PIER
, in Building, denotes a mass of stone, &c, opposed by way of fortress, against the force of the sea, or a great river, for the security of ships lying in any harbour or haven. Such are the Piers at Dover, or Ramsgate, or Yarmouth, &c.
Piers are also used in Architecture for a kind of pilasters, or buttresses, raised for support, strength, and sometimes for ornament.
Circular Piers, are called Massive Columns, and are either with or without caps. These are often seen in Saracenic architecture.
Piers, of a Bridge, are the walls built to support the arches, and from which they spring as bases, to stand upon.
Piers should be built of large blocks of stone, solid throughout, and cramped together with iron, which will make the whole as one solid stone. Their extremities, or ends, from the bottom, or base, up to highwater mark, ought to project sharp out with a saliant angle, to divide the stream. Or perhaps the bottom part of the Pier should be built slat or square up to about half the height of low-water mark, to encourage a lodgment against it for the sand and mud, to cover the foundation; lest, being lest bare, the water should in time undermine and ruin it. The best form of the projection for dividing the stream, is the triangle; and the longer it is, or the more acute the saliant angle, the better it will divide it, and the less will the force of the water be against the Pier; but it may be sufficient to make that angle a right one, as it will render the masonry stronger, and in that case the perpendicular projection will be equal to half the breadth or thickness of the Pier. In rivers where large heavy craft navigate, and pass the arches, it may perhaps be better to make the ends semicircular; for though this figure does not divide the water so well as the triangle, it will better turn off, and bear the shock of the craft.|
The thickness of the Piers ought to be such as will make them of weight, or strength, sufficient to support their interjacent arch, independent of the assistance of any other arches. And then, if the middle of the Pier be run up to its full height, the centring may be struck, to be used in another arch, before the hanches or spandrels are filled up. They ought also to be made with a broad bottom on the foundation, and gradually diminished in thickness by offsets up to low-water mark.
To find the thickness FG of the Piers, necessary to support an arch ABM, this is a general rule. Let K be the centre of gravity of the half arch ADCB, A = its area; KL perpendicular to AM the span of the arch, OB its height, and BC its thickness at the crown: then is the thickness of the pier
Some authors pretend to give numbers, in tables, for this purpose; but they are very erroneous. See my treatise on the Principles of Bridges, sect. 3.
