Flat plate/pipe column
structure for
low-and medium-rise buldings
Those hard-to-manage
columns can be hidden
in the partitions of an
architectural layout
BY FREDERICK P. WIESINGER AND
EUGENE P. HOLLAND,
WIESINGER-HOLLAND LTD.,
STRUCTURAL ENGINEERS,
CHICAGO, ILLINOIS
he structural procedure described here was devised as
a solution to the difficult
task of fitting columns into
the architectural layout of low- and
medium-rise reinforced concrete
buildings. It consists of a thin reinforced concrete flat plate supported
on pipe columns and has been used
successfully by the authors in
countless apartments in the Chicago area.
Traditionally, low-rise apartment
buildings of two to five stories have
been constructed of ordinary building materials—wood stud interior
walls, wood joists and exterior masonry walls. The much debated
four-plus-one buildings allowed by
the city of Chicago have been built
with these materials for years, as
have thousands of these buildings
in the suburban areas. Precast voided concrete planks bearing on masonry walls, with topping to provide
a level floor surface are also extensively used, and unless the architect
or developer accepts the joint lines
between planks, a ceiling is also required. Needless to say, the cost of
this structure far exceeds the cost of
o rd i n a ry construction, thereby requiring the developer to reduce his
T
profit margin. Bar joists with a concrete slab and ceiling are sometimes
used but appear to be in the same
cost category as the voided plank
construction.
The spring and summer of 1968
saw a tremendous leap in the cost of
lumber. Architects, contractors and
developers began searching for less
costly altern a t i ve s. The flat
plate/pipe column idea seemed
worthy of investigation, and the
Dana Point Apartments in Arlington
Heights, Illinois (a $15,000,000 fourand five-story development) offered
an excellent opportunity for contractors and subcontractors to try
new procedures. The data presented here describe some of those
used.
An obvious starting point for the
flat plate/pipe column scheme is
the three-and-five-eighths-inches
space available in the interior of a
standard partition because it restricts the column size to nominal
three-inch pipe, with three and onehalf-inches outside diameter. Such
columns can be located without
causing bulges, and it is not necessary to place them into closets and
c o rn e r s. Thus they can be placed
relatively close to one another. Consequently, the largest necessary
span is usually determined by the
width of the living room. Preliminary calculations for the Dana Point
development indicated that the
loads would be within the capacity
of the pipe columns.
Compared to conventional flat
plate construction, smaller spans
lead to a thinner slab and to reduction in reinforcing. Article 2101 (e)
of ACI 318-63 specifies at least fiveinch thickness, for which a maximum span of 15 or 16 feet is feasi-
Figure 1 Pipe Columns
Figure 2 I-beam capital
Figure 3 Sleeve extends through
slab to receive the column above.
ble. Furthermore, it is clear that the
support size required by peripheral
shear in the slab can readily be obtained by welding a head on top of
the column.
The structure for Dana Point I is a
five-inch thick cast-in-place reinforced concrete flat plate, supported
on pipe columns with I-beam column capitals three and one-half
inches wide, six inches deep and
two feet long below the slab (Figure
1). The columns vary from threeinch diameter standard to extra
heavy pipe entirely within the conventional partitions, as do the capitals (Figure 2). The partitions provide the necessary fire protection
for both columns and capitals. A
sleeve is welded to the top of the
capital. This extends through the
slab to receive the column above
( Fi g u re 3). The slab is reinforced in
accordance with the requirements
of the American Concrete Institute
Code, utilizing a uniform mat of
welded wire fabric in the bottom of
the slab, with supplemental bottom
reinforcing bars where re q u i re d .
The top reinforcing is conventional
and consists of reinforcing bars. The
exterior masonry walls are loadbearing as well as resistant to lateral
f o rc e s. Floor covering (carpet and
tile) is applied directly to the slab.
The underside of the slab is painted.
At Dana Point, the contractor
elected to place the columns from
the deck above by simply dropping
them through slots cut in the deck
into the sleeves below. The original
intention was to weld the columns
to the sleeves but when the cost became known it was decided that the
only reason for the welding would
be long-standing engineering custom. The weld was omitted and
gravity alone does the job. After the
forms for holes, plumbing sleeves
and such were in place, the deck
was covered with the bottom mat.
Electrical conduits were put in place
next, followed by the top reinforcement. Concrete was placed in the
usual fashion.
Additional advantages devised
With the successful completion of
the first phase of Dana Point, the advantages of the flat plate/pipe column became apparent to other developers. Because of this growth in
interest, the authors’ firm was
prompted to study further the various parameters of the procedure
and arrive at variations that not only fit the different types of architecture but also facilitate construction
and offer further reduction in costs.
Experience with building the loadbearing walls, which required a
great deal of coordination and field
control as well as use of highstrength masonry materials, led to
providing pipe columns in the exterior walls. The exterior masonry enclosure, acting as infilled shear panels between the columns, provided
the lateral bracing.
Bracing
A further technique was the use of
scattered braced bents. Steel pipe Kbracing (Figure 4), located in the
partitions between columns, provides the braced-bent action. The
bents must be placed at varying locations at each floor level to avoid
the accumulation of wind compression and tension forces in the
columns supporting the braced
panels. It developed that it is possible to base the design on the criterion of never allowing more force on
the column due to wind than the
amount that can be absorbed by the
33 percent increase in allowable
stresses for wind. Simple formulas
have been developed that determine the number of bays per floor
that must be braced to accomplish
this. For example, for a six-story
building there is a single-K brace
needed on the ground floor for
e ve ry 40 linear feet of wind exposure, one for every 45 feet on the
second floor, and gradual increases
to one for every 126 feet on the floor
below the top floor.
Reinforcement
The speed and ease with which
the bottom reinforcement (welded
wire fabric and supplemental bars)
could be placed pointed up the slow
and laborious procedure for setting
the top reinforcement, which, on
the first buildings, consisted entirely of bars. Attempts at using welded
wire fabric for top re i n f o rc e m e n t
proved to be uneconomical initially
because of the ACI Code requirement for providing middle strip
negative reinforcement (Figure 5).
The most efficient way to place
welded wire fabric in this location is
to extend the top re i n f o rc e m e n t
from over the column heads to the
middle strip areas. Although this allows for simplified setting, the total
amount of welded wire fabric is increased considerably, thus increasing cost. This factor led to the study
of eliminating the top steel in the
middle strip areas and providing for
the total negative moment by increasing the reinforcement over the
column head.
Such departures from the standard methods specified in the ACI
Code, and indeed from the standard
practice, could be justified only on
the basis of thorough, detailed
analysis, research, and expensive
tests. The Portland Cement Association committed the necessary test
funds, expertise and manpower.
The results of the tests were subsequently reported. * They proved
conclusively that for apartment
loading with short spans and thin
slabs the middle strip negative moment area can be left entirely without reinforcement. Thus on subsequent
projects
the
top
reinforcement consisted entirely of
a piece of welded wire fabric combined with a few reinforcing bars
over the columns (Figure 6). This
arrangement proved to be most
economical and convenient.
In-slab shearhead devised
The below-slab shearhead as discussed proved to be somewhat of a
nuisance, due to the fact that
columns located adjacent to a doorway required a soffit to enclose the
head. The possibility also existed
that the shearhead would be misori* Cardenas, Alex E.; Kaar, Paul H.,
“Field Test of a Flat-Plate Structure,”
ACI Journal, January 1971, pp. 50-59.
ented with respect to the partition.
To counter this problem, an in-slab
shearhead was devised (Figure 7)
and has been used in most of the
later developments.
Figure 4 Steel pipe K-bracing
Random placement of columns
Other considerations re q u i re d
d i f f e rent kinds of innova t i o n s. It
became apparent, for instance,
that on some projects it is not desirable to locate columns on a grid
p a t t e rn. Ra t h e r, they must be
placed in a completely scattered or
random fashion. This situation is
not cove red by the ACI Code and
thus special design pro c e d u re s
were developed.
Cost comparisons
Sixteen projects totaling over four
million square feet have been constructed with the above described
p ro c e d u re s. Detailed cost comparisons made by the developers for
s e ve ral sizes of bays have been updated to the latter part of 1971 (Figure 8). While it is hardly possible to
generalize, it can be concluded that
costs with the flat plate/pipe column method are presently about
fifty cents per square foot more than
o rd i n a ry (wood) construction. A
number of developers decided that
superior quality, lower maintenance
costs, improved sound transmission
p ro p e rt i e s, and reduced fire insurance rates are worth the cost differential, even where local building
codes allow ordinary construction.
On the other hand, when the
comparison was made with any of
the other building types, such as
voided slab, bar joist or conventional flat plate, the flat plate/pipe column method proved to be the more
economical, in the majority of cases, if no more than one- or two-hour
fire rating is required. For more than
two-hour ratings, the cost of fireproofing may tip the balance in the
direction of concrete columns. In
such cases, precast or cast-in-place
columns have been used, keeping in
mind the fact that the economic advantages of the method presented
in this article are due, to a significant extent, to the use of thin slab
FLAT PLATE TOP REINFORCEMENT
Figure 5 Top reinforcement at column head
and between columns
ACI Code
Figure 6 Top reinforcement at column head
only
Wiesinger-Holland
Figure 7
IN-SLAB SHEARHEAD
Figure 8
FLAT-PLATE COLUMN
STRUCTURAL COST
COMPARISON
Structural cost per square foot
of supported slab. Includes
footings, slab on grade, pipe columns,
wind bracing, and structural slabs.
Does not include land, fees or
general contractors’ overhead and
profit. Transfer for parking
not considered.
reinforcement as described. The
same consideration is applied to
buildings that are too tall for pipe
columns to support the loads.
Most recent innovations
Fu rther innovations in the flat
plate/pipe column method are
now being utilized. On-site precast
s l a b s, apartment size or slightly
smaller, are being lifted to form the
apartment floors. The slabs for another thre e - s t o ry, twe l ve - u n i t
building are supported on bearing
walls. The change from the bearing
wall to the pipe column type of
support will be made on two projects now in the design stage. Sixs t o ry, one hundred-unit apartments in a Chicago suburb and
t h re e - s t o ry, twe n t y-unit apartments in New York will utilize special multiple-purpose inserts that
function as a lifting device as well
as the column connector. Developers are projecting a minimum fifty
cents per square foot savings by use
of the on-site precast slabs. The
large size panels allow all joints to
fall at the partition lines, thus elim-
inating the need for topping, and
also eliminating the problems connected with joinery. Casting of the
panels stacked on top of one another will save the expense of forming and shoring and is expected to
lead to economies in field labor
and materials handling—eve n
when measured against the cost of
erection.
PUBLICATION #C720380
Copyright © 1972, The Aberdeen Group
All rights reserved