Chap. XXIII.
1355
troduce struts and binding-pieces in the position, where such a change in the arrangementis likely to occur.
The timber employed in the construction of a bridge requires to be considered ina different manner to that in ordinary buildings: besides the weight of Jhe framing androadway, there is a constant vibration, from the movement of carriages and passengers overit, and the timbers so acted on affect one portion at a time. As the load passes over, thetimber immediately bearing it bends, and recovers itself when the weight is removed :experience, however, proves that we must not permit any portion of the structure to bendbeyond its powers of elasticity or its ability to recover the original form ; constant actionupon timber in the course of time destroys its flexibility, and experiment teaches us thatthe elasticity of timber decreases with time, and particularly when subjected to the effects ofa momentary action by a load passing over it. But the momentary load is less dangerousto its strength than that which acts continually, so that in valuing the strength of a pieceof timber to support the heaviest weight to which it may be subjected when immovable,there will be no danger in applying it to a bridge. Timber of good quality and properlyseasoned endures a great length of time, particularly if protected from moisture and theworm : the alternations of humidity and dryness occasion a constant change in the movementof the fibres, and in the course of a few years an entire disorganisation.
'When trees are growing, that portion of the wood nearest the heart is the strongest, butwhen their growth ceases, the other portions acquire the same strength, and when on thedecline, the change commences at the heart, so that the outer parts, or those nearest thebark, are the most to be depended upon : when decay has commenced at the pith orheart, it extends by degrees until the whole timber perishes; too much care cannot betaken in the selection of timber, and it is well to observe the dimensions of the trees in theforests, that afford the supply, and to choose only those which are beneath the averagediameter, so as to be certain that decay has not already commenced. Timber is often cutdown the middle for the purpose of ascertaining if the heart be sound ; when the concentricrings are destroyed there is less resistance offered to pressure than before.
Where timber is free from attacks by the worm, and no change has taken place atthe heart, humidity and alternate dryness are the only effects we have to guard against:the fibres on the exterior become more saturated with moisture, which they take in from theatmosphere, and again readily yield w r hen the air is heated, and the surface being that portionmore exposed to frequent alterations, its disorganisation will commence first, the fibres losetheir elasticity, and, no longer exerting any resistance, a diminished scantling is the result.
Timbers employed in bridges may be considered as losing a portion of their scantlingevery year, and consequently their strength : the bridge in the course of time, no longer ableto support its load, must fall, and this ruin is delayed so long as the proportion givento the timbers admits of the annual loss, without reducing its scantling below the minimumthat would bear the weight imposed upon it: in consequence of the cost increasing withthe extra scantling of the timbers, we are restrained from making them perhaps so large asthey ought to be in the first instance; true economy, however, consists in using thequantity of timber sufficient to bear the required weight.
There can be no difficulty in ascertaining the weight or load to which any piece oftimber is subjected, after the rules already given: these may be applied to timber eithersingly or collectively, but they must be carefully analysed and calculated.
Of Piers -These are usually composed of one, and sometimes of several rows of
piles, driven in the direction of the current; but the best plan is to drive the firstso that their heads may be cut off immediately below the water, and on them to constructa platform of timber, into which the posts which carry it may be framed : this constructionhas a considerable advantage, as the parts above water, which more frequently go to decay,can be easily repaired, whilst that constantly immerged endures for a greater length of time.
There are various methods of uniting these timbers together; one is by iron pins, about3 feet in length, passed through the horizontal timbers into both the upper and lower piles;
Fig.2194.
Fig. 2196.
Fig. 2195.
PIERS.
PLAN OP CROSS TIMBERS.
the horizontal timbers are also strapped to them by alternate horizontal and vertical ironbars, the upper timbers of the pier being level with the surface of the w r ater.