SITE SPECIFIC SEISMICITY STUDIES
The objective of doing a site-specific study is to establish acceleration parameters corresponding to Maximum Considered Earthquake.
The outputs are as follows:
1. Identify seismic sources possible to generate strong ground shaking at the site;
2. Characterize the identified earthquake sources in terms of their location and geometries,
maximum earthquake magnitudes, and frequency of earthquake occurrence;
3. Characterize source-to-site ground motion attenuation;
4. Generate uniform – seismic hazard curves corresponding to 10% and 2% probabilities of
exceedance in 50- year exposure or as specified.
GEOSEED utilizes both probabilistic and deterministic approaches in its studies.
EXAMPLE:
Seismic Design Parameter Assessment for Water Tanks
Seismic design of liquid-containing structures needs to incorporate earthquake-induced dynamic effects of the impulsive mode of the tank-liquid system and the convective modes of sioshing of the stored liquid which occur at differing natural vibration.
The current specialty design code for water tanks, ACI 350.3-06 (“Seismic Design of Liquid-Containing Concrete Structure”), specifies design seismic loads based on the so-defined maximum considered earthquake in which the spectral response acceleration shall be taken to be the lesser of the probabilistic maximum considered earthquake spectral response acceleration and the deterministic maximum considered earthquake spectral response acceleration.
DESIGN ACCELERATIONS
The ground motion caused by earthquake is generally characterized in terms of ground surface displacement, velocity, and acceleration. Acceleration is generally used because it is directly related to the dynamic forces that earthquake induce in the soil mass. The measure of the cyclic ground motion is represented by the maximum horizontal acceleration at the ground surface amax. The maximum horizontal acceleration at the ground surface is also known as the peak horizontal ground acceleration (PHGA or PGA).
ACTIVE FAULTS WITHIN 150 KM RADIUS OF THE SITE
Table 1. Five active fault lines were considered based on the MMEIRS Report (2005).
Site Specific Elastic Design Response Spectrum
A site-specific elastic design response spectrum based on the geologic, tectonic, seismologic and soil characteristics associated with the specific site were developed using the model of Villaraza (1986). The spectra were developed for 0.5% and 5% damping ratios for a 475- and 2475- year return periods.
Horizontal Spectral Response Acceleration Spectrum
Based on the existing ground conditions without improvement or use of concrete piles, the Elastic Acceleration Design Response Spectrum for a 475-year return period at 5% damping is shown in Figure 4a. To reduce the seismic amplification due to the resonant frequency of the tank’s long period in sloshing and the soft soil period as shown in Figure 4b, ground improvement is necessary. The use of piles will improve ground soil type from SE to SD for seismic effects.
Since the Project Site is closest to a strike-slip fault West Valley Fault. Simulation was done using the Villaraza (1986) model. PGA=0.29g was obtained for M6.8 and epicentral distance of 40km. PGA=0.32g was used to conform to the Provision of NSCP 2010. The vertical acceleration should be considered in the design as provided in the ASCE 7-05 Section 15.7 or whatever governing Codes that will give a higher value for design. The minimum value to be used for the design of building structures shall two-thirds of the PGA or 0.10g minimum as provided in the NSCP 2010.
STRUCTURAL RESTORATION / REHABILITATION
The need to improve the ability of an existing building to withstand seismic forces arises from evidence of damage and poor behavior during an earthquake. These improvements consist of repair, restoration and retrofitting.
Repair is typically concerned with bringing back the architectural shape of the building as well as its aesthetic features. Restoration on the other hand, is the restitution of the strength of the building. This involves structural repairs to load-bearing elements. Lastly, retrofitting is the addition of new structural elements not present during the original construction of the building. Retrofitting is necessary especially if large components of the structure have failed.
Points considered:
• Building Rehabilitation.
• Restoration Scheme.
• Structural Modelling Scheme.
• Materials Compatibility.
• Structural Stability Check.
STRUCTURAL EVALUATION
The objective of structural evaluation is to assess the effect of loads on an existing structure. Structural data may be obtained through observation and testing prior to structural analysis.
Points considered:
• Actual Building Condition.
• AS-BUILT Plans, Materials Test Results, and Specifications.
• Testing of Materials.
• Loading Conditions.
• Building Code Structural Requirements.
• Structural Dynamic Analysis to check building behaviour during earthquakes.
• Foundation Analysis.
STRUCTURAL DESIGN
Structural design is the application of physical laws and empirical knowledge of the structural performance of different materials and geometries. It utilizes relatively simple structural design elements to build complex structural systems integrated into the architectural design.
Points considered:
• Architectural & Engineering Requirements.
• Building Code Requirements.
• Structural Design and Analysis (Elastic/Inelastic Behaviour).
• Foundation Analysis.