stresses due to this change may be neglected. Cut pieces of wood will warp 

 toward the sun unless restrained or dried before use. 



d. Wave and Current Effects . Because wood has less strength than some 

 other commonly used structural materials, a larger wood member is needed to 

 adequately protect the wood against the force developed by water currents 

 and waves even where a solid face is presented to the wave and current 

 forces. The resilient characteristic of wood, however, allows wood members 

 to absorb impact energy and rebound intact better than concrete and steel. 



e. Effects of Severe Temperature and Ice . Temperature effect upon wood 

 strength is immediate and its magnitude depends on the moisture content of 

 the wood. If the exposure is above normal atmospheric conditions for a 

 limited period and the temperature is not excessive, wood can be expected 

 to recover essentially all its original strength. Air-dry wood can be 

 exposed to temperatures of about 65.6° Celsius (150° Fahrenheit) for a 

 year or more without significant permanent loss of most of its strength 

 properties. Ice or freezing conditions will impact mechanically by causing 

 fiber failure and thus loss of strength through a reduction of section. 



f . Marine Organisms . As discussed in subsection 6, Destructive Biota, 

 the principal marine organisms that cause wood destruction in the coastal 

 zone are Teredoes, Limnoria, Poria and Merulius. Most of these animals 

 attack wood as free-swimming organisms. They bore an entrance hole in the 

 wood, attach themselves and grow in size as they bore tunnels into the wood. 

 Wood structures are protected from these animals by proper treating with 

 creosote or coal-tar solutions or by a protective enclosure. 



g. Periodic Wetting and Drying . Wood in a marine environment should 

 always be protected from excessive moisture or water and therefore has 

 little change in its structural or mechanical properties. If the preserva- 

 tives eventually leach out of the wood cells, then alternate expansion and 

 contraction of the wood cells can result in gradual and slow deterioration. 

 Wood structures have a history of long service life even when subjected to 

 alternate wetting and drying. 



h. Wind Erosion . Wood being a relatively soft construction material as 

 compared to concrete or metal, it can be eroded by wind action. Wind does 

 not erode wood directly but strong winds picking up particles of sand or 

 other materials will cause a wood surface to wear. Erosion of this kind 

 will usually take place near the ground line. 



i. Effects of Burrowing Animals . Marine animals will burrow into wood 

 very rapidly unless the wood is protected by appropriate preservatives. 

 Wood is sufficiently soft as to offer little resistance to burrowing attack 

 and in addition serves as a food source to the animals. Onshore termites 

 are very destructive to wood. These attacks, if left unchecked, will result 

 eventually in the loss of all structural properties of wood. 



j . Effects of Flora . There are no reported effects of flora growth on 

 wood. 



k. Fire . Wood, when exposed to fire, forms a self-insulating surface 

 layer of char and thus provides a degree of its own fire protection. 



274 



