Research & Development

1. Tensile coupon tests to determine material properties of steel

In order to determine the material properties of the Kiwicolour steel, tensile coupon tests were conducted for both the G300 and G550 grade of steel coils. The tensile coupons were prepared from the centre of the cladding sheets tested herein, in accordance with ISO 6892-1:2009. Five of each coupon were obtained from the longitudinal and transverse directions of each of the cladding profiles. The coupons were tested in an Instron 4469 tensile testing machine which has a capacity of 50 kN. A calibrated extensometer of 50 mm gauge length was used to determine the tensile strain of the coupons. The test machine and the average stress-strain graphs for the steel is shown in the figures below:

2. Laser scanning of TRS cladding/roofing profiles using a 2D laser scanner:

A laser scanner assembly was used to measure the initial geometric measurements present in the TRS roofing & cladding profiles. As can be seen from the Figure below, the laser scanner is comprised of a 5500 x 2500 x 1500 mm steel frame which supports a travelling platform mounted on precision rails in the longitudinal (5500 mm) direction. The platform supports a stepper motor, which allows the displacement-controlled motion using a rack and pinion system. The platform is designed to have a precision shaft in the transverse (2500 mm) direction which guides a move-able laser scanner. The laser scanner records reading at every 0.0001 mm. A photograph of the imperfection measurements setup is shown in the Figures below. A typical plot of the initial geometric imperfections versus length is shown in the Figure below. These imperfections were used in the numerical model, developed for each of the roofing/cladding profiles.

3. Full scale static and cyclic wind load testing of TRS Roofing & Cladding profiles:

The Roofing Store profiles were tested under static wind uplift and cyclic pressure in a rectangular pressure box. The figure below shows a photograph of the pressure box used in the experimental tests. A vacuum pump and pressure loading actuator (PLA) was used to simulate the wind uplift & cyclic pressure inside the pressure box.

Local failure of the screws/clips followed by the global failure of the roofing/cladding assembly was expected, when subjected to wind uplift/cyclic pressure, which is controlled by load per screws/clips at central and edge supports. Sensors were used to measure the deflection at central support and mid-pan of the interlocking claddings. As shown in the Figure below four mounting frames were used to place the sensors and load cells above the interlocking cladding panels. Three pressure sensors of each 20 kPa capacities were used to measure the uplift/cyclic pressure inside the pressure box. the screws/clips. Products were tested to the point of failure so we could achieve a serviceability level. An experimental set up is shown in the Figure below:

Wind loading procedure

TRS roofing/claddings were loaded under two different types of wind pressures as discussed below:

Static uniform uplift pressure

All the test specimens were loaded under a uniform static uplift pressure. The uplift pressure was applied at a constant rate until one of the screws/clips failed or the cladding assembly popped up globally. Initially, the cladding deflection increases linearly with uplift pressure. As the pressure is increased beyond a certain value (which depends on the profile), non-linear behaviour was observed i.e. the deflection of claddings increases non-linearly with static wind uplift pressure.  This non-linear behaviour was obtained from the transverse and longitudinal strain gauge readings, which were installed near the vicinity of the critical central screws/clips.

Cyclic pressure

Cyclic tests were conducted by means of applying sinusoidal loads to the test specimens, as shown in the Figure below. The sinusoidal pressure was applied at a frequency of 1.2 Hz for all cladding profiles. The constant amplitude cyclic tests were conducted for various peak/maximum cyclic loads equal to different percentages (100%, 80%, 70%, 60%, 50% and 40%) of the measured static wind uplift capacity with approximately zero minimum cyclic load. Cyclic pressure was applied to the claddings as per the AS/NZS 1170.2.  

4. Finite element modelling of TRS roofing/calling profiles under wind loading:

General

The finite element program ABAQUS (2017) was used to develop nonlinear elastic-plastic finite element models for each of the TRS roofing/cladding profiles under static wind uplift pressure. The finite element models were developed based on centre line dimensions of the cross sections of the cladding profiles. For the screw fasteners, only the screw head was modelled. The symmetry boundary condition was applied along the free edge of the adjacent claddings. Only one intermediate span was modelled for all experiments to reduce the complexity of the FE model and computational time. The screws were modelled as experiments. Specific modelling issues are discussed in the following sections.

Geometry and material properties

An elastic-plastic model was used. The material properties were taken from the results of tensile coupon tests. As per the ABAQUS manual, the engineering material curve was converted into a true material curve.

Type of element and finite element mesh

Since the TRS roofing/claddings were subjected to combined effects of in-plane membrane and bending actions, the finite elements must be able to represent such behaviour and deformation of the claddings. Therefore, S4R5 shell elements were used to model the CFS TRS claddings. S4R5 quadrilateral shell elements allow each node to have five degrees of freedom both along translational and rotational directions. S4R5 elements are also shear flexible and accounts for in-plane membrane and bending actions, allowing the CFS interlocking claddings to deform in the longitudinal and transverse directions. Three dimensional eight-noded continuum elements were used to model the screw head. A mesh size of 8 mm by 8 mm was found to be appropriate for the cladding profiles, based on the results of a convergence study. For the screw head, a mesh size of 5 mm by 5 mm was used. The number of elements across the length and width of the claddings was verified through a mesh sensitivity analysis.

Boundary conditions and load application

Symmetry boundary conditions were applied along the longitudinal edges of the FE model to simulate the boundary conditions in the experiments. Therefore; all the nodes were constrained along the in-plane transitional direction which is orthogonal to the plane of symmetry. Additionally, all the in-plane nodes at the longitudinal edges were constrained against rotation. The transverse edge near the end support was considered to be free. All three translations were constrained against at the top layer nodes of the screw head. Constraint boundary conditions between the top layer of the screw head and the bottom face of cladding profiles. For this purpose, master-slave contact pair option available in the ABAQUS library, was used between the bottom surface of the clips and the top surface of the screw head. A uniform surface pressure load, available in ABAQUS library, was applied across all shell elements to consider static wind uplift pressure.

Analysis method

Two different methods of analysis were used: elastic buckling and nonlinear static RIKS analysis method. Elastic buckling analysis was used to obtain the eigenvectors for modeling the initial imperfections. Nonlinear static RIKS analysis was used to apply the static wind uplift pressure on CFS cladding profiles. The RIKS method can predict the post-buckling behaviour of CFS claddings, therefore static RIKS was preferred over the general static as the analysis method.

Validation of the finite element model

The FE model was validated against the test results of each of TRS cladding profiles under static uniform wind uplift loading. Significant mid-pan deflections were observed in the cladding profiles from the FE analysis.  Very good correlation was observed between the experimental and finite element results, both in terms of failure modes and wind-load capacities.

5. Weather tightness testing in accordance with NZ building cose E2/VM1:

This work was funded by “"NZ GOVERNMENT FUNDING FOR R&D-Callaghan Innovation”

Weather tightness testing of TRS interlocking panels were conducted in accordance with the NZ building code (E2/VM1 testing)

Test Rig:

Size of the weather test rig: 2.4 m X 3.6 m.

Vertical and horizontal movement joints were included in the test specimens.

The test specimens were mounted and sealed into a simulated building frame in the same manner and by the same fixings that were intended to attach the facade to the building structure.

The internal finishes and linings were installed where they contribute to the air seal. The air seal of the test sample was continued to the air seal of the test chamber.

All framing members and other interconnected joints in the facade were sealed at the specimen boundaries to minimize the effects that surrounding construction on the test performance of the specimens.

To allow the observation of any water penetration, a proportion of the internal wall lining was made using transparent material of enough structural capability and similar airtightness to resist the applied wind pressures.

Apparatus:

a) An externally mounted chamber of a size to fit the test sample of the building facade. The fit was such that it sealed the perimeter of the test specimens against the air and water penetration.

b) A reversible air pump with controls to pressurize and depressurize the chamber.

c) Water sprays

d) An orifice plate or laminar flow elements or other airflow measuring device calibrated to a traceable standard.

e) A manometer capable of measuring air pressure to an accuracy of ±2% of measuring range. The manometer used during the cyclic water test had a fast response time of at least 0.05s.

Experimental tests:

E2/VM1 is the test method for proving the weathertightness of wall claddings on low-rise buildings within the scope of NZS building code.

It was derived from AS/NZS 4284:2008 (Testing of building facades), which, in turn, was derived from work by CSIRO in Australia in the 1970s. The Australian work utilised an old aero engine and spray nozzles to simulate wind driven rain. Such engines are still used occasionally, but generally, water penetration tests have moved to a more repeatable method utilising a pressurised box, with the test specimen forming one side of the box. This approach is used in both the AS/NZS 4284:2008 and E2/VM1.

AS/NZS 4284:2008 is used to test multiple aspects of a facade’s performance, including its structural strength. E2/VM1 is intended only to look at the water penetration and water management of a facade system. In AS/NZS 4284:2008, the water penetration tests consist of spraying the specimen with a known amount of water and subjecting it to a steady pressure and then cyclic pressure tests. Failure generally occurs when uncontrolled water is visible on the inside surface of the facade.

The water penetration criterion used in AS/NZS 4284:2008 is like other tests around the world and assumes the use of structural materials that do not absorb water. However, this is not necessarily suitable for typical low-rise residential construction, which will often include materials that do absorb water.

Residential systems may have a damaging leak, but this may not manifest as water visible on the inside surfaces. For this reason, E2/VM1 has a slightly different set of water penetration tests and a slightly different failure criterion.

As well as water penetration tests equivalent to those in AS/NZS 4284:2008, E2/VM1 requires that holes be drilled in the cladding and the tests repeated. After that, the wet wall test is performed, where the pressure is applied across the cladding itself rather than the whole wall. Failure is when the cavity is breached or, in the case of the wet wall test, when water penetrates the cladding.

For low-rise buildings, E2/VM1 can be used to demonstrate that a cavity-based wall system can meet the requirements of E2 and can be suitable for medium-rise buildings based on the procedure described below.

The full-scale experiments were conducted on the test site of 12 Hutur drive, Kiwi Steel. In order to perform the tests, following steps were followed in accordance with the current design guidelines:

Test temperature: Room Temperature of ~18°C

Preconditioning-

Apply a preconditioning loading to the external face of the test sample for a period of 1 minute of positive pressure, followed by a period of 1 minute of negative pressure (suction). The loading shall be 1515 Pa.

Series 1 Static Pressure Water Penetration-

The water penetration test by static pressure shall be conducted in accordance with Clause 8.5 of AS/NZS 4284 and at the maximum test pressure of 455 Pa.

Series 2 Water Management Testing-

Tests 2 was repeated, as per the AS/NZS 4284 Clause 9.9 in at least 4 places, as noted below:

o Through the window/wall joint at 3/4 height of both window/door jambs,

o Immediately above the head flashing,

o Through the external sealing of the horizontal and vertical joints

o Above any other wet wall penetration detail. The introduction of defects is intended to simulate the failure of the primary weather defence/ sealing. It must only penetrate to the plane of the back of the wet wall so the water management of the cavity can be assessed.