Custom Search
|
|
|
||
 (3) Welded Wire Fabric. Welded wire fabric, designated as A185 by
ASTM, is used to reinforce the flanges of tee and double tee sections. In
conventional design, welded wire fabric is sometimes used as shear
reinforcement, but it is not used for blast design which requires closed
ties. The static design strength, fy, of welded wire fabric is equal to
its yield stress, 65,000 psi.
(4) Prestressing Tendons. There are several types of reinforcement
that can be used in prestressing tendons. They are designated by ASTM as
A416, A421, or A722, with A416, grade 250 or grade 270, being the most
common. The high strength steel used in these types of reinforcement can
only undergo a maximum elongation of 3.5 to 4 percent of the original length
before the ultimate strenth is reached. Furthermore, the high strength
steel lacks a well defined yield point, but rather exhibits a slow
continuous yielding with a curved stress-strain relationship until ultimate
strength is developed (see Figure 61). ASTM specifies a fictitious yield
stress, fpy, corresponding to a 1 percent elongation. The minimum value
of fpy depends on the ASTM designation, but it ranges from 80 to 90
percent of the ultimate strength, fpu.
c. Dynamic Strength of Materials. Under the rapid rate of straining of
blast loads, most materials develop higher strengths than they do when
statically loaded. An exception is the high strength steel used in
prestressing tendons. Researchers have found that there was very little
increase in the upper yield stress and ultimate tensile strengths of high
strength steels under dynamic loading. The dynamic design strength is
obtained by multiplying the static design strength by the appropriate
dynamic increase factor, DIF, which is as follows:
(1) Concrete:
Compression
DIF
=
1.25
Diagonal tension
DIF
=
1.00
Direct shear
DIF
=
1.10
Bond
DIF
=
1.00
(2) Non-Prestressed Steel Reinforcement:
Flexure
DIF
= 1.10
Shear
DIF
= 1.00
(3) Welded Wire Fabric:
DIF
= 1.10
(4) Prestressed Reinforcement:
DIF
= 1.00
d. Ultimate Strength of Precast Elements. The ultimate strength of
non-prestressed precast members is exactly the same as cast-in-place
concrete members and as such is not repeated here. For the ultimate
strength of beams and columns see paragraphs 3 and 4 respectively of Section
3 of this manual. For the ultimate strength of slabs see Chapter 5 of
NAVFAC P-397.
(1) Ultimate Dynamic Moment Capacity of Prestressed Beams. The
ultimate dynamic moment capacity Mu of a prestressed rectangular beam (or
of a flanged section where the thickness of the compression flange is
greater than or equal to the depth of the equivalent retangular stress
block, a, is as follows:
2.08-219
|
 
|
|
|
||
