Spiral staircase structures

The main element of the staircase, which allows you to achieve the main goal of moving vertically, is the step. The shape and position of the steps determine two main types of staircase structures – straight and spiral staircases. Steps, the width of which increases from one end to the other, mounted on a central post or otherwise and arranged in a circle, forming a spiral staircase.


Fig. 1.
Comparative characteristics of the areas of spiral and straight stairs with steps 1000 mm long (arrows indicate the direction of movement):
1 – spiral staircase;
2 – straight staircase;
a – intermediate platform

The design of the spiral staircase has both advantages and disadvantages. The main advantage of a spiral staircase is the smaller (compared to a straight staircase) area occupied by it.

In most cases, the spiral staircase fits into a circle, the center of which is the support post, and the radius is determined by the length of the step. With a step length of 1000 mm and the same riser height (even without taking into account a possible intermediate platform), a spiral staircase takes up about one and a half times less space than a straight one (see Fig. 1).

But already with a step length of 1500 mm, with other characteristics being equal, the areas occupied by a spiral and a straight staircase are compared. A further increase in the length of the step leads to a significant increase in the area on which the spiral staircase is located.


Fig. 2.
Winder tread width
1 – width on the path of movement of the right leg;
2 – width on the path of movement of the left leg

It should also be borne in mind that the optimal path of movement along the spiral staircase runs approximately in the middle of the march, while the ability to support the railing should be provided. It follows from this that the length of a step in a spiral staircase has finite dimensions.

In this regard, the main disadvantage of the spiral staircase is revealed – the tread width is too small on the line of movement. This is what determines the common practice of performing steps without risers. It should be borne in mind that the width of the tread of a spiral staircase is not the same in the places where the left and right legs rest on it (see Fig. 2).


Fig. 3.
Spiral staircase “samba” with treads “duck step”
1 – tread;
2 – railings

For a leg that is closer to the center post of the spiral staircase and, accordingly, rests on a smaller tread section, there is an increased risk of slipping, especially when moving down.

The absence of a riser allows part of the foot to be positioned behind the inner edge of the tread in the upward direction. It is in connection with the above circumstances that spiral staircases are often used as additional and decorative ones and much less often as the main ones..


Fig. 4.
Spiral staircase on curved bowstrings
1 – bowstrings

The original design of the spiral staircase, which eliminates the problem of insufficient tread width and the associated inconvenience, is known as “samba” (see Fig. 3). In such a spiral staircase, treads with a complex configuration “duck step” are used. Usually these treads are used in straight stairs with a large angle of inclination..

Since in this case the treads have unequal width along their length, they are in some way run-in, and alternately oppositely directed. The use of treads “duck step” in the construction of a spiral staircase, where all steps are by definition run-in, requires an ingenious design solution.


Fig. five.
Spiral staircase without a central post and bowstrings (click to enlarge)
1 – tread;
2 – balusters;
3 – handrails;
4 – railing posts

When designing spiral staircases, one should proceed from the fact that the width of the tread on the line of movement (i.e., in the middle of the march) cannot be less than 200 mm, and at a distance of 150 mm from the central post it must be at least 100 mm.

When using a spiral staircase as the main tread, a length of at least 800 mm is made, and the opening in the ceiling must have a diameter of 2000 mm.

If the spiral staircase plays an additional role, then the tread length of 550–600 mm is quite sufficient and requires an opening of up to 1400 mm. This design of a spiral staircase is compact, and its main area of ​​application is to provide access to the attic..

In addition, the parameter that determines the convenience of the spiral staircase in operation is the height of the passage, which cannot be less than 2000 mm..

There are several types of supporting structures for spiral staircases and, accordingly, methods of fastening steps to them. You can make a spiral staircase based on bowstrings (support beams located on the sides of the steps).

Fig. 6.
Spiral staircase with central support post (click to enlarge)
1 – flange at the lower end
central rack;
2 – central support post;
3 – winders;
4 – support brackets;
5 – handrails;
6 – handrail posts

In this case, the bowstrings have a curved spiral shape and are made of glued wooden elements (see Fig. 4). It should be noted that the manufacture of such support structures is associated with great difficulties and requires special skills..

In addition, it is possible to make a spiral staircase without any support beams at all (see Fig. 5). In such a design, the run-in steps are connected to each other, as well as to the railings, which in turn are attached to the balusters (the initial and final railing posts).


Fig. 7.
Flange for fixing the B-pillar to the floor
1 – flange;
2 – central post;
3 – lower wooden bushing;
4 – metal washer;
5 – anchor bolts

Also, the first and last steps are attached to the balusters. The last step, in turn, is fixed on the floor. Loads in such a spiral staircase are distributed in a certain way throughout the structure.

The most traditional and commonly used type of spiral staircase is the center support structure (see fig. 6). Since such a rack is the supporting structural basis of a spiral staircase, it is most rational to make it from a thick-walled metal pipe with a diameter of 50 mm.


Fig. 8.
The arrangement of the elements of the spiral staircase on the central pillar
1 – central post;
2 – tread;
3 – wooden bushings;
4 – intermediate metal washers

The central post of the spiral staircase should be located strictly vertically – this is controlled using a construction plumb line. Particular attention must be paid to fixing the central pillar to the floor, since the attachment point takes the load from the weight of the spiral staircase itself and people moving along it.

The support post can be concreted into the floor (using guy ropes) or fixed with anchor bolts.

Anchoring fasteners can also be used when anchoring the support leg to a wooden floor. To use anchors, the lower end of the post must be equipped with a perforated flange (see Fig. 7). A flange is installed on the anchor bolts fixed in the floor, and then the nuts are tightened on the bolts.


Fig. nine.
Scheme for constructing treads for a spiral staircase (click to enlarge)

To place the steps of the spiral staircase on the central pillar, holes are pre-drilled in them, which are in the narrow part of the tread and have a diameter corresponding to the diameter of the central pillar.

Now you need to place the treads at the correct vertical distance from each other. This distance is determined by the height of the riser.

To solve this problem, bushings are used that have a size corresponding to the height of the riser minus the thickness of the board from which the tread is made (see Fig. 8). In addition, a metal washer is located between the bushing and the tread, which serves as an interlayer between the wooden parts of the spiral staircase and protects them from deformation at the points of contact.

Fig. ten.
Interposition and fastening of adjacent treads of a spiral staircase
1 – upper tread;
2 – lower tread;
3 – the place of attachment of the support bracket

The thickness of the washers should be taken into account when making the bushings. If this is not done, then the real height of the spiral staircase may exceed the design one by several centimeters..

The bushings are made of wood and have internal holes with a diameter corresponding to the diameter of the center post. All bushings are made the same, with the exception of the lower one, in the manufacture of which one should take into account the peculiarities of the place where the support post is attached to the floor.

Usually, the height of the riser in the construction of a spiral staircase is 180-200 mm, which is slightly higher than a similar size for a straight staircase. In practice, this value can vary slightly in one direction or another. It depends on the distance between the slabs (from the level of the finished floor of one floor to the level of the finished floor of the next floor) and the number of steps.

The main difficulty in designing a spiral staircase is determining the shape and size of the treads. Unlike a straight staircase, where the treads have the same width and are rectangular in plan, a spiral staircase implies the use of winder steps, and the width of such steps increases in the direction from the central pillar to the outer contour.

Fig. eleven.
A variant of the design of the spiral staircase railing
1 – tread;
2 – fences made of rectangular bars

First of all, you should determine the dimensions of the opening in the floor (the length of the tread depends on this) and the distance between the floors (this determines the number of steps). It is also necessary to set the angle of rotation of the spiral staircase.
Suppose we are designing a spiral staircase with a 360-degree rotation angle, i.e. the end of the march is parallel to its beginning.
We define the length of the tread as 1000 mm. Having determined the number of steps (for example, there will be 16 of them), we divide the circle in the drawing by the corresponding number of radii (the drawing is carried out in the appropriate scale). At first glance, we have in front of us an image of our spiral staircase from above, but this is not so.

It should not be forgotten that in the construction of a spiral staircase, the treads are “overlapping” and overlap each other in the plan. With the indicators we have chosen, the width of the tread, if it is determined in the drawing from one radius to the next, will be less than the required 200 mm.

The radii that divide the circle in the drawing, we conventionally designate as the center lines (axis of symmetry) of the tread and from one of them we will build the projection of this element of the spiral staircase (see Fig. 9). This line should be divided in half (point A) and through the specified point draw a segment perpendicular to the line.

This segment centered at point A will mark the middle of the tread, where its width should be at least 200 mm. The optimal tread width in our case will be 220 mm. The length of the segment should correspond to the scale required in this place for the width of the tread. The ends of the segment are denoted as A1 and A2.

Further, from the center of the circle, we postpone on the center line of the tread, the distance corresponding to the regulated scale of 150 mm, and denote this as point B. In this place, the width of the tread cannot be less than 100 mm.

Let’s draw a perpendicular segment centered at point B. The length of this segment corresponds to the scale of the tread width. The ends of the segment will be denoted as B1 and B2. Now let’s draw two straight lines through points A1, B1 and A2, B2.
It should not be forgotten that the tread in the narrow part must be equipped with a hole for the central post, and the strength of the step cannot be reduced. To do this, the tread is made in a shape resembling a classic keyhole..

Around the support leg there is a round tread section with a radius of 80–100 mm. We will construct a corresponding circle (to scale) in the drawing, while its center will coincide with the center of the main circle. Let’s denote the points of intersection of the lines passing through the points A1, B1 and A2, B2, and both circles as C1, C2, D1, D2.

Thus, the required tread contour consists of segments C1-D1, C2-D2, a small arc C1-C2 and a large arc D1-D2. Now you can convert the resulting dimensions to a real scale and make a correctly constructed tread.
The tread is made of solid wood and is typically 50 mm thick. There are designs with treads, the thickness of which decreases with distance from the central pillar, but the implementation of such elements is very laborious. Usually, the treads are overlapped and, when viewed from above, partially overlap each other.

This allows supporting brackets to be installed between the rear edge of the wide part of the lower tread and the front edge of the wide part of the upper tread, which increases the reliability of the structure (see Fig. 10). In addition, holes are pre-drilled at the wide ends of the tread for the installation of fences.

The assembly of the spiral staircase should be carried out in a specific order. After installing and securing the central post, bushings and treads are alternately installed on it, alternating with metal washers. Then the treads strung on the rack are fanned out and they take their places around the circumference.
A spiral staircase can have both counterclockwise and clockwise ascent directions, but the latter is most common. Particular attention should be paid to the correct orientation in the room of the first and last steps.
Further, between the wide parts of the treads, support brackets are installed and fixed. After that, the last sleeve is put on and tightened with a nut on the upper end of the central post (equipped with a thread).
It is advisable to supplement this nut with a decorative head. The last step of the spiral staircase is fixed to the floor with fasteners and metal elements.

Finally, handrail posts and spiral staircase handrails are installed. The design of the handrails can have a curved outline (which is quite laborious in execution) or consist of straight bars (see Fig. 11).

The resulting spiral staircase is quite capable of carrying the corresponding loads and fulfilling its purpose.

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