Evening star., 1922-02-12

termine the strength of a reinforced
concrete slab when all important
factors governing it's construction
are known, such as the actual COM
pressive strength of the concrete. the
thickness of the concrete and the
size and actual locati of the steel
reinforcement. Assuming that the
reinforcing steel in the concrete slab.
which is herein described as having
caused the initial failure. was placed
at it's most effective position. namely
'3-S inch from the bottom of the
slab. and assuming that the slab had
an average thickness of three inches
and that the compressive strength of
the concrete was approximately 1900
pounds per square inch. it is de-
termined that the steel at the point
marked w" In figure 2 was stressed
to its elastic limit by the normal
loads which 1t was required to carry
The exact weight of the ceiling and
Stag roofing supported by the slab
at this point is. unfortunately. inde-
terminate at this time.
Sees Chance for Drift.
Referring again to figure l, the at
tention of the reader Is called to the
Iact that a ventilator. approximately
feet in diameter and feet high, is To-
Gated directly over the center of the
stage roof, and to the north of this
mouse, approximately feet high, with
proportions approximately as indicat-
ed. The roof slab. which is believed
to have failed, is approximately one
root below the adjacent roof slab COW
er1ng the stage. 1t will be recalled
that the Slight wind which brought
the unprecedented snowstorm of Jan
uary 20 was from a northeasterly di-
rection during practically the entire
afternoon and evening. 1t will be
readily seen that a calm area would
prevail In the vicinity of the point
marked wv" both due to the fact
that this roof was one foot lower than
the adjoining roof, and also due to
the structures upon the roof. This, to-
gether with the raised wall and cor
nice on the west side, would, in all
phrooaoi1ity aoin. cause & drift to form at
which is also problematical.
Concrete Is a most valuable substi-
tute for steel and wood in construc-
tion work. However greater factors
OF safety must be allowed in concrete
than In other types of construction
material, for the reason that its ulti-
mate strength depends upon many i'm
portant factors, such as the condition
and quality of cement used; the ac-
tual quantity OF cement in proportion
to sand and gravel; the relative por-
ttons of cement, sand and gravel, de-
peHo!hS upon tuelE respective sizes;
the cleanliness of the sand and gravel,
and the consequent adhesive and bond
ing strength Of the cement in the con-
crete; the completeness of the mixing
of the concrete; the consistency of the
concrete as regards its moisture con-
tent and its freshness at the time of
pouring; the amount of pudding or
tamping done to secure compactness
of the concrete; the exact location of
the steel in the finished concrete slab,
and also the degree Of disturbance, If
any, of such concrete during its period
of initial set.
Cannot Blame Wall.
Since so many contributing factors
affect the ultimate strength Of a fin
ished piece of concrete, it is common
practice to allow from two to three
times greater factor of safety in this
material than Is customary in steel
structures. This point is emphasized d
for the reason that apparently the
same factors of safety have been OBJ
served in the design of this concrete
roof slab as were used in other forms
Of material employed in this construc-
ton.
The writer cannot agree with re-
ports to the effect that the Columbia
road, or west wall, was the direct
cause Of this failure, due to it's move
ment in an outward direction, caused
by temperature changes Of the roof
slab or by weakness Of the wall itself,
for several very conclusive reasons.
First. Truss TIL rested in this wall
on q twenty-inch I-beam, set in the
hollow tile and secured only by NOR
tar Had this wall moved outward
appreciably before the failure this
I-beam would have been disturbed in
it's seating, or there would have been
evidence Of scoring on its top surface.
Second. Had this wall moved out
ward appreciably before the failure a
decided leak would have been caused
in the roof adjacent to the inner side
Of the wall, since the concrete roof
slab rested on a ledge Of hollow tile
only from one to three inches wide,
and the slope Of the roof carried ali
the run-off water west of truss TIL
to the point OF intersection of the roof
with the western wall.
Believes Crack Was New.
Third. The two-inch crack which
appears at the top of the proscenium
wall and the west or Columbia road
wall extends downward to the floor,
showing very plainly on the finished
wall Of the theater below the ceiling.
Had this crack existed prior to the
failure it is almost certain that It's
evidence would have been reported.
5 Fourth. a similar crack In