final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 107
The following is a description of what is believed to be the most probable
failure mechanism:
About five minutes before the collapse, during the final concrete placement
in the outside stem forms, there is evidence that a form-tie broke and the form s
began to spread. Further failure was probably arrested as adjacent ties took up
the load and the form was carefully filled with concrete to the top.
Minutes later, when the operator put the vibrator into this form, a complete
rupture of the form probably occurred with several ties failing. The form would
not have opended widely, however, because the inside form was braced solidly
· to the second stem form and the outside form was held tightly to the extended
soffit form. The pressure of the fluid concrete would produce a large horizontal force pushing the soffit form outward.
Th e horizontal outward force, it is believed, pulled open the joint in the soffit
form under the second girder stem and concrete began to fall down onto the
working platform.
Opening of the joint probably caused the inside 4 in x 8 in stringer to roll onto
its side, dropping the soffit form several inches and in so doing increasing the
horizontal thrust.
The timber falsework frame under the two exterior stems would transfer all
the horizontal thrust loads down to the tops of the two outer steel falsework
stringer girders. It should be pointed out that the 2 in X 10 in planks in the
working platform were not nailed and offered no resistance to a spreading
force. Also the X-bracing between the steel girders which was merely butted
up to them, would not offer any resistance to a spreading force.
Olmeni;lon1 on
! of route 210
,
7 •
174-8 1/g
230'-o•
I
lBQnt 2
I•
varies; but
75-o·
1=
t
route 210
75'-o'
I
Abutment 1
Abutmllnt I,
Bent 3
Elevation
Typical Section A-A
Plan
108
