ADVANCED SEISMIC ANALYSIS METHODS AND APPLICATION TO EARTHQUAKE DAMAGED BUILDINGS STRENGTHENING DESIGN

Transcription

1 ADVANCED SEISMIC ANALYSIS METHODS AND APPLICATION TO EARTHQUAKE DAMAGED BUILDINGS STRENGTHENING DESIGN Zheng Ping Wu ABSTRACT: Using advanced modal response spectrum methods, the current practice of the New Zealand standards and the guidelines/regulations of the national and regional authorities, this paper presents the investigations on the buildings subjected to seismic damages and proposes respective strengthening methodologies. Two engineering cases were investigated: one five story office building and one L-shaped two storey retail building. Detailed strength capacities in terms of New Building Standard (NBS) as well as the overall behavior of the buildings were achieved based on the detail modal response spectrum analysis. Strengthening was designed successfully based on the latest engineering standards and regulations. It was found to be imperative to employ advanced modal response spectrum analysis for all the horizontally and/or vertically irregular buildings. Further researches were recommended: (a) to refine the formula for seismic shear distribution to roof in New Zealand standard, and; (b) to better understand the energy dissipation mechanism in the connection details of the concentric braced frames. KEYWORDS: Modal response spectrum analysis (RSA), concentric braced frame (CBF), New Zealand standards. INTRODUCTION A comprehensive structural assessment for an existing building is always a complex task, especially for an earthquake damaged existing tall buildings. They were constructed decades ago and normally only limited engineering documents are available. For a structurally irregular building, it requires the structural engineers even more to utilize an advanced analysis tool such as commercially available software ETABS or SAP 2000 to carry out the full modal response spectrum analysis. Indeed, the response of any building under the coarse seismic actions is very complex. It is hard to understand the overall structural response of the building without detail computer analysis of the whole structure. It is required to collect sufficient modal responses of the structure before an almost true and full response of the building could be achieved. Using the advanced modal response spectrum analysis methods, the current practice of the New Zealand Standards and the guidelines/regulations of the national and regional authorities, the purpose of this paper is threefold: To study the structural layout of the building and its necessity using advanced analysis tool when carrying out structural seismic response assessment. To assess the building s structural response under the seismic actions and propose respective repair and strengthening methodologies. This is to bring the earthquake damaged building back to its intended service while being able to sustain the code required seismic actions. To investigate the earthquake resistance capacity of the individual element and to carry out its strengthening design if needed. Engineering projects used are the comprehensive structural assessments and strengthening methodologies for two buildings in Christchurch damaged in the September 200 and February 20 earthquakes and aftershocks. One is a five storey reinforced concrete office building and another is an L-shaped two storey reinforced concrete commercial retail building. This paper also outlines the criteria used in the modal analysis, and; the guidelines/ regulations in relations to the seismic modal analysis and the strengthening design. For both engineering projects, it was aimed to establish: a) the current condition of the building structures, including its seismic resistance strength of the individual structural elements and the building as whole, and; b) the repair and strengthening methodologies. Based on the site investigation and the detail modal analysis, the comprehensive assessment for the building s strength capacity was achieved, from which repair and strengthening methodologies were designed successfully. Different strengthening concepts were adopted for these two buildings. While individual column strengthening was chosen for the five storey building and the upper level of the two storey L-shaped building, the concentric bracing frames were adopted for the ground floor of the two storey building. Discussions were given to the seismic load distribution to the roof level and the plastic energy dissipation design of concentric braced frame, whereby further researches were recommended. Zheng Ping Wu, Harrison Grierson Consultants Ltd, Christchurch. z.wu@harrisongrierson.com

2 2. STRUCTURAL SEISMIC ANALYSIS In this paper, modal response spectrum analysis [] (RSA) was used. It is an approximate method of dynamic analysis. For a single degree freedom system (SDOF) with the same damping ratio and different natural frequencies, it gives the maximum (peak) response (acceleration, velocity or displacement) when responding to a specific seismic excitation. For a structure with n- degree of freedom, it is transformed to n single-degree systems, whereby response spectra principles could be applied to the systems with multiple degrees of freedom. In general, for a multi-degree freedom (MDOF) system subjected to ground seismic action, its equation of motion is expressed as [ M ]{ uɺ } [ C]{ uɺ } + [ K]{ u} = [ M ]{ B} uɺ g + () Where [ M ] is the mass matrix. By neglecting the mass coupling effect, it is a diagonal or uncoupled mass matrix in the form of tributary lump masses to the corresponding displacement degree of freedoms. [ K ] is the stiffness matrix. [ C ] is the damping matrix accounting for all the energy dissipating mechanism in the structure. { B } is the displacement transformation vector defining the degrees of freedoms that the seismic action applies. In general term, the displacement { u }, velocities { } { uɺ } uɺ acceleration ɺ of the structure and the ground motion uɺ ɺ g are all function of time. In explicit matrix form, the mass, damping and stiffness are expressed as the follows. [ M ] m 0 = 0 0 m m nn (2) values of forces and deformations over the duration of the earthquake-induced excitation directly from the earthquake response spectrum without undertaking response history analysis of the structure. By doing so, the dynamic analysis is reduced to a series of static analyses. For each mode, the static analysis for a structure subjected to forces, f n, produces the respective modal response, φ n. It is then multiplied by the spectral ordinate, A n, to obtained the peak modal response r no, i.e. r no { φ n } A n = (5) In order to find out the modal response φ n of the structure, [ C ] and uɺ ɺ g are set to be zero in Equation (), it then becomes [ M ]{ uɺ } + [ K]{ u} = 0 ɺ (6) It is further rearranged to ω n (7) 2 [ K] [ M ] { φ } = 0 Where { φ n } is the deflected shape matrix, i.e. dimensionless natural mode shapes. Solution to this equation is obtained using its corresponding natural frequencies ω by setting i 2 [ K] ω [ M ] = 0 (8) Having achieved the mode shapes { φ n }, the maximum (peak) response can be established using the method shown in Equation (5) or graphically shown in Figure below. [ C] c c = cn 2 c c c 2 22 n2 c n c 2n cnn (3) [ K] k k = kn 2 k k k 2 22 n2 kn k 2n knn For a multi-degree of freedom (MDOF) system, it is often accurate enough for a general structural engineering application not to carry out a response history analysis. These structures are often excited by a single component of the ground motion at one time (e.g. acceleration in either x-x or y-y direction), where multiple support excitation is not considered. In other words, the simultaneous action of other two components is not considered. Also, all the supports of the building structure are assumed to be excited simultaneously by the same excitation. Based on these assumptions, the response spectrum analysis procedure calculates the peak response (4) Figure : Resultant response and modal components Mode shapes of low-order mathematical expression tend to provide the greatest contribution to structural response. As orders increase, mode shapes contribute less, and are predicted less reliably. It is reasonable to truncate analysis when the number of mode shapes is sufficient. In the above procedure, one fact is worthwhile to be noted that, although the response spectrum analysis solves a series of static analyses, it is still a dynamic analysis procedure due to that it adopts the vibration properties in its procedure development. These properties are natural frequencies, natural modes and damping ratio. These are the dynamic related nature of the structure. It also uses the dynamic characteristics of the ground motion through its response (design) spectrum. One of the main advantages of RSA is that these dynamic features have been done in

3 developing earthquake response spectrum, whereby the earthquake excitation has been characterized by the smooth design spectrum. 3. ANALYSIS CRITERIA FROM THE CODES AND STANDARDS In order to ensure the reliability of the structural seismic analysis, especially the commonly used modal response spectrum methods, the structural design codes and standards of every country/region provide a full set of criteria that governs and verifies the results of the computer analysis. In New Zealand codes, these are mainly given in AS/NZS 770.5: 2004 [2]. They are a) the mass participation ratio; b) the base shear ratio, and; c) methods of the modal combination. In addition, NZS 30 [4] requires: d) reduction factor for the reinforced concrete structural members: beams, columns, walls and floor slabs. 3. MASS PARTICIPATION RATIO For the modal response spectrum analysis, it is required by AS/NZS 70.5:2004, that sufficient number of modes shall be included to ensure the minimum 90% of the total mass participated in the dynamic calculation. It is particularly important for each of the structure s orthogonal principal directions. 3.2 BASE SHEAR RATIO Theoretically, the design spectrum used in the modal response analysis consists of pairs of values: period versus acceleration or period versus displacement. These acceleration or displacement values obtained from the geological data for the particular site have often been normalized. It means that the values of acceleration or displacement have been divided by a number (i.e. normalization factor) which represents some reference value. One of the commonly used normalization factors is 'g', the gravity acceleration. In order to reinstate the actual seismic magnitude, a scale factor is required in the computer analysis. It can be initially calculated as the follows for the units of kn-m. S p Scale factor = 9. 8 (6) Where S p k µ is the structural performance factor. In accordance with AS/NZS770.5:2004, k µ is given as the follows. For soil classes A, B, C and D k = µ For s µ T 0. 7 (7a) For soil class E ( µ ) T = For 0.4 s T < 0. 7 s (7b) k = µ For s µ.0 s T <. 5 (7c) ( µ ) T For 0.4 s T. 0 s = + < 0.7 and T (7d). 5 s If kip-in units are used in the computer analysis, 9.8 shall be replaced by (in/sec2) in Equation (6). After initial analysis, this initial calculated scale factor should be reviewed based on the resulted base shear due to all modes (i.e. the sufficient number of modes that achieves 90% mass participation). The scale factor shall then be adjusted to a value such that the dynamic base shear reaches more than 80% of the base shear calculated using static equivalent method. 3.3 MODAL COMBINATION To achieve the maximum (peak) response of the structure under the ground seismic actions, various modal combination methods are available, namely: i) square root of the sum of the square (SRSS); ii) root mean square method; iii) complete quadratic combination (CQC) method, and; iv) absolute sum (ABSSUM) method. Research had shown that ABSSUM gives always an overestimate the response. Commonly adopted ones are hence CQC and SRSS methods. In AS/NZS 70.5:2004, it is recommended that: a) When the modal responses for different modes are not coupled, SRSS shall be used; and b) When the modal responses for different modes are coupled, CQC combination method shall be used. In practice, due to the complex of the structure layout, CQC shall be used in most situations. In engineering application, the seismic actions in two or more orthogonal horizontal directions are often analyzed and combined for design. To combine the effects of these orthogonal directions, either SRSS combination method or by using the load combinations could be used. 3.4 MEMBER REDUCTION FACTOR For the concrete structural members, cracking shall be taken into account in the seismic response analysis to obtain the reliable computer results. This can be facilitated by using the effective section properties for the respective forces, for which the guidelines could be found in Section 6 of NZS30: Part2:2006. For the applications presented in this paper, the following reduction factors were adopted: For wall: a).0 (i.e. no reduction) for the horizontal axial forces, shear forces of both in-plane and out-of plane; b) 0.33 for vertical axial forces; and c) 0.25 for the in-plane and out-of-plane moment forces. Reduction factor of 0.8 and 0.4 were used for the concrete columns and beams, respectively. 4. EARTHQUAKE DAMAGED BUILGINGS ASSESSMENT Two cases were analyzed using the modal response spectrum method based engineering software ETABS: one two storey L-shaped commercial retail building and one 5 storey office building. Both buildings were damaged in September 200 and February 20 earthquakes and aftershocks. The purpose was to carry out structural assessment and propose strengthening methodologies to bring the buildings back to service while meeting current statutory requirements of the

4 structural strength capacity, i.e. minimum 67% strength of New Building Standard (NBS). For the two storey L-shape building, its L-shape layout produces the horizontal torsion deformations under seismic actions. On the first floor level, there are terminations of the masonry walls for the stair wells at the both ends of the L-shape and the terminations of the centre core walls. All these together produce both horizontal and vertical structural irregularity of the building. For the five storey office building, its core walls were arranged on one side of the building, which makes the structure subjecting to large stiffness eccentricity in horizontal plan, hence producing torsion deformation under seismic actions. In vertical elevation, the irregular window openings and the reinforced concrete wall filled external wall have all accumulated up the vertical irregularity. Hence, in accordance with Section C4.5 of NZS70.5 Supp :2004, both buildings shall be analyzed using a rigorous method for its seismic response. For both cases, accidental eccentricity considered was ± 0. times the plan dimension of the structure perpendicular to the action of the seismic acceleration. Based on AS/NZS70.0:2002 [3], following load combinations were analyzed for the earthquake effects to the building structures. ) G +Ψ E Q + E x-direction E y-direction 2) G +Ψ E Q E x-direction + E y-direction Where Ψ E is the earthquake combination factor for the live loads. concrete floor slabs throughout all floors providing diaphragm actions. Core-wells were arranged on one side of the building at the front and rear for the access stairs and the lifts. External wall were of brick infill to the frames, except the rear walls were of reinforced concrete wall on 3 rd and 4 th floor. The front wall was of brick veneer wall supported on the frame beams. The foundations were separate footings for the internal columns and strip footings on the perimeters. The internal footings were tied in both directions by the gird beams in both directions. The building had gone through the alterations on the ground floor and the top floor in around 995 and Summary of the Earthquake Damages Site inspections were carried out in July and August 203 to identify the damage extents to the building. It was found that the damage was of substantially structural to the reinforced concrete walls and the columns throughout. Typical damage cracks are shown in Figure 3. a) Horizontal cracks in column at the beam s soffit 4. APPLICATION I: FIVE STOREY OFFICE BUILDING As shown in Figure 2, the five storey building, approximately 20 m x 30 m in plan, was constructed in 952 as an extension from its two storey existing factory building next. It was the main building of the entertainment complex called Sol Square in the centre b) Cracks in reinforced concrete stair well wall Figure 3: Typical earthquake damage Figure 2: Five storey reinforced concrete framed office building (Typical floor plan layout) of Christchurch before the major earthquakes. It had a reinforced concrete beam and column frame system to resist vertical and horizontal loads with reinforced 4..2 Structural Assessment and Strengthening Design Based on the existing drawings and the site inspection, ETABS models was set up to analysis the structural response under the seismic actions, as shown in Figure 4. To achieve 95% mass participation ratio, 20 modes were included in the calculation. The first five modes are shown in Figure 5 to Figure 9. It was seen that the building undergoes substantial twisting in the primary responses. The transverse direction of the core wall (i.e. x-direction) subjects to large deformation as a result of the twisting. It was also shown that the external reinforced concrete parapet wall on each floor level was weak in its out-of plane direction during response to the earthquake. Based on the ETABS results, the beams and columns were all checked. The results were expressed as the strength capacity over the force demands in terms of

5 percentage of the New Building Standard (%NBS). Figure 0 shows the results for the frame at the front wall. It was Figure 4: ETABS analysis model Figure 8: Mode 4 Figure 5: Mode Figure 6: Mode 2 Figure 9: Mode 5 found that the building was weak in resisting lateral seismic loads. This is quite common for the old buildings in Christchurch. Historically, the old buildings were designed based on much smaller seismic resistance requirements than it is in the current practice. The engineer at old time found that the building design was actually governed by the vertical gravity load other than the seismic lateral loads. To strengthen the building, two schemes were proposed for the upper structure: ) to strengthen all weak columns; and 2) to strengthen the selected columns in combination of adding several shear walls. The first scheme maintains the current seismic resistant frame system; whilst in the second scheme, the shear walls would be added and become more effective in reducing the twisting of the building. Figure shows the typical strengthening design for the columns. Both schemes require the foundation strengthening. For the foundation strengthening, a raft foundation was proposed. It aimed to reduce the bearing pressure as the geotechnical investigation found that the bearing capacity of the ground was actually much lower than the required capacity, especially for the service limit state. By using the raft foundation, it would also improve the building behavior during seismic induced liquefaction events by bridging over the liquefied zone in the instance of strong earthquake. Figure 7: Mode 3

6 Figure 0: Strength %NBS for the front frame Figure 2: L-shaped two storey retail commercial building (Ground floor plan layout) Figure : Front and rear frame column strengthening These proposed strengthening designs were carried out successfully and presented to the client in time. However, due the significant cost of the strengthening, the decision was made to demolish the building for the redevelopment by its new owner. 4.2 APPLICATION II: L-SHAPED TWO STOREY RETAIL COMMERCIAL BUILDING As shown in Figure 2, the L-shaped two storey commercial retail building located in the north of Christchurch was constructed in 987 with the ground floor being un-reinforced concrete slab-on-grade. The first floor was constructed of 75 mm reinforced concrete topping on 75 mm thick precast planks spanned on reinforced concrete beams. The north and west ends of the building have precast concrete and reinforced masonry concrete walls surrounding stair wells for accessing to the upper floor. The central lift core located on one side of the L shape corner was constructed using reinforced concrete masonry. The lateral load resisting system for the lower floor consists of the concrete shear walls located in the middle and two building ends, whilst the upper floor s lateral resistance is dependent on the cantilever capacity of the individual columns due to the fact that there was no roof bracing constructed. The building is founded on 50mm square by 9.5 m long precast reinforced concrete driven piles. a) Cracks in reinforced concrete column at first floor beams soffit b) Vertical cracks in ground masonry wall Figure 3: Typical earthquake damage 4.2. Summary of the Earthquake Damages Series of site inspections were carried out since the major earthquake in September 200 and the interim detail engineering evaluation (DEE) reports were produced for the client to monitor the status of the building and to assess the building s suitability for its service continuity. The latest site inspections were conducted in May and June 203. Structural damages are: a) substantial subsidence of the un-reinforced concrete ground slab; b) horizontal cracks in the columns at the soffit of the first

7 floor beams, and; c) cracks in the ground masonry walls. Figure 3 shows the typical damages Structural Assessment and Strengthening Design Based on the existing drawings obtained from the Christchurch City Council and the site inspection, ETABS model as shown in Figure 4 was set up to analysis the structural response under the seismic actions so as to establish the strength status of the building. Figure 6: Mode 2 Figure 4: ETABS analysis model At the beginning of computer analysis, 5 modes were included in the calculation. However, this could only achieve around 75% mass participation. After few trials, 90 modes were included in calculation. It achieved more than 97% mass participation for the lateral translational actions and more than 95% mass participation for the rotational action in all x-x, y-y and z-z direction. To include so many modes in achieving the required mass participation shows that the structure s seismic response was mathematically highly loosely scattered, or structurally extremely irregular. It proves again that the modal response spectrum analysis was imperative in such Figure 7: Mode 3 Figure 8: Mode 4 Figure 5: Mode Figure 9: Mode 5

8 Figure 20: Mode 6 structural layout. Figure 5 to Figure 20 show the first six modes. It was seen that the primary modes exhibited substantial twisting and open-up/close-down of the L shape. With regards to the base shear and the share distribution to the storey levels, Table and Table 2 show the seismic load distribution to the roof and the first floor calculated based on the ETABS calculation and the static equivalent methods. Table : Seismic Loads Based on ETABS (kn) DL SDL LL Mass: DL Seismic Total + SDL LL force for ETABS seismic force Roof First Storey (NBS). It would explain very well why the upper columns and walls did not crack during the two major Christchurch earthquakes in 200 and 20. Based on the static equivalent calculation, the strength would be around 40%NBS. If that were the case, the upper columns should have been failed. There should be at least some hairline cracks on the surface of the upper columns, which was not the case based on the detailed series of site inspections. To strengthen the upper columns and wall to 00%NBS, box jacketing to the half height of the upper columns and steel member (PFC) strengthening to the full height of the upper wall were adopted. Based on the detailed site inspections and comprehensive structural analyses, concentric braced frames (CBF) on the ground floor were selected together with the ground tie beams system. Figure 2 shows the strengthening plan layout, where the CBFs were designed to enhance the lateral seismic load resistance from the upper level to the ground. The tie beams were designed as the collectors to ensure the diaphragm actions of the first floor being transferred to the CBFs. They were to be fixed to the transverse reinforced concrete frame beams and the first floor slabs. These collector beams also worked as perimeter ties of the building. Figure 22 shows the typical layout of the CBFs. The connection details to the base were given in Figure 23, where clear space of 40 mm (i.e. two times of the connection plate thickness) was given to facilitate the plastic deformation of the connection during seismic events. Table 2: Seismic Loads Distribution: static equivalent (kn) Height (m) Mass x Height 0.08 x Base shear 0.92 * Storey Contribution seismic ratio force Based on Cd(T) x mass Roof First Storey Based on the ETABS calculation, the primary period T = second and the total mass ( G + Ψ E Q ) for seismic action is kn. The resulted C d (T ) = 0.696, and the base shear = kn, respectively. The base shear from ETABES is kn. Hence the ratio of ETABS calculated base shear to that of the static equivalent method was 89.5%. It is greater than the code required 80%, hence satisfactory. With regards to the seismic shear distributed to the roof, it was seen that the static calculation was 43.3% greater than that of the ETABS calculation. However, the ETABS result agrees very well with the results of the roof mass multiplies C d ( T ). Based on the ETABS results, the strength checks were carried out. It is found that the existing columns had approximately 60% of the current New Zealand Standard required strength, i.e. 60% New Building Standard, Figure 2: Strengthening layout plan Based on the geotechnical investigation, the site is subjected to liquefaction in the layers from.8 m to 2.6 m and 7.0 m to 8.2 m. Both layers are well within the depth of the piles. It was hence imperative to ensure the robustness of the critical columns in the strong earthquake events. As such, ground tie beams were designed to bridge these critical columns to the adjacent pile foundations. Figure 24 shows the details of the ground beams at the location of these critical columns, which in

9 this case was defined as the columns located close to the location of the CBFs. mode distribution fails to account for the effect of the higher modes. It tends to increase the shear in the upper storeys. It defines the shear distribution as given in Equation below. n F = + i Ft 0.92V Wi hi / ( Wi hi ) (8) i= Figure 22: Conccentric bracing frame (CBF) Where F t = 0. 08V at the top level and zero elsewhere. V is the total base shear. W i and hi are the storey mass and its height from the base. Similar consideration has been given in the Uniform Building Code (UBC) [5] and in the National Building Code of Canada (NBCC) [6, 7]. In addition, NBCC and UBC recognize the influence of the building s primary period, which reflects the overall stiffness of the building structure. The distribution of shear is given as n F = + i Ft ( V Ft ) Wi hi / ( Wi hi ) (9) i= Figure 23: Strengthening layout plan Figure 24: Ground tie beam details considering liquefaction 5. DISCUSSION AND FURTHER RESEARCH RECOMMENDATION 5. VERTICAL DISTRIBUTION OF THE BASE SHEAR The total base shear is distributed to each storey in according to the contribution of the storey mass production with its height from the base. For a building with the uniform floor mass and uniform storey heights, the distribution shape is an inverted triangle. Furthermore, AS/NZS code for seismic action recognizes that the first Where Ft is defined as follows. F t = 0 T 0. 7 (0a) F t = 0.07 T V 0.7 < T < 3. 6 (0b) F t = 0. 25V T 3. 6 (0c) It is seen that, in NBCC and UBC, extra distribution to the roof level increases with the increases of the primary period or the decrease of the overall building stiffness. Furthermore, it is worthwhile to note that UBC has been replaced by the new International Building Code (IBC). Its Section 63 uses the earthquake forces calculation of ASCE 7 [8]. It defines the vertical distribution of the base shear as below. Wi Fi = V () W i This distribution refers to the effective mass only. By comparing these four codes, it is found that the static equivalent method formula given in NZS70.5:2004 gives the greatest forces for the lower period building and the lowest force for the higher period building. In the second case study of this paper, it shows the overestimation was substantially around 45%. As this formula is a primary base for the daily seismic engineering design. It would be of greatly worthwhile to investigate further on this issue to avoid both under estimate and over estimate of the base shear distribution to the roof level. 5.2 CONCENTRIC BRACED FRAMES In order to resist the seismic lateral forces, concentrically braced frames (CBFs) [9, 0] or eccentrically braced frames (EBFs) are the commonly used. Historically EBFs have been developed to accommodate the architectural requirements for openings, where its bracing members are required to be offsite from the column or avoid the intersecting with the floor beams. In design, both frames need the appropriate selection of its local (e.g. plate/wall

10 thickness) and global (i.e. member itself) slenderness of the bracing members such that adequate post-buckling inelastic deformation could be facilitated. Apart from this, the difference lies in their connection details. Figure 25 and Figure 26 are the different connection configurations for EBFs and CBFs. While the EBFs use the link element, the CBFs use the linear or elliptical clearance to establish a plastic hinge zone to dissipate the energy when subject to seismic actions. (a) EBF s connection I (b) EBF s connection II Figure 25: Different connection configurations of EBFs (a) CBF s connection I (c) CBF s connection III (b) CBF s connection II Figure 26: Different connection configurations of CBFs From these connections layout, it is found that the connections for CBFs are simplest and most cost effective. It hence becomes a practical and economical structural solution for many applications. In HERA Report R4-76, Figure 26 (a) is recommended as the connection detail for the CBFs. However, if the complete understanding could be established for the connection shown in Figure 26 (b), it would made the CBFs much more preferred seismic resistant bracing frame. 6. CONCLUSION AND REMARKS Using advanced modal response spectrum analysis, the current practice of the New Zealand standards and the guidelines/regulations of the national and regional authorities, this paper presents the investigations on the structural response subject to the seismic actions and proposes respective repair and strengthening methodologies. Two engineering cases were investigated: one five story reinforced concrete office building and one L-shaped two storey reinforced concrete commercial retail building. Detailed member capacities in terms of New Building Standard (NBS) as well as the overall behavior of the buildings were achieved based on the detail modal response spectrum analysis results. Strengthening of the building s overall capacity as well as the individual member were designed successfully based on the latest engineering standards, guidelines and regulations. It was found to be imperative to employ modal response spectrum analysis for all the buildings with vertical and/or horizontal irregularities so as to establish its reliable structural response under seismic actions. This is true even for the two storey irregular buildings. In addition, discussion was given to the seismic shear distribution to the roof level and the plastic energy dissipation design of the concentric bracing frame connections, from which recommendations for further research were given. REFERENCE [] Chopra A.K.: Dynamics of Structures theory and applications to Earthquake Engineering, 3 rd edition, Person Prentice Hall, New Jersey, [2] AS/NZS 70.5:2004, Structural Actions Part 5: Earthquake Actions-New Zealand, the Council of Standards New Zealand. [3] AS/NZS 70.0:2002, Structural Actions Part 0: General Principles, the Council of Standards Australia and the Council of Standards New Zealand. [4] NZS 30: Part 2006: Concrete Structures Standard, Part the Design of Concrete Structures, the Standards Council, New Zealand. [5] UBC 997: Uniform Building Code, International Conference of Building Officials, California, USA. [6] NBCC. 995: National Building Code of Canada, Institute of Research in Construction, National Research Council of Canada, Ottawa, Canada. [7] Humar J.M. and Mahgoub M.A. 2003: Determination of Seismic Design Forces by Equivalent Static Load Method, Can. J. Civ. Eng. Vol.30, pp [8] ASCE/SEI 7-0: Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers/Structural Engineering Institute, Virginia, USA, 200. [9] HERA Report R4-76: Seismic Design Procedures for Steel Structures, New Zealand Heavy Engineering Research Association, Manukau City, New Zealand. [0] Sabelli R., Roeder C.W. and Hajjar J.F.: Seismic Design of Steel Special Concentrically Braced Frame Systems a guide for practicing engineers, NEHRP Seismic Design Technical Brief No.8, National Institute of Standards and Technology, (NIST) GCR , US. Department of Commerce.