Page 3 - Open Access - November

TECHNICAL PAPER

cores sustained the large plastic strain. Qu et al. conducted Design yield axial forces of BRBs in tension (P ty) and compression
[19]
a numerical investigation on the use of a zigzag BRB in RC (P cy) can be determined from the material yield stress (ơ yBRB) and
buildings and concluded that the BRBs are efficient in reducing core cross-section area (A BRB) as follows:
the responses of the building, even if the nonlinearity of brace T=P ty=ϕσ yBRB A BRB (2)
connections is considered. Furthermore, strength demands for
C=P cy=ϕβσ yBRB A BRB (3)
the brace connections are significantly influenced by the higher
modes of the system after brace yielding. Qu et al. studied
[20]
the effectiveness of double K-braced RC frame with BRBs under
cyclic loading.

The review of literature concluded that BRBs have a good
potential to be used as strengthening elements in the RC
frames. These braces can be fitted at the open ground stories to
improve the lateral strength and displacement ductility and to
contribute to the energy dissipation capacity of the RC frames.
However, a design procedure to determine the required sizes
of BRBs to carry the desired lateral load is not available in the
context of seismic strengthening of RC frames. In addition, it
is necessary to evaluate the effective of BRBs in controlling the
seismic response of low-to-high rise RC frames.

2. ScoPE And oBJEctIVES
The main objectives of this study are (i) to develop a design (a)
framework for determining the required sizes of BRBs as the
strengthening elements in a soft-story RC frame, and (ii) to
evaluate the seismic performance of the strengthened low-
to-high rise RC frames under earthquake simulated loading
conditions. BRBs are designed based on the expected
seismic demand on the OGS buildings frames depending on
their seismic weights. Thus, force-based design method has
been adopted in this study to find required sizes of BRBs for
different frames. Both nonlinear static and dynamic analyses are
conducted to evaluate the seismic response of strengthened
frames.

3. dESIGn oF StrEnGtHEnEd FrAMES
It is proposed to install BRBs in all bays of the OGS frames. It is
assumed that the seismic weight of the frames will not change
significantly due to the installation of BRBs only in the open (b)
ground stories of the study frames. The design procedure
adopted in this study has been discussed in the following Figure 2: (a) BRBs located at ground story level of a frame,
sections: (b) Axial forces in BRBs under the action of lateral loading.
Strength reduction factor, Ø , can be taken as 0.9. The
3.1 determination of BrB Sizes compression strength of BRB at any displacement cycle is higher

Figure 2 shows the BRB strengthened RC frame and the BRB than the corresponding tension strength primarily due to the
forces under lateral loading. Assuming the total design story Poisson’s effect. The ratio of compression to tension strengths
shear (V BRB) is resisted by BRB forces, the tension (T) and of BRB at any displacement excursion is defined as compression
compression (C) forces in BRBs inclined at angle of θ can be strength adjustment factor (β). For the known values of Ø, β
expressed as follows: and ơ yBRB, the required area of BRB core can be determined as
follows:
V BRB = (T+C) cosθ (1) V BRB (4)
A BRB =
(1+β)ϕσ yBRB cosθ

The IndIan ConCreTe Journal | november 2019 25