Effect of Openings on Structural Performance of Unreinforced Masonry

7/31/2019 Effect of Openings on Structural Performance of Unreinforced Masonry

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Effect of Openings on Structural Performance of Unreinforced MasonryInfilled Frames

Ghassan Al-Chaar1, Gregory E. Lamb2, AND Daniel P. Abrams3

Abstract

Door or window openings in masonry infill panels can reduce the lateral strength and stiffness of infill-framesystems. In an effort to study these effects, a series of tests were conducted on half-scale test structuresconsisting each of three stories and three bays. Infill panels of the control structure were solid with no openingswhile panels of the second structure were perforated with window and door openings of varying size and location.The test structures were designed to replicate typical building practice of the early 1950s with little or no seismicdetailing of frame reinforcement. The test structures were subjected to cyclic in-plane lateral forces to study theirstrength and deformation capacity under seismic excitation. The cyclic loading was chosen to apply displacementdemands on the structures, representative of those that are expected to occur during strong earthquake motions.Test results discussed in this paper are presented in terms of observed changes in strength, stiffness anddeformation capacity of both test structures. Damage patterns and propagation of cracks in the concrete frameand masonry infill during loading are illustrated and discussed in terms of measured histories of force anddeflection. Experimental results supported by analytical studies are used to estimate overall reductions instrength, deformation capacity and stiffness due to the presence of openings in the panels.

Keywords: Cyclic Loading, Multibay, Multistory, Openings, Reinforced Concrete, Seismic, Stiffness, Strength,Unreinforced Masonry.

Introduction

A half-scale three-story, three-bay reinforced concrete (R/C) frame infilled with concrete masonry units (CMU) wassubjected to a cyclic loading protocol in order to better understand the structural behavior of this complex system(Al-Chaar 2001). The experimental model was constructed using design philosophies of the 1950s. Therefore,the reinforced concrete frame lacked the seismic detailing required of more modern building codes. Figure 1illustrates the dimensions and reinforcing of the half-scale models used in this experiment.

1Structural Engineer, U.S. Army ERDC-CERL, P.O. Box 9005, Champaign, IL 61822

2Structural Engineer, U.S. Army ERDC-CERL, P.O. Box 9005, Champaign, IL 61822

3Hanson Engineers Professor of Civil Engineering, University of Illinois, 205 N. Mathews, Urbana, IL 61801


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Figure 1. Experimental Model Dimensions and Reinforcement.

Lateral loading on the third floor was based on a displacement-controlled cyclic loading protocol whichrepresented demands from an actual earthquake. Figure 2 depicts the loading protocol used where the buildingdrift ratio refers to the third floor displacement over the entire height of the half-scale model. The lateral loads onthe first and second floors were force-controlled and followed a certain loading ratio dependent upon the resultingforce on the third floor from the cyclic loading history. The load ratios gave an approximate inverse triangulardistribution of lateral seismic force.

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0.5

1

1.5

2

0 20 40 60 80 100 120 140 160 180

Time (min)

BuildingDriftRatio(%)

Figure 2. Cyclic Loading Protocol.


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Material Properties

Ideally, both the fully infilled and perforated infill experimental models would have contained the exact same

material properties. However, due to variance in construction and materials, the fully infilled model wasconstructed with materials of fairly weak structural properties, while the perforated infill model had much strongermaterial properties. Table 1 gives the concrete cylinder properties and masonry prism test results for bothmodels. In addition to the masonry and concrete variance, small differences in reinforcing steel properties existedas well.

Table 1: Concrete Cylinder and Masonry Prism Properties.

Concrete Cylinder Properties

Model Fully Infilled Perforated Infillf'c(psi) Ec(ksi) f'c(psi) Ec(ksi)

Floor 1 4845 3968 5716 4294

Floor 2 4365 3766 7223 4844Floor 3 2619 2917 7771 5025

Masonry Prism Properties

Model Fully Infilled Perforated Infillf'm(psi) Em(ksi) f'm(psi) Em(ksi)

Floor 1 787 590 1599 1199

Floor 2 978 734 1497 1123

Floor 3 873 655 1425 1068

Experimental Results

Subjecting the experimental models to the loading protocol in Figure 2 resulted in the hysteretic behavior for thefully infilled and perforated infill models shown in Figures 3 and 4, respectively.

For the fully infilled model, the peak lateral load occurred during the 21st cycle at a corresponding first floor storyshear of 32.21 kips. The second and third floors had story shears of 24.90 and 14.17 kips, respectively, duringthe positive stroke of this cycle. A summary of peak load, story shear, and associated drift ratio for the fully infilledmodel is given in Table 2.

Table 2: Load-Deformation Response of the Fully Infilled Model.

Peak Load (kips) Story Shear (kips) Drift Ratio (%)

Floor + - + - + -

3 14.17 13.90 14.17 13.90 0.07 0.05

2 10.73 9.18 24.90 23.08 0.21 0.35

1 7.31 5.94 32.21 29.02 0.12 0.09


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0

20

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-3 -2 -1 0 1 2 3

Drift Ratio (%)

BaseShear(kips)

Figure 3. First Floor Load-Displacement Relationship for the Fully Infilled Model.

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20

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-4 -3 -2 -1 0 1 2 3

Drift Ratio (%)

BaseShear(kips)

Figure 4. First Floor Load-Displacement Relationship for the Infill Model with Openings.


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For the infill model with openings, the peak lateral load occurred during the 35th cycle with a resultant first floorstory shear of 71.50 kips. Recall, the material properties for the model with openings were significantly greaterthan the fully infilled model. Table 3 summarizes the load-deformation behavior of the perforated infill model.

Table 3. Load-Deformation Response of the Perforated Infill Model.

Peak Load (kips) Story Shear (kips) Drift Ratio (%)

Floor + - + - + -

3 33.60 30.07 33.60 30.07 0.53 0.55

2 22.80 21.42 56.40 51.49 1.20 1.10

1 15.10 14.59 71.50 66.08 0.80 0.60

After subjecting the experimental models to the cyclic loading protocol in Figure 2, both models were severelydamaged. Figure 5 illustrates the cracking patterns observed in the fully infilled and perforated infill models,respectively. Note how bed joint sliding is prevalent in the fully infilled model indicative of weak materialproperties. For the perforated infill model, the openings acted as crack initiators with cracks propagating from the

corners of the openings.

Figure 5. Cracking Observed in the Fully-Infilled Model (Top), Infill with Openings (Bottom)


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Analytical Strength and Stiffness Prediction

An analytical method utilizing eccentric equivalent struts in conjunction with nonlinear pushover analyses wasutilized to predict the strength and stiffness of infilled frames (Al-Chaar 2002). This method relies on thedevelopment of plastic hinges to capture the nonlinearities of the structural system. Using this method, a

theoretical prediction of strength and stiffness is compared to the actual experimental results for the fully infilledmodel in Figure 6. The theoretical prediction yielded an ultimate capacity, initial stiffness, and displacement atultimate load that closely resemble those values found experimentally. The value of initial stiffness was within3.5% of the experimental value while giving a displacement at ultimate load was within 4.3%. In addition, theultimate load found theoretically was within 4.4% of the experimental value.

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Displacement (in)

BaseShear(kips)

Experimental Data Theoretical Prediction

Figure 6. Experimental and Theoretical Comparison for the Fully Infilled Model.

Development of a Strength Reduction Factor for Perforated Panels

To determine the correct strut width to use when openings are present in an infill panel, it is necessary to

determine a strut width reduction factor to multiply to the original strut width for a fully infilled panel. Pushoveranalyses were run for full-scale 3-bay by 3-story R/C infilled frames containing perforated panels. The strut widthswere modified until ultimate load capacities given matched those given by FEA. The FEA data used for thisrelationship were corrected to account for a change in shear coefficient once experimental results were calibrated(Al-Chaar 2002).

All FE models were created and executed using the ANSYS program (1998). The various configurations of bareand infilled reinforced concrete frames were subjected to in-plane loading until structural failure occurred. The


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analyses use non-linear material behavior for all the real physical components. The reinforcing bars includeelastic-plastic material behavior. Note the reinforcing bars are included explicitly in the model, and not averagedacross a section of the frame. The concrete, mortar, CMU infill, and brick infill use material failure surfaces thataccount for material tensile failure and compressive material crushing. The material behavior also includes sheartransfer for cracked material. Figure 7 illustrates several of the FE models used in the development of a strength

reduction factor for perforated panels. The FE model on the left shows a door and window opening in infill panel1A, where 1 represents the floor level and A represents the left bay. Using this convention, the middle bay isrefered to as B and the right bay, C. A total of six FE models with door/window openings in single panels and twoFE models with large and small window openings in every panel were used in this research.

(a) (b)

(c) (d)Figure 7. FE Models showing door/window opening in panel 1A (a), in panel 1B (b), in 60% openings in all panels(c), and 22% openings in all panels (d).

Six points were plotted to find a correlation between percent of open infill and percent of original strut width withthe former on the abscissa and the latter on the ordinate. Only uniform and first floor door/window opening frameswere used since they were the only frames that showed significant changes in ultimate capacity (Al-Chaar 2002).The data points used are shown in Table 4 followed by the plot shown in Figure 8. The numerical interpretation ofthis plot becomes the following equation:


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16.16.0)(

2

1 +

=

panel

open

panel

open

iA

A

A

AR

[Eq. 1]

Where:

Aopen= area of the openingApanel= area of the infill panel

The variable, (R1)i, is the in-plane reduction factor that accounts for the presence of infill openings. This reductionfactor is multiplied to the width of the strut and results in a fairly accurate estimate of system strength andstiffness. This analytical method should be used as an alternative to FEMA 273.

Table 4. Summary of Data used to develop Equation 1.

Model Area of Openings Reduced Strut WidthArea of Panel Original Strut Width

Bare Frame 1 0Large Openings 0.52 0.32

1A Door/ Window 0.31 0.52

1B Door/ Window 0.31 0.63

Small Openings 0.22 0.68

Fully Infilled 0 1

y = 0.5889x2- 1.5952x + 1.0041

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

Infill Area Ratio

StrutWidthRatio

Strut Width Reduction Factor = 0.6 * x 2 - 1.6 * x + 1

where x = (Area of Openings) / (Area of Panel)

Figure 8. Strut Width Ratio, (R1)i, versus infill area ratio (Aopen/Apanel).


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This equation was applied to determine modified strut widths for the full scale 3x3 R/C frame with CMU infill forvarious opening configurations. Pushover analyses were then run for these models shown in Figure 7 and Figure9, with the modified strut widths. The values of ultimate load capacity given by the pushover analysis resulted in amaximum error of 12.1 percent compared to the FEA. However, most data points were within 3 percent. These

pushover capacities and their corresponding percent differences compared to FEA are tabulated in Table 5.

(a) (b)

(c) (d)

Figure 9: Triple-Bay Triple-Story Models with Openings at One Panel. (a) Openings in Edge Bay of

Top Story, (b) Openings in Interior Bay of Top Story, (c) Openings in Edge Bay of Interior Story, (d)

Openings in Interior Bay of Interior Story.


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Table 5. Capacities from modified struts.

Opening Opening Nonlinear FEA Pushover %

Location Description Capacity (kips) Capacity (kips) Difference

--- --- 231.7 224.5 3.1

3A door/window 223.8 224.4 0.33B door/window 221.1 225.0 1.8

2A door/window 212.9 207.5 2.5

2B door/window 227.0 199.5 12.1

1A door/window 204.9 192.1 6.2

1B door/window 207.2 188.5 9.0

ALL large window 114.8 116.4 1.4

ALL small window 174.0 172.7 0.7

If the area of the openings exceeds 60 percent of the area of the infill panel, if is recommended that the effect of

the infill should be neglected and the strut width reduction factor should be set to zero.

In addition to correlating the strength reduction factor for perforated panels to FEA data, this relationship was alsocompared to the experimental results for the half-scale infill model with openings. The strut width reduction factor(Eq 1) was used to calculate the theoretical capacity of the perforated infill model. Figure 10 illustrates theexperimental and theoretical comparison for the infill model with openings using pushover analysis with eccentricequivalent struts. The theoretical prediction for ultimate strength was very accurate with only 6.3% difference inthe positive stroke direction and 1.4% difference in the negative stroke direction. However, the initial stiffness anddisplacement at ultimate load were not as accurate as was predicted for the fully infilled model. In order toaccurately predict the displacements for an infill panel with openings, the deformation capacityof the equivalentstrut must be modified to account for the loss of stiffness due to the openings. Further research is necessary to

determine the appropriate deformation capacity of struts representing perforated infill panels.

Comparison of Fully Infil led and Perforated Infill Models

In order to appropriately compare both the fully infilled model and the perforated infill model, an analysis neededto be performed on both structures with the same material properties. Therefore, a structural analysis model ofthe perforated infill model was constructed using the weaker material properties of the fully infilled model (Table1). This model was constructed using eccentric equivalent struts (Al-Chaar 2002) and incorporating the strengthreduction factor for perforated panels.


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Displacement (in)

BaseShear(kips)

Experimental Data Theoretical Prediction

Figure 10. Experimental and Theoretical Comparison for the Infill Model with Openings.

Figure 11 shows the theoretical comparison of the fully infilled model and the perforated infill model using thesame material properties. In general, the results of this analysis agree with the expected outcome. For instance,the initial stiffness of the perforated infill model was 46% less than the fully infilled model. In addition, the ultimate

capacity was 15% less for the infill model with openings. However, the displacement at ultimate load for theperforated infill model was only 88% of the value for the fully infilled model. Table 6 summarizes these results.

Table 6. Load-Deformation Summary of Infill Models with the same Material Properties.

Initial Displacement Ultimate

Stiffness at Ultimate Capacity

Model (kips/in) (in) (kips)

Fully Infilled 411 0.24 33.6

Perforated 189 0.21 28.6


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0

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30

35

40

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Displacement (in)

BaseShear(kips)

Fully Infilled Model Perforated Infill Model

Figure 11. Theoretical Comparison of the Fully-Infilled Model and the Infill Model with Openings using the sameMaterial Properties.

Conclusions

This paper provides an alternative method to FEMA 273/356 for analyzing infill panels with openings. Thismethod offers a simple empirical relationship to account for the reduction in strength and stiffness based on thearea of openings in each panel. Using the eccentric equivalent strut method in a structural analysis was shownpreviously to accurately predict the ultimate capacity, initial stiffness, and displacement at ultimate for fully infilledpanels. This paper shows that the same eccentric equivalent strut method can be applied to predict the strengthof perforated infill panels.

References

Al-Chaar, G., Evaluating Strength and Stiffness of Unreinforced Masonry Infill Structures, U.S. ArmyCorps of Engineers, Construction Engineering Research Laboratories, ERDC/CERL TR-02-01, January 2002.

Al-Chaar, G., Lamb, G., Abrams, D., Seismic Behavior of a Multistory and Multibay Frame-Infill System,

Proceedings of the Ninth Canadian Masonry Symposium, New Brunswick, Canada, June 2001.

Al-Chaar, G. and Abrams, D., Parametric Studies on Seismic Behavior of Frame-Infill Systems,Proceedings of the Ninth Canadian Masonry Symposium, New Brunswick, Canada, June 2001.

ANSYS Release 5.5.1, ANSYS, Inc., 275 Technology Drive, Canonsburg, PA 15317, October 1998.

FEMA 273, NEHRP Guidelines for the Seismic Rehabilitation for Buildings, FEMA, October 1997.

Effect of Openings on Structural Performance of Unreinforced Masonry

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