7 Replies Latest reply on Jul 15, 2013 4:19 PM by Chris Saville

    Max load capacity of material rack - please help

    Chris Saville

      I too am new to simuation and I have to try and work out the max material carrying load on a rack made from 1020 cold rolled and welded assy.

       

      Can anyone help so I have time to have a go at thia please?

      Assy.jpg

        • Re: Max load capacity of material rack - please help
          Jared Conway

          Hi Chris, looks like a good one for weldments and/or mixed meshing. Where are you stuck so far? If you're stuck getting started I'd draw up a quick outline of what the load is, how it distributed, how this is going to be "held" in space in real lfie and if you were going to calculate something by hand, how you would do it and what assumptions you are ok with. It would also be good to discuss the level of accuracy that you need. That has an effect on the approach you'll want to take.

          • Re: Max load capacity of material rack - please help
            Chris Saville

            Thanks Jarad - I am as you said - stuck getting started. The rack simply holds bar stock, plate etc, it is free mounted on a concrete floor and an absolute approximation of the capacity on the arms is what I am being asked - I need to do some reading methinks.

              • Re: Max load capacity of material rack - please help
                Jared Conway

                i'd also go for some of the tutorials in sim.

                 

                if this was me. i'd probably look at starting simple (just analyze the arms) and then worry about a full analysis of the structure after.

                 

                how we generally help customers in your sitaution is set them up with at training class and then one-on-one mentoring. the training gets them the foundation and then the one-on-one helps them see how it applies directly to their problem. sometimes they have a need for the results immediately so we'll solve the problem as an analysis consulting problem, and then come back and teach them when the project is done.

              • Re: Max load capacity of material rack - please help
                Stewart Mills

                Hi Chris,

                 

                I have a bit of experience engineering these types of structures and for whatever reasons I'll offer you some considerations...

                 

                Typical ASTM type standards suggest designing for types of use or categories.  For example, if the structure is something you will use and is in your control as to how it is used/inspected then the category requires less restriction.  The other end is if this structure is used where you have no idea as to how it is really being treated/inspected then the category is more strict.  Of coarse, the categories are meant to keep people safe not the structure so, you can make that decision as to what category is needed.  In addition, when testing the structure you may consider loading it to 125% of the rated max load capacity then inspect the structure and welds for yield and/or cracking. 

                 

                Some comments on the design...

                • the bottom six gusset tubes (the X shape) could be changed to one vertical tube, on both sides of the column, between the bottom and the first T.  This change may allow more volume capacity for the rack while providing the same structural support
                • the center column has no foundation support and may be considered a weak link when only two columns are loaded while supporting shorter stock materials.  This issue can be resolved by adding a footer cross-beam similar to the two end columns.  You may consider removing the two center footers, as seen in your original image above, as these appear not to be loaded directly

                 

                Some thoughts on identifying the maximum load capacity...

                • non-symmetric loading is probable.  That is, load one end column and the center column on one side
                • shelving is also probable in an uncontrolled use.  That is, say an end column and the center column are loaded with stock which looks like an attractive shelf.  Now, plates and couplings and what-not are stacked at one end of the shelf...
                • As Jared suggested, start your analysis of stress by considering a cantilever of one of the proposed T columns loaded at the end of the cantilever with a single load then a distributed load.  One approach is to set the problem up using known material yield stress and tube cross-section geometry then back out the max load for each case
                • I suspect the worst case will be when a set of the top T tubes are non-symmetrically loaded causing higher stress and/or deflection of the vertical support tubes just above the bottom T.  Deflection may become more of an issue depending on how tall this structure is...
                • For this problem, I would not model the welds unless they are critical but based on the image above they appear not to be.  Good practice is to not put welds into bending...only shear, tension or compression and you have done that
                • Base your failure criteria on how to keep people safe; a 2x factor is pushing it when compared to most standards for potentially dangerous situations...dropping a thousand pounds of steel on an unsuspecting technician for example.  A factor of 2.5 is a typical minimum for stresses in a controlled environment and higher for an environment where you will not have control of how the structure is being used

                 

                While my comments do not explain how to do this in SolidWorks Simulation they may help you formulate an approach while using SW Simulation for virtual prototyping and virtual testing.  I suspect if you start with a simple structure, say one T column and perform a simple cantilever loading then verify the results on paper you will be ready for a more complex model.  In general, simulation of these types of structures follows a method:

                • Make sure all items in assembly have a material assigned
                • Suppress any items not being considered in the simulation
                • Create a Study
                • Apply boundary Conditions or Fixtures such as fixed (6 dof restrained) or slider (2 dof restrained) etc...
                • Apply connections if needed...(SW Simulation will automatically create bonded contact sets by default making it relatively simple to get a quick look at how an assembly will respond to loads)
                • Apply loads
                • Mesh or...
                • Run (if not meshed the run command will mesh the model)

                 

                Hope this helps,

                 

                -Stewart

                  • Re: Max load capacity of material rack - please help
                    Chris Saville

                    Thanks Stewart, is this what you meant......? Thanks for the input very helpfull

                    ASSY 2.jpg

                      • Re: Max load capacity of material rack - please help
                        Stewart Mills

                        Yes, if this works better for you regarding vertical supports instead of the gussets.  The foot under the structure is a great addition while I might consider removing the intermediate footers but I'll leave that up to you and your situation...

                         

                        Now, if we consider the upper two T's and we load the upper T with a cantilever condition, then it may be easy to imagine the max bending type stress will be where the vertical tube(s) connect to the lower T.  Again, deflection from bending may become more of a design criteria than actual stress but this will depend on how tall the structure is...

                         

                        A method to reduce the deflection in the bending direction is to stiffen the substructure.  Maybe similar to what was done on the lower T by adding verticals to help support the bend of the cantilever extensions of the T.  Just a thought but you will know what is best for your situation.  Now, as we stiffen the substructure the stresses regarding the cantilevers will most likely go up as the structure becomes less flexible.  This in general isn't a problem because we are now basing our criteria more on stress and less on unpredictable loading and flexure which is why it is good practice to label the max rated load on the structure...

                         

                        Best of luck...