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DYNAMIC RELATIVE DISPLACEMENTS OF PRECAST-SLAB SYSTEM FLOATING ON SHEAR-WALLS DURING EARTHQUAKE (INVESTIGATION OF POUNDING/SLIDING DISTANCES)
It has been widely accepted that pounding between building is an undesirable phenomenon
That should be prevented or mitigated . In one structure that must be separated into units because of temperature movements or for other reasons should have separations so detailed as to avoid the possibility of hammering . Floating support is commonly used in precast slab –system at expansion joints that transferring vertical lad while allowing lateral movement through neoprene pad seating .
This paper investigates the seismic time-history pounding and /or sliding distances between precast concrete slab system and its supporting shear -wall . due to their floating connection . therefore . the effective horizontal displacement are determined and relatively compared for chosen plan finite element models simulating this system . the analysis is carried out by using the transient-dynamic Muddle OF ADINA computer program package under the effect of relatively long time-acceleration earthquake –record of ( EL –CONTRO 1940) considering the studied variables of building height . relative inertia of walls and mass of the precast-slab system . the results indicate different oscillation-amplitude of the parts consisting the studied structure , and hence different pounding /sliding distances .
1- pounding and/or skidding distances are preformed between precast –slab system and its lateral –load-supporting system (shear-walls) under seismic loading , if their junction is floating type using neoprene pad each part of this system vibrates with different amplitude along the ground motion (dynamic horizontal relative displacements) , depending on its dynamic properties . the difference slab and wall displacements in each direction is the pounding and /or distance .
2- Graphs are presented for the time-history of relative horizontal displacements at upper floor level of the studied cases of the slab-wall system using ADINA dynamic computer program package the graphs show maximum pounding / sliding distance value of 9 cm .
3- The limitation of using floating-support in precast-structures without providing special details for adequate seismic resistance can be decided based on the values of pounding and sliding distances of the concerned building using similar Graphs as presented in this paper
4- The pounding /sliding distance is affected mainly by the building stiffness in addition to other factor such as type of neoprene pad , ground motion intensity and floor masses .
SEISMIC PERFORMANCE OF COUPLED WALLS OF PRECAST HYBRID BEAM-WALL CONNECTIONS
In precast concrete structures , the connections between components , in particular beam-to-(column/wall) affect the strength , stability and lateral load distribution in the structure . to remain competitive , precast concrete structures must be erected quickly and simply by using practical connections .
Lateral stability of precast structures is commonly depending on the shear walls which as flexural cantilevers in most cases . the precast floor system are intended to provide diaphragm action between walls during earthquake by using practical connections between walls and ( beam/slabs) precast hybrid connection can provide coupling action between the individual walls as well as diaphragm action . this wall-beam connection improves the seismic performance of the coupled walls at suitable wall-stiffness by creating coupling moment across the two walls rather then the individual wall flexural action .
A dynamic numerical analysis is carried out in this paper to investigate the seismic performance of two equal walls connected by a precast beam of common practice hybrid wall-beam connection considering the effect of wall stiffness , Neoprene pad seating only and hinged connection are also investigated . The result are plotted in time history graphs for normal force and bending moment at wall base which show major effect of hybrid wall beam connection at higher wall stiffnesses , which attributed to vibration mode effect.
The in hand dynamic study , invastgates seismic preformance of hybrid coupled equal-walls in which practical precast hybrid beam-wall connections are used to couple the two concrete sherr walls . the result are compared , in the time history chart , with other types of wall-floor connections that used in precast structures , which are hinged and neoprene pad seating connections , the abave discussion of results shows the following Conclusions. which are applicable only for the studie modeld and ground motion .
1- The base shear force developed in uncoupled shear walls due to ground motion is effected by the wall-mode of vibrtion which depends on the wall stiffnedd , the base shear increases as the wal stiffnessin crease , A suitable wall depth should be chosen to fulfill the architectural deisgn requirement , wall strength , serviceability requirement , and minimum base force of dynamic time-history analysis.
2- Precast hybrid wall-beam connection is not pushover matter in dynamic analysis and contruction , Using this connection should be decided based on dynamic analysis results of maximum drift, wall-movement, and normal-force at different wall stiffnessess , Actually this connection provides low value of coupling moment of approximately 11% of the developed wall-base moment at wall stiffness ratio (D/H) of 0.143, while providing high coupling moment values at lowe wall-stiffnesses
3- The seismic preformance of the wall-system is affected by both single wall stiffness and wall-beam conection , the precast hybrid conection provide over-coupling to the walls of higher stiffness and hence increases the wall base moment .
4- The hybrid coupled wall of wall stiffness (D/H) of 0.143 and hight (H) of 28 m is an over-coupled system , the studied wall-beam connections . of this wall case shows lower wall wall base-moment even lower than the uncoupled walls . this attributed to more suitable vibration mode effect for this hinged wall- system .
Dynamic Analysis on Earthquake Resistance of Jointed Precast Wall-Boundary Columns System
Seismic resistance of building depends on either flexible or stiff lateral load resisting systems. This paper investigates the interaction structural behavior of jointed – precast- concrete –system consisting of cantilever shear wall and adjacent "gable" gravity load –resisting equal-columns. This combined system is arranged through point connection of limited ductility and different arrangements between the wall and the adjacent columns. A dynamic study is carried to investigate improving the earthquake – resistance of the system to introduce suitable base reaction values at acceptable roof drift. This task is performed by trying different system-stiffness using different wall- columns connection locations for different column sizes.
Practically, the point connections are formed by bolting bars-in-sockets protruded and lapped in separately concreted narrow strip packing between the prefabricated wall and each of the two columns. However, common uniform staggered connection arrangement along the full height is investigated, in addition to two suggested locations which are: floor levels, and floor levels only.
The results indicate that the systems vibrate differently and hence developing different inertia forces (base shears). The resistance of seismic overturning moment is participated by bending in wall, and coupling moment between the normal tension and compression forces created in the boundary- columns. Seismic response time-history charts are provided to investigate the studied variables that produce desired values of design parameters and may help for modeling similar future problems for proper results of dynamic analysis.
KEY WORDS: Dynamic analysis, seismic resistance, jointed-system, precast-concrete, cantilever wall, boundary columns, inertia force, coupling moment.
Dynamic study is carried out on jointed – precast-system of wall and equal- adjacent boundary columns. The seismic behavior of the system is in investigate at different connection point location and columns sizes. Actually, the values and directions of the of the developed inertia-forces depend on the load-transfer- mechanism between the jointed components which affecting the system-vibration. The following conclusions ar summarized :
1- The shearing action of the seismic-inertia-force is mainly resisted by the web (wall) of the precast jointed-system, while the overturning action is resisted by coupling moment between the tension and compression forces developed in the boundary –columns increases as the column size increase.
2- The path of floor-diaphragm-action should be connected to the cantilever shear wall only if it is not intended to increase the wall stiffness by providing full height uniform connections with the adjacent-boundary-columns.
3- The precast wall-boundary columns system jointed at floor levels only develops more (roof draft, base shear, and overturning moment) during earthquake. This effect considerably decreases by adding mid floor connections, reaching to the lowest design parameters at uniform connections along the full height.
4- The presented time history charts can be used as guide for pre-arranging future jointed precast wall-boundary columns system to expect fortunate results for the design parameters when carrying a dynamic analysis.
Dynamic soil-structure-interaction analysis of long column-necks embedded in seismically excited soil media
Construction of building in some areas of arid environment requires providing under-ground columns (long-necks) supported on footings resting on natural sand-layer of low surface level related to the road level. This difference in level may be used for parking purpose, creating seismic soft story , or backfilled as it happens in most cases. This typical example, encountered in north Sinai of Egypt where thick sand –layer over hard soil is found in some places. The ground motion used in the dynamic interaction analysis is assumed to take place in such hard layer.
This paper investigates the dynamic-soil-structure- interaction effect on long column-necks embedded in under-ground backfilling and their super-structural ordinary-frame that similar to the current buildings in north Sinai. This interaction system is modeled by finite element of interface surfaces between the soil and the sub-structure, considering repeatable side boundaries at the ends of a chosen wide soil-media to avoid reflection of waves. transient dynamic analysis is carried out by using (ADINA)computer program package.
Results are presented in time-history charts for : the inertial- forces (shear-forces) created at the top of column-necks, and roof-drift of the super- structural frame due to :type and thickness of the supporting sand- media, backfilling/open-space around the column-necks and their embedment height in soil, tie beam or no tie between footings. In addition, the vertical settlements of footings and the free-field motion in soil-media are presented as wall. The results show more roof-drift of the structural –frame due to soil – structure interaction analysis and increase in its natural period compared to the studied fixed case at bottom of footings, as recommended by the uniform building code (UBC 97 ), For seismic loading
Key words:Dynamic soil-structure interaction, embedded column necks, tie beam, super- structural frame, finite element, shear force, intertial force, footings, north Sinai.
Dynamic soil-structure-interaction study is carried for an ordinary 5-story structural-frame supported on column-necks of relatively long heights to accommodate the unleveled nature of arid areas. These under-ground columns are studied in both cases of open-spacing and backfilling around them. In addition to supporting footings connected with and without tie beams. The supporting soil –layer is either loose or dense of 40M thickness over an assumed bed-rock exited by a recorded Earthquake motion. The fixed base condition at bottom of footings is also studied. Time history charts are presented in this study for :roof-drift of the super- structural frame, shear forces at top of outer and inner column- necks, and free-field motion inside the soil layer and the following conclusions are summarized:
1- roof-drift of the structural frame, is mainly affected by the type of supporting soil-media.
2- columns-necks having total height of (2.5M, and 3.5) and and embedment – height of (2m, and 3m) inside under-ground backfilling may be considered of similar seismic shear-force response of difference in maximum absolute values of ( 5 to 15% ) for the case of no tie beam between the supporting footing.
3- The stiffness of the supporting soil-media is the major factor developing higher inertial- force in the outer columns- necks compared to loose one.
4- The maximum absolute values ofshear-force developed in the columns- necks for the fixed case at bottom of footing (rocky soil condition) , due to the recorded earthquake, are very less than the case of supporting soil-media due to free-field motion, and vice versa for the roof-drift of the structural frame. This rsult assures that the soil over the bedrock amplifies the incident Earthquake waves rather than damping it.
Three-dimensional effect on dynamic analysis of generic frames:
Seismic resistance due to space-bracing of stair-slabs
This paper investigates the generic (one-bay-wide) frame of typical building (5- and 7- story heights) in three-dimensional (3D) and two dimensional(2D). the analysis is performed through linear time-history dynamic analysis under the effect of ground motion in the direction parallel to the frame plane. The frames are supposed to be at the stair-case location in the building where the lateral resisting systems (frames or walls) are commonly provided and at any place inside the building as well.
Actually two typical frames are modeled in space and connected transversely with: 1) the inclined and horizontal slabs of the stair (flights and landings) that may provide space- bracing to the supporting frames, 2) the horizontal floor-slabs as common case. On the other hand, corresponding plane frame without any equivalent transverse connection is studied in 2D dynamic analysis for comparison purpose. All the studied frames have typical stifnesses and masses and modeled by finite-elements instead of line-elements to allow more joints for slab-frame interaction simulating the prototype.
The study investigates the 3D action of the frame-systems in addition to the effects of 3D and 2D modeling on the capacity evaluation of the frame. The results show common torsional effects and transverse reactiions at supports especially for the space-bracing of the stair slabs. However, the main objectives of this study concerns with the direction parallel to the frame planes in order to compare the 3D and 2D results.the results are investigated for all floors and time-history charts are presented for alternate story-drifts and the base-shear forces of the frames that showing more stiffness of the frame-sytstem due to the stair-bracing- slabs.
Key words: three-dimensional modeling, dynamic analysis, generic frame, space-bracing stair slabs. Floor diaphragm seismic resistance.
The 3D and 2D dynamic analysis are carried out for the generic frames (one bay width) of 5 and 7 story heights considering the space-bracing effect of the stair-slabs in addition to the effect the common horizontal-floor-diaphragm. Time-history charts are presented for the story-drifts and the base-shear-forces and the following conclusions are summarized:
1- The three-dimensional modeling shows considerable difference in analysis results in the direction of the ground motion compared to the two-dimensional one, due to the effect of the spce –bracing of the stair-slabs and the horizontal-diaphragm action of the floor-slabs. This difference varies according to the variation of the dynamic properties of the studied system and its global stifnesses.
2- The space-bracing effect of the stair-slabs increases the frame stiffness and hence increases the base-shear force especially for the studied system of lower height (5-story).
3- The proper choice of structural modeling leads to suitable dynamic behavior of the structural-system to provide acceptable results that clearly show the effect of the studied parameet. However, the two studied frames are typical in behavior for effect of horizontal slab diaphragm, while difference in results is found for the space bracing of different flight directions and missing ones at lower and upper floors.