final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 118
An analysis of th e 23rd floor slab indi ca tes th at its m ost likely m ode of fa ilure
was in shear aro und one or m ore column s in section 3 of the floor slab. The
strength of the 23rd floor slab on th e day of collapse has been fo und to be of a
m agnitude th at com plete or partial rem oval of shoring undern eath th e slab
would have pro duced a shear fa ilure in the slab. The weight of de bris resulted
in failure in th e slabs below and ca rried thr ough th e height of the buildi ng.
Shoring in Section 3 of the 22nd storey
Examination of physical evidence and employees' statements indicate that
the 22nd storey forms were being removed on the day of the incident. Occupational Safety and Health Administration (OSHA) regulations require adherence to engineer's specifications and local building codes in determining
length of time for forms to remain in place. The engineer's requirements were
expressed in the form of a note on the structural drawings. This note required
the "slab being poured to be shored for two floors and back propped at centre
of span each way and at centre of bay on next floor down". The architect's
specifications required that "In all cases, two floors shall be fully shored".
The removal of the 23rd storey forms left only one storey of form work in place
under the recently poured 24th floor.
Premature removal of 22nd storey forms
The length of time forms were required to be leftinplacewasnotexplicitly stated
by the engineer, architect or local code. In such instances OSHA regulations
provided minimum curing times. The 4-day old 23rd floor slab had spans
exceeding 10 ft. The forms removed on the 22nd storey were in an area with
spans exceeding 20 ft and therefore, according to the regulations, should have
been in place for 10 days of temperatures exceeding 50°F.
Lateral bracing
OSHA regulations require the design of braces and shores to resist all foreseeable lateral loads. Minimum value of 100 pounds per foot of floor edge or
2 % of the total dead load of the floor, whichever is the greater, is required. No
evidence has been found which indicates that lateral load was considered in
the design of forms. The lateral bracing provided ( about 2 nominal 3 in x 4 in
timbers per 16 ft, would not provide this resistance.
Shoring out of plumb
OSHA regulations allow a maximum deviation of ½ in per 3 ft out of plumb.
Deviations of shoring exceeding these limits were found on the 23rd and 24th
storeys.
Damaged shoring
OSHA regulations required removal of damaged or weakened shoring. Onsite inspection after the incident indicates this was not done on the 23rd and
24th storeys.
Inspection
OSHA regulations require inspection immediately before, during and after
placing concrete. Either this was not done or deficiencies in the shoring were
not corrected.
119
