Chapter 2

Why do timber frames fail? 3/5.

2.3 Loading failure - timber unable to take load or is later overburdened.

Loading failures are relatively uncommon in early timber-frames. The sections are over-engineered with nearly square sections and the armature of the external cladding and the roof battens normally prevents any failure under normal load. Loading failures usually occur after the timber has been sufficiently weakened by decay, shrinkage or by reduction in section by removal of timber.

A typical loading failure would result in racking along one or more axes. The loosened pegged joints allow sufficient rotation for the timbers to move under their own weight. (Fig 2.9).

Windbraces in the Granary at Cressing Temple

Fig. 2.9. The interior of the granary at Cressing Temple. The pegs in the butt purlin roof have all loosened over the years since it was erected in 1623. The building has badly racked and successive timber windbraces and iron tie rods have been inserted to arrest the movement.

2.4 Movement - thermal - weather/ wind - accidental - changing loads
(new roof/ wall materials).

Movement accounts for most joint failures in timber framed buildings. Even in a building that is well protected from the buffeting of the weather it will be subject to movement from the changing forces of the wind, rain and snow on its roof structure.
Appendix 4 contains details of a series of observations of the normal movement of a timber-framed building. It can seen that the cyclical movement is constant and considerable.

No building is free from thermal movement. External temperatures and heat generated from within by heating systems, cooking, electrical appliances and body heat all lead to constant thermal movement of the frame as its elements expand and contract. Extremes of changing temperature can actually lead to catastrophic failure from the build up of internal stresses within a localised point. (Fig 2.10).

Stress fracture in timber

Fig.2.10. A cracked beam in The Rectory, Kelvedon Hatch. The calamitous failure (highlighted) was caused by its proximity to a powerful modern stove which, over a period of 20 years, heated and cooled the timber to beyond its endurance.

The walls and floors will also be subject to the movement induced by the occupants of the building walking, opening and closing doors, moving and placing furniture and other activities. Accidental impact loads will also affect the frame in varying degrees, from a slight knock to a vehicular impact to a falling tree.

Changing the loads that the building was designed for will also lead to movement. Inserting a new floor or firestack, re-roofing, extending or remodelling will all induce movement. Subsidence or ground heave may affect the frame. The modern practice of underpinning always leads to movement in a timber framed building usually signified by cracking render. (For this reason Historic Buildings Advisors in Essex will invariably oppose underpinning). Changing the external cladding from one type to another will also lead to movement. (Fig. 2.11).

Boarded walls in the Wheat Barn at Cressing Temple

Fig 2.11. A tableau in the Wheat Barn at Cressing Temple shows how the barn was originally walled with vertical boards. Around 1620 AD the walls were completely dismantled and infilled with brick nogging. A significant amount of ironwork was then introduced to arrest the subsequent movement.

2.5 Decay and Erosion

Click for Index

src=/bin/counter.pl?h=1 width=1 height=1>