Precast Concrete Arch/ Structural Details, J. V. Tyrrell
CASE S1O -
Inattention to structural details
Symptoms: Structural failures
Collection of Facts: A large precast concrete arch with a steel tie rod was
designed to be supported by concrete piers. The arch was cast in two segments
to be connected at the crown during erection. The piers were designed as
cantilever members with spread footings proportioned to take lateral thrust on
the piers during erection, before the tie was connected. Although the soil
bearing capacity was adequate, no consideration was given to the effect that a
small settlement might have on the position of the arch seats.
The tie was designed as a tension member and a rod of minimum size was
selected. The rod was strong enough to resist the maximum load but again no
consideration was given the effect of elastic stretch on the piers. The
contractor devised an erection scheme that called for several arches to be
erected before the first tie rod was placed. This work was performed without
shoring, and the process was observed by the A&E when about three arches were
in place without the tie. The ROICC mentioned that the piers appeared to have
moved outward slightly and a check showed a movement a little over one inch.
The A&E said that this was not a problem but should be watched.
The next day, a failure during erection caused injury to the construction
crew and extensive structural damage. The primary cause was determined to be
the failure to immediately connect the tie rods or alternatively provide
adequate shoring. The structural calculations were then reviewed and it was
found that even with erection completed and the tie in place, the stability
was only marginal.
Solution: There are no complete solutions to these problems. Designers must
try to think about the function of the entire system and not just crank out
design for the major factors such as shear and moment. The designer should
try to foresee potential problems in erection. When erection is complicated,
unusual, or involves major structures, the contractor should submit an
erection plan which is structurally checked. As you can see, not paying
attention to details continues to be a principal cause of structural failures.