5 - Modern Steel Construction

• are very decorative," Mead explained. In
addition, the openness of the system translates
into savings on the amount of light
fi xtures needed .
•
•
Lafayette Square
I PARKING STRUCTURES I
A project recently completed by Mrs is
the eight-story, 878-car La fayette Square
parking garage in Bridgeport, CT. "Lafayette
Square presented Mrs with a unique
design problem," according to Mead. "Because
of the size of this garage, the client
wanted a double-threaded hel ix concept to
minimize the number of circuits the driver
would have to make. Also, the double- r
threaded helix would provide one-way
traffic for more efficient traffic flow."
Compounding the problem was the
sites small footprint of 174' -wide-by-20Tlong.
"The solution was to provide a threebay
structure with two 56' exterior bays
and a 62' center bay," Mead explained. "By
ra mping all four sid es of the structure, we
were able to keep the slope of the ramps to
an acceptable 5%."
In contrast, most double-helix garages
only ramp two sides, primarily due to
drainage problems. "When you ramp all
four sides, you create a flat area. We tapered
the top flange of our beams so it
slopes W'/ft," Mead said. The supporting
castellated beams were fa bricated with a
horizontal bottom fl ange and a top fla nge
sloping from 32" deep on each end to 29"
deep at the center of the flat area. "With a
precast concrete structure you're dealing
with a plane that can' t be warped. And
with poured-in-place concrete, it would be
very difficult and very expensive to crea te
a slope."
680 Tons Of Steel
The $6.7 million structure required 680
tons of steel. Structural steel fabrica tor on
this project, and most Mrs structures, was
AISC-member Mulach Steel Corp., Bridgeville.
Accord ing to MPS, the concrete slab is
the major problem area in parking structures.
While a 4,500 p.s.i. concrete mix is
recommended by the Portland Cement Association
when de-icer is present, Mrs
opts fo r a minimum 7,000 p.s.i. concrete
mix, including silica fume admix fo r all
supported deck slabs.
"This mix greatly enhances the durability
of the concrete by strengthening it, adding
to its electrica l resistivity, and preventing
the passage of water and de-icing
chemica ls into it."
.. _ ............. .
. ...... " • ... W., 1 . • _
. . .. . ... . ••• • _ ..... , to .....
Further d urability is provided by using
a two-way post-tensioning system. If hairline
cracks should occur, they are then kept
tightly closed by the compressive stresses
of the post-tenSioned tend ons. Post-tensioning
also red uces the required slab
thickness to 5" at 18' spans. After the concrete
slab has cured, a penetrating sealer is
applied, producing a moisture repelling
surface that hard ens to resist abrasion.
The steel also is coa ted and the paint has
a life expectJ ncy of at least 15 yea rs. This
long life is accomplished first by shot-bla t
clea ning the steel and applyi ng 2.5 dry
mils of zinc-rich epoxy in the shop, then 3
to 5 dry mils of epoxy polymide in the fie ld
for the finish coat. The coating is provided
by Tnemec. D
Tire exlerior of IAfayelle
Square was desig1led to
blelld i" with
lle;ghbori/lS bllildi"8s.
The dOlllJie-helix desigll
is best illustrated byall
isometric view .
These castellated IN!ams
weigh between 60 al1d
70 lbslft .
Modern Steel Construction Septemt>cr-Oclobr 1990 51