New-York tribune., 1908-11-12
THE BRIDGE BLUNDER
Continued from arst page

aSessrs. Boiler & Hodge treat this matter in
their report very briefly. They say merely;
We made stress sheet for the loads called
for in the specifications: But II M2s EoT
DFrNT THE STRUCTURE COULD MOI
SAFELY CARRY THESE LOADS. so we mao 10
find the maximum safe carrying capacity OF the
structure.
The Capitalization is ours and is fully just-
fed. we submit by The Importance Of the state
rnent Here is a stupendous bridge-cne Of the
greatest IN the world-designed 10 carry safely
"a certain loading and contracted for on that
hasis on completion il is found to be abso-
lutelv unsafe 10 carry this load. Surely the
public is entitled TO ask who has blundered ana
why IL is not to receive what IT has paid for
turning now 10 Professor Burr's report. we
hnd il much more explicit on the condit''n OF
overstrain in the bridge He states that the spec
.fsed loading the structure would stress some
Of the bottom chord members OF the Island span
obouf % per cent more than the specifications
permit Reduced TO figures this means that some
Of these compression members concerning whose
strength the profession is in grave doubt any
way1 would carry. if the congested loading We're
spplied. about thirty thousand pounds a square
i1ch Oess the usual deduction of quantity de-
pendent upon the column length and size).
And this IS no: an isolated of overstress
TIE nickel-steel eyebars in the chords of this
same span would also be overstressed about %
Il.
$1,000 pounds q square inch It may be noted
here that the specified elastic limit OF these
rickeI-steel eyebars was only $4,000 pounds
square inch.
Even this is not quite the worst that Professor
Burr finds. We quote as follows from his re-
port:
The overstresses In number Of the carbon
steel eye-bars would range from 10 to 10 per
cent in some members in all parts of the struct-
Ure except in the Queens anchor arm, where 50
per cent is reached in enc case. There would be
aisc some similar oVerstreSses IN rINeted ten
sion members in the same parts OF the bridge
rising above 80 per cent IN a single instance
Few main posts in all the bridge would be over
stressed much 25 per cent In the Queens
anchor there is post which would be
overstressed 80 per cent. The Manhattan rocker
srr, would be overstressed nearly 20 per cent in
tension and 80 per cent in compression, while
the Queens rocker arm would be subjected to
an excess Of 10 per cent IN tension and Il per
cert in compression.
Few main posts in all the bridge would re
overstressed as much as % per cent is. we
take It a bit OF veiled sarcasm the part Of
Professor Burr. for every engineer knows that
II would take precious Few such overloaded
members 20 condemn the structure.
II must be remembered in reading the reports
Of Messrs. Boiler & Hodge and Professor Burr
that they are rendered 70. the Department of
BRIDGES which IS responsible for the design and
erection Of the BlackwelIs Island Bridge. Com
mon courtesy, therefore. forbade anything Of
condernnatory tone in the reports. and confined
the writers 10 a bare statement OF facts
C1Iriously enough this absence of any con-
oernnatory tone in the wording, coupled with
the technical character Of most Of the report
has led many of the New York newspapers to
regard these reports quite Of reassuripg
character. and they publish extracts from the
reports under such headings as 'QueensbeTo
Bridge safe" II is needless for to sav-to
cur readers that the bare facts given in these
reports are more condemnatory to those Who
understand their fun significance than any mere
words could possibly be.
And il should be noted here that au-the in
formation desired concerning the bridge has not
yet come 10 light stress sheet has been
given out and is printed in this issue showing
what loads the various members would carry
under the loading recommended by Messrs. Boil
Ter & Hodge. We still lack the stress sheet
showing what stresses would be imposed
these members if fAe loads fired ia the specif-
cations applied. 1t ought to he most illu-
minating. II may shed some light, for example,
the puzzng question why a bridge with a
dead weight of over 5000 lbs per lin. # can
carry live load of only 1000 lbs. per lin. #.
N We said above that the report contains mate
fIrial that will astonish the engineering profess
. sion Surely the facts Just quoted justify this
statement Here is q huge bridge structure buit
by great and wealthy city. There was no need
and thought Of attempting to economic in
the design. done from force Of necessity
az Quebec The city bridge department has had
or its STAFF some engineers Of high professional
reputation. and the structure has been built by
firm Of long experience and high standing
We're these engineers ignorant Of the stress con-
ditions in the structure? Did they knowingly
trsnsEress the limitations for safe stress fixed
by the specifications? Would they have delib-
ersfely taken the responsibility of opening the
bridge 70 public use in this condition regardless
Of the terrible object lesson at Quebec a year
ago?
A multitude of questions Of this sort come to
mind II is a thousand pities that the history
Of this bridge cannot be investigated by another
Eoleate Commission as was the history Of the
Quebec Bridge. The engineering profession
might then gain some further knowledge as to
how to design-and how not 10 design-great
cartever bridges.
Adopting this C20O0O per square inch) as the
safe working stress for the metal. Messrs. Boiler
& Hodge find that the main trusses as they
now stand, safely carry a live load Of about
2coc lbs. per lineal foot each. and by taking off
about 900 lbs per lineal foot Of the dead load
the capacity be raised to 1900 lbs. live load
per ILL., 1 foot BY assuming loading Of only
80 lbs. per Sc # on the roadways and side
walks, and specifying that trolley cars shall he
spaced least a car-ength apart. Nessrs Boil
Ter & Hodge conclude that the bridge may be
safely opened to pedestrian highway and trolley
car traffic on four tracks but that the elevated
railway tracks cannot he used. Professor Burr
takes of more Of the dead load. spaces the cars
farther apart and then concludes that one pair
Of elevated tracks might safely be opened for