Case Study of Interstory Drift on a Typical Building in Chile

Isidora Foster Building (Fig. 13)

Project overview: slender floor plan, 25 floors above grade, eight floors under grade, total height: 80 m (262 ft) above grade. Completed in 2004, location: El Golf area, Las Condes, Santiago.

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Project name, location, and date

Torre Parque Arauco loca­tion: Las Con – des, Santiago two buildings, each with 22 floors above grade, com­pleted 2009

Nueva Las Condes (NLC 6&8) location: Las Condes, Santiago two buildings, each with 21 floors above grade, completed 2010

Height and
width of
frames

Frame sizes: 600 x 3240 mm,

1200 x 3240 mm,

1800x 3240 mm, total height 60 m above grade, setting blocks

Frame size 1380x 3300 mm, setting blocks at bot­tom

Project name, location, and date

Isidora Magda­lena building location: Las Condes, San­tiago one building with 25 floors above grade, com­pleted 2008

Magdalena con Riesco build­ing location: Las Condes, Santiago one building with 25 floors above grade, com­pleted 2009

Project name, location, and date

Las Torcazas con Apoquindo building loca­tion: Las Con – des, Santiago one building with 18 floors above grade, completed 2008

Clinica BiCen – tenario build­ing location: Las Rejas con Alameda, San­tiago one building with 21 floors above grade

Height and
width of
frames

Frame size 1300x 3200 mm, setting blocks at bot­tom

Frame size 1250x 3500 mm, setting blocks at bot­tom

FIG. 13—Isidora foster building.

Description of curtain wall system:

• CW type: four-sided SSG system; window wall type, anchored between slab with about 20 mm clearance,

• CW area: 7000 m2 (75,000 ft2),

• Interstory height: 3200 mm (10.5 ft), glass width: 1300 mm (4.3 ft),

• Glass type: IG units of AN 6 mm/12 mm, AS/AN 4 mm (vision and spandrel),

• Was reported to have suffered no damage during the earthquake.

Seismic conditions and characteristics of the building:

• Seismic zone no. 2: effective acceleration value A0 = 0.3 g,

• Soil type II, Ro = 11,

• Fundamental period of the building, T = 2.5 s (courtesy Rene Lagos Engineers, Santiago),

• Response modification factor (R*) = 10.6 (from Fig. 12, with T = 2.5 s and above soil and seismic zone conditions). However, to satisfy the base shear stress requirements mentioned previously, the calculated drift must be multiplied by R* and divided by 3.0 to arrive at the actual estimated drift. Therefore, the final R* value to use is 10.6/3 = 3.53.

Lateral displacement calculation (worst case):

• Assuming the total interstory drift allowed by the NCh433 code = 3 % (which is the sum of lateral displacement of 2 % and torsional effect of 1 %),

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• The calculated allowable interstory drift is then = 3 % of 3200 mm = 9.6 mm (0.38 in.),

• And the actual interstory drift will be 9.6 mm x 3.53 = 33.9 mm (1.33 in.),

• Considering that the four-sided SSG glass will rotate about its center of mass, we can calculate that the shear displacement at the location of highest strain (corner) is = 16.9 mm.

Lateral displacement calculation (from dynamic spectral analysis; courtesy Rene Lagos Engineers, Santiago):

• Calculated interstory drift ratio (drift/height) = 0.00081 (by FEM software),

• NCh433 calculated interstory drift = 0.00081 x 3200 mm = 2.6 mm (with reduced seismic response spectra),

• Therefore, the actual interstory drift is 2.6 mm x 3.53 = 9.2 mm (0.36 in.)

Summary Relative to Isidora Foster Building

The maximum allowable interstory drift according to NCh433 would be 33.9 mm (1.33 in.). If we compare the actual estimated drift value for this building (9.2 mm) with the maximum allowable (33.8 mm), we find significant reserve capacity.