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TRANSAMERICA PYRAMID, San Francisco

 
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Dr. N. Subramanian
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PostPosted: Wed Aug 22, 2012 1:13 pm    Post subject: TRANSAMERICA PYRAMID, San Francisco Reply with quote

TRANSAMERICA PYRAMID, San Francisco


Buildings are complex structures. They are made of multiple elements and components that are stressed and interact with one another when shaken by an earthquake. Buildings vary widely in size, geometry, structural system, construction material, and foundation characteristics. These attributes influence how a building performs when the ground shakes.

The 1989 Loma Prieta earthquake (magnitude 6.9) set San Francisco’s Transamerica Pyramid swaying and rocking. Though the top floor of the 49-story Transamreica Pyramid swayed more than one foot from side to side in the 7.1-magnitude 1989 San Francisco earthquake, no one was injured nor the building damaged.

An array of 22 sensors (small arrows) installed by the U.S. Geological Survey in the steel-frame structure documented that the horizontal displacement on the 49th floor of the building was five times the 1a inches measured in the basement, as indicated by the recordings (red lines). No significant twisting of the building was measured due to the symmetry of the building about its vertical axis. Located 60 miles from the epicenter of the quake and designed to withstand even larger shocks, the building was undamaged. Earthquake records from buildings, such as those from the Transamerica Pyramid, allow engineers to verify mathematical models used to predict deformation of a structure from a given pattern of shaking of its foundation.



Figure 1. Transamerica Pyramid.
Introduction
The Transamerica Pyramid is the tallest skyscraper in the San Francisco skyline and one of its most iconic. Although the building no longer houses the headquarters of the Transamerica Corporation, it is still strongly associated with the company and is depicted in the company's logo. Designed by architect William Pereira and built by Hathaway Dinwiddie Construction Company, at 260 m , upon completion in 1972 it was among the five tallest buildings in the world.  Designed by architect William Pereira, it faced considerable opposition during its planning and construction, and was sometimes referred to by detractors as "Pereira's Prick".[6]

Design
The land use and zoning restrictions for the parcel limited the number of square feet of office that could be built upon the lot, which sits at the northern boundary of the financial district.

The building is a tall, four-sided pyramid with two "wings" on either side to accommodate an elevator shaft on the east and a stairwell and a smoke tower on the west. The top 64.6 m  of the building is the spire. There are four cameras pointed in the four cardinal directions at the top of this spire forming a virtual observation deck. Four monitors in the lobby, whose direction and zoom can be controlled by visitors, display the cameras' views 24 hours a day. An observation deck on the 27th floor was closed after the September 11, 2001 attacks, and replaced by the virtual observation deck. The top of the Transamerica Pyramid is covered with aluminum panels. During the holiday season of Thanksgiving, and Independence Day, a bright, white light is seen on top of the pyramid. The building was built on a special base platform that allows it to reduce shaking from earthquakes. While it gradually reduces shaking, some shaking still intrudes the building.

The Pyramid has 49 stories and a 212 foot spire. The height from the apex to the ground is 845 feet. The base width is 175 feet.
The pyramid is flanked by a set of structures that look like wings. These structures start at the 29th floor and are both for form and function. The eastern wing is an elevator shaft. The western wing has a stairwell.

The unique structural feature of this tapered building is the truss system above the first floor, as shown in Figures 2 and 3.
Note that that photos in Figures 2 and 3 are courtesy of the Earthquake Engineering Research Center, University of California, Berkeley.

       


Figure 2. Truss System at Base of the Transamerica Building

The truss system supports both vertical and horizontal loading. The building is carefully engineered to take large horizontal base shear forces. Note that the nearby San Andreas and Hayward Faults are sources of major earthquakes.
       
Figure 3. Horizontal X-bracing
The overhead X-bracing resists torsional movement of the building about its vertical axis.
In addition to the exterior frames, four interior frames in each direction extend up to the 17th floor and two interior frames in each direction continue to the 45th floor.
The building is constructed of steel, reinforced concrete, and glass.
Loma Prieta Earthquake
The Transamerica Pyramid successfully withstood the Loma Prieta earthquake as reported in Reference 2:
The 1989 Loma Prieta earthquake of magnitude 7.1 struck the Santa Cruz Mountains in California. Sixty miles away, in downtown San Francisco, the 49 story Transamerica building shook for over a minute. During the quake, the top story swayed over 12 inches from side to side. Yet the building was undamaged and no one was seriously injured.
Modal Natural Frequencies
The natural frequencies of the Transamerica Pyramid are given in Table 1. These frequencies were obtained by measurement as documented in Reference 2.

Table 1. Transamerica Pyramid Natural Frequencies
_______________________________________________
Mode         East-West           North-South          Torsion
                         (Hz)                    (Hz)                  (Hz)
________________________________________________     
1                   0.330                   0.337            0.447
     
2                  0.616                   0.630            0.814

3                 0.843                    0.880           1.06

4                  1.10                      1.11              1.27
     
5                   1.50                       1.52            -
     
6                  1.72                        1.73           -
_________________________________________________     
The first mode shape is shown in Figure 4.


Figure 4.

Other details of the Building
    

  • The building's façade is covered in crushed quartz, giving the building its pure white color.
  •     The four-story base of the building contains a total of 16,000 cu yd (12,000 m3) of concrete and over 300 mi (480 km) of steel rebar.
  •     It has 3,678 windows.
  •     The building's foundation is 9 feet (2.7 m) thick and was the result of a 24-hour continuous concrete pour.
  •     Only two of the building's 18 elevators reach the top floor.
  •     The original proposal called for a 1,150 ft (350 m) building, which would have been for one year the second-tallest completed building in the world. The proposal was rejected by the city planning commission on the grounds that it would have interfered with views of San Francisco Bay from Nob Hill.[4]
  •     The building occupies the site that was the temporary home of A.P. Giannini's Bank of Italy after the 1906 San Francisco earthquake destroyed its office. Giannini founded Transamerica in 1928 as a holding company for his financial empire. Bank of Italy later became Bank of America.
  •     There is a plaque commemorating two famous dogs, Bummer and Lazarus, at the base of the building.
  •     The hull of the whaling vessel Niantic, an artifact of the 1849 California Gold Rush, lay almost exactly beneath the Transamerica Pyramid, and the location is marked by a historical plaque outside the building (California Historical Landmark #8Cool.
  •     The aluminum cap is indirectly illuminated from within to balance the appearance at night.
  •     The two vertical external extensions allow preservation of useful interior space at the upper levels. One extension is the top of elevator shafts while the other is a smoke evacuation tower for fire-fighting.[8]
  •     A glass pyramid cap sits at the top and encloses both aircraft warning light and a seasonal white beacon.
  •     At certain times of the year the glass cap will briefly cast a reflected sunlight gleam onto traffic crossing the San Francisco – Oakland Bay Bridge


References
1. Western States Seismic Policy Council Earthquake Quarterly - Spring 1998.
2. The Recorder, Vol. II - Issue 1, Kinemetrics, Inc.

3. http://www.vibrationdata.com/earthquakes/Pyramid.htm
4. http://pubs.usgs.gov/fs/2003/fs068-03/irr.html
5. http://en.wikipedia.org/wiki/Transamerica_Pyramid
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