Logo of the European Union
Valuate
online! Download
catalog

Static requirements

Strona główna » For specialists » Static requirements

Choosing substructure system

The main task of the elements of ventilated facade substructure is to transfer the loads to the main bearing structure of a building. In order to chose the right system the main thing to consider are the strength criteria, which are described in the Guidelines for the European Organization for Technical Approvals (EOTA) – ETAG 034 and European Assessment Document – EAD 090034-00-0404.  These are the documents on the basis of which National Technical Assessment and European Technical Assessment for the ventilated facade substructure are issued. They regulate the scope of tests to be performed for substructure elements and describe their methodology and interpretation.

Resistance tests

It is crucial to test brackets resistance against vertical and horizontal forces, described in detail in annex E to ETAG 034 and annex H to EAD 090034-00-0404. During the tests brackets are subjected to increasing loads as well as cyclic vertical and horizontal loads. The results of the tests are presented in the form of a graph of force as a function of displacement.

During measurements the following results should be recorded:

For vertical load:

  1. Load value for causing residual distortion on the bracket measured at the head of wing (after returning to zero)
    equal to: \Delta L = \frac{0,2\times L}{100}

    where L _{x}^{} is the length of the perpendicular wing to the support

  2. Loads values that cause a displacement under load of 1 and 3 mm respectively measured at the head of wing

  3. Loads values that correspond to a failure. Failure is defined by any one of the following events:
    • Any bracket breaks
    • Any bracket presents a significant permanent deflection
    • Any fixing breaks

For horizontal load:

  1. Load value that causes a residual distortion on the bracket measured at the head of wing (after returning to zero) equal to 1 mm
  2. Loads that correspond to a failure. Failure is defined by any one of the following events:
    • Any bracket breaks
    • Any bracket presents a significant permanent deflection
    • Any fixing breaks

Recorder results are then used in an equation:

F _{u,5}^{}=F_{mean}-k_{n} \times S

where:

  • F _{u,5} – characteristic breaking force giving 75% confidence that 95% of the test results will be higher than this value
  • F _{mean} – means breaking force, either under tension or shear
  • k _{n} – variable as a function of the number of test specimens for 5% (p = 0,95) with 75% confidence level when the population standard deviation is unknown (see table below)
  • S – standard deviation of series under consideration
Number of specimens 3 4 5 6 7 8 10 20 30 ∞
Variable kn 3,37 2,63 2,33 2,18 2,10 2,00 1,92 1,76 1,73 1,64

The above results are the strength criteria for the selection of brackets.  Criterion no. 2 for vertical load refers to the servicbiility limit state. The decision on the permissible deflection (1 mm or 3 mm) should be made by the designer based on the static and visual analysis of the entire system. The remaining criteria refer to the ultimate limit state. In order for the solution to be considered safe and approved for use in a given project and in certain conditions all of the above conditions need to be met. This means that design calculation loads must be less than the values calculated using the above method. This can be represented by the formula below:

F_{obl} < F_{u,5}

where F_{obl} means design calculation loads

It should be noted that the above condition should be met for all criteria. In many cases only one condition is met – most often the one regarding occasional displacement of 3 mm.

If substructure systems consists of any kind of hooks despite profiles and brackets then strength tests similar to those described above should also be performed for them. There tests are described in detail in annex D to EAD 090034-00-0404.

BSP system solutions

Elements of BSP systems are tested in certified institute – Institute of Building Technology (ITB) in accordance with ETAG 034 and EAD 090034-00-0404 according to the above description. To download the results of the research follow the link below.

Go to: Tests and reports

 

Static calculations of aluminium substructure

Individual static calculations should be made for each design of ventilated facade in order to collect loads, determine the layout and spacing of substructure elements and count their strength. In case of aluminium substructure design there is unfortunately relatively few standards describing the rules of their design. The Eurocode 9: Designing of aluminium structures, describes them in the most detail. It is one of the ten European Standards established by the European Committee for Standardization, which are in force in the Member States of European Union and are gradually replacing national standards in accordance with Recommendation 2003/887/EC. It is assumed that Eurocode European Standards constitute an overreaching set of design and construction regulations, on the basis of which all structural calculations should be made. Usually specialised computer software is used in calculation process. Unfortunately, very few of the programs currently available on the market can calculate and dimension aluminium structures in accordance with Eurocode 9. This is due to the fact that aluminium structures, unlike steel structures, started to appear in projects relatively recently.

In order to verify static calculations, it is recommended to perform laboratory tests, thanks to which it is possible to safely establish and confirm the strength of the individual elements of the substructure and the entire solution of attaching the facade cladding.

Individual static calculations of the BSP substructure

For each workshop design of the BSP substructure, individual static calculations are made to confirm that static requirements have been met. Our company has an RFEM program (Dlubal Software) that has implemented the European Standard Eurocode 9: Designing of aluminium structures. Sample calculations are available for download below.

 

Sample calculations: