Technical Portal for Sustainable Architecture
How are thermal expansions of the substructure managed?
When designing a ventilated façade, the metal substructure (usually aluminum or stainless steel) serves as a static transition element between the building envelope and the external cladding. Since the latter is exposed to extreme seasonal and daily temperature variations, the materials undergo significant geometric dimensional variations.
Ignoring or underestimating the linear thermal expansion coefficients during roughing and installation can lead to cracking, structural distortion, and, in the worst cases, the failure of the fastening systems.
This article analyzes the kinetic criteria and calculation solutions for compensating for thermal movements of the substructure.
Calculating linear expansion: material coefficients
Each metal responds to temperature changes (𝚫 T) according to its own coefficient of linear thermal expansion (α). Aluminum, the most commonly used material for substructures, has a high coefficient (α approximately 23 x 10^-6 \ x K^-1, which translates to an elongation of approximately 2.3 mm per linear meter over a thermal gradient of 100 °C. The designer must calculate the overall geometric displacement (𝚫 L) using the fundamental equation:
𝚫 L = L_0 x alpha x 𝚫 T
This calculation allows for the correct sizing of expansion joints and mechanical tolerance points along the height of the building.
Kinematics of constraints: fixed-point brackets and sliding-point brackets
The key strategy for managing vertical expansion consists of geometrically differentiating the anchoring systems (brackets). Each vertical upright must be secured to the supporting structure using:
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A single fixed point (PF): Typically positioned at the top or center of the upright, it supports the static load (self-weight) and dynamic loads (wind), preventing vertical sliding but allowing rotation.
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Multiple sliding points (PS): Distributed along the upright's axis, they feature elongated vertical slots. The fixing screws are tightened at the center of the slot, leaving the necessary slack so that the metal profile can freely expand or contract without transmitting parasitic stresses to the chemical or mechanical anchor to the wall.
Fraction joints and interruptions of uprights
Vertical mullions cannot be continuous along the entire height of the building; they must be interrupted and divided into elements of a defined length (usually no more than 3-4 meters). At the intersection between two consecutive mullions, a vertical expansion joint of a calculated width must be provided (typically between 10 and 15 mm). To ensure static continuity against horizontal forces (wind pressure/depression), internal connecting sleeves or alignment pins are used, which join the profiles while leaving them free to slide axially relative to each other.
Cladding plate fixing tolerances
The movement of the substructure must not affect the grid of the external finishing panels (whether made of stoneware, fiber cement, HPL, or natural stone). Whether using exposed fastening systems (using clips or rivets) or concealed systems (rear-panel brackets or mules), the retaining components must incorporate dimensional tolerances. For example, in rivet-on aluminum systems, the holes in the panel are classified as "fixed holes" (equal to the diameter of the rivet) and "slotted/oversized holes" (with a larger diameter), using special centering bushings to prevent the clamping from blocking the relative movement between the panel and the profile.
Pathologies caused by incorrect kinematic constraints: deformations and mechanical stresses
What happens if the system is blocked due to an installation error, such as incorrectly tightening the sliding points? The impediment to free thermal expansion instantly transforms into internal mechanical stresses, either compressive or tensile (kinematic thermal stresses).
These forces cause buckling of the upright (deflection and buckling), ovalization of the holes, shearing of the connecting screws, and localized cracking of the cladding panels due to stress concentrations at the anchor points.
