Chapter 4. Experimental and analytical investigation of an exterior reinforced
4.5 Results and discussion
4.5.1 Experimental results
joint subjected to shock loading
specimens is in the range of 45-50 ms. Therefore, the t Td N ratio is around 0.72 and the loading is thereby considered impulsive. For impulsive loading, the peak maximum response is observed in the forced vibration phase as shown in Figure 4.10 to Figure 4.12. The maximum transverse displacement for the specimens with seismic detailing is smaller than that of their non-seismic counterparts. The maximum displacements are presented in Table 4.3. For specimen type BS (beam weak in shear), the peak displacement is reduced by 35% because of seismic detailing. Similar behavior is observed for specimens CS (column weak in shear) and BF (beam weak in flexure).
Thus, seismic detailing of the beam-column joint enhances its resistance to shock loads.
Figure 4.10 Transient displacement profile for specimens: a) BSNS, and b) BSS under shock loading
Figure 4.11 Transient displacement profile for specimens: a) BFNS, and b) BFS under shock loading
Figure 4.12 Transient displacement profile for specimens: a) CSNS, and b) CSS under shock loading
Table 4.3 Maximum displacements recorded in the beam-column joint tests
ii) Acceleration response
The acceleration time histories are acquired using shock accelerometers that are installed at three distinct locations along the length of the beam. The location of the three accelerometers is shown in Figure 3.9. The first accelerometer is located exactly at the rear face of the region of shock loading. The acceleration time histories near the inflection point of the beam for the specimens designed for gravity and seismic loading are shown in Figure 4.13 to Figure 4.15, respectively. The peak shock accelerations acquired from the recordings are presented in Table 4.4.
Table 4.4 Peak shock accelerations recorded in the beam-column joint tests Specimen
type
Non-Seismic Seismic
BSNS BFNS CSNS BSS BFS CSS
A1 (m/s2) 1411.2 823.2 1097.6 1960.0 1607.2 1646.4
The seismic specimens behave stiffer compared to their non-seismic counterparts due to ductile reinforcement detailing and thereby experience higher accelerations.
Correspondingly, the peak displacement of the seismic specimens is lower compared to Specimen
type
Non-Seismic Seismic
BSNS BFNS CSNS BSS BFS CSS
δ1 (mm) 13.20 8.66 13.98 7.32 7.23 7.62
δ2 (mm) 9.77 6.65 10.24 5.53 5.52 5.65
δ3 (mm) 6.39 4.54 7.24 3.48 3.99 4.23
joint subjected to shock loading
that of their non-seismic counterparts. However, the specimen types CSNS and CSS are designed for the column that is weak in shear. These trends are observed from the peak values presented in Table 4.4.
Figure 4.13 Acceleration–time histories for specimens: a) BSNS, and b) BSS under shock loading
Figure 4.14 Acceleration–time histories for specimens: a) BFNS, and b) BFS under shock loading
Figure 4.15 Acceleration–time histories for specimens: a) CSNS, and b) CSS under shock loading
iii) Evolution of cracks near beam-column joint
A visual inspection is conducted to examine the evolution of cracks in the test specimen. The crack patterns are carefully studied to qualitatively evaluate the behavior of the beam-column joint. The crack patterns at the beam column junction for all the six specimens are marked and discussed below.
a) Specimen BSNS and BSS
For specimens BSNS and BSS, the crack pattern at the beam-column joint is presented in Figure 4.16. The specimens are designed with weak shear reinforcement (BS). As a result, the shear cracks in the D-region are severe for both specimens. The shear cracks extend all over the beam-column joint. Due to the provision of additional shear reinforcement for the BSS specimen, the development of shear cracks in this specimen is relatively smaller compared to its non-seismic counterpart (BSNS).
Additional horizontal cracks are observed in the BSNS specimen at a distance of 260 mm from the beam-column joint due to larger transverse reinforcement spacing.
Figure 4.16 Crack pattern and failure mechanism of specimens: a) BSNS, and b) BSS
b) Specimen BFNS and BFS
The crack pattern for BFNS and BFS specimens is presented in Figure 4.17. The specimen designed with non-seismic detailing (BFNS) has a larger shear crack in the D-region compared to that of the specimen designed with seismic detailing (BFS). The shear cracks in the D-region of BFNS started propagating from the beam junction and ended at the column exterior face. On the other hand, the specimen BFS shows significantly less cracking. This implies that the seismic reinforcement detailing has contributed in resisting the shock loads by arresting the shear cracks. The transverse reinforcement spacing for BSNS and BFNS specimens is 180 mm and 110 mm, respectively. The larger reinforcement spacing for the BSNS specimen resulted in severe shear cracks in the beam-column joint compared to the BFNS specimen, which can be observed in Figure 4.16 and Figure 4.17. Similar behavior is observed for the specimens with seismic detailing (BSS and BFS).
joint subjected to shock loading
Figure 4.17 Crack pattern at the beam-column joint for specimens: a) BFNS, and b) BFS
c) Specimen CSNS and CSS
The crack pattern for the CSNS and CSS specimens is presented in the Figure 4.18. It is observed that the cracks formed in the D-region are not confined to the beam-column junction for the CSNS specimen. The cracks initiate diagonally in the beam-column junction and further propagate along the height of the column. The longitudinal crack propagation in the column is due to relatively larger tie spacing.
However, the specimen with seismic detailing (CSS) has confined the cracks to the beam-column junction by restricting further propagation. Therefore, it is inferred that the specimens with seismic detailing are more effective in resisting shock loads.
Figure 4.18 Crack pattern and failure mechanism of specimens: a) CSNS, and b) CSS