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nitiation
of Failure
Failure initiated at the “weak-link” of the system
– the anchor bolts on the eastern face, which were installed in the 1882
construction (Figure 6). The failure
mode was separation within the anchor bolt system by one of two distinct modes.
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F |
ailure Mode 1 – Separation at the Boundary of 1882 and 1900 Construction – Expansion Bearing Anchor Bolt Collar Coupling
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This mode accounts for approximately 3/4 of
observed separation failures. All collar
couplings observed at the site exhibit a radial cracking pattern. The equiangular cracks completely penetrate
the collar couplings. The observed
couplings either remained engaged on the bearing assemblies or are found loose
within the debris field. Coupling
fractures show evidence of fatigue fracture with secondary fractures occurring
by overload presumably during the collapse.
Many of the couplings were fractured by fatigue prior to the time of the
collapse incident. |
single tower. 3 of 4 anchor bolts remain and this condition evidences failure via separation within three collar couplings. |
The 1900 construction provided washers
“surrounding” the collar couplings. As a result, the collar couplings and
associated cracking were “hidden” from view.
The cracks within the collar couplings could only have been visually
observed if the restraining bolts and washers were removed during an inspection
cycle. Due to the complete penetration cracking of the couplings, a majority of
the collar couplings were judged to be ineffective and could not transmit
uplift forces to the substructure.
Therefore, for analysis purposes, no uplift capacity was
attributed at the locations where the 1882 anchor bolts remain in-situ, without
attachment of collar couplings.
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F |
ailure Mode 2 – Ductile Failure (Separation) within the existing 1882 anchor bolts – Expansion Anchor Bolts.
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This mode accounts for approximately 1/4 of observed failures. Fractographic examination of fractured original 1882 anchor bolts showed that the fracture resulted from tensile overload and was a fully ductile fracture. The estimated tensile capacity of one 1882, 1-1/4 inch anchor bolts at failure, assuming a 20% corrosion loss, is 30 tons. Based on the observed, 3:1 ratio of collar coupling separation to ductile anchor bolt failures, an uplift capacity of 30 tons per tower can be estimated. This capacity establishes a lower bound, critical wind speed of 94 mph, which was sufficient to initiate failure (see Figure 5). Failure was sudden and catastrophic. |
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V |
ulnerability
to winds from the East
Note that the structure was highly vulnerable;
specifically to winds from the east due to the “separation” within the collar
couplings of the “expansion” tower bearings.
The anchor bolts on the “fixed” tower bearings - opposite face – did not
employ collar couplings; hence, an uplift capacity of 152 tons (without section
loss due to corrosion) can be estimated for the west legs.
Note that the failure of anchor bolts at the fixed
bearings was similar to Mode 2 of the anchor bolt at the expansion bearings.
The anchor bolts at the fixed bearings failed by simple tensile overload
(ductile fracture) within the stem of the bolt.
(See Figure 7.) TABLE 1 Tensile
Capacity of Component Parts of the Expansion Bearing Anchor Bolt System 1 1882 Wrought Iron Construction 1900 Steel Construction Nominal Heavy Upset Threaded Rod Diameter Tensile Capacity of Original 1882 Anchor Rod (Based on Tensile Area at Threads)2 Tensile Capacity of Original 1882 Anchor Rod
(Based on Nominal Area of Tie Rod)2 Tensile Capacity of Original 1882 Anchor Rod
(Based on Nominal Area of Tie Rod, Assuming 20% Section Loss)2 Tensile Capacity of Anchor Bolts added in 1900 (Based on Tensile Area at Threads)3 Tensile Capacity of 2 ¼” O.D. Coupling added in
1900 (Based on Tensile Area at Threads)4 1 ¼“ 87.1 kips 76.1 kips 60.9 kips5 (30 tons) 84.3 kips (42 tons) 99.3 kips (50 tons) 1 Values given
represent “limit states” for the purpose of determining collapse load and are
not intended to reflect rational design parameters. 2 Tensile capacity is
based on a Tensile Strength (Fu) of 62 ksi
determined from material tests conducted by ATLSS/Lehigh University, 3 Tensile capacity is
based on a Tensile Strength (Fu) of 60 ksi
determined from material tests conducted by the Center for Advanced
Technology for Large Structural Systems by ATLSS/Lehigh University,
Bethlehem, Pennsylvania. 4 Tensile capacity is
based on a Tensile Strength (Fu) of 55 ksi
determined from material tests conducted by ATLSS/Lehigh University, 5 Limit state capacity is governed by the
weakest element of the system, which in this case is the 1882 anchor bolts. Note: Post-collapse site investigations and material
tests indicate that corrosion loss was significant in the original 1882 heavy
upset threaded rod anchors. Fractographic examination of fractured original 1882
anchor rods showed that the fracture resulted from tensile overload and was
fully ductile fracture. Coupling
fractures show evidence of fatigue fracture with secondary fractures
occurring by overload, presumably during the collapse. It appears that at least some of the
couplings were completely fractured by fatigue at the time of the
collapse. It is assumed that 3 out of
4 couplings at the expansion rollers under the windward columns of each tower
had failed prior to the collapse of the viaduct (e.g. a 25% effective
efficiency), and that the original anchor rods experienced 20% section
loss. Therefore, the calculated uplift
capacity for each tower at the time of collapse is: “At collapse” uplift capacity/tower = 62 ksi x π x (1.25”)2 /4 x 80% (due to 20%
corrosion reduction) x 4 bolts x 25% (effective efficiency) = 60.9 kips/tower =
30 tons/tower Should all component parts of the anchor bolt
system be fully effective, the limit state for uplift capacity at a tower is
governed by the lesser of the weight of the tower foundation or the system
capacity without corrosion at 100% efficiency. Therefore the calculated “upper bound”
uplift capacity for a tower at the windward legs is: “Upper bound” uplift capacity/tower =< 2 x 334
kips/masonry pedestal = 668 kips kips (334 tons) “Upper bound” uplift capacity/tower =< 62 ksi x π x (1.25) 2 /4 x 4 bolts x 100%
(effective efficiency) = 304.4 kips (152 tons) governs E pisode 1 – Collapse of Spans 19 – 23, Towers 10 – 11 The following occurred in the sequence indicated. ① Tornado touches down – easterly winds
grow rapidly – local wind speeds (from the east) exceed 90 mph – as wind speeds
grow, the towers oscillate laterally at their natural frequency as a unit until
the wind lock bolts shear and the rails pull apart. ② “Separation” failures occur within the
“expansion” anchor bolt system of Towers 10, 11, 12, 13 and 14. ③ Small rotation about the “fixed”
bearings of towers 10, 11, 12, 13 and 14 occurs followed rapidly by tensile
failure of the “fixed” anchor bolts. The
rails separate at Tower 12. ④ Tower sections 10 and 11 and attaching
girders separate at the wind locks (of Towers 9 and 12) and become
airborne. Total collapse of this segment
is rapid. Wooden decking, rails and
structural steel collapse as a unit. The
wooden decking and rails come to rest in the immediate vicinity of the girders. ⑤ Towers 12, 13 and 14 initially become
airborne and “jump” a small distance north and westward. Towers 12, 13 & 14 momentarily come to
rest in the upright position on the ground but do not initially
catastrophically collapse. The rails
momentarily hold the towers and momentarily prevent immediate catastrophic
collapse of Towers 12, 13 & 14. E pisode 2 – Collapse of Spans 7 – 18, Towers 4 - 9 The following occurred in the sequence indicated. ➀ Tornado moves northward – easterly
winds grow rapidly – local wind speeds (from the east) exceed 90 mph. As wind speeds grow, all towers vibrate at
their respective natural lateral frequencies. ➁ Wooden decking
and rails (spans 1 – 18) separate from the structure. (The rails, separated at Tower 12 during
episode 1, ride down with Towers 10 and 11 pulling ties and decking from Towers
9 through 4.) ➂ “Separation” failure occurs in sequence
within the “expansion” bearings of Towers 9, 8, 7, 6, 5 and 4. Tower 9 fails, momentarily, followed by
failure of Tower 8, etc. ➃ Small rotation about the “fixed”
bearings of Towers 9 thru 4 occurs in sequence followed rapidly by tensile
failure of the “fixed” anchor bolts. ➄ In rapid but distinct sequence, Towers
9, 8, 7, 6 and 5 individually become airborne, pivot approximately 90° clockwise about
the base of the fixed bearings and strike the ground upon impact. Collapse is progressive from south to north. ➅ Span 7 and Tower 4 are initially
restrained by Tower 3; however, after fracture of the connection to Tower 3,
rapid collapse and clockwise twist of the tower occurs. Tower 3, although
standing, is visibly distorted. E pisode 3 – Collapse of Spans 24 – 29 and Towers 12, 13 and 14 The following occurred in the sequence indicated. ➀ Tornado moves northward – rapid and
confined inflow winds attack from the south. ➁ The wooden
decking and rails (spans 24 – 29) separate from the structure. The wooden decking undergoes lift and falls
in a northeasterly direction. The rails
remain attached to Tower 15. ➂ Spans 25 and 27 and Towers 12, 13 and
14, having separated from the bearings during Episode 1, now become airborne
and are displaced to the north and east.
The displacement induces torsional buckling of
the columns of Tower 12. Collapse is
total. Towers 12 & 13 twist 90° counterclockwise
and collapse on top of previously collapsed Tower 11. Tower 14 twists and collapses separately. ➃ Span 29 oscillates (laterally several
times) at the Tower 15 connection, eventually separating and rotating upside
down before impact. The rails remained
attached and “hang” from Tower 12.
(Subsequent to Board of Inquiry Investigation, the “hanging” rails were
cut and allowed to drop to the ground.) Note: From a
forensic perspective, the most puzzling aspect of the debris field is the
location and orientation of Towers 12, 13 & 14 after collapse. They (a) clearly were effected by the winds
during Episode 1, (b) lie on top of all Episode 1 debris and (c) are oriented
in a direction opposing all other structure debris. Interpretation of the debris clearly
indicates the effects of inflow wind throughout the collapse cycle. Evidence of initial motion westward and
northward, during Episode 1, includes the angled orientation of the fixed
bearing base plate, as illustrated in the photograph below as well as the
impression marks on the ground surface from the temporary landing (Photo
17). Evidence of the effects of inflow
from southerly winds includes the overall northward vector orientation of the debris
field.
As a result, the structure was much less
vulnerable to winds from the west – the prevailing wind direction. Winds from
the west would result in base compressive and tensile forces, reversed in
orientation from the forces effects illustrated in Figure 5.
