Seismic of structural elements. In this paper, first

Seismic
Rehabilitation by Steel Jacketing Method Affected by Different Base Support
Conditions Using Pushover Analysis

 

Abstract

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Rehabilitation of vulnerable
existing structures against earthquakes is essential. Structures need to be
retrofitted for a variety of reasons, such as change of use, structural elements
corrosion, design code changes, and the lack of match between the computational
results and construction. One of the most common and economical ways of retrofitting
structures in developing countries such as Iran is the steel jacketing method
which increase the stiffness and strength of structural elements. In this
paper, first the seismic vulnerability of a three dimensional four story
residential building was proved and then the effect of base support conditions
on structural retrofitting by steel jacketing method was studied using pushover
analysis. It was observed that the behavior of the retrofitted structures
depends on their base support conditions. According to pushover curves the retrofitted
structures with pinned and fixed base support conditions showed similar
behavior in the linear zone and presented a different behavior in terms of stiffness
and ductility in the nonlinear region.

Keywords

Pushover analysis;
Seismic rehabilitation; Steel structures

Introduction

Alcocer and Jirsa (1993)
stated that seismic rehabilitation is a suitable way to increase people’s
safety and to protect their investments in vulnerable structures. To obtain
this goal, various investigations have been carried out by researchers, but
most of them are focused on retrofitting reinforced concrete structures. Sheykh
(1994) studied on a method for obtaining the amount of confined steel for
rectangular columns. Migliacci et al. (1983) studied on a way to
increase the strength and energy absorption of retrofitted connections by a
closed steel frame at the corners of the column by pre-stressed straps. A model
was provided by Corazao and Durrani (1989) to increase the stiffness, strength
and ductility of the beam-to-column connections by confined concrete jacketing
surrounded by steel plates. Hoffschild et al. (1993) studied another
method which was using a grouted circular steel jacket to increase the
cross-sectional moment capacity.

Pushover analysis was
utilized to rehabilitate considered structure by steel jacketing method. In the
pushover analysis the structure is subjected to a specific lateral load
pattern. In this type of analysis, the criterion is the amount of displacement
of the structure, so the force increases to a point that the desired
displacement is provided. Although nonlinear time history analysis is more
accurate than the pushover analysis and Ali Vatanshenas (2017) study showed
that by time history analysis it is possible to determine the directivity
effect of the earthquake on the structure, pushover analysis was used. The
pushover analysis has various merits such that the pushover analysis provides
the nonlinear responses quickly with low computational complexity and also
there are no response dispersion in this analysis and no complicated
interpretation in this type of analysis. In the pushover analysis by applying a
predetermined displacement all structural responses to different displacements
are analyzed within that range. Krawinkler and Seneviratna (1997) mentioned
that with pushover analysis it is possible to estimate the deformation demands
for the components that do not have a brittle behavior and should absorb energy
by plastic deformation, moreover forcing a structure to apply the desired
behavior by the lateral load pattern is a very unique feature that can be
accomplished through the pushover analysis.

Structural vulnerability
assessment

The considered structure
in this study should provide the life safety level of performance under the
influence of the design base earthquake, so that the failure occurs in the
structure due to the earthquake but the extent of the failure should not menace
the life safety of occupants. Skokan and Hart (2000) mentioned that considering
the seismic demand assessment of low and medium height structures has a higher
accuracy in this paper a three-dimensional four-story steel structure with a
concentrically braced frame system that has residential use is considered.
Before starting the rehabilitation by steel jacketing method, structural
vulnerability must be proven. The main components of the studied structure i.e.
the elements that resist the earthquake force in order to achieve the desired
level of performance are braces and columns beside the braces in which the
axial compressive and tensile forces were created. As shown in Fig. 1 a
hypothetical lateral static force was applied to the structure to determine these
main components of the structure.

 

Figure 1. The main components of considered structure
was determined by a hypothetical lateral static force

For investigating the vulnerability
of the considered structure following assumptions were considered: the monitor
point was selected as the center of the mass of the roof, the lateral load
pattern was determined according to the first mode of the structure. The amount
of target displacement is obtained by equation (1) based on Fema (2005) where C0
is the correction coefficient for the spectral displacement of one degree of
freedom system to the roof displacement of several degrees of freedom, C1
is the correction coefficient to apply system’s inelastic displacement, C2
is the correction coefficient for the effects of reduction of stiffness and
structural members strength, C3 is the correction coefficient to
show the displacement of the P-? effect, Sa is the spectral
acceleration and Te is the effective period obtained by equation (2)
based on Fema (2005) Where, Ki and Ke are obtained by the
pushover diagram shown in Fig. 2. After analyzing the studied structure, it was
illustrated in Fig. 3 that the structure in both directions of X and Y needs to
be retrofitted.

 

Figure 2. The
Ki and Ke parameters in the pushover diagram

Figure 3. The structure in both X and Y directions
needs to be retrofitted

The effect of steel
jacketing method on seismic rehabilitation

Seismic rehabilitation
goal is balancing the structure’s demand and capacity; therefore, the
structure’s demand should be reduced or its capacity should be increased for
rehabilitation. When rehabilitation is carried out by increasing the structural
capacity, it is referred to as retrofitting. As shown in Fig. 4 one of the most
common methods of retrofitting is the use of steel jacketing in which the
vulnerable section is retrofitted by increasing strength and stiffness. After
retrofitting the vulnerable columns by steel jacketing method no plastic hinge
was created in any column even though the hinges created in the braces went
beyond the life safety performance level. However, in the case of
concentrically braced frame the strip color shown in Fig. 5 and Fig. 6 is not a
valid criterion for the acceptability of the braces and their acceptable
displacement should be checked based on their acceptance criteria at the life
safety performance level for all created hinges. It was observed that all
hinges created in braces provided the desired performance level.

 

Figure 4. A retrofitted element by added steel plates
surround it

The impact of the steel jacketing method was
considered in two modes. The first mode is shown in Fig. 5 when the structure
connection to the foundation is assumed to be completely rigid, and the other
case is shown in Fig. 6 when the structure connection to the foundation is
assumed to be pinned. The created hinges, in the last step of pushover analysis
of the retrofitted structures were dependent on the base supporting conditions
and in the retrofitted structure with pinned support conditions, the hinges
created in the first floor had a more critical condition than the second floor
while the opposite was observed in the structure with rigid support conditions.

 

Figure 5. Hinges created in the structure with rigid
support after rehabilitation by steel jacketing method

Figure 6. Hinges created in the structure with pinned
support after rehabilitation by steel jacketing method

Comparison of obtained
pushover diagrams

The total capacity of the structure was indicated
by the base shear-roof displacement diagrams. The results obtained from the
pushover diagrams of the non-retrofitted and retrofitted structures by steel
jacketing method with rigid and pinned support conditions were compared
according to the Fema (2005). As shown in Fig. 7 and Fig. 8 It was observed
that the retrofitted structures by steel jacketing method had higher slope in
the linear region which means that the retrofitted structures by steel
jacketing method were stiffer than the non-retrofitted structure and the slope
of the linear region in the case that the retrofitted structure had the rigid
support conditions was not different with the case that the support conditions were
pinned thus they have equal stiffness. In the obtained diagrams, retrofitted
structures showed more brittle behavior than the base structure and the
non-retrofitted structures showed more ductile behavior because of the greater
distance between its yield and final displacement. It was observed that along Y
direction which structures had less stiffness than the X direction, the
retrofitted structure with pinned support conditions showed better ductility.
It was also observed that in nonlinear region the stiffness of the retrofitted
structure with rigid support conditions was more than the structure with pinned
support conditions.

 

Figure 7. pushover diagrams of the base and
retrofitted structures by the jacketing method in X direction

 

Figure 8. pushover diagrams of the base and
retrofitted structures by the jacketing method in Y direction

Conclusion

It was observed that in the case that the structure
had pinned support conditions, the hinges created on the first floor were more
critical than the second floor while the opposite was observed in the structure
with the rigid support conditions. After reviewing the pushover diagrams it was
observed that the retrofitted structures behaved stiffer than the base
structure and there was no significant difference between the stiffness of the
two retrofitted structures in the linear region but in nonlinear region the
structure retrofitted with the rigid support conditions behaved stiffer than
the one retrofitted with the pinned support conditions.

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