CASESTUDY ON SETTLEMENT OF A BUILDING· SelectedBuilding : Leaning tower of San Francisco , theMillennium Tower.
It is constructed between 2005 and 2009.· GeneralDetails : Property Value Height 196.6m Floor above ground 58 Floor below ground 4 Number of apartment 440 Tower gross floor area 106,933 m2 · Typeof soil : · Siteconditions : The tower is constructed on the south sideof Mission street between Beale street and Fremont street.
This complexconsists of two structures Tower and mid rise separated by a seismic joint. Bothstructures are of cast and situ construction using post tensioned slabs forfloors above ground level. Most importantly , San Francisco is located inseismically active zone with seven sub parallel fault lines surrounding thecity. In 2010 , construction of Tunnel and transit terminal started near tothis building that dewatered the soileffectively.· Aboutstructure : As mentioned earlier , this tower consist of tower and midrise , both have different foundations. The tower foundation consist of 10 footthick pile cap supported by about 950 precast concrete piles, approximately 80feet in length. Whereas mid rise supported on mat foundation of 6 to 8feet varying thickness. The matfoundation of mid rise resist the uplift hydrostatic pressure while tower haveto face these types of pressure.
Therefore, settlement expected to towerbuilding. Expected settlement for this building by structural engineer is about12 inches during its whole life time . The settlement of this tower indifferent years are given below : Year Settlement (inches) 2008 6 2009 9 2016 16 2017 Almost 17 According to the above details , itis can also be observed that more settlement occurs from 2009 to 2016.
· Reasonsof Settlement : Following are the reasons of settlement occur in building : 1. Poor integrity between two structures(having different foundations).As there foundations are different so thereresponse are different against uplift hydrostatic pressures and other soilconditions etc.
2. The dewatering of soil during 5 yearsby the construction of tunnel near to this tower. The leaning of this tower dueto settlement is shown in the figure given below : 3. By 2014 and 2016 research report,he leaning of tower isconsidered as safe and not at risk during an earthquake event by Millenniumpartners and they declared that the settlement is due to the increased in stress on pile’s cap byapproximately 50 % at some locations in mat foundation (which may be due toseismic pressure).
· Solutionfor settlement : The solution to ensure the safety of tower during strongearthquake is invoved the reinforcement of foundation mat cap.They proposed torun the reinforced walls from the core of the building into two basementlevels, creating a triple decker foundation .· References: http://sfgsa.org/sites/default/files/Document/MillenniumTowerSafetyReviewReport.pdf