Boxoffice-November.24.1951

I tonnage
I
member
:
Crete.
'
I
I tensile
I
I The
,L ment—usually high-tensile strength steel
°, *
^'
I wires— is stretched so as to place the con-
Crete in compression. This enables the cono
' Crete to eliminate or better withstand the
f
stresses to which it is subjected
f
,*'
""^i I WIRE
'"');
*'*!
1
under heavy loads.
REDUCES STEEL TONNAGE
amount of wire used as reinforcement
In prestressed concrete is only a frac-
I tlon — usually about one-fourth — of the
of steel reinforcing bars needed
for a corresponding structure or structural
of conventional reinforced con-
While the unit cost of the steel wire
is higher than that of bars, the amount
and weight used is considerably less, so
I
that in the long run, materials costs are
usually less for prestressed concrete. The
most important features of prestressed concrete
are that it breaks down previous
limitations on the spans and loads for
which a concrete structure can be economically
designed, and that the concrete
is made free from tension and from cracking.
Also, of great importance today, is
the fact that there is
the weight of
steel required.
Prestressed concrete, like
actually a saving in
thin-shell roof
construction, has found its greatest use in
England and on the European continent.
Wartime and postwar materials shortages
made its use practical for rebuilding and
restoration work and for new construction.
While prestressing techniques had
been used in this country in construction
of circular tanks and pipe, it was not until
the start of the Walnut Lane Bridge in
Philadelphia In late 1949 that prestressed
concrete was used for other types of structures.
Since that time, interest has pyramided
among American engineers. More is
being learned about prestressing practices
and procedures, and it is almost certain
that it will find increasingly wide application
in this country in the coming years.
METHODS THOROUGHLY PROVED
Both thin-shell roof construction and
prestressed concrete are thoroughly proved
methods of construction. Both save steel;
both require less concrete than conventional
reinforced concrete: both are eminently
practical for theatre construction.
An excellent example of how steel can
be saved through use of these types of construction
is a hangar at Karachi, Pakistan.
Not unlike a theatre, the hangar required
a large unobstructed ground floor area,
and in addition an unusually wide door
opening for the admission of large planes.
The roof was built of barrel shells of concrete
only two and one-half inches thick,
utilizing much less steel than would normally
have been required. The prestressed
concrete girders over the hangar doors
were called upon to span openings 200 feet
wide, and required 10 tons of steel each.
But if steel girders of the same span and
depth had been used, it was estimated that
they would have weighed as much as 100
tons each ... or fen times as much! Thus
in the two girders alone there was an
estimated saving of ninety per cent in steel.
In addition to these two methods of
construction, there are several general
ways in which steel can be saved, more or
less common sense ideas that will occur to
most architects. They may not .seem
dramatic, but this does not detract from
I heir effectiveness.
A few steel-saving suggestions include:
elimination of wasteful overdesign in walls,
columns, beams and floor slabs: reduction
of roof and floor span lengths where
possible: greater use of arches: use of concrete
masonry for bearing walls as well as
for "back up" and interior partitions: Increasing
the strength of the concrete to
reduce the amount of compressive reinforcement
needed: use of control joints to
reduce wall reinforcement, and the use of
plain concrete wherever possible, such as
in footings and below surface construction
where mass concrete sections may often
be used to eliminate all steel.
GOOD DESIGN CAN SAVE STEEL
Sometimes ways of saving steel are
readily apparent. For example, in World
War 11 the beforementioned methods and
similar procedures were used in the redesign
of five military warehouses from
steel frame to reinforced concrete, and
resulted in a net saving of 67 per cent of
the tonnage required in the all-steel design.
Such large savings as this cannot usually
be hoped for in the construction of the
average theatre, where a goodly amount of
concrete is normally employed. But substantial
savings can be made through good
design and through careful attention to
small as well as large items. No saving,
regardless of how inconsequential it may
seem at the time, should be overlooked.
Nor should an idea be discarded as not
worthy of redesign until every portion of
the theatre has been carefully checked.
The difference between a completed
structure and one never started may lie in
overlooking the obvious or in discounting
as "new fangled" procedures and methods
which have been thoroughly proved.
NPA Building Restrictions
Apply to
Used Materials
Exhibitors who plan to build new theatres
and have been able to secure used
materials must still comply with NPA restrictions.
Steel and copper which may have been
secured from razed buildings must be
counted as a part of the allowable quantities
permitted under NPA Order M4-A,
namely, two tons of carbon steel and 200
pounds of copper.
This recent order removed the cost
ceiling for new theatre construction which
is now limited only by the amount of restricted
materials used.
Brick masonry is one of the most common
constructions used for permanent exterior
walls. It provides enduring and
sturdy structural elements.
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