CASE Expected settlement for this building by structural

CASE
STUDY ON SETTLEMENT OF A  BUILDING

·       
Selected
Building : Leaning tower of San Francisco , the
Millennium Tower.It is constructed between 2005 and 2009.

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·       
General
Details  :                                                   

  

Property

Value

Height

196.6m

Floor above
ground

58

Floor below
ground

4

Number of
apartment

440

Tower gross floor
area

106,933 m2

·       
Type
of soil :   

·       
Site
conditions :   The tower is constructed on the south side
of Mission street between Beale street and Fremont street. This complex
consists of two structures Tower and mid rise separated by a seismic joint. Both
structures are of cast and situ construction using post tensioned slabs for
floors above ground level. Most importantly , San Francisco is located in
seismically active zone with seven sub parallel fault lines surrounding the
city. In 2010 , construction of Tunnel and transit terminal started near to
this building that dewatered the  soil
effectively.

·       
About
structure :  As mentioned earlier , this tower consist of tower and mid
rise , both have different foundations. The tower foundation consist of 10 foot
thick pile cap supported by about 950 precast concrete piles, approximately 80
feet in length. Whereas mid rise supported on mat foundation of 6 to 8
feet  varying thickness. The mat
foundation of mid rise resist the uplift hydrostatic pressure while tower have
to face these types of pressure. Therefore, settlement expected to tower
building. Expected settlement for this building by structural engineer is about
12 inches during its whole life time . The settlement of this tower in
different years are given below :

Year

Settlement (inches)

2008

6

2009

9

2016

16

2017

Almost 17

According to the above details , it
is can also be observed that more settlement occurs from 2009 to 2016.

·       
Reasons
of 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 there
response are different against uplift hydrostatic pressures and other soil
conditions etc.

2.     The dewatering of soil during 5 years
by the construction of tunnel near to this tower. The leaning of this tower due
to settlement is shown in the figure given below  :

 

 

 

 

 

 

 

3.     By 2014 and  2016 research report,he leaning of tower is
considered as safe and not at risk during an earthquake event by Millennium
partners and they declared that the settlement is due to the  increased in stress on pile’s cap by
approximately 50 % at some locations in mat foundation (which may be due to
seismic pressure).

 

·       
Solution
for settlement :  The solution to ensure the safety of tower during strong
earthquake is invoved the reinforcement of foundation mat cap.They proposed to
run the reinforced walls from the core of the building into two basement
levels, creating a triple decker foundation .

·       
References
:           http://sfgsa.org/sites/default/files/Document/MillenniumTowerSafetyReviewReport.pdf

 

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