02.06 Loading

Categories: Rainscreens

Introduction
This Section deals with loading on rainscreen walls.  These differ from face sealed walls as the loads, including wind loading, are carried by either, or both, the rainscreen and the backing wall.

A full description of loading on walls is given in Package 04.
 

Wind loading
Walls are conventionally designed to withstand a wind load defined as the difference between the external wind pressure and the pressure inside the building that occurs as a result of infiltration or significant openings in the building envelope.

P = q x ( C_pe  -  C_pi )

where:

P is the loading on the wall;
q is the site wind pressure;
C_pe is the external pressure coefficient;
C_pi is the internal pressure coefficient.

In the case of a wall with ventilated cavities it is not clear what peak pressures act across the air barrier and the rainscreen respectively.

• For rainscreen systems designed with sealed joints, all of the wind loading is likely to be carried by the rainscreen.
• For a pressure-equalised system some of the pressure difference across the wall occurs at the air barrier and some at the rainscreen.
• For all except face-sealed systems the full wind load may be carried on the air barrier.

In the absence of better information it should be assumed that the full wind loading may act on either the air barrier or the rainscreen.

For a pressure-equalised wall the peak pressure difference across the rainscreen will be reduced.  It may be assumed that the peak pressure difference across the rainscreen is limited to two-thirds of the peak external pressure, i.e.

P = 2 / 3 x  q x C_pe

Provided that the wall is designed as a pressure equalised wall, Section 02.05.

Note that rainscreens are designed on the basis of C_pe not (C_pe - C_pi ), the value normally given by structural engineers.

However the pressure difference acting across the air barrier should still be taken as the full pressure difference acting across the wall.  Note that wind pressures are transient and that the peak pressures on the rainscreen and the air barrier do not occur simultaneously.  For this reason the two peak pressures do not sum to the peak pressure difference across the wall.

Detailed calculation or measurement may well show that the peak pressure difference across the rainscreen is considerably less than two-thirds of the peak external pressure.

The design wind pressure is also used to determine the test pressures for water penetration resistance, Section 01.03.
 

Calculated wind pressures
The site wind pressures for the wall shall be calculated in accordance with the current national code for the determination of wind load on buildings.

Knowledge of the anticipated wind pressures, currently calculated using BS 6399: Part 2, is fundamental to the design of the wall and to the establishment of the peak pressures acting on the wall.

Wind loading is covered in greater detail in Section 04.04.
 

Overcladding
Overcladding or recladding an existing building may change the magnitude and location of the forces carried by its primary structure.  The ability of the structure to carry the new loading shoul be verified by a structural engineer.

Loading may change because:

• Overcladding may transfer the wind loading to the original building as point loads at the fixings, not as a uniform pressure;
• The surface area may increase;
• The weight of the overcladding may increase the dead loads.

The increase in loading may or may not be significant depending on the form of the original structure and the type of overcladding.
 

Wind resistance - cyclic loading
Panels, framing members, fasteners and fixings of a rainscreen wall should take into account the effect of wind load fatigue.  Fatigue is not normally a design limiting requirement.  However, wind fatigue loads may be significant when flexible components interface with more rigid components or where brittle panels are overly restrained by their fixings.

There should be no reduction in performance after the effective wind pressure has been applied in the following sequence from BRE Digest 346, 7:
 

The above sequence of 6401 cycles of loading represents the significant cycles of wind loading that will occur during the design life of the building.

Alternatively a simpler but more onerous test can be carried out by applying a 100% effective wind pressure over 10000 cycles.
 

Dead load and live loads
Dead loads and live loads in general are described in Section 04.02.  When considering a rainscreen wall it is essential that loads are allocated appropriately to either the rainscreen, the air barrier or the structural elements (framing or backing wall).

The dead weight of the cladding panels, particularly for overcladding, will lie in a plane outside the primary structure.  Fixings and brackets will generally have to carry moments due to this off-set.

Cladding panels used in a rainscreen may be wetted from both faces and when calculating the dead weight of panels the saturated weight should be used.
 

Transferred loads
When overcladding an existing structure the primary building structure should be capable of carrying all existing loads and the additional loading imposed by the overcladding.  Re-analysis of the existing primary structure may have to be as detailed as that undertaken for a new structure.