Scholarly article on topic 'Effect of reinforcing steel debonding on RC frame performance in resisting progressive collapse'

Effect of reinforcing steel debonding on RC frame performance in resisting progressive collapse Academic research paper on "Civil engineering"

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{Debonding / "Progressive collapse" / "Catenary action" / "Moment frame"}

Abstract of research paper on Civil engineering, author of scientific article — Waleed Mohamed Elsayed, Mohamed A.N. Abdel Moaty, Mohamed E. Issa

Abstract This paper presents the experimental program performed to study the effect of reinforcing steel debonding on progressive collapse resistance of moment resisting frame designed and detailed in accordance with the Egyptian code provisions for seismic design. Half-scale specimens of the first story were extracted from the frame structure prototype. Each specimen represented a two-bay beam resulting from the removal of middle supporting column of the lower floor. In all specimens, the exterior two short columns were restrained against horizontal and vertical displacements and a monotonic vertical load was applied on the middle column stub to simulate the vertical load of the upper stories. Gradually increasing vertical load at the location of the removed column is continuously applied and increased up to failure. The cracking patterns, strains and the deformations at selected locations of reinforcing steel and concrete are recorded for further analysis. Different debonded reinforcement ratios, places and length are examined in this study to evaluate its effect on the collapse resistance performance of the frame. The effect of debonding on the distribution of reinforcing steel strain is evaluated. The nonlinear response of the frame to the removal of the column is evaluated and the amount of energy absorbed during the course of deformation is calculated.

Academic research paper on topic "Effect of reinforcing steel debonding on RC frame performance in resisting progressive collapse"

HBRC Journal (2015) xxx, xxx-xxx

Housing and Building National Research Center HBRC Journal

http://ees.elsevier.com/hbrcj

Effect of reinforcing steel debonding on RC frame performance in resisting progressive collapse

Waleed Mohamed Elsayed *, Mohamed A.N. Abdel Moaty, Mohamed E. Issa

Structural Engineering Department, Faculty of Engineering, Cairo University, Egypt Received 4 December 2014; revised 10 February 2015; accepted 19 February 2015

KEYWORDS

Debonding; Progressive collapse; Catenary action; Moment frame

Abstract This paper presents the experimental program performed to study the effect of reinforcing steel debonding on progressive collapse resistance of moment resisting frame designed and detailed in accordance with the Egyptian code provisions for seismic design. Half-scale specimens of the first story were extracted from the frame structure prototype. Each specimen represented a two-bay beam resulting from the removal of middle supporting column of the lower floor. In all specimens, the exterior two short columns were restrained against horizontal and vertical displacements and a monotonic vertical load was applied on the middle column stub to simulate the vertical load of the upper stories. Gradually increasing vertical load at the location of the removed column is continuously applied and increased up to failure. The cracking patterns, strains and the deformations at selected locations of reinforcing steel and concrete are recorded for further analysis. Different debonded reinforcement ratios, places and length are examined in this study to evaluate its effect on the collapse resistance performance of the frame. The effect of debonding on the distribution of reinforcing steel strain is evaluated. The nonlinear response of the frame to the removal of the column is evaluated and the amount of energy absorbed during the course of deformation is calculated.

© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

Progressive collapse has been of great concern to structural engineers, especially with the wide publicity of recent cases.

* Corresponding author.

Peer review under responsibility of Housing and Building National Research Center.

According to ASCE 7 [1], Progressive collapse is ''the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it''. The initial local failure can be occurred when the structure subjected to abnormal loadings, which they were not explicitly designed for. The abnormal loading can be blast, vehicle impact, gas explosion or mistakes in the design or during construction. When the structure fails to redistribute the load of the failed elements to the neighboring elements, progressive collapse occurred. One of the earliest recorded incidents is the collapse of Ronan Point apartment (London, 1968), due to gas explosion. This is followed by

http://dx.doi.org/10.1016/j.hbrcj.2015.02.005

1687-4048 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

the failure of Skyline Plaza (Virginia, 1973) due to mistakes during construction, the terrorist attacks on the Murrah Building, (Oklahoma, 1995) and the World Trade (New York, 2001). Precautions can be taken in the new design of structures to confine the effect of the local failure and resist progressive collapse. According to Department of Defense (DoD) 2005 guidelines [2], two general approaches are used for reducing the possibility of progressive collapse: Direct Design and Indirect Design. For the Indirect Design approach, the structure resistance to progressive collapse is considered implicitly through the provision of minimum levels of strength, continuity and ductility. Direct Design incorporates explicit consideration of resistance progressive collapse through two methods. One is the Alternate Path method, which requires that the structure be capable of bridging over a missing structural element, with the resulting extent of damage being localized. The other method is the Specific Local Resistance method, which seeks to provide sufficient strength to resist a specific threat.

There are a few researches which studied the effect of reinforcement detailing in resisting progressive collapse. Yi et al. [3] carried out an experimental study on a four-bay and three-story one-third scale model of reinforced concrete frame. They concluded that, failure resulting from progressive collapse of RC concrete frame structure was ultimately controlled by the rupture of reinforcing steel bars in the floor beams. They claimed that, if the strain of the tensile steel bars can be distributed more uniformly along the length, the deformation capacity of the beams can be enhanced so as to further improve the load-carrying capacity of the beam through catenary mechanism. Sasani and Sagiroglu [4] studied numerically the progressive collapse resistance of RC frame structural system designed against different levels of natural hazards such as winds and earthquakes. The study demonstrated that the vulnerability of the frame structures against progressive collapse depends heavily on their resistance to natural hazards and following to the loss of the supporting column, in spite of satisfying the current structural integrity requirements, premature beam bottom bars fracture can occur. And they claimed that such bar fracture can be avoided if the minimum beam bottom continuous bars are set equal to the minimum flexural reinforcement. However, in another study by Yu and Tan [5] it was concluded that, seismic detailing has no obvious advantage in developing catenary action since it focuses mainly on enhancing the shear resistance. Sasani et al. [6] Studied experimentally and analytically the removal of the load bearing element of a 10-story reinforced concrete structure. They identified that, the modulus rupture of concrete is an important parameter in limiting the attained vertical displacement following the removal of first floor column. In an experimental study, Sasani and Kropelnicki [7] found that, by satisfying the integrity requirements of ACI-318 code, the catenary action developed in spite of the rupture of the bottom reinforcements of the beam. Corley [8] though a discussion about the bombing of the Murrah Federal Building in Oklahoma City as a case study, concluded that damage due to blast can be significantly reduced by using seismic detailing in the structure.

This paper presents an experimental program developed to study the effect of reinforcement debonding on the progressive collapse resistance of moment resisting frame designed and detailed in accordance with the Egyptian code provisions for seismic design. Half-scale specimens of the first story were

extracted from the frame structure prototype. Each specimen represented a two-bay beam resulting from the removal of middle supporting column of the lower floor. Different reinforcement debonded length, debonded reinforcement ratios and places are examined in this study to evaluate their effect on the collapse resistance performance of the RC frame. Moreover, the effect of reinforcement debonding on the behavior of RC frames with different concrete strength and different reinforcement properties and details are studied. The nonlinear response of the frame to the removal of the column is evaluated and the amount of energy absorbed during the course of deformation is calculated for the different configurations.

The experimental test results presented in this paper are used as basis for verifying numerical models that are developed to perform further parametric study on the progressive collapse resistance of RC frames. The general-purpose finite element program of LS-DYNA [9] is used to perform static nonlinear analysis on the test specimens where the center column was pushed down under displacement control until failure occurred. The finite element model details, material models and parameters affected models behavior are not discussed in this experimental study but, they are detailed in Ref. [10].

Experimental program

The experimental program is designed to study the effect of reinforcement debonding on the progressive collapse resistance of moment resisting frame designed and detailed in accordance with the Egyptian code provisions for seismic design (ECP 203-2007) [11]. Reinforcement debonding is the removal of bond between reinforcing steel bar and the surrounding concrete and it was performed by placing the required bar length into a plastic tube and closing the tube ends by adhesive tape, as shown in Fig. 1.

Twelve half-scale specimens of the first story were extracted from the frame structure prototype, and only eight specimens are reported in this paper. Fig. 2 shows the prototype frame and the extracted specimens. Each specimen represents a two-bay beam after the removal of the middle supporting column at the lower floor.

The parameters studied in this experimental program are as follows:

• The effect of reinforcement debonding on the progressive collapse resistance of RC frames designed and detailed in accordance with seismic design provisions.

Fig. 1 Debonding of reinforcing steel bars.

' 4.00 T 4.00 'Y 4.00 X 4.00

Elevation

Fig. 2 Prototype building frame.

• The effect of reinforcement debonding ratio and place on the behavior and mode of failure of RC frames with different reinforcing steel properties.

• The effect of reinforcement debonding on the behavior and mode of failure of RC frames with different concrete com-pressive strength.

The prototype building considered in this study is a seven story office building located in Cairo. The typical story height is 3.0 m and the ground floor height is 4.0 m as shown in Fig. 1. The structural system of all floors is solid slabs and projected beams. The building was designed and detailed in accordance with the Egyptian code provisions for seismic design. The following loads were considered for the design of prototype: (i) self-weigh of the floor with slab thickness 120 mm and beams in addition to super imposed dead loads for flooring equals to 1.50 kN/m2; (ii) live loads 3.0 kN/m2; (iii) equivalent dead load for walls on the floor beams: 10.0 kN/m for the exterior walls and 5.0 kN/m for the interior walls; (iv) earthquake lateral loads as per Egyptian Code (ECP-201) [12]. The building is considered to be located in Cairo in seismic zone 3, with design ground acceleration ag = 0.15 g. A compressive strength of 350 MPa for concrete and a yielding strength of 360 MPa for the reinforcing steel were considered in the design of the members. The section of the columns in the prototype structure was 400 x 400 mm and the longitudinal

reinforcing ratio was q = 1.0%. The cross section of the beams was 250 x 500 mm in all stories and longitudinal reinforcing ratio was 0.71% for the mid span bottom reinforcement and 0.89% for top reinforcement at the negative moments locations.

The test specimen represents a half scale model of two adjacent beam spans resulting from the removal of middle supporting column of the first story in prototype building, Fig. 3. All specimens had the same concrete dimensions and varied in, reinforcement debonding length and place, reinforcement ratio (resulting from lap splice), reinforcement steel properties, reinforcement details and concrete compressive strength. All specimens represent frames are designed and detailed in accordance with the Egyptian code provisions for seismic design (ECP 203-2007) [11]. ECP 203-2007 provides provisions for the ductile reinforced concrete (RC) frames to have the ability to dissipate the energy produced from the lateral loads. These provisions quantify the longitudinal bottom and top reinforcements of the frame beams, stirrups spacing along the beam span and prevent the lap splice in the beam-column joints. The ECP 203-2007 also quantifies the longitudinal and transverse reinforcement of the column and the beam-column joint.

The test specimens are designated as S2, S3, S4, S6, S7, S8, S10 and S12, as shown in Table 1. The specimen S2 represents the control specimen, where no reinforcement debonding takes place. For S3, 50% of the bottom steel bars (in the cross section adjacent to middle column) of S3 are debonded in a

Fig. 3 Test specimen concrete dimensions (mm).

Table 1 Test specimens properties.

Test specimen Fcu (MPa) Longitudinal bars and reinforcement ratio Ties ф@ mm Debonding

Top bars (RFT%) Bottom bars adjacent to middle column (RFT%) Bottom RFT adjacent to middle column Top RFT adjacent to end columns

S2 43.7 ЗФ10 (0.78%) 6Ф10 (1.57%) ф6 @50 No No

S3 41.0 ЗФ10 (0.78%) 6Ф10 (1.57%) ф6 @50 Yes No

S4 39.8 ЗФ10 (0.78%) 6Ф10 (1.57%) ф6 @50 Yes Yes

S6 43.4 ЗФ10 (0.78%) 3Ф10 (0.78%) ф6 @50 Yes Yes

S7 41.0 ЗФ10 (0.78%) 3Ф10 (0.78%) ф6 @50 No No

S8 38.1 ЗФ10 (0.78%) 6Ф10 (1.57%) ф6 @50 Yes Yes

S10 41.2 Зф1З (1.33%) 6/13 (2.66%) ф6 @50 Yes Yes

S12 81.2 ЗФ10 (0.78%) 6Ф10 (1.57%) ф6 @50 Yes Yes

distance of one and half times of the beam depth measured from the face of middle column in the two spans. 50% of the bottom bars (in the cross section adjacent to middle column) of S4 are debonded for a distance of one and half the beam depth measured from the middle column; however, all of its top reinforcement bars are debonded for a distance of one and half the beam depth measured from the end columns faces. The bottom reinforcement of the specimen S6 is continuous through the two adjacent beam spans and has no lap splice in the middle column zone. The total bottom reinforcement of S6 is debonded throughout the two beam spans, while the top reinforcement is debonded in a distance of one and half the beam depth measured from the face of the end columns in the two sides. Due to the full debonded bottom RFT of S6, two steel angles are used to prevent the

slippage of bottom RFT. S7 is the same as S6 but has no debonding in the top or the bottom reinforcement. Specimen S8 is the same as the specimen S6 but the debonded length of the bottom RFT is implemented in the distance between the two mid spans of beams. An additional bottom RFT equal to the area of the main bottom RFT was added in the length between the two beams mid spans. The additional steel bars are debonded in the length of one and half the beam depth measured from the middle column in the two beam sides, so the total bottom RFT of S8 is debonded next to the middle column. S10 is the same as the specimen S4; however; its main reinforcement is mild steel instead of high tensile steel to study the effect of debonding on the performance of RC frames if mild steel is used. Specimen S12 is the same as the specimen S4; however; high strength concrete is used. Figs. 4-7 show

06 /60mm

Fig. 4 Reinforcement details and instrumentation of specimen S2.

06/60mm 06/60mm 06/60mm

Fig. 5 Reinforcement details and instrumentation of specimen S3.

06/60mm 06/60mm 06/60mm

Fig. 6 Reinforcement details and instrumentation of specimen S4.

06/60mm 06 /60mm 06 / 60mm

Fig. 7 Reinforcement details and instrumentation of specimen S6.

the typical reinforcement details of specimens S2, S3, S4 and In all specimens, the exterior two short columns were

S6. The properties of the used reinforcing steel are shown in restrained against horizontal and vertical displacements during

Table 2.

the test and a monotonic vertical load was applied on the

Table 2 Properties of reinforcing steel.

Nominal diameter (mm) Grade Type Actual area (mm2) Yield strength (MPa) Ultimate strength (MPa) Elongation (%)

6 240/350 Mild 28.3 348.50 510.10 32.2

10 360/520 High tensile 78.5 558.00 709.50 15

13 240/350 Mild 132.8 318.70 490.70 31.25

y///////////////////M^

Fig. 8 Test setup.

middle column stub to simulate the vertical load of the upper floors. Fig. 8, shows the test setup and the specimen in the loading frame.

Electrical strain gauge type FLA-6-11-1L of gauge length 6 mm was used to measure strain in the reinforcing steel bars. Strain gauges were bonded to the reinforcing bars at predefined locations as shown in Figs. 3-6. For concrete, electrical strain gauge type PL-60-11-1L was used to measure strain on concrete surface at top surface of the beam near columns. A linear variable displacement transducer (LVDT) was attached to each specimen under the middle column stub to measure the vertical displacement produced due to the applied vertical load. A computer controlled data acquisition system consists of 16 channels with maximum sampling rate 5 kHz that was used to collect and record data from different sensors (load, displacement and strain measurements). The sampling rate used in the test was 2 Hz. All specimens were tasted under applied vertical downward load to simulate the gravity load acting on the location of the removed middle column. The data acquisition system recorded continuously the readings of the load cell, LVDT and strains in reinforcing steel and concrete surface. The test continued under increasing monotonic vertical loading until the failure of specimen or reaching the maximum actuator stroke. The failure of specimen was attained when the rupture of reinforcement occurred.

Experimental test results

Cracking patterns and modes of failure

Cracks were observed and marked during test for all specimens to follow cracking history until failure mechanism was reached. For specimen S2, the first flexure crack developed at the negative moment zone adjacent to the right column support at load 15 kN. The positive moment zone adjacent to the middle column stub showed first crack at load 35 kN. With increasing load, flexural cracks spread along the beam and propagated vertically. After reaching the maximum load of 107.4 kN, crushing of the concrete at the compressive zone adjacent to the middle column stub was observed and the crack at the end of the lab splice of the bottom reinforcement became wider. This wide crack initiated vertically and then propagated diagonally. At the failure of specimen S2, the top reinforcement ruptured adjacent to the right and the left column supports. The cracking pattern of specimen S2 and top reinforcement rupture are presented in Figs. 9 and 10, respectively.

For specimen S3 the first crack was observed at load of 20.0 kN at the debonded zone adjacent to the middle column stub. With the increase of applied load, the flexure cracks spread along the specimen in the tension zones. At load about

Fig. 9 Cracks pattern of specimen S2.

Fig. 10 Rupture of top reinforcement of specimen S2.

Fig. 11 Cracks pattern of specimen S3.

50 kN a flexure crack was observed in the tension zone at the end of lab splice of the bottom reinforcement. The crack initiated vertically and with the increase of the applied load propagated diagonally. As the applied load increased, the specimen experienced large deformation and the tension cracks spread along the beam. By the end of the test, the tension cracks at the end of bottom reinforcement lab splices became wider and penetrated the compression zone. As the maximum load reached, failure of the compression zone was observed. At later stage of test, rupture of the top reinforcement adjacent to the face of the end column support was occurred. Figs. 11 and

Fig. 12 Rupture of top reinforcement of specimen S3.

12 show the cracking pattern and rupture of top reinforcement of S3, respectively.

The same as S3, the first crack in S4 initiated at the debond-ed zone adjacent to the middle column stub at load of 20.0 kN. At load of 50.0 kN a crack initiated at the end of lab splice of the bottom reinforcement and propagated diagonally. Tension cracks propagated along the beam in the tension zone at the sides of the middle column stub while only two main wide cracks observed in the top debonded reinforcement area in the right and left end column supports. A compression failure at the top compression zone occurred at the maximum load of 97.0 kN next to the middle column stub. The maximum load maintained constant for a while then, started to decrease gradually to the minimum value of 72.50 kN at displacement about 305 mm then increased again to 99.40 kN before the test stopped. No rupture of reinforcement was observed due to the effect of debonding bottom and top reinforcement. Fig. 13 presents specimen S4 after test.

The behavior of S6 was different from the preceding specimens where only four main cracks produced during the test, two cracks were at the right and the left of the middle column stub initiated at the bottom surface of the beam and propagated vertically. The other two cracks were adjacent to the face of the right and the left end column supports initiated at the top concrete surface. The first crack observed at load of 10.0 kN adjacent to the middle column stub. That early appearance of tension cracks in concrete was due to the relative movement of concrete to bottom steel bars as a result of full debonding of the main bottom steel. At load of 60.0 kN sever concrete crushing in the bottom concrete compression zones adjacent to end columns occurred. The maximum carried load was 61.40 kN at a vertical displacement of 61.54 mm. The carried load started to decrease gradually to the minimum value of 42.80 kN at displacement about 176.60 mm then increased again to 92.20 kN before the end of the test. Because of the debonding of bottom and top reinforcement, the specimen experienced large deformation and no reinforcement rupture was observed. Fig. 14 presents S6 after test.

The specimen S7 started to crack at load about 20 kN then cracks spread along the beam length with increasing load. At load 65 kN, crushing in the concrete compression zones adjacent to middle column stub was observed. At displacement of 220 mm rupture of the total bottom reinforcement bars occurred.

Fig. 13 Cracks pattern of specimen S4.

Fig. 14 Cracks pattern of specimen S6.

The first flexural crack initiated in specimen S8 at load of 10.0 kN. Main cracks located at negative bending moment zone adjacent to the right and left column supports, positive moment zone adjacent to middle column stub and the zone of the end debonded length of the bottom reinforcement. Crushing in concrete at the compression zone adjacent to middle column stub was observed at load about 86.0 kN. After the specimens attained its maximum load capacity, a small reduction was occurred in the carried load and then the carried load resumed ascending again reaching 136.60 kN at the end of the test. Due to the debonding bottom and top reinforcing succeeded to distribute the high reinforcement strain on a larger length and prevented the rupture of reinforcement.

The first crack initiated in the specimen of S10 at load of 17.0 kN adjacent to middle column stub face. A vertical crack was observed at the end of bottom lab splice at load about 31.0 kN then, propagated diagonally with increase of the applied load. Crushing in concrete at compression zone adjacent to middle column stub and crushing in concrete around the hooks of the bottom reinforcement were observed at load of 100.0 kN. At a later stage of the test, spalling of concrete around the bottom and top lap splices and opening of the hook were occurred. By the end of the test, slippage of bottom and top reinforcement was observed and no rupture of reinforcing steel was occurred. Fig. 15 shows crack pattern of S10.

The first crack was observed at tension zone adjacent to right column support in the specimen of S12 at load of 9.0 kN. At load of 50.0 kN and displacement of 17.0 mm, crack observed at the bottom reinforcement splice zone. Compression failure in concrete adjacent to middle column

Fig. 15 Cracks pattern of specimen S10.

Fig. 16 Cracks pattern of specimen S12.

stub occurred at load of 92.0 kN and displacement of 53.0 mm. At displacement of 275.0 mm and load of 90.0 kN, splitting in concrete at the right compression zone occurred. Finally, the actuator reached its maximum stroke and no rupture reinforcement occurred. Fig. 16 shows the crack patterns of S12 at the end of the test.

Load-displacement behavior

The load-displacement curves of all specimens are shown in Figs. 17-19. As the test specimens have the same dimensions and test setup the flexural strength capacity will be referred by the maximum resisted load. The maximum flexure strength of specimen S2 was 107.40 kN and the corresponding displacement was 61.1 mm. After the maximum strength was attained,

a О 60

4 •»V.i •• ф » ,1"

-S2 ......S4

0 50 100 150 200 250 300 350 400 450 Displacement (mm)

Fig. 17 Load-displacement curve of S2-S4.

140 120 100

T3 § 60 J

0 50 100 150 200 250 300 350 400 450 Displacement (mm)

Fig. 18 Load-displacement curve for S6 and S7.

140' 120' 100'

g 80 ■

40 20 0'

0 50 100 150 200 250 300 350 400 450 Displacement (mm)

Fig. 19 Load-displacement curve for S8, S10 and S12.

the specimen showed softer resistance to the applied load with the increase of vertical displacement due to geometrical and material nonlinearity. When the applied load reached 95.20 kN, and vertical displacement of 171.8 mm, a sudden drop in the applied load occurred due to the rupture of reinforcing steel bars.

For the specimen S3, the maximum flexure capacity was 94.50 kN and occurred at displacement of 74.36 mm. After the maximum flexural capacity was reached, the load-carrying value started to decrease gradually with the increase of vertical displacement. At displacement of 223.5 mm, sudden drop in the applied load occurred due to the rupture of the top reinforcing steel bars.

The response of S4 was the same as S3 from starting loading to the maximum flexure strength then the flexure strength of S4 reduced rapidly compared to S3. After S4 resistance reached its minimum value at displacement of 296 mm, the load-displacement curve started ascending again and the specimen was able to sustain higher load. This increase in specimen resistance occurred due to developing catenary action where, the applied vertical load redistributed to the two edge columns by axial tension forces in the beams. Due to debonding of the bottom reinforcement at the maximum positive moment zone and the top reinforcement at negative moment zone, the specimen was able to develop catenary action without rupture of reinforcement.

For S6, the maximum flexure strength reached 61.40 kN at displacement of 61.54 mm. The resistance of specimen

decreased gradually with the increase of displacement to reach 42.70 kN at displacement of 167 mm; thereafter, the load-displacement curve started ascending again and the specimen was able to sustain higher load. At the end of test, and due to debonding of reinforcement, specimen attained load about 150% of the maximum flexural strength capacity without rupture of reinforcement. This increase in specimen resistance occurred due to developing catenary action.

Specimen S7 showed maximum flexural capacity of 77.50 kN at displacement of 115.40 mm. The load-carrying value of specimen decreased gradually until the rupture of bottom reinforcement. The rupture of reinforcement produced a sudden drop of specimen resistance at displacement of 190 mm. By the end of the test, the specimen loss was 60% of its maximum capacity due to the rupture of reinforcing steel bars.

The maximum flexural strength of S8 was 86.90 kN at displacement of 57.7 mm then flexural strength almost remained constant up to displacement of 173.5 mm then, a small reduction occurred. After the specimen resistance reached its minimum value, the load-displacement curve started ascending again and the specimen was able to sustain higher load with the increase vertical displacement. The full debonding of the main reinforcement succeeded to distribute the high tensile strain produced in the bottom and top reinforcement through the debonded bar length and prevent the rupture of reinforcing bars. Due to developing catenary action, the maximum load capacity reached 137.50 kN at displacement of 410 mm by the end of the test.

The maximum flexural capacity of specimen S10 was 109.80 kN and the corresponding displacement was 40.17 mm. The resistance of specimen reduced gradually to load 78.70 kN and the measured displacement was 216.8 mm. The resistance of specimen gradually increased again due to the developing catenary action. The maximum measured load by the end of the test was 114.10 kN at displacement of 420 mm.

For S12, the maximum flexural strength was 93.40 kN and occurred at displacement of 51.71 mm then, capacity of the specimen reduced gradually to minimum value of 82.60 kN at displacement 193.20 mm. The specimen resistance increased gradually due to formation of catenary action mechanism. The maximum measured load was 119.20 kN at displacement 396.40 mm.

Analysis of test results

Flexural strength analysis

The use of debonded reinforcement bars significantly affected the load-displacement behavior of the tested specimens. Elongation of the free length of debonded bar results in larger deflection and consequently, greater crack width in beams where debonding took place. Tables 3 and 4 present the flexure strength and the reduction in flexure strength due to debond-ing of the reinforcement bars, respectively.

From Table 4, it is clear that debonding of the reinforcement steel bars reduces the maximum flexural strength of the test specimens except the S10, the specimen of mild steel in the main reinforcement. The flexural strength reductions are 12.04%, 8.78%, 19.09% for S3, S4 and S8, respectively

Table 3 Summary of the load and displacement results of the test specimens.

Specimen P max (kN) D (at P max) (mm) D max (mm) P (at D max) (kN)

S2 107.40 61.11 213.77 50.50

S3 94.50 66.67 261.70 56.10

S4 98.0 65.56 413.22 99.60

S6 61.40 61.54 347.50 92.20

S7 76.60 89.74 240.66 31.00

S8 86.90 57.69 409.83 137.60

S10 109.80 40.17 420.10 114.10

S12 93.40 51.71 396.40 119.20

Table 4 % Reduction in strength due to debonding of

reinforcement.

Specimen P max (kN) Reference specimen % Reduction

S3 94.5 S2 -12.04

S4 98.0 S2 -8.78

S6 61.40 S7 -19.85

S8 86.90 S2 -19.09

S10 109.80 S2 + 2.23

S12 93.40 S2 -13.04

compared to S2. However, the strength reduction for S6 is 19.85% compared to S7, the specimen of the same reinforcement ratio. Moreover, the flexural strength reduction percent depends on the debonded reinforcement ratio where, for the specimens which have 50% of the bottom reinforcement debonded (S3, S4 and S12), the reduction of the flexural strength range is 8.78-13.04%; however, for the specimens which have total bottom RFT debonded (S6 and S8), the reduction of the flexural strength range is 19.09-19.85%.

Effect of RFT debonding on strain distribution

Figs. 20 and 21 show the strain distribution of the bottom reinforcement of S6 and S7, respectively. It is obvious that, S6 was able to redistribute the tensile strain along the debonded length of reinforcing steel bars. The tensile strain of the bottom reinforcement at the mid span in the right and left beam was almost the same as the tensile strain adjacent to the middle column stub. However, the tensile strain of the bottom reinforcement of S7 was varied along the span. Bottom RFT reached

-a a 60

-Mid Span Right ---- Mid Span Left

Strain (1x10-3)

140 120 100 80 60 40 20 0

Mid S pan Right Joint pan Left

* piv..........

Fig. 20 Strain distribution of the bottom RFT of S6.

Strain (1x10)

Fig. 21 Strain distribution of the bottom RFT of S7.

yield strain next to the middle column stub whereas, strained to small value at the mid span of the right and left beams. The strain concentration at the maximum bending moment zone led to the rupture of the reinforcing steel bars at the location of maximum tensile strain, which does not occur in S6.

Displacement ductility

In general, ductility is the ability of the reinforced concrete member to sustain large inelastic deformations without excessive deterioration in strength or stiffness. The displacement ductility is used here to evaluate the performance of the test specimens. The displacement ductility factor iA is calculated according to Park [13] using the measured displacement at the middle of the specimen as: iA = Af/Ay where Df is the displacement at 80% of the ultimate load on the descending branch of load-displacement curve or the displacement at the rupture of reinforcing steel, whichever occurred first. Ay is the yield displacement; it can be calculated as the secant stiffness at 0.75 of the ultimate load, as shown in Fig. 22.

By referring to load-displacement curves of the test specimens, it can be observed that except S2 and S7 there was not a clear point of failure, because, after the ultimate load was reached a small reduction in the resisted load was occurred with the increasing displacement then, the specimen resumed carrying load by developing catenary action. According to Park definition of Af a significant portion of ductility will be ignored by neglecting the displacement after that corresponding to 80% of the ultimate load in the descending branch of the load-displacement curve. Moreover, for the specimens S8 and S12, their minimum resisted load in the descending branch of the load-displacement curve was greater than 80% of the

Dispalcement

Fig. 22 Determination of yield and maximum displacements and initial stiffness.

Table 5 Displacement ductility factors.

Specimen Dy (mm) Af (mm) lA

S2 35.328 176.92 5.0

S3 33.048 223.45 6.76

S4 41.829 380 9.34

S6 46.155 348 7.54

S7 51.283 189.39 3.69

S8 30.769 380 12.35

S10 30.769 380 12.35

S12 34.757 380 10.93

Table 6 Initial flexural stiffness.

Specimen Stiffness (kN/m)

S2 3040.91

S3 2859.48

S4 2408.42

S6 1282.80

S7 1536.51

S8 2824.89

S10 3568.49

S12 2754.95

ultimate load. Thus, Df used in the calculation of displacement ductility will be taken as the displacement when the rupture of reinforcement occurred or the displacement corresponding to 20% rotation of the clear span of beam. The displacement corresponding to 20% rotation is 380 mm.

Table 5 presents the displacement ductility factors (iD) of the tested specimens. By comparing the ductility factors of

53, S4, S8, S10 and S12 to S2, it can be observed that, the ratio of debonded RFT and debonding places affects significantly the specimens' ductility. The displacement ductility of S3, which had 50% of the bottom RFT in the beam cross section adjacent to middle column debonded, is increased by 35.2%; however, debonding 50% of the bottom RFT adjacent to middle column and total top RFT next to the end columns increase the ductility factor by 86.8%, 147% and 118.6% in

54, S10 and S12, respectively. Debonding the total bottom RFT along the beam span of S6 and top RFT next to end columns increases the ductility factor by 104.3% compared to S7. Debonding RFT in the high compressive strength specimen S12 increased the ductility factor by 17% compared to the normal compressive strength concrete specimen S4.

Flexural stiffness

Stiffness is the property that quantifies and controls the deformations of structural elements under the action of applied loads. In other words, it is the quantity that relates the applied loads to the structural element deformation. The stiffness of test specimen is computed based on the secant stiffness to the load-displacement curve at a load of about 75% of the ultimate load, as shown in Fig. 22. From Table 6 it is clear that, debonding of reinforcement steel reduced the flexural stiffness of test specimens. The flexural stiffness of partially

S3 S4 S6 S8 S10 S12

Fig. 23 Percentage of the absorbed energy increase of the test specimens.

RFT debonded specimens; S3 and S4 reduced by 5.97% and 20.8%, respectively compared to S2. However, the RFT debonding of the mild steel specimen S10 has no effect on its initial stiffness because, the bond between mild steel and concrete depends mainly on the end bar hook not on the bar surface. For specimens that had its total bottom and top RFT debonded; S6 and S8, the reductions of flexural stiffness are 16.51%, 7.1% compared to S7 and S2, respectively. For high compressive strength specimen S12, the reduction of the flexural stiffness is 9.4% compared to S2; however; its flexural stiffness is greater than S4, the normal compressive strength concrete specimen with the same conditions, by 14.4%.

Energy absorption

Absorbing energy is significantly required for the RC structures to resist progressive collapse and prevent the local

failure from spread throughout the structure. The absorbed energy by the specimen during test is calculated as the area enclosed by the load-displacement curve. In spite of the reduction of flexural strength due to RFT debonding, it permits the specimen to undergo large deformation without rupture of reinforcing steel bars thus, it increases the area enclosed by the load-displacement curve. Fig. 23 presents the increase of the absorbed energy of the debonded RFT specimens as a percentage of that of normal bond specimens S2 and S7. The increase of absorbed energy of S3 is only 12.37% due to the early rupture of the bonded top RFT; however, the absorbed energy of S4 increased by 70.98% which is almost the same as S12 (high strength concrete) compared to S2. The increased absorbed energy of S6 is only 24.22% compared to S7 due to the significant reduction of its flexural strength. The increase of the absorbed energy of S8 and S10 are 91.27% and 95%, respectively.

Formation of catenary action mechanism

Catenary action is the mechanism by which structure redistributes the load carried by the failed element to the neighboring elements through axial tension force developed in the bridging beams at large displacement. The debonding of Reinforcing steel redistributed the maximum tensile strain along the debonded length and permitted the specimens to undergo large deformation without the rupture of RFT. The experimental results showed that the debonded bottom and top reinforcement specimens; S4, S6, S8, S10 and S12 were able to produce catenary action without the rupture of RFT; however, the bonded RFT specimens; S2 and S7, and the bottom debonded RFT only; S3 failed to develop catenary action before the rupture of RFT.

Conclusions

Experimental evaluation of the effect of reinforcement debonding on the performance of reinforced concrete frame in resisting progressive collapse is considered in this research. Half-scale specimens of the first story were extracted from the frame structure prototype. Progressive collapse loading is simulated by gradually increasing vertical load at the location of the removed column continuously applied and increased up to failure. Twelve half-scale specimens tested, only eight specimens are reported in this paper. The parameters studied in this experimental program include: the effect of reinforcement debonded ratio and place on the performance of RC frames with different reinforcing steel properties and different concrete compressive strengths. The cracking patterns, strains and the deformations at selected locations are recorded for further analysis. The nonlinear response of the frame to the removal of the column is evaluated and the amount of absorbed energy during the course of deformation is calculated.

The main conclusions drawn from the experimental test results of the effect of debonded main beam RFT on progressive collapse resistance of RC frames are as follows:

• Improves characteristic properties required to prevent progressive collapse such as; the displacement ductility and energy absorption.

• Debonding main beam RFT is able to redistribute the high tensile strain of the RFT bars on longer bar length preventing the rupture of reinforcing steel bars.

• Enhances the deformation capacity of the beam, and that permits the occurrence of large deformation without the rupture of RFT. Therefore, specimen is able to produce catenary action which assumed to be the last alternative load path that can be implemented by the structure to prevent progressive collapse.

• Reduces the flexural strength and the initial stiffness of the beam and the reduction values depend mainly on the percentage of debonded RFT in the cross section; however the effect of the debonding length is not clear in this research.

• Debonding the mild steel has no negative effect on the flex-ural strength of flexural stiffness of Specimens.

• The main effects of debonding of high compressive strength concrete are as follows:

• Increases the ductility by 17% compared to normal com-pressive strength concrete with the same reinforcement ratio.

• The initial flexure stiffness of high strength concrete beam is increased by 14.4% and the absorbed energy is increased by 10.4% compared to normal strength concrete beam. However, there is no significant effect of compressive strength of concrete on the beam ultimate flexural strength.

Conflict of interest

There is no conflict of interest.

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