Page 421 - Forensic Structural Engineering Handbook

12.12 MATERIAL-SPECIFIC FORENSIC ANALYSES

A more defensible way of determining material properties is to take cores from the con-
2
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crete, as recommended in ACI 318 and ASTM C42. Following ASTM procedures, cores
can be used to determine the modulus of elasticity as well as the strength of concrete in
place. As noted in ACI 318, section 5.6.4, concrete design strength can be taken as the core
strength divided by 0.85.
To analyze structures, computer programs available from the Portland Cement
Association are useful. Two-way slabs may be checked by the program ADOSS, and flex-
ural members may be checked by the program BEAM.

Time-Dependent Properties
When time-dependent properties are of concern, values for use in analysis may be obtained from
tests of cores or may be estimated. Core tests to determine creep and shrinkage properties, fol-
lowing procedures of ASTM C 512 and C 157, provide the most reliable data but are relatively
time-consuming and expensive. Analytical methods for determining time-dependent properties
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are given in reports of ACI Committee 435. A simple direct approach is given in Ref. 16.
Inelastic Properties
Design procedures of ACI 318 limit redistribution of negative moment at a support to a
maximum of 20 percent. However, for lightly reinforced flexural members, full redistribu-
tion of moments can generally be achieved. Under this condition, capacity of a flexural
member can be determined assuming that nominal moments M develop at each end and at
N
the location of maximum positive moment. When special confinement reinforcement is
placed in beams, a common practice in seismic regions, calculations often show that full
redistribution is realized even in heavily reinforced flexural members.
Large amounts of redistribution are also possible in two-way reinforced-concrete slabs.
Strength based on yield line analysis may be calculated following the procedures of Ref. 17.
In general, calculated capacity based on yield line analysis will significantly exceed capacity
indicated by design procedures such as those of ACI 318. 2
Once capacity has been calculated by yield line analysis, shear strength and torsion
capacity must also be checked. In many cases, either shear or torsion may control capacity.
Note that yield line analysis does not consider either deflection or cracking at service
loads. When serviceability is the prime consideration, specific calculations of deformations
should be done.

METHODS OF TESTING MATERIALS,
COMPONENTS, AND ASSEMBLIES

Visual inspection of the structure and review of documents are important first steps in any
investigation. Often, this step will indicate likely causes of the distress. However, it is usu-
ally necessary to determine physical properties of materials, components, and assemblies
in order to arrive at final conclusions.

Field Load Tests
To obtain physical dimensions and material properties for use in an analytical investiga-
tion, it may be necessary to take cores of the concrete and obtain representative samples
of the reinforcement. In addition, locations of reinforcement within the concrete and