Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 8 DESIGN FOR EXTREME EVENTS 8.1 INTRODUCTION Extreme Event Limit States involve events with a low probability of occurrence. Such events are considered to be unique events whose return period maybe significantly greater than the design life of the structure (AASHTO 2014). The Extreme Event Limit State is intended to ensure the survivability of the structure during such an event. This Limit State is to protect against collapse of the structure and loss of life some damage or loss of functionality maybe acceptable in such a case. The resistance factors for extreme events when analyzing lateral resistance areas discussed in Chapter 4 (Brown et al. 2010). The load factor is 1.0 (AASHTO 2014). Extreme Event Limit States for design of deep foundations include the following The check flood for scour • Earthquakes/seismic events Loading from ice Vessel collision Vehicle collision Note that the check flood is an extreme event, or super flood, with an estimated return period of approximately 500 years. It is not to be confused with scour from the year flood, which is evaluated as part of the Strength and Service Limit States. Arneson et al. (2012) provides detailed information regarding scour development near bridge piers and abutments. Kavazanjian et al. (2011) presents detailed information regarding seismic analysis and design of foundations for transportation facilities. AASHTO (2014) design specifications provide additional detail regarding the determination of vessel impact, vehicle impact, and ice loads. 8.2 EXTREME EVENT SCOUR (CHECK FLOOD) Structures that are constructed over or adjacent to bodies of water and are subject to scour must be designed for the extreme scour event or check flood. This typically corresponds to a year event return period. Arneson et al. (2012) provides details on design assessments of the scour conditions. AASHTO (2014) indicates that the foundation must be designed for the applied loads on the structure as well as any debris loads during the flood event. Based on AASHTO, (2014), there is no separate load combinations that is applicable to the extreme event scour condition. However, load factors specific to extreme event conditions apply, and potentially deeper or wider scour conditions must be considered. The design for the extreme scour condition differs from the year scour condition as follows 1. The scour prism for the extreme flood condition is typically deeper and wider than for the year flood condition. Referring to the process outlined in Chapter 5 for design of laterally loaded deep foundations, this will require updating the geotechnical conditions for analysis (Block 5). 2. The load factors for the structural loads are different for the Extreme Limit States compared to the Strength and Service Limit States. In general, load factors for Extreme Limit States are 1.0, whereas 117 load factors for Strength and Service Limit States maybe greater than 1.0. Refer to Chapter 4 for the specific load factors and load combinations for each Limit State. 3. Resistance factors differ for the axial resistance for the Extreme Event Limit State. A resistance factor of 1.0 is applicable for axial and lateral resistance and 0.8 for uplift resistance for Extreme Event Limit States. Although a resistance factor of 1.0 is also used for p-y analyses for other Limit States, the often greater factored loads for the extreme Limit State may impact the design of the deep foundations in terms of foundation type, length or size. Also, as discussed in Chapter 4, other considerations maybe applicable to the selection of resistance factors, and a value of less than 1.0 maybe considered for the Extreme Event Limit States. Deep foundations should be designed such that they are deep enough to account for the loss of resistance during the extreme scour event. As outlined in Chapter 5, the design for lateral loads should include a pushover analysis to check the overall stability of the deep foundation elements and verify that the foundations are embedded deep enough to avoid a pushover failure and behave as ductile elements under lateral loads, even under extreme conditions. Additional considerations for extreme scour conditions with regard to load combinations and development of the scour profile can be found in Arneson et al. (2012). Specific considerations for scour related to drilled shafts can be found in Brown et al. (2010) and for driven piles in Hannigan et alb Download 6.03 Mb. Share with your friends:
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