wind

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 multi-layer 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.

 

Fibre-cement

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

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.

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) Example calculation of rainscreen cladding panel thicknesses for different types of materials

Cladding panel dimension properties

► Cladding panel length: a = 1,2 m
► Cladding panel width: b = 0,6 m

 

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

Static scheme of rainscreen cladding panel

 

Support restraints

► 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)

 

Bending moment

Rainscreen cladding – M = 0,307 kN.m/m

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 (as explored above):

Material	Hardness	Bending moment M(kN.m/m)	Average Bending strength (MPA)	Design Bending strength σRd(MPa)	Panel thickness (cm)
Limestone	Medium hard	0,307	2÷21	3,8	2,2
Granite	Hard, durable	0,307	3÷30	4,6	2,0
Ceramic	Very hard	0,307	Vast range	50	0,6
Fibre-cement	Hard, stable	0,307	22÷32	26	0,8

 

The illustrations below demonstrate the stress distribution on rainscreen cladding panels made of the four materials with the resulting optimal thicknesses (as explored above).