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The wooden beams of one framing cannot be loaded by loads of any quantity.
Why can the wooden beams not be loaded by loads of any quantity?
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For the loading of such ceilings a continuous surface load is assumed. The basis in housing construction is a working load of 2.0 kN/m2.
Such load is reached in exceptional cases only. It would mean that about 48 persons would stay at the same time on a wooden beam ceiling of a room with an area of 16 m2 (about 0.75 kN/person).
When the wooden beam ceiling is loaded, the wooden beams deflect. Such deflection will always occur with loads but it must not exceed 1/300 of the span.
The uniformly distributed, allowable (safe) load on a wooden beam ceiling, with due consideration to the deflection of the wooden beams, is determined through the moment of inertia.
The table for cross sections of wooden beams shows sections for coniferous wood (NH) of grade II (Gk II) for clear widths of rooms (w) from 2.0 m to 5.0 m as well as for distances of wooden beams of 600 mm, 800 mm and 1000 mm.
The sections of the wooden beams refer to a total load (dead load and working load of the wooden beam ceiling) of 4.0 kN/m2.
Table 3: Table for cross sections of wooden beams
This table shows cross sections of wooden beams (NH, Gk II) for
a load of 4.0 kN/m2.
(dead load = = 2.0 kN/m2, working
load = 2.0 kN/m2)
The dead load of the wooden beam ceiling corresponds to that of a heat and sound-insulating ceiling.
|
Clear width of room |
Distances of wooden beams | ||
| |
600 mm |
800 mm |
1000 mm |
|
in mm w |
b/h in cm2 |
b/h in cm2 |
b/h in cm2 |
|
2000 |
6/12 |
8/12 |
10/12 |
|
2100 |
8/12 |
10/12 |
12/12 |
|
2200 |
8/12 |
10/12 |
10/14 |
|
2300 |
10/12 |
12/12 |
10/14 |
|
2400 |
12/12 |
8/14 |
12/14 |
|
2500 |
12/12 |
10/14 |
14/14 |
|
2600 |
10/14 |
10/14 |
14/14 |
|
2700 |
10/14 |
12/14 |
12/16 |
|
2800 |
12/14 |
12/14 |
12/16 |
|
2900 |
12/14 |
14/14 |
14/16 |
|
3000 |
14/14 |
10/16 |
16/16 |
|
3100 |
10/16 |
12/16 |
16/16 |
|
3200 |
12/16 |
14/16 |
14/18 |
|
3300 |
12/16 |
14/16 |
14/18 |
|
3400 |
14/16 |
16/16 |
16/18 |
|
3500 |
14/16 |
16/16 |
16/18 |
|
3600 |
16/16 |
14/18 |
18/18 |
|
3700 |
16/16 |
14/18 |
14/20 |
|
3800 |
14/18 |
16/18 |
16/20 |
|
3900 |
14/18 |
16/18 |
14/22 |
|
4000 |
16/18 |
18/18 |
14/22 |
|
4100 |
16/18 |
18/18 |
16/22 |
|
4200 |
12/20 |
14/20 |
16/22 |
|
4300 |
14/20 |
16/20 |
18/22 |
|
4400 |
14/20 |
18/20 |
18/22 |
|
4500 |
16/20 |
18/20 |
18/22 |
|
4600 |
16/20 |
20/20 |
20/22 |
|
4700 |
14/22 |
20/20 |
22/22 |
|
4800 |
14/22 |
16/22 |
18/24 |
|
4900 |
16/22 |
18/22 |
^0/24 |
|
5000 |
16/22 |
18/22 |
20/24 |
Coniferous wood (NH), such as spruce, pine wood grade II (Gk II) means:
- wood free from wood pest,- wood with knots having a diameter of less than 1/3 of the wood width but not exceeding 70 mm,
- the sum of all knot diameters over 150 mm wood length per area of cut must not exceed 2/3 of the wood width.
If the cross sections of wooden beams shown in the table are not in stock, other sections may be used.
Other cross sections, however, would mean to calculate them through the section modulus.
Assuming that wooden beams are mainly used with rectangular section, the small face is always used as bearing surface.
If the width of the beam shall be changed, the height of the beam must be changed, too.
The section modulus for wooden beam sections is calculated by means of the formula:
W = section modulus referred to the x axis
b = width of the wooden beam
h = height of the wooden beam
If the height of the beam shall remain unchanged, the formula is to be converted to give b as follows:
If the width of the beam shall remain unchanged, the formula is to be converted to give h as follows:
If the result is a decimal fraction, it is to be rounded off to the next greater size of wooden beams.
For example:
b = 13.4 cm - - - b = 14 cm
b = 14.1 cm - - - b = 16 cm
h = 21.5 cm - - - h = 22 cm
h = 20.5 cm - - - h = 22 cm
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