Civil Engineering Reference
In-Depth Information
1978; however, for noncompact members, the allowable bending stress was
given as F
b
= 0.6 F
y
.
In 1975, the sixth edition of API RP2A gave the allowable bending stress as
(for F
y
in MPa):
F
b
=
0
:
66 F
y
for D/t
≤
22,750/F
y
(7.7a)
F
b
=
0
:
66 F
yr
for D/t
>
22,750/F
y
(7.7b)
where F
yr
is the reduced yield stress as given in
Equation (7.4)
.
In the eleventh edition (1980), F
yr
was replaced by F
xc
, and the limiting
value was changed to 60, i.e., the high rotational capacity of the structure
provides a ductile failure mode exhibiting very gradual load decay and the inter-
mediate rotational capacity provides a semi-ductile failure mode.
F
b
=
0
:
66 F
y
for D/t
≤
60
(7.8a)
F
b
=
:
>
0
66 F
xc
for D/t
60
(7.8b)
The seventeenth edition (1987) substantially changed the allowable bending
stress from a maximum value of 0.66 F
y
to 0.75 F
y
, and allowance for local
buckling was formulated explicitly in the expressions. Formulations for three
regions are now provided in RP2A; they can be classified according to rota-
tional capacity:
●
high rotational capacity; ductile failure mode exhibiting very gradual load
decay;
●
intermediate rotational capacity; semi-ductile failure mode;
●
low rotational capacity; little post-yield ductility, susceptible to local buck-
ling and rapid load decay.
Because an unstiffened tubular member with a very high D/t ratio is unlikely
to be practical in offshore structures, in the eighteenth edition the upper limit for
D/t was changed to 300.
7.6.4 Shear
The sixth edition of API RP2A recommended for the first time that, for tubular
members, the applied beam shear stress should be evaluated using one half of
the gross cross-sectional area. However, not until the seventeenth edition was
the allowable shear stress recommended by AISC introduced, as presented in
Chapter 3
. The seventeenth edition also introduced an expression for the
applied torsional shear stress (for torsional shear calculation, see
Chapter 3
).
In the eleventh edition (1980), the equation for acting hoop stress from
hydrostatic pressure was modified slightly by replacing the constant by
ρ
,
where
ρ
is the density of seawater.
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