PSI - Issue 44

Maria Teresa De Risi et al. / Procedia Structural Integrity 44 (2023) 966–973 De Risi, Ricci, Verderame / Structural Integrity Procedia 00 (2022) 000–000

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(7+8+7) steel strips were adopted. The second strengthened specimen (“CAM4”) was aimed at investigating the specific influence of a lower value of f c ; then, it had the same beam longitudinal reinforcement of specimen NS (i.e., (3+3) 16 mm bars); it was strengthened with (5+5+5) steel strips. As far as material properties are concerned, f c was equal to 16.3 MPa (lower than in the previous campaign, as explained above), f y and f t were equal to 502 MPa and 597 MPa for 16 mm diameter bars used as longitudinal reinforcement and 538 MPa and 620 MPa for 8 mm diameter bars used as transverse reinforcement, respectively, and, finally, steel strips used for strengthening had a 0.2% proof stress (CEN, 2004) equal to f 0.2 = 389 MPa. The same setup and loading protocol of the previous experimental campaign were adopted. Moreover, the same axial load (290 kN) was adopted, too, thereby leading, due to the lower f c , to a higher axial load ratio (ν=0.20). Table 1 reports a summary of all the tested specimens.

Table 1. Experimental matrix.

column section ( mm × mm )

A s,column

f c ( MPa )

N c ( kN ) 290 290 290 290 290 290

ν (-)

Joint stirrups

Strength. strips

Test ID

beam section ( mm × mm )

A s,beam

NS

300 × 500 300 × 500 300 × 500 300 × 500 300 × 500 300 × 500

(3+3) Φ16 (3+3) Φ16 (3+3) Φ16 (5+5) Φ16 (5+5) Φ16 (3+3) Φ16

300 × 300 300 × 300 300 × 300 300 × 300 300 × 300 300 × 300

(3+3) Φ16 (3+3) Φ16 (3+3) Φ16 (5+5) Φ16 (4+4) Φ16 (3+3) Φ16

32.2 32.2 32.2 16.3 16.3 16.3

0.1 0.1 0.1 0.2 0.2 0.2

No

- -

S

Yes

CAM1

No No No No

5+5+5

2NS

-

CAM3 CAM4

7+8+7 5+5+5

3. Experimental results 3.1. Global response

The force-displacement global response of the specimens is reported in terms of beam shear-versus-beam drift, the latter calculated as the ratio between the displacement at beam’s end and L b . Figure 2 reports beam shear-beam drift hysteretic responses and corresponding envelopes, along with the expected values of beam shear at first yielding (V b,y ) and maximum (V b,max ), characteristic points at joint cracking, beam’s first yielding, peak resistance and conventional ultimate condition (20% strength decay). Specimen NS showed the onset of hairline diagonal joint cracking at 1.0%, followed by the attainment of the peak load, very close to the predicted beam flexural yielding, at 1.5%. Then, a rapid softening occurred, due to the progress of damage in the joint panel, up to the total collapse, with buckling of longitudinal reinforcement on the outer face of the joint panel. These observations allow defining the response of this specimen as characterized by a BJ-failure. Specimen S showed a ductile response, controlled by the development of a flexural plastic hinge at beam’s end. Hairline diagonal cracking was observed in joint panel, starting from 1.0% drift (as in specimen NS). The observed peak load was close to the predicted beam flexural strength, V b,max , at 6.0% drift. The test was terminated at 8% drift, with a strength drop about equal to 20%, caused by concrete cover spalling and buckling of longitudinal reinforcement at beam’s end. Specimen CAM1 showed a response very similar to the specimen S, with a ductile response controlled by the development of a flexural plastic hinge at beam’s end. The onset of diagonal cracking was delayed compared to specimen S (i.e., at 2% instead of 1%), and apparently this cracking was less developed, too. Specimen 2NS showed the onset of hairline diagonal joint cracking at 0.75%, followed by the attainment of the peak load, well below the predicted beam flexural yielding, at 2.0%. Then, a rapid softening occurred, due to the progress of damage in the joint panel, up to the total collapse. These observations allow defining the response of this specimen as characterized by a J-failure. Specimen CAM3 showed the onset of hairline diagonal joint cracking at 1.0%, followed by the attainment of the peak load, very close to the predicted beam flexural yielding, at 3.0%. Then, a rapid softening occurred, due to the progress of damage in the joint panel, up to the total collapse. These observations allow defining the response of this specimen as characterized by a BJ-failure. Finally, specimen CAM4 showed the onset of hairline diagonal joint cracking at 1.0%, followed by the attainment of peak load, very close to the predicted beam flexural strength, at