Evening star., 1922-02-12
BY COL. r. Nj. ANDERSON,
Engineer In Charge Of Concrete Ves
sci Construction, War Department.

Engineer in Charge Of Concrete ves-
sci Construction. War Department.
Written for The Sunday starl
The writer has read with intense
interest and enlightenment the de-
tailed and mastery report by Theo-
dore Condron covering the probable
causes of the knickerbocker disaster,
as well as the thorough analysis re-
ported by the Army and Navy en-
gineers. The defects reported cannot
be disputed after careful analysis of
the various details of the structure.
From personal examination. however.
the writer is firmly of the opinion
that the initial cause of the disaster
has not yet been reported.
Reports so far published have en-
phaSized the weakness of the various
structural steel members, as we'll as
the arrangement of their assembly
and the weakness of the supporting
walls and fastenings. No serious crit
.icisms have been made of the con-
crete roof slab except brief reference
to the method Of placing reintoreihg
steel on supporting beams MY only
purpose. therefore, in making further
report as to the probable cause OF this
disaster is to bring to the attentlon
of the profession, and the public gen.
orally. what appears to have been the
weakest point of the structure.
For the benefit of those unfamiliar
with the technical details of rein
forced concrete slab construction the
writer will endeavor to describe IN
non-technical terms and by simple
drawings how the original roof was
constructed and the probable success
sion of failures which caused this
terrible disaster.
Differ Over Cause.
Figue 1 is an outline plan OF the
theater building, showing the steel
roof structure which supported the
concrete roof slab, with notations,
showing sizes of individual steel
opinion that Bal. connecting columns
N2 and C-3. together with its support
over the main part of column CO.
was the weakest part of the steel
structure and probably caused the
initial failure. The Army and Navy
engineers report that in all probabil-
ity the tile wall supporting l beam
B-21 in the Columbia road or west
wall at the left of the stage was
crushed under it's load, thus allowing
this beam to fall, leaving the concrete on
roof slab between beams B-2O and
B-22 unsupported. which caused it's
failure and consequent topping of
the entire roof structure
After careful examination of such
records as are available in the office
of the inspector of buildings. and ex-
animation of such materials as are
available at the structure. as we'll as
examination of the remaining part OF
the structure, it has been determined
that the entire roof was composed OF
one continuous reinforced concrete
slab resting directly upon the beams
and trusses, shown in figure ]. At
the surrounding walls, which project
from one to two feet above the roof.
and between beams and trusses, this
slab rested on a ledge Of hollow tile
from one to three inches wide. In
order to pour this roof slab in place
the spaces between the top fianges OF
beams and trusses were hoored over
with boards to receive the soft con-
crete and later removed when the con-
own weight.
Reinforced With Mesh.
This slab varied in thickness from
two and one-half to three and one
half inches, and was reinforced by
three-sixteenth-inch wires. spaced
three inches apart, running in an
easterly and westerly direction across
the shortest spans of the roof. In
northerly and southerly direction
these slabs were reinforced by wires
one-eighth of an inch in diameter
spaced twelve inches apart. This steel
reinforcement is a commercial prod
uct known as Clinton wire mesh and
is manufactured in various weights
and widths for such purposes. The
small cross wires are welded to the
heavier longitudinal wires at all in
tersections in order to keep the spac-
ing of the steel uniform while plac-
ing the concrete.
The wire mesh reinforcement used
in this roof structure is said to have
been four feet In width, and is ship
ped in large rolls. This wire mesh
is unrolled and laid upon the roof
forms before the concrete is poured.
and an effort is made to have this
steel embedded in the finished con-
Gxete slab about three-eighths OF an
inch from its lower surface. 1t has
been found, however, that some of
this steel was as much as one and
three-eighths inches from the lower
surface. and an engineer will readily
appreciate that such increase in the
height of steel tremendously decreases
It's effectiveness and ultimate strength
Of the slab.
This concrete roof slab was somewhat
higher directly over the center OF the
roof, SO as to shed water to the east and
west walls. The slope Of this roof ap-
pears to have been approximately four
inches in ten feet. The entire slab was
covered with tar paper. Tapped at joints
and tarred SO as to be rainproof, and
then covered with about one-half inch
cnrrTAT NOTIFIES