At designing of ladders it is necessary to meet following conditions:
- the ladder should correspond to type of movement which to it intends to provide;
- to correlate the sizes of a ladder to space available for its placing;
- the ladder should correspond to difference of levels which are overcome with its help;
- correctly to choose and place support;
- even the smallest ladder should be absolutely safe and as much as possible convenient;
- it should correspond to the functions and the general architectural decision of a building.
The Ladder consists of marches and the platforms which quantity depends on the general constructive architectural decision of object. As a ladder march is called the inclined part of a ladder which consists of a number of steps and connects two platforms. Basic elements of marches are prostup (a horizontal part of a step) and podstupen (a vertical part of a step). Their sizes, in turn, depend on a bias and the general height of a ladder.
At erection of ancient architectural constructions builders adhered to following main principle: that descent and lifting were as much as possible convenient, a parity between distance on which the person moves forward, and in distance on which it falls or rises, should be exact. .
More often width appear corresponds to width of a human step. It makes 60-64 This size see corresponds to a step which the person moves on a horizontal surface, and can indirectly define approximate width of steps at movement on a ladder. By practical consideration the width of a step can be calculated under the formula 2h + sz=60-64 sm where for h the height podstupeni in centimetres undertakes, and for sz - width svesa podstupeni in centimetres.
With the help fig. 72 it is possible to define a step parity on a flat surface and on a ladder.
a Fig. 72. A parity of width of a step on a plane and on a ladder :
And - a garden ladder; - terrasnaja a ladder; in - an external ladder
More often the sizes of ladder steps are designated in the form of product or fraction (an example: 17 X 29, 17/29) where numerator and mnozhimoe - the size podstupeni, and a denominator and a multiplier - the size appear.
By means of the resulted formula and drawing it is possible to calculate easily the sizes rope, ladders and gangways. For ladders the proportion 28/7 (steps settle down with a small inclination), for footboards - 31,5/0 (steps settle down vertically) is characteristic.
Very much an important point is exact definition of height and width of a ladder. Small discrepancy in some millimetres in total result leads to a deviation in some centimetres.
It is necessary to consider also that very abrupt and very flat ladders are inconvenient at operation, and in certain cases can lead to traumas.
Following proportional parities between the sizes zaglushiny and svesom are most often used: ladders at an input - 15/33-16/31, garden ladders - 10/45-12/40. .
As a rule, steps on ladder marches settle down perpendicularly to an axial line of a ladder march. Thus it is necessary to consider that the length of steps of a ladder should be equal to width of a march. If it is a question of console ladders the steps which have been not closed up in a wall mean.
So far as concerns ladders, it is often possible to meet a combination "чрсхцэ№х ё=ѕяхэш". Zabezhnymi the steps which normal width is observed only in an average line of a march are called. Their internal edge already, and external - is wider. On ladders with sites bow-shaped (zabezhnyh) steps and on abrupt often do not suffice a place for a normal arrangement of foot. To correct such inconvenience, it is necessary to strengthen a return notch of a surface podstupeni, to arrange slanting podstupen or to make a ladder without podstupenej (of one prostupej). .
On a ladder steepness share on abrupt and flat . Ladders with a bias to 38 °, and abrupt - from 38 to 45 ° are called as flat.
External ladders can be made of a tree, a brick, ready concrete blocks etc.