Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.61
Relaxation, a gradual decrease in load or stress under a constant strain, is a
significant concern in the application of steel tendons to prestressing (Art. 9.104).
With steel wire or strand, relaxation can occur at room temperature. To reduce
relaxation substantially, stabilized, or low-relaxation, strand may be used. This is
produced by pretensioning strain at a temperature of about 600�F. A permanent
elongation of about 1% remains and yield strength increases to about 5% over
stress-relieved (heat-treated but not tensioned) strain.
4.41.4 Hardness of Structural Steels
Hardness is used in production of steels to estimate tensile strength and to check
the uniformity of tensile strength in various products. Hardness is determined as a
number related to resistance to indentation. Any of several tests may be used, the
resulting hardness numbers being dependent on the type of penetrator and load.
These should be indicated when a hardness number is given. Commonly used hardness
tests are the Brinell, Rockwell, Knoop, and Vickers. ASTM A370, ‘‘Mechanical
Testing of Steel Products,’’ contains tables that relate hardness numbers from
the different tests to each other and to the corresponding approximate tensile
strength.
4.41.5 Fatigue of Structural Steels
Under cyclic loading, especially when stress reversal occurs, a structural member
may eventually fail because cracks form and propagate. Known as a fatigue failure,
this can take place at stress levels well below the yield stress. Fatigue resistance
may be determined by a rotating-beam test, flexure test, or axial-load test. In these
tests, specimens are subjected to stresses that vary, usually in a constant stress range
between maximum and minimum stresses until failure occurs. Results of the tests
are plotted on an S-N diagram, where S is the maximum stress (fatigue strength)
and N is the number of cycles to failure (fatigue life). Such diagrams indicate that
the failure strength of a structural steel decreases with increase in the number of
cycles until a minimum value is reached, the fatigue limit. Presumably, if the
maximum stress does not exceed the fatigue limit, an unlimited number of cycles
of that ratio of maximum to minimum stress can be applied without failure. With
tension considered positive and compression, negative, tests also show that as the
ratio of maximum to minimum stress is increased, fatigue strength is lowered significantly.
Since the tests are made on polished specimens and steel received from mills
has a rough surface, fatigue data for design should be obtained from tests made on
as-received material.
Tests further indicate that steels with about the same tensile strength have about
the same fatigue strength. Hence the S-N diagram obtained for one steel may be
used for other steels with about the same tensile strength.
4.42 HEAT TREATMENT AND HARDENING OF
STEELS
Heat-treated and hardened steels are sometimes required in building operations. The
most familiar heat treatment is annealing, a reheating operation in which the metal