Wood Beams - Load Tables

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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WOOD
BEAMS
Wood
Beams
-
Load
Tables
The tables for beam loads are based
on
solid wood
beams
of rectangular cross section, surfaced
4
sides
to
standard dressed dimensions,
as
given
on
pages
25
and
26.
The compression edge is supported throughout the
beam length
to
prevent lateral displacement and lateral support is provided at each end at points of bearing
to
prevent rotation. Beams are single span and are loaded uniformly for their entire lengths. The data are
presented with span lengths in feet for the various sizes of beams with
load
capacities based
on
a range of
bending
stress,
F,
values. Data provided for each span and
nominal
size of beam are as follows:
W
w
F,
E
=
tofal
load
in pounds, uniformly distributed, based
on
Fb
=
load
per linear foot of
span,
WIL
=
minimum horizontal shear design value, psi, required to resist the horizontal shear stress induced by
=
required modulus of elasticity design value, psi,
if
deflection under load
W
is limited
to P/360
load
w
Use
of
Tables
To
use
the tables, secure from appropriate reference
(see
page
28),
the bending design value,
F,,
appropriately adjusted for duration of load, service condition, size factor or other applicable modification factors,
and refer
tot
he span length involved.
If
the
total load
W
is
known read down the column under the appropriate
F,
heading
to
find
a
matching
design load
W
and then red across the page
to
see
the required beam size. If the
beam size is known read across the page
to
the column under the appropriate
F,
heading
to
find the design load
W.
Before selecting a size of
beam
it
is
advisable
to
check the board measure, bm, in several sizes which qualify
in order
to
fmd the one which had the least amount of lumber and thus is the most efficient.
After determining he required
beam
size, or design load,
W,
in the manner
just
described, it is
necessary
to
check the horizontal shear,
F,,
and the modulus of elasticity,
E,
to
make sure that the induced
or
required
values do
not
exceed the respective values allowed for the species and grade of lumber
to
be
used.
It
is
good
practice to consult the local lumber supplier(s) before finalizing a beam design,
to
determine what
sizes, species and grades are
on
hand or can be readily secured.
Use of
the
tables
is
illustrated in the
two
examples which follow.
Example
1.
Assume a
span
of
14'-
0"
for a species and grade of lumber having a fiber bending stress,
F,,
value
of
1400
psi
to
carry
a
total load
of
So00
pounds. The problem
is
to
determine the
size
of beam required.
Turn to the page
on
which the
14'-
0"
span is listed and, under the column headed
1400,
read down until
the total load of
SO00
pounds is reached. Then read to the left
to
note the size of beam required.
In
this
case.,
the required
size
is a nominal
6
by
12
member having a total load capacity,
W,
of
8082
pounds
or
a load per
foot, w, of
5-77
pounds.
WOOD
BEAMS
The apparent horizontal shear design value,
Fy,
required for the load of
8082
pounds is 96
psi and the modulus of elasticity, E, required to limit deflection to P/360 under the same load is
1,530,000 psi. Thus, the nominal
6
x
12
beam selected t o carry the
8,000
pounds must have
values of not less than
Fb
=
1400
psi,
Fv
=
96 psi and
E
=
1,530,000 psi.
If
deflection control is
not important for the case under consideration the required E value may be ignored.
If
Fv
is
critical, the adjustment procedure below may be applied.
Example
2.
Assume a. span
of
15
'-0"
and a beam size of nominal
8
by
12
with a fiber
bending design value, Fb, of
1600
psi. The problem is to determine the total load capacity,
W,
of
the beam.
Turn t o the page on which the
15""
span is listed and read down the left column
until
the
8
by
12
size is reached. Then read across to the right t o the column headed 1600 where it is
shown that the total load capacity,
W,
is
1 1
755 pounds, the load per foot, w, is
784
pounds, the
apparent shear,
Fv,
required for the load of 11755 pounds is
102
psi and the modulus
of
elasticity,
E,
for a deflection limit of ai 360 under the same load is 1,878,000 psi.
In both Examples
I
and
2,
the total load,
W,
includes live and dead load.
To
determine the
allowable live load which may be super imposed on the beam, the weight of the construction
materials should be deducted from the total load,
W.
Adjustment
of
Modulus of Elasticity
As previously stated, the modulus of elasticity values listed in the tables are based
on
limiting the initial deflection due to total live and dead load,
W,
to P/360. Where other
deflection limits are acceptable the tabular values of
E
may be adjusted by multiplying them by
the following factors:
For limit of t i 300
0.833
For limit of
PI240
----
0.667
For limit of
!?/I80
0.500
When it is appropriate t o design for deflection due t o live load only (see page 34), the
determine the live load supported by the beam, either from known design
loads
or
by subtracting the weight of supported construction materials from
the tabulated total load,
W,
multiply the tabular value of E by the ratio of the live load divided by the
tabulated total load,
multiply the resulting value of
E
by the applicable deflection limit factor,
if
the limit is other than ai360.
required value
of
E may be calculated as follows:
(a)
(b)
(c)
Adjustment of Shear Stress
When the tabulated horizontal shear value, Fv, exceeds the shear design value for the
member, the tabular value may he multiplied by:
t o adjust Fv by neglecting that portion of the load within a distance from either support equal t o
the depth of the beam.
I f
the adjusted Fv still controls member design, use of the detailed shear
NOOD BEAMS
SIZE
OF
BEAM
design procedure in the National Design Specification
for
Wood Construction, available from the
National Forest Products Association, may be considered.
Fb
900
1000 1100
1200
1300
1400 1500 1600 1800 2000
Interpolation
of
Tabular Values
Design loads and induced values
of
F, and E
for
bending stresses intermediate
of
those
listed
in
the column headings may be determined through straight-line interpolation.
Example. For a nominal
6
by
12
beam with Fb
of
1200
on
20- 0"
span:
W
=
4849,
w
=
242,
F,
=
57
and E
=
1,878,000.
For
a
beam of the same size
on
the same span with Fb of
1300,
the preceding values
for
W,
w,
F, and
E
are multiplied by
I3/
12.
W
459
510
561
613
664
715
766
W
115
128
140
153
166
179
191
2 x 4
66
73
80
88
95
102
109
2
926
1029
1131
1234
1337
1440
1543
W
766
851
936
1021
1106
1191
1276
W
191
213
234
255
276
298
319
66
73
80
88
95
102
109
926 1029 1131 1234 1337 1440 1543
E
F V
3 x 4
WOOD BEAMS-SAFE LOAD TABLES
817
919
1021
204
230
255
117
131
146
1646 1851 2057
1361
1531
1701
340
383
425
117
131
146
1646
1851
2057
4'-
0" SPAN