The structure is exposed to near-field ground motions from the Italian Aquila and Norcia earthquakes, due to the similarity of the geotectonic environment between Greece and Italy. After implementing the Chilean Code provisions and examining the building in the Y direction only, the results have shown a significant improvement in the response of the building and a reduction in the damage limit states reached for most of the records. Finally, it is concluded that damage intensity estimation is mostly affected by the energy contained in the ground motion, which is introduced as the “energy flux” index.
INTRODUCTION
In Chapter 3, a description of the building and its seismic modeling is given, including the building structure, material characteristics, typical reinforcement detailing of structural elements, applied loads, seismic characteristics, and eigenvalue and pushover analyzes are also performed. In Chapter 5, a description is given of the new provisions of the Chilean seismic code, which are used to mitigate the damage achieved, the mitigation is in the form of strengthening the building with the required number of shear walls in the Y direction only, and also the value of own. and pushover analyzes are performed. The displacement value indicates the structural damage state of the building, the results are illustrated in plots and tabular forms for each earthquake showing the interstory movement (maximum roof displacement/building height) and the corresponding default limit damage state.
NEAR-FAULT GROUND MOTIONS
- Statement of Problem
- Near-Fault effects
- Directional effects
- Parameterization of Near-Fault Ground Motion
- Pulse characteristics
- Effect of the pulse on the acceleration response spectrum
The figure shows a snapshot of the rupture front at a given time (from Somerville et al. 1997a). The occurrence of this effect depends on the fracture process and on the geometric configuration of the fault and site. In dip-slip events, forward conditions occur for locations located near the up-dip projection of the fault plane.
BUILDING DESCREPTION
- Building Layout
- Modelling of the building using Seismostruct
- Materials
- Concrete
- Steel Reinforcement
- Modeling Elements
- Beams
- Columns
- Walls
- Loading
- Seismic Characteristics
- Eigenvalues Analysis
- Static Pushover Analysis
- Theory and purpose
- Static pushover in seismostruct
The reinforced concrete building consists of beams which are simulated as T-sections in the interior spaces while considered as L-sections in the perimeter of the building. The cross section of a typical beam is shown in Figure 3.7 and the side view in Figure 3.8. The slope of the thrust curves gradually decreases as the lateral displacement of the building increases.
GROUND MOTIONS RECORDS
L’Aquila earthquake 2009
The coordinates of the vertices of the rupture plane and the hypocenter are reported in Table 4.1. The main shock was recorded by the stations of the National Accelerometric Network (RAN) of the Italian Civil Protection, in figure 4.1 the projection of the crack surface with the epicentral location, the code of the RAN stations, their location class Eurocode 8 (EC8) is presented. and some towns and villages badly damaged. Baker was used to analyze the L'Aquila data; Table VII shows the pulse identification results for the data of Table VI;.
Norcia earthquake 2016
The first strong earthquake of the sequence (Mw 6.0) struck central Italy at 01:36:32 GMT near Amatrice, causing scattered building collapse and around 300 casualties. Four events were caused by normal faulting, the dominant style of faulting in the area, and all have NW-SE or NNW-SSE extension and SW dip. The location of the three epicenters along with events of magnitude greater than 4.0 is shown in Figure 4.2.
Red boxes are surface fault projections: fault geometries are preliminary to the Ussita and Norcia events. A map of the various epicenters together with the stations for which we detected visible impulses in the strike-normal (fault-normal, FN) component can be seen in Figure 4.4 (note that no impulsive ground motions were detected in the case the blow Mw5.9). Of all the investigated data belonging to the Mw6.0 shock, the six ground motions recorded in Amatrice (AMT), Norcia (NRC), Norcia Le Castellina (NOR), Montreal (RM33), Monte Fema (FEMA) and Fiastra (MNF) exhibited impulsive characteristics over a range of orientations, as expressed by a pulse index (PI) score above 0.85 (see Baker, 2007).
The record at Amatrice revealed two distinct pulses, one fault-normal dominant (FN) and the other a longer fault-parallel (FP) pulse. On the other hand, the Norcia record was found to contain a pulse with a period of 2.09 s mostly towards orientations lying between FN and FP, without being decidedly dominant in any perpendicular/parallel strike direction. Finally, the ground motions recorded at the Fiastra and Montreal stations were found to contain pulses in the FN direction with Tp of 1.4 s and 1.2 s, respectively, also suggesting effects of rupture orientation, despite the lower velocity amplitude due to greater distance from error and consequent weakening.
In the following figures, a polar graph is presented for each station, showing the result of the PI on azimuth and the time histories of the velocity in the most important directions (original signal and extracted pulse superimposed).
SEISMIC DESIGN OF RC SHEAR WALL STRUCTURES IN CHILE
- Introduction
- Building code provisions in Chile
- Typical Construction Method and Assumptions
- Implementation of Chilean code assumptions in the case study
- Strengthened building layout
- Eigen value analysis
- Pushover analysis
Therefore, severely damaged concrete buildings correspond to almost the stock of newer buildings of 9 or more stories in south-central Chile. The limits are set at the base shear (Section 6.3.7), which cannot be less than IAoP∕6g and must not exceed ICmaxP, where Cmax is 0.35SAo∕g for RC wall buildings, and P is the weight total seismic load of the building (total dead load and minimum 25% of live load for typical buildings and minimum 50% for assembly sites). NCh433.Of96, Section 5.9 limits the relative displacements between two consecutive stories, measured at the center of mass in each direction, to 0.002hs, where hs is the story height.
Calderón (2007) used the parameter dnp = Aw∕W, where W is the number of floors multiplied by the weight of the floor, to account for the effect of increasing building height. For preliminary design, some engineers select the wall area in each of the building's principal directions to limit the average wall shear stress to about 6 kgf/cm2. In newer buildings (Figure 5.1b), the walls are usually arranged along a central corridor in the long direction of the building with several rectangular walls in the short direction of the building.
Ratios of Aw∕Af of 1%, 2% and 3% in each principal direction of the building were taken into account, while tributaries were estimated for fairly typical walls in the transverse (short) direction of the building based on an assessment of structural drawings for several damaged buildings built around 2005. Due to the large number of walls used in typical buildings, the amount of longitudinal boundary reinforcement is light compared to that in the United States. The change made to the building is illustrated in the following subdivision, assigned by Pushover and Eigenvalue Analysis, which characterizes the building.
In the calculation of the eigenvalue analysis, the efficient Lanczos algorithm [Hughes, 1987] is used for the estimation of structural natural frequencies and mode shapes.
DATA ANALYSIS AND RESULTS
Identification of damage limit states for RC buildings
Methodology
Dynamic time history Results for the original building
- Aquila earthquake in X-Direction
- Norcia earthquake in X-Direction
- Aquila earthquake in Y-direction
- Norcia earthquake in Y-Direction
Dynamic time history results of strengthened building
- Aquila Earthquake in Y-Direction
- Norcia Earthquake in Y-Direction
SUMMARY AND CONCLUSIONS
Summary
In Seismostruct, the efficient Lanczos algorithm [Hughes, 1987] is used to evaluate the structural natural frequencies and mode shapes in the calculation of the eigenvalue analysis. The fundamental period of the original building has been determined to be 0.61 sec. A static non-linear pushover analysis is then performed to calculate the building yield acceleration (ay), which is later used in collecting the appropriate ground motion records that are expected to cause significant damage. As a result of the pushover analysis, the yielding force of the building in the Y direction was 10332 KN and since the mass of the building was 2709 tons, the yielding acceleration is estimated to be about 3.8 m/sec2.
Next, a nonlinear inelastic dynamic time history analysis is performed, using a sample of 16 near-fault ground motion records from the Aquila and Norcia earthquakes in Italy, to obtain the displacement time history of each record, which will be used to determine the damage limit states determine achieved according to “T. Consequently, the existing building was strengthened by a certain number of shear walls to make the ratio of shear wall in one main direction (Y) of the building to the floor area (Aw/Af) about 3% in each floor. check the improvement or improvement of the building seismic response taking into account the reduction of the existing shear walls and core reinforcement according to the new Chilean code assumptions and provisions. Five shear walls were introduced in the Y direction with a total area of 3.5 m2, with minimum required light reinforcement in longitudinal and transverse directions consisting of 10 mm diameter bars spaced at 20 cm (ρv = 0.39%) and 8 mm diameter bars spaced at 20 cm (ρt = 0.25%), respectively.
Horizontal web beams are typically placed outside vertical beams and anchored to the wall edge with 90-degree hooks, as shown in the image below. For the reinforced building, the fundamental period of the structure from the eigenvalue analysis is 0.56 sec. In addition, a static nonlinear pushover analysis is performed to compare the yield displacement after amplification with the original one.
The results showed that the displacement after strengthening is 6.4 cm, while it was 15 cm for the original building.
Conclusions
As observed in the figures below, all the Aquila records have moderate damage conditions, with energy flux and acceleration amplitude ranging between 400~500 cm2/s and 300~400 cm/s2 respectively, except for AQK which has energy flux of 1400 cm2/s. , which is the station with the strongest pulse-like signal in FN direction as it is the further station from the epicenter than the others. While for Norcia records, the damage limit conditions reached were moderate and extensive, as the energy flux for the moderate damage condition varies between 400~500 cm2/s, and for the extensive damage condition varies between 1500~3000 cm2/s. After strengthening the building in Y direction according to the new Chilean code of practice provisions (NCh433.Of96; INN 1996) with the minimum required longitudinal and transverse reinforcement which is 10 mm diameter bars spaced at 20 cm (ρv = 0.39%) and 8mm diameter bars spaced 20 cm (ρt = 0.25%) respectively, making the ratio of shear walls in that direction (Y) of the building to the floor area (Aw/Af) about 3% in each floor, and the building simply set. to the same earthquake records previously mentioned performing nonlinear inelastic dynamic time history analyses, it is found that the building behavior in terms of damage limit states is improved as illustrated in the following tables 7.3 and 7.4 respectively.
For Aquila records, behavior is improved from moderate to light damage, and for Norcia records, behavior is improved from extensive-moderate to moderate-light damage. As noted in the previous tables, the behavior has improved for most of the records, except for records (T1201_DATA1) and (ACC_DATA2), since the dominant period of the impulse is almost twice as long as the elastic period of the structure. Therefore, when the structure behaves inelastically, its period approaches the pulse period and a high response gain is achieved, "resonance can occur", as shown in the following figures.