At fi rst step, gravity load was applied to the structure. When gravity loads are applied to structures with fi bre sections, it is possible for concrete cracking to occur (steel yield or concrete crushing should not occur).
Concrete cracking is a nonlinear event, so it is often necessary to specify that the gravity load analysis is nonlinear. In this structure the behaviour is nonlinear, so nonlinear analysis was used. The results show that along with concrete cracking, some of the slender concrete coupling beams behave inelastic under gravity load so these coupling beam need to be strengthened or changed to steel coupling beam. Shear wall are highly loaded under gravity loads which shows that the system is mostly behave like bearing wall system rather than pure shear wall. Figure 26 shows the state of concrete compression strain and the area where coupling beams show nonlinear behaviour.
At second step, time histories applied to the building and acceptance criteria (Table 6) were evaluated at each time step. Results generally show that most of the nonlinear and earthquake energy dissipation happened through the nonlinear behaviour of slender and deep concrete coupling beams and yielding of steel and crushing of concrete did not happen in the shear walls and all the steel members remained elastic, this was the desired behaviour and our building essentially met the assumed performance objectives.
DM (2009)7 requires maximum 3.5% inter-story drift under level E3 earthquake. Maximum inter-story drift under all time histories were evaluated and it was seen that the maximum inter-story drift is around 0.006 which is well below the limit. Figure 27 shows the envelope of inter-story drifts.
Shear walls need to remain essentially elastic in shear under earthquake load. The envelope of analysis results show that the maximum demand over capacity ratio (D/C) is 0.45 in the central wall which meet the acceptance criteria. Figure 28 shows the envelope of shear force and the capacity of central shear wall.
Performance based seismic design of tall building structures: case study
Figure 26 - State of concrete compression strain and the area where coupling beams show nonlinear behaviour under gravity load.
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Review of the dissipated energy showed that in-elastic energy is still small as compare to elastic and viscous energy (Figure 33) which indicate that building does not have ductile behaviour due to some weak points and heavy gravity loads applied to the shear walls (Bearing Wall System) but it has good over-strength factor due to primary design of elements for full gravity load and limited inter-story wind drift. Although the applied load is 4 times more than the anticipated Concrete compression strain remained
under the assumed concrete crushing strain as shown in Figure 29.
Story shears are compared in Figure 30, in this fi gure reduced elastic story shear due to the UBC design spectrum input with R (ductility factor) equal to 4.5 is compared to maximum story shear carried by shear walls only and shear walls plus steel elements under Lander 000 MCE event.
Review of the dissipated energy by different mechanism provides
It can be seen that most of the energy is dissipated by elastic strain energy and also viscous damping; small amount of energy is dissipated by in-elastic energy through inelasticity of concrete coupling beams.
To understand the exact locations of hinge formations and nonlinear behaviour of the structure, Lander000 time history which creates the maximum drift was magnifi ed four times and applied to the structure.
Analysis stopped at 13.44 seconds
Performance based seismic design of tall building structures: case study
Figure 27 - The envelope inter-story drift ratios for MCE earthquakes (Acceptance limit 3.5%)
Figure 28 - The envelope of shear force and the capacity of central shear wall
Figure 29 - Envelope of the concrete compression strain at critical location Figure 30 - Story shear comparison-Lander 000 MCE event
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The weak points can be avoided or strengthen. Based on this, it was decided to add confi ned boundaries for the fi rst 15 levels of shear wall by adding confi ned boundary elements wherever possible. At the location where side element meet the concrete shear wall, steel member imbedded inside the concrete to provide smooth load transfer to the shear wall.
The analysis of the animations provided us the exact behaviour of the building through the entire time duration and specifi cally it showed that above the shear walls where the lateral load resisting system is just provided by steel arch, is moving very much out of phase as compare to the shear walls movement, in other word, the arch is much more fl exible than the shear wall. It was decided to add steel bracing above the main truss all the way up to the steel arch to avoid this out of phase movement. Figure 34 shows the proposed new braces.
Performance based seismic design of tall building structures: case study
Figure 31 - Dissipated energy by different mechanisms-Lander 000 MCE event
Figure 32 - Concrete crushing under four times stronger earthquake
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Performance based seismic design of tall building structures: case study
Figure 33 - Dissipated energy by different mechanisms-scaled up four times Lander 000
Figure 34 - The proposed new braces
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Conclusion
In this study, structural system of Icon Hotel for gravity and lateral loads were presented. Performance based seismic design approach was selected to evaluate the seismic behaviour of the building for anticipated future earthquake. In this regards nonlinear time history analysis was used and modelling assumptions were presented.
Following conclusions were made:
Some of the slender coupling (1)
beams behave inelastic under gravity loads. These coupling beams need to be strengthened or changed to steel beams.
Scaled time histories applied to (2)
the structure and acceptance criteria were evaluated at each time step. Results show that steel member generally remain elastic and building essentially meet the desired performance objectives.
Analysis of animation through (3)
each time steps revealed that steel arch is more fl exible than shear walls. To avoid the out of phase movement of top portion, additional steel bracing added to the structure above the shear walls.
Review of the dissipated energy (4)
by different mechanism showed that most of the earthquake energy is dissipated by elastic strain energy and some part by viscous damping. This behaviour indicates the low ductility of system and comparably high over-strength of bearing wall system.
Scaled Lander 000, which creates (5)
the maximum inter-story drift, was magnifi ed 4 times and applied to the structure. Although the applied load was 4 times more than the anticipated load but concentration of concrete crushing in specifi c areas lead us to identify the weak points. Additional confi ned boundary zone added to fi rst 15 levels of shear walls to avoid sudden concrete crushing and more ductile behaviour.