Vulnerability Studies to Existing Building due to Seismic Activities in Sabah

This paper was presented to the 1st National Colloquium on Wind and Earthquake Engineering 2017 and published in INTERNATIONAL Journal of Civil Engineering & Geo-Environment.

Authors:

Noor AzudinIr. Hj. Mohd Noor Azudin bin Hj. Mansor, Director of Engineering of Civil and Structure Division.  Subject Matter Expert (SME) in Structures.

Ir. Mohd Noor Azudin bin Haji Mansor has been responsible for structural analysis, design and supervising of government buildings and infrastructure works since 1986 for over 30 years of experience. He has represented JKR for 18 technical committees and presented 11 technical papers in national and international conferences.

Co-authors: Ir.Lee Choon Siang, Ir. Amisah binti Ahwang, Mohd. Assyarul bin Saadun, Juranie binti Dumatin; Cawangan Kejuruteraan Awam dan Struktur, Ibu Pejabat JKR Malaysia

Decorative Divider 01

ABSTRACT

The “Vulnerability Studies to Existing Building due to Seismic Activities in Sabah” is the first Malaysia government initiated study for major government buildings and facilities on the safety and structural capacities subjected to local impacts of seismic activities in Sabah. The study was based on the cabinet meeting decision dated 19th June 2015 as the concerns on the building safety was raised after the Ranau earthquake on 5th June 2015. Subsequent from the cabinet meeting decision dated 19th June 2015, Ministry of Works Malaysia had instructed Jabatan Kerja Raya Malaysia (JKR Malaysia) to carry out the vulnerability study in collaboration with Jabatan Kerja Raya Sabah (JKR Sabah).

This study is intended to evaluate, to determine structural performance level, identify structural deficiency and providing appropriate retrofitting proposal for those buildings under the risk of damages under predicted intensity of seismic activities. Twelve major districts in Sabah has been identified within this study based on criteria of population, density and levels of potential seismic risk. Total number of fifty-four (54) buildings are evaluate under this exercise under four phases of studies as summarised below:

  1. Phase I – Data Collection
  2. Phase II -Evaluation by FEMA1541 and ASCE41-032 (Tier 1)
  3. Phase III- Develop Demand/Capacity Analysis (DCA) and Fragility
    Evaluations
  4. Phase IV-Retrofitting Proposal

1.0 Introduction

On the 5th June 2015, a strong earthquake with recorded moment magnitude of 6.0 had struck Ranau, Sabah (refer Figure1-1, source USGS). The epicenter was located at 10 km below the ground, 14km away in northwest of Ranau town. The earthquake caused the biggest casualties and structural damage in the Malaysian history.Fig 1-1 Shakemap Sabah

Eighteen (18) lives were taken during this quake. Most of them were killed on the Mount Kinabalu during the climbing activities. The earthquake also affected about 200 families in Ranau and Kota Belud who suffered after subsequent mudslides destroyed their homes, farms and plantations as well as disrupted water supply. Cracks were reported at the residential, commercial buildings, resorts and hotels as well as religious places of worship. Even structures, which are normally used for the emergency such as the hospital, schools and police department were not spared.

Typical damages reported were brick wall shear failure cracks, cracks on columns and beams, roof failure, failure of supporting columns or tilts, concrete spalling, and shattered windows. There were worries about whether continuous swaying of buildings and vibrations of some parts of the non-structural elements such as doors and windows, will continuously happen and affects the integrity of the whole buildings.

2.0 Phase I –Data Collection

For the purpose of gathering information of the selected buildings, JKR Malaysia Headquarters had conducted several sessions of meetings and discussion with JKR Sabah whom are regarded as the local authorities and representative for the local communities. With the constant supports from JKR Sabah, most of the required information had successfully acquired without much difficulties.

2.1 As Built Measurements and Soil Information

For those buildings without as-built drawings, several on-site measurements were conducted to verify the plan information and to assess the buildings condition. Total number of 24 buildings were visited in order to carry out dimensional measurement for component structural layout as well as measurement for major structural members.

Since the soil conditions cannot be readily identified by visual methods in the field, existing soil investigation (SI) information for related location was first collected from JKR Sabah during the planning stage and put into a readily information for use during rapid visual screening (RVS). During the screening, or the planning stage, this soil type was documented on the Data Collection Form.

However, for building with no existing SI report available, new SI works was conducted to determine the soil type. There are twenty-six (26) buildings with existing SI reports out of fitty-four (54) buildings. Due to constraint of time and budget, new SI works was limited to eight (8) numbers of SI (bore log) and ten (10) numbers of Mackintosh Probe (M.Probe).

Table 2-1 shows from eight (8) numbers of borehole results.

Table 2-1 New SI

3.0 Phase II ‐Evaluation by FEMA154 and ASCE 41‐03 (Tier 1)

For Phase II evaluation stage, a total of numbers fifty-four (54) identified buildings were inspected based on FEMA 154 Rapid Visual Screening Procedure. These building are mainly public facilities consist of hospitals, police stations, schools, government quarters and offices. Only two types of FEMA 154 Modified data collection forms were used in this exercise which is the Low and Moderate level of seismicity according to local seismicity level as proposed under macrozonation map developed earlier by JKR-UTM collaboration research in 2010.

FEMA 154 was opted for this exercise as it is a simplified format which offers an ease-to use method to collect information. It also provides a scoring system which enable the users able to determine the potential risk of large numbers of buildings under seismic action within short periods and limited resources.

3.1 FEMA154 Screening Implementation

For steps were involved in the planning and performing RVS of potentially seismically hazardous buildings. First, the seismicity level was determined from the macrozonation map based on two-thirds of the values from a 2,475-year average return period (corresponding to ground motions having a 2% probability of exceedance in 50 years).

By interpolation approach, the seismicity intensity for 1.0s and 0.2s period obtained from the map are tabulated in Table 3-2 and Table 3-3 respectively (not shown). However, the seismicity level for Ranau/Kundasang region were in the low seismicity since this earlier map did not capture effects from the recent Ranau earthquakes. Based on the recorded ground motion intensity during Ranau earthquake, it was decided that Ranau/Kundasang region shall be considered as moderate seismicity.

3.2 Summary of Findings for RVS Data Collection Form

The Buildings involved in this screening varies from 3 to 8 stories. The number of stories for these buildings is shown in Figure 3-1. From RVS Data Collection Form, it is observed that there are 32 buildings were in the category of seismically hazardous and 22 buildings were in the category of acceptable seismic performance with passing score of 2.0. there were twenty five (25) buildings located in the moderate seismicity area and twenty-nine (29) more in the low seismicity region. However, no high seismicity region was recorded in Sabah. It was also observed that the percentage of soil type D are much higher than soil type E, which is 83% and 17% respectively.

Fig 3-2 Building BenchmarkFigure 3-2 shows building benchmark consisting of pre-code, vertical irregularity and plan irregularity. Vertical irregularity shows the higher percentage with 58%, followed by plan irregularity with 33% and pre-code is 9%. These indicated vertical irregularity is one of the prominent factors that can influence the possibility of building failed.

3.3 ASCE 41‐13 (Tier 1)

Tier 1 seismic assessment was performed in accordance with ASCE 41-13, Seismic Evaluation and Retrofit of Existing Buildings. The evaluation is based on as-built structural drawings provided by JKR Sabah and supplemented by site visits, to perform limited observations of the existing condition of the structures. No material testing or removal of building finishes was performed. All the buildings have been evaluated for the expected structural performance using ASCE 41-13 procedures.

3.3.1 Scope of Work

Tier 1 screening was required for all buildings to determine the selected building’s risk due to seismic activity. It is also to quickly identify buildings that comply with the provision of this standard. If the building meets the benchmark building criteria, it is deemed to meet the structural requirements of this standard for the specified level of performance. In this report, all data collected are sufficient to conduct a Tier 1 evaluation. The evaluation process in Tier 1 screening Phase is shown in Figure 3-3.

However, before we proceed to Tier 1 Screening process, we have to determine Level of Performance and Level of Seismicity for each building. It is important part to identify the Tier 1 checklist. In this study, there are 5 checklists were used and completed according to the targeted building performance expected.

For building that does not comply with the criteria outlined in the checklist, further investigation by Linear and Non-Linear structural analysis was done.

3.3.2 Level of Performance

Performance Level shall be defined before conducting a seismic evaluation using this standard. All the buildings were evaluated based on the Life Safety (LS) Performance Level. The Immediate Occupancy (IO) Structural Checklist was only filled up for specific critical buildings with high importance such as Hospitals, Schools, Police Stations and Fire Stations only.

3.3.3 Level of Seismicity

Fig 3-3 Tier 1The Level of Seismicity of the building also be categorised as Very Low, Low, Moderate, or High in accordance to this ASCE 41-13 standard. Building’s seismicity level was obtained from local macrozonation map recommended by JKR-UTM3. The level of seismicity can be defined based on values shown in Table 3-4 .

3.3.4 Selection of Checklists

All these buildings are classified as Building Type C1: Concrete Moment Frames. Thus, the structural checklist associated with building Type C1 are used. A Tier 1 Evaluation of all these buildings involves completing the following checklist;

a. Basic Checklist for Building Type C1.
b. Life Safety Basic Configuration Checklist.
c. Immediate Occupancy Basic Configuration Checklist.
d. Life Safety Structural Checklist for Building
Type C1.
e. Immediate Occupancy Structural Checklist for
Building Type C1.

Each of the evaluation statements on the checklists shall be marked “compliant” (C), “noncompliant” (NC), or “not applicable” (N/A). Compliant statements identify issues that are acceptable according to the criteria of this ASCE 41-13, while non-compliant statements identify issues that require further investigation.

3.3.5 Pseudo Seismic Forces

The Pseudo Seismic force, in a horizontal direction of a building, shall be calculated in accordance with below equation.

V=CSaW

Where;

V= Pseudo seismic force
C = Modification Factor (Refer to table 4.8, ASCE
Standard 41-13)
Sa = Spectral acceleration at the fundamental
period of the building,
W = Effective seismic weight of the building

3.3.5.1 Spectral Acceleration, Sa

The calculation of Spectra Acceleration is based on the formula, Sa = Sx1/T, where Sa < Sxs

Foundamental period,         T = Cthnβ

Sxs = FV/Ss
Sx1 = FV/S1

Where; Ss = response spectrum ordinates for short (0.2s) and S1= long (1s) periods, in the direction of maximum horizontal response.

3.3.5.2 Distribution of Story Shear Forces

The pseudo seismic force calculated above shall be distributed vertically accordance with following equation. Shear EquationShear Equation 2

Where, Wx = Portion of the total building weight W located on or assigned to floor level

Wi = Portion of the total building weight W located on or assigned to floor level x
k = 2.0 for T > 2.5 seconds
= 1.0 for T < 0.5 seconds
Hx = Height in ft from the base to floor level x
Hi = Height in ft from the base to floor level i
V = Pseudo lateral load
Vj = Story shear at story level j

3.3.5.3 Shear Stress in Concrete Frame Columns

The shear stress check provides a quick assessment of the overall level of demand on the structure. The concern is the overall strength of the building. The average shear stress in the columns of concrete frames shall be computed in accordance with below equation.

Shear Stress

Where,

nc = Total numbers of columns
nf = Total numbers of frames in the direction of loading
Ac = Summation of the cross-sectional area of all columns in the story under consideration
Vj = Story shear computed in section 4.4.2
Ms= System modification factor; Mx shall be taken as equal to 2.0 for building being evaluated to the Life Safety Performance Level and equal to 1.3 for buildings being evaluated to the Immediate Occupancy.

3.5.4 Column Axial Stress Caused by Overturning

Typical columns design only take into account the gravity load hence they may have limited additional capacity to resist seismic forces. Where axial forces caused by seismic overturning moments are added, the columns may crush due to excessive axial compression. Where both demand are large, the combined effect of gravity and seismic forces must be calculated to demonstrate compliance. The axial stress of columns in moment frames at the base subjected to overturning forces, Pot, shall be calculated in accordance with below equation.

Column Axial

Where,

nf = Total numbers of frames in the direction of loading
V = Pseudo seismic force
hn = Height (ft) above the base to the roof level
L = Total length of the frame (ft)
Ms = System modification factor; Mx shall be taken as equal to 2.0 for building being evaluated to the Life Safety Performance Level and equal to 1.3 for buildings being evaluated to the Immediate Occupancy.
Acol = Area of the end column of the frame.

3.3.6 Results and Findings

A list of deficiencies identified by evaluation statements for which the building was found to be non-compliant have been compiled upon completion of the Tier 1 Checklists. Further evaluation requirements have been determined once the checklists have been completed. Out of 54 buildings that have been recorded, there are thirty-two (32) buildings (59%) that have been analyzed in Tier 1 Screening Phase.

3.3.6.1 Vertical Irregularity (Soft Story)

Based on Figure below, the total of buildings having “Soft Story” are twenty-five (25) buildings. These multi-story buildings with weak and/or open front wall lines creating a “soft-story” performed poorly than other buildings and may be subjected to structural failure during or after earthquake. Buildings that are most vulnerable have been identified with the following criteria:

  • Column Axial Stress Caused by Overturning is more than 0.3fc psi
  • Shear Stress in Concrete Frame Columns is more than 100 lb/in2

3.3.6.2 Basic Checklist

The following is an example of completed Basic Checklist. In this report, most of buildings have complied with the basic checklist items such as load path and wall anchorage.

Fig 3-4

The wall anchorage check was based on the assumption of load wall has been transferred through slab diaphragm towards the adjacent beams. The typical top and bottom bars in slabs shall always having 900 bent down at the edge connected to adjacent beams. Calculation based on this typical detailing in Malaysia Practice, rebar size of R8 to R10 was widely used in construction shows sufficient capacity for wall anchorage.

3.3.6.3 Column Axial Stress Caused by Overturning

All 32 buildings were found to have adequate column axial stress especially caused by overturning and complied with the requirements of 30fc psi in the standard.

3.3.6.4 Shear Stress in Concrete Frame Columns

Figure 3-6 shows the results for shear stress in concrete frame columns. Only two (4) buildings are found to be noncompliance to the shear stress requirements. Twenty-eight (28) buildings were found to be in compliance and exceeding the column shear stress requirement. The limiting value of shear stress should be less than the greater of 100 lb/in2 or 2 \/f’c.

Fig 3-6

3.3.7 Findings from ASCE Tier 1 Screening

By conducting more evaluation items in the ASCE Checklist, it can be determined that most of the buildings that have ‘Soft Story’ issues did not comply with requirements of Shear Stress Check. These buildings will be further analysed by using Column Axial Stress Check and Shear Stress in Concrete Frame Columns Check.

In the Summary of the ASCE Tier 1 Screening Phase as in this report, there are several items have been deemed “noncompliant” and represent potential structural deficiencies and thus requiring further analysis which is by using Structural Non-Linear analysis.

4.0 Phase III‐ Develop Demand/Capacity Analysis (DCA) and Fragility Evaluations

Most buildings that are designed for seismic resistance using elastic analysis will experience significant inelastic deformations under large scale earthquakes. Modern performance-based design methods require ways to determine the realistic behaviour of
structures under such conditions. Enabled by advancements in computing technologies and available test data, nonlinear analyses provide the means for calculating structural response beyond the elastic range. Under both codes ASCE 41-13 and MS EN 1998-14, Non-linear static analysis (Pushover) is recommend for those buildings which are defined with irregularities. ACT-40 developed by Applied Technology Council provided the basic guidance of Pushover analysis shall be implemented.

The two key elements of the Pushover procedure are the demand and capacity. Demand is a representation of the seismic response expressed by the response spectrum diagram. While the capacity of a structure in the inelastic state could be defined by the load displacement curve through series of applied incremental load.

Both demand and capacity curve are independent in nature. As the demand increases, the structure would yield eventually, as the stiffness decrease and prolonging the period. Since the seismic acceleration depends on period, demand will also change as the structure yields. By capacity spectrum conversion method, the capacity and demand spectrum was converted to matching format as indicated in Figure 4-1. By overlapping both diagrams, an intersection point of both curve will represent the building’s actual performance point as shown in Figure 4-2.

Performance Point

4.1 Pushover Analysis

Under the Pushover Analysis exercise, a total of fifteen (15) numbers of building were analysed. The demand capacity curves are shown below in Figure 4-3 to Figure 4-5.

It is noticed that for some buildings Nos. 2, 3, 5, 12 and 14, the performance point was found to be located within the elastic zone under MS EN 1998 response spectrum. This behavior may due to the buildings uniform shape and with minimal irregularities. Some of them even remaining within the linear elastic when intersecting with much higher intensity Malaysian National Annex response spectrum. Whereas the building no. 6 shows the most flexible structural behavior and having most of the performance point at large deformation level.

Fig 4-3Fig 4-4Fig 4-5

4.2 Fragility Curves

The incremental dynamic analysis (IDA) was carried out by performing series of nonlinear response history analyses based on ground motions that are systematically scaled to increasing earthquake intensities until collapse occurs. Incremental dynamic
analysis yields a distribution of results at varying intensities that can be used to generate a collapse fragility.

In this study, six (6) types of recorded ground motion as listed in Table 4.1 were adopted in the development of fragility curve.

Among the fifteen (15) analysed buildings, two (2) hospitals (building nos. 8 & 9) shows the highest probability of damages which is 100% and 40% respectively at Operational Performance level under 0.2g PGA. However, both buildings are expectected to be able to perform by less than 3% of fragility under Life Safety performance level as shown in Figure 4-6.

Fragility curve offer the great flexibility of hazard assessment for a typical building subjected to various performance level and seismic demand. It can be utilised as a powerful tool to provide early estimate over vast area of affected region with ease within short period of time.

Fig 4-6

4.2.1 Phase IV‐Retrofitting Proposal

From total numbers of 54 government buildings selected for this study, 4 buildings were ranked as the most vulnerable building having insufficient capacity to resist earthquake forces and need to be further investigated for their strength requirement and suggested to be retrofitted.

Since there was no local document available to be referred, the retrofitting works described in this report were solely based on FEMA 5476: Techniques for the Seismic Rehabilitation of Existing Buildings. The rehabilitation techniques for building Type C1 will be considered in this report.

4.3 Seismic Deficiencies and Rehabilitation Measures

Failure to meet stipulated criteria in seismic evaluation will identify certain seismic deficiencies. In general, deficiencies were categorized into 7 main categories:

  1. Global strength
  2. Component detailing
  3. Global stiffness
  4. Diaphragms
  5. Configuration
  6. Foundations
  7. Load path

In the traditional sense of improving the performance of the existing structure, there are 3 basic classes of measures taken to retrofit a building:

  1. Add elements, usually to increase strength or stiffness.
  2. Enhance performance of existing elements, increasing strength or deformation capacity.
  3. Improve connections between components, assuring that individual elements do not become detached and fall, a complete load path exists, and that the force distributions assumed by the designer can occur.

5.0 Retrofitting Techniques

For this particular report, all the four (4) buildings that were ranked as the most vulnerable building showing deficiencies specifically in the categories of global strength, global stiffness and configuration. The most common deficiencies found were vertical irregularity (soft story) and plan irregularity. Therefore, this report will emphasize the retrofitting technique for these deficiencies.

5.1 Add Steel Braced Frame (Connected to a Concrete Diaphragm)

Addition of steel diagonal braced frames to an existing concrete frames building is a method of adding strength and/or stiffness to the structural system. The steel braces can be added without significant increase to the building weight. Any variety of the diagonal brace configurations may be used, as well as a variety of brace member section types. Figure 5-1 shows several common configurations.

5.2 Add Concrete or Masonry Shear Wall (Connected to a Concrete Diaphragm)

Addition of shear walls to an existing concrete frame building is a common method of adding significant strength and/or stiffness to the structure. The new walls may be of cast-in-place concrete, shotcrete or fully grouted concrete masonry unit (MCU) construction. Figure 5-2 shows typical concrete wall connection to concrete slab.

5.3 Enhance Column with Fiber‐Reinforced Polymer Composite Overlay

The use of a fiber-reinforced polymer (FRP) overlay with columns has proven to be an efficient rehabilitation technique in the building construction industries. Columns are overlaid with unidirectional fibers in a horizontal orientation, thus providing shear strengthening and confinement similar to that provided by hoops and spirals used with circular columns, and stirrups and ties used with rectangular columns.

The confinement enhances the concrete compression characteristics, provides a clamping action to improve lap splice connections, and provides lateral support for column longitudinal bars. Figure 5-3 shows a typical seismic retrofit of columns using FRP composites.

Fig 5-3

Figure 5-3 Seismic retrofit of columns using FRP composites

5.4 Enhance Concrete Column with Concrete or Steel Overlay

Adding a fiber reinforced polymer (FRP) composite overlay to a concrete column is a recent approach to addressing seismic deficiencies. Adding a concrete or steel jacket is a more traditional method of enhancing a deficient concrete column. Figure 5.4 shows an examples of concrete and steel jacketing for a rectangular column.

Fig 5-4Figure 5.4 Concrete and steel overlays for concrete column

 

6.0 Conclusion

Generally, most buildings under the study was found to behave linearly elastic under the Push-Over analysis. However, the deficiencies such as inadequacies of shear stress discovered from ASCE 41-13 checklist shall be retrofitted. In order to have the appropriate choice of retrofitting option, multiple techniques and procedures mentioned above could be applied with some deeper thought in term of constructability, cost, targeted seismic performance as well as the aesthetical value aspect.

References;

1 FEMA 154 Rapid Visual Screening of Buildings for Potential Seismic Hazards
2 ASCE41-13 Seismic Evaluation and Retrofit of Existing Buildings
3 Seismic Design Guideline for Concrete Building in Malaysia 2010- JKR-UTM.
4 MS EN 1998-1 Eurocode 8: Design of structures for earthquake resistance.
General rules, seismic actions and rules for buildings
5 ATC-40 Volume 1, Seismic Evaluation and Retrofit of Concrete Buildings,
Applied Technology Council, 1996
6 FEMA 547 Techniques for the Seismic Rehabilitation of Existing Buildings
7 Malaysia National Annex to MS EN 1998-1: 2015, Eurocode 8
8 JKR-Uni-Technologies Sdn. Bhd. report: Structural Vulnerability Studies of
Existing Buildings Due to Seismic Activities in Sabah 2016

 

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