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close this bookManufacture of Ceilings - Course: Timberwork techniques. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.)
View the document(introduction...)
View the document1. Purpose of Ceilings
View the document2. The Wooden Beam Ceiling
View the document3. The Framing
View the document4. The Attic and Collar-beam Framing
View the document5. The Pitch Size
View the document6. Load on Wooden Beams
View the document7. The List of Timber
View the document8. The Wall Plate
View the document9. Flooring Sleepers
View the document10. Special Constructional Recommendations

6. Load on Wooden Beams

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