Cantilevered bolt staircase
Supporting bolt stairs are prefabricated - stair systems in which treads are connected by supporting bolts.
Basics
Support bolts are the metallic connectors that connect the steps to one another in a tensile and pressure-resistant manner. Wall anchors are metallic supports which, depending on the type of approval, connect every step or every third or fourth step to the wall in a tensile and pressure-resistant manner. The treads are an essential part of bearing bolt stairs.
The total load of the stairs is partly carried over the steps to the entry and exit areas and the walls. Depending on the number of steps per run, the treads must have a certain strength class and withstand a certain bending and torsional load. Since the torsional load on supporting bolt stairs is often a multiple of the bending load on the treads, proof of torsional load on the treads is sufficient.
To improve the strength of the treads, they often consist of two partial panels, which are glued together in a sandwich process with glass fiber mats and epoxy resin. In addition to improving the load-bearing capacity, the treads have a defined residual load-bearing capacity due to the bonding, so that local material failure does not lead to a sudden total loss of the load-bearing capacity of the steps.
Types
Single bolt stairs (WE1)
In the case of single-bolt stairs, the steps are integrated into the wall at least 7 cm. On the wall-free side, the steps are connected to each other by a support bolt.
Two-bolt stairs (WF2)
The steps are connected to the wall and on the wall-free side by a support bolt. It is common to use a wall anchor for each step on the wall, which is integrated into the stairwell wall. Alternatively, there are also systems in which only every third or fourth step with rigid support bolts and reinforced wall anchors at the beginning and end of this area or a steel beam are used. These systems are particularly advantageous in the area of wall openings.
Spiral stairs
Carrying bolt stairs can also be designed as spiral stairs . The treads are clamped together between height-adjustable spindle pots by means of a tension cord (spindle). On the outside, the steps are connected to each other by a support bolt. Spindle pots, tension cord and support bolts are made of steel. Since the design of the spiral staircase made of natural stone is not regulated by a standard, proof of this is necessary in individual cases or the use of a European approval.
In most cases, spiral staircases require less floor space than normal bearing bolt stairs and are therefore also suitable when there is little space available. Due to the elegant, curved appearance, spiral staircases are often planned directly in the living area and especially in maisonette apartments.
Legal requirements
The dimensioning, manufacture, monitoring and installation of internal and external supporting bolt stairs with straight and spiral runs is regulated in DIN 18 069. However, this DIN may only be used for steps made of reinforced concrete. In practice, however, steps made of artificial stone or natural stone have been used almost exclusively for many years. For all of these stairs, the usability for the intended use must be proven by a European technical approval or in individual cases by the approval of the highest building authority.
Different approvals are required depending on the design of the pinned staircase. The construction types of the supporting bolt stairs differ according to the type, arrangement and number of the supporting bolts and wall anchors per step as well as the mounting of the steps on the wall.
If bearing bolt stairs with natural stone steps are planned and installed in Germany and the European Union, in practice a European technical approval (ETA) must be applied. This approval stipulates all the conditions and rules that must be taken into account when dimensioning, manufacturing and installing a bolted staircase. The procedure for applying for, preparing and issuing approvals for bearing bolt stairs is standardized in the European guideline ETAG 008-01.
Since the process up to the granting of an approval is time-consuming and usually associated with high costs, many stonemasons take advantage of the opportunity to use existing approvals. The following requirements are necessary to use an existing license:
- The authorization user must have been authorized in writing by the authorization holder. The address of the licensing user must be deposited with the responsible supreme building authority.
- The technical documentation and the prescribed approval rules must be observed. Only products specified by the approval holder (e.g. support bolts and epoxy resin adhesive) may be used.
- The approval may only be used by companies and personnel trained by the approval holder.
Planning and design of bearing bolt stairs
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The number of steps per flight of stairs and the width of the steps are limited for bolted stairs. The floor height and the dimensions of the stairs are fixed by the planner or by the building floor plan. The permitted number of steps per run essentially depends on:
- Floor plan type 16G / 16V / 16VV / 16H
- Step thickness (62, 72 or 82 mm)
- Strength class of the step material (I – VI)
- Material steps: cast stone or natural stone
- Geometry of the entry or exit step straight or spiral
- Step length
With this information, which should already be available when planning a staircase, it can be determined whether and at what point additional support of the staircase on the wall-free side is necessary.
Step width and tread on stone stairs
The maximum permissible step width is primarily dependent on the thickness of the steps and the shape of the stair floor plan. The maximum running width varies depending on the approval. A maximum running width of 105 cm with a maximum distance of 5.5 cm from the edge is typical. In the case of wider steps that cannot be covered by an approval, static evidence must be provided in each individual case, which, however, in the simplest case can be based on an existing approval.
With straight steps, the tread of the step must not be less than 21 cm and should not exceed 29 cm. Basically, the step rule also applies here: twice the incline and once the step should be between 59 cm and 63 cm in the average step.
Strength classes of natural stone and cast stone
Depending on the tensile and compressive strength of the stone materials, these are divided into different strength classes, mostly into six classes. The strength class of the steps has a major impact on the number of steps that can be planned without additional support. The position of the intermediate support depends on the type of step, the strength class of the step, the step thickness, the material of the step, as well as the step length and geometry. With artificial stone as the step material, more free steps are often possible than with natural stone with the same strength class.
The strength parameters of a type of natural stone or artificial stone vary widely. This is due to both the quality of the epoxy resin used and the treatment used during extraction. For example, the stone loses 1–2 strength classes if the raw block is mined with the help of explosives. For these reasons, the strength class of natural stone can only be estimated without obligation.
Drilled holes in the steps
The position of the drill holes for the support bolts relative to the edge of the step is specified in the approvals. The diameter of the hole depends on the type of lifting pin. The minimum diameter can be found in the technical data of the respective lifting pin type. In the case of the rigid stair bolts with pre-tensioning, this is 7 cm from the side and 7 cm from the front and back. For the "small" support bolts without pre-tensioning, this is 5.5 cm from the side and from the front and back. This results in an overlap of the steps of 11 cm or 14 cm.
Support bolt with covered sleeve
In most cases, the support bolts are pushed through the hole provided in the steps during assembly and screwed tight from above. However, there is also the possibility of fastening support bolts in a blind hole without pre-tensioning so that the step surface is not visually interrupted. The threaded sleeves must then be fastened in the steps so that they are resistant to tension and pressure.
Carrying bolt stairs in the outside area
If carrying bolt stairs are planned in the outdoor area, the material used for the steps and carrying bolts must be suitable and approved for use outdoors. In most cases, steps made of bonded natural stone are not approved for outdoor use. Most European approvals only apply to internal stairs.
Soundproofing with pinned stairs
In residential buildings with more than two apartments, the rated standard impact sound level of a maximum of 58 dB is permissible for stairs according to DIN 4109. Depending on the building mass and design, bearing bolt stairs generate a standard footfall sound level when used, which is above the maximum permissible value for apartment buildings. There are various soundproof wall anchor systems that can be used to reduce the impact sound level generated when walking on the pinned staircase to below the maximum permissible value. Even small sound bridges at the entrance, exit or over the railing can significantly reduce the desired impact sound insulation. Due to the many anchoring points between stairs, railings and building, special training and advice is usually advantageous before installing these soundproofing elements.
Anchoring the entry and exit step
The stairs are connected to the structure via the walls and the entry and exit steps. Since a large part of the load acts directly on the entry and exit steps, these must be anchored in a particularly stable manner. Depending on the type of floor plan and the length of the run, loads of more than 10 kN can be expected on the entry and exit steps.
The anchoring of the entrance and exit steps must be tensile and compressive, as these are subjected to torsion. For secure anchoring, the step should definitely be attached to the wall side, better still on both sides, with a stable step plate. Alternatively, the first step can also be attached with two wall anchors and a threaded pin glued into the unfinished floor. As with the single-bolt staircase, the entry and, in particular, the exit step could also be integrated into the masonry at least 7 cm. In this case, however, proof of the permissible masonry pressures and the minimum wall load must be provided.
The exit step is often secured against twisting on the side of the stairwell, i.e. on the wall-free side, with a Z-angle. If there is no exit step, the penultimate step can also be connected to the raw ceiling via Z or L angles and two wall anchors. This ensures that the tread of the entire staircase is lengthened by the otherwise necessary overlap at the last step (approx. 12 cm). By omitting the last step, a price advantage of 6% - 8% can result in the calculation, depending on the number of steps.
Drilling and mounting the wall anchors
For two-bolt stairs, holes of at least 56 mm and 110 mm deep must be provided for the wall anchors. The wall anchors are to be integrated into the anchor holes at least 100 mm deep in accordance with the static requirements. As an alternative to the classic installation of wall anchors, dry installation with bondable anchor bodies has become increasingly popular. The anchor bodies, which are made of sound-absorbing plastic, are glued together with the wall anchors in the wall using a fast-curing, high-strength expansion resin. The wall anchors can be loaded after just a few minutes.
literature
- Richard Watzke (ed.): The stone mason - manual for training and practice . Callwey-Verlag, Munich 2013, ISBN 978-3-7667-2028-3 , p. 600 .