Further example of incorrect finite element modelling

Collaborative reporting for safer structures. Report 1145 Further example of incorrect finite element modelling

A CROSS report has prompted further concerns about finite element modelling – in particular, the modelling of masonry walls as part of a reinforced concrete framed building

CROSS report 1073 Concern over modelling of concrete frame building for construction stage reminded a reporter of errors that came to their attention when checking a full-building finite element model. It included loadbearing and non-loadbearing walls, which were intended to be constructed in masonry. The primary part of the structure was of in-situ reinforced concrete columns and slabs including a transfer slab. There were some beams, but in the main it was flat slab construction. The walls had been modelled with full attachment to the slabs with no releases other than a moment release for transverse bending. The reporter contends that this form of model will give grossly incorrect results in most circumstances and is a worrying situation. There are at least two effects at play – which, according to the reporter, do not seem well understood by less experienced staff and some more experienced designers.

Firstly, walls will arch, and they are usually stiff enough to arch even when the masonry is much weaker and less stiff than the frame. Indeed, this can sometimes be seen in practice. The result is that the beam or slab that the engineer thinks is supporting the wall is not receiving that load, especially near the middle of the span, so slab or beam bending moments may be significantly incorrect.

Secondly, a full connection (where in-plane shear can be transferred) will inevitably cause the wall and slab or beam to behave compositely. If the wall is of substantial height, say a storey height, it will have appreciable stiffness. The effect is to introduce non-conservative errors. Shear outputs will be affected too; however, it depends on how the wall is attached to columns, other walls or other elements. It might be expected that most of the shear would get back to the ends, but that would not be the case if the wall has been inadvertently attached to the column, remarks the reporter.

The reporter contends that design errors will propagate regardless of whether the designer designs manually, or allows the software to generate the reinforcement from the model. They experimented with various software packages to try to evaluate if there are any easily implemented ‘workarounds’. The necessary releases can be made, but this is not always straightforward. It may also be complicated by how the software makes these releases. For example, some software can release the longitudinal shear along an edge, but still have the end/corners attached. Getting all this right is possible, but it is fiddly and – the reporter fears – beyond many who are routinely creating these models.

Another thing that might tempt the user is to ‘soften’ the wall material. However, because the wall is likely so stiff, it would have limited effect. A further idea might be to directly adjust the element stiffness matrix to make it incapable of interacting in this way. Doing so, however, could result in elements that are badly conditioned and may require much study to validate whether they are behaving and interacting as expected.

The reporter would ideally like to see a special wall element that is compression-only and unable to interact in the sense of longitudinal shear at the edges and corners. The reporter has only seen one software system that might implement something like this. Wall elements can be arranged to form vertical strips – these act like a series of columns that are effectively in contact to form a wall. They might gather transverse load, such as wind, by using dummy load collection elements, which are quite common.

The reporter suggests that if a designer is not confident that they can predict behaviour or interpret the results, they stick to modelling masonry walls as loads only so that unwanted interactions cannot occur. This is inconvenient but tends to be on the safe side. The reporter is concerned that there is insufficient training for these roles.

Further reading

CROSS report 886 Unconservative design of flat slab due to software modelling issues: bit.ly/CROSS_886

CROSS report 1073 Concern over modelling of concrete frame building for construction stage: bit.ly/CROSS_1073

CROSS report 1139 Connection fixity considerations for steel frame modelling: bit.ly/CROSS_1139

CROSS report 1144 Incorrect modelling of a cantilever: bit.ly/CROSS_1144

Comments

The reporter raises several very valid points and emphasises that designers should understand the medium they are working in and that analytical methods are approximations of true performance. These approximations and their influence need to be well understood. CROSS report 886 Unconservative design of flat slab due to software modelling issues reviewed similar issues where masonry was modelled as part of the supporting structure – this report also noted the improper use of (or over-reliance on) computer modelling, with the potential for results to be divorced from reality. As noted by the reporter, CROSS report 1073 explored a number of further issues related to modelling, including the importance of verification processes.

The idea that engineers can do all analyses and design in a single model should be challenged. Software marketers who may not have sufficient oversight from those who actually understand how the programs work may be tempted to over-sell what can be modelled. Designers who have been taught by engineers, however, are more likely to understand why modelling masonry as part of a steel or concrete structure is very difficult.

Finite element analysis (FEA) in its standard form only addresses linear elastic materials. There are tricks that some software developers employ to allow the use of tension or compression-only 1D elements in such an analysis, by negating their action when they are subject to axial forces in the “other” direction. These options are not suitable for compression or tension-only 2D elements, so in these instances advanced nonlinear explicit solvers are needed. These programs are usually beyond the budgets of all but a few specialist engineering firms.

FEA requires engineers to understand all the materials being modelled; however, masonry is perhaps not addressed in many engineers’ academic journeys and therefore many designers do not appreciate how modelling should take account of the engineering properties and use of masonry on site. The reporter is right to be concerned about the education basis of modelling. Arching of masonry, for example, must be carefully considered, especially when the masonry is built upon a flexible support. When each course is laid, the mortar can carry a little compression (though not much, hence the limits on how high masonry can be built in a day) and less shear. Hence arching is unlikely to occur for the wall’s self-weight unless the supporting structure is propped during the wall’s construction. What is then likely to happen is that subsequent loading on an under-stiffness supporting slab or beam will try to pull it away from the masonry above, causing the masonry to attempt to arch and certainly to crack.

We also have changes in time and temperature to contend with. In this example, we have an interaction between masonry and concrete, both of which exhibit time changes by creep and shrinkage and expansion, and in the case of these two materials to differing degrees and possibly in opposite directions. Squashing masonry in as shear panels and not allowing soft top packs (for vertical frame shrinkage) has caused problems, but will also certainly invalidate any FEA stress predictions. It must be remembered that the detailing of the joints between the concrete and masonry will allow the designer’s intent to be achieved. If, however, the designer is not clear about how the structure behaves, how are they able to decide the details needed for a safe structure?

Key Learning Outcomes

For civil and structural design engineers:

• model masonry walls as loads only, other than in exceptional cases

• use the simplest model that is reasonably practicable

• remember that analytical methods are approximations of true performance

• be aware of time- and temperature-related changes such as creep, shrinkage and expansion

• attributes such as ductility are fundamentally important for the safety of structures

• ensure that those using specialist software programs understand the materials being modelled; and

• it is good practice to carry out sense checks and validate all analysis and design outputs.

FEA of masonry can be used with great care to gain an insight into how forces can flow through masonry, but should not be relied on for the design or detailed understanding of it. Only in exceptional cases should masonry be represented in steel and concrete FEA models as loads, with independent analysis and design checks being made on the masonry panels themselves.

Finally, we should not forget that originally all analysis was linear elastic, but alarm spread when real stresses were measured in frames and it was realised there was little correlation. Out of that came the concept that design ought to be based on ultimate loads. Design then largely evolved into a procedure meaning ‘do it this way and, by long experience, we know the frame will be good enough’. Although this process might break down with more complicated buildings, the concept of ultimate design is quite adequate provided the structure is, as intimated by the reporter, ductile enough. However, that quality of ductility is elusive and not directly calculable, and is achieved largely by detailing – which is not always under the main designer’s control. Some may consider it fundamentally wrong to excessively manipulate analyses to try and control stresses on the premise that stress governs everything when we know that is not the case in real life. Other attributes such as ductility are fundamentally important for the safety of structures.

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