Seismic vulnerability assessment of bare and masonry ... · determination of strength of infilled...

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING

Volume 5, No 4, 2015

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

Research article ISSN 0976 – 4399

Received on September, 2014 Published on May, 2015 392

Seismic vulnerability assessment of bare and masonry infilled reinforced

concrete frame structures Saim Raza1, M. Khubaib Ilyas Khan2

Department of Civil Engineering, National University of Sciences and Technology (NUST),

44000, Islamabad, Pakistan

[email protected]

doi: 10.6088/ijcser.2014050036

ABSTRACT

Construction of masonry infilled RC frames is common practice in many countries. Infills

contribute to the performance of structures which is generally neglected in analysis and

design by considering it as non-structural element. The purpose of this study is to evaluate

band compare the seismic vulnerability of bare and masonry infilled RC frames. Different

types of infill materials have been considered in this study. These infill materials include

brick masonry, solid concrete block masonry and hollow concrete block masonry.

Experimental testing has been performed to determine the compressive strength of the

masonry infill. Three, five, seven and nine story, RC frames designed under gravity loads

have been considered in this study. Seismic Vulnerability assessment is conducted to evaluate

the seismic hazards corresponding to various levels of damage in the building. Perform-3D

has been used as an analytical tool for this purpose. Cyclic pushover analysis followed by the

capacity spectrum method and seismic vulnerability assessment framework proposed by

(Kyriakides, 2007) have been used to derive the vulnerability curves for bare and masonry

infilled RC frames. The vulnerability curves show an improvement in overall performance of

RC frames with the inclusion of infill panel.

Keywords: Bare RC frames, Masonry infilled RC frames, Brick masonry, solid concrete

block masonry, hollow concrete block masonry, Capacity spectrum method, seismic

vulnerability curves.

1. Introduction

The multistory buildings construction is associated with infilled frames. In developing

countries like Pakistan masonry is used as infill wall because in comparison to other

construction materials it is easy to make, locally available and cheaper. The most commonly

used masonry infills are burnt clay brick masonry, Hollow and solid concrete blocks masonry.

The infill wall serves both as partitioning wall and also contributes to the performance of

reinforced concrete frame structures. Past researches have shown that presence of infills have

altered the global response of structures significantly under seismic loading (Degefa, 2005).

However infill walls are regarded as non-structural component although they contribute to the

structural performance and behavior during an earthquake. They contribute to the lateral

stiffness of the structure and significantly increase the lateral strength. Nevertheless exact

behavior of the infilled structures is unknown due to the complex and unpredictable behavior

of the masonry infill under lateral loading. Thus building codes do not provide clear

guidelines to include the effects of infill walls in design which cannot be ignored as they alter

the overall performance of structures. This study aims at determining the extent of

vulnerability of reinforced concrete frame structures to earthquake damages, with and without

inclusion of infill walls.

Seismic Vulnerability Assessment of Bare and Masonry Infilled Reinforced Concrete Frame Structures

Saim Raza and M. Khubaib Ilyas Khan

International Journal of Civil and Structural Engineering 393

Volume 5 Issue 4, 2015

Figure 1: Damage to masonry infilled reinforced concrete frame during earthquake

1.1 Previous Research

Analytical and experimental studies on seismic response of infilled frames started from mid-

1950.Various research studies have concluded that some of structural responses such as roof

displacement, fundamental period, inter storey drift ratio and beam and column member

forces usually reduce and base shear increases with the inclusion of infill wall in analyses.

One of the pioneer researchers in this field Polyakov, (1958) suggested that infill panel can be

considered equivalent to diagonal bracing. (Holmes, 1961) practically overtook this

suggestion and replaced the infill panel by an equivalent pin-jointed diagonal strut which had

the properties of infill panel i.e. same thickness as infill panel and a width equal to one-third

of diagonal length of infill. Polyakov was the first to describe the action of infill as an

equivalent diagonal strut. Mainstone, (1971) developed eight equations for equivalent strut,

four equations for strut width and four for strength equations. These equations were based on

three full scale tests on brick infill and twenty one small scale model brick and micro

concrete tests. Dhanasekar, (1986) studied the behaviour of infilled frame under an in-plane

load. The results from biaxial tests on half scale solid brick masonry was used to develop a

material model for brick and the mortar joints which were then used to construct non-linear

Finite Element Model. The results showed that the Young’s modulus of elasticity of the infill

has a significant influence on the behaviour of the infilled frame. However, the influence of

Poison’s ratio was fond insignificant on the behaviour of structure. It was also reported

that the infill wall failed due to shearing along the diagonal length of the wall and hence

the influence of the compressive strength of infill material was not observed. Valiasis et al.,

(1989) conducted tests on concrete frames infilled with masonry walls. The infill wall was

not connected to surrounding frame. The experimental results revealed that infill wall

increased building strength by 50%. Moreover experiment disclosed that additional strength

disappeared at small lateral loads. El-Dakhakhni et al., (2006) concluded that it is not always

safe to ignore the frame-infill interaction in seismic areas, since infill walls can significantly

increase the lateral stiffness by its diagonal action. Thus seismic demand of structure changes

due to reduction in natural period of the composite structure. Kaushik, (2008) conducted a

comparative study of the seismic codes, especially design of infilled frame structures. The

study revealed that the most of modern seismic codes lack the important information required

for the design of such buildings. Kircher et al., (2006) suggested that concrete frames,

including those with and without infill, represent one of the three major sources of seismic

risk in the earthquake prone zone (the other two sources being URM bearing wall buildings

and soft-story wood-frame structures) because approximately 80% of the cost of

damages to structures from earthquakes is due to damages of the infill walls and the

consequent damages of doors, windows, electrical and hydraulic installations.





Kyriakides, 2007) used analytical procedures to develop the seismic vulnerability curves of

buildings in order to investigate their behavior when exposed to earthquakes. He concluded

that although empirical assessment curves are easy to derive but they cannot describe

unusual buildings, also the expert Judgment method is not trustworthy due to the inherent

uncertainties in the building performance. Analytical procedures are most suitable when

past records of building damage are not available; working in detail and near to exact

information is required. However, the models obtained from analytical methods are to be

verified by empirical models.

2. Methodology

Capacity Spectrum Method as described in (ATC-40, 1996) is used for the seismic

vulnerability assessment of structures. Non-linear Static cyclic pushover analysis has been

performed on all proposed structures. Perform-3D has been used as analytical tool due to

availability of inbuilt diagonal strut module for infill panel modeling and also its ability to

conduct non-linear analysis. The detailed methodology is explained in flow chart below.

Figure 2: Seismic Vulnerability Assessment (Flow Chart)





3. Experimental testing

As mentioned earlier burnt clay brick masonry, solid concrete block masonry and hollow

concrete block masonry is being considered in this study. The results of compressive strength

test of these masonry materials are shown below

Table 1: Compressive strength of brick masonry prisms

Sr. No. Crushing Load (Kips) Compressive Strength (KSI)

Solid Block Hollow Block Solid Block Hollow Block

1 37.52 35.62 0.41 0.39

2 30.62 31.92 0.33 0.35

3 34.85 45.41 0.38 0.49

Average 34.33 37.65 0.37 0.41

Table 2: Compressive strength of Concrete Hollow and Solid block masonry prisms

Sr. No. Crushing Load

(Kips)

Compressive

Strength (KSI)

1 23.16 0.61

2 28.78 0.77

3 25.85 0.70

Average 25.93 0.69

(a) (b)

Figure 3: (a) Compression testing assembly for concrete block masonry (b) Cracked Sample

after testing

3.1 Design of proposed building in SAP2000

In this research the influence of the infill panel in infilled concrete frame structures with

varying number of storeys while keeping number of bays same is studied. All structures are

hypothetical regular moment resisting frame structures. Soil Structure Interaction is ignored

and a raft foundation is considered representing fix supports at the base of structure in

models.





Table 3: Geometric parameters of buildings

Bay Size 20 ft.

First Storey Height 13 ft.

Typical Storey Height 12 ft.

Roof Slab Thickness 5 inch.

Typical floor Slab Thickness 6 inch.

Table 4: Material used for buildings design

Concrete f’c=3000 psi

Steel (Reinforcement) ASTM A615 Grade 60 (Fy =60 Ksi)

All the buildings are considered as office buildings and are designed in SAP2000

under gravity loading, with live loads being taken from (Uniform building code, 1997). All

the buildings have same number of bays in two directions i.e. 3 bays x 3 bays and number of

storeys change as 3, 5, 7 and 9 storeys. Typical plan is shown below:

Figure 4: Typical plan of buildings

3.2 Structural Modeling in perform-3D

For non-linear analysis and seismic vulnerability assessment of RC frame structures, 2D

models of RC frames are exported from SAP2000 to Perform-3D. To make the frames

behave as 2D, restraints are applied at all nodes. All nodes except the foundation nodes are

free to translate in H1 or X and V or Y direction and free to rotate in H2 or Z direction. The

foundation nodes have fixed supports. For modeling beams and columns of RC frames,

FEMA beam concrete type and FEMA column concrete type are used. F-D relationships,

Deformation Capacities and strength loss parameters are inputted using the guidelines of

(FEMA356, 2000). However in order to determine inelastic strength properties of RC beam

and columns an analytical tool XTRACT has been used. Elastic perfectly plastic (EPP)

behavior for F-D relationships of FEMA concrete beams and FEMA concrete column is used.





Figure 5: Basic F-D relationships for RC Beam type Components (Perform-3D)

3.3 Modeling of infill panel

Figure 6: Infill panel diagonal strut model

Infill panel element of Perform-3D has been used to model masonry infill. Each infill panel

element consists of one infill panel component. The diagonal strut model consists of two

struts, each of which resists compression force only. The actions and deformations are the

compression forces and compression deformations of the struts, as shown in figure (Perform-

3D User guide, 2006). In this study Compression failure mode has been considered for

determination of strength of infilled frames. For compression failure of the equivalent

diagonal strut, a modified version of the method suggested by (Stafford-Smith and Carter,

1967) can be adopted (FEMA 306, 1996). The shear force (horizontal component of the

diagonal strut capacity) is calculated as:

VC =atinf f’m90cosθ

Where: a= Equivalent strut width

tinf = Infill thickness

fm90 = Expected strength of masonry in the horizontal direction,

which may be set at 50% of the expected stacked prism

strength fm.





Figure 7: F-D relationship for Infill panel strut model

3.4 Analysis and results

For this study a displacement controlled cyclic pushover analysis was performed in

Perform3D. In perform-3D, it is easy to visualize the cyclic and hysteric behavior of the

structure and investigate the post peak behavior with the effects of strength and stiffness

degradation. So after the modeling was completed cyclic pushover analysis was performed on

bare and infilled RC frame structures. For this purpose gravity and fifty pushover load cases

were defined in analysis phase in Perform3D. For pushover analysis triangular distribution as

given in (Uniform building code, 1997) was used. In each step the maximum drift was

increased by 0.002 than the preceding step starting from 0.002 in first step in H1 direction (In

plane loading). Each next step pushes the structure in opposite direction than the previous

step and uses the stiffness at the end of previous pushover load case. In analysis drifts keep

on increasing until either the structure fails or maximum allowable push in Perform3D is

reached i.e. 10%. After the completion of analysis we can see the hysteresis loop between

base shear and roof displacement. Using this hysteresis we can get the capacity or backbone

curve which is used to develop the vulnerability curves. Hysteresis loops for 3 bay 3 storey

and 3 bay 9 storey bare and brick infilled RC frames are shown in below figure 8. It can be

clearly seen from hysteresis plot that with the inclusion of infill panel base shear has

increased and roof drift is reduced indicating an increase in strength and lateral stiffness of

structure with inclusion of masonry infill. After the formation of hysteresis loops for all the

structures considered in the study, capacity spectrum method mentioned in (ATC-40, 1996)

was used to get the performance point of the structures which is actually the intersection

point of capacity spectrum and response spectrum. These performance points are then used to

determine the hazard level of the structure by determining the PGA at each performance point

(Kyriakides, 2007). Finally a plot between peak ground acceleration (PGA) and damage

index (DI) is made which is the vulnerability curve.





Figure 8: Hysteresis loops for 3 bay 3 storey (3B3S) and 3 bay 9 storey (3B9S) bare and

brick infilled (BI) RC frames

Figure 9: Vulnerability Curve for Bare and Infilled RC Frame

3.5 Seismic Vulnerability Curves

The results of seismic vulnerability assessment in the form of vulnerability curves for three,

five, seven and nine storey bare and masonry infilled RC frames are shown in above figure 9.





Each graph presents vulnerability curve for bare RC frame, burnt clay brick infilled (BI) RC

frame, Solid concrete block infilled (SB) RC frame and Hollow concrete block infilled (HB)

RC frame. It can be seen from the vulnerability curves of all frames that brick infilled RC

frame offers most resistance then comes hollow block infilled RC frame followed by solid

block infilled RC frame and finally bare frame has least seismic resistance.

4. Discussion

It can be clearly seen from the results of seismic vulnerability assessment i.e. seismic

vulnerability curves that brick infilled frames offer greatest resistance to earthquakes of all

the models considered in this study. Brick infill has better earthquake performance because of

its higher compressive strength and less weight as compared to solid and hollow concrete

block infill. Similarly hollow concrete block has better earthquake resistance than solid block

because it is lighter as compared to solid concrete block while bare frame has least

earthquake resistance because of its lesser lateral stiffness. A comparison of seismic

resistance of bare and infilled RC frames is presented below in the form of bar charts for

three, five, seven and nine storey RC frames. Here the PGA at 100% damage of bare frame is

compared with the PGA at 100% damage of burnt clay brick infilled, solid concrete block

infilled and hollow concrete block infilled frames respectively.

(a) 3 bay 3 storey (b) 3 bay 5 storey

(c) 3 bay 7 storey (d) 3 bay 9 storey

Figure 10: PGA comparison at 100% damage of Bare and Infilled concrete Frames (a) 3 bay

3 storey (b) 3 bay 5 storey (c) 3 bay 7 storey (d) 3 bay 9 storey

A close look at this comparison suggests that as the number of storeys are increasing the

effect of infill is reduced as the PGA increase for three bay three storey brick infilled frame is

38% as compared to three bay three story bare frame while PGA increase for three bay nine

storey brick infilled frame is just 12.5% as compared to three bay nine story bare frame





indicating that influence of infill is more significant in the frames with lower number of

storeys. Following figure shows the percentage increase in PGA at 100% damage of five,

seven and nine storey brick infilled RC frame structures as compared to the three storey brick

infilled RC frame which complements the result described above.

Figure 11: Comparison of PGA at 100% damage of Brick Infilled Concrete frames with

variation of number of storeys

5. Conclusion

Following conclusions can be drawn from the results of Seismic Vulnerability Assessment:

1. Infilled RC frames can resist more PGA at 100 % damage thus are less vulnerable to

earthquake damages as compared to bare RC frames.

2. It can be concluded from the vulnerability curves of storey analysis that low story RC

bare frames such as three and five story collapse suddenly while the failure of higher

story RC bare frames such as seven and nine storey is more gradual.

3. It is also observed that with the inclusion of infill panel the collapse of RC frames has

become more gradual.

4. For all structures considered in this study it has been observed that brick infilled

frames are most resistant to earthquakes, then comes the hollow concrete block

infilled frames and last are solid concrete block infilled RC frames.

5. It has been noticed that the effect of infill walls is more significant in low storey

frames and as the height of structure increases the effect of infill wall reduces.

Acknowledgement

The authors acknowledge NUST Institute of Civil Engineering for providing all the lab

facilities and support for this research.





6. References

1. Mekonnen Degefa., (2005), Response of Masonry Infilled RC Frame under

Horizontal Seismic Force, Master thesis, Addis Ababa University

2. Polyakov, S.V., (1960), On the interaction between masonry filler walls and enclosing

frame when loaded in the plane of the wall”, in Earthquake Engineering, Earthquake

Engineering Research Institute, San Francisco, pp. 36-42.

3. Holmes M., (1961), Steel frames with brickwork and concrete infilling. Proceedings

of the Institution of Civil Engineers, 19(4), pp 473-478

4. Mainstone, R. J., (1971), On the Stiffness and Strength of infilled frames

5. Dhanasekar, M. and Page, A. W., (1986). "The Influence of Brick Masonry Infill

Properties on the Behavior of Infilled Frames" Proceedings of the Institution of Civil

Engineer, 81(4), pp 593-605

6. Valiasis, T., Stylianidis, K., (1989), Masonry infilled R/C frames under horizontal

loading –Experimental results, European Earthquake Engineering, 3, pp 10-20

7. El-Dakhakhni W., (2000), Experimental and analytical seismic evaluation of

concrete masonry infilled steel frames retrofitted using GFRP laminates.

Electronics Theses DSpace at Drexter University Libraries

8. Kaushik HB, Rai D, Jain SK., (2008), A Rational Approach to Analytical

Modeling of Masonry Infills in Reinforced Concrete Frame Buildings.

Proceedings of the 14thWorld Conference on Earthquake Engineering, Beijing, China

9. Kircher, C.A., R.V. Whitman, and W.T Holmes., (2006), HAZUS Earthquake Loss

Estimation Methods, Natural Hazards Review, pp 45-59

10. Kyriakides N., (2007), Vulnerability of RC buildings and risk assessment for Cyprus,

PhD Thesis, Department of Civil and Structural Engineering, University of Sheffield,

UK.

11. ATC-40., (1996), Seismic evaluation and retrofit of concrete buildings, Volume-1,

California, Redwood City.

12. FEMA 356., (2000), Prestandard and commentary for the seismic rehabilitation of

buildings, Federal Emergency Management Agency, Washington, D.C., 2000.

13. PERFORM 3D., (2006), Nonlinear Analysis and Performance Assessment for 3D

Structures, user guide, Computers and structures Inc., Berkeley, California, 2006.

14. Stafford Smith, B., (1967), Methods for predicting the lateral stiffness and strength of

multi storey infilled frames, Building Science, 2(3), pp. 247-257

15. FEMA 306, (1998), Evaluation of Earthquake Damaged Concrete and Masonry wall

buildings, Federal Emergency Management Agency, Washington, D.C., 2000

16. Uniform building code., (1997), Volume 2, International code council

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