The Analysis of Fabricated Steel Structures.
How you join components together is a perennial problem for designers. Each of the joining methods (welding, bolting, adhesives etc.) have their advantages and disadvantages, so if the choice is free how do you choose the right one? In many cases the method of joining is dictated by the material its thickness and size and where the components are assembled (on site or in a factory). But the question of how many or how much of your joining method is a trickier question.
Technology also impacts your joining methods. In the 1900’s if you wanted to permanently join two thick steel plates together the most common method was hot riveting. One hundred years later this is a pretty rare joining method. In recent years considerations of recycling have also come into play, with the increasing use of design methods promoted by the IDSA Okala Guide (you can find an old one here http://www.idsa.org/okala-ecodesign-guide )
Structural analysis (or FEA) cannot tell you which joining method is the most eco-friendly or cost efficient but it can tell you if your joining method will fail. So it is an ideal tool to answer the question of “how many or how much”
Let’s consider some fabrication methods.
Using adhesives to join components together where there is a significant area is a very common manufacturing process, especially when you don’t want the parts to be separated. Below is a simple test case of two thin plates tested in tension, for clarity the plates are shown with a gap.
This is possibly one of the most common fabrication methods. In SolidWorks Simulation the analysis of a bolted fabrication is a pretty straight forward. Consider this design of a splice plate. What we want to know is how many bolts to use and what kind of preload we should use?
Hot riveting in conceptually similar to the bolt methodology except the preload is defined by a thermal load. Let’s look at the splice plate again, but this time with hot rivets. The process of hot riveting ensures a tight fit by assembling the components when the rivet is at a high temperature and as it cools the rivet shrinks drawing the components together. We can simulate this behavior with a negative thermal load on the rivets.
The analysis of weld is possibly one of the trickiest forms of fabrication analysis, which is a shame as this is a very common fabrication method. The difficulty in analyzing welds with FEA lies in the fact that FEA generally assumes homogeneous and defect free materials. When components are welded this is not the case. The welding process not only affects the weld bead but also the materials that are being joined, locally changing their material properties (the heat affected zone). The weld itself is also prone to imperfections and inclusions which act as stress raisers. This is why welding standards are so widely used. Generally the effort needed to get a good FEA analysis of a weld is not worth the increase in accuracy over welding standards. But if you do feel the need to analyze a weld you should employ specific analysis methodologies.
- The default parent components cannot be bonded together
- The load path can only be through the welds
- Create multi body parts for the heat affected zones
- Consider the extend of the weld penetration
These consideration are shown in a simple example below
With SolidWorks Simulation one can also look at spot welding for thin or sheetmetal components. Let’s look at the model we used for the adhesive test, but this time we can join the two sheets with a series of spot welds, defined by datum points.
Spot Welded Tension Test Piece: With SolidWorks Simulation the spot weld size can be defined and the solution quickly calculated.
Defining Spot Weld Connectors
Spot Weld Stress Results
In many cases you need to mix and match FEA methodologies to mimic a particular fabrication case. But as long as you are systematic this should present no problem to SolidWorks Simulation.
If you have to analyze large structures, SolidWorks Simulation has a great tools called sub modeling. This allows you to analyze the large scale problem, and then focus is on small scale problems such as welds or joints.
With SolidWorks Simulation you can look as granular as you want or as far field as you desire. So the next time you wonder how many or how much during fabrication do yourself a favor run a quick analysis. It's much better to know rather than guess.
Stephen Endersby is a Senior Product Manager at DS SolidWorks specializing in simulation. Stephen is a passionate advocate of simulation in the design process and believes anyone can learn to use and gave value from SolidWorks Simulation