ICF Structural Design
Many view Insulated Concrete Forms (ICF) simply as an alternative to brick or timber. However, ICF is a highly versatile construction method offering significant advantages in structural design. This versatility is often misunderstood by architects, builders, and even many structural design consultants who may not be aware of the possibilities introduced by recent changes in design standards. This page delves into the modern technologies structural design consultants can leverage to produce sophisticated ICF structural engineering drawings.
Insulated Concrete Form Design Considerations
Structural Design of Buildings Practice
Many new ICF houses are built to very modern designs with large open areas and cantilevers. Insulated Concrete Forms Design can be very useful in these circumstances because when it is reinforced the amount of structural steelwork required can be substantially reduced – especially if the Structural Design Consultant is experienced in the design of ICF.
Find out more about how using ICF can simplify your building project.
Building Regulation – Structural Compliance
Part A, Structure
The UK Building Regulations do not refer to ICF as a construction method and so the design must follow the principles set out in Part A.
Section 2 of Part A deals with the structural design of the building by setting out the size of structural elements based on cavity wall construction.
ICF walls without reinforcement are deemed to provide the equivalent structural strength and so if the structural engineer drawings comply with the Section 2 limits then calculations are not required and it will not need to be reinforced (unless it is necessary to stabilise the forms during concreting).
LOADING
A1. (1) The building shall be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground:
(a) safely; and
(b) without causing such deflection or deformation of any part of the building, or such movement of the ground, as will impair the stability of any part of another building.
(2) In assessing whether a building complies with sub-paragraph
(1) regard shall be had to the imposed and wind loads to which it is likely to be subjected in the ordinary course of its use for the purpose for which it is intended.
GROUND MOVEMENT
A2. The building shall be constructed so that ground movement caused by:
(a) swelling, shrinkage or freezing of the subsoil; or
(b) land-slip or subsidence (other than subsidence arising from shrinkage), in so far as the risk can be reasonably foreseen, will not impair the stability of any part of the building.
Part A States:
In simple terms, the Guidance is saying that if any building or structure lies within the limits set out in Section 2, structural calculations are not required.
However, if the building or structure does not fall within the limits set out in Section 2, calculations are required to show that it will sustain the loadings specified in A1/2 in accordance with the Codes of Practice specified in Section 1.
Part A, Section 2
If the structural elements of a building comply with Part A Section 2, plain concrete ICF can be used without requiring structural engineer calculations.
If the Structural Elements do not comply with Part A Section 2, Structural Engineer Drawings will be required to demonstrate the Structure is safe by providing the calculations required based on Eurocode 2: BS EN 1992-1-1:2023 Design of concrete structures. In particular, the structural design consultant must show that the building is safe under wind loading.
This Apartment Block was designed and built in 2023 and demonstrates how ICF can be used as the main structural framework without the need for structural steel. The reinforced concrete beams over the open areas at ground floor level support the unreinforced ICF walls above. The walls on the North and South elevations are reinforced to resist the wind loading but are not generally reinforced as would be the case if the design was carried out to BS8110
Technical Advantages:
Structural Design Consultants are justifiably nervous of accepting commissions for the design of an ICF structure because the Designer must select the Method of Design that will suit the structure. This is completely different from the way in which Engineers are trained because they are normally told by the Architect to provide a structural steel or reinforced concrete solution. Historically, the design was then carried out in accordance with BS8110 (Structural Concrete) or BS5950 (Structural Steel).
These Codes have been withdrawn and BS8110 has been replaced by Eurocode 2: BS EN 1992-1-1:2023 Design of concrete structures. This has left Structural Engineers with a huge learning curve and many have just followed exactly the same design methods with adjustment of the material and load factors and the concrete detailing rules from the new Code.
BS8110 was based on design rules for every type of structural element such as beams, slabs and columns. The new code just provides general principles and allows far greater discretion on how to design a structure. This allows far greater scope for innovative structural design of buildings in an exciting new material like ICF.
But this freedom to experiment requires very experienced Structural Engineers!
The design flexibility provided by Eurocode 2: BS EN 1992-1-1:2023 allows the production of Structural Engineer Drawings using ICF as structural framing for apartment and office blocks without the large tonnage of reinforcement and time consuming shuttering required for a conventional reinforced concrete frame.
When shutters are stripped from a concrete wall the exposure to oxygen makes the chemical reaction that hardens the concrete occur in about 4 weeks and there is very little subsequent strengthening. The concrete in an ICF form cures over about a year and reaches strengths about twice that of shuttered concrete. And in the forms that have lots of wire ties (as Polysteel forms have) there is virtually zero cracking.
This allows the structural design of the building to take full advantage of very high concrete strengths, including allowable tension in the concrete that was never allowed in the old British Code BS8110.
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