Page 416 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.7

TABLE 12.1 Comparison of ACI 318-10 with Current Requirements
1910 Current
2, psi 2,000 18,000
f c
f , maximum psi 650 N/A
c
f , maximum psi 20,000 41,000
s
f y , maximum psi 50,000 100,000
Pages 12 400 w/commentary

Up until the mid-1900s, working stress design was used almost exclusively. The use of
low design stresses and low design strengths for concrete generally created robust struc-
tures. Due to lack of sophisticated analytical tools, continuity was often provided but
ignored in analysis. Consequently, reinforced-concrete buildings and bridges built prior to
World War II usually have reserve flexural and compressive strengths.
During and after the 1940s, designers made significant efforts to reduce the amount of
materials used and to take advantage of higher-strength concrete and reinforcement. In
1963, building codes were changed by adding strength design procedures to take advantage
of newer materials.
Possible Deficiencies. Research that was carried out in the 1950s and 1960s demon-
strated that previous requirements for shear and bond provided less safety than previously
believed. As a result, shear and bond requirements have been made more restrictive in
recent years.
Due to a lack of understanding about the relationship between shear and moment when
applied to reinforced concrete, codes prior to 1963 required stirrup spacing that was only
partially effective. Also, allowable shear stresses on the concrete sometimes exceeded
strengths that are now known to be available.
Bond of deformed reinforcing bars is another issue where older structures may not meet
the more restrictive requirements of recent codes and specifications. Although bond
designed by current ACI 318 requirements results in much longer lap splices and develop-
ment lengths, there is no documented evidence of failures attributed directly to bond design
requirements of the past.

CRITICAL CHARACTERISTICS OF CONCRETE
STRUCTURES

Plain Concrete
Plain concrete is a structural material with high compressive strength and low tensile
strength. When loaded to destruction, plain concrete exhibits low amounts of energy dissi-
pation. However, under normal conditions, it is a very durable and economical structural
material.
From the point of view of structural performance, plain concrete is defined in Ref. 2 as
follows: “Plain concrete—structural concrete with no reinforcement or with less reinforce-
ment than minimum amount specified for reinforced concrete.” (Minimum reinforcement
for reinforced concrete is specified in Ref. 2.)
In general, minimum reinforcement required for concrete to be considered “reinforced” is
that amount necessary to provide strength after cracking that equals that of the cross-section