Articles
"Load Transfer to Hang
Five Floors Is a Success", Construction.com, (c) Nadine M. Post, 3/27/06
Getting up at the crack of dawn on a chilly Saturday to witness subtle
structural gymnastics turned out to be rewarding for the team building a
12-story addition to the Children’s Hospital of Philadelphia. The jacking
operation that would hang five floors from a 10th-floor truss system went off
without a glitch.
The exercise, though complicated, wasn’t the overarching challenge of the
$200-million West Tower expansion project, which includes the 225,000-sq-ft
northwest addition. The biggest headache was having to keep the hospital fully
functional. That resulted in a $364-per-sq-ft cost, almost double that of a
“greenfield” hospital, say officials with construction manager L.F. Driscoll
Co., Bala Cynwyd, Pa.
The “hanging floors” system was needed to create a fourth-floor operating
room free of interior columns, says Ted Przybylowski, project executive for the
local architect, Robert D. Lynn Associates. The scheme would also provide a
double-high-ceiling cafeteria on the first floor, free of interior columns.
Driscoll could not hang the five floors without building them first on
temporary supports. That would have been easy enough to do, except that the
hospital’s loading dock, which had to remain active throughout, was directly
below the site for the addition. Erection columns could not go through the
loading dock.
Another restriction was that none of the columns of the addition, during
construction or otherwise, could penetrate the existing post-tensioned concrete
beams of the ceiling of the loading dock.
The solution, provided by project structural engineer Cagley Harman &
Associates, was a system of erection trusses under the third floor to
temporarily support the upper floors. The trusses would bear on permanent
supports.
The addition has a 190x85-ft footprint. To resist lateral loads, the steel
structure has braced frames in both directions. In the long direction within the
west face, there is a so-called Super-X truss, with a 96-ft clear span at the
base, in the middle of its 140-ft length and 145-ft height. The Super X consists
of a lower truss, 28 ft deep and 26 ft above grade, and an upper truss, 20 ft
deep and 190 ft long, at the 10th mechanical floor. Additional braces connect
the trusses, forming the Super X. The Super X derived its odd shape to conform
to the architecture of the glass curtain wall.
The 10th-floor trusses running east-west, which hang five floors and support
four above, consist of seven rows of trusses, 20 ft deep. They span 54 ft
between the Super X’s upper truss and interior columns.
Hangers to support the suspended floors from the trusses were initially
designed as columns. The lowest lengths, on the third and fourth floor, were
temporary. They bore on the 54-ft-long erection trusses. The top chord of each
was designed to remain as a 36-in.-deep beam.
“The hangers were working as compression members until the load transfer,
when they turned into tension members,” says John G. O’Donovan, project engineer
for the King of Prussia, Pa.-based structural engineer.
Workers from steel contractor Samuel Grossi & Sons Inc., Bensalem, Pa., first
installed shores on which to erect the lower portion of the Super X. Using a
tower crane, they then erected the necessary framing and the seven 13-ft-deep
erection trusses, in the short direction, under the third floor. They then
erected the remaining superstructure.
The structural engineer specified the load that was going to be required of
the hydraulic jacking system and incorporated a 1-in. gap in a splice in the
fourth floor erection columns to accommodate anticipated truss deflections
during the transfer. The deflections were a combination of the temporary truss
rebounding when the load was removed and the downward deflection of the
superstructure due to the permanent dead load.
For the synchronous jacking of a pair of erection columns at a time, Grossi
installed two 150-ton jacks per erection column. After loosening nuts of the
bolted splices, workers put 350 kips maximum force on the jacks, increasing
pressure in 500-psi increments, to take loads off the erection trusses. After
removing appropriate bolts, workers reduced jacking pressure in 500-psi
increments. Surveyors, who monitored movement as the load was transferred to the
10th-floor trusses, measured 1/4-in. deflection and 1/8-in. rebound on each
column pair—exactly the engineer’s prediction. The operation, which began March
18, was finished March 20. Workers have already removed 50 tons of temporary
steel.
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