Are you a main contractor considering prefabricated steel for the first time, and want to know if it will be structurally calculated?

If you’re used to working with subcontractors who use in situ poured concrete, you’ll know that they don’t usually complete the design work.

Unfortunately this includes a vital part of any project; the structural calculation.

If you’re like most of our clients, a subcontractor who can take on that design risk is an appealing prospect.

As a subcontractor specialising in prefabricated steel, I’m going to talk you through how this method of below-ground construction can eliminate the risk for you and your team.

Why a prefabricated steel substructure can reduce risk

Many ground work subcontractors simply don’t have the structural ability to complete the contractor’s design portion (known as CDP) for a below-ground substructure.

However, for main contractors, this is so important, as you’ll want to ensure you employ someone with that ability. Otherwise, they could be lacking the insurance, skillsets or experience for your project. That’s why many subcontractors are solely employed to complete the installation of a design, rather than be involved in the design itself.

So, what makes prefabricated steel different?

Well, with a prefabricated steel subcontractor, they can complete a full design package – designed by their own engineers – which is then issued to your engineer for feedback.  After your final comments, the substructure can be manufactured (which can take around four – six weeks), and then installed on-site in a matter of days.

How can you ensure your substructure is structurally calculated?

In the early stages of engaging with a subcontractor who specialises in prefabricated steel, the best way to ensure your substructure will be structurally calculated is to issue them with a ground investigation (GI) report for the project.

You should also provide them with a design intent. This will allow for the substructure to be fully calculated. The design intent should specify the exact design requirements the structure has to achieve.

Once your subcontractor has this, they’ll be able to design your bespoke prefabricated steel substructure to suit your exact specifications and needs.

In my experience, clients who have worked through the structural design process with us usually turn into repeat business. Once they realise that it completely de-risks a difficult activity on-site, they’re able to see more of the value of prefabricated steel.

Advice from a specialist subcontractor

If you want the best results, including a substructure that’s structurally calculated for your project and is to be watertight for over 100 years, I’d recommend getting in touch with a subcontractor who specialises in prefabricated steel and has the experience for the job.

This has to be done early in the project; ideally in the design or tender stages. A good subcontractor should be able to talk you through your options and take the time to understand your needs, as well as the specific contract requirements of the project.

So far, we’ve never come across a project designed for prefabricated steel that couldn’t be structurally calculated – and I’ve seen us complete some very extreme designs. With over 35 years’ experience, we’ve encountered many different design conditions and constraints, as well as site constraints.


I hope this blog has helped shed some light on structural calculations for prefabricated steel substructures. This is something a specialist subcontractor should be able to provide you with, taking on all the risk of this vital aspect of your project.

Just be sure to engage them early on in the design or tender process. They should be able to advise you on your next steps, and what they’ll need from you in order to start the process.

NOTE: Remember, your specialist subcontractor will need to be issued with a GI report and design intent.

Do you have any specific questions about structurally calculated prefabricated steel substructures? Let me a comment below. I’d be happy to reply!