Page 224 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Steel Frame Building With Masonry Infill Walls Chapter 4 217

KL s ffiffiffiffiffiffiffiffiffiffi
2
r 2 2π E 6440
β ¼ ,F:S: ¼ 1:67 + 0:375β 0:125β ,C c ¼ ¼ p ffiffiffiffiffi
C c F y F y
2
KL 12π E 105 10 5
if > C c ! F a ¼
2
r KL ¼ KL 2
23
r r
where:
P CL ¼lower-bound compression strength of the column;
A ¼column cross-sectional area;
K ¼effective length factor determined in accordance with AISC [12];
L¼laterally unbraced length of the member; and
r ¼governing radius of gyration.
The lower-bound axial strength of all the column sections in the example build-
ing is presented in Table 4.5. The lower-bound flexural strength of all the col-
umn sections in the example building which is presented in Table 4.6 is
determined based on the method in Section “Flexural and Shear Strength of
Columns,” except for replacing F yL as the lower-bound yield strength of the
material with F yE .

4.4.2.3 The Retrofit Solution
As previously stated, because of the high length of the walls in longitudinal direc-
tion, these walls cannot be regarded as infill walls; therefore, the building is seis-
micallyvulnerable.However,thewallsintransversedirectioncanprovidestiffness
and strength to the building because they meet the requirement for infill panels.
Based on these results, the following retrofit solution for the building is proposed:
l Longitudinal direction: inclusion of concentric steel braces to the building.
l Transverse direction: considering the existing infill panels as the lateral
load-bearing system; if not adequate, retrofitting these panels with shotcret-
ing is considered.
In this example, only the retrofit of the building in a transverse direction which
is related to the retrofit of infill walls is presented. The retrofit design process of
adding concentric steel braces to the building, which is recommended for this
example building in the longitudinal direction, is presented in a separate exam-
ple in Chapter 5.

4.4.2.4 Modeling of the Building
For modeling of the building, ETABS is used. It is assumed that the example
building is located in an area with very high seismicity with the design PGA
of 0.35g according to Iranian Code of Practice for Seismic Resistant Design
of Buildings (Standard 2800) [13]. The soil condition is III according to