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Reducing costs using high-strength steel in conjunction with GMNIA analysis – Stressfield Oy

High strength steels are not yet widely used even though there lies a significant cost-cutting opportunity. Often they are seen as an exotic option for traditional steel grades with their strength increase just compensating for the extra cost of the steel per tonne. This is especially true when using same traditional structural design methods as with low-strength steels. The real gain comes only after pairing these modern high strength steels with modern GMNIA analysis to really shave off all unnecessary material from the product.

In many cases the analysis is needed to fill the requirements set to the product by national standards. With a little extra effort the basic analysis can be extended to more beneficial analysis. So, instead of seeing the strength calculation as a mandatory extra burden of the project, why not utilize it as a cost-cutting measure as well!

Extra effort on advanced analysis Pays back in the end

Applications of GMNIA analysis

High strength steel structures are not the only place where GMNIA analysis can be utilized. It can be used whenever there are thin members included in the design. Examples of such products are

  • Load bearing plated structures
  • Lifting devices
  • Crane booms and girders
  • Stiffening plates and webs
  • Air ducts and channels
  • Plated machine beds and fixtures

However, in this article we focus on pairing high strength steels with GMNIA analysis.

What are the benefits of using high-strength steels?

Simple. Reduced usage of steel. Even though the high strength steel is more expensive than lower strength steel, you can reduce plate thicknesses to end up with less steel consumption and overall cheaper manufacturing cost. This is important specially now with high steel prices. Furthermore, lighter structures allow higher payloads, are easier to handle and cheaper to ship. By reducing the plate thicknesses, the amount of welding is also reduced leading in additional savings in material and labor costs.

How to gain the full benefits of high-strength steels?

Higher strength steels allow for reduced plate thicknesses and overall more slender structures. However, with the reduced plate thickness comes stability issues, which were not a huge problem with heavy low-strength steel design. If the new, slender structure is designed using the same methods that were used in designing the low-strength steel structure, you will end up with either stability issues or under-utilizing the capacity of the high-strength steel.

Two beams with same load bearing capacity. S235 beam fails by reaching its plastic capacity (left), while high-strength steel slender design fails by local buckling of the lower flange and web (right). In this example the high-strength steel solution cuts material costs by 35%.

The solution is a clever usage of GMNIA-analysis. This type of analysis gives very accurate information of the structural behavior which leads to better and more economic design. The design is improved by removing structurally unnecessary material. This way the full potential of the higher strength of the steel can be utilized and the overall material usage will be reduced.

GMNIA analysis

GMNIA stands for Geometrically and Materially Nonlinear Analysis with Imperfections. In practice it means that we take the standard analysis as a basis and extend it to include material nonlinearities (plasticity) and geometric nonlinearities (membrane action). However, no product is ideally perfect. Tolerances allow for imperfections in structures. Also, residual stresses are caused by the manufacturing processes. And even if we had the most perfect structure, it still buckles at some point. That’s why we introduce initial imperfections to the analysis model. The model now predicts accurately the actual product’s structural behavior under different loading conditions.

As we now better understand the structural behavior of the product, we can change the design accordingly. For optimal usage of the material, we can reduce material from low-stressed areas. This can lead to stability issues, which can be fixed by adding stiffeners to the problematic area or changing the overall design. The aim is to utilize as much of the material strength as possible without losing stability of the structure.

Case study: lifting device

A lifting beam is to be designed. The beam must fulfill the requirements of crane standard EN 13155, so it has to be analyzed anyways. Three material options were compared: S355, S420 and S700 (high strength steel).

The lifting beam is supposed to carry 5t weight
Failure of S355 beam by yielding. Stiffener highlighted in orange.
Failure of S700 beam by local buckling of the web. Stiffener highlighted in orange.

The stiffening plates are extended according to GMNIA analysis in order to give more support to the problematic area. The GMNIA-analysis is then re-run for the improved design.

It is very important to visually inspect the initial imperfections of the GMNIA analysis as they play vital role in the results. Incorrect choice of initial imperfections can increase the calculated load-bearing capacity unrealistically.

The equivalent initial imperfection state that is applied to the GMNIA analysis is usually generated by combining multiple linear buckling modes. Thus, the actual imperfections do not have to be measured from the actual product and they do not have to be monitored at the production line other than following the general manufacturing tolerances.

Failure of improved S700 beam design by local buckling of the web. Stiffener highlighted in orange.

After the modification to the stiffeners the required capacity of the beam is reached. The plate thicknesses are reduced 25% compared to the original design.

The relative mass for analyzed steel grades are shown in figure below. The mass of the structure was reduced by 41%, and when accounting the higher price of higher steel grades, the material cost is reduced by 22%. The best result has been achieved by altering the product design during analysis. The original design weighted 600 kg. In this case the high strength option saves 22% in material costs compared to S355, which is more than the extra cost of GMNIA analysis on top of the mandatory analysis.