All components must be taken into account when designing a façade with dry installation technology:
- thermal insulation
- type of stone material
- types of static loads
- selection of structural anchors for façade installation.
An important element of the stone façade is the insulation of the structural wall. The most commonly used insulation material is 100 x 60 cm stone wool slabs.
(e.g.: Ventirock F Plus by Rockwool). The wool slabs are covered on one side with a black glass fibre veil.
They are fixed to the structural walls, black veil on the outside, using facade dowels with a driven pin with plates. Each mineral wool slab should be fixed to the substrate with the appropriate number of dowels in accordance with the mineral wool manufacturer's guidelines. When installing the façade on anchors set in mortar, the installation of the insulation covers the holes drilled for the anchors in the structural wall. To install the anchor in the wall, a hole with a diameter of approx. 15 cm must be cut in the mineral wool. Once the anchor has been installed, the cut-out section of mineral wool is returned to its place, ensuring the continuity and tightness of the insulation.
Anchors are set into structural walls using quick-setting cement mortar. Their embedding depth is a minimum of 8 cm and depends on their overhang and the material of the wall. For reinforced concrete walls, this depth generally does not exceed 10 cm, for brick walls 12 cm. The anchors are made of stainless steel, which guarantees the safety and durability of the stone cladding for decades. It should be noted at this point that thermal bridges created by the anchoring of façade slabs must be taken into account in the calculation of the UC(max) heat transfer coefficient for the wall. Anchors set with fast-setting cements can, in extreme cases, significantly reduce the insulating capacity of the wall if they are heavily compacted.
The main element of the façade, however, is the stone slabs. They are the ones that cause the static load for the entire wall, as well as carrying the dynamic loads caused by wind pressure and suction. Najważniejszym obciążeniem, jakie należy uwzględnić przy obliczeniach elewacji, jest jednak ciężar samych płyt kamiennych. The table below shows the specific weights of the different stone types.
- thermal insulation
- type of stone material
- types of static loads
- selection of structural anchors for façade installation.
An important element of the stone façade is the insulation of the structural wall. The most commonly used insulation material is 100 x 60 cm stone wool slabs.
(e.g.: Ventirock F Plus by Rockwool). The wool slabs are covered on one side with a black glass fibre veil.
They are fixed to the structural walls, black veil on the outside, using facade dowels with a driven pin with plates. Each mineral wool slab should be fixed to the substrate with the appropriate number of dowels in accordance with the mineral wool manufacturer's guidelines. When installing the façade on anchors set in mortar, the installation of the insulation covers the holes drilled for the anchors in the structural wall. To install the anchor in the wall, a hole with a diameter of approx. 15 cm must be cut in the mineral wool. Once the anchor has been installed, the cut-out section of mineral wool is returned to its place, ensuring the continuity and tightness of the insulation.
Anchors are set into structural walls using quick-setting cement mortar. Their embedding depth is a minimum of 8 cm and depends on their overhang and the material of the wall. For reinforced concrete walls, this depth generally does not exceed 10 cm, for brick walls 12 cm. The anchors are made of stainless steel, which guarantees the safety and durability of the stone cladding for decades. It should be noted at this point that thermal bridges created by the anchoring of façade slabs must be taken into account in the calculation of the UC(max) heat transfer coefficient for the wall. Anchors set with fast-setting cements can, in extreme cases, significantly reduce the insulating capacity of the wall if they are heavily compacted.
The main element of the façade, however, is the stone slabs. They are the ones that cause the static load for the entire wall, as well as carrying the dynamic loads caused by wind pressure and suction. Najważniejszym obciążeniem, jakie należy uwzględnić przy obliczeniach elewacji, jest jednak ciężar samych płyt kamiennych. The table below shows the specific weights of the different stone types.
Table 1. Specific gravity of stone
These characteristics, in conjunction with the distance of the façade slab from the structural wall in which the anchors are embedded (known as the overhang), form the basis for calculating and selecting suitable anchors for the façade.
The connecting element between the anchor and the stone slab is a bolt which, through an ABS sleeve, forms a flexible joint that allows compensation for any movement the slab may experience during the use of the façade.
A very significant load on the façade is the forces caused by wind pressure and suction. The pressure is caused by the impact of the wind against the outer plane of the stone slab, while suction is the result of air movement in the ventilation void. These forces add up. The wind load (W) is determined by the formula: W = Cp x Q where: Q [kN/m2] - wind pressure dependent on wind speed and height of structure; Cp - aerodynamic pressure factor.
The aerodynamic wind pressure factor depends on the angle of impact of the airflow on the stone slabs, the exposure of the façade (façade corner or centre slabs) and the height of the façade. In general, for ventilated façades, it is higher for wind suction than for wind pressure and ranges from -3.2 to +1.0. For this factor to be achieved, an adequate ratio of open joint area to façade area must be maintained. German standards set this value at a minimum of 0.75 per cent, which is provided by a joint 8 mm wide.
The wind pressure above 100 m for a 165 km/h hurricane is 1.3 kN/m2. These dynamic loads are not so important for the selection of anchors on which the stone slabs are installed. Instead, they are extremely important when calculating the correct thickness of stone slabs.
We know that different rocks have different elasticities and have very different resistance to bending. Diabase is the most resilient, with a bending resistance factor of 15 to 25 N/mm2. The weakest in this respect are tuffs and limestones, for which the ratio varies between 2 and 8 N/mm2. Sandstones, dolomites and marbles have factors of 6 to 15 N/mm2, while for all granites, gabbro, syenites, diorites and quartzites the factor is 10 to 22 N/mm2. Calculations should be carried out as for a bilaterally supported beam loaded with a F force at mid-span.
The connecting element between the anchor and the stone slab is a bolt which, through an ABS sleeve, forms a flexible joint that allows compensation for any movement the slab may experience during the use of the façade.
A very significant load on the façade is the forces caused by wind pressure and suction. The pressure is caused by the impact of the wind against the outer plane of the stone slab, while suction is the result of air movement in the ventilation void. These forces add up. The wind load (W) is determined by the formula: W = Cp x Q where: Q [kN/m2] - wind pressure dependent on wind speed and height of structure; Cp - aerodynamic pressure factor.
The aerodynamic wind pressure factor depends on the angle of impact of the airflow on the stone slabs, the exposure of the façade (façade corner or centre slabs) and the height of the façade. In general, for ventilated façades, it is higher for wind suction than for wind pressure and ranges from -3.2 to +1.0. For this factor to be achieved, an adequate ratio of open joint area to façade area must be maintained. German standards set this value at a minimum of 0.75 per cent, which is provided by a joint 8 mm wide.
The wind pressure above 100 m for a 165 km/h hurricane is 1.3 kN/m2. These dynamic loads are not so important for the selection of anchors on which the stone slabs are installed. Instead, they are extremely important when calculating the correct thickness of stone slabs.
We know that different rocks have different elasticities and have very different resistance to bending. Diabase is the most resilient, with a bending resistance factor of 15 to 25 N/mm2. The weakest in this respect are tuffs and limestones, for which the ratio varies between 2 and 8 N/mm2. Sandstones, dolomites and marbles have factors of 6 to 15 N/mm2, while for all granites, gabbro, syenites, diorites and quartzites the factor is 10 to 22 N/mm2. Calculations should be carried out as for a bilaterally supported beam loaded with a F force at mid-span.
Rys.2. Schemat obciążenia belki
Wind load forces are also very important in determining the resistance of a stone slab to anchor bolt pullout. These forces are transferred directly from the whole slab to four bolts embedded in its sides. A diagram of the destructive bolt pull-out is presented in the figure.
Fig. 1. Diagram of damage in case of bolt removal
The following table shows as an example the destructive forces on the stone at different wall thicknesses of the opening (b) in the facade slab. These are only auxiliary values, as the actual value is determined on a case-by-case basis by carrying out appropriate technical tests on samples of the specific stone material from which the façade will be constructed.
Table 2. Destructive forces on the stone depending on the thickness of the hole wall.
Często więc bywa, że zbyt długie lub zbyt wysokie płyty montowane na narożnikach elewacji, a zwłaszcza na wysokościach powyżej 20 m, wymagają właśnie z tych dwóch powodów większej grubości.
Do obliczania elewacji kamiennych służą odpowiednie programy komputerowe.
Schemat obliczeniowy jest zawsze ten sam. Kotwa nośna umiejscowiona w fudze pionowej (kotwienie boczne) przenosi obciążenie połowy lewej i połowy prawej płyty. Kotwa nośna umiejscowiona w fudze poziomej (kotwienie dolne) przenosi obciążenie od brzegu płyty do połowy odległości do drugiej kotwy nośnej dla tej płyty. Zasadę tą przedstawia poniższy schemat.
Fig. 3. Anchor loading scheme for façade slabs
The second important element for the calculation is the anchor overhang, i.e. the distance of the axis of the anchor bolt from the wall in which the anchor is embedded. With the above data, the anchors are selected from the Roko Systems product catalogue. The prepared design drawing carries the dimensions of the stone slabs, the numbers of the slabs, the positioning of the anchors with their markings, the distances of the bolt holes from the corner of the slab and the positioning of the scaffolding anchors.