THE EFFECT OF ONE-WAY ARCHING ON ENHANCING THE PERFORMANCE OF CONCRETE ... · PDF file15th International Brick and Block Masonry Conference Florianópolis – Brazil – 2012 MATERIALS

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

THE EFFECT OF ONE-WAY ARCHING ON ENHANCING THE PERFORMANCE OF CONCRETE BLOCK WALLS SUBJECTED TO

BLAST LOADING

Abou-Zeid, Badr1; El-Dakhakhni, Wael2; Razaqpur, Ghani3 and Foo, Simon4 1 PhD, Senior Engineer, Structural Department, SNC-Lavalin group, Cairo, Egypt. [email protected]

2 PhD, Martini, Mascarin and George Chair in Masonry Design, McMaster University, Canada. [email protected] 3 PhD, Professor, McMaster University, Hamilton, L8S 4L7, Ontario, Canada. [email protected]

3 PhD, Senior Engineer, Real Property Branch, Public Works & Government Services, Canada. [email protected]

New standards for blast protection of buildings have been recently developed in the USA and Canada. In this regard, both standards are considering unreinforced masonry (URM) walls as particularly vulnerable to blast events and may not be used in blast resisting structural systems. In this paper, the effectiveness of enforcing arching action to as a cost-effective hardening technique for vertically-spanning one-way URM walls under blast loads is experimentally investigated. A total of eight full-scale concrete-block URM walls were subjected to blast loads generated by high explosives. Enforcing URM walls to arch between rigid supports significantly enhanced their out-of-plane blast resistance compared to similar non-arching (flexural) URM walls. Moreover, no fragments or debris were observed on the leeward side of the arching walls, indicating the potential of the proposed hardening technique in reducing the hazard level for the occupants of buildings with exterior URM walls. A simple bi-linear moment-rotation relationship is developed to model arching URM walls. The model takes into account the masonry material strength, thrust forces, and wall geometry. Nonlinear time-history analyses were performed using a single-degree-of-freedom (SDOF) model. The model takes into account the rocking phenomenon and the second order effects. Responses generated by the SDOF model were verified with available experimental data.

Keywords: Arching action, Blast loads, Concrete masonry, Dynamic stability, Out-of-plane capacity, Moment-Rotation relationship, rocking phenomenon, SDOF models. INTRODUCTION The focus of the current study is to explore the effects of enforcing composite action between the infill panel and the surrounding frame as a simple and economical approach for hardening existing buildings or for new infill-frame structures design to resist explosions. The concept of a wall acting as an arch can be explained as follows: after the formation of the first cracks in a simply-supported URM infill wall its out-of-plane resistance would depend on the compressive strength of the masonry rather than the mortar tensile strength. This was first



described by McDowell et al. (1956) who developed a theory based on arching action of a one-way URM wall butted against rigid supports. This theory assumes that after the development of tension cracks at the ends and the centre of the wall, the wall acts as two rigid segments (Fig.1), with each segment rotating about its end until either the masonry crushes or the two segments snap-through. The strength enhancement is caused by the large in-plane compressive resistance developed at the ends and the centre of the wall, a resistance that will be controlled by the crushing strength of masonry.

BLA

ST L

OA

DCRACK

CRUSHING

CRUSHING

CRACK

CRACKC

RU

SHIN

G

RIGIDSUPPORT

RIGIDSUPPORT

twhw2

hw2

P

P

P

P

Fig. 1 Arching Mechanism The behavior of masonry walls subjected to free-field explosions has recently been experimentally investigated by a number of investigators (Carney et al. 2005 and Trevor et al. 2008). Most of these studies were intended to predict the out-of-plane capacity of the tested specimens or to evaluate the strength increase when fiber reinforced polymers were used as a retrofit technique. Other researchers, including Monk (1958), Gabrielsen et al. (1974) and Baylot et al. (2004), evaluated the effect of frame flexibility on the infill wall capacity. The goal of the current study is to investigate the behavior of face-shell mortared full-scale unreinforced hollow concrete masonry walls under blast loads and to examine the wall-frame interaction during blast events. Another goal was to evaluate the effectiveness of the arching action in enhancing the out-of-plane blast capacity of one-way URM walls as well as the potential of hazard mitigation for occupants of building with exterior arching URM walls. EXPERIMENTAL PROGRAM Eight full-scale unreinforced concrete masonry walls were constructed and tested under free-field blast loads generated by substantial quantities of high explosives. All walls were built by an experienced mason using standard 190-mm (nominal 20 cm) two-cell concrete blocks. The dimensions of each wall were 2.5 blocks wide (990 mm) by eleven courses high (2,190 mm). All walls were face-shell mortar bedded with 10 mm mortar joint concavely tooled. Each wall was built over 12.5 mm (1/2”) steel plate, and all the walls were constructed in a running bond fashion to simulate common construction practice in North America.



MATERIALS The Type-S mortar flow was measured and three 51mm cubes were cast for each mortar batch to determine its compressive strength. The average mortar flow was found to be 125% with a coefficient of variation (COV) of 5%. The mortar mixing and tests were performed according to the CSA A179-04 (CSA 2004a). Tests of randomly selected 60 mortar cubes resulted in an average mortar compressive strength of 17.9 MPa with 8.7% COV. Tests to determine the compressive and the flexural tensile masonry strengths normal to the bed joints were also conducted. A total of eleven 4-course high prisms with face shell mortar bedding were constructed at the same time as the walls. Out of which, five masonry prisms were tested according to the requirements of the CSA S304.1 (CSA 2004b) to determine the masonry compressive strength. Average values of compressive strength, elastic modulus and tensile strength (using the bond wrench test) of the masonry prisms were 25.9 MPa, 22.9 GPa and 0.37 MPa, respectively, with COV of 7.1%, 22.6% and 16.9%, respectively. TEST MATRIX Table 1 presents the test matrix divided in to three groups; Group I was composed of five walls (W1 to W5) and each wall was subjected to a single shot with Wall W1 being incapable of developing arching action (through changing the top wall boundary conditions) and thus serving as the control wall. The purpose of this group was to understand the response of arching URM walls under actual blast loads and to evaluate the enhancement to the walls’ out-of-plane flexural capacity. Group II was comprised of Walls W6 and W7 and was designed to investigate the influence of pre-cracking on the out-of-plane capacity of arching walls. These walls were subjected to five and three successive shots, respectively. Group III consisted of Wall W8 only which was designed to investigate the debris hazard of arching URM wall under severe blast loads.

Table 1: Experimental matrix of free field blast tests

Group Shot # Specimen# Charge size ANFO (kg) Standoff distance (m)

I

1 W1 50 15.0 2 W2 100 15.0 3 W3 150 15.0 4 W4 200 15.0 5 W5 250 15.0

II

6

W6

15 19.4 7 25 19.4 8 50 19.4 9 75 19.4

10 153 17.0 11

W7 30 20.0

12 100 20.0 13 250 17.0

III 14 W8 30 3.0



TEST-SETUP One of the main problems with free field blast testing is the blast wave clearing effect. This phenomenon is usually accompanied by a pressure drop and high level of uncertainty regarding blast wave parameters (Smith et al., 1999). In order to reduce the clearing effect and to simulate the interior space behind the walls in a typical building, ISO steel containers were used to house the walls. The nominal external dimensions of the container were 6,100 mm (20’) long, 2,400mm (8’) wide, and 2,600 mm (8’6”) high (see Fig. 2).

Fig.2 Test-setup

The length of the container was also checked for adequacy to eliminate the wrap-around effect. The latter phenomenon occurs when a part of the reflected pressure wave from the target front face propagates around the target’s edges and result in increased pressure on its rear face. Two hollow rectangular steel sections 76.2 mm (3”) wide and 50.8 mm (2”) high, welded to a 6.35 mm (1/4”) steel plate, were used to snug fit the URM wall top and bottom against the container’s steel frame in order to restrain free rotation of the wall at its end and enforce vertical arching. In addition, the backside of the URM wall was laterally supported near the top and the bottom courses to eliminate sliding of the specimen during its inward motion. Moreover, two external steel clamps were used at the top and the bottom to prevent the wall pull-out of the specimen during the negative pressure phase or rebound.



As discussed earlier, to develop arching action the wall end supports should be able to resist the thrust force without significant movement (i.e. unyielding supports), otherwise, the effectiveness of the arching mechanism would be compromised. To increase the rigidity of the container frame in the vertical direction, two steel columns were welded to the top and bottom beams of the container frame and were designed to carry a significant level of the thrust force. DISPLACEMENTS RESPONSE Sample displacement time histories at the 5th, 6th, and, 9th courses of Walls W1 and W2 are plotted in Fig. 3. A positive displacement value indicates inward wall displacement. Points A, B, C and D on the response curves indicate maximum and minimum displacement values in inward and outward responses. In all wall specimens, except the control Wall W1, the maximum wall mid-height displacement (Point A) was attained with the 5th, 6th, and 9th course displacement being almost in-phase. In addition, the 5th and 6th course displacements were nearly equal while the 9th course displacement was half of the 6th course displacement. This response indicates rigid-body rocking at the mid-height of the wall. On examining the relationship between the 9th and the 5th course versus the 6th course displacements, an approximately linear variation of the 5th, 6th, and 9th course displacements from zero up to Point A (maximum inward displacement) was noticed. This response suggests an in-phase displacement of the three courses. After Point A, the response curves show a highly non-linear variation. This deviation from linearity indicates that the displacement of the 9th and the 6th courses were out-of-phase indicating initiation of the wall damage. The calculated average ratio of the 9th to the 6th course displacement (at the same time as Point A) for Walls W2 to W5 was 0.46 versus an exact value of 0.47 assuming a rigid body rocking motion. On the other hand, the ratio of the 5th to the 6th course displacement was found to be 0.97, which would result if the wall was rocking as two rigid segments, with an upper segment of 1,200 mm (6-blocks) and a lower segment of 1,000 mm (5-blocks) high.

0

10

20

30

40

50

60

0 50 100 150 200 250 300Time (msec)

Dis

plac

emen

t (m

m)

A

B

C

D

Free Vibration Phase

9th 6th

5th

-40

-20

0

20

40

60

80

100

0 50 100 150 200 250 300Time (msec)

Disp

lace

men

t (m

m)

A

B

C

D

Free Vibration Phase

6th

5th

9th

(a) Wall W1 after Shot#1 (b) Wall W2 after Shot#2

Fig. 3 Displacements at the 5th, 6th, and 9th courses



The deflected shapes of the upper segments of Walls W1 to W5 were captured at the same time as Point A in Fig.3 and are plotted in Fig. 4. The wall top end displacement was taken equal to zero due to the imposed end conditions as discussed earlier. Again, the deflected shapes for the arched walls showed no significant bulging at the 9th course indicating a rigid a rigid-body rocking. On the other hand, the deflected shape of Wall W1 shows a kink at the 9th course which approximately matches the actual location of the crack observed above the 8th course in this wall rather than above the 5th course as observed in Walls W2 to W5. The control Wall W1 may be considered as pinned at the top and fixed at the base and subjected to a uniformly distributed pressure. For this loading case and boundary conditions, the maximum positive bending moment will occur above the wall mid-height. The assumption that the higher the impulse (or pressure), the larger the displacement does not seem applicable to Walls W3 and W4, where the central displacements are approximately the same, even though they were subjected to blast loads resulting from different charge sizes. Figure 4 illustrates the proposed deflected shape for vertically spanning one-way simply-supported URM walls that are capable of developing arching action when subjected to blast loading. The proposed deflected shape represents a simple mechanism with a hinge at mid-height of the wall.

-100

100

300

500

700

900

1100

0 50 100 150 200Displacement (mm)

Hal

f the

Wal

l Hei

ght (

mm

)

9th Course LVDT

6th Course LVDT

W5 W2

W4

W3

W1

Δhw/2

hw/2

Fig. 4 Comparison of maximum mid-height displacements of walls W1-W5

SINGLE-DEGREE-OF-FREEDOM (SDOF) MODEL DEVELOPMENT Several methodologies were developed to investigate the damage of different members subjected to blast loadings. Among these, are SDOF analysis and Pressure-Impulse (P-I) diagrams. The former can generate the complete response while the latter only give the peak response. In addition, an accurate SDOF models that determine the peak response throughout the time-history is the first step in developing accurate P-I diagrams. The deflected shape of a one-way arching URM wall vertically spanning between unyielding supports is illustrated in Fig. 5 (a). The deflected shape is based on the assumption that the wall displacement over its lower half varies linearly with 0=bottomu at the bottom and u = hw! 2 at the wall mid-height. Similarly, it varies linearly over its upper half between u at the wall mid-height and 0=topu at the top of the wall. Figure 5 (b) illustrates the inertial force IF distribution. At all



times, IF has a distribution identical to the wall’s deflected shape. Considering the upper half of Fig. 5 (b), the inertial force 2IF is calculated as the product of the mass and the acceleration and is equal to mw 2( )! hw

!!! 4( ) = mwhw!!! 8 . The latter force is applied at a distance of hw 6 from the

base of the upper triangle. An additional moment of Io ! !!! = mw 2( )! hw 2( )2"

#$% 12! !!! = mwhw

2 !!! 96 is also induced for each rigid wall segment to

account for the inertial moments about each segment’s centroid. Figure 5 (c) shows the blast forces, arching thrust forces of the wall, wall’s self-weight, wall’s inertial forces and, inertial moments about its centroid, and damping forces.

mw2

hw4

..

mw2

g

F(t)2

..Io

P

Rd2

Rd2

P+mw2

g

P+mwg

hw4

.c

I o= mw/2 (hw/2)12

2

mw2

hw4

..

mw2

g

F(t)2

..Io hw

4

.c

hw2

hw2

hw2u=

O1

O

(a) (c)

hw6hw6

FI2

FI2

FI2

mw2

hw4

..=

(b)

Fig. 5: Arching action and SDOF restoring forces mechanism Based on the above rationale, the dynamic equation of motion at the wall mid-height can be shown to be given by:

8)()(

1624

22wwww htFMchhm

=++ θθθ

(1)

It should be mentioned that Eq. (1) generates the complete response of the wall’s both forced and free vibration phases. This is performed because ( )tF is comprised of two successive parts, where the forcing pulse is followed by another pulse with zero force amplitude and at least has the same loading time as the previous pulse.



SDOF MODEL VALIDATION In order to validate the SDOF analytical model, the developed equation of motion (Eq. 1) was solved for each blast shot using the Newmark constant acceleration method (Clough and Penzien, 1993) to obtain the full displacement time-history response of each wall. Dynamic analyses of the arching walls were performed for the 250 milliseconds duration of the measured response of the wall in the test, using a time step of 0.01 millisecond, which was the sampling rate of the data acquisition used during the tests. It should be mentioned that the effect of strain rate on the strength of the arching walls was not considered in the dynamic analysis. The latter approach is based on the fact that finding an equivalent strain rate that covers all fibers in all sections for the entire load history is neither an easy nor an accurate task. This is because of the several uncertainties involved and the several assumptions to be made that cannot be verified by the authors from the tests conducted. In addition, the availability of information related to strain rate effects on mortar bond strength and masonry compressive strength for URM in literature is very scarce. In addition, as failure usually occur due to arch instability, any enhancement to the masonry compressive strength or the mortar tensile bond strength would be insignificant in terms of the arching URM wall response. Figure 6 compares the experimental peak displacements to those obtained by the SDOF analysis for the 13 blast shots. The mean ratio of the experimental to the predicted peak displacements is 0.92, with a coefficient of variation of 17%, indicating that the model with max5.0 θθ =cr is capable of predicting the experimental results with a reasonable accuracy.

34

5

2

6 78

9 10

1112

13

0

25

50

75

100

125

0 25 50 75 100 125

Experimental Displacement (mm)

SDO

F D

ispl

acem

ent (

mm

)

Fig. 6: Comparison of peak inward displacements: experimental versus SDOF model prediction



CONCLUSIONS The development of the new standards for blast protection of buildings in USA and Canada is currently underway. The focus of the new standards will be to propose different levels of protection for existing and new construction in order to meet certain performance criteria. In this regard, both standards are considering URM walls as particularly vulnerable to blast events indicating that URM may not be used in blast resisting structural systems. In this paper, the effectiveness of enforcing arching action as a cost-effective hardening technique to enhance the blast capacity of URM walls was evaluated by free-field explosion test of eight full-scale walls. Based on the experimental results and the test results analyses, the following conclusions can be drawn: 1. The out-of-plane strength of the arching URM walls is governed by stability requirements rather than the compressive strength of masonry. 2. The capacity of URM walls subjected to out-of-plane blast pressure was significantly enhanced due to the arching action and the resulting large in-plane forces. 3. Arching action reduced the hazard resulting from flying debris in the tested URM walls by increasing the frictional forces between the masonry courses. 4. The deflected shape at peak inward displacement of all arching walls was found to resemble a symmetrical three-hinged arch. 5. Although arching is known to increase the out-of-plane resistance of URM walls, it also enhanced the shear resistance due to the increased friction between the masonry courses caused by the induced thrust force. 6. The developed SDOF model can be used to capture the displacement time-history response of vertical one-way arching URM walls subjected to blast loading. The model accurately predicts the displacement amplitudes, but with a phase lag when compared to the experimental data. The latter is attributed to the formation of multiple cracks, which are dictated to higher mode effect. ACKNOWLEDGMENTS The following organizations are gratefully acknowledged for their support towards this study: the Chemical, Biological, Radiological/Nuclear and Explosives Research and Technology Initiative (CRTI project 06-015TD) for the financial support, the Canadian Explosives Research Laboratory (CERL) for conducting the blast tests, the Canadian Armed Forces for the use of the test range, the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Concrete Masonry Producers Association (CCMPA) for donating the masonry blocks, the Canada Masonry Design Center (CMDC) for building the walls, and the Center for Effective Design of Structures (CEDS) at McMaster University funded through Ontario Research and Development Challenge Fund (ORDCF), a program of the Ministry of Research and Innovation (MRI) for their financial support.



REFERENCES American Society for Testing and Material (ASTM), “Standard test method for compressive strength of masonry prisms.” C1314-06, West Conshohocken, PA. Baylot, J.T., Bullock, B., Siawson, T.R., Woodson, S.C., 2004. “Blast Response of Lightly Attached Concrete Masonry Unit Walls.” Journal of Structural Engineering, ASCE, 131(8), August 2005, pp. 1186-1193. Biggs J.M. (1964),” Introduction to Structural Dynamics.” McGraw-Hill, New York. Canadian Standards Association (CSA) (2004a).” Mortar and Grout for Unit Masonry.”CSA A179-04, Mississauga, ON, Canada. Canadian Standards Association (CSA) (2004b).” Design of Masonry Structures.” CSA S304.1-04, Mississauga, ON, Canada. Carney P., and Myers J., (2005). “Out-of-Plane Static and Blast Resistant Capacity of Unreinforced Masonry Wall Connections Strengthened with Fiber Reinforced Polymers.” Center for Infrastructure Engineering Studies, University of Missouri-Rolla, Rolla, MO. Clough R.W., and Penzien J., (1993), “Dynamics of Structures.” McGraw-Hill, New York, USA.

Gabrielsen B., and Wilton C. (1974). “Shock Tunnel Tests of Arched Wall Panels.” URS 7030-19, URS Research Co., Contract No. DAHC20-71-C-0223. McDowell E. L., McKee K. E., and Sevin E. (1956). “Arching Action Theory of Masonry Walls.” Proceedings of the American Society of Civil Engineers, Journal of the Structural Division, Vol. 82, No. ST2, pp. 915-1 to 915-18. Monk C. B. (1958). “Resistance of Structural Clay Masonry to Dynamic Forces.” Research Report No.7, Structural Clay Products research Foundation, Geneva, Illinois. Smith P.D., Rose T.A., and, Saotonglang E.,(1999),” Clearing of Blast Waves from Building Facades.” Proceedings of the Institution of Civil Engineers:Structures and Buildings, pp.193-199, Thomas Telford Limited, London. Trevor, D. Hrynk, and John J. Myers, 2008. “Out-of-Plane Behavior of URM Arching Walls with Modern Blast Retrofits; Experimental Results and Analytical Models.” Journal of Structural Engineering, ASCE, Vol. 134, No. 10, October 2008, pp. 1589-1597.

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THE EFFECT OF ONE-WAY ARCHING ON ENHANCING THE PERFORMANCE OF CONCRETE ... · PDF file15th International Brick and Block Masonry Conference Florianópolis – Brazil – 2012 MATERIALS