Be safe and save – optimizing the thickness of rainscreen cladding panels
Building great, functional facades starts with optimizing the façade panels. The optimized thickness saves on cost for materials, as well as for the fixing system itself. It also makes for a safer façade and a faster construction process, as the resulting lighter panels are quicker and easier to install.
So, how do we achieve optimal panel thickness? In this article, we will look into all the factors that affect the thickness of façade panels. Rainscreen cladding panels should be designed to comply with safety regulations, but also to save redundant material, taking into account the self-weight, high wind pressures and induced seismic forces.
I. Cladding panel design
Minimal rainscreen cladding panel thickness for secure fixing with standard methods and systems is obtained after precise calculations for:
1. Wind loads and self-weight
When considering an external multilayer wall such as a ventilated façade, wind loads present a complex load pattern. They are a key factor for safe, practical and economically viable design. We will discuss wind load analysis, design and best practices in an upcoming article.
2. Type of material used
Depending on the material of the rainscreen cladding panels, façade engineers calculate the minimum, optimal thickness. All materials have their unique characteristics, especially “non-man-made” ones. Properties of the materials, as well as geometrical data are some of the key variables that determine structural reliability.
In this article, we explore four material types – two natural (granite and limestone) and two “man-made"(ceramic and fibre-cement) for a panel with dimensions 1200mm/600mm to determinate relative minimal thickness.
1) Natural, or “non-man-made" materials - Limestone & Granite
Stone is strong enough to resist hygrometric differential movement, deflection, vibration and creep of the support concrete structure, altogether with weathering and deterioration of the cladding stones. Stone safety factors can vary widely because of its natural origin and non-homogeneous structure. In time, stone can change its physical properties due to heating – freezing cycle and saturation. Therefore stone standards are more conservative - with high factors and assuming the lowest flexural strength values.
2) Artificial, or “man-made" materials - Ceramic & Fibre-cement
‘Man-made’ materials consist of natural stone fragments, clay, cement, fibres such as glass and cellulose. Additional chemicals and water serve to achieve a more consistent mixture. During the heating or drying process, they transform into microscopic bubbles which are visible in the end product. Due to the controlled nature of the manufacturing process, the safety factor is more predictable and substantially lowered.
a) Ceramic (dry pressed)
Ceramic is obtained by wet grinding of clayish raw materials, granite and metamorphic, feldspar-containing rocks and ceramic pigments. Complex shapes are molded in a compacter and then go through high temperature sintering. The panels in the example below are made from two thin slabs with reinforcement of fibreglass blanket between them.
Fibre cement is a modern reinforced material. The base mixture can be of cement, sand, cellulose and water (autoclave) or cement, lime, synthetic fibres and water (air-cured). The mixture is rotated in different cylinders, vacuum processed, pressed to obtain density, as well as the required thickness and, lastly, air-cured or autoclave cured.
3. Stresses according to applicable standards
This case study will consider the combination of panel self-weight and wind loads applied on the corner façade segment for a building 40 m high in an urban area. Initially, we’ll also look at an optional computational concept for rainscreen cladding panel design.
1) Bending moment due to wind load on rainscreen cladding panel
Considering the characteristic wind pressure, the design bending moment per unit of width in the panel’s mid-span cross-section is given by:
- Span length: l (m)
- Characteristic wind pressure: w (kN/m2)
- Partial safety factor for variable actions: γ_f = 1,5 (according to Eurocode)
Note: A computer program for static analysis can calculate the bending moment more accurately.
2) Bending strength of cladding panel
a) Characteristic bending strength „ σRk”
Usually, the expected lower value of material property or product is unfavourable, and the 5% (lower) fractile is then considered as the characteristic value.
b) Design bending strength „ σRd”
Partial safety factor for material: γ_m≈ 2 (according to DIN 18516 - 3)
3) Thickness of the rainscreen cladding panel
4) Еxample calculation of rainscreen cladding panel thicknesses for different types of materials
a) Cladding panel dimension properties
► Cladding panel length: a = 1,2 m
► Cladding panel width: b = 0,6 m
b) Wind load
Project location: Berlin, Germany (according to Eurocode → terrain IV)
Building height: 50 m
► Characteristic wind pressure for corner areas: w = 1,30 kN/m2
c) Static scheme of rainscreen cladding panel
► Support 1 – simply supported (Ux = 0; Uy = 0; Uz = 0)
► Support 2 – simply supported (Uy = 0; Uz = 0)
► Support 3 – simply supported (Uz = 0)
► Support 4 – simply supported (Uz = 0)
d) Bending moment
e) Rainscreen cladding panel thickness for different types of materials
The rainscreen cladding panel thicknesses shown in the table below are calculated by the derived formula above:
|Material||Hardness||Bending moment M(kN.m/m)||Average Bending strength (MPA)||Design Bending strength σRd(MPa)||Panel thickness (cm)|
|Ceramic||Very hard||0,307||Vast range||50||0,6|
The illustrations below demonstrate the stress distribution on rainscreen cladding panels made of the four materials with the resulting optimal thicknesses (as explored above).