New-York tribune., 1908-11-12

traffic provided that the heads of the trans are
spaced at least thousand feet apart
Is IL safe to permit this great bridge to be
used under such conditions? Is there sumclent
assurance that regulations as TO the spacing OF
trains will be actually observed without inter
mission? Professor Burr cites practice on the
old BROOKLYN Bridge in the spacing apart OF cars
as justification for the adoption Of similar pre-
cautionary measures on the Blackwell's Island
structure on the Brooklyn Bridge, however,
the car spactng was adopted in view Of limita-
tions in the floor system. IF through neglect
cars should he punched too closely together, the
worst to be expected would be injury of some of
the floor members or possibly of some OF the
stiffening truss members. II is almost certain
that any distress in these members from such
overlooking would become evident before serf-
Gus accident occurred. But on the Blackwelfs
Island Bridge. when official neglect-elways
possibility if not proba bity-permits cars
and trains to crowd closer than the limits fixed
above the result will be that the main truss
members iriV be 0rcrstressed
Let US not forget just what this means, It
means that unit stresses cf 10000 lbs
square inch. with no dllotrGnce for snow loads,
for wind for reverse stresses. for impact or for
secondary stresses. such the bending of mem
hers due 10 their own weight, mag be eTceeded
if trolley cars are allowed to closer than a
car length apart. Let not forget. either. that
experience at Quebec shows that important
mem bers of a truss may he loaded almost to the
point Of final failure and yet show very little
external evidence of their condition.
- We believe this matter should receive more
thorough consideration. at least before the
bridge is opened to the public There is an
other question however that seems TO us Of
even greater importance. That question is the
strength Of the compression members in the
BlackweIl's Island Bridge. VIII these safely
carry the unit stresses which the experts have
assumed? What margin of strength will still
remain when such load is applied to them?
Both the expert reports give special attention
TO the design Of these members, and we have
studied carefully their discussions of the
strength We fsil to find in this discussion any
ADEQUATE assurance that these posts and chord
members can carry safely the load # is pro-
posed to place upon them Both reports con-
clude that these compression members at
least as strong against bending horizontally as
vertically. hut the reasoning on which this
clusion is based is fatally defective in that it
leaves wholly out of account the strengthen
lack of strength--in the column latticing It is
needless 10 say that the strength of these huge
columns depends as absolutely the strength
of this Iatticing as did the unfortunate structure
at Quebec. The interests al stake are too great
and too important TO rest on the mere judgment
Of any man or set Of men as to whether this
column latticing is strong enough or not
Theories of stresses are valuable and neces-
earv. but they must be reinforced by actual test
or we shall fall into more such disasters that
which hefell the profession August %, 1907.
We call the especial attention of our readers
to Professor Burr's report this matter. We
quote:
Adverse comment has been made on the
heavy compression lower chords of this bridge
and that design therefore has been scrutinized
With great care il viii be found by
examination of the stress sheet that there are
other COMPRESSION members of the trusses which
sustain higher unit stresses with greater ratio
Of length over radius of gyration. There
he no apprehension. therefore. that these loirer
chord compression members arc IN Gnp T0B nr-
safe Iitalics ouTs-Ed. Engineering News"1
Of less unit carrying capacity than other main
compression members of the trusses.
And 60 the lower chord members are safe be-
cause the posts are weaker than the chords are,
We doubt whether assurances of safety based
on such reasoning will convince anybody. Evi-
IDENTIFY il failed 10 convince Professor Burr him
self. for elsewhere in the report we find him rec
olnmending that the Bridge Department have
made and tested to destruction not less than six
pairs Of structural steel compression members
similar in cross section to typical compression
members of the Blsckwell's Island Bridge and
as large as can be tested in the largest testing
machine in this country. Why go to this trouble
and expense if these compression members are
20 safe
Professor Burr's recommendation for tests ia
good as far as it goes, but it does not go far
enough Let frankly face the fact that neither
the public nor the engineering profession can
have confidence in the safety Of the Rlaclwelrs
Island Bridge until soIle of its full size members