New-York tribune., 1908-11-12


follows
TIE INVESTIGATION OF TIE BLACK
WELLS ISLAND CANTILEVER BRIDGE
Ever since the fan Of the great cantfever for
1he St Lawrence bridge at Quebec the Black
wells Island cantilever bridge now approaching
completion over the East River at New York
City has been under suspicion While the
BisckUe1's Island bridge has clear spans Of only
I,2Sfo feet which is only two-thrds that at
tempted a. Quebec. il vas designed 70 carry
very much heavier I've load. and conseQuentty
the dimensions % its members were quite as
iarEe Is those OF the bridge at Quebec
IN The BlackweII's Island bridge there are
compression members designed TO carry even
Erester loads than were carried by the falling
chords II The Quebec Bridge: and the design OF
The compression members in these two great
bridges IS closely similar We speak adyisedly.
therefore IN saying that the QUebec disaster
raised cloud Of doubts as 20 the safety Of the
RlackWelrs Island structure
These doubts we're felt sure. at the
time Of the disaster by those responsible for the
BlackTreIl-s Island bridge. and II fact engineers
REPRESENTING rue coppan.- building the bridge
and the New YORK city Bridge Department were
early on The ground Al Quebec. There was talk
Al that time OF q thorough re-ezamination OF tls
stresses in the bridge and also of plan feT
suhject5nE sc7re q the big compression members
haze been laid the shelf. and the reoomputa-
1ion Of stresses seems 70 have progressed at
ver7 leisurel- pace.
Meanwhile month after month passed away
and the erection steadily progressed The can
tie,er arms we're joined that the main t-usSes
we're completed and the floor ana Toadways were
nj in place. So far as The public was informed,
il appeared probable that the bridge would be
finished and opened for traffic exactly as # the
disaster n the similar structure al Quebec had
r.@ver occurred.
Most fortunately. however as is now evident,
-1@ public uneas{ness concerning the structure
4rz1r, made such a course impossible. The Ne"
York City revspapers- particularly Ihe IT'D
-..e- voiced a public demand that the bridge
sror1d not ne opened 10 traffic until some assur-
sn,ez as 70 i1s strength and safety were OBJ
tanned
A. result Of the pubic agitation II was
finally decided 10 have two EXPERT lhNesIlsa
7Icrs and reports made upon the bridge one oS
re firm ar Boiler &, Hodge. consulting Engl-
7.ee-g and the other by Professor Nn- h burr
~ Columbia University. Not since the quebec
Bridge fell has anything appeared Of such In
7e-esr 10 bridge engineers generally as these Two
-ar,--s They contain information astonishing
to me last degree
The F7lsclp-ell-s Island bridge. it should he
sKflained was designed al the outset Tor
-..e-gesred loading Experience with the oio
F3r,oslvn Bridge over the East River had made
&C. -,-.7 7-alz. clear al the tine the BlackwelIs
Island bridge was projected that bridge IN
rhe heart OF New York City Is daily subjected
t. heavy crowds during the rush hours, and
ma AZ any time. through temporary blocking
Of ths traffic or other causes, be densely packed
with cars teams and people Even If sucn con-
zeered loading should occur only once or twice
n score Of years. Il goes without saying that
the bridge ought TO be made perfectly safe 70
carry its Ioad at the time when the greatest
number 90 human lives are dependent on lis
strength
An expcrt board OF bridge engineers which re-
ported in 1873 on the design Of the Manhattan
Bridge ove- the East River advised that the
structure should be Droportioned FOR con
Eested loading. defined as a possible load when
la,.IG only occur II special pains we're taken TO /
produce il. and ar=o for smaller regular /
loading such ss may occur under unusua! condi- /
5ious oT crowding al any time.
The Board further advised that this con
eeyted loading combined with dead load and I
Hiad pressure should produce ne stresses any
where reaching the elastic limit Of the ma /
re-iaI- This recommendation has been inter
ITeted % mean that unit stresses for this con-
Ee~red load lust under the elastic limit Of the /
metal ere approved by this Board; bat such
ar Interpretation is wholly unwsrranted.
f. The design Of the Blackwell's Island Bridge
rersmrnended 10y the board for TIE design of the
Manhattan SUSPENSION bridge was adopted. and I
Tho resulting unit stresses from dead and live
load aIore tritH 40 aje'reteo for lriHd pressure.
ss9sr Ioad ~ sec0Rdarg stresses IA thc 'aeH-
bc-s were fixed al 24.vO0 pounds square inch
Tor ordinary structural steel
Al tBe present time the consensus - englneer-
Ing opinion Is doubtless that these unit stresses
are beyond the limits Of safety. and. in fact
sT
even if any engineers approve such high stresses
they wiH agree that such stresses represent
imlt which should by possibility be e@-
ceeded and that II any members are stresseu
Hsteriall. above this limit il is cause for seil-
o1s alarm
These fac.s were of course, perfectly vel
hnoRn 10 the designers and huiiG@rs o. '''e
bridge ard yet the computation Of the stresses
ContIBa-d On fourth pss&