70 



Structural Requirements 



The design of a structure is often controlled by considerations which affect its 

 potential physical life but are quite independent of the length of time the structure is 

 to serve, or its economic lite. 



An instance of this already noted is the case of ferry slips, which must be Iniilt of 

 flexible piling. Since timber is best suited to this use, and for that purpose is commonly 

 used untreated, such structures have a limited physical life. 



Another instance is that of trestles and wharves without superstructures, in which 

 tke piling can be renewed with comparative ease. A relatively short-li\ed piling is 

 likely to be selected for such structures, giving low first cost and avoiding the possibility 

 of economic loss if changes are found desirable in the future. 



Short-li\ed piling usually requires considerable maintenance, however, and in 

 structures where the piles are inaccessible or where maintenance would be very ditficult, 

 the short-lived material is uneconomical and long-li\'ed or permanent types are 

 indicated. 



The general axiom that "a building is only as good as its foundation" may be 

 paraphrased by its converse, "the foundation need only be as good as the building" — 

 unless there is definite assurance that the same foundation can continue to serve 

 future conditions after the superstructure is rebuilt. If this assurance is lacking, it is 

 likely to be assumed that the foundation can be rebuilt at the same time as the super- 

 structure. Under average conditions the foundation should be capable of service at 

 least equal to that of the superstructure: tempt>rary, permanent, or fireproof buildings, 

 as the case may be, should be supported by piling of comparable qualities. 



Structural requirements may necessitate the strongest and the most permanent 

 materials regardless of all other considerations. This will obtain in the case of seawalls, 

 bridge piers, substructures for buildings having excessive floor loadings, and similar 

 cases. 



The limitations of available materials may eliminate short-lived types, as, for 

 example, where the depth of water is too great for the use of available timber piling. 



The conditions of exposure, such as in "ocean exposure," may eliminate short- 

 lived material and recjuire the strongest and most permanent available. 



INSTALL.-XTIOX AND CONSTRUCTION PRACTICE 

 Preliminary Investigation of Substrata 



Test piles and borings should be utilized to ascertain the condition of the sub- 

 strata. This is necessary that correct lengths be secured and the piles driven properly. 

 It is particularly necessary where partial length protection is to be used, the portion 

 above the mud line being protected and that below unprotected. To accomplish this 

 the exact penetration should be known beforehand so that the protection will be 

 carried to the proper point. Alternating layers of hard and soft strata often lead to 

 mistakes in driving. During this operation when the pile strikes a hard layer the pene- 

 tration decreases, which is normally an indication that the driving can be stopped; 

 a moment later the pile may pass through the hard layer and sink rapidly through a 

 soft formation. After this the pile is in danger of being overdriven with the expectation 

 of penetrating another layer. 



Such overdriving is dangerous for all types: in reinforced concrete it develops 

 cracks through which water can enter and corrode the reinforcing steel; in protected 

 timber piling it causes checks and flaws which open the way for borer attack and may 

 render the protection practically worthless. Preliminary investigations will show 

 whether or not jetting should be used, to relieve the driving and consequent effects. 



