HISTORIC IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.

Appendix 4.

Observing the movements of iron-ties in a timber-framed structure.

This was a year-long study to highlight the cyclical movements that are undergone by the timber-frame of the Wheat Barn at Cressing Temple. A section of one of the trusses that was known, by previous observation, to move was monitored with two-weekly readings being recorded. The section was particularly relevant as the tie-beam had failed historically and had been re-enforced with wrought-iron L-ties at each end. Thus by measuring the displacement of the failed tenons the cyclical movement exerted upon the L-ties could be measured.

Method.

A series of measuring points was selected and galvanised nails tapped in, one either side of the gaps between the timbers. The distances between each pair of nails was measured with a Moore & Wright vernier caliper and recorded.

Reading were taken on a two-week basis as and when access was practical.

Sample.

The two points measured were those on the tie beam on the north side of Truss 5 of the Wheat Barn at Cressing Temple. Here shown on the left of the human scale figure. The two L-ties are those securing the tie-beam to the wall top-plate and to the arcade post.

The end of the tie-beam where it met the top-plate was of particular concern. Where the dovetail had failed, the tenon on the upstand had been over-stressed and the upstand had consequently split down its grain and slowly rotated outwards. The L-tie had been inserted to arrest further movement. A slip gauge was placed (by the author) on the upstand to register further movement after some local repairs in 1995 caused it to twist further out.

Results.

Three sets of readings were taken on each visit. A record sheet, comprising of an identifying drawing and date and readings column was produced and duly filled in. The readings were taken by scaling a ladder and using vernier calipers to measure the distance ‘outside to outside’ of the pairs of nails.

Table of Readings.

Date	Point A	Point B	Point C
08 06 01	96.8	106.4	67.9
25 06 01	97.0	106.3	68.0
06 07 01	97.1	106.4	67.9
27 07 01	97.4	106.4	68.0
80 08 01	97.2	106.3	67.9
20 08 01	97.2	106.4	67.9
03 09 01	97.3	106.4	67.7
13 09 01	97.2	106.5	68.1
28 09 01	97.1	106.4	68.0
18 10 01	96.9	106.2	67.9
12 11 01	96.8	106.3	67.9
28 11 01	96.8	106.2	67.8
10 12 01	96.6	106.1	67.6
21 12 01	96.6	106.2	67.6
08 01 02	96.0	105.9	67.6
01 02 02	96.3	105.9	67.3
15 02 02	96.6	106.2	68.0
28 02 02	96.4	106.3	68.1
15 03 02	96.6	106.3	67.9
26 03 02	96.6	106.2	67.8
10 04 02	96.7	106.2	68.0
23 04 02	96.6	106.5	67.9
02 05 02	96.5	106.4	67.8
15 05 02	96.4	106.2	67.8
29 05 02	96.6	106.4	68.0
17 06 02	96.7	106.4	68.1
28 06 02	96.8	106.3	68.0
11 07 02	96.9	106.3	67.9
22 07 02	97.3	106.4	68.0
01 08 02	97.4	106.7	67.9
13 08 02	97.5	106.6.	68.1
28 08 02	97.6	106.9	68.0

Conclusions

The table shows that the joints are far from static and that even over a bi-weekly cycle of readings the movement can be quite discernible. The movement can be attributed to a number of factors. Thermal expansion and contraction, wind pressure and changing humidity. Vehicular traffic might have a slight impact but the building is quite isolated. However 30.000 people a year visit the barn and this would definitely induce movement.

End