Civil Engineering Reference
In-Depth Information
TABLE 3.5
Values for Q
u
Brace Load
Joint
Classification
Axial
Compression
In-plane
Bending
Out-of-plane
Bending
Axial Tension
1.2
Q
g
but
1.2
Q
g
K
(16 + 1.2
γ
)
β
≤
40
β
T/Y
30
β
2.8 + (20 +
0.8
1.6
but
γ
)
β
2.5 + (4.5 +
0.2
1.6
≤
2.8 + 36
β
1.2
2.6
(5 + 0.7
γ
)
β
γ
)
β
X
23
β
for
β ≤
0.9
[2.8 + (12 +
0.1
20.7 + (
β −
0.9)
γ
)
β
]
Q
β
(17
γ −
220) for
β >
90
D. Linear interpolation
between the limiting values of the above two Q
g
expressions may be used for
−
Preferably, the overlap should not be less than 0.25
β
0.05 when this is otherwise permissible or unavoidable.
F
yb
= yield stress of brace or brace stub, if present (or 0.8 times the tensile
strength, if less), in ksi (MPa).
The Q
u
term for tension loading is based on limiting the capacity to first
crack. The Q
u
associated with full ultimate capacity of tension-loaded Y and
X joints is given in API RP2A.
The X joint, axial tension, Q
u
term for
0.05
<
g/D
<
β >
0.9 applies to coaxial braces (i.e.,
e/D
0.2 where e is the eccentricity of the two braces). If the braces are not
coaxial (e/D
≤
>
0.2), then 23
β
should be used over the full range of
β
.
Chord Load Factor Q
f
The chord load factor Q
f
accounts for the presence of nominal loads in the chord
and is calculated from the following equation.
"
!
−
#
−
FSM
ipb
M
p
FSP
c
P
y
y
F
3
A
2
Q
f
=
1
+
F
1
F
2
(3.97)
The parameter A is defined as:
t
FSP
c
P
y
"
#
2
2
FSM
c
M
p
A
=
+
(3.98)
where P
c
and M
c
are the nominal axial load and bending resultant in the chord;
P
y
is the yield axial capacity of the chord; and M
p
is the plastic moment capacity
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