Manufacture of Ceilings - Course: Timberwork techniques. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.)
 (introduction...) 1. Purpose of Ceilings 2. The Wooden Beam Ceiling 3. The Framing 4. The Attic and Collar-beam Framing 5. The Pitch Size 6. Load on Wooden Beams 7. The List of Timber 8. The Wall Plate 9. Flooring Sleepers 10. Special Constructional Recommendations

### 5. The Pitch Size

The wooden beams of a wooden beam ceiling should be placed at equal distances so as to equally distribute the load on all beams.

If the distances between the wooden beams are not shown in the drawing, the pitch size is to be calculated.

Figure 12

Determination of the pitch size

(1) bay 1, (2) bay 2, (3) bay 3

Lr length required for the calculation, BL length of wooden beam, T pitch size (beam spacing), B clear width of the room, L length to be distributed, a bearing length, b1, b2 width of wooden beams, c minimum distance of wooden beams from the wall, 1, 2, 3, 4 wooden beams

The pitch size depends on the thickness of the flooring and is calculated to formula (1).

T = pitch size (distance of wooden beams)
Lr = length required for the calculation
nF = number of bays

The number of bays is determined by means of formula (2).

Tzul = pitch size depending on the thickness of the flooring

20 mm thick flooring - - - pitch size Tzul = 600 mm
24 mm thick flooring - - - pitch size Tzul = 800 mm
30 mm thick flooring - - - pitch size Tzul = 1000 mm

If the resulting nF is a decimal fraction, it is to be rounded off to bring it up to the next integer number.

The rounded-off number of bays is provided with a superscript mark for identification.

For example: nF = 6.4; n’F = 7

The length to be distributed is determined by deducting the wooden beams’ minimum distance from the wall at both ends from the clear width of the room.

L= B - 2c (3)

L = length to be distributed
B = clear width of the room
c = minimum distance of the wooden beams from the wall, normally 20 mm

For the calculation of the pitch size, the arithmetical length is required. It results from formula (4):

b = wooden beam width

Formula (5) gives the number of wooden beams to be placed over a calculated distance:

nB = n’F + 1 (5)

The length of the wooden beams is calculated to formula (6):

BL = w + 2a (6)

BL = length of wooden beam

w = clear width of the room

(in straining direction of the beams)

a = bearing length

(a = at least 150 mm)

Why should the wooden beams have equal distances?

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Why should the wooden beams have equal distances?

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Why must the wooden beams have a distance of a least 20 mm from the wall?

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What does the formula nB = n’F + 1 say?

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Why must nF be rounded off to the next integer number?

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