ACEE-P-136

Naveed Anwar

CEO AIT Consulting Bangkok, Thailand

Deepak Rayamajhi

External ConsultantAIT ConsultingBangkok, Thailand

CASE STUDY FOR PERFORMANCE BASED DESIGN OF 50BUILDING WITH DUCTILE CORE WALL SYSTEM

Abstract The case study building is 50story of below grade parking (extending approximately 13 m below consists mainly of residential units, and a terrace and amenity deck. The ground level contains retail and back of the house space. This isthree towers, connecting at the podium level (2nd floor level). The total floor area for Phase 1 is approximately 79,000 m2. It is a reinforced concrete building, which is laterally braced by ductilecore wall system together with buckling restrained braces. Firstly, the preliminary design is performed against Design Basis Earthquake (DBE) (as defined by ASCE 7-05) in accordance with the code based by performance based approach against the seismic hazard which is likely to happen, so that predictable and safe performance is achieved. Two levels of performance are checked; Serviceable/Operational Level performance under 43probability of exceedance in 30 years) and Collapse Prevention Level performance under Maximum Considered Earthquake (MCE) with 2475exceedance in 50 years). Response spectrum analysis is conducted for Design Basiresponse spectra. For the evaluation against collapse during extremely rare events, nonlinear time history analysis is performed for the site specific ground motion records. Average of seven ground motion approach is uslateral force resisting system. Keywords: Maximum Considered

Naveed Anwar

AIT Consulting Bangkok, Thailand

Thaung Htut Aung

Projects Coordinator AIT ConsultingBangkok, Thailand

Deepak Rayamajhi

ernal Consultant AIT Consulting Bangkok, Thailand

CASE STUDY FOR PERFORMANCE BASED DESIGN OF 50BUILDING WITH DUCTILE CORE WALL SYSTEM

The case study building is 50-story tower (about 166.8 meters above the story of below grade parking (extending approximately 13 m below the consists mainly of residential units, and a terrace and amenity deck. The ground level contains retail and back of the house space. This is the Phase 1 of the entire project which is comprised of three towers, connecting at the podium level (2nd floor level). The total floor area for Phase 1 is

. It is a reinforced concrete building, which is laterally braced by ductilecore wall system together with buckling restrained braces.

Firstly, the preliminary design is performed against Design Basis Earthquake (DBE) (as defined 05) in accordance with the code based design procedures. Then, building is designed

formance based approach against the seismic hazard which is likely to happen, so that predictable and safe performance is achieved. Two levels of performance are checked; Serviceable/Operational Level performance under 43-year return period earthquake (50%probability of exceedance in 30 years) and Collapse Prevention Level performance under Maximum Considered Earthquake (MCE) with 2475-year return period (2% of probability of

Response spectrum analysis is conducted for Design Basis Earthquake Level and Service Level response spectra. For the evaluation against collapse during extremely rare events, nonlinear time history analysis is performed for the site specific ground motion records. Average of seven

is used to check the demands in the primary structural members of the lateral force resisting system.

onsidered Earthquake, Design Basis Earthquake

Thaung Htut Aung

Projects Coordinator AIT Consulting Bangkok, Thailand

CASE STUDY FOR PERFORMANCE BASED DESIGN OF 50-STORY

ground level) and 3½-the grade). The tower

consists mainly of residential units, and a terrace and amenity deck. The ground level contains the Phase 1 of the entire project which is comprised of

three towers, connecting at the podium level (2nd floor level). The total floor area for Phase 1 is . It is a reinforced concrete building, which is laterally braced by ductile

Firstly, the preliminary design is performed against Design Basis Earthquake (DBE) (as defined procedures. Then, building is designed

formance based approach against the seismic hazard which is likely to happen, so that predictable and safe performance is achieved. Two levels of performance are checked;

year return period earthquake (50% probability of exceedance in 30 years) and Collapse Prevention Level performance under

year return period (2% of probability of

s Earthquake Level and Service Level response spectra. For the evaluation against collapse during extremely rare events, nonlinear time history analysis is performed for the site specific ground motion records. Average of seven

ed to check the demands in the primary structural members of the

arthquake, Service/Frequent

earthquake, Performance Based Design, Ductile behaviour, Brittle behaviour

Introduction The case study tower is a residential tower located in Makati City, Philippines. It is built of reinforced concrete and 166.8 meters (50-story) tall above the ground level with 11x11 meter plan area. Reinforced concrete bearing walls, gravity columns and post-tensioned flat slabs are utilized for gravity load resisting system. The lateral load resisting system consists of reinforced concrete bearing wall coupled with outrigger columns, connected by the buckling restrained braces. The building has 3½-story of below grade parking, resting on the mat foundation. Fig. 1: Building Plan

Seismic Performance Objectives The specific performance objectives for the design of the building for three levels of earthquake hazards are shown in the following table.

Table 1: Performance Objectives

Level of Earthquake Seismic Performance Objective

Frequent/Service: 50% probability of exceedance in 30 years (43-year return period), 2.5% damping

Serviceability: Structure to remain essentially elastic with minor damage to structural and non-structural elements

Design Basis Earthquake (DBE): As defined by ASCE 7, Section 11.4, 5% damping

Code Level: Moderate structural damage; extensive repairs may be required

Maximum Considered Earthquake (MCE): 2% probability of exceedance in 50 years (2475-year return period), 2 to 3% damping

Collapse Prevention: Extensive structural damage; repairs are required and may not be economically feasible

Design Approach To demonstrate that the design is capable of providing code equivalent seismic performance, a three- step analysis and design procedure is performed.

Step 1 – Preliminary design phase

Step 2 – Serviceability check

Step 3 – Collapse prevention check at MCE Level

Preliminary Design Phase In this phase, elastic response spectrum analysis and design are performed in accordance with the code based design approach by using appropriate load factors and strength reduction factors against the gravity loads, wind load and seismic load. Site specific response spectrum for DBE

level is used for the preliminary design phase. Structural components to be remained elastic are designed by applying the appropriate amplification factors.

Serviceability Check Primary response characteristics such as story drift, coupling beam and shear wall capacity ratios are checked against the demands resulting from the response spectrum analysis using service level response spectrum with 43-year return period.

Collapse Prevention Check at MCE Level Design verification is performed by non-linear response history analysis (NLRHA) against the MCE level earthquakes. The initial design is modified as required in order to meet the acceptance criteria. The following structural elements are checked in anticipation of non-linear response: Core wall coupling beams Core wall flexural response Slab Outrigger beam The following structural elements are checked to remain essentially elastic during the non-linear response history analysis: Core wall shear Diaphragms Basement walls Foundations Columns

Loading Criteria

Gravity Load The minimum loading requirements have been taken from Table 4-1 of ASCE 7-05. Live loads are reduced where permitted in accordance with Section 4.8 of ASCE 7-05. In addition to the uniform slab loads, a superimposed dead load is applied along the perimeter of plan to account for the weight of the cladding system.

Wind Load Wind load is determined in accordance with ASCE 7-05. The design wind speed for the case study building is 200 kph and the exposure type is B.

Seismic Load Frequent/Service Level Earthquake For the performance evaluation at Service Level seismic hazard, the following service level site specific response spectrum is used.

Fig. 2: Response

Maximum Considered Earthquake

Seven pairs of site specific ground motions are used to conduct the nonlinear analysis. Average of demands from seven ground motions approach is used fat MCE level. The response spectra of seven ground motions are shown in figure

Fig. 3: Response Spectrum for Fault Normal and Fault Parallel Earthquakes at MCE Level

Modelling and Analysis ToolsA complete full three-dimensional finite element model is created which includes the tower and the whole podium. The modelling and analysis of building for evaluation and design at Service Level earthquake and DBE level are carried out in with the specified material properties and appropriate stiffness modifiers for the structural components. For the MCE level performance evaluation, nonlinear threePERFORM-3D (Version 4.0.4) computational platform.

Response Spectrum at Service Level Earthquake (2.5% damping)

arthquake Level

ground motions are used to conduct the nonlinear Average of demands from seven ground motions approach is used f

The response spectra of seven ground motions are shown in figure

for Fault Normal and Fault Parallel Earthquakes at MCE Level

Modelling and Analysis Tools dimensional finite element model is created which includes the tower and

The modelling and analysis of building for evaluation and design at Service Level earthquake and DBE level are carried out in ETABS 9.5 computational platform. An elastic model is created with the specified material properties and appropriate stiffness modifiers for the structural

For the MCE level performance evaluation, nonlinear three-dimensional model is created in ersion 4.0.4) computational platform.

arthquake (2.5% damping)

ground motions are used to conduct the nonlinear response history Average of demands from seven ground motions approach is used for design evaluation

The response spectra of seven ground motions are shown in figures below.

for Fault Normal and Fault Parallel Earthquakes at MCE Level

dimensional finite element model is created which includes the tower and

The modelling and analysis of building for evaluation and design at Service Level earthquake platform. An elastic model is created

with the specified material properties and appropriate stiffness modifiers for the structural

dimensional model is created in

Material Models Concrete

In nonlinear model, effect of confinements is taken into account for the compressive strength and ductility of concrete. Mander’s (1994) confinement model is used to determine the confinement effect. In PERFORM 3D, concrete material is modelled with tri-linear backbone curve. Tensile strength of concrete is neglected. Material cyclic hysteretic degradation is not considered in the model.

Reinforcing Steel

In nonlinear model, reinforcing steel material is modelled with tri-linear backbone curve. Yield strength is taken as 1.15 times nominal strength and the ultimate strength is estimated as 1.5 times expected strength with approximately 1% of strain hardening.

Shear Wall Fiber modeling technique is used to model the flexural behavior of the core wall. PERFORM-3D shear wall element is used to model the nonlinear behavior of shear wall. Basically, two parallel fibre sections are used to model the shear wall. The first fibre section consists of only uniformly distributed steel (Steel only) and the second fibre section consists of both concrete and boundary zone steel reinforcement. For the uniformly distributed steel, auto-size fiber elements are used whereas for latter one, fixed size fibre elements are used. Shear behaviour in the wall is modelled with elastic material properties.

Fig. 4: Fibre Modelling of Reinforced Concrete Shear Wall

Coupling Beam In this building, two types of coupling beams are present. First one is deep beam having span to depth ratio of 1.9 (span/depth < 4), and second one is slender beam having span to depth ratio of 4.3 span/depth > 4). Since deep beams are dominated by shear behaviour, they are modelled for shear deformation controlled while the slender beams are modelled for flexural deformation controlled. The deep coupling beam is modeled with elastic frame section with a nonlinear shear hinge located at mid span of the element. The capacity of the shear hinge is calculated based on the diagonal reinforcements. The elastic stiffness of the deep beams is reduced to 0.16EIg. The shear capacity of diagonal reinforcement is calculated based on formula provided in ACI 318-08. The

ultimate point is taken as the 1.33 times of the yielding capacity. The slender coupling beam is modeled with two moment hinges placed at the ends of the beam. The capacity of the moment-curvature hinges are calculated based on the longitudinal reinforcements provided in the beams. The deformations capacities are taken from ASCE 41-06 for the flexural coupling beams. The elastic stiffness of the slender beams is reduced to 0.5EIg.

Columns and Girders The columns and girders are modelled as elastic frame member. The capacities of these members are checked against the forces extracted from MCE analysis. The elastic stiffness of the columns and girders are reduced to 0.7 EIg and 0.5Ig respectively.

Floor Slab In the tower portion, the floor is modeled as rigid floor diaphragm. The slab is not modeled for the tower portion. However, equivalent “slab outrigger beams” are modeled in order to study impact of slab to core and column only. Slab outrigger beams are modeled with nonlinear hinges at both the ends of the beam. Moment-curvature type of hinge is used to model nonlinearity in the slab-beam. The moment capacity of the slab beam is calculated based on the reinforcement in the slab. However, the performance of the moment hinges is not specifically reviewed. At the podium and basements level, the slabs are modeled without rigid floor diaphragm. Slabs in the podium and basement are modeled using shell element. The elastic flexural stiffness of the slabs and equivalent slab-beams are reduced to 0.5EIg.

Support/ Foundation The base of the reinforced concrete shear wall is modeled as pinned at the location of mat whereas the columns and basement walls are modeled as fixed support. Furthermore, for this analysis, the basement walls are also restrained by lateral springs in the lateral direction to take into account the restraining effect of lateral soil. In order to consider the flexibility of the diaphragm, the stiffness of the ground floor and below–grade diaphragms are reduced to 0.1 Ag.

Buckling Restrained Braces BRBs are used in the main lateral force resisting system in order to enhance the performance of the building. The intended benefits of using BRB in this building are to reduce the story drifts and lateral displacement as well as to participate in the outriggering effect on the overturning moment in the tower. Moreover, BRBs can reduce the base shear in the building by dissipating the energy. Two different levels of designed forces are used for the BRBs in this building. One is from level 19th - 23rd floor and another from 43rd - 47th floor. 16 BRBs are used in the building. Eight BRB’s are located at 19th - 23rd floor and rests are at 43rd - 47th floor, principal minor.

Analysis Results

Modal Analysis The natural periods of the building are 5.75 s and 4.86 s in principal directions with 0.40 and

0.42 modal participating mass ratios.

Mode 1 Mode 2 Mode 3

Fig. 5: Mode Shapes

Base Shear The base shear is compared between DBE level response spectrum analysis and average of MCE level nonlinear response history analysis in the following table. The base shear is calculated above the podium level and considered the tower portion only. The seismic weight of the tower above the podium level is 616,900 kN.

Table 2: Base Shear Comparison

Load Cases Base Shear

(KN)

% of Seismic Weight

DBE level (Along principal major dir.) 21,012 3.56

DBE level (Along principal minor dir.) 22,691 3.84

MCE level (Along principal major dir.) 47,892 7.76

MCE level (Along principal minor dir.) 46,462 7.53

It is found that the base shear calculated from the MCE analysis in average is approximately two times higher than DBE level base shear.

Story Shear and Story Moment Story shears and story moment distributions are nearly triangular shape, showing the dominance of first modes in each principal direction. Furthermore, the story shear at the basement level is generally decreased in most of the time history except some time histories where the story shear has increased. This may happened due to the irregular distributions of basement walls and supports.

Story Drifts The average story drifts distribution is higher in maximum story drifts envelopes for both acceptable limit against MCE level earthquakes

x

x

Fig. 6: Story Shear

Fig. 7: Story Moment

drifts distribution is higher in minor direction than drifts envelopes for both principal directions are less than 3%

acceptable limit against MCE level earthquakes.

direction than major direction. The directions are less than 3% which is

Y

Y

Axial Strain Shear WallThe flexural capacity of shear wall is evaluaconcrete materials. The strain in steel fibre and concrete fibre are checked against the acceptable strain limits. The compression strain of MCE analysis is increased by 2 times and compared with the limit set in the performance criteria.

Findings

Service Level PerformanceAt service level earthquake, all storey drifts are less than 0.5%. The response of the columns and coupling beams in shear and moment, shear walls in flexure and shear and buckling rebraces in axial direction are within the elastic limit. The capacity of each element at service level is higher than the corresponding demand in the element. There are a few elements in which the demand exceeds the capacity such as deep coupling b

MCE Level PerformanceThe design base shear (shear calculated above the podium) is approximately 3.5% and 3.8% in each principal direction, which is higher than the minimum limit of 3%, set by the LATBSDC2008 guidelines. Furthermore, the dynamic base shear calculated from the average of seven time histories is approximately two times higher than the design base shear, which is typical in high rise buildings. Moreover, from the above seismic performance evaluationthe R factor of 6 for the design purpose is a good estimation for the code based design of this

x

Y

Fig. 8: Story Drift

Wall The flexural capacity of shear wall is evaluated in terms of the yielding of steel and crushing of concrete materials. The strain in steel fibre and concrete fibre are checked against the acceptable strain limits. The compression strain of MCE analysis is increased by 2 times and compared with

mit set in the performance criteria.

Performance At service level earthquake, all storey drifts are less than 0.5%. The response of the columns and coupling beams in shear and moment, shear walls in flexure and shear and buckling rebraces in axial direction are within the elastic limit. The capacity of each element at service level is higher than the corresponding demand in the element. There are a few elements in which the demand exceeds the capacity such as deep coupling beams shear, however, which is permissible.

Performance The design base shear (shear calculated above the podium) is approximately 3.5% and 3.8% in each principal direction, which is higher than the minimum limit of 3%, set by the LATBSDC

idelines. Furthermore, the dynamic base shear calculated from the average of seven time histories is approximately two times higher than the design base shear, which is typical in high rise buildings. Moreover, from the above seismic performance evaluation, it seems that choosing the R factor of 6 for the design purpose is a good estimation for the code based design of this

ted in terms of the yielding of steel and crushing of concrete materials. The strain in steel fibre and concrete fibre are checked against the acceptable strain limits. The compression strain of MCE analysis is increased by 2 times and compared with

At service level earthquake, all storey drifts are less than 0.5%. The response of the columns and coupling beams in shear and moment, shear walls in flexure and shear and buckling restrained braces in axial direction are within the elastic limit. The capacity of each element at service level is higher than the corresponding demand in the element. There are a few elements in which the

eams shear, however, which is permissible.

The design base shear (shear calculated above the podium) is approximately 3.5% and 3.8% in each principal direction, which is higher than the minimum limit of 3%, set by the LATBSDC-

idelines. Furthermore, the dynamic base shear calculated from the average of seven time histories is approximately two times higher than the design base shear, which is typical in high

, it seems that choosing the R factor of 6 for the design purpose is a good estimation for the code based design of this

building. From the storey shear and storey moment plots of seven time histories, in average the results demonstrate that the building is mainly dominated by first fundamental modes in both X and y direction. From these results, it seems that higher modes are not significantly affecting response. Flexural deformation capacity of shear wall, evaluated by the axial strain of the fibres, is within the acceptable limit. All the columns including the outrigger columns remain essentially elastic under MCE earthquake. On average, all the BRB’s have ductility demand less than 9, the limit set by ASCE41, and indicates that all the BRBs satisfy the performance criteria.

References [7] ACI Committee 318. 2008, “Building Code Requirements for Structural Concrete and

Commentary (ACI 318-08)”, American Concrete Institute

[8] Los Angeles Tall Buildings Structural Design Council. 2008, “An Alternative Procedure for Seismic Analysis and Design of Tall Buildings Located Los Angeles Region”, Los Angeles Tall Buildings Structural Design Council