Page 419 - Forensic Structural Engineering Handbook

12.10 MATERIAL-SPECIFIC FORENSIC ANALYSES

wires or strands, very slender members can be constructed. Also, prestressed members can
be designed to have little or no cracking under service loads, thereby improving their overall
durability and appearance.
Bond and anchorage of pre-stressed elements are important in prestressed concrete.
Contaminated strands in pretensioned concrete can cause loss of bond, with a resulting
potential for large deformations and possible failure. Although strands can become conta-
minated in the field, failure to clean strands of lubricants used during manufacturing has
been identified as the cause of distress in several jobs.
Elastic and/or time-dependent movements sometimes may cause damage to precast
concrete structures. When prestress is applied and continued for more than 2 years there-
after, structural elements shorten. Failure to properly consider this movement in the design
can cause damage to connections, columns, and the prestressed member.

Precast Concrete
Structural elements may be precast either in a remote factory, or at the job site. Precast con-
crete may be either ordinary reinforced or prestressed. In general, structural considerations
are the same for precast as they are for cast-in-place concrete.
The major differences between precast and cast-in-place concrete are methods of con-
necting structural elements together. Most connections of precast concrete have little or no
continuity. Connections often are a source of distress in precast buildings. Major items of
potential distress include damage caused by unanticipated restraint, inadequate bearing
because of construction tolerances, and failure to provide a suitable load path.
Transportation and erection of precast concrete often are sources of distress. When pre-
cast members are being lifted, inadequate anchorage of lifting embedments can result in
pullout of the embedment. Improper lifting of tilt-up and other site-cast elements can cause
excessive cracking. Also, impact of precast elements against other parts of a structure can
cause spalling of concrete.

Steel-Concrete Composite Deck
In recent years, a popular method of constructing floors in steel-frame buildings has been to
use metal deck with a thin concrete topping. Metal deck can be designed either to be com-
posite with the thin topping or to be noncomposite and thereby carry the entire load in the
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metal deck. According to recommendations of the Steel Deck Institute, a small amount of
reinforcement should be placed in the concrete deck. The recommended amount of rein-
forcement falls below that recommended by ACI 318 to qualify for reinforced concrete.
However, the reinforcement called for is often enough to carry full dead load plus live load
with some margin of safety even if the metal deck is lost due to corrosion or other damage.
Metal deck supporting reinforced concrete is highly susceptible to corrosion due to the
chlorides in concrete. When chlorides are present, corrosion can quickly and severely dam-
age the metal deck. When metal deck is needed as part of the structural system, remedial
measures may include addition of reinforcement either below or above the concrete slab.
Added reinforcement must be protected against corrosion.

Formwork, Shoring/Reshoring
The location of formwork establishes the bottom location for all cast-in-place concrete.
Where deflections may be of concern, the forms may be cambered to offset initial dead load
and creep deflections. While sometimes used on beams and one-way slabs, camber is seldom