Page 438 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.29

Copper/copper sulfate half cell
Volmeter Wet sponge

6''
Lead wires

Alligator
Reinforcing mat
clip 12''
Steel pin

12''
FIGURE 12.25 Schematic diagram of concrete test slab and of electrical circuit for measuring potentials.
(Courtesy of CTLGroup.)

TYPES AND CAUSES OF COMMON
NONPERFORMANCE AND FAILURES

Construction Failures
Many times, the largest loads that ever go on a structure occur during construction.
Consequently, collapses and other damage are most frequent during the construction phase.
Formwork collapse, premature removal of formwork, and failure of formwork support are
the most common causes of construction collapse.
Modern building codes are calibrated to provide economical and safe structures.
Designs that follow these codes seldom fail either during or after construction under grav-
ity or wind loading. However, seismic loading has caused failures in buildings of all types
of materials.
In most cases where there is a structural failure or where concrete buildings perform
poorly, the reason can be attributed to misinterpretation of the code or mathematical blun-
ders or faulty construction. An example of such a blunder is failure of the Cocoa Beach,
28
Florida, Apartments during construction. Investigations disclosed that the slabs had not
been checked for shear. Consequently, inadequate shear strength was provided, and a col-
lapse occurred.

Overload
In prestressed concrete roof structures, sometimes ponding can become a source of over-
load. For very shallow sections, deflections caused by the weight of rainwater on the roof
can cause enough sag that additional water accumulates until failure occurs. Although