"Fire resistance of connections in timber structures"

This paper describes a series of experiments to obtain the embedment strength of wood at elevated temperatures. The results will be used in Johansen’s yield equations to predict the fire resistance of nailed, screwed, and bolted timber connections. Johansen’s yield equations are used in Europe and USA to predict the ultimate strength of bolted or dowelled connections in timber members, but only at ambient temperatures. Recently, several researchers have begun to investigate the embedment strength of bolted or dowelled connections at elevated temperatures applicable to fire conditions. Embedment strength varies with temperature. Recent studies have proposed a tri-linear relationship for bolted connections in LVL, following embedding tests carried out at high temperatures in accordance with the ISO standard. This research extends the investigation to determine if the model is accurate for other types of connections such as screws and nails. All connection specimens had exposed steel side plates. Bolts, nails and screws were tested by loading in shear. Temperatures ranged from ambient to 250oC. The programme was divided into two different heating regimes. In Part 1, screwed connections were tested in tension parallel to grain, using a constant temperature furnace with 2 hours pre-heating time. In Part 2, compression tests were performed on nailed, bolted and screwed connections both perpendicular and parallel to grain, after several hours of heating for long enough to obtain a constant temperature throughout the wood. The results from short-term heating tests (Part 1) and longer-term heating tests (Part 2) show very different answers. The two hour heating tests showed a tri-linear decline of the embedment strength, as found by earlier experiments, while the longer-term oven tests shows a more linear decline. The difference is attributed to different moisture profiles in the wood. The paper shows how this information can be used in design of timber fasteners for fire resistance.