concrete structure, with buckling restrained braces included. Computation was made for one recorded accelerogram. The aim of the study was to highlight the advantages and disadvantages of using BRBs together with a moment-resisting frame structural system for improving the seismic response of buildings. Parameters like displacements and stresses were carefully evaluated and then some comparatively studies were made in order to establish the efficiency of the buckling restrained braces. Introduction In a country like Romania, where about two-thirds of the territory is affected periodically by strong earthquakes, the proper design of buildings to seismic actions is an essential prerequisite for the safety of the population. During a major earthquake, a large amount of energy is fed into a structure. The manner in which this energy is consumed determines the level of damage. The design criteria stipulated in building codes, including Romanian Standards, are based on the philosophy of capacity design approach. The damage to the buildings and other associated costs for Northridge and Kobe are estimated to be more than US$50 billion and US$150 billion, respectively. These earthquakes have clearly shown that conventional construction, even in technologically advanced and industrialised countries, is not immune to destruction. Braced steel frames are known to be economical and effective in controlling lateral deflections due to wind or moderate earthquakes. During a major earthquake, these structures do not perform that well. A brace in tension stretches during severe shock and buckles in compression during reversal of load. On the next application of load in the same direction, this elongated brace is not effective even in tension until it is taut again and is stretched even further. As a result, the energy dissipation degrades very quickly and the structure may collapse. Moment-resisting frames are favoured for their earthquake resistance capability because properly detailed frames have stable ductile behaviour under repeated reversing loads. However, these structures are very flexible and it is often economically difficult to develop enough stiffness to control storey drifts to prevent non-structural damage. The reinforced concrete building that is studied in this paper was designed following the Romanian design code P100-1/2013. This code contains most of the Eurocodes guidelines. The building was designed to be built in Bucharest and its destination was considered offices.

2 TRANDAFIR Alexandru / FE Structural modelling All analyses reported in this paper were conducted using SAP2000 program, developed by CSI. The geometry The building that is analysed has 10 stories. Each story has 3 meters height with a total height of 30 m. In this paper only the suprastructure was analysed and it was considered fixed at the ground floor level. The structure has 5 bays: 6 meters, 3 meters and 4 meters. Figure 1. Floor plan Loads The following loads were considered in this study: the self weight of the structural elements was automatic computed by the program, live load was taken 2 kn/m 2. Seismic load The accelerogram (figure 2) used for time history analysis was recorded during the earthquake from Vrancea in 1977, because the accelerogram from 1977, component N-S, recorded at INCERC, Bucharest, is the most severe, with PGA=0.2g, in this paper are presented only the results for this seismic input. In figure 3 is presented a comparison between design spectrum according to P100/2006 and the response spectrum of Vrancea 77 N-S accelerogram.

3 TRANDAFIR Alexandru / FE Figure 2. Vrancea 1977 Accelerogram Figure 3. Accelerations Response Spectrum Materials For the columns, beams and slabs the concrete used was C45/55 and the reinforcement used was BST500S. For the buckling restrained braces the steel used was S235. The stiffness of all the concrete elements was reduced to 50% by decreasing the value of the modulus of elasticity. Characteristics of C45/55 are according to Eurocode 2. Structural elements The columns and beams were modelled as frame elements with a rigid connection between them so the beams are moment resistant. All the braces, including BRBs, were modelled as pinned elements. The behaviour of the buckling restrained braces was modelled using axial hinges, with a length of the hinge equal to 4 m, although the element is 7 m long. In order to model the real behaviour of the BRBs their rigidity was multiplied with 1.4

4 TRANDAFIR Alexandru / FE Figure 4. Buckling restrained braces behaviour Equivalent lateral force method Lateral force method of analysis was used for preliminary design. The seismic action is represented by the elastic response spectrum: the reference peak ground acceleration amounts to a g = 0.3g, the value of the period T C = 1.6s, the building is classified as importance class II and the corresponding importance factor amounts to γ I = 1.2. The behaviour factor q, which depends on the type of the structural system, regularity in elevation and plan, and ductility class (DCH) was considered The seismic coefficient value was The following structural elements were determined: columns 70x70 cm, beams 50x30 cm. For designing the BRBs equation (1) was used: (1) Where A req is the required area of the steel core of the buckling restrained brace, N Ed is the axial force determined using a seismic behavior factor of q=6.75, γ M0 is a partial safety factor equal to 1.0 and f y = 235 N/mm 2, the strength capacity of steel S235. Only rectangular sections were considered for the steel core. Table 1. BRBs design Floor N Ed [kn] f yd A req [cm 2 ] A eff [cm 2 ] Validation [N/mm 2 ] Base

5 TRANDAFIR Alexandru / FE Dynamic analysis reports Top displacement The hazard level was defined so: 1 for a reference return period of 40 years (scale factor 0.6) and 4 for a reference return period of 975 years (scale factor 1.8). Figure 5. Top displacement (m) Vrancea 1977 Column axial force at the base section Figure 6. Vertical section through the building The BRBs increased the level of stress in column 23 who belong to the braced frame by 20% to 30%. Figure 7. Compressive stress (kn) in column 23

6 TRANDAFIR Alexandru / FE Column bending moment at the base section The BRBs manages to reduce the maximum bending stress in column 23 by 15%. Figure 8. Bending moment (knm) in column 23 First floor beams The following graphic provides better information on dynamic behavior of frame BRB system susceptible to low intensity dynamic action. Figure 9. Bending moment (knm) in beam 36 Plastic hinges The plastic hinges were modelled according to FEMA 365. The number of plastic hinges formed was 1200, 2 for each structural element. In the stage 2 stress level (scale factor 1) the status of plastic hinges appeared on the structure is different for the two models. The elements of the initial structure are starting to have plastic deformations very fast, eventually reaching Collapse Prevention deformations. Acceptance criteria for this performance level (CP) according to FEMA 356 is radians for beams and columns also. In the structure with BRBs the plastic hinges reached Life Safety deformations. Figure 10. Define plastic hinge for beams

7 TRANDAFIR Alexandru / FE Figure 11. Define plastic hinge for columns Concluding remarks After analysing the behaviour of two structures, one with BRBs and the other with simple moment resisting frames, a few conclusions can be made. The maximum story drift was reduced by using BRBs. In addition, a better distribution of the drifts along the height of the structure was obtained by adding these elements. This can be a solution for avoiding the possibility of appearing a weak story mechanism. Another advantage of using BRBs is that the top displacement was reduced by approximately 10%, so in case of close structures the top displacement can be limited using this kind of system. After adding buckling restrained braces in that frames, beams presented a better behaviour, plastic rotations decreased more than 10 times and the maximum moments recorded in plastic areas were reduced. The axial stress at the bottom column section had varied according to their connection to the BRB. Column 17 has no direct connection with the BRB the stress decreased by 11%. The bending moment in column 17, 23 decreased by 15% and in column 27 by 9%. A small disadvantage of the BRBs is that they can increase the level of stress in other structural elements who belong to the braced frames. References ACI (2002). Building code requirements for structural concrete and commentary. American Concrete Institute. Bordea S., Dubina D. (2009). Retrofitting/upgrading of reinforced concrete elements with buckling restrained bracing elements.. In: Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering, Computers and Structures Incorporated (2015). CSI Analysis Reference Manual for SAP2000. Berkeley, California, USA. Computers & Structures Inc. Dunai, L. (2011). Type testing of buckling restrained braces according to EN Budapest University of Technology and Economics. Eurocode 0 (2002). Basis of structural design. European Committee for Standardization. Eurocode 1 (2002). Actions on structures Part 1-1: General actions Densities, self-weight, imposed loads for buildings. European Committee for Standardization.

8 TRANDAFIR Alexandru / FE Eurocode 2 (2002). Design of concrete structures Part 1-1: General rules and rules for buildings. European Committee for Standardization. Federal Emergency Management Agency (2000). FEMA 356: Prestandard and commentary for the seismic rehabilitation of buildings. Federal Emergency Management Agency P (2013). Romanian Seismic Design Code-Part I: Design Provision for Buildings. Technical University of Civil Engineering Bucharest Sabeli, R., Lopez, W. (2004). Design of Buckling-Restrained Braces Frames. Starseismic. Preliminary design of BRBF system. Use of equivalent force method. Vamvatsikos, D., Cornell, C.A. (2002). The Incremental dynamic analysis and its application to performance-based earthquake engineering. In: Proceedings of the 12th European Conference on Earthquake Engineering; London, UK, Paper No. 479.