This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
A joint is the place where two different portions or pieces of a structure meet or unite, and it consequently forms the division or union between them, as well as a constituent factor of the structure it helps to consolidate. A joint is either simple or compound, fixed or movable, and however perfect it may be it is almost invariably a source of weakness. In this light the monolithic description of concrete is superior to masonry; but, as an opposite instance, the superiority of a beam sawn asunder down the middle, reversed, and bolted together again, over a jointless one, is worthy of note. As a general rule all parts of a joint should be equally strong, the strength of the entire joint equal if possible to that of the parts joined, and its form, when used in framing or other analogous combination, such as to direct the pressures, as far as skill will allow, along the axes of the component pieces. Moreover, a joint should never be of a complicated nature, but its parts, reduced to the smallest practicable number, ought to be so devised as to resist in the most scientific manner the particular kind of stress that each will be tried by, whilst as a whole it satisfies the object of its formation through eco nomic construction, fitness, and soundness.
By a simple joint is meant one consisting of nothing beyond the contact of meeting surfaces, such as an abutting joint in carpentry, without fastenings of any kind - thougl it may otherwise ill-devisedly be of an elaborate and intri cate character; and by a compound joint is meant one in which a cementing medium, or some kind of attachment, is used to assist weight or pressure in keeping the contiguous parts from moving or racking - the first step towards separation and ruin. A mortar joint forms a common illustration of the latter class, for the mortar not only binds the bricks together, but before setting affords a conveniently yielding bed to enable each brick to take its bearing, so to speak, and thus to be in its best position to resist external forces. Such a joint is necessarily a compound one, since the mortar joint proper between any two bricks is compounded of the two joints formed by the adhesion of the intervening mortar to the surface of both bricks. Either a compound or a simple joint may be fixed or movable, the sliding joint in joinery affording a familiar instance of the simple movable variety. In construction generally joints are for the most part compound and fixed as well, but whether in engineering works or in the carcase or finishing of a building, the simple fixed description is of common occurrence ; as instanced, for example, in the bed joint at the bearing end of a flitch beam seated on a stone template, and in the key joint between the plasterer's rendering and the wall's rough surface. The movable sort, which includes hinged, expansion, adjustable and lifting joints, appears occasionally in carpenters', joiners', smiths', and plumbers' work.
A compound joint consists, as has been said, of something besides the meeting or united surfaces, and this may he either a cementing medium such as mortar, cement, asphalte, glue, white lead, solder, sulphur, lead, cast zinc, putty, clay, etc, or else a fastening or connection consisting of one or more of the following: - hoops, straps with or without gibs and cotters, cast or wrought iron shoes and sockets, screw bolts, wedges, dowels, cramps, bolts, pins, keys, treenails, rivets, plates, nails, spikes, sprigs, screws, etc. Sometimes one or other of these may be present in conjunction with a cementitious material, but in most instances it is found advisable, owing to the disturbing influence of settlement, shrinkage, expansion, impact, etc, not to trust alone to superincumbent pressure or the operation of gravity for the maintenance of joints, but to fortify simple or interlocking contact by the extraneous aid of a third substance.
And here it may be observed that a joint is wrongly defined when it is said to be the space occupied by the substance between two pieces of material. The surfaces of course must also be taken into account, else how can joints be rightly named, as is often the case, after the particular manner in which they are wrought, the common dovetail, for instance, furnishing a well-known illustration ? It is equally wrong to define a joint to be an interstice, for besides being frequently distinguished by the way in which they are filled, joints ought to be full and close, excepting when a little slackness is left purposely in framing and elsewhere to allow for the inevitable settlement when the load is felt. The evils arising from punching involve much loose riveting, not only in the booms of girders but likewise at the junctions of the cross-bracing ; but this is accidental, and the reason why joints in wood are so often open is because careless setting out, bad fitting, and imperfect seasoning cause unequal bearing or an excess of shrinkage, and preclude the surfaces from closing or keeping close either when at rest or under the stress of pressure, or when tried by variations of temperature or the hygrometric condition of the air. Quirks, mouldings, and architraves in joinery are notoriously useful in concealing the interstice, or air-space, arising from the shrinkage of insufficiently seasoned stuff ; which air-space reminds one of the spot on the dinner-table that ought to have been occupied by the haunch of venison, which not turning up in due course, left it to the spot in question simply to indicate where the joint ought to have been.
The nomenclature of joints is for the most part determined by their shape, character, position, and mode of execution or finish; from the particular kind of cementing material, or fastening, or connection employed for preventing severance; and likewise from the structure to which they belong. We thus have rebated joint, coarse joint, bed joint, wiped joint, flushed joint, mortar joint, pinned joint, strap joint, arch joint. It is rarely, however, that a name is derived from that of the materials united, though the term metal joints is generically used to denote the whole class of joints between metallic surfaces. Jointing and joining are synonymous with joint, but the latter is not frequently used, whilst the former is commonly employed for the sham joints in stucco to imitate masonry. Amongst plumbers connection often stands for joint.
Omitting altogether the consideration of those cases in which a heavy draw is made upon the designer's art to combine strength and elegance in complicated joints, as, for example, where the extremities of level, upright, and inclined members cluster together for mutual support, perhaps there is no other branch of labour in the whole round of a general builder's business, which indicates and tests his responsibility and capacity so much as the execution in a workmanlike manner of the particular species of work now under notice. Excepting with regard to his own individual trade he is quite left in this respect to the skill, honesty, or pleasure of the craftsmen in the others; and when the number and variety of the joints are considered which they have thus to make, as well as the difficulties and obstacles which often stand in the way of making them sound and durable, it seems almost incredible that anybody, however practical yet incompetent to make them, and at the same time lacking the knowledge of the theory of their construction, should presume to guarantee their fitness and reliability. And this is more particularly the case since bad joints do not of necessity hurriedly disclose themselves. On the contrary, it often happens that storm and tempest, frost and drought, have to be encountered before strain, vibration, or fluctuation of dimensions demonstrate their indifferent or even beggarly character. Thus the question arises, can work performed under such conditions and supervision, unchecked by the scientific and general knowledge of an architect or engineer, produce carcases and finishings or habitable dwellings that if sold are not usually sold as being superior in construction and finish to what they really are ? And again, assuming this to be the case, does the unwary purchaser suffer most by ignoring a surveyor, or by the builder being his own architect? Leaving these inquiries for the inquisitive to probe, it remains to describe succinctly and alphabetically under each trade the joints by which diversified units and substances are aggregated into a perfect structure. Within the narrow compass, however, of a brief explanation, it will not be feasible to particularise all the modifications and applications of each sort of joint that have sprung from widespread use influenced by individual fancy, local custom, or the effects of the ever restless desire for change and novelty, which is as active amongst the building fraternity as elsewhere, though perhaps more under control, owing to the risk to life and property attachable to a departure from the well-beaten track.