PSI - Issue 70

Abhishek Badalia et al. / Procedia Structural Integrity 70 (2025) 121–128

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The stiffness requirement at the ground storey appears to be unmet with the current configurations. Installing bracings along each bay at the ground storey level can prevent the formation of a soft storey mechanism. 6.2.4 Seismic weight and base shear Among all braced frame arrangements, the seismic weight is nearly constant, although the weight of braced frames is slightly higher than that of unbraced frames. This is as expected because the addition of braces slightly raises the structure’s total mass. The efficiency of the bracing system in enhancing seismic performance is demonstrated by the observed decrease in base shear from the unbraced to the braced configuration, which contrasts with seismic weight. For almost all bracing types, base shear values remain constant. This suggests that braces help reduce the lateral force demand on the structure, particularly X-bracing. This is very often as base shear depends upon seismic weight and horizontal acceleration coefficient (Ah). Ah is inversely related to the response reduction factor, which is 3 for OMRF and 4 for braced frames as mentioned in IS 1893 (Part 1):2016. Also, bracing enhances lateral stiffness, reducing the structural drift and improving energy dissipation, thereby reducing the overall seismic force demand. 6.2.5 Time-Period and frequency The findings presented in the graphs shown in Figures 5 and 6 indicate that implementing various bracing configurations reduces the period and increases the frequency of the reinforced concrete frame across different oscillation modes. Lower stiffness and greater deformation susceptibility are indicated by the unbraced frame's highest period and lowest frequency. On the other hand, X-braced and X-intersecting braced topologies exhibit superior stiffness enhancement, offering the greatest frequency increase (about 90%) and the most substantial reduction in period (nearly 46%). While diagonal bracing exhibits the least improvement among the retrofitted cases, chevron and V-bracing also increase structural performance, albeit to a significantly lesser extent.

2

0 1 2 3 4 5 6

1.5

1

0.5

Time period (seconds)

0

Frequency (Cycles/Sec)

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

Mode of Oscillation

Mode of Oscillation

Unbraced

X-Braced

X Int.-Braced Diagonal-Braced

Unbraced

X-Braced

X Int.-Braced Diagonal-Braced

Chevron-Braced V-Braced

Chevron-Braced V-Braced

Fig. 5: Variation of period in different modes

Fig. 6: Variation of Frequency in different modes

The structure's period decreases as its stiffness increases. The data clearly show that the unbraced RC frame has the longest period of any vibration mode, indicating more flexibility and less rigidity. X-braced and X-intersecting braced configurations show the greatest reduction (about 46%), followed by Chevron and V-bracing (41%), while diagonal bracing shows the least reduction (37%). The installation of bracing shortens the period. Bracing increases structural stiffness and reduces the oscillation period under dynamic loads. Higher frequencies result from increased stiffness since frequency is the reciprocal of period. Among all the modes, the unbraced frame has the lowest frequencies, demonstrating its flexibility. Structural frequency increases significantly with bracing: X-braced and X intersecting braced (~90%), Chevron (~74%), and V-braced (~71%). diagonal bracing results in an approximate 61% increase in frequency. Higher frequencies imply that the modified frames will be more seismically resistant and experience less deformation. Chevron and V-bracing configurations provide moderate improvements. Diagonal bracing, while beneficial, is the least effective among the bracing options considered.