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SEISMIC RETROFITTING WITH COST EFFECTIVENESS: STEEL BRACED FRAME, STEEL MOMENT FRAME, CONCRETE SHEAR WALL, CONCRETE MOMENT FRAME
Author: Han Sang Kim | Size: 2.19 MB | Format:PDF | Quality:Unspecified | Publisher: Han Sang Kim | Year: 2012 | pages: 211
Located in the heart of Los Angeles, USC is considered an earthquake prone zone, and
there is a vital imperative to ensure the safety of all students and faculties during such
an event. In order to prevent casualties, buildings must be checked and assured to
perform under design loads.
Under the current California Building Code (CBC), engineers must design all structural
members for new buildings to meet CBC 2010 standards. However, designing to simply
meet the standards cannot ensure the safety of the building and its occupants as seismic
loads in earthquakes cannot be predicted and can exceed building code expectations.
When retro‐fitting of existing buildings (designed under earlier, less stringent codes) is
considered, the CBC 2010 standards may be applied with the same caveat.
This study considers structural retrofit systems for existing buildings, taking into account
economic as well as structural factors. Two USC buildings, Waite Phillips Hall (WPH), a
12‐story classroom/office building, and Webb Tower (WTO), a 14‐story residential
building, were chosen as case‐studies to represent mid‐size buildings constructed in the
Los Angeles area. Four hypothetical structural systems for retrofit were studied: steel
braced frame, steel moment frame, concrete shear wall core, and concrete moment
frame systems. In accordance to design earthquake loads, the different systems’
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MITIGATING SHRINKAGE CRACKING OF CONCRETE IN BRIDGE DECKS THROUGH INTERNAL CURING
Author: Daniel Robert Goad | Size: 4.69 MB | Format:PDF | Quality:Unspecified | Publisher: Daniel Robert Goad | Year: 2013 | pages: 88
As the need for durable, long lasting infrastructure increases, new methods and
techniques are being explored to prolong the service life of roads and bridges. One method to
reduce shrinkage and early age cracking in concrete is internal curing. Internal curing supplies
water to concrete, using pre-wetted lightweight aggregate (LWA), as needed throughout the
process of hydration to reduce self desiccation, which leads to cracking. This research project
analyzed two types of coarse LWA, expanded clay and expanded shale. The mixtures were
developed specifically for use in bridge decks and adhered to specifications of the Arkansas State
Highway and Transportation Department (AHTD). The concrete mixtures contained LWA at
rates of 0, 100, 200, and 300 lb/yd3. The research was divided into two phases. The first phase
measured autogenous and drying shrinkage in both plastic and elastic states using embedded
vibrating wire strain gages (VWSG) cast in concrete prisms. The expanded clay LWA mixtures,
with the 300 lb. replacement rate yielding the best results, were most effective in reducing
shrinkage. Compressive strength decreased as the amount of LWA included in the mixture
increased. However, all mixtures surpassed the 28 day compressive strength specified by AHTD.
The second phase of the research project measured plastic shrinkage cracking in thin concrete
test slabs. Methods and materials were investigated to produce consistent plastic shrinkage
surface cracks of the concrete slabs. The extent of plastic shrinkage that occurred was quantified
by measuring the total crack area of the test slabs. Implementation of 300 lb. of expanded clay
LWA did not reduce the crack lengths, but did reduce the average crack widths experienced by
the test slabs due to plastic shrinkage.
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IMPROVING THE BEHAVIOR OF SPECIAL CONCENTRICALLY BRACE FRAMES WITH CAST STEEL INSERTS
Author: Kristen M. Ward | Size: 6.79 MB | Format:PDF | Quality:Unspecified | Publisher: Kristen M. Ward | Year: 2012 | pages: 147
A Cast Modular Ductile Bracing System (CMDB) has been developed as an alter-
native to special concentrically braced frames. The CMDB system introduces cast
components at the ends and center of the brace in an attempt to produce a system
with reliable strength, sti ness, and deformation capacity. A cruciform cross-section
has been chosen for the cast component geometry, which is specially detailed to en-
hance energy dissipation and increase low cycle fatigue life thereby reducing the
likelihood of fracture. In this dissertaion, capacity design parameters are estab-
lished that describe the axial strength and
exural strength of the cast components
relative to the main hollow structural section member. These parameters are varied
in 2D nite element models to understand the nature of the system and identify the
best performing designs. The cruciform shape of the casting is varied to produce
better performance and self-centering enhancements are introduced. 3D FE models
of the CMDB system and a typical special concentrically braced frame, in combi-
nation with fracture indices, are used to compare the expected low cycle fatigue life
of the two systems. The dynamic performance of the system is assessed through
nonlinear nite element anaylses and conclusions are drawn.
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Base isolation systems are preferably applied in low-rise buildings since the
vibration period of the structure can be shorter, so that it performs more rigidly to
maximize the benefit from the isolation system. Mid-rise and high-rise base-isolated
buildings are expected to have different and variable response characteristics compared to
low-rise base-isolated buildings, and may not perform as well as low-rise or relatively
stiffer base-isolated buildings. However, in this study, the benefit of seismic isolation in a
9-story is comparable or even better than a low-rise (3-story) isolated building that was
investigated thoroughly in a previous study (Sayani et al. 2011) with respect to peak floor
acceleration, peak story drifts, and peak plastic rotation.
A pair of 9-story isolated and conventional buildings were designed by
Forell/Elsesser, using the same design philosophy as 3-story buildings that were
investigated earlier. Both 9-story and 3-story buildings were designed to satisfactorily
meet the current building code standards (ASCE 2005). OpenSees and SAP 2000 were
used to develop analytical models of the 9-story buildings for independent purposes. The
SAP model was used primarily for model analysis and to perform a design check with
standard response spectrum analysis procedures. The OpenSees model was used for
nonlinear pushover analysis and nonlinear response history analysis to suites of ground
motions representing various probability of occurrence events.
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The P-Delta effect occurs when a frame or structure is laterally displaced by either seismic or wind loadings. As the structure is being displaced laterally, gravity begins to act on the members causing a secondary effect on the forces and moments which in turn cause additional displacement.
The purpose of this study is to investigate the P-Delta effect on multi-story steel framed structures with welded flange plate connections. The traditional method regarding the calculation of the P-Delta effect assumes the lateral displacement of the steel columns is due to lateral deflection only and does not take joint rotation into account. In this study, a finite element analysis using computer models will be used to investigate the additional lateral displacement from the P-Delta effect due to the addition of joint rotation.
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The P-delta effect is a second order effect experienced by any structure when subjected to lateral loads like earthquake or wind loads, and is originated by an additional destabilizing moment generated due to the gravity acting on the laterally deflected member further displacing it. For the purpose of this research, displacement is considered as the study parameter to analyze the second order P-Delta effects.
The main objective of this study is to investigate effects of forces causing P-Delta effects on Single Story Single Bay Steel Moment Frames with Reduced Beam Section Connection (RBS). FEMA-350 and AISC Seismic Design Manual suggest that, if the specified conditions are satisfied, there is no need to provide additional panel zone reinforcements as continuity and doubler plates. This study makes an effort to observe the effects of panel zone strength in formation of plastic hinges and in shifting fracture zone away from the column face on frames with RBS connections under P-Delta effects and find whether further increasing the stiffness of panel zone will have beneficial outcome or not.
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Inelastic behavior in steel special moment frames occurs through the development of plastic hinges at locations near the ends of the beam. The main objective of using a reduced beam connection is to force the formation of plastic hinges to be formed at the reduced beam section rather than at the ends of the beam which otherwise would lead to brittle failure of the beam-column connections. The beam has two reduced beam sections, each located at a certain distance from the face of the column, so that the plastic hinges are formed symmetrically at each of this section. When acted upon by lateral loads, the maximum moments occur at the ends of the beam. Therefore, the plastic hinges form at the reduced beam section. However, when a frame is subjected to a combination of gravity and lateral loads, the plastic hinge formation at one of the reduced beam section is not so clear and further analysis has to be done to study the effect. FEMA 350 indicates that the desired plastic hinge location is only valid for beams with gravity loads representing a small portion of the total flexural demand. If gravity demands significantly exceed 30% of the girder plastic capacity then further plastic analysis of the frame should be performed to determine the appropriate hinge locations.
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This research investigates the analytical lateral load responses of unbonded posttensioned
cast-in-place concrete special structural walls with bonded or debonded
longitudinal mild steel reinforcement under the action of monotonic lateral load. Two sets
of generalized closed-form expressions are derived for estimating the lateral load
responses, base moment and lateral drift, of the walls for critical limit states. First set of
equations is for cast-in-place special structural walls with bonded longitudinal
reinforcement and the second for cast-in-place special structural walls with
predetermined length of debonded longitudinal reinforcement at the base of the wall.
Analytical models for walls, with the two configurations of longitudinal reinforcement,
using fiber elements are also presented for nonlinear analysis of walls under monotonic
lateral load. In general, results of the closed-form expressions are in good agreement with
the results of analytical models.
The analytical models and the closed-form expressions are also used for investigating
effects of six design parameters in the lateral load response of the walls. The parameters
considered are area of post-tensioning steel with constant initial prestress; initial prestress
in post-tensioning steel with constant area of post-tensioning steel; constant initial
prestress force with varying area and initial prestress; area of boundary longitudinal
reinforcement; area of web longitudinal reinforcement; and spacing of web longitudinal
reinforcement.
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Hi friends
Please share this article; The Stability of Liquid-Filled Cylindrical Shells Under Dynamic Loading
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